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Respiratory Support in Pediatric Surgery
Symposium on Respiratory Care in Surgery
Respiratory Support in Pediatric Surgery
Arvin I. Philippart, M.D.,':' Ashok P. Sarnaik, M.B., B.S., t and Walter M. Belenky, M.D.t
Major recent advances in pediatric surgical care have resulted largely from improvements in supportive care. Total parenteral nutrition and new techniques of respiratory assistance both account for much of that improvement. The improved survival of progressively smaller neonates with respiratory distress over the past decade has been the result of the application of constant distending pressure, the availability of continuous flow ventilation, and the development of ventilators specifically designed for this group of patients. No longer is one limited to assisted ventilation utilizing insensitive ventilators with large external circuits or controlled ventilation with its attendant difficulties. The availability of respiratory support without mechanical ventilation and of continuous flow ventilation has increased not only the flexibility in support techniques but also patient survival. The complexities of neonatal pulmonary support are emphasized by the fact that all of these infants are critically ill with multisystem disease. A characteristic scenario is the 1000 gm infant with respiratory distress syndrome; patent ductus arteriosus with congestive heart failure and hypoperfusion; renal ischemia; and necrotizing enterocolitis with superimposed gram-negative septicemia. Although it is necessary for clarity of this presentation to isolate issues in respiratory support, it is critical to appreciate that all necessary therapeutic modalities must be integrated. Managing end organ failure in the patient as described without initially correcting the etiologic cardiopulmonary disease is fruitless. Likewise, accurate volume administration and temperature '''Associate Professor of Surgery, Wayne State University School of Medicine, and Chief, Department of Pediatric General Surgery, Children's Hospital of Michigan, Detroit, Michigan t Assistant Professor of Pediatrics, Wayne State University School of Medicine, and Director, Intensive Care Unit, Children's Hospital of Michigan, Detroit, Michigan !Chief, Department of Otorhinolaryngology, Children's Hospital of Michigan, Detroit, Michigan
Surgical Clinics of North America- Vol. 60, No. 6, December 1980
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control and appropriate management of sepsis, coagulopathy, and acidbase balance are critical to optimal cardiopulmonary function and, therefore, survival. Early skepticism in the application of these techniques and the quality of infant obtained has been largely alleviated. Controversies still exist despite progress. These controversies include techniques of ventilatory support, airway management, and monitoring. The major emphasis in this discussion will be the neonate, for several reasons. This patient population creates the greatest anxiety in surgeons unfamiliar with their care. These patients present unique pulmonary disease, rapid physiologic changes, unfamiliar equipment, and variations in the accepted monitoring techniques used with older patients. Nevertheless, comprehension of the techniques in this age group is applicable to the older child with whom the occasional children's surgeon is more comfortable.
PHYSIOLOGY OF NEONATAL RESPIRATION The newborn is an obligate nose breather (Fig. 1). Nasal resistance to airflow is significantly less in the neonate than it is in the adult. The initial evaluation of labored respiration in this age group should include the evaluation ofthe nasopharyngeal airway for anomalies or secretions.
Figure 1. In this lateral magnification study of a normal laryngeal and supralaryngeal airway, uvular closure of the oropharynx is apparent, with dominance of the nasopharyngeal airway posteriorly.
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t Figure 2. Comparison of representative pulmonary compliance curves in healthy individuals (A) and in those with diffuse alveolar atelectasis (B). X= normal functional residual capacity (FRC) andY= FRC in diffuse atelectatic states. Shaded areas indicate pressures at which the compliance is maximal.
DISTENDING PRESSURE (em H20)
Because neonates are nursed on their backs, their relatively larger head size produces anterior flexion of the neck and airway that may stimulate soft tissue obstruction. In that circumstance, the neck should be placed in gentle extension with a roll under the shoulders. This may frequently produce a significant reduction in the work of breathing. The neonatal lung is not fully developed at the time of birth. Peripheral airways develop by progressive dichotomous branching with continual remodeling. By 28 weeks of gestation saccules first appear distal to transitional ducts. At birth there are generally three generations of such saccules capped by a terminal saccule representing approximately the eighteenth division. Shallow depressions in the wall of the saccule have been considered primitive alveoli but are thin walled, flattened, and do not contain elastic tissue. 5 Further postnatal branching occurs until seven to eight years of age and 22 to 23 respiratory divisions. Comprehension of the differences between neonatal pulmonary mechanics and those of the adult are necessary for an understanding of respiratory support. Functional residual capacity (FRC), that volume of air within the lung at end tidal expiration, plays an important role in minimizing changes in alveolar oxygen tension (PAo 2) during inspiration and expiration, thereby preventing arterial desaturation. The reduction in FRC is greater in neonates with poor pulmonary compliance than in adults as a result of the highly compliant neonatal chest wall. Additional factors that decrease FRC are atelectasis (infant respiratory distress syndrome), fluid (pneumonia, pulmonary edema), and muscle fatigue. 13 The FRC has a major effect on pulmonary compliance (Fig. 2). At the
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extremes of FRC, tidal breathing occurs in the more horizontal portion of the compliance curve, thereby requiring greater work of respiration for the same volume of ventilation. The desire to shift the curve to the vertical slope has generated the current concept of constant distending pressure. Overdistention from excessive airway pressure or regional air trapping (bronchopulmonary dysplasia or anomalies) reduces compliance and diaphragmatic excursion. Closing capacity is that lung volume at which the most dependent airways close. Such airway closure occurs at higher lung volumes in the newborn than in the adult, leaving the neonate more susceptible to air trapping and venous admixture. 12 The bellows effect of the chest wall and diaphragm of the neonate also differs from that of the adult. As mentioned, the chest wall is significantly more compliant. The ribs of the neonate are more transverse with less excursion than in the adult. Much of neonatal sleep is rapid eye movement (REM) sleep in which intercostal muscle activity is minimal. As a result, neonatal ventilation has greater reliance on diaphragmatic excursion than does adult ventilation. Neonatal surgical illness such as bowel obstruction or abdominal wall defects will, therefore, have a marked effect on diaphragmatic excursion and thereby decrease FRC. Cheyne-Stokes ventilation may not be an abnormal finding in a premature infant. Other less obvious aberrations in rate are frequent. Apnea is a consideration in all premature infants, particularly those who are ill. This is a major consideration in selection of methods of respiratory support. The pulmonary circulation of the neonate also differs from that of the adult. Fetal circulation is a right-sided dominant circulation with right-to-left shunting across the foramen ovale and ductus. This phenomenon is the result of high resistance pulmonary arterioles with thick muscular walls. These vessels are highly reactive and constrict in response to hypoxemia, hypothermia, acidosis, 15 and ventilationperfusion imbalance. Such elevated resistance decreases rapidly in the normal neonate over the first week to 10 days. However, the superimposition of cardiopulmonary disease prolongs that period of high resistance. Active pulmonary vasoconstriction results in increased central shunting with resultant hypoxia and acidosis, myocardial dysfunction, and the diminished production of surface active materials. The syndrome ofpersistant pulmonary hypertension in the neonate is still poorly understood 10 but is a primary circulatory phenomenon helped little by variations of ventilation.
