VENTILATION IN THE PATIENT WITH UNILATERAL LUNG DISEASE

MECHANICAL VENTILATION

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VENTILATION IN THE PATIENT WITH UNILATERAL LUNG DISEASE Allen R. Thomas, MD, and Tracey L. Bryce, MD

One of the more difficult ventilator problems confronting physicians in the critical care setting is the management of patients who have unilateral lung disease (ULD). Conventional ventilatory support that applies the same flow and pressure to both lungs fails to produce adequate gas exchange in many clinical situations in which the pathology involves one lung exclusively or predominately. Application of traditional methods to improve ventilatory support may result in worsening of the gas exchange and injury to the more normal lung. The aim of this article is to review the ULDs in which traditional ventilator support has been reported to be ineffective or detrimental and review modalities and methods of support developed and reported to manage successfully patients who have these conditions. Particular emphasis is placed on the role of lateral positioning and independent-lung ventilation (ILV) in these situations. TYPES OF LUNG INJURY

A variety of disease processes have been reported to produce severe unilateral lung pathology in the critical care setting (Table 1).ULD becomes a management problem during mechanical ventilation when significant differences in the mechanical properties of the two lungs produce difficulties that hinder the physician’s efforts to oxygenate and ventilate the patient adequately. To help understand the mechanisms by which ULD produces management problems, these processes are considered in the context of the underlying pathophysiology and the differences in the mechanical properties of the two lungs-specifically whether the pathologic process produces an increase or a decrease in the compli-

From the Department of Medicine (ART), the University of Arizona/Maricopa Medical Center Integrated Residency in Anesthesiology (TLB), Maricopa Medical Center, Phoenix; and the Mayo Medical School-Scottsdale (ART), Scottsdale, Arizona

CRITICAL CARE CLINICS VOLUME 14 * NUMBER 4 * OCTOBER 1998

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Table 1. UNILATERAL LUNG DISEASES ASSOCIATED WITH RESPIRATORY FAILURE

Unilateral parenchymal disease with decreased compliance of the involved lung Pulmonary contusion Unilateral pneumonia Reexpansion edema Repetfusion edema Aspiration Refractory atelectasis Massive hemorrhage Unilateral lung problems associated with increased compliance of the involved lung Bronchopleural fistula Single-lung transplant in obstructive lung disease Unilateral hyperinflation Other reported unilateral problems Massive unilateral pulmonary embolism Large pleural effusions

ance of the involved lung. This classification is also thought to be useful since the underlying pathophysiology dictates and directs the therapeutic approach in each of the major types of ULD. The literature addressing the ventilatory management of ULD consists of case reports and case series frequently mixing diseases of varying pathophysiology, with the common link being the presence of disease exclusively or predominantly in one lung and the utilization of unconventional methods of support. Understandably, there have been no controlled patient series dealing with these problems. Without a systemized approach to describe the underlying pathophysiology, it becomes difficult to compare patients, therapeutic methods, and outcomes. The simplified scheme utilized should provide some direction for those confronting similar patient problems. There are obvious limitations to a simplified classification when applied to a widely disparate group of patients, particularly with the numerous other problems typically encountered in a critically ill patient requiring mechanical ventilation. Unilateral Lung Disease with Decreased Compliance on the Involved Side

The largest collection of case reports concerning problematic ULD deal with those diseases that result in decreased compliance in the involved lung and redistribution of ventilation away from the affected side. These compliance changes typically result from diseases that produce alveolar filling, interstitial edema, or atelectasis in the involved lung. Patients with pulmonary contusion, unilateral pneumonia of various origins, reexpansion edema, and refractory atelectasis account for most of the reported cases of this type of unilateral disease which have proven refractory to conventional ventilatory management. The largest number of reported cases in this group are those with pulmonary contusion (Fig. 1).This is understandable since external chest trauma is the most likely means of producing a massive injury to one lung. Other lung diseases that are acquired through the airways or hematogenously are more likely to involve both lungs if the insult is large. Problematic cases of unilateral pulmonary contusion tended to respond well to treatment with most reported patients surviving.Io6 Severe pneumonia is a common problem in the intensive care setting, but

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Figure 1. Chest radiograph of a trauma victim with a right hemothorax and extensive rightsided consolidation from a lung contusion. The patient had marked hypoxemia refractory to supplemental oxygen but improved with lateral positioning.

overwhelming unilateral pneumonia is relatively rare. Much more common is severely asymmetrical pneumonia (Fig. 2), which can produce the same gas exchange abnormalities seen in unilateral disease. Patients who have severe unilateral pneumonia reported to require unconventional support do poorly with over 50% mortality.'06 Many different organisms have been reported to produce severe unilateral pneumonia, and aspiration is a frequently noted assoc i a t i ~ n .67,~ 81, ~ ,88, 93, 94, If18The other diseases noted in Table 1 are from reported cases that gained attention because of the significant alterations in gas exchange that required intervention beyond conventional mechanical ventilation. When occurring in one lung, these diseases produce a decrease in compliance of the involved lung with maldistribution of tidal ventilation when equal pressure and flow are applied to both lungs. The involved lung receives less of the tidal volume and the more compliant normal lung receives a greater portion. Perfusion is frequently less affected and the resulting ventilation-perfusion mismatch produces significant oxygenation defects as the result of intrapulmonary shunt and shunt effect.2s,52, 98 Patients who have severe abnormalities of this type are typically refractory to high F ~ o ~44,. sy,~ "', ~ y8, , Failure of normal protective mechanisms, particularly hypoxemic vasoconstriction, to compensate adequately for decreased ventilation in one lung and improve ventilationperfusion matching may be a major determinant of which patients will require unconventional techniques as opposed to those who respond to conventional support. In some patients who have severe ULD, conventional mechanical ventilation may not provide satisfactory support. The application of positive-pressure volumes and positive end-expiratory pressure (PEEP) may overexpand the more compliant normal lung and exacerbate the ventilation-perfusion mismatch by diverting blood from the normal to the affected lung.2s,sy Failure to respond to supplemental oxygen and a paradoxical deterioration with PEEP are typical of

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Figure 2. Chest radiograph of a patient with extensive left-sided pneumonia and multiple segments involved on the right. The patient was refractory to supplemental oxygen and had significant deterioration in gas exchange with the use of PEEP. Lateral positioning was transiently beneficial until the patient developed generalized disease and ARDS which eventually improved with PEEP.

severe noncompliant unilateral lung injuries?,24, ~ 5 , 65,89 In addition to a PEEPinduced worsening of hypoxemia, other detrimental effects reported include PEEP-induced hyperinflation of the normal lung with collapse of the affected lung,', 25 bar~trauma,"~ and PEEP-induced reduction of cardiac output and blood 65, lo* The rate at which these problems with unilateral alterations in pressure.25, compliance occur and require special management cannot be determined from the literature. Unilateral Lung Disease with Increased Compliance on the Involved Side

As opposed to the first group of diseases, the other major group of difficult ULDs are those that produce redistribution of ventilation toward the involved side either from air leakage or an increase in the compliance in the affected lung. The commonly reported processes include bronchopleural fistula (BPF), single-lung transplantation in patients who have obstructive lung disease, and unilateral hyperinflation. BPF is a relatively common critical care problem with many potential causes. Approximately two thirds of all cases of BPF are related to surgical procedure~'~ with the remainder being caused by several pathologic processes in the chest, including necrotizing pneumonia, tuberculosis, lung abscess, chest trauma, and empyema. Severe presentations of these lung problems are more likely to be associated with BPF, and these patients are also more likely to require critical care management and ventilatory support. Additionally procedures typically

