Emergency mechanical ventilation at moderate altitude

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Wilderness and Environmental Medicine, 6,283-287 (1995) ORIGINAL ARTICLE Emergency mechanical ventilation at moderate altitude MARTIN ROEGGLA, MD*, GEORG ROEGGLA, MD, ANDREAS WAGNER, MD, BETTINA

1 Wilderness and Environmental Medicine, 6, (1995) ORIGINAL ARTICLE Emergency mechanical ventilation at moderate altitude MARTIN ROEGGLA, MD*, GEORG ROEGGLA, MD, ANDREAS WAGNER, MD, BETTINA EDER, MD, and ANTON N. LAGGNER, MD DepartmentofEmergency Medicine, University of Vienna, Vienna, Austria Portable emergency ventilators are commonly used in the prehospital setting in the transport of critically ill patients in hypobaric environments. The aim of our trial was to evaluate the influence on minute ventilation and blood gas analysis of moderate altitude (3000 m) compared to 171 m in healthy volunteers during mechanical ventilation with the Draeger Oxylog ventilator. At 3000 m, the delivered minute volume increased by 9.8% in the air mix mode and by 14.6% in the no air mix mode. PaOz at 3000 m altitude decreased by 33.3% in the air mix mode, and no statistical change was observed in the no air mix mode. PaCOz at 3000 m altitude decreased by 9.0% in the air mix mode and by 12.8% in the no air mix mode. These changes are of sufficient magnitude and importance to require monitoring of minute volume to prevent barotrauma or volume-related trauma and to monitor oxygenation by pulse oximetry during emergency mechanical ventilation at moderate altitude. Key words: mechanical ventilation, moderate altitude Introduction Portable emergency ventilators are commonly used in the prehospital setting in the transport of critically ill patients. Exposure to a hypobaric environment is not uncommon in alpine rescue at moderate or high altitude, in helicopter transport, or in aircraft transport between hospitals. In the study, 36.4% of all patients transported in Swiss helicopter series were mechanically ventilated [1]. Gas-powered emergency ventilators are constructed to be lightweight, robust, and simple to operate [2]. Most do not have an integrated device to control the delivered minute ventilation. Therefore, the user must rely on the accuracy of the ventilator in delivering the preset volume to assure adequate ventilation without unnecessary risk of barotrauma. A study at low altitude with difference commercially available emergency ventilators using artificial lungs reported considerable differences between preset and delivered minute ventilation, especially when used at low minute volumes [3]. A study on the function of an emergency ventilator and a test lung in a decompression chamber reported an increase of minute ventilation at hypobaric conditions [4]. The target of our trial was to evaluate the influence of moderate altitude on minute ventilation and blood gas analysis in healthy volunteers during mechanical ventilation with a commercially available portable emergency ventilator at 3000 m. 'To whom all correspondence should be addressed Chapman & Hall

284 Roeggla et al. Methods Six healthy male volunteers without history of cardiac or pulmonary disease and with normal lung function took part in a controlled trial after informed consent.

2 284 Roeggla et al. Methods Six healthy male volunteers without history of cardiac or pulmonary disease and with normal lung function took part in a controlled trial after informed consent. The subjects underwent special training during which they learned how to relax both during inspiration and expiration [5]. They were advised to relax both the rib cage and abdomen. Therefore, the subjects learned to accommodate the airway pressures, allowing increased lung volume without substantial dyspnea. Age was years (mean 22.7 ± 1.0), and all subjects had a normal body weight (78.3 ± 2.0 kg) for their height (1.84 ± 0.04 m). They were free of any medication or drug consumption for at least 4 weeks before and during the study. The protocol was designed as a paired trial and consisted of two consecutive periods of mechanical ventilation lasting 15 min each at 171 m and at 3000 m altitude with a 3-day interval between both altitudes. Ventilation was performed via mouthpiece while the nose was occluded by a clip. The ventilator (Oxylog, Draeger, Luebeck, Germany) was on the following setting throughout the trial: respiratory rate 12 min-i and minute volume 8 liters min- 1 (tidal volume approximately 10 ml kg-i body weight). The oxylog was preset to "air mix" (FI02 = 0.55) for the first 15 min and then changed to "no air mix" (FI02 = 1.0) for the second 15 min at both altitudes. Minute volume was registered at the end of each period with a spirometer (Draeger, Luebeck, Germany). Capillary blood samples were obtained from the arterialized ear lobe at the same times [6]. Samples were taken between 12 A.M. and 1 P.M. under similar conditions, ambient temperature was constant at 21 C at 171 m (air-conditioned university clinic), and 18 C at 3000 m altitude (mountain restaurant). Blood samples were stored in an ice-water-containing cooling box and arterial oxygen partial pressure (Pa02) and arterial carbon dioxide partial pressure (PaCOz) analyzed on a BGE Blood Gas Analyzer within 4 h (Instrumentation Laboratory, California, USA). Statistics After consideration of the essential test preconditions, blood gas analysis and minute volume data at both investigation altitudes were compared by t-test. Probability levels less than 0.05 were considered significant. Results All six volunteers managed to cooperate according to the study protocol. Pa02, PaC02, and minute volume at both investigation altitudes in the air mix mode are shown in Table 1, and in the no air mix mode in Table 2. The delivered minute ventilation at 3000 m Table 1. PaOZ, PaC02(in mm Hg), and minute volume (in liters) at 171 m altitude and at 3000 m altitude of six subjects during ventilation with the oxylog in the air mix mode. 171 m altitude 3000 m altitude Mean diff 95%CI p Pa ± ± <0.01 PaC ± ± <0.05 Minute ventilation 8.13 ± ± <0.01 Data are reported as mean:±: SD, mean difference and 95% CI for the difference.