CLINICAL ASSESSMENT A sequence of clinical findings provides evidence of increasing · respiratory distress. Tachypnea is an early and important sign. Although there are broad ranges of normal rate, persistant quiet rates of more than 60 in the premature neonate, 40 in the term neonate, and 30 in the infant are warning signs. Flaring nares is also a frequent early sign. Chest wall
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retractions occur as a result of increasingly negative intrapleural pressure and the marked compliance of the chest wall. Grunting is an adaptive phenomenon produced by expiration against a partially closed glottis in an attempt to generate positive end-expiratory pressure. For that reason, the grunting infant who requires respiratory support should have end-expiratory pressure applied to the airway. Cyanosis is a widely sought but frequently misinterpreted finding. Differentiation between peripheral and central cyanosis is important. Peripheral cyanosis, in the absence of central cyanosis, results from polycythemia and diminished peripheral perfusion commonly seen in hypothermia. Central cyanosis is a much more ominous finding that may result from either cardiac or pulmonary disease. Cough is a rare symptom in the neonate but common in the older child. The neonatal cough mechanism is poorly effective because of easily collapsing central airways with increased pleural pressures. A chest x-ray film is important early in the evaluation of respiratory distress. The following disease entities requiring rapid intervention should be sought: pneumothorax, diaphragmatic hemia, or aspiration pneumonia as evidence of intestinal obstruction. More commonly, the findings are diffuse, symmetric parenchymal disease, as found in infant respiratory distress syndrome and meconium aspiration syndrome (MAS).
MONITORING TECHNIQUES Blood gas determinations are critical to the diagnosis and management of respiratory insufficiency. Arterial C0 2 pressure (Paco 2 ) and Pao 2 are the indices of adequacy of ventilation and oxygenation, respectively. Numerous techniques and routes have evolved for such monitoring. Intra-arterial monitoring in the neonate contains all the hazards seen in the adult plus the technical difficulties of small vessels. Right-to-left shunting through the ductus as well as retinal sensitivity to oxygen produce unique complications in the neonate. In the presence of shunting, oxygen tension in the aortic arch is significantly higher than that below the ductus. Therefore, application of all monitoring techniques must take this discrepancy into account. The most widely used route, and therefore the most common source of complication, is the umbilical artery catheter. Although still controversial, placement of the proximal end in the distal thoracic aorta or just above the aortic bifurcation appears to produce little difference. 2 The advantages of this route are ease of insertion and ready access to arterial blood. Although disadvantages are significant, they can be minimized with great care. A major disadvantage to the umbilical artery catheter is the discrepancy between oxygen tension in the aortic arch and the distal aorta in the presence of right-to-left shunting through the ductus. Thromboembolic complications are significant and have included loss of perfusion to mesenteric vessels, kidneys, legs, and distal spinal cord with resultant paraplegia. 11 For this reason, we prefer the distal
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aortic locus of the tip. These complications can be minimized by the use of continuous mechanical infusion of only isotonic solutions containing small doses of heparin. Bolus flushing techniques should not be used, and a poorly functioning catheter should be removed. Other peripheral arteries utilized include the radial and temporal arteries. Radial artery lines should be inserted only when an intact ulnar artery is demonstrated by palpation or Doppler assessment after radial artery occlusion. Retrograde flushing in this location may produce loss of the extremity or even vertebral artery embolization. Temporal artery lines, like radial artery lines and unlike umbilical artery lines, have the advantage of monitoring partial pressures of oxygen in the aortic arch. Despite these advantages, they are inconvenient and can produce scalp loss or embolization to the ophthalmic or cerebral circulation. In our large nursery, we have found it rarely necessary to resort to indwelling lines in recent years. Alternative monitoring techniques that avoid the hazards of intravascular catheters include equipment for monitoring capillary gas and transcutaneous oxygen. Both have the disadvantage that abnormally low values are obtained in hypoperfusion states. However, in the presence of normal perfusion and adequate harvesting techniques, the correlation between capillary gases and arterial gases is excellent.s Despite the disadvantages of expense and inaccuracy in low perfusion states, transcutaneous electrodes have recently been popularized because they provide continuous monitoring and accurately assess the Pao 2 of the aortic arch when placed on the right anterior area of the chest. 7 Monitoring of arterial pH is also critical. Acidosis produces pulmonary arterial vasoconstriction as well as myocardial depression in the neonate. Respiratory alkalosis will diminish cardiac output, decrease cerebral blood flow, diminish oxyhemoglobin dissociation, and increase airway resistance with diminished pulmonary compliance.n
TECHNIQUES OF RESPIRATORY SUPPORT Administration of Oxygen The purpose of increased inspired oxygen is the elevation of Pao 2 to 60 to 70 torr in hypoxemic infants with adequate ventilation. Indications for this simplest form of support include lesser degrees of pulmonary parenchymal disease and congenital heart disease, or administration of oxygen may be part of the process of weaning from mechanical ventilation. Administration through a small hood covering the head and neck is the preferable technique in neonates in order to avoid wide swings in the fraction of inspired oxygen (Fi0 2) when the isolette is open. All inspired gas must be warmed and humidified to prevent heat loss and drying of secretions. Flow rates in such hoods must be at least 5 liters per minut~ to prevent C0 2 rebreathing. Neonatologists will add further measures if an Fi0 2 greater than 0.6 is required to maintain adequate Pao 2 • Because major surgical procedures produce pharmacologic depression, chest wall splinting, abdominal distention, extracellular fluid shifts, and in-
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creased oxygen requirement, surgical neonates require earlier application of respiratory support (constant distending pressure or mechanical ventilation).
Constant Distending Pressure Physiologic changes occurring in postoperative neonates include atelectasis, decreased FRC, and ventilation-perfusion abnormalities with venous admixture. These are the results of superimposition of operative changes on both normal and abnormal lungs. It was in this physiologic setting that Gregory first applied constant distending pressure in the management of respiratory distress syndrome.• The resultant increase in FRC improved pulmonary compliance and arterial oxygenation and diminished venous admixture. The function of constant distending pressure is the prevention of further atelectasis rather than the opening of closed respiratory units. Alternative techniques of administration include a nasotracheal tube or nasopharyngeal routes utilizing nasal prongs, face masks, or nasopharyngeal catheters. Routes other than an endotracheal tube are largely inappropriate in the postsurgical neonate, as nasopharyngeal routes produce distention of the gut undesirable in the patient after thoracic or abdominal surgery. Use of constant distending pressure requires an appreciation of the therapeutic goals and physiologic alterations produced as well as periodic determinations of blood gases. The pressure selected is based on the pathologic process and the response of the infant. In general, the least amount of pressure that will accomplish the therapeutic goals is best. High pressures (greater than 8 em of water) should be avoided, unless necessary, to avoid compromise not only in systemic venous return but also in pulmonary vascular perfusion. Although controversy exists, the application of normally therapeutic levels of constant distending pressure (4 to 6 em of water) rarely produces circulatory changes in the normovolemic patient. In the patient who requires ventilatory support, there is frequently little difference in cardiac output between the use of constant distending pressure in the form of continuous positive airway pressure (CPAP) as opposed to mechanical ventilation with the application of positive end-expiratory pressure (PEEP). Frequent monitoring of gas levels is required because changes in pulmonary function may be rapid after institution of such therapy. The best rule of thumb is that less pressure is preferable as long as the Fi0 2 can be reduced to levels infrequently associated with pulmonary oxygen toxicity.
Mechanical Ventilation Nowhere is judgment required more than in the assessment of the child who has failed with the previously mentioned methods of support utilizing increased Fi0 2 and constant distending pressure. The one absolute indication for mechanical ventilation is apnea. Neonatologists will use mechanical assistance with C0 2 retention above 60 torr and pH 7.2 or less or Pao 2 below 50 torr with an Fi0 2 of 0.8. VENTILATORS. Three types of ventilators are currently available.
