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Preliminary Communication |

Respiration During Snow Burial Using an Artificial Air Pocket FREE

Colin K. Grissom, MD; Martin I. Radwin, MD; Chris H. Harmston, ME, MSE; Ellie L. Hirshberg, MD; Thomas J. Crowley, MD
[+] Author Affiliations

Author Affiliations: Pulmonary Division, LDS Hospital (Dr Grissom), University of Utah (Drs Grissom, Radwin, and Hirshberg), and Black Diamond Equipment Ltd (Mr Harmston), Salt Lake City, Utah; and TJ Crowley Corp, Aurora, Colo (Dr Crowley).


JAMA. 2000;283(17):2266-2271. doi:10.1001/jama.283.17.2266.
Text Size: A A A
Published online

Context Asphyxia is the most common cause of death after avalanche burial. A device that allows a person to breathe air contained in snow by diverting expired carbon dioxide (CO2) away from a 500-cm3 artificial inspiratory air pocket may improve chances of survival in avalanche burial.

Objective To determine the duration of adequate oxygenation and ventilation during burial in dense snow while breathing with vs without the artificial air pocket device.

Design Field study of physiologic respiratory measures during snow burial with and without the device from December 1998 to March 1999. Study burials were terminated at the subject's request, when oxygen saturation as measured by pulse oximetry (SpO2) dropped to less than 84%, or after 60 minutes elapsed.

Setting Mