- Email: [email protected]
Biphasic extrathoracic cuirass ventilation for resuscitation
American Journal of Emergency Medicine (2005) 23, 488 – 491
www.elsevier.com/locate/ajem
Biphasic extrathoracic cuirass ventilation for resuscitation Ilan Gur MDa, Ephraim Bar-Yishay PhDb, Ron Ben-Abraham MD, DEA, MHAc,* a
Neonatal Intensive Care Unit, Bikur Holim Hospital, Jerusalem, Israel Pulmonary Function Laboratory, Hadassah University Hospital, Jerusalem, the Hadassah Faculty of Medicine, Hebrew University, Jerusalem, Israel c Department of Anesthesiology and Critical Care Medicine, Tel-Aviv Sourasky Medical Center, the Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel b
Received 13 October 2004; accepted 24 October 2004
Abstract Purposes: The MRTX portable lightweight respirator (MRTX) provides noninvasive respiratory support using biphasic extrathoracic ventilation via a cuirass fitted around the patient’s chest. Methods: MRTX was applied with or without full protective gear, on adult volunteers simulating nerve agent (NA) victims by nonmedical caregivers. Assessment was made based on scores for correct positioning of the cuirass, quality of seal, and rapid ness. Results: For the unprotected and protected personnel, the respective median (F95% confidence interval) scores for correct positioning of the cuirass were 2 (1.4-1.9) and 1 (1.2-1.8) (n = 15 per group, P = NS); quality of seal scores were 2 (1.5-2.0) and 2 (1.3-1.8) ( P = NS); and mean (FSD) time required for instituting mechanical ventilation was 90.5 F 10.9 and 100.3 F 7.9 seconds ( P b .05). The respirator was activated at first attempt 11 times in the group of 15 without protective gear and 8 times in the group of 15 with protective gear ( P = NS). Discussion: Biphasic cuirass ventilation is an easily learned and rapidly applied method suitable for use by nonmedical personnel, even when wearing cumbersome protective gear. D 2005 Elsevier Inc. All rights reserved.
1. Introduction Chemical warfare using nerve agents (NA) can cause high mortality rates, as demonstrated in the terrorist attacks against urban populations in Japan in 1994 and 1995 and in the military use of NA by Iraq against Iran and the Iraqi Kurdish population during the 1980s. The high toxicity of
T Corresponding author. E-mail address: [email protected] (R. Ben-Abraham). 0735-6757/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.ajem.2004.10.004
NA derives from their powerful ability to irreversibly inhibit acetylcholinesterase, which regulates the activity of both the nicotinic and muscarinic synapses. The resultant accumulation of acetylcholine at the neuro-effector junctions leads to disruption of normal synaptic transmission in both the peripheral and central cholinergic systems, leading to the clinical manifestations of severe cholinergic crisis [1]. The immediate cause of death is usually rapid progressive respiratory failure brought about by NA’s poisonous and depressive action on the central respiratory center and on the neuromuscular junction and airways.