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They are classified by the characteristic that terminates the inspiratory phase (that is, inspiratory pressure, tidal volume, inspiratory time). Each ventilator has its advantages and disadvantages. In pressure-cycled ventilators, gas flow is terminated when a preselected peak inspiratory pressure is reached. Such ventilators were used extensively in neonates in the 1960s and are rarely, if ever, appropriate now. Their advantages were simplicity of design and presumptive minimization of pulmonary barotrauma. However, changes in total pulmonary compliance as well as variable compliance of specific respiratory units are so rapid as to preclude their successful use. The volume ventilators were the next instruments designed to deliver a preselected tidal volume. Use of such ventilators is appropriate with proper recognition of the limitations of machine performance and patient application. Machine-delivered inspiratory volume must compensate for changes in pulmonary compliance, volume loss as a result of compliance of the external circuit, and air leak around the endotracheal tube. Cuffed tubes to minimize leak are never appropriate in neonates and rarely appropriate in the older child. Increases in inspiratory volume are required to overcome the changes in the external circuit and the tube leak. One can calculate losses in the external circuit but not around the tube. Another major technical limitation is that the inspiratory flow cannot be regulated below 50 ml per second, negating application in the small neonate. The availability of an assist mode makes this ventilator useful in the larger and more vigorous infant or child whose inspiratory flow exceeds 50 ml per second. However, application of the assist mode requires that the child be able to generate sufficient negative pressure to trigger the machine. Machine insensitivity, as in the pressure ventilators, further precludes the use of this mode in the small neonate and the infant who is less vigorous. Although the assist mode is preferred, controlled ventilation with paralysis can be advantagous in occasional circumstances. 17 Time-cycled ventilators provide a constant flow of gas of a predetermined duration. These ventilators are the most recently available and possess a number of advantages, particularly for the small neonate with poor pulmonary compliance. Precisely controlled inspiratory:expiratory ratios provide prolongation of inspiratory time, necessitating low flow rates, which are unavailable with volume respirators. Preset maximum inspiratory pressures are available to reduce barotrauma. The continual gas flow allows spontaneous ventilation superseding preset ventilator rates without the danger of C0 2 rebreathing. INTERMITTENT MANDATORY VENTILATION. In the past, asynchronOUS breathing during mechanical ventilation necessitated rebreathing of expired gas or wasteful expenditure of energy to generate inspiratory pressures in a closed system. The availability of continual gas flow in a ventilator circuit with an expiratory valve has allowed both spontaneous and mechanical ventilation. 9 Such a combination of spontaneous breathing with periodic mechanical ventilation is called intermittent mandatory ventilation (IMV). Intermittent mandatory ventilation is the currently preferred technique of ventilatory support of neonates and small infants with rapidly changing pulmonary compliance.
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With such a system, the level of ventilatory support can be varied by preselecting respiratory rate, inspiratory to expiratory ratio, and volume or pressure. The major advantage of IMV is the ability to vary the level of support, which becomes useful in the weaning process. There are two safety features in this system. A pressure pop-off valve prevents abnormal elevation of pressure in the airway in the face of asynchronous breathing. There is also an ability for neonates with diminished total lung volumes (for example, those with diaphragmatic hernias and defects of the abdominal wall) to set their own respiratory rates. No current technology for ventilatory assist exists that supersedes all problems of asynchronous respiration. However, IMV is the currently preferred technique for managing asynchrony. Although newer ventilators are reputed to provide synchronized IMV, our limited experience has found this to be of little value.
Ventilator Preparation As previously mentioned, pressure ventilators are rarely, if ever, appropriate. Volume ventilators are appropriate in the larger, more vigorous child, particularly when used in the assist mode. Time-cycled ventilators are preferred in neonates and smaller, less vigorous patients. TIDAL VoLUME. The initial calculation of tidal volume for a child with presumed normal lung volumes is 7 ml per kg. The range may be very broad, depending on primary pulmonary pathology and type of ventilator. In the child with markedly elevated diaphragms or a diaphragmatic hemia, significant reductions are necessary. In the latter circumstance, particularly, pulmonary hypoplasia is a large part of the anomaly, and tidal volumes in the nature of 4 to 5 ml per kg should be instituted initially. If a volume ventilator has been selected, the tidal volume should be augmented by the calculated volume lost to compliance of the extemal circuit. Frequent analyses of blood gases are necessary initially because of changes in pulmonary compliance consequent to the introduction of mechanical ventilation and the ever present leak around the endotracheal tube. The appropriate end point in all mechanical ventilation is adequate alveolar ventilation, and the selection of increase in rate or tidal volume will be dealt with later. INFLATION PRESSURE. With volume respirators, inflation pressure depends upon tidal volume and dynamic pulmonary compliance. Sudden changes in inflation pressure are important clinical indices of sudden changes in the patient such as selective bronchial intubation, airway plugging, bronchospasm, or other causes of rapid changes in compliance. A maximum inflation pressure can be established with a pop-off valve at 40 to 45 em of water. In time-cycled ventilators, rate, inspiratory to expiratory ratio, and inflation pressure are preset, and the tidal volume delivered to the patient depends upon dynamic compliance. Endotracheal tube leaks are, to a certain extent, compensated for by delivery of increased volume until the preselected inflation pressure is reached. Selection of the pressure limits at the time of initially instituting mechanical ventilation requires clinical judgment based on assessment of pulmonary compli-
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ance. Under the usual circumstances, levels are established at 2.5 to 30 em of water, with adjustments made on the basis of blood gas analysis. However, those infants with severely reduced pulmonary compliance will require higher initial pressure in order to establish adequate ventilation. RESPIRATORY RATES. Adjustments in rate are determined coincident with tidal volume to provide a desired minute ventilation and are best indicated by Pao 2 • The initial setting of rate is determined by the age of the child, since rates vary widely, as discussed under normal physiology. High respiratory rates are less effective in increasing alveolar ventilation than are increases in tidal volume. Increases in respiratory rate increase ventilation of dead space as well as alveolar ventilation. On the other hand, increases in tidal volume will increase alveolar ventilation without increasing ventilation of dead space. Furthermore, decreasing the rate will increase ventilation of alveolar units with slow time constants that would be inadequately ventilated with shorter inspiratory times coincident with more rapid rates. INSPIRATORY FLOW RATES AND INSPIRATORY TO EXPIRATORY RATIOS. Optimal inspiratory to expiratory ratios vary with pulmonary pathology. The classic inspiratory to expiratory ratio of 1:2 can be used initially in patients with relatively normal lungs. However, in patients with markedly diminished compliance, prolongation of inspiratory time has been shown to increase arterial oxygenation as well as to potentially reduce the risk of pulmonary barotrauma. 14 Restrictions on inspiratory to expiratory ratio appear when inspiratory time is sufficiently prolonged so that expiration is decreased to the point of markedly elevating FRC, diminishing compliance, and trapping gas. Such control of inspiratory to expiratory ratio is most easily obtained with time-cycled ventilators. In volume ventilators, the desired inspiratory to expiratory ratio can be achieved by controlling the inspiratory flow rate and thereby the inspiratory time.
AIRWAY MAINTENANCE Normal Airway Critical to an understanding of airway support in the pediatric patient is a knowledge of the anatomic features that differ from those of the adult. Airway size and tracheal length for appropriate age groups are listed in Table 1. The critical limitation of the airway is the subglottic area, which is only 5 mm in diameter in the normal neonate. As a result of small cross-sectional area, any changes producing edema produce rapidly apparent symptoms that are all the more critical as the airway decreases in size. Two millimeters of edema in a 6 mm subglottic area will reduce the functional airway to one-third of the normal airway size. The importance of tracheal length is emphasized by the ease with which neonates with inappropriately placed or mobile tubes can develop carinal trauma, selective bronchial intubation, or tracheal extubation. This is
~
01
"''1:1
:;:; ~ 0
Table 1.
~
Size of Tracheal Tube in Relation to Age and Dimensions of Airway
r:Fl
d
ENDOTRACHEAL TUBE SIZE''
MEASUREMENTS
TRACHEOTOMY 3
Age
Glottis Diameter AP (mm)
Subglottic' Diameter (mm)
Tracheal Diameter (mm)
Tracheal 3 Length (em)
Internal Diameter (mm)
External" Diameter (Fr.)