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associated with critical care management, including central-line placement and mechanical ventilation, particularly ventilation with high transpulmonary pressures and overinflation, can be complicated by pneumothorax and the development of BPF. Massive air leak through a BPF may result in inadequate alveolar ventilation and respiratory insufficiency. Inadequate ventilation of the noninvolved lung can produce significant ventilation-perfusion mismatch and arterial hypoxemia. Efforts to improve gas exchange with increased minute ventilation may result in increased air leakage as the pressure gradient across the defect increases. Incremental increases in PEEP to improve oxygenation may also result in increased leakage through the BPF. Increased flow through a BPF interferes with healing and closure of the defect.13,82 Conversely, ventilatory measures to reduce the air leak through a BPF by reducing minute ventilation, PEEP, and inspiratory time may adversely affect ventilation and oxygenation. Very large air leaks (>500 mL per breath) and respiratory acidosis (pH<7.30) are associated with a poor outcome in patients who have BPF and are maintained on ventilatory Single-lung transplant (SLT) has been used to improve the functional capacity and quality of life in patients who have advanced obstructive lung disease. After the transplantation procedure, there is marked asymmetry of the mechanical properties and vascular resistance of the two lungs. The native lung with persistent obstructive disease is typically highly compliant with increased vascular resistance, whereas the transplanted lung has a normal to low compliance and normal vascular resistance. Significant gas exchange difficulties arise when ventilation is preferentially distributed to the native lung and perfusion to the transplanted lung. Postoperative problems, including reperfusion edema and acute rejection, can further decrease the compliance of the transplanted lung exacerbating the difference in mechanical properties of the two lungs and increasing gas exchange problems. Mechanical ventilation in this situation produces dynamic hyperinflation of the native lung and compression with frequent atelectasis of the transplanted Progressive intrinsic PEEP with hyperinflation of the native lung may lead to increased vascular resistance in that lung and further redistribution of blood flow to the transplanted lung exacerbating the ventilation-perfusion mismatch and intrapulmonary shunt. Recent progress and experience with SLT has decreased postoperative ventilatory problems resulting from mechanical discrepancies. Effective measures include graft volume selection with the donor lung having a greater vital capacity than the native lung and limitation of mechanical ventilation and PEEP with return to spontaneous ventilation as rapidly as possible.57 Unilateral lung hyperinflation similar to that occurring with SLT has been observed in patients who have chronic obstructive lung disease during mechanical ~ e n t i l a t i o n 63, . ~ ~ ,Io6 Frequently, there is concomitant disease in the contralatera1 lung, resulting in a decreased compliance in that lung. The authors attribute the unilateral hyperinflation to the development of intrinsic PEEP in the involved lung during mechanical ventilation. Pneumothorax and hemodynamic compromise are observed complication^.^^, 64 Rarely, other diseases with involvement of one lung have been reported to produce significant gas exchange problems complicating conventional ventilator management.74Massive unilateral pulmonary embolism with obstruction of a main pulmonary artery has been described to produce ventilatory failure and problems with effective gas exchange and ventilator management.'I2 Whereas the previously discussed problems produce ineffective gas exchange primarily from a maldistribution of ventilation resulting from widely variable mechanical properties of the two lungs, in pulmonary embolism, the primary defect is a

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severe maldistribution of perfusion with severe ventilation-perfusion mismatching producing problems with ventilation as well as oxygenation. Cases of patients who have massive unilateral pulmonary emboli not adequately supported with conventional mechanical ventilation who benefited from lateral positioningRor ILV112have been reported. MANAGEMENT

When confronted with a clinical situation in which a patient who has one of the disease processes previously discussed requires mechanical ventilation, the critical care practitioner may employ a variety of modalities to support the patient. Although it is tempting to consider unconventional and appealing techniques to aid the difficult patient, sound general management and conventional ventilatory techniques are sufficient to provide adequate support in the majority of patients who have ULD.52,82 Experience would dictate that adequate gas exchange can be managed in most situations without resorting to technically difficult and potentially hazardous maneuvers. The frequency of problematic ULD and the rate of utilization of unconventional techniques (lateral positioning and ILV) to support patients who have these problems is impossible to determine from the medical literature. The relative paucity of reports would support the contention that these modalities are infrequently required. Most cases and series reported in the past 20 years describe unconventional modalities applied in situations in which conventional therapy was deemed unsuccessful in meeting the therapeutic goals. Very little is written about successful application of conventional techniques in ULD, particularly the relatively new pressure-targeted modes of ventilation. The relative infrequency of reports of unconventional forms of ventilator support in the past decade. This would suggest that either the techniques being considered are becoming commonplace and no longer deemed worthy of reporting or that newer ventilator management strategies are more effective in supporting patients who have ULD and the use of less conventional modalities is in fact declining. There are no data to support either contention. Inherent in the sound management of all patients who have major gas exchange problems is attentive monitoring in an intensive care setting with the ability to closely follow arterial blood gases and other indices of gas exchange. The need for more invasive forms of monitoring is determined by the individual case and the practices of the particular unit. Invasive hemodynamic monitoring is frequently advisable when unconventional forms of support are employed to follow the effect of the selected intervention on cardiac output and intrapulmonary shunt (Qs/Qt). Ventilatory Support of Patients Who Have Low Compliance Unilateral Lung Disease: Conventional Ventilation

As previously discussed, a variety of processes, most frequently, unilateral pneumonia and lung contusion, produce a state where the involved lung is less compliant than the unaffected lung. Institution of mechanical ventilation in this setting requires understanding of the potential adverse effects of positive-pressure ventilation and alternative modalities and maneuvers that can be employed to improve gas exchange. The criteria for initiation of ventilatory support in the patient who has ULD

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are the same as criteria utilized for other patients who have the same pathologic processes (pneumonia, contusion, and other conditions) in more typical distributions. Access to the lower respiratory tract and the ability to clear the airway are important considerations when gas exchange is tenuous. Mobilization of secretions and improvement in the distribution of ventilation may be as beneficial as the application of ventilatory support in some situations.65The selection of initial ventilatory mode for the patient who has ULD is best determined by the knowledge and familiarity of the operator. Conventional ventilatory modes apply the same pressure and flow to both lungs. The involved lung with lower compliance receives less of the delivered tidal volume regardless of the mode employed. In problematic cases, this results in failure to achieve the desired gas exchange, particularly acceptable levels of oxygenation. Application of PEEP in this situation may result in overinflation of the more compliant uninvolved lung with a resultant decrease in perfusion to those lung units and a redistribution of blood flow to the affected lung. This exacerbates gas exchange problems by worsening the ventilation-perfusion mismatch and increasing Qs/Qt. Attempts to address the deteriorating gas exchange by increasing minute ventilation and PEEP may further exacerbate the situation by hyperexpanding the uninvolved lung and producing increased maldistribution of ventilation and perfusion while increasing the risk of barotrauma and other complications outlined earlier. Recognition of this process is important to avoid injury to the patient and to direct attention to other interventions. The adverse response to PEEP has been used by some authors as a criteria for consideration of alternative forms of therapy.3, 25, 44, 108 The management of ULD with conventional ventilatory methods is largely empiric with cautious observation of the patient's response to serial manipulations of pressure, volume, and flow parameters. In most circumstances acceptable gas exchange can be achieved. If the initial mode selected fails to achieve adequate gas exchange, consideration should be given to using a mode that produces a lengthened inspiratory time and utilizes a decelerating flow profile. Utilizing this technique, Giordano et a144reported success in the management of a patient who had extensive unilateral pneumonia. Similar techniques have been widely employed in the management of diffuse lung disease.2,72, i"5 Pressuretargeted modes of ventilation, particularly pressure control ventilation (PCV), are frequently used with long inspiratory phases, including inverse ratio ventilation. PCV inherently has a decelerating wave profile."15 Decelerating inspiratory flow can also be successfully incorporated with volume-cycled modes.72,I' The decelerating flow pattern and inspiratory prolongation have been shown to improve gas exchange in patients who have acute respiratory distress syndrome (ARDS) by improving distribution of ventilation within the lungs.2, The improved distribution is thought to result from limitation of maximal regional pressures among lung units with heterogeneous time constants.*,'I The variability in lung time constants in ULD has been shown to be almost entirely caused by the differences in compliance between the two lungs."4 Thus ventilation with a decelerating flow pattern and inspiratory prolongation may improve the distribution of ventilation between the two lungs in ULD with favorable changes in gas exchange. High-frequency ventilation is a generic term encompassing multiple methods of ventilatory support with higher than normal breathing frequencies. The clinical systems include high-frequency jet ventilation (HFJV), high-frequency positive pressure ventilation (HFPPV), and high-frequency oscillation (HF0).y5 Several of these methods of high-frequency ventilation have been used in the management of ULD. Kahn et a15hreported the successful management of two

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patients with high-frequency ventilation through a single-lumen endotracheal tube. Although different techniques of high-frequency ventilation were employed in each of the patients (HFJV and HFPPV), both proved effective in improving gas exchange. In the series of Crimi et a1,3O one patient who had unilateral pneumonia failed HFJV and required differential ventilation.