Extrathoracic ventilation of victims of nerve agent poisoning Irritation of both the upper and lower airways, their blockade by secretion, toxic pulmonary edema, and severe bronchoconstriction are all parts of the effects of the NAinduced severe parasympathetic overstimulation that increases airway resistance and poses mechanical obstruction to ventilation. Initial resuscitative efforts for NA-poisoned victims focus upon immediate respiratory support to prevent instant death from hypoxia. Current therapeutic protocols stress the need for urgent laryngoscopy and intubation, with concomitant provision of positive pressure ventilation until signs of muscle paralysis disappear [1]. In the event of a sudden and unexpected need to resuscitate multiple victims who are highly diverse in age and size and in an atmosphere of chaos, there will inevitably be a shortage in personnel who are well trained in airway management. Moreover, the cumbersome protective gear, including thick rubber gloves, worn by the rescue teams will inevitably present difficulties in performing a laryngoscopy. The MRTX respirator (MRTX; Medivent, London UK) is a lightweight, easy to operate, portable respirator which can provide proper artificial ventilation by means of external high-frequency oscillation (EHFO) via a cuirass that is tightly fitted around the patient’s chest. The cuirass consists of a clear, flexible plastic enclosure surrounding the chest and abdomen. Its borders are covered by a soft foam rubber, which creates an airtight seal around the patient. By choosing the appropriate cuirass size, the apparatus is capable of ventilating a wide range of different-sized subjects, from infants to the obese adult. The cuirass is connected to a computerized power unit by a wide-bore tube and the respiratory parameters are controlled by a feedback mechanism between the two. The power unit works by creating cyclic pressure changes inside the cuirass. The negative pressure (vacuum) creates chest expansion-inhalation. The positive pressure creates chest compression-exhalation. Thus, both inspiratory and expiratory phases are actively controlled, and the chest is oscillated around a variable negative baseline pressure. The system was found to be effective in a variety of clinical settings, with pressures of 25 to +15 cm H2O, inspiratory/expiratory (I/E) ratios of 1/1 to 1/3, and frequencies of 60 to 150 cpm [2-4]. In an experimental study in cats intoxicated with organophosphate [5], EHFO was reported to be capable in keeping animals alive until spontaneous recovery from the poisoning occurred. In addition, excessive bronchial secretions, typical of organophosphate intoxication, drained spontaneously through the mouth making airway protection and suction unnecessary. We hypothesized in the present preliminary study that proper respiratory assistance using the portable MRTX respirator can be provided to adult volunteers simulating NA victims by a nonmedical rescue team, even when wearing protective gear.
489
2. Methods 2.1. Participants An institutional review board approval for the study was obtained as well as verbal consent from each of the participants. Two groups of 15 nonmedically trained personnel participated in this prospective comparative study. All were members of nonmedical extrication teams with similar professional skills. Members were randomly assigned to either one of the 2 groups. They were asked to connect the MRTX respirator to adult volunteers (age range, 25-40 years; weight, 70-100 kg) who simulated a NA-poisoned patient and who was lying fully dressed on a stretcher. One group wore ordinary clothing while treating the bpatientQ and the other group wore complete antichemical warfare gear, which included a full-body plastic suit, rubber gloves, and a gas mask with an attached filter. Both groups were given 15 minutes to read the instruction manual for operating the MRTX respirator as well as a short demonstration on its use.
2.2. Study protocol The ease and efficacy of operation of the MRTX by both groups was prospectively and comparatively assessed by the following score parameters which were validated in preliminary experiments: 1.
2.
3. 4.
Scoring for correct positioning of the cuirass around the simulated NA-poisoned victim’s chest using a scale of 0 to 2 (0 = cuirass positioned on the abdomen, 1 = cuirass partially positioned on the chest, 2 = cuirass correctly positioned around the chest). Scoring for air leakage using a scale of 0 to 2 (0 = audible and tactile leak of air around the rim of the cuirass, 1 = tactile leak only, 2 = no air leak [ie, an adequately tight fit of the cuirass to the body]). Recording the number of times the respirator was actually activated on the first attempt. Measurement of the time needed for instituting respiratory assistance. This included removing the cuirass from its box, fixing it onto the patient’s chest, connecting it to the power unit, switching on the respirator, and inspecting that chest inflation is symmetrical.
2.3. The MRTX portable respirator The MRTX consists of a flexible, lightweight plastic cuirass that covers the anterior part of the chest and upper abdomen. It has a wide foam rim, which creates an airtight seal, and a back plate, which is secured by straps. The cuirass is connected by wide-bore flexible tubing to a mobile and microprocessor-controlled power unit. The power unit contains a turbine that generates an oscillatory pressure that operates over a wide range of frequencies (ie, 6-1200 cpm) and pressures ( 50 to +50 cm H2O). The
490 frequency, I/E cuirass chamber pressures, and the I/E ratio are automatically set and controlled by negative feedback loop [6] which makes it possible to achieve high frequencies. This is in contrast to regular negative pressure ventilation, which depends on passive recoil of the chest during the expiratory phase, thus, limiting the attainable frequencies.
2.4. Statistical analysis Timing events are presented as means F SD with differences among groups analyzed using the 2-tailed unpaired Student t test. All scores are presented as medians and 95% confidence intervals with differences analyzed using the nonparametric independent 2-group comparisons Mann-Whitney U test. v 2 Tests were used to compare categorical values. P b .05 was considered significant.