TUBE
Length (em)
Suction Catheter
'1:1 '1:1 0 :xl >-l
z "1:1 01
tl
~
Premature infant < 1500 gm Term neonate 1-3 mo. 3-6 mo. 6-12 mo. 2 yrs. 3-5 yrs. 5-6 yrs. 6-8 yrs. 8-10 yrs. 10-12 yrs. 12-14 yrs. 14-16 yrs.
4.0 7
5.0-5.5 6.0
13-25
6 6.5 7.0 7.5 9.5
5.0-7.1 5.5-8.0 5.5-9.0 7.0-9.0
2.5
12
8
6
00
3.0 3.0 3.0-3.5 3.5 4.0 4.5 5.0 5.5 6.0 6.5 6.5 7.0
14 14 14-16 16 18 20 22 24 26 28 28 30
10 10 10-12 12 14 16 16 18 18 20 20 22
6 6 6-8 8 8 8 10 10 10 12 12 12
0 0 0 0-1 2 2-3 3 3-4 4 4 5 6-8
>-l :xl H
C'l
r:Fl
d
:xl 0 01
:xl ><:
'·'Add 2-3 em for nasotracheal tube.
.....
(J1 N)
t.C
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best avoided by care in tube selection, frequent auscultatory evaluation, and chest x-ray films for location of the tube.
Pathology of Airway Intubation An ongoing eight year study of all patients coming to postmortem examination who have had airway intubation in this institution has allowed reconstruction of a sequence of pathologic changes associated with intubation. The initial observation is that any form of airway manipulation will produce trauma of varying degrees. Tracheal changes are the same whether a nasotracheal tube or a tracheotomy is used. The latter obviously spares the glottic and subglottic areas. Early histologic changes of epithelial ulceration, necrosis, and inflammation appear in the first hour or two and extend to the perichondrium within 24 hours. Extension of the perichondritis with subsequent inflammation of cartilage itself may occur at approximately one week. These histologic findings underlie the subsequent development of stenosis and malacia. Characteristics predisposing to more extensive injury include large tube size, inadequate fixation of tube, diminished peripheral tissue perfusion and oxygenation, and traumatic suctioning techniques. A guide to tube size is provided in Table 1, but the experienced physician relies on his or her assessment for a proper fit. No attempts should be made to provide a tight fit, particularly in neonates. The presence of a leak around the tube can be compensated for by ventilatory management and is associated with less airway trauma. Because tube fixation is important, nasotracheal tubes are preferred in practically all circumstances other than emergency oral intubation. Little can be done to avoid the effects of hypoperfusion and hypoxia other than an earlier decision for tracheotomy than might be the case in the normally perfused and normally oxygenated patient. Despite this categorization of histologic findings and underlying etiologies, decisions regarding routes of airway maintenance in neonates are based heavily on extensive clinical experience and apparent ability of the neonate to repair such pathologic changes in more satisfactory fashion than does the adult.
Artificial Airways The classic indications for the insertion of an artificial airway of either type have always been relief of upper airway obstruction, provision of control of ventilatory support, and enhancement of tracheobronchial toilet. The method of airway intervention is much less clear, and much more controversial, than it is in the adult group. As a result, the following schema is the approach used in this institution and accepted by many, but not all, similar facilities. In the premature or term neonate requiring prolonged respiratory support, nasotracheal intubation is always the initial choice and can be maintained, with optimum care, for several months if required. variation from this route arises in the child with airway anomalies or a recognized need for a much longer term artificial airway. In that circumstance a tracheostomy is performed. In the older child, endotracheal intubation is continued for a much shorter period (72 hours) before conversion to a tracheostomy. Two disease processes occurring in
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older infants and young children in which airway management is controversial are croup and epiglottitis. In this institution, all children with acute epiglottitis are intubated with a nasotracheal tube with subsequent removal in approximately 48 hours. When this is done with care by experienced people, difficulties are avoided. No tracheostomy for acute epiglottitis has been done in this institution in the last eight years. 1 Likewise in croup, the availability of racemic epinephrine inhalations with or without steroids has obviated the need for tracheostomy in the past five years. When tracheostomy is required, several controversial decisions are necessary. Tracheostomy in the child should be performed in the operating room with airway control over an endotracheal tube or bronchoscope. The classic argument surrounding vertical or transverse skin incisions is different in the pediatric age group. A vertical skin incision allows the tracheostomy tube to settle in its optimal position, avoiding angulation and tracheal pressure. Cutaneous scarring appears equal no matter which incision is used. A single vertical tracheal incision through one tracheal ring (the third) is usually a good guide to appropriate selection of tube size. These rules may require modification at the time of the procedure. The type of tube selected is also controversial. Silastic tubes have the advantage of causing less tracheal trauma. However, standard metal Jackson/Holinger tubes with inner cannulas are frequently preferred, as they offer the advantages of a wider selection of the tube length and curvature and also allow frequent changes of the inner cannula, obviating the need for frequent total tube changes. The current raging controversy surrounding the use of the metal or Silastic tracheostomy tubes remains unresolved. Several successful experiences exist with each type of tube. New Silastic tracheostomy tubes provided with inner cannulas may achieve the ideal result of producing less tracheal trauma as well as having the added safety of an inner cannula. Tube obstruction by respiratory secretions is a danger that must always be considered in the pediatric tracheotomy. Tube fixation in a short, fat neck of a young child is critical. When the tapes are tied, the neck should be in flexion. When tied in extension, the tapes are too loose, allowing dislocation. Postoperative care must be rigid in both the hospital or outpatient setting. Proper humidification of inspired air, including ultrasonic nebulization, and the periodic instillation of small volumes of normal saline followed by suctioning enhance removal of secretions. Suctioning technique is also critical, as the suction catheter itself can produce tracheal trauma. As a result, suction tubes are carefully measured to extend minimally beyond the indwelling tracheal tube, be it a tracheotomy or an endotracheal tube.
SUMMARY The impact of recent developments in neonatal respiratory care on patient survival has been great. The emphasis placed on this group of patients in this article reflects the major management problems that remain in pediatric respiratory support. The presence of controversy in
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some areas emphasizes the lack of universally successful therapeutic modalities. Nevertheless, sufficient progress has been made that successful surgical procedures in the 1000 gm premature infant should be the rule and no longer the exception. Comprehension of techniques of respiratory support is necessary for integrated and successful care of the neonate with multisystem disease. Such comprehension can then be applied to the less complex physiology of the older child.
REFERENCES 1. Bottenfield, G. W., Arcinue, E. L., Sarnaik, A. P., eta!.: Diagnosis and management of acute epiglottitis- report of 90 consecutive cases. Laryngoscope, in press. 2. Feinauer, L. R., Atherton, S. 0., and Jung, A. L.: High vs. low placement of umbilical artery catheters. Clin. Res., 23:142A, 1975. 3. Fearon, B., and Whelen, J. S.: Tracheal dimensions in the living infant. Ann. Otol. Rhino!. Laryngol., 76:964, 1967. 4. Gregory, G. A., Kitterman, J. A., Phibbs, R. H., et a!.: Treatment of the idiopathic respiratory-distress syndrome with continuous positive airway pressure. N. Engl. J. Med., 284:1333, 1971. 5. Hislop, A., and Reid, L.: Development of the acinus in the human lung. Thorax, 29:90, 1974. 6. Holinger, P. H., et a!.: Subglottic stenosis in infants and children. Ann. Otol. Rhino!. Laryngol., 85:591, 1976. 7. Huch, R., Lubbers, D. W., and Huch, A.: Reliability of transcutaneous monitoring of arterial Po, in newborn infants. Arch. Dis. Child., 49:213, 1974. 8. Kama, P., and Poland, R. L.: Monitoring critically ill newborn infants with digital capillary blood samples: An alternative. J. Pediatr., 92:270, 1978. 9. Kirby, R. R.: Intermittent mandatory ventilation in the neonate. Crit. Care Med., 5:18, 1977. 10. Levin, D. L., Heymann, M.A., Kitterman, J. A., et al.: Persistent pulmonary hypertension of the newborn infant. J. Pediatr., 89:626, 1976. 11. Neal, W. A., eta!.: Umbilical artery catheterization: Demonstration of arterial thrombosis by aortography. Pediatrics, 50:6, 1972. 12. Nelson, N. M.: Respiration and circulation after birth. In Smith, C. A., and Nelson, N. M. (eds.): The Physiology of the Newborn Infant, 4th ed. Springfield, Ill., Charles C Thomas, 1976. 13. Polgar, G., and Weng, T. R.: The functional development of the respiratory system. Am. Rev. Respir. Dis., 120:625, 1979. 14. Reynolds, E. 0. R., and Taghizadeh, A.: Improved prognosis of infants mechanically ventilated for hyaline membrane disease. Arch. Dis. Child., 49:505, 1974. 15. Rudolph, A.M., and Yuan, S.: Response of the pulmonary vasculature to hypoxia and H+ ion concentration changes. J. Clin. Invest., 45:399, 1966. 16. Smith, R. M.: Anesthesia for Infants and Children, 4th ed. St. Louis, C.V. Mosby Company, 1960. 17. Stark, A. R., Bascom, R., and Frantz, I.: Muscle relaxation in mechanically ventilated infants. J. Pediatr. 94:439, 1979. 18. Thibeault, D. W., and Gregory, G. A.: Neonatal Pulmonary Care. Menlo Park, California, Addison-Wesley Publishers, 1979. A. I. Philippart, M.D. 3901 Beaubien Detroit, Michigan 48201
Respiratory Support in Pediatric Surgery
Arvin I. Philippart, M.D.,':' Ashok P. Sarnaik, M.B., B.S., t and Walter M. Belenky, M.D.t
Major recent advances in pediatric surgical care have resulted largely from improvements in supportive care. Total parenteral nutrition and new techniques of respiratory assistance both account for much of that improvement. The improved survival of progressively smaller neonates with respiratory distress over the past decade has been the result of the application of constant distending pressure, the availability of continuous flow ventilation, and the development of ventilators specifically designed for this group of patients. No longer is one limited to assisted ventilation utilizing insensitive ventilators with large external circuits or controlled ventilation with its attendant difficulties. The availability of respiratory support without mechanical ventilation and of continuous flow ventilation has increased not only the flexibility in support techniques but also patient survival. The complexities of neonatal pulmonary support are emphasized by the fact that all of these infants are critically ill with multisystem disease. A characteristic scenario is the 1000 gm infant with respiratory distress syndrome; patent ductus arteriosus with congestive heart failure and hypoperfusion; renal ischemia; and necrotizing enterocolitis with superimposed gram-negative septicemia. Although it is necessary for clarity of this presentation to isolate issues in respiratory support, it is critical to appreciate that all necessary therapeutic modalities must be integrated. Managing end organ failure in the patient as described without initially correcting the etiologic cardiopulmonary disease is fruitless. Likewise, accurate volume administration and temperature '''Associate Professor of Surgery, Wayne State University School of Medicine, and Chief, Department of Pediatric General Surgery, Children's Hospital of Michigan, Detroit, Michigan t Assistant Professor of Pediatrics, Wayne State University School of Medicine, and Director, Intensive Care Unit, Children's Hospital of Michigan, Detroit, Michigan !Chief, Department of Otorhinolaryngology, Children's Hospital of Michigan, Detroit, Michigan
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control and appropriate management of sepsis, coagulopathy, and acidbase balance are critical to optimal cardiopulmonary function and, therefore, survival. Early skepticism in the application of these techniques and the quality of infant obtained has been largely alleviated. Controversies still exist despite progress. These controversies include techniques of ventilatory support, airway management, and monitoring. The major emphasis in this discussion will be the neonate, for several reasons. This patient population creates the greatest anxiety in surgeons unfamiliar with their care. These patients present unique pulmonary disease, rapid physiologic changes, unfamiliar equipment, and variations in the accepted monitoring techniques used with older patients. Nevertheless, comprehension of the techniques in this age group is applicable to the older child with whom the occasional children's surgeon is more comfortable.
PHYSIOLOGY OF NEONATAL RESPIRATION The newborn is an obligate nose breather (Fig. 1). Nasal resistance to airflow is significantly less in the neonate than it is in the adult. The initial evaluation of labored respiration in this age group should include the evaluation ofthe nasopharyngeal airway for anomalies or secretions.
Figure 1. In this lateral magnification study of a normal laryngeal and supralaryngeal airway, uvular closure of the oropharynx is apparent, with dominance of the nasopharyngeal airway posteriorly.
RESPIRATORY SUPPORT IN PEDIATRIC SURGERY
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t Figure 2. Comparison of representative pulmonary compliance curves in healthy individuals (A) and in those with diffuse alveolar atelectasis (B). X= normal functional residual capacity (FRC) andY= FRC in diffuse atelectatic states. Shaded areas indicate pressures at which the compliance is maximal.
DISTENDING PRESSURE (em H20)
Because neonates are nursed on their backs, their relatively larger head size produces anterior flexion of the neck and airway that may stimulate soft tissue obstruction. In that circumstance, the neck should be placed in gentle extension with a roll under the shoulders. This may frequently produce a significant reduction in the work of breathing. The neonatal lung is not fully developed at the time of birth. Peripheral airways develop by progressive dichotomous branching with continual remodeling. By 28 weeks of gestation saccules first appear distal to transitional ducts. At birth there are generally three generations of such saccules capped by a terminal saccule representing approximately the eighteenth division. Shallow depressions in the wall of the saccule have been considered primitive alveoli but are thin walled, flattened, and do not contain elastic tissue. 5 Further postnatal branching occurs until seven to eight years of age and 22 to 23 respiratory divisions. Comprehension of the differences between neonatal pulmonary mechanics and those of the adult are necessary for an understanding of respiratory support. Functional residual capacity (FRC), that volume of air within the lung at end tidal expiration, plays an important role in minimizing changes in alveolar oxygen tension (PAo 2) during inspiration and expiration, thereby preventing arterial desaturation. The reduction in FRC is greater in neonates with poor pulmonary compliance than in adults as a result of the highly compliant neonatal chest wall. Additional factors that decrease FRC are atelectasis (infant respiratory distress syndrome), fluid (pneumonia, pulmonary edema), and muscle fatigue. 13 The FRC has a major effect on pulmonary compliance (Fig. 2). At the
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extremes of FRC, tidal breathing occurs in the more horizontal portion of the compliance curve, thereby requiring greater work of respiration for the same volume of ventilation. The desire to shift the curve to the vertical slope has generated the current concept of constant distending pressure. Overdistention from excessive airway pressure or regional air trapping (bronchopulmonary dysplasia or anomalies) reduces compliance and diaphragmatic excursion. Closing capacity is that lung volume at which the most dependent airways close. Such airway closure occurs at higher lung volumes in the newborn than in the adult, leaving the neonate more susceptible to air trapping and venous admixture. 12 The bellows effect of the chest wall and diaphragm of the neonate also differs from that of the adult. As mentioned, the chest wall is significantly more compliant. The ribs of the neonate are more transverse with less excursion than in the adult. Much of neonatal sleep is rapid eye movement (REM) sleep in which intercostal muscle activity is minimal. As a result, neonatal ventilation has greater reliance on diaphragmatic excursion than does adult ventilation. Neonatal surgical illness such as bowel obstruction or abdominal wall defects will, therefore, have a marked effect on diaphragmatic excursion and thereby decrease FRC. Cheyne-Stokes ventilation may not be an abnormal finding in a premature infant. Other less obvious aberrations in rate are frequent. Apnea is a consideration in all premature infants, particularly those who are ill. This is a major consideration in selection of methods of respiratory support. The pulmonary circulation of the neonate also differs from that of the adult. Fetal circulation is a right-sided dominant circulation with right-to-left shunting across the foramen ovale and ductus. This phenomenon is the result of high resistance pulmonary arterioles with thick muscular walls. These vessels are highly reactive and constrict in response to hypoxemia, hypothermia, acidosis, 15 and ventilationperfusion imbalance. Such elevated resistance decreases rapidly in the normal neonate over the first week to 10 days. However, the superimposition of cardiopulmonary disease prolongs that period of high resistance. Active pulmonary vasoconstriction results in increased central shunting with resultant hypoxia and acidosis, myocardial dysfunction, and the diminished production of surface active materials. The syndrome ofpersistant pulmonary hypertension in the neonate is still poorly understood 10 but is a primary circulatory phenomenon helped little by variations of ventilation.
CLINICAL ASSESSMENT A sequence of clinical findings provides evidence of increasing · respiratory distress. Tachypnea is an early and important sign. Although there are broad ranges of normal rate, persistant quiet rates of more than 60 in the premature neonate, 40 in the term neonate, and 30 in the infant are warning signs. Flaring nares is also a frequent early sign. Chest wall
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retractions occur as a result of increasingly negative intrapleural pressure and the marked compliance of the chest wall. Grunting is an adaptive phenomenon produced by expiration against a partially closed glottis in an attempt to generate positive end-expiratory pressure. For that reason, the grunting infant who requires respiratory support should have end-expiratory pressure applied to the airway. Cyanosis is a widely sought but frequently misinterpreted finding. Differentiation between peripheral and central cyanosis is important. Peripheral cyanosis, in the absence of central cyanosis, results from polycythemia and diminished peripheral perfusion commonly seen in hypothermia. Central cyanosis is a much more ominous finding that may result from either cardiac or pulmonary disease. Cough is a rare symptom in the neonate but common in the older child. The neonatal cough mechanism is poorly effective because of easily collapsing central airways with increased pleural pressures. A chest x-ray film is important early in the evaluation of respiratory distress. The following disease entities requiring rapid intervention should be sought: pneumothorax, diaphragmatic hemia, or aspiration pneumonia as evidence of intestinal obstruction. More commonly, the findings are diffuse, symmetric parenchymal disease, as found in infant respiratory distress syndrome and meconium aspiration syndrome (MAS).
MONITORING TECHNIQUES Blood gas determinations are critical to the diagnosis and management of respiratory insufficiency. Arterial C0 2 pressure (Paco 2 ) and Pao 2 are the indices of adequacy of ventilation and oxygenation, respectively. Numerous techniques and routes have evolved for such monitoring. Intra-arterial monitoring in the neonate contains all the hazards seen in the adult plus the technical difficulties of small vessels. Right-to-left shunting through the ductus as well as retinal sensitivity to oxygen produce unique complications in the neonate. In the presence of shunting, oxygen tension in the aortic arch is significantly higher than that below the ductus. Therefore, application of all monitoring techniques must take this discrepancy into account. The most widely used route, and therefore the most common source of complication, is the umbilical artery catheter. Although still controversial, placement of the proximal end in the distal thoracic aorta or just above the aortic bifurcation appears to produce little difference. 2 The advantages of this route are ease of insertion and ready access to arterial blood. Although disadvantages are significant, they can be minimized with great care. A major disadvantage to the umbilical artery catheter is the discrepancy between oxygen tension in the aortic arch and the distal aorta in the presence of right-to-left shunting through the ductus. Thromboembolic complications are significant and have included loss of perfusion to mesenteric vessels, kidneys, legs, and distal spinal cord with resultant paraplegia. 11 For this reason, we prefer the distal
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aortic locus of the tip. These complications can be minimized by the use of continuous mechanical infusion of only isotonic solutions containing small doses of heparin. Bolus flushing techniques should not be used, and a poorly functioning catheter should be removed. Other peripheral arteries utilized include the radial and temporal arteries. Radial artery lines should be inserted only when an intact ulnar artery is demonstrated by palpation or Doppler assessment after radial artery occlusion. Retrograde flushing in this location may produce loss of the extremity or even vertebral artery embolization. Temporal artery lines, like radial artery lines and unlike umbilical artery lines, have the advantage of monitoring partial pressures of oxygen in the aortic arch. Despite these advantages, they are inconvenient and can produce scalp loss or embolization to the ophthalmic or cerebral circulation. In our large nursery, we have found it rarely necessary to resort to indwelling lines in recent years. Alternative monitoring techniques that avoid the hazards of intravascular catheters include equipment for monitoring capillary gas and transcutaneous oxygen. Both have the disadvantage that abnormally low values are obtained in hypoperfusion states. However, in the presence of normal perfusion and adequate harvesting techniques, the correlation between capillary gases and arterial gases is excellent.s Despite the disadvantages of expense and inaccuracy in low perfusion states, transcutaneous electrodes have recently been popularized because they provide continuous monitoring and accurately assess the Pao 2 of the aortic arch when placed on the right anterior area of the chest. 7 Monitoring of arterial pH is also critical. Acidosis produces pulmonary arterial vasoconstriction as well as myocardial depression in the neonate. Respiratory alkalosis will diminish cardiac output, decrease cerebral blood flow, diminish oxyhemoglobin dissociation, and increase airway resistance with diminished pulmonary compliance.n
TECHNIQUES OF RESPIRATORY SUPPORT Administration of Oxygen The purpose of increased inspired oxygen is the elevation of Pao 2 to 60 to 70 torr in hypoxemic infants with adequate ventilation. Indications for this simplest form of support include lesser degrees of pulmonary parenchymal disease and congenital heart disease, or administration of oxygen may be part of the process of weaning from mechanical ventilation. Administration through a small hood covering the head and neck is the preferable technique in neonates in order to avoid wide swings in the fraction of inspired oxygen (Fi0 2) when the isolette is open. All inspired gas must be warmed and humidified to prevent heat loss and drying of secretions. Flow rates in such hoods must be at least 5 liters per minut~ to prevent C0 2 rebreathing. Neonatologists will add further measures if an Fi0 2 greater than 0.6 is required to maintain adequate Pao 2 • Because major surgical procedures produce pharmacologic depression, chest wall splinting, abdominal distention, extracellular fluid shifts, and in-
RESPIRATORY SUPPORT IN PEDIATRIC SURGERY
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creased oxygen requirement, surgical neonates require earlier application of respiratory support (constant distending pressure or mechanical ventilation).
Constant Distending Pressure Physiologic changes occurring in postoperative neonates include atelectasis, decreased FRC, and ventilation-perfusion abnormalities with venous admixture. These are the results of superimposition of operative changes on both normal and abnormal lungs. It was in this physiologic setting that Gregory first applied constant distending pressure in the management of respiratory distress syndrome.• The resultant increase in FRC improved pulmonary compliance and arterial oxygenation and diminished venous admixture. The function of constant distending pressure is the prevention of further atelectasis rather than the opening of closed respiratory units. Alternative techniques of administration include a nasotracheal tube or nasopharyngeal routes utilizing nasal prongs, face masks, or nasopharyngeal catheters. Routes other than an endotracheal tube are largely inappropriate in the postsurgical neonate, as nasopharyngeal routes produce distention of the gut undesirable in the patient after thoracic or abdominal surgery. Use of constant distending pressure requires an appreciation of the therapeutic goals and physiologic alterations produced as well as periodic determinations of blood gases. The pressure selected is based on the pathologic process and the response of the infant. In general, the least amount of pressure that will accomplish the therapeutic goals is best. High pressures (greater than 8 em of water) should be avoided, unless necessary, to avoid compromise not only in systemic venous return but also in pulmonary vascular perfusion. Although controversy exists, the application of normally therapeutic levels of constant distending pressure (4 to 6 em of water) rarely produces circulatory changes in the normovolemic patient. In the patient who requires ventilatory support, there is frequently little difference in cardiac output between the use of constant distending pressure in the form of continuous positive airway pressure (CPAP) as opposed to mechanical ventilation with the application of positive end-expiratory pressure (PEEP). Frequent monitoring of gas levels is required because changes in pulmonary function may be rapid after institution of such therapy. The best rule of thumb is that less pressure is preferable as long as the Fi0 2 can be reduced to levels infrequently associated with pulmonary oxygen toxicity.