Lateral Positioning

Most patients being supported with mechanical ventilation in the critical care setting are maintained in the supine position. Several reports have documented improved gas exchange, particularly oxygenation, when the patient who has ULD is placed in a lateral decubitus position with the involved lung in the nondependent position. Benefit has been reported in patients who have unilateral pneumonia,42,53, 89, 92, 93 lung contusion,” and atelectasis.”, 92, 93 No improvement was noted in a patient who had unilateral pleural effu~ion.4~ This positioning maneuver presents a potentially simple method of improving gas exchange in patients who have ULD and who are not adequately managed with conventional methods. The positive effect of lateral positioning on oxygenation was first reported by Zack et al”’ who evaluated the effect of lateral positioning on gas exchange in a diverse group of ambulatory and hospitalized patients. Among their patient group they noted that the patients who had predominately or exclusively ULD had improved oxygenation when the healthy lung was placed in the dependent position. This observation was confirmed by Remolina et a192in a study of patients who had ULD, including one being supported with mechanical ventilation. The oxygenation benefit was greater in this report of selected ULD patients. Subsequent reports have extended this observation of positional improvement in oxygenation in other patients who have predominately ULD receiving mechanical 53, 93 In two reports, the decision to utilize lateral positioning was prompted by the deterioration of gas exchange with the application of PEEP.”, 89 A similar improvement in oxygenation has been noted with the Trendelenburg position in patients who have bilateral lower lobe The response to lateral positioning in ULD results from favorable changes in the distribution of ventilation and perfusion with a net improvement in overall gas exchange. Perfusion of the lung regardless of position has been shown to be gravity dependent in normal subjects5, In the supine position, there is a uniform vertical distribution of blood flow within the lung in normal spontaneously breathing5,58 and mechanically ventilated patients.29The gravitational effect is also noted in the lateral decubitus position. Perfusion is quantatively greater in the dependent lung and the distribution is uniform in that lung. In the upper lung, the perfusion distribution is vertical? In both the supine and the lateral decubitus positions, the distribution of ventilation shows a vertical gravity-dependent d i s t r i b ~ t i o nThere . ~ ~ is greater overall vertical distribution of perfusion so that the net ventilation-perfusion relationship favors increased ventilation relative to perfusion in the upper lung and increased perfusion relative to ventilation in the dependent lung. This positional dependence of regional ventilation-perfusion relationships has been demonstrated in experimental models and normal humans.5,42* 58 Despite these positional changes in ventilation and perfusion, very little change in arterial blood gases is noted in

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In the critical care setting additional factors must be weighed when the use of mechanical ventilation and lateral positioning in patients who have ULD is considered. In spontaneously breathing patients, diaphragmatic excursion significantly affects the distribution of ventilation in the lateral decubitus position. The dependent diaphragm has a greater di~placement;~,R9 which has been shown to be a major factor in the increased ventilation to the dependent lung.2y The increased displacement is attributable to increased passive expiratory movement produced by gravitational hydrostatic forces from the abdominal contents. This produces a stronger active contraction in the dependent diaphragm that has a shorter radius of curvature and an increased resting muscle fiber length. Consequently, during an active inspiration in a laterally positioned normal patient, more of the tidal volume enters the dependent lung. When inspiration is passive as in some modes of mechanical ventilation, the diaphragm no longer contributes to lung inflation and becomes a flaccid partition supporting the abdominal hydrostatic forces. In this circumstance applied positive pressure is opposed by the added resistance of the abdominal contents. This results in decreased compliance and decreased ventilation to the dependent lung. In the nondependent lung the resistance from the abdominal contents is much less and ventilation is relatively increased. The net effect in the passively ventilated patient is that dependent-lung ventilation is decreased and nondependent-lung ventilation is increased. This has been documented in human 49, ho With the diminution of the gravity-dependent distribution of ventilation during passive mechanical ventilation, the range of regional ventilation-perfusion relationships increases with greater ventilation relative to perfusion in the nondependent lung and greater perfusion relative to ventilation in the dependent lung.*" This is typically associated with a small decrement in oxygenation from the increased shunt effect in the dependent lung.29 Most studies evaluating the physiologic effects of lateral positioning have been conducted in normal patients'O, 49, or patients who have bilateral lung di~ease.~, 5o Clinical reports of the use of lateral positioning in patients who ULD have documented improvement in oxygenation with the unaffected lung in the dependent position. Several reports have documented deterioration of oxygenation with the affected lung in the dependent position.42,92 The proposed mechanism for improvement in gas exchange is reduction of intrapulmonary shunting.4z,43, 53, 92, In-depth evaluation of ventilation-perfusion relationships with lateral positioning in ULD has been reported in one study. Gillespie and Rehder42studied gas exchange with the multiple inert-gas elimination technique in four patients who had ULD and who required mechanical ventilation. Patients were selected because of radiographic evidence of ULD and were studied in both lateral positions, i.e., with the good lung both dependent and nondependent. Significant variability was noted in the range of ventilation-perfusion ratios and was accounted for by nonhomogeneity of disease among the patients. Oxygenation improved in all patients who had the good lung dependent. Their studies confirmed that reduction of intrapulmonary shunting was responsible for the improvement in two patients. No significant change in shunt was noted in other patients, and improved oxygenation was demonstrated to be caused by an overall improvement in ventilation-perfusion matching. The decision to utilize lateral positioning to improve gas exchange in the ventilated patient who has ULD must be made in the context of the individual patient situation. A beneficial response is not predictable from clinical findings, laboratory data, ventilatory parameters, or radiographic patterns other than the appearance of disease exclusively or predominately in one lung. Most patients reported to have been treated with lateral positioning had persistent hypoxemia

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refractory to high Fio, and PEEP. Several had a deterioration in gas exchange in response to PEEP.", Hy A trial of lateral positioning with the good lung being dependent should be attempted with careful monitoring of oxygenation, ventilator volumes and pressures, and blood pressure. No initial changes are necessary in ventilatory parameters. There have been no comparative reports of various ventilator modes during lateral positioning. Most reported patients were being supported with a volume-targeted mode. Improvement in oxygenation is typically seen within a matter of minutes and is not associated with a significant change in Pco~. The response may be variable in patients who have active diaphragmatic movement and who are spontaneously triggering the ventilator compared with those who are paralyzed or are being ventilated in a passive mode. A suboptimal response in a patient who has active diaphragmatic function may be an indication to paralyze and sedate the patient to determine if further redistribution of the ventilation-perfusion relationships may be beneficial, while the physician gives particular attention to recognizing the differences in actively and passively ventilated patients. In limited study, half of the patient responses to lateral positioning were the result of a decrease in right-to-left intrapulmonary shunt and half were because of an improvement in ventilationperfusion matching?, If lateral positioning is found to improve gas exchange, then an effort should be made to maintain the patient in that position. Ventilator settings can be modified in response to arterial blood gas results. Frequently, an initial improvement in both oxygenation and, to a lesser degree, ventilation is noted. PEEP may be better tolerated because of the decreased compliance of the dependent lung and allow for a reduction in Fio, if necessary. Although conceptually a rather simple maneuver, lateral positioning in the mechanically ventilated patient can present many challenges and problems. A variety of patient and nursing considerations limit its application. Patient habitus, orthopedic appliances, traction apparatus, traumatic injuries-especially to the chest, thoracostomy tubes, surgical wounds, and patient tolerance-all present problems in repositioning and maintaining a critically ill patient in a lateral position. Routine nursing care likewise becomes much more difficult with the patient in a lateral position. Specialized nursing needs and procedures may preclude positioning in some patients. Specialized beds that greatly simplify patient positioning and easily allow return to a supine position for required intermittent care and procedures are available.65,96 Improvement in oxygenation has been documented during lateral positioning in specialized rotational beds.22 If a patient is found to benefit from lateral positioning in an empiric trial, then strong consideration should be given to placing the patient on a specialized bed (Fig. 3) for the time this modality is used. Reports in the literature document treatment with lateral positioning for periods of as long as 10 days.89 The use of lateral positioning does not preclude the use of hemodynamic monitoring with pulmonary artery catheters. Effective monitoring is frequently necessary in complicated patients. If not already present in a patient who has ULD and is being maintained in a lateral position, then placement of a pulmonary artery catheter or other central line may require temporary placement in the supine position. Once the line is established, the patient can be placed in the lateral position and reliable hemodynamic data obtained. Experimental models suggest that correlation of pulmonary artery wedge pressure with left atrial pressure may improve during mechanical ventilation with lateral positioning, provided the tip of the catheter is below the level of the left atrium.47 A potential risk with lateral positioning is the spread of infection from a nondependent lung with pneumonia to the dependent normal lung. Similarly,

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Figure 3. The Roto Rest Delta specialty bed has been successfully used to laterally position patients with unilateral lung disease (Courtesy of Kinetic Concepts, Inc., San Antonio, TX).

blood and other secretions may collect in the dependent lung in other situations in which lateral positioning is used. It is difficult to determine the mechanism of spread of an infection but presumed extension of infection to the dependent lung in patients being managed with lateral positioning has been r e p ~ r t e d . ~ ~ Attentive respiratory and nursing care with efforts directed to secretion mobilization should minimize this problem. Care must also be taken to provide proper support for the patient being maintained in the lateral position to prevent peripheral nerve injury and skin breakdown in pressure-bearing areas. Specialized beds again help in this regard. The need for maintaining a patient in a lateral position is dependent on the evolution of the underlying process. Oxygenation should be continuously monitored and periodic assessment should be made of the continued benefit of lateral positioning. Other indices of gas exchange may also be helpful in evaluating ongoing benefit. Improvement in gas exchange is frequently seen before radiographic improvement, but periodic radiographs are necessary to evaluate the progress of the disease process. When minimal difference is noted in oxygen parameters in the lateral or the supine position, lateral positioning can be discontinued. Progression to bilateral disease may also render lateral positioning ineffective and may be an indication to return to a conventional supine position.