3 Emergency mechanical ventilation atmoderate altitude 285 Table 2. Pa02, PaC02(in mm Hg), and minute volume (in liters) at 171 m altitude and at 3000 m altitude of six subjects during ventilation with the oxylog in the no air mix mode. 171 m altitude 3000 m altitude Mean diff 95%CI p Pa ± ± n.s. PaC ± ± <0.01 Minute ventilation 8.13 ± ± <0.01 Data are reported as mean ± SD, mean difference and 95% CI for the difference. altitude increased to be statistically significant by 9.8% in the air mix mode and by 14.6% in the no air mix mode. Pa02 at 3000 m altitude decreased to be statistically significant by 33.3% in the air mix mode compared to 171 m altitude, whereas no statistical change was observed in the no air mix mode. PaCOz at 3000 m altitude decreased to be statistically significant by 9.0% in the air mix mode and by 12.8% in the no air mix mode. Discussion The oxylog is a time-cycled, volume-constant ventilator which features pneumatic logic controls and only requires pressurized oxygen to function. It can deliver either 100% oxygen in the no air mix mode or 55% oxygen in an air mix mode by activating a venturi injector. The Draeger oxylog is a small, reliable ventilator which is widely used in the transportation of critically ill patients in Austria. According to the manufacturer's specification, it can function within its operational parameters at ambient pressures of mbars. The oxylog is used in two situations with hypobaric pressures outside the recommended range: in alpine helicopter rescue services due to the geographic circumstances, and in patient transport by airplane, as cabin pressure in a pressurized ambulance aircraft can decrease below 600 mm Hg [7]. Minute volumes were measured with a volumetric device that has not yet been specially validated for measurements in hypobaric environments. A similar device has shown to be reliable in hypobaric and hyperbaric conditions [4,8]. Furthermore, accuracy was checked with a I-liter test syringe at both altitudes. The main reason for the elevated minute volume at moderate altitude can be explained by the construction of the ventilator. Control of minute volume or tidal volume is done via a pressure regulator which delivers a pressure of approximately bar above ambient pressure, depending on the minute volume setting. The mass flow depends on the absolute pressure with gas flowing via resistance and injector, which together give the flow to the patient's lungs. This mass flow decreases with increasing altitude, and tidal volume calculated at 1 atm pressure would be lower. As the volume expands according to Boyle-Mariotte's law, it must be multiplied by the corresponding factor, for example by a factor of 2 at an altitude of 5486 m [4]. The patient's lung mechanics are altered at high altitude due to reduced gas density. Pulmonary compliance is elevated and resistance is decreased in spontaneously breathing subjects [9,10]. A reduction of resistance and elevation of compliance in a test lung model increased minute ventilation with the oxylog at lowland (11] altitude; thus the same effect is to be expected at moderate altitude in human subjects. Although it was not possible to record the mechanics of ventilation in detail at moderate altitude, we must presume that