3. Results Slightly better scores were achieved by the 15 operators not wearing protective gear as compared with the group of 15 operators with protective gear, but differences did not reach significance. For the unprotected and protected personnel, the respective median (F95% confidence interval) scores for correct positioning of the cuirass were 2 (1.4-1.9) and 1 (1.2-1.8) (n = 15 per group, P = NS). For quality of seal, the scores were 2 (1.5- 2.0) and 2 (1.3-1.8) ( P = NS). Mean (FSD) time required for instituting mechanical ventilation was 90 5 F 10.9 and 100.3 F 7.9 seconds ( P b .05). The respirator was activated at first attempt 11 times in the group of 15 without protective gear and 8 times in the group of 15 with protective gear ( P = NS). Initial inadequate placement of the cuirass and failure to switch on the respirator were rapidly corrected and respiratory support could be speedily provided in all cases.
4. Discussion The results of the present study, although descriptive in nature, indicate that the cuirass can be rapidly and easily applied to apneic victims by gear-protected caregivers and, if necessary, biphasic external cuirass-based ventilation can be instituted by the portable MRTX respirator. Thus, this method of ventilation might serve as an effective tool in case of need for urgent resuscitative respiratory support to adult victims of NA poisoning. Nonmedical personnel, with no previous experience with patient ventilation, could efficiently operate the device and respiratory support could be rapidly instituted, even when wearing bulky protective gear. Its simplicity of use and the need for fewer medical staff members compared with a hand-operated respirator (ie, selfinflating bag) are additional benefits. There are earlier reports on resuscitative respiratory support having been given by intubation followed by positive pressure ventila-
I. Gur et al. tion for mass casualties from NA poisoning [7], but there are none on the use of any type of negative pressure ventilation in such a scenario. The MRTX respirator is a portable, relatively small (14 14 18 cm), lightweight (3 kg including battery), battery-operated rugged respirator. It is a modern version of the Hayek Oscillator that has long been used for administering respiratory support for both children and adults [2-4]. Its small dimensions and lightweight permit it to be easily carried by rescue personnel even under difficult conditions (ie, the wearing of cumbersome protective gear, the need to carry other equipment, etc), making it easily deployed and capable of operation both indoors and outdoors. The power supply can suffice for a period up to 3 hours. Mass casualties can be anticipated from the exposure of a population to nonconventional warfare agents. The safest mode of airway protection in NA poisoning is probably endotracheal intubation [1], but this may not be the case for intubation performed by physicians who are unskilled in the procedure and who must perform it in the prehospital setting under chaotic conditions. The application of biphasic cuirassbased negative pressure ventilation using the MRTX respirator may spare the need for laryngoscopy and intubation, both exacting procedures that may well be hampered by the impaired vision and manual dexterity of protected medical personnel. In addition, cross-contamination of the acute rescue team can cause ocular signs and symptoms, such as severe miosis, dim vision, and persistent rhinorrhea which, although not lethal, can compromise even the most proficient physician’s ability to function [1]. Another potential benefit for obviating laryngoscopy is sparing the use of muscle relaxants, which might act unpredictably in cases of underlying systemic inhibition of acetylcholinesterase [8]. Because systemic toxicity of NA is invariably associated with severe bronchorrhea [8], repeated active drainage of bronchial secretions is a major component of the respiratory care of the intoxicated patient. Clearance of secretions was shown to be enhanced by using biphasic cuirass-based negative pressure EHFO because of reduction in sputum viscosity and enhancement of ciliary’s clearance [5,9]. In the prehospital setting where massive numbers of casualties need to be treated and when medical personnel is expected to be too limited to be available for performing airway suctioning for every patient that needs it, periodic emergency ventilation using the MRTX respirator might serve as a useful solution. The EHFO is a relatively new modality of noninvasive ventilation which controls the inspiratory and expiratory phases of respiration, both of which are active [10]. The peakinspiratory chamber pressure and the end-expiratory chamber pressure are controllable as well. The inspiratory pressure will always be negative to expand the chest and inflate the lungs, but the expiratory pressure may be negative, atmospheric, or positive to produce an end-expiratory lung volume above functional residual capacity. Tidal volume is determined by the pressure difference between the expiratory