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independent-Lung Ventilation When conventional ventilatory management and lateral positioning have not produced the desired response, consideration can be given to ILV. ILV in the critical care setting is an extension of techniques developed and widely used in the operating suite. The Carlens double-lumen tube (DLT) was introduced in 1949 and was widely used in thoracic surgery. Many problems, including difficult placement, red rubber composition with mucosal irritation, small lumen size with increased resistance to air flow, difficulty with secretion mobilization, low-volume-high-pressure cuffs, and tracheal trauma produced by the carinal hook limited use of this tube in the operating area and precluded its use for long-term management of critical care patients. New production techniques, including the introduction of more flexible polyvinyl chloride tubes, improved tube design, and the transition to low-pressure-high-compliance cuffs, led to increased operative use. With effective DLTs separation and control of each lung were obtained. This not only served to prevent the spillage of purulent material or blood into the uninvolved lung but also provided perioperative control of individual lung inflation and ventilation. Effective independent-lung ventilation was a major advance in thoracic surgery used exclusively in the operating suite for many years. In 1976 Glass et a145reported the use of ILV in the postoperative setting for prolonged support of patients who had ULD. Powner et a P were the first to report the use of ILV in the critical care area for treatment of a patient who had unilateral pneumonia. Numerous subsequent reports and series have demonstrated the utility of ILV in the treatment of ULD unresponsive to conventional ventilator management.* The technique has been applied in the support of patients who have all of the ULDs listed in Table 1. By effectively controlling the pressure and the flow directed to each lung, the altered physiology resulting from the widely disparate mechanics of each lung can be compensated for and improved gas exchange achieved. Patient Selection

As with lateral positioning and other specialized methods of patient support, before deciding to use ILV in the patient who has ULD the physician must take into account the condition of the patient as well as the technical difficulties imposed by ILV. Most authors of case reports cite failure of conventional ventilatory management as the indication for ILV in ULD. The criteria used to determine failure are frequently not clearly specified. Demonstration of adverse redistribution of blood flow in response to PEEP with angiography" or radioisotope techniquess9was one of the indications for ILV used in early reports in conjunction with evaluation of arterial oxygenation and Qs/Qt. Subsequent studies have focused on the failure to achieve adequate gas exchange in response to high Fio, and PEEP. A PAo,/Fioz ratio less than 150 has been used in several studies as the indication for ILV in patients who have LJLD.11*115In a recently published review10h of ILV, it was proposed that the criteria summarized in Table 2 be used in patients who have unilateral disease. In addition to listed criteria, the patient must also be a candidate for reintubation with a DLT. Many of the problems and complications associated with ILV are related to the use of the *See References 1, 3, 16, 17, 27, 31, 41, 51, 52, 55, 65, 74, 77, 81, 94, 98, 102, 110, and 113-115.

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Table 2. CRITERIA FOR INDEPENDENT-LUNG VENTILATION IN UNILATERAL LUNG

DISEASE Radiographically apparent unilateral or asymmetrical lung disease with one of the following: 1 . Hypoxemia refractory to high Fio, and generalized PEEP 2. PEEP-induced deterioration in oxygenation or shunt fraction 3. Overinflation of the noninvolved lung with or without collapse of the involved lung 4. Significant deterioration in circulatory status in response to PEEP

DLT. Finally, support of the patient using ILV requires an increased level of proficiency and experience at multiple levels in the team caring for the patient, including nursing, respiratory, and physician staff. Continuous availability of experienced support may affect the decision to use ILV in many circumstances. Double-Lumen Endotracheal Tubes

When the decision is made to utilize ILV the first step is to secure independent access to each lung by placing a DLT. Double-lumen endotracheal tubes are essentially two separate tubes bonded together. One tube is shorter and provides ventilation to the trachea; the other tube is longer and is designed to be passed into the main-stem bronchus. Each lumen has a separate cuff, a proximal cuff on the tracheal and a distal cuff on the bronchial portion. Tubes are designated either right or left sided, depending whether the right or the left main-stem bronchus is to be intubated. Historically, several varieties of DLTs have been utilized for lung separation. These include the Carlens, White, Bryce-Smith, and Robertshaw. The Robertshaw tube and its modifications are the preferred DLTs today, although occasionally, a Carlens tube may be used (Fig. 4). The characteristic feature of the Carlens tube is the presence of a carinal hook used to aid in and maintain proper placement; however, complications associated with this hook resulted in the Carlens tube losing popularity. Frequently encountered problems included increased difficulty and laryngeal trauma on intubation, amputation of the hook, tracheal and bronchial trauma, and malpositioning owing to the hook. In addition, the cross-sectional shape of each lumen of the Carlens tube is oval, which produces greater flow resistance and can cause difficulty in passing a suction catheter. The modern plastic Robertshaw tube (Fig. 4) is the most frequently used DLT in the operating and in critical care areas for patients requiring independent access to each lung. This tube offers several advantages over the older Carlens tube. The clear plastic allows observation of respiratory moisture and secretions. The bronchial cuff is colored bright blue for easy recognition during fiber-optic bronchoscopy, and each lumen has a radiopaque line for chest radiographic identification. Both the tracheal and endobronchial cuffs are a highvolume-low-pressure design to minimize airway trauma, and the carinal hook was eliminated to reduce laryngeal and airway injury further. Finally, each lumen of the tube is D-shaped, allowing large internal-to-external diameter ratios to provide easier suctioning and lower air-flow resistance. The Robertshaw tubes are available in five sizes with the following corresponding internal diameters for each of the lumens: 41 French (I.D. 6.5 mm), 39 (6.0), 37 (5.5), 35 (5.0), and 28 (4.5). The largest size tube that can comfortably pass through the glottis should be used (Table 3). This generally corresponds to

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Figure 4. Design of the (A) left- and (B) right-sided Robertshaw and the (C) Carlens double lumen endotracheal tubes showing relative placement of the two cuffs in the airways for each design as well as the carinal hook characteristic of the Carlens tube.

a 41 French for most adult men, and a 39 French for women, although there is some correlation of appropriate sizing with patient height.I5,20, 21 Although both right- and left-sided DLTs are available, a left-sided tube is the preferred choice in most clinical situation^.'^, lo3 The anatomical arrangement of the right main-stem bronchus, with minimal distance between the tracheal carina and the orifice to the right upper lobe, makes placement of a cuffed bronchial lumen on the right side problematic. Available right-sided DLTs are designed with a ventilation slot that must be aligned with the right upper lobe orifice. Since there is considerable variation in the position of this orifice and in the length of the right main-stem bronchus, correct sizing and positioning of a right-sided tube may be extremely difficult and inadequate right upper lobe ventilation frequently occurs.15Because of the very fine tolerances dictated by airway anatomy, right-sided tubes are much more prone to displacement with patient movement. Therefore, a right-sided tube should only be used when there is a specific contraindication (such as lesions of the left main-stem bronchus,

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including strictures, tumors, or tracheal-bronchial disruptions) to left-sided placement that would preclude safe passage of the endobronchial portion of a left-sided tube. Insertion of a Robertshaw double-lumen endotracheal tube is similar to simple intubation with a few special modifications. Usually, a curved (MacIntosh) blade is used since it provides a larger pharyngeal area through which to pass the tube. The tube should be well lubricated to prevent tearing of the cuffs on the teeth during passage, and the provided stylet should be used to guide placement. Initially, the tube is passed with the distal curvature pointing anteriorly then as the distal end passes through the larynx, the tube is rotated 90" (towards the desired bronchus). The stylet is simultaneously removed, and the tube is advanced until moderate resistance is met. A DLT may also be passed through a tracheostomy, although in this situation the tracheal cuff may be at or partially protruding from the stoma. Multiple reports document successful use of DLTs through a tracheostomy." 31,sl, "I3 In several circumstances more effective long-term stabilization of the tube was achieved with placement through a tracheostomy.", Io2 After successful intubation, the tracheal and bronchial cuffs should be inflated (with no more than 2 to 3 mL in the endobronchial cuff) and correct positioning confirmed with both auscultation and fiber-optic bronchoscopy. Using auscultation, there should be ipsilateral loss of chest movement and breath sounds with clamping of the bronchial lumen. Conversely, clamping of the tracheal lumen should cause loss of chest expansion and breath sounds on the opposite side. A detailed protocol for determining proper DLT placement by auscultatory means has been p~blished.~", lo3 Several studies in thoracic surgery patients have looked at the reliability of auscultatory techniques for correctly placing DLTs with direct bronchoscopic observation for verification. Even when auscultatory findings suggested the tube was properly placed, 48'%"" to 83%.' of the time the tube required repositioning when placement was assessed by bronchoscopy. It has been recommended that bronchoscopic verification of DLT position be routinely done in the operating room,'" I' but the necessity of this practice is not uniformly accepted.4hIn the critical care setting, particularly in the patient who has ULD, auscultatory methods of verifying DLT placement may not be practical or reliable. Fiber-optic bronchoscopy is a simple and effective means of confirming DLT placement and is recommended for patients in the critical care area."" A standard 4.9-mm outside diameter bronchoscope will pass through the lumens of 39 and 41 French tubes with adequate lubrication and can be utilized to assess tube placement in most adult patients. However, a pediatric bronchoscope (3.6- to 4.2-mm outside diameter) is needed to pass through the lumens of 37 French Table 3. DOUBLE-LUMEN ENDOTRACHEAL TUBE SIZE Tube Size (French)

Circumference (mm)

35

38

37 39 41

40 44 45

Data from references 23, 78, and 106.

Lumen Diameter (mm)

Clinical Use

5 5.5 6 6.5

Large children Small adult Medium adult-most women Large adult-usual male

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and smaller DLTs and occasionally in larger tubes in which angulation may slightly decrease lumen diameter.lo3To assess correct positioning of a left-sided tube, the physician should pass the bronchoscope down the tracheal lumen to visualize the left bronchial cuff (colored blue) just distal to the carina. Any herniation of the cuff into the trachea should be corrected by deflation of the cuff and incremental advancement under direct visualization. Inability to visualize the right main bronchus through the tracheal lumen suggests placement at too great a depth and should be corrected by incremental withdrawal until the right bronchial lumen and the left bronchial cuff can be visualized. When correct placement is confirmed, the bronchoscope should be passed through the bronchial lumen to assess for any excessive luminal narrowing caused by overinflation of the cuff. The assessment of correct right-sided tube placement is more complicated. The bronchoscope is passed through the tracheal lumen of the tube to ensure that the tracheal opening is above the carina. Often, the right bronchial cuff is not visible by this approach. The bronchoscope is then passed through the bronchial lumen to visualize the right upper lobe ventilation slot aligned with the right upper lobe orifice and the distal lumen opening positioned above the right-middle and the lower-lobe carina. Identification of proper alignment with the right-upper lobe is frequently difficult because of the small size of the ventilation slot and the sharp angulation of the upper-lobe bronchus. Even with bronchoscopically confirmed positioning, double-lumen endotracheal tubes are susceptible to displacement. Movement as small as 16 to 19 mm with left-sided tubes and 1 to 8 mm with right-sided tubes can cause malposition and inadequate ventilation. This can occur with any patient movement, including flexion or extension of the head (which can cause as much as 28 mm of movement of the tube proximally or distally). For this reason most patients intubated with a DLT for ILV require heavy sedation and not infrequently muscle relaxation. With the extended use of a DLT for patient support in the critical care unit, any movement of the patient or the ventilator apparatus may and frequently does result in tube displacement. Particular attention to tube position during any movements of the patient or the ventilator circuit with purposeful stabilization of the tube prevents many of these problems. Tube slippage and migration may also occur during the course of ventilatory support with repetitive application of positive pressure. Significant changes in volume return, airway pressures, or compliance may indicate tube displacement, particularly if they occur over short intervals and are not explained by other changes in patient status. At any point that the tube position is uncertain, bronchoscopy should promptly be used to reconfirm tube position and reposition as required. Other techniques of monitoring DLT position and migration have been utilized in the operating room setting. Reported methods include continuous spirometry utilizing flow-volume and pressure-volume loops,'2,99 dual end-tidal CO, monit0ring,9~and bronchial cuff pressure monitoring6 None of these techniques have been studied in the critical care area or utilized in long-term patient management. The technique of bronchial cuff pressure monitoring recently reported by Araki et a16 is simple and could be easily applied to patients being supported with ILV. In their study, serial measurements were made of the bronchial cuff pressure and a falling cuff pressure was seen as the DLT migrated proximally. Cuff pressure changes indicated displacement before spirometric or capnographic changes were noted. In the critical care area, serial measurements of cuff pressures should be routinely made and a falling bronchial cuff pressure might be used as an indication for bronchoscopic verification of tube position. Although modern double-lumen endotracheal tubes are safe and relatively easy to use, complications do exist, and they must weighed against the benefits

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of ILV. Most of the complications reported in the literature involve the older Carlens tubes,19but these same complications, with the exception of those caused by the carinal hook, may occur with the newer tubes. As discussed above, malpositioning of the tube may occur and impede adequate ventilation. Tracheobronchial tree damage or disruption may occur, especially in those patients who have preexisting congenital or acquired bronchial-wall abnormalities. Potential problems encountered in the critical area include bronchial-wall infiltration by tumor or infection, poor tissue quality secondary to alcoholism, sepsis or drug abuse, and distortion of the bronchial tree by tumors or enlarged lymphatic glands.I4 Although the modern tubes have high-volume-low-pressure cuffs, a common underlying feature of bronchial damage is the correlation with excessive air volume, and consequently, pressure, in the endobronchial cuff." Therefore, it is prudent to choose an appropriately sized tube, avoid overinflating the endobronchial cuff, and correct displacement whenever it is suspected. Being the equivalent of two endotracheal tubes, the DLT must have a smaller internal diameter in each of the limbs in comparison with a standard endotracheal tube. This may produce difficulties passing suction catheters to clear secretions, particularly through the longer and angulated bronchial lumen. The use of pediatric-size catheters may be necessary to clear secretions effectively. Difficulties with effective secretion clearance through a DLT has resulted in the discontinuance of ILV and reintubation with a standard endotracheal tube.yx,loH Additionally, the smaller lumen size may produce increased flow resistance necessitating higher delivery pressures. The added expiratory flow resistance may also produce dynamic hyperinflation and auto-PEEP. In spontaneously breathing patients this could significantly increase the work of breathing. Techniques of Independent-Lung Ventilation

The establishment of independent control of each lung with a DLT permits application of flow and pressure to each lung to address the variation in mechanical properties effectively. A wide variety of systems and techniques have been reported in the effort to provide effective ventilatory support in patients who have ULD. Although these systems and techniques vary in their method, the primary goal in all cases reported in the diseases with low compliance in the involved lung has been the application of differential PEEP. Uniformly, PEEP at a higher pressure has been applied to the involved lung. Experimental 48, 79, Io4, lo7 have confirmed that improvement in gas exchange, ventilatory mechanics, and cardiovascular function is dependent on effective differential PEEP. In these models, application of generalized PEEP to both lungs and selective PEEP to the uninvolved lung is associated with deterioration of oxygenation, increased shunt, and impaired cardiac function. Selective PEEP applied to the involved lung uniformly improves gas exchange and cardiovascular function. In many reports, a similar improvement in gas exchange with application of selective or differential PEEP is cited for patients who failed to respond or deteriorated with generalized PEEP.24,52, 65, 98, 113 Differential PEEP was shown to be the primary determinant of a favorable response. Studies by Pace et a17y,xoand East et a136in an experimental model of acute unilateral lung injury examined methods of allocating lung volume between the two lungs during differential ventilation. In their studies PEEP was randomly applied and volume to each lung was determined with three schemes controlled by a computer-driven ventilator system. Groups were ventilated with equal volumes to each lung, equal driving pressure to each lung, or volume distributed to achieve equal end-tidal CO, from each lung. The method of partitioning tidal volume had minimal effect on