Mechanical Ventilation Nowhere is judgment required more than in the assessment of the child who has failed with the previously mentioned methods of support utilizing increased Fi0 2 and constant distending pressure. The one absolute indication for mechanical ventilation is apnea. Neonatologists will use mechanical assistance with C0 2 retention above 60 torr and pH 7.2 or less or Pao 2 below 50 torr with an Fi0 2 of 0.8. VENTILATORS. Three types of ventilators are currently available.
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They are classified by the characteristic that terminates the inspiratory phase (that is, inspiratory pressure, tidal volume, inspiratory time). Each ventilator has its advantages and disadvantages. In pressure-cycled ventilators, gas flow is terminated when a preselected peak inspiratory pressure is reached. Such ventilators were used extensively in neonates in the 1960s and are rarely, if ever, appropriate now. Their advantages were simplicity of design and presumptive minimization of pulmonary barotrauma. However, changes in total pulmonary compliance as well as variable compliance of specific respiratory units are so rapid as to preclude their successful use. The volume ventilators were the next instruments designed to deliver a preselected tidal volume. Use of such ventilators is appropriate with proper recognition of the limitations of machine performance and patient application. Machine-delivered inspiratory volume must compensate for changes in pulmonary compliance, volume loss as a result of compliance of the external circuit, and air leak around the endotracheal tube. Cuffed tubes to minimize leak are never appropriate in neonates and rarely appropriate in the older child. Increases in inspiratory volume are required to overcome the changes in the external circuit and the tube leak. One can calculate losses in the external circuit but not around the tube. Another major technical limitation is that the inspiratory flow cannot be regulated below 50 ml per second, negating application in the small neonate. The availability of an assist mode makes this ventilator useful in the larger and more vigorous infant or child whose inspiratory flow exceeds 50 ml per second. However, application of the assist mode requires that the child be able to generate sufficient negative pressure to trigger the machine. Machine insensitivity, as in the pressure ventilators, further precludes the use of this mode in the small neonate and the infant who is less vigorous. Although the assist mode is preferred, controlled ventilation with paralysis can be advantagous in occasional circumstances. 17 Time-cycled ventilators provide a constant flow of gas of a predetermined duration. These ventilators are the most recently available and possess a number of advantages, particularly for the small neonate with poor pulmonary compliance. Precisely controlled inspiratory:expiratory ratios provide prolongation of inspiratory time, necessitating low flow rates, which are unavailable with volume respirators. Preset maximum inspiratory pressures are available to reduce barotrauma. The continual gas flow allows spontaneous ventilation superseding preset ventilator rates without the danger of C0 2 rebreathing. INTERMITTENT MANDATORY VENTILATION. In the past, asynchronOUS breathing during mechanical ventilation necessitated rebreathing of expired gas or wasteful expenditure of energy to generate inspiratory pressures in a closed system. The availability of continual gas flow in a ventilator circuit with an expiratory valve has allowed both spontaneous and mechanical ventilation. 9 Such a combination of spontaneous breathing with periodic mechanical ventilation is called intermittent mandatory ventilation (IMV). Intermittent mandatory ventilation is the currently preferred technique of ventilatory support of neonates and small infants with rapidly changing pulmonary compliance.
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With such a system, the level of ventilatory support can be varied by preselecting respiratory rate, inspiratory to expiratory ratio, and volume or pressure. The major advantage of IMV is the ability to vary the level of support, which becomes useful in the weaning process. There are two safety features in this system. A pressure pop-off valve prevents abnormal elevation of pressure in the airway in the face of asynchronous breathing. There is also an ability for neonates with diminished total lung volumes (for example, those with diaphragmatic hernias and defects of the abdominal wall) to set their own respiratory rates. No current technology for ventilatory assist exists that supersedes all problems of asynchronous respiration. However, IMV is the currently preferred technique for managing asynchrony. Although newer ventilators are reputed to provide synchronized IMV, our limited experience has found this to be of little value.
Ventilator Preparation As previously mentioned, pressure ventilators are rarely, if ever, appropriate. Volume ventilators are appropriate in the larger, more vigorous child, particularly when used in the assist mode. Time-cycled ventilators are preferred in neonates and smaller, less vigorous patients. TIDAL VoLUME. The initial calculation of tidal volume for a child with presumed normal lung volumes is 7 ml per kg. The range may be very broad, depending on primary pulmonary pathology and type of ventilator. In the child with markedly elevated diaphragms or a diaphragmatic hemia, significant reductions are necessary. In the latter circumstance, particularly, pulmonary hypoplasia is a large part of the anomaly, and tidal volumes in the nature of 4 to 5 ml per kg should be instituted initially. If a volume ventilator has been selected, the tidal volume should be augmented by the calculated volume lost to compliance of the extemal circuit. Frequent analyses of blood gases are necessary initially because of changes in pulmonary compliance consequent to the introduction of mechanical ventilation and the ever present leak around the endotracheal tube. The appropriate end point in all mechanical ventilation is adequate alveolar ventilation, and the selection of increase in rate or tidal volume will be dealt with later. INFLATION PRESSURE. With volume respirators, inflation pressure depends upon tidal volume and dynamic pulmonary compliance. Sudden changes in inflation pressure are important clinical indices of sudden changes in the patient such as selective bronchial intubation, airway plugging, bronchospasm, or other causes of rapid changes in compliance. A maximum inflation pressure can be established with a pop-off valve at 40 to 45 em of water. In time-cycled ventilators, rate, inspiratory to expiratory ratio, and inflation pressure are preset, and the tidal volume delivered to the patient depends upon dynamic compliance. Endotracheal tube leaks are, to a certain extent, compensated for by delivery of increased volume until the preselected inflation pressure is reached. Selection of the pressure limits at the time of initially instituting mechanical ventilation requires clinical judgment based on assessment of pulmonary compli-
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ance. Under the usual circumstances, levels are established at 2.5 to 30 em of water, with adjustments made on the basis of blood gas analysis. However, those infants with severely reduced pulmonary compliance will require higher initial pressure in order to establish adequate ventilation. RESPIRATORY RATES. Adjustments in rate are determined coincident with tidal volume to provide a desired minute ventilation and are best indicated by Pao 2 • The initial setting of rate is determined by the age of the child, since rates vary widely, as discussed under normal physiology. High respiratory rates are less effective in increasing alveolar ventilation than are increases in tidal volume. Increases in respiratory rate increase ventilation of dead space as well as alveolar ventilation. On the other hand, increases in tidal volume will increase alveolar ventilation without increasing ventilation of dead space. Furthermore, decreasing the rate will increase ventilation of alveolar units with slow time constants that would be inadequately ventilated with shorter inspiratory times coincident with more rapid rates. INSPIRATORY FLOW RATES AND INSPIRATORY TO EXPIRATORY RATIOS. Optimal inspiratory to expiratory ratios vary with pulmonary pathology. The classic inspiratory to expiratory ratio of 1:2 can be used initially in patients with relatively normal lungs. However, in patients with markedly diminished compliance, prolongation of inspiratory time has been shown to increase arterial oxygenation as well as to potentially reduce the risk of pulmonary barotrauma. 14 Restrictions on inspiratory to expiratory ratio appear when inspiratory time is sufficiently prolonged so that expiration is decreased to the point of markedly elevating FRC, diminishing compliance, and trapping gas. Such control of inspiratory to expiratory ratio is most easily obtained with time-cycled ventilators. In volume ventilators, the desired inspiratory to expiratory ratio can be achieved by controlling the inspiratory flow rate and thereby the inspiratory time.