4 286 Roeggla et al. an alteration of compliance and resistance contributed to the increase of minute ventilation. Arterialized ear lobe blood samples have proved to be accurate and reliable, with no statistical difference to arterial PaOz and PaCOz [6,12]. PaOz and PaCOz are not influenced by a four hour interval of analysis [13]. Sampling kit and the delay in analysis were identical at both investigational altitudes. The decrease of PaCOzin both ventilation modes at 3000 m altitude compared to 171 m altitude is caused by the increase in minute ventilation. The decrease of PaOz in the air mix mode at 3000 m altitude compared to 171 m altitude is explained by the addition of hypobaric hypoxic environmental air to the inspiratory gas mixture. What is the clinical relevance of our data? The relatively small, but statistically significant increase in minute volume could be of importance even in brief alpine rescue service flights of instable patients with elevated risk of barotrauma or volume-related trauma, such as is seen in asthmatic patients. In airplane flights, artificial ventilation is often performed for many hours. It is, therefore, important not to expose patients, even with no underlying pulmonary problem, to risk factors for lung injury like high tidal volume and airway pressure [14]. These effects could potentially be increased by events such as sudden loss of cabin pressurization. Potential damage can be minimized by preventing overdistention of the lung by monitoring the delivered minute ventilation with mechanical ventilation under conditions of reduced barometric pressure. Monitoring of end tidal COz by capnometry is not yet routine in prehospital emergency medicine. With regard to the decrease of PaOz at 3000 m altitude in the air mix mode, patients with severe cardiorespiratory diseases are at elevated risk of hypoxia compared to the situation at low altitude. The air mix mode should, therefore, not be used without monitoring of oxygenation by pulse oximetry. References 1. Demartines, N., Castelli, 1., Scheidegger D., and Harder F. Development of the helicopter rescue concept in the Basel region. Schweiz Rundsch Med Prax 1992; 81, Branson, RD. and McGough, E.K. Transport ventilators. Problems Cnt Care 1990; 4, Heinrichs, W., Metzlufft, F., and Dick, W. Accuracy of delivered versus preset minute ventilation of portable emergency ventilators. Cnt Care Med 1989; 17, Thomas, G. and Brimacombe, J. Function of the Draeger oxylog ventilator at high altitude. Anaesth Intens Care 1994; 22, Leithner, c., Podolsky, A, Globits, S., Frank, H., Neuhold, A, Pidlich, J., Schuster, E., Staudinger, T., Rintelen, C., Roeggla, M., Glogar, D., and Frass, M. Magnetic resonance imaging of the heart during positive end-expiratory pressure ventilation in normal subjects. ent Care Med 1994; 22, Spiro, S.G. and Dowdeswell, 1.R Arterialized ear lobe blood samples for blood gas tensions. Br] Dis Chest 1976; 70, Parson, c.j. and Bobechko, W.P. Aeromedical transport: its hidden problems. Can MedAssoc] 1982; 126, Youn, B.A and Myers, RA.M. Volume monitor for mechanical ventilation in the hyperbaric chamber. Cnt Care Med 1989; 17, Mansell, A., Powles, A, and Sutton, J. Changes in pulmonary PV characteristics of human subjects at an altitude of5366 m.]applphysiol1980, 49, Mognoni, P., Saibene, F., and Veicsteinas, A Ventilatory work during exercise at high altitude. Int] Sports Med 1982; 3,33-6.

Emergency mechanicalventilation atmoderatealtitude 287 11. Nolan, J.P., Anaes, F.e., and Baskett, P.J. Gas-powered and portable ventilators. Prehospital DisasterMed 1992; 7, 25-34. 12. Pitkin, AD.

5 Emergency mechanicalventilation atmoderatealtitude Nolan, J.P., Anaes, F.e., and Baskett, P.J. Gas-powered and portable ventilators. Prehospital DisasterMed 1992; 7, Pitkin, AD., Roberts, e.m., and Wedzicha, J.A. Arterialised earlobe blood gas analysis: an underused technique. Thorax 1994; 49, Mathur, U.S., Manchada, A, Singh, V., Rishi, J.P., and Athaiya, V. Comparative study of capillary and arterial blood gas values in plastic and glass syringes at various intervals in normal and asthmatic subjects. Indian J Chest Dis Allied Sci 1989; 31, Parker, J.e., Hernandez, L.A, and Peevy, K.J. Mechanisms of ventilator induced lung injury. Crit Care Med 1993; 21,

Lung Volumes and Capacities

Lung Volumes and Capacities Normally the volume of air entering the lungs during a single inspiration is approximately equal to the volume leaving on the subsequent expiration and is called the tidal volume.