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overall gas exchange, whereas selective PEEP to the involved lung improved gas exchange in all groups. The findings in the experimental model are analogous to reported human experience in which improvement in gas exchange has been accomplished by a wide variety of methods and is dependent on the application of PEEP preferentially to the involved lung. The use of selective or differential PEEP is the critical factor in improving gas exchange during ILV in low-compliance ULD. An array of systems and techniques have been used to provide ILV and differential PEEP in ULD refractory to conventional therapy. These systems can be considered in five categories:

1. Continuous positive airway pressure (CPAP) to each limb of the DLT with spontaneous ventilation 2. Differential ventilation and PEEP applied with a single ventilator and a flow divider 3. Independent ventilation with two synchronized ventilators 4. Ventilation with two independent asynchronous ventilators 5. A combination of different modalities The least complex systems are those that employ independent CPAP circuits applied to each limb of a DLT in a spontaneously breathing patient. Two reports of successful use of independent CPAP circuits in patients who have ULD appear in the literature.31,108 Levels of CPAP to be applied were determined by analysis of pressure-volume curves31or incremental increases in pressure to the involved lung monitoring the hemodynamic response and gas exchange, including oxygenation and Qs/Qt until an optimal response was obtained.Ios Both studies document improved gas exchange with eventual improvement and successful extubation. This technique may prove useful in select patients who are capable of spontaneous ventilation in the face of severe illness and intubation with a DLT. Since most patients are heavily sedated and many are paralyzed to facilitate intubation and management in the critical care setting, this technique would appear to have limited applicability. Several techniques have been described to provide differential ventilation and PEEP with a single ventilator in a patient intubated with a DLT. A singleventilator system supplies synchronous ventilation to the two lungs while providing the ability to regulate PEEP levels independently. Most single-ventilator systems utilize dual-ventilator circuits attached to a T connection on the inspiratory port of the ventilator and the independent PEEP devices on the expiratory limb of each circuit. Since volume would be expected to be preferentially delivered to the more compliant uninvolved lung, most reported systems use an inline flow resistor in the circuit to the noninvolved lung; this resistor can be used to increase resistance in that circuit and improve distribution of ventilati~n.~~, 88 A similar technique involves the use of a bronchial blocker incorporated in a standard endotracheal tube to act as a flow resistor and to redistribute ventilation during ventilation with a single ventilator.34,54 Others have used timed-cycled ventilators equipped with dual inspiratory-expiratory modules in which the main unit serves as a synchronizing mechanism for the parallel 52 or have used extensively modified volume ventilators to provide dual Although single-ventilator systems have been successfully used in many patients, their use has been discouraged", 52 because of several problems that may develop. Problems and disadvantages include the complexity of the circuits, particularly the long lines involved, which are prone to leak and disconnect. Monitoring of ventilatory function and mechanics is difficult and frequently inadequate. If the in-line resistance is placed in the circuit so that it also creates

VENTILATION IN THE PATIENT WITH UNILATERAL LUNG DISEASE

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expiratory flow resistance, then there is the potential for introducing increased PEEP with hyperinflation and barotrauma to the uninvolved lung. The majority of reported cases of ILV use two ventilators with each connected to one lumen of the DLT. This arrangement permits independent control of flow, volume, and pressure to each lung. Intrinsic systems in each ventilator simplify monitoring of delivered volumes and pressures as well as the determination of ventilatory mechanics for each lung. Initial reports of ILV incorporating dual ventilators used systems in which the inspiratory cycles of both ventilators were synchronized. This involved customized timing circuits that were developed by the reporting institution specifically for this purpose.1h,24, y8 Subsequently commercial ventilators have been produced that can be linked in a master-slave configuration for use in synchronized ILV (Fig. 5). Asynchronous ILV (AILV) was initially reported by Hillman and Barber? With this technique, two ventilators are used as with synchronous ILV; however, no attempt is made to synchronize the inspiratory cycles. Each system is considered an independent entity. This simplifies the management of each ventilator and the system as a whole while increasing the flexibility and management options. Different modes and rates of ventilation in addition to pressures and volumes can be applied to each lung to maximize response. In some cases different types of ventilators were used on each of the limbs of the system.’02 Initial concerns about adverse cardiovascular effects with asynchronous ventilation proved unfounded. The potential for decreased systemic or pulmonary venous return, increased pulmonary vascular resistance, and decreased cardiac output exists when the lungs are being randomly ventilated, particularly when the two systems are out of phase. Although pulmonary artery and wedge

Figure 5. A patient receiving independent lung ventilation with two synchronized Servo 9OOC ventilators in a master-slave configuration (Siemens Corp., Danvers, MA).

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THOMAS & BRYCE

pressures were noted to be difficult to interpret, cardiac output determinations and systemic pressures were stable when compared with values obtained before AILV.51Subsequent experimental study confirmed no significant difference in gas exchange or hemodynamic measurements when synchronous and AILV were compared in an animal model of acute ULD.35Asynchronous ventilation has been well tolerated in reported clinical use?, 77, The effect of asynchronous ventilation on the respiratory drive has not been reported, and patient-triggered modes of ventilation should be used with caution. Since most patients being supported in this fashion are typically heavily sedated and frequently paralyzed, ventilator-driven modes are most commonly used during AILV. From reported cases, AILV is well tolerated and has been successfully used for periods of as long as 13 days.I7 Nishimura et a176reported a modified differential lung ventilation technique in which a catheter was passed through the standard endotracheal tube and positioned in the main-stem bronchus of the involved lung in a hypoxemic patient who had atelectasis and was poorly responsive to oxygen and PEEP. The catheter was used to supply HFJV to the involved lung in conjunction with conventional ventilation through the endotracheal tube. This technique was effective in improving gas exchange and reexpanding the atelectatic lung. ILV with different types of ventilatory support to each limb of a DLT has been reported with multiple configurations. In all reports, conventional positivepressure support is applied to the noninvolved lung with tidal volume and rate determined by individual patient requirements and tolerance. In three rep0rts,4~, 84, CPAP was applied to the involved lung without tidal ventilation. Pressure . applied ranged from 5 to 20 cm"O ~ a t e r . 4 ~These systems were successful in improving gas exchange in patients who had lung contusions4,110 and atelecta~ i s . 4More ~ commonly reported is the use of high-frequency ventilation to the involved lung in conjunction with conventional support of the noninvolved lung.30* 69* 71 Although the most commonly reported use of ILV with combined conventional and high-frequency ventilation has been in the management of patients who have BPF, these systems have also been successfully used in patients who have unilateral pneumonia, lung contusion, and h e m a t ~ m a .69~ ~ , HFJV is the method most commonly used to ventilate the involved lung, although HFPPV has also been successfully Successful intraoperative use of combined conventional-high-frequency ILV has also been reported.70,73 Although the combination of high-frequency and conventional ventilation has been successful in providing ILV, the superiority of these systems has not been demonstrated. Considering the complexity of high-frequency ventilator systems and the technical problems involved with ILV, this combination would appear to have very limited applicability in this setting. Ventilator Management with Independent-Lung Ventilation

Once the decision to use ILV has been made, the methods of ventilatory support must be determined. None of the techniques outlined previously have been demonstrated to be superior, and the decision as to which method to use is dependent on local experience, equipment availability, expertise, and patient status. For those who have no previous experience with ILV, AILV provides the most flexibility in terms of equipment and relative ease of use since each circuit can be manipulated independently. Additionally, monitoring of ventilatory mechanics is generally easier with independent ventilators. The discussion of ventilator management will assume AILV is the method in use, although the