AIRWAY MAINTENANCE Normal Airway Critical to an understanding of airway support in the pediatric patient is a knowledge of the anatomic features that differ from those of the adult. Airway size and tracheal length for appropriate age groups are listed in Table 1. The critical limitation of the airway is the subglottic area, which is only 5 mm in diameter in the normal neonate. As a result of small cross-sectional area, any changes producing edema produce rapidly apparent symptoms that are all the more critical as the airway decreases in size. Two millimeters of edema in a 6 mm subglottic area will reduce the functional airway to one-third of the normal airway size. The importance of tracheal length is emphasized by the ease with which neonates with inappropriately placed or mobile tubes can develop carinal trauma, selective bronchial intubation, or tracheal extubation. This is
~
01
"''1:1
:;:; ~ 0
Table 1.
~
Size of Tracheal Tube in Relation to Age and Dimensions of Airway
r:Fl
d
ENDOTRACHEAL TUBE SIZE''
MEASUREMENTS
TRACHEOTOMY 3
Age
Glottis Diameter AP (mm)
Subglottic' Diameter (mm)
Tracheal Diameter (mm)
Tracheal 3 Length (em)
Internal Diameter (mm)
External" Diameter (Fr.)
TUBE
Length (em)
Suction Catheter
'1:1 '1:1 0 :xl >-l
z "1:1 01
tl
~
Premature infant < 1500 gm Term neonate 1-3 mo. 3-6 mo. 6-12 mo. 2 yrs. 3-5 yrs. 5-6 yrs. 6-8 yrs. 8-10 yrs. 10-12 yrs. 12-14 yrs. 14-16 yrs.
4.0 7
5.0-5.5 6.0
13-25
6 6.5 7.0 7.5 9.5
5.0-7.1 5.5-8.0 5.5-9.0 7.0-9.0
2.5
12
8
6
00
3.0 3.0 3.0-3.5 3.5 4.0 4.5 5.0 5.5 6.0 6.5 6.5 7.0
14 14 14-16 16 18 20 22 24 26 28 28 30
10 10 10-12 12 14 16 16 18 18 20 20 22
6 6 6-8 8 8 8 10 10 10 12 12 12
0 0 0 0-1 2 2-3 3 3-4 4 4 5 6-8
>-l :xl H
C'l
r:Fl
d
:xl 0 01
:xl ><:
'·'Add 2-3 em for nasotracheal tube.
.....
(J1 N)
t.C
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best avoided by care in tube selection, frequent auscultatory evaluation, and chest x-ray films for location of the tube.
Pathology of Airway Intubation An ongoing eight year study of all patients coming to postmortem examination who have had airway intubation in this institution has allowed reconstruction of a sequence of pathologic changes associated with intubation. The initial observation is that any form of airway manipulation will produce trauma of varying degrees. Tracheal changes are the same whether a nasotracheal tube or a tracheotomy is used. The latter obviously spares the glottic and subglottic areas. Early histologic changes of epithelial ulceration, necrosis, and inflammation appear in the first hour or two and extend to the perichondrium within 24 hours. Extension of the perichondritis with subsequent inflammation of cartilage itself may occur at approximately one week. These histologic findings underlie the subsequent development of stenosis and malacia. Characteristics predisposing to more extensive injury include large tube size, inadequate fixation of tube, diminished peripheral tissue perfusion and oxygenation, and traumatic suctioning techniques. A guide to tube size is provided in Table 1, but the experienced physician relies on his or her assessment for a proper fit. No attempts should be made to provide a tight fit, particularly in neonates. The presence of a leak around the tube can be compensated for by ventilatory management and is associated with less airway trauma. Because tube fixation is important, nasotracheal tubes are preferred in practically all circumstances other than emergency oral intubation. Little can be done to avoid the effects of hypoperfusion and hypoxia other than an earlier decision for tracheotomy than might be the case in the normally perfused and normally oxygenated patient. Despite this categorization of histologic findings and underlying etiologies, decisions regarding routes of airway maintenance in neonates are based heavily on extensive clinical experience and apparent ability of the neonate to repair such pathologic changes in more satisfactory fashion than does the adult.
Artificial Airways The classic indications for the insertion of an artificial airway of either type have always been relief of upper airway obstruction, provision of control of ventilatory support, and enhancement of tracheobronchial toilet. The method of airway intervention is much less clear, and much more controversial, than it is in the adult group. As a result, the following schema is the approach used in this institution and accepted by many, but not all, similar facilities. In the premature or term neonate requiring prolonged respiratory support, nasotracheal intubation is always the initial choice and can be maintained, with optimum care, for several months if required. variation from this route arises in the child with airway anomalies or a recognized need for a much longer term artificial airway. In that circumstance a tracheostomy is performed. In the older child, endotracheal intubation is continued for a much shorter period (72 hours) before conversion to a tracheostomy. Two disease processes occurring in
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older infants and young children in which airway management is controversial are croup and epiglottitis. In this institution, all children with acute epiglottitis are intubated with a nasotracheal tube with subsequent removal in approximately 48 hours. When this is done with care by experienced people, difficulties are avoided. No tracheostomy for acute epiglottitis has been done in this institution in the last eight years. 1 Likewise in croup, the availability of racemic epinephrine inhalations with or without steroids has obviated the need for tracheostomy in the past five years. When tracheostomy is required, several controversial decisions are necessary. Tracheostomy in the child should be performed in the operating room with airway control over an endotracheal tube or bronchoscope. The classic argument surrounding vertical or transverse skin incisions is different in the pediatric age group. A vertical skin incision allows the tracheostomy tube to settle in its optimal position, avoiding angulation and tracheal pressure. Cutaneous scarring appears equal no matter which incision is used. A single vertical tracheal incision through one tracheal ring (the third) is usually a good guide to appropriate selection of tube size. These rules may require modification at the time of the procedure. The type of tube selected is also controversial. Silastic tubes have the advantage of causing less tracheal trauma. However, standard metal Jackson/Holinger tubes with inner cannulas are frequently preferred, as they offer the advantages of a wider selection of the tube length and curvature and also allow frequent changes of the inner cannula, obviating the need for frequent total tube changes. The current raging controversy surrounding the use of the metal or Silastic tracheostomy tubes remains unresolved. Several successful experiences exist with each type of tube. New Silastic tracheostomy tubes provided with inner cannulas may achieve the ideal result of producing less tracheal trauma as well as having the added safety of an inner cannula. Tube obstruction by respiratory secretions is a danger that must always be considered in the pediatric tracheotomy. Tube fixation in a short, fat neck of a young child is critical. When the tapes are tied, the neck should be in flexion. When tied in extension, the tapes are too loose, allowing dislocation. Postoperative care must be rigid in both the hospital or outpatient setting. Proper humidification of inspired air, including ultrasonic nebulization, and the periodic instillation of small volumes of normal saline followed by suctioning enhance removal of secretions. Suctioning technique is also critical, as the suction catheter itself can produce tracheal trauma. As a result, suction tubes are carefully measured to extend minimally beyond the indwelling tracheal tube, be it a tracheotomy or an endotracheal tube.
SUMMARY The impact of recent developments in neonatal respiratory care on patient survival has been great. The emphasis placed on this group of patients in this article reflects the major management problems that remain in pediatric respiratory support. The presence of controversy in
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some areas emphasizes the lack of universally successful therapeutic modalities. Nevertheless, sufficient progress has been made that successful surgical procedures in the 1000 gm premature infant should be the rule and no longer the exception. Comprehension of techniques of respiratory support is necessary for integrated and successful care of the neonate with multisystem disease. Such comprehension can then be applied to the less complex physiology of the older child.
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