Extrathoracic ventilation of victims of nerve agent poisoning and peak inspiratory chamber pressure and frequency: increasing this pressure difference increases tidal volume, whereas increasing the frequency reduces it. Because minute ventilation is the product of frequency and tidal volume, minute ventilation increases as long as the increment in frequency is higher than the decrease in tidal volume [5]. With both the inspiratory and expiratory phases being controlled, high respiratory rates (up to 1200 oscillations per minute) may be achieved, in contrast to normal negative pressure ventilation (which does not have an active component of expiration). Optimal carbon dioxide removal is achieved with a rate of 90 per minute in adults with normal lungs. An I/E of 1/1 is optimal, although changing the I/E ratio may be needed for optimal ventilation in patients with inspiratory or expiratory obstruction [5]. Trials have proved the efficacy of EHFO in ventilating normal and sick lungs [3,4]. Potential preservation of cardiac output by EHFO compared with conventional positive pressure ventilation [3] might be preferred in light of the negative inotropic effects induced by NA [8]. Although EHFO can actively aid in secretion clearance in a forceful manner, a fact that probably reduces the chances for aspiration [5], adequate separation of the digestive and respiratory tracts is not maintained with this method. Hence, it would be prudent to exchange the extrathoracic ventilation that had been provided by the cuirass by the placement of an endotracheal tube as soon as possible. This is especially true when respiratory failure is prolonged and support is needed for an extended period. Our study is based on a descriptive model. For obvious reasons, experimental studies using models of NA intoxication are difficult to conduct even in animals because of the extreme lethal potency of these agents and the need for special well-equipped laboratory and personnel familiar with the use of NA. In conclusion, the methodology of instituting biphasic extrathoracic cuirass-based ventilation via an MRTX porta-
491
ble respirator is easily learned and rapidly applied by nonmedical personnel, even those wearing cumbersome, full-body antichemical protective gear. This method, when used for urgent respiratory support, saves the need for laryngoscopy and aids in secretion removal. Our encouraging preliminary findings warrant further assessment as for its beneficial role in resuscitation of mass casualties from NA poisoning.
References [1] Ben Abraham R, Rudick V, Weinbroum AA. Practical guidelines for acute care of victims of bioterrorism: conventional injuries and concomitant nerve agent intoxication. Anesthesiology 2002;97: 989 - 1004. [2] Petros AJ, Fernando SS, Shenoy VS, et al. The Hayek Oscillator. Nomograms for tidal volume and minute ventilation using external high frequency oscillation. Anaesthesia 1995;50:601 - 6. [3] Penny DJ, Hayek Z, Redington AN. The effects of positive and negative extrathoracic pressure ventilation on pulmonary blood flow after the total cavopulmonary shunt procedure. Int J Cardiol 1991;30:128 - 30. [4] Shekerdemian LS, Schulze-Neick I, Redington AN, et al. Negative pressure ventilation as haemodynamic rescue following surgery for congenital heart disease. Intensive Care Med 2000;26:93 - 6. [5] Hayek Z, Sohar E. External high frequency oscillation—concept and practice. Intensive Care World 1993;10:36 - 40. [6] Dilkes MG, Hill AC, McKelvie P, et al. The Hayek Oscillator: a new method of ventilation in microlaryngeal surgery. Ann Otol Rhinol Laryngol 1993;102:455 - 80. [7] Morita H, Yanagisawa N, Nakajima T, et al. Sarin poisoning in Matsumoto, Japan. Lancet 1995;346:290 - 3. [8] Weinbroum AA, Rudick V, Paret G, et al. Anaesthesia and critical care considerations in nerve agent warfare trauma casualties. Resuscitation 2000;47:113 - 23. [9] King M, Phillips DM, Gross D, et al. Enhanced tracheal mucus clearance with high frequency chest wall compression. Am Rev Respir Dis 1983;128:511 - 5. [10] al-Saady NM, Fernando SS, Petros AJ, et al. External high frequency oscillation in normal subjects and in patients with acute respiratory failure. Anaesthesia 1995;50:1031 - 5.
www.elsevier.com/locate/ajem
Biphasic extrathoracic cuirass ventilation for resuscitation Ilan Gur MDa, Ephraim Bar-Yishay PhDb, Ron Ben-Abraham MD, DEA, MHAc,* a
Neonatal Intensive Care Unit, Bikur Holim Hospital, Jerusalem, Israel Pulmonary Function Laboratory, Hadassah University Hospital, Jerusalem, the Hadassah Faculty of Medicine, Hebrew University, Jerusalem, Israel c Department of Anesthesiology and Critical Care Medicine, Tel-Aviv Sourasky Medical Center, the Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel b
Received 13 October 2004; accepted 24 October 2004
Abstract Purposes: The MRTX portable lightweight respirator (MRTX) provides noninvasive respiratory support using biphasic extrathoracic ventilation via a cuirass fitted around the patient’s chest. Methods: MRTX was applied with or without full protective gear, on adult volunteers simulating nerve agent (NA) victims by nonmedical caregivers. Assessment was made based on scores for correct positioning of the cuirass, quality of seal, and rapid ness. Results: For the unprotected and protected personnel, the respective median (F95% confidence interval) scores for correct positioning of the cuirass were 2 (1.4-1.9) and 1 (1.2-1.8) (n = 15 per group, P = NS); quality of seal scores were 2 (1.5-2.0) and 2 (1.3-1.8) ( P = NS); and mean (FSD) time required for instituting mechanical ventilation was 90.5 F 10.9 and 100.3 F 7.9 seconds ( P b .05). The respirator was activated at first attempt 11 times in the group of 15 without protective gear and 8 times in the group of 15 with protective gear ( P = NS). Discussion: Biphasic cuirass ventilation is an easily learned and rapidly applied method suitable for use by nonmedical personnel, even when wearing cumbersome protective gear. D 2005 Elsevier Inc. All rights reserved.
1. Introduction Chemical warfare using nerve agents (NA) can cause high mortality rates, as demonstrated in the terrorist attacks against urban populations in Japan in 1994 and 1995 and in the military use of NA by Iraq against Iran and the Iraqi Kurdish population during the 1980s. The high toxicity of
T Corresponding author. E-mail address: [email protected] (R. Ben-Abraham). 0735-6757/$ – see front matter D 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.ajem.2004.10.004
NA derives from their powerful ability to irreversibly inhibit acetylcholinesterase, which regulates the activity of both the nicotinic and muscarinic synapses. The resultant accumulation of acetylcholine at the neuro-effector junctions leads to disruption of normal synaptic transmission in both the peripheral and central cholinergic systems, leading to the clinical manifestations of severe cholinergic crisis [1]. The immediate cause of death is usually rapid progressive respiratory failure brought about by NA’s poisonous and depressive action on the central respiratory center and on the neuromuscular junction and airways.
Extrathoracic ventilation of victims of nerve agent poisoning Irritation of both the upper and lower airways, their blockade by secretion, toxic pulmonary edema, and severe bronchoconstriction are all parts of the effects of the NAinduced severe parasympathetic overstimulation that increases airway resistance and poses mechanical obstruction to ventilation. Initial resuscitative efforts for NA-poisoned victims focus upon immediate respiratory support to prevent instant death from hypoxia. Current therapeutic protocols stress the need for urgent laryngoscopy and intubation, with concomitant provision of positive pressure ventilation until signs of muscle paralysis disappear [1]. In the event of a sudden and unexpected need to resuscitate multiple victims who are highly diverse in age and size and in an atmosphere of chaos, there will inevitably be a shortage in personnel who are well trained in airway management. Moreover, the cumbersome protective gear, including thick rubber gloves, worn by the rescue teams will inevitably present difficulties in performing a laryngoscopy. The MRTX respirator (MRTX; Medivent, London UK) is a lightweight, easy to operate, portable respirator which can provide proper artificial ventilation by means of external high-frequency oscillation (EHFO) via a cuirass that is tightly fitted around the patient’s chest. The cuirass consists of a clear, flexible plastic enclosure surrounding the chest and abdomen. Its borders are covered by a soft foam rubber, which creates an airtight seal around the patient. By choosing the appropriate cuirass size, the apparatus is capable of ventilating a wide range of different-sized subjects, from infants to the obese adult. The cuirass is connected to a computerized power unit by a wide-bore tube and the respiratory parameters are controlled by a feedback mechanism between the two. The power unit works by creating cyclic pressure changes inside the cuirass. The negative pressure (vacuum) creates chest expansion-inhalation. The positive pressure creates chest compression-exhalation. Thus, both inspiratory and expiratory phases are actively controlled, and the chest is oscillated around a variable negative baseline pressure. The system was found to be effective in a variety of clinical settings, with pressures of 25 to +15 cm H2O, inspiratory/expiratory (I/E) ratios of 1/1 to 1/3, and frequencies of 60 to 150 cpm [2-4]. In an experimental study in cats intoxicated with organophosphate [5], EHFO was reported to be capable in keeping animals alive until spontaneous recovery from the poisoning occurred. In addition, excessive bronchial secretions, typical of organophosphate intoxication, drained spontaneously through the mouth making airway protection and suction unnecessary. We hypothesized in the present preliminary study that proper respiratory assistance using the portable MRTX respirator can be provided to adult volunteers simulating NA victims by a nonmedical rescue team, even when wearing protective gear.
489
2. Methods 2.1. Participants An institutional review board approval for the study was obtained as well as verbal consent from each of the participants. Two groups of 15 nonmedically trained personnel participated in this prospective comparative study. All were members of nonmedical extrication teams with similar professional skills. Members were randomly assigned to either one of the 2 groups. They were asked to connect the MRTX respirator to adult volunteers (age range, 25-40 years; weight, 70-100 kg) who simulated a NA-poisoned patient and who was lying fully dressed on a stretcher. One group wore ordinary clothing while treating the bpatientQ and the other group wore complete antichemical warfare gear, which included a full-body plastic suit, rubber gloves, and a gas mask with an attached filter. Both groups were given 15 minutes to read the instruction manual for operating the MRTX respirator as well as a short demonstration on its use.
2.2. Study protocol The ease and efficacy of operation of the MRTX by both groups was prospectively and comparatively assessed by the following score parameters which were validated in preliminary experiments: 1.
2.
3. 4.
Scoring for correct positioning of the cuirass around the simulated NA-poisoned victim’s chest using a scale of 0 to 2 (0 = cuirass positioned on the abdomen, 1 = cuirass partially positioned on the chest, 2 = cuirass correctly positioned around the chest). Scoring for air leakage using a scale of 0 to 2 (0 = audible and tactile leak of air around the rim of the cuirass, 1 = tactile leak only, 2 = no air leak [ie, an adequately tight fit of the cuirass to the body]). Recording the number of times the respirator was actually activated on the first attempt. Measurement of the time needed for instituting respiratory assistance. This included removing the cuirass from its box, fixing it onto the patient’s chest, connecting it to the power unit, switching on the respirator, and inspecting that chest inflation is symmetrical.
2.3. The MRTX portable respirator The MRTX consists of a flexible, lightweight plastic cuirass that covers the anterior part of the chest and upper abdomen. It has a wide foam rim, which creates an airtight seal, and a back plate, which is secured by straps. The cuirass is connected by wide-bore flexible tubing to a mobile and microprocessor-controlled power unit. The power unit contains a turbine that generates an oscillatory pressure that operates over a wide range of frequencies (ie, 6-1200 cpm) and pressures ( 50 to +50 cm H2O). The
490 frequency, I/E cuirass chamber pressures, and the I/E ratio are automatically set and controlled by negative feedback loop [6] which makes it possible to achieve high frequencies. This is in contrast to regular negative pressure ventilation, which depends on passive recoil of the chest during the expiratory phase, thus, limiting the attainable frequencies.
2.4. Statistical analysis Timing events are presented as means F SD with differences among groups analyzed using the 2-tailed unpaired Student t test. All scores are presented as medians and 95% confidence intervals with differences analyzed using the nonparametric independent 2-group comparisons Mann-Whitney U test. v 2 Tests were used to compare categorical values. P b .05 was considered significant.
3. Results Slightly better scores were achieved by the 15 operators not wearing protective gear as compared with the group of 15 operators with protective gear, but differences did not reach significance. For the unprotected and protected personnel, the respective median (F95% confidence interval) scores for correct positioning of the cuirass were 2 (1.4-1.9) and 1 (1.2-1.8) (n = 15 per group, P = NS). For quality of seal, the scores were 2 (1.5- 2.0) and 2 (1.3-1.8) ( P = NS). Mean (FSD) time required for instituting mechanical ventilation was 90 5 F 10.9 and 100.3 F 7.9 seconds ( P b .05). The respirator was activated at first attempt 11 times in the group of 15 without protective gear and 8 times in the group of 15 with protective gear ( P = NS). Initial inadequate placement of the cuirass and failure to switch on the respirator were rapidly corrected and respiratory support could be speedily provided in all cases.
4. Discussion The results of the present study, although descriptive in nature, indicate that the cuirass can be rapidly and easily applied to apneic victims by gear-protected caregivers and, if necessary, biphasic external cuirass-based ventilation can be instituted by the portable MRTX respirator. Thus, this method of ventilation might serve as an effective tool in case of need for urgent resuscitative respiratory support to adult victims of NA poisoning. Nonmedical personnel, with no previous experience with patient ventilation, could efficiently operate the device and respiratory support could be rapidly instituted, even when wearing bulky protective gear. Its simplicity of use and the need for fewer medical staff members compared with a hand-operated respirator (ie, selfinflating bag) are additional benefits. There are earlier reports on resuscitative respiratory support having been given by intubation followed by positive pressure ventila-
I. Gur et al. tion for mass casualties from NA poisoning [7], but there are none on the use of any type of negative pressure ventilation in such a scenario. The MRTX respirator is a portable, relatively small (14 14 18 cm), lightweight (3 kg including battery), battery-operated rugged respirator. It is a modern version of the Hayek Oscillator that has long been used for administering respiratory support for both children and adults [2-4]. Its small dimensions and lightweight permit it to be easily carried by rescue personnel even under difficult conditions (ie, the wearing of cumbersome protective gear, the need to carry other equipment, etc), making it easily deployed and capable of operation both indoors and outdoors. The power supply can suffice for a period up to 3 hours. Mass casualties can be anticipated from the exposure of a population to nonconventional warfare agents. The safest mode of airway protection in NA poisoning is probably endotracheal intubation [1], but this may not be the case for intubation performed by physicians who are unskilled in the procedure and who must perform it in the prehospital setting under chaotic conditions. The application of biphasic cuirassbased negative pressure ventilation using the MRTX respirator may spare the need for laryngoscopy and intubation, both exacting procedures that may well be hampered by the impaired vision and manual dexterity of protected medical personnel. In addition, cross-contamination of the acute rescue team can cause ocular signs and symptoms, such as severe miosis, dim vision, and persistent rhinorrhea which, although not lethal, can compromise even the most proficient physician’s ability to function [1]. Another potential benefit for obviating laryngoscopy is sparing the use of muscle relaxants, which might act unpredictably in cases of underlying systemic inhibition of acetylcholinesterase [8]. Because systemic toxicity of NA is invariably associated with severe bronchorrhea [8], repeated active drainage of bronchial secretions is a major component of the respiratory care of the intoxicated patient. Clearance of secretions was shown to be enhanced by using biphasic cuirass-based negative pressure EHFO because of reduction in sputum viscosity and enhancement of ciliary’s clearance [5,9]. In the prehospital setting where massive numbers of casualties need to be treated and when medical personnel is expected to be too limited to be available for performing airway suctioning for every patient that needs it, periodic emergency ventilation using the MRTX respirator might serve as a useful solution. The EHFO is a relatively new modality of noninvasive ventilation which controls the inspiratory and expiratory phases of respiration, both of which are active [10]. The peakinspiratory chamber pressure and the end-expiratory chamber pressure are controllable as well. The inspiratory pressure will always be negative to expand the chest and inflate the lungs, but the expiratory pressure may be negative, atmospheric, or positive to produce an end-expiratory lung volume above functional residual capacity. Tidal volume is determined by the pressure difference between the expiratory
Extrathoracic ventilation of victims of nerve agent poisoning and peak inspiratory chamber pressure and frequency: increasing this pressure difference increases tidal volume, whereas increasing the frequency reduces it. Because minute ventilation is the product of frequency and tidal volume, minute ventilation increases as long as the increment in frequency is higher than the decrease in tidal volume [5]. With both the inspiratory and expiratory phases being controlled, high respiratory rates (up to 1200 oscillations per minute) may be achieved, in contrast to normal negative pressure ventilation (which does not have an active component of expiration). Optimal carbon dioxide removal is achieved with a rate of 90 per minute in adults with normal lungs. An I/E of 1/1 is optimal, although changing the I/E ratio may be needed for optimal ventilation in patients with inspiratory or expiratory obstruction [5]. Trials have proved the efficacy of EHFO in ventilating normal and sick lungs [3,4]. Potential preservation of cardiac output by EHFO compared with conventional positive pressure ventilation [3] might be preferred in light of the negative inotropic effects induced by NA [8]. Although EHFO can actively aid in secretion clearance in a forceful manner, a fact that probably reduces the chances for aspiration [5], adequate separation of the digestive and respiratory tracts is not maintained with this method. Hence, it would be prudent to exchange the extrathoracic ventilation that had been provided by the cuirass by the placement of an endotracheal tube as soon as possible. This is especially true when respiratory failure is prolonged and support is needed for an extended period. Our study is based on a descriptive model. For obvious reasons, experimental studies using models of NA intoxication are difficult to conduct even in animals because of the extreme lethal potency of these agents and the need for special well-equipped laboratory and personnel familiar with the use of NA. In conclusion, the methodology of instituting biphasic extrathoracic cuirass-based ventilation via an MRTX porta-
491
ble respirator is easily learned and rapidly applied by nonmedical personnel, even those wearing cumbersome, full-body antichemical protective gear. This method, when used for urgent respiratory support, saves the need for laryngoscopy and aids in secretion removal. Our encouraging preliminary findings warrant further assessment as for its beneficial role in resuscitation of mass casualties from NA poisoning.
References [1] Ben Abraham R, Rudick V, Weinbroum AA. Practical guidelines for acute care of victims of bioterrorism: conventional injuries and concomitant nerve agent intoxication. Anesthesiology 2002;97: 989 - 1004. [2] Petros AJ, Fernando SS, Shenoy VS, et al. The Hayek Oscillator. Nomograms for tidal volume and minute ventilation using external high frequency oscillation. Anaesthesia 1995;50:601 - 6. [3] Penny DJ, Hayek Z, Redington AN. The effects of positive and negative extrathoracic pressure ventilation on pulmonary blood flow after the total cavopulmonary shunt procedure. Int J Cardiol 1991;30:128 - 30. [4] Shekerdemian LS, Schulze-Neick I, Redington AN, et al. Negative pressure ventilation as haemodynamic rescue following surgery for congenital heart disease. Intensive Care Med 2000;26:93 - 6. [5] Hayek Z, Sohar E. External high frequency oscillation—concept and practice. Intensive Care World 1993;10:36 - 40. [6] Dilkes MG, Hill AC, McKelvie P, et al. The Hayek Oscillator: a new method of ventilation in microlaryngeal surgery. Ann Otol Rhinol Laryngol 1993;102:455 - 80. [7] Morita H, Yanagisawa N, Nakajima T, et al. Sarin poisoning in Matsumoto, Japan. Lancet 1995;346:290 - 3. [8] Weinbroum AA, Rudick V, Paret G, et al. Anaesthesia and critical care considerations in nerve agent warfare trauma casualties. Resuscitation 2000;47:113 - 23. [9] King M, Phillips DM, Gross D, et al. Enhanced tracheal mucus clearance with high frequency chest wall compression. Am Rev Respir Dis 1983;128:511 - 5. [10] al-Saady NM, Fernando SS, Petros AJ, et al. External high frequency oscillation in normal subjects and in patients with acute respiratory failure. Anaesthesia 1995;50:1031 - 5.