VENTILATION IN THE PATIENT WITH UNILATERAL LUNG DISEASE

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principles can be applied to other systems and methods with appropriate adjustment. Initial ventilator parameters to be specified need to include mode of ventilation, minute ventilation (rate and tidal volume), Fio,, and PEEP level in each circuit. Since most patients intubated with a DLT for ILV will be sedated and frequently paralyzed, the initial modes(s) will usually be a ventilator-cycled mode that provides total support. Initial rate and volume are determined by patient size and expected ventilatory requirements. Since most patients requiring ILV will have been supported with conventional ventilation before ILV, the total minute ventilation and Fio, requirements will have been established and can serve as starting points though the distribution of ventilation and ventilation/perfusion (V/Q) match will undoubtedly change. It is understood that subsequent adjustment and fine tuning of ventilator parameters is necessary after initiating ILV. The initial tidal volumes typically reported are 5 to 8 mL/kg applied to each lung.’ In the experimental studies of Pace et a17y,8o and East et aP6 the group that was ventilated with equal tidal volume to each lung tended to have a better overall response, supporting the findings of Hasan et a1A8 In reported cases, equal distribution of tidal volume is the most commonly used management scheme, although frequent adjustments are necessary in response to pressure levels and blood gas data. Reduction of volume in the involved lung because of excessive pressures frequently necessitates compensatory increases on the contralateral side to maintain desired gas exchange. Recent reports with pressure-targeted modes”5,h7 in ILV achieved adequate gas exchange, although few details of ventilator management are supplied. In one study, pressure levels to each lung were adjusted to maintain approximately equal tidal volumes in each lungss The determination of appropriate PEEP levels to apply to the involved and the noninvolved lung have been made with several methods. Carlon et aP4used progressive increments of PEEP applied to the involved lung until the compliance of the two lungs approximated each other. Subsequent changes were made based on blood gas and cardiac output data. Details of the rationale for these changes are not supplied. Siege1 et alY8used a computer-based system with continuous monitoring of pressure, volume, and flow data to determine the tidal volume and PEEP combination that produced the maximum compliance in each lung. Efficacy was correlated with blood gas and hemodynamic data. Other studies have attempted to allocate PEEP based on pressure-volume curves and compliance data.31In most reports, patients are given trials of PEEP with levels of 0 to 5 cm water (reported range 0 to 18 cm water) applied to the noninvolved lung and levels of 10 to 20 cm water (reported range 5 to 22 cm water) applied to the involved lung. Subsequent changes are made based on clinical, oxygenation, hemodynamic, and radiographic responses with gas exchange and hemodynamic improvement being the primary goals. During support with ILV, the primary therapeutic goal is to maintain adequate systemic oxygenation and hemodynamic stability. Arterial blood gases and hemodynamic parameters reflect the global efficiency of the lungs and pulmonary circulation. These values do not directly reflect the characteristics or status of either lung independently, making it difficult to use these values for specific monitoring of the changes in the properties of either lung other than as reflected in total function. Several mechanical parameters have been used to follow the properties and performance of each lung during ILV. Individual lung compliance (Ct), peak-airway pressure (Pawpcak), differential time constants, and inspiratory and expiratory airway resistance have been shown to reflect changes in the properties and the performance of the individual lungs.’, 52, yH, ‘I4These values are indicative of changes in mechanical properties in each lung that occur

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THOMAS & BRYCE

as the underlying disease evolves or changes are made in ventilation parameters. Improvement in Ct and Pawpeak in the involved lung is noted during improvement in lung function and is a favorable sign.52,98, 114 In addition to asymmetry in ventilator mechanics, variability in the distribution of pulmonary perfusion has been shown to contribute to the V/Q mismatch that occurs with ULD as previously discussed. In addition, perfusion distribution is also significantly affected by positive-pressure ventilation, particularly PEEP. Zandstra and 115 reported the use of differential carbon dioxide production St~utenbeek"~, (Vco,) and end-tidal CO, concentration (ETco,) to provide indirect assessment of the differential pulmonary perfusion in a group of patients who had pulmonary contusions. They concluded that both C0,-derived variables were useful in monitoring the differences in pulmonary perfusion during ILV and reflected improvement as recovery occurred. The VCO, was slightly better in following the evolution of the underlying process, particularly in more severe cases, but it is technically more difficult to monitor on a continuous basis compared with ETco,. They do not report the effect of changes in PEEP on differential VCO, or ETco,, but presumably perfusion changes introduced by changes in ventilator parameters, particularly PEEP, would be reflected to a degree in these values although changes in Vd/Vt will also contribute. The C0,-derived values in conjunction with the mechanical parameters and blood gas data can be used to achieve improved V/Q matching and optimize gas exchange during ILV11*115In experimental models of ILV in ULD when ventilatory methods were compared, a technique that was designed to provide ventilation distributed to achieve equal ETco, from both lungs has been successfully used and in one study produced the best gas exchange.8O Weaning from Independent-Lung Ventilation

Most patients supported with ILV show immediate improvement in oxygenation with the effective application of differential PEEP. As with other patients supported with mechanical ventilation, periodic reassessment of gas exchange and ventilator parameters is necessary as the process evolves. The need for continuous use of ILV is determined by clinical, mechanical, and radiographic progress. ILV has been successfully used for periods of 1 to 13 days in ULD and for periods of as long as 35 days in SLT patients.4O Improvement eventually leads to the need to make the transition to conventional ventilator support in the course of withdrawing ventilator support. Similarly, progression of the disease to involve both lungs in a more symmetrical radiographic and physiologic pattern would also be an indication to terminate ILV and return to conventional support. Most reported patients successfully managed with ILV were reintubated with a single-lumen tube when deemed appropriate and weaned from ventilator support with standard methods. This course is preferable given the problems with the DLT, including high flow resistance that would become a factor during weaning and the difficulty with tube displacement as sedation is withdrawn. Nevertheless, the DLT can be configured to function as a singlelumen tubelo3and potentially used throughout the course of ventilatory support. The decision to terminate ILV is made when the patient shows significant resolution of the asymmetry of the disease process physiologically and radiographically. At the point where a transition to conventional ventilation is being considered, a trial of ventilation with identical settings in the circuits to each lung is valuable to assess residual differences in physiology. When a majority of the goals and criteria outlined in Table 4 are met, successful transition to conventional ventilation would be expected. These parameters are a distillation

VENTILATION IN THE PATIENT WITH UNILATERAL LUNG DISEASE

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Table 4. CRITERIA FOR TERMINATING INDEPENDENT-LUNG VENTILATION 1. Difference in PEEP in two circuits < 5 cm H,O with stable Pao, 2. Pawpeak difference < 5 cm H,O on identical settings in both circuits 3. Compliance difference between the two lungs < 10 mUcm H,O 4. Ratio of end-tidal CO, (ETco,) or CO, production (Vco,) 2 0.88 5. Total minute ventilation < 12 Umin (sum of the two systems) 6. Radiographic improvement with decreased asymmetry Data from references 1, 52, 98, 113,and 114.

of criteria from reported studies and have not been subjected to scientific study. As such, they serve as guidelines to be considered in the context of another patient and practice variables in each case. Ventilatory Support of Patients with Low-Compliance Unilateral Lung Disease

Ventilatory support of patients who have ULD and asymmetric physiology favoring redistribution of ventilation away from the normal or uninvolved lung may present significant challenges to the critical care physician. BPF in the ventilated patient is a relatively frequent critical care problem. Management by conventional means as well as the indications and use of unconventional modalities needs to be understood by all actively involved with critical care ventilator management. The management of postoperative lung transplant patients is beyond the scope of this article and is relevant to relatively few practitioners. ILV has been successfully used to manage and support SLT patients who developed hyperinflation of the native lung in the postoperative period.7,40,86, l w The cases of unilateral hyperinflation during mechanical ventilation reported in the literature were successfully managed by changes in the ventilator parameters and other conservative measures and did not require ILV. As noted, the unilateral hyperinflation was thought to be a manifestation of intrinsic PEEP, and measures to lower intrinsic PEEP were effective in resolving these 63 These measures included patient sedation, reduction of the minute ventilation by decreasing rate and tidal volume, and adjustment of inspiratory flow to promote longer expiratory times. Other measures to facilitate expiratory flow, such as the use of a larger endotracheal tube, may be useful. In one case, unilateral hyperinflation resolved with the removal of an in-line suction apparatus.37 Bronchopleural Fistula

The management of BPF involves the integration of treatment of the underlying cause of the fistula, effective chest-tube management, measures to address the defect primarily, and frequently ventilator management. Although ventilatory support may be necessary when a patient who has a BPF develops respiratory failure, more frequently, barotrauma from mechanical ventilation is a causal factor in the development of a BPF and positive airway pressure contributes to persistent air leakage. Management of BPF in the ventilated patient has been the subject of multiple excellent reviewsI3,**, 87 that consider all aspects of fistula management. The primary goal of ventilator management in this situation is to

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THOMAS & BRYCE

provide adequate support to the patient while reducing the air flow through the defect. Flow through the fistula is determined by the pressure gradient across the defect so that maneuvers to reduce airway pressure are utilized in an effort to minimize that pressure gradient.I3,82 Modes that support spontaneous patient efforts, particularly modes with low inspiratory pressures such as pressure support, are favored. Airway pressure produced by mechanical breaths is minimized by reducing the number and volume to the lowest level that provides adequate ventilation. PEEP should be avoided if possible or kept at the lowest level needed to provide acceptable gas exchange. Fistula flow in response to PEEP changes may be a factor in determining optimal PEEP levels. Finally, inspiratory time may be shortened during mechanical breaths by increasing inspiratory flow in patients who have parenchymal defects producing the fistula. Increased inspiratory pressures associated with increased inspiratory flow may be detrimental in patients who have proximal airway defects and result in increased air leakage. In the majority of patients who have BPF during mechanical ventilation, the manipulations outlined will be sufficient to provide adequate ventilation. In a retrospective review of 39 ventilated patients with a BPF,S3 conventional ventilator management with appropriate adjustments provided adequate ventilatory support in most patients with only 2 of the 39 patients developing severe respiratory acidosis (pH<7.30). Other measures and means of support were thought to be only rarely required.8z,R3 In the patient with a BPF and who fails management by conventional techniques, a variety of methods and techniques have been reported to improve gas exchange and diminish air leak. Complete review and discussion of the appropriate utilization of these measures have appeared elsewhere.13,82, 87 Although its role remains controversial, high-frequency ventilation has been used fairly extensively in patients who have BPF.13, It appears to have the greatest success in supporting patients who have normal lungs and a proximal BPF.I3 Lateral positioning and ILV have been applied to the management of ventilated patients with a BPF and who were believed to fail management with conventional techniques. Lad6 reported decrease in fistula flow in ventilated patients with a BPF when positioned with the involved lung in a dependent position. As previously discussed, ventilation to the dependent lung is diminished in the passively ventilated patient in a lateral position. The use of this technique, however, is limited by difficulties with chest-tube management in the dependent lung. The tube may be compressed or kinked leading to pneumothorax. Utilization of a specialized rotational bed (Fig. 3) would be advisable in this circumstance to provide adequate support to the patient and to prevent tube obstruction as well as allow adequate visualization of the chest tube. ILV has been utilized in the management of BPF when massive air leak results in inadequate ventilation and refractory hypoxemia. As previously noted, problematic BPF typically occurs in the setting of other lung processes for which the patient may require mechanical ventilation. The decision to use ILV must be made in the context of the individual patient situation as well as the local setting parameters previously discussed. The criteria utilized to initiate ILV in reported cases are summarized in Table 5. A DLT is placed as previously described to secure independent access to each lung. AS with ILV in unilateral parenchymal disease, a variety of configurations have been reported to provide ILV in patients who have BPF. The goal in all cases is to minimize air leak through the BPF by minimizing the pressure gradient across the fistula. This is accomplished by ventilating the involved lung with reduced pressure by minimizing tidal volume, rate, inspiratory flow, and

VENTILATION IN THE PATIENT WITH UNILATERAL LUNG DISEASE

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Table 5. INDICATIONS FOR INDEPENDENT-LUNG VENTILATION IN VENTILATED PATIENTS WHO HAVE BRONCHOPLEURAL FISTULA

Any of the following that are unresponsive to usual maneuvers during conventional

ventilation 1. Air leak exceeding 50% of the delivered tidal volume 2. Hypercapnic respiratory acidosis (pH < 7.30)

3. Refractory hypoxemia, particularly in patients in whom PEEP increases exacerbate 4.

air leak Persistent lung collapse despite optimum catheter drainage

PEEP. Frequently, compensatory increases in ventilation to the noninvolved lung provide adequate gas exchange. Single-ventilator systems with dual circuits and in-line flow resistors to control the distribution of ventilation have been successfully used in BPEZh, 28 Most frequently, dual-ventilator systems with synchronous or asynchronous control are used. The involved lung receives a lower rate and volume than the noninvolved lung and little or no PEEP.17,25, 33, 'I Combined modalities have also been reported. Wendt et alno reported the successful use of conventional ventilation to the normal lung with CPAP applied to the lung with the BPF in a patient who failed AILV. Several authors report the use of conventional ventilation to the noninvolved lung in conjunction with high-frequency ventilation to the lung with the BPF.30,38, These techniques were generally successful in supporting the patients while allowing the BPF to close. As with unilateral parenchymal disease, none of these methods of ILV has been shown to be superior, and asynchronous dual-ventilator ILV is most frequently employed because of the relative ease of use and equipment availability. Ventilator management in BPF is directed toward minimizing pressure in the circuit supplying the lung with the BPF while providing sufficient support via the noninvolved lung if possible. The mode of ventilation and parameters used are dependent on the preferences of the operator, the volume of air leak through the fistula, and the concomitant patient problems, particularly accompanying pulmonary processes. Low rates, small tidal volumes, little or no PEEP, and short inspiratory times should be used in the circuit with the BPF. The noninvolved lung can be supported with the same or an alternative mode while attempting to maintain adequate global gas exchange. Excessive PEEP or dynamic hyperinflation in the noninvolved lung may result in a redistribution of pulmonary blood flow and deterioration of gas exchange. Subsequent changes are made based on blood gas data; mechanics, including airway pressures; and the volume of air flow through the fistula. The mechanical and other parameters used to assess the relative performance of each lung in ILV with parenchymal disease are of limited value in this situation because the air leak precludes the actual determination of compliance or use of co,-derived variables. Once the air leak has ceased, an effort should be made to equalize the settings in each circuit while avoiding further barotrauma and reopening of the fistula. This equilibration is frequently done with a stepped series of changes in the ventilator settings with the values for each circuit approaching one another. Once equal or relatively proximate settings are achieved, the patient can be switched to conventional support for weaning or ongoing support.

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Other Considerations In addition to techniques to address the ventilatory asymmetry in ULD, new interventions are being explored to improve oxygenation by addressing the perfusion aspects of the mismatch that contributes to the hypoxemia in many lung diseases. The use of inhaled nitric oxide (NO) is a relatively recent technique that has been used to improve ventilation-perfusion matching in a variety of lung diseases. NO is a potent vasodilator that is delivered as a part of the inhaled gas mix and presumably distributed to well-ventilated regions of the lung. Selective vasodilatation in those regions produces a redistribution of blood flow to these well-ventilated areas, improving ventilation-perfusion matching and arterial oxygenation. NO use in a patient being supported with ILV was first reported by Badesch et a17 in a postoperative lung-transplant patient. In their experience, inhaled NO had no effect on gas exchange, although the patient reported was transplanted for primary pulmonary hypertension and may not have been responsive to inhaled NO. Subsequently, Johannigman et a155reported a unilateral pulmonary contusion patient who was being supported with ILV and who received inhaled NO successively to each lung and then to both lungs simultaneously. The use of NO to the normal lung and to both lungs simultaneously was associated with an improvement in oxygenation. Oxygenation deteriorated when NO was administered to the injured lung alone. The authors attribute the improvement to redistribution of blood flow to betterventilated areas of the lung and to reduction in mismatch and shunt. Note that oxygenation improved with NO administration to both lungs raises the possibility of using a trial of inhaled NO to improve gas exchange in patients who have refractory ULD before subjecting them to more invasive measures. This treatment presents an exciting possibility and awaits further study. Agents such as NO may provide beneficial effects in redistributing pulmonary blood flow; however, a word of caution also needs to be interjected about the use of vasodilator drugs in patients who have ULD and difficult oxygenation. Several of these agents are known to reverse hypoxemic pulmonary vasoconstriction and their administration in this setting has the potential to worsen gas exchange by increasing the perfusion to marginally or unventilated portions of the lung. This phenomenon has been observed in our ICU and presents difficult management problems. Additionally, deterioration in gas exchange not explained by other factors in a patient who has ULD should prompt a review of the patient’s medications. SUMMARY

Severe ULD presents a challenge in ventilator management because of the marked asymmetry in the mechanics of the two lungs. The asymmetry may result from significant decreases or increases in the compliance of the involved lung. Traditional ventilator support may fail to produce adequate gas exchange in these situations and has the potential to cause further deterioration. Fortunately, conventional techniques can be safely and effectively applied in the majority of cases without having to resort to less familiar and potentially hazardous forms of support. In those circumstances when conventional ventilation is unsuccessful in restoring adequate gas exchange, lateral positioning and ILV have proved effective at improving and maintaining gas exchange. Controlled trials to guide clinical decision making are lacking. In patients who have processes associated with decreased compliance in the involved lung, lateral posi-

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tioning may be a simple method of improving gas exchange but is associated with many practical limitations. ILV in these patients is frequently successful when differential PEEP is applied with the higher pressure to the involved lung. In patients in whom the pathology results in distribution of ventilation favoring the involved lung, particularly BPF, ILV can be used to supply adequate support while minimizing flow through the fistula and allowing it to close. The application of these techniques should be undertaken with an understanding of the pathophysiology of the underlying process; the reported experience with these techniques, including indications and successfully applied methods; and the potential problems encountered with their use. Fortunately, these modalities are infrequently required, but they provide a critical means of support when conventional techniques fail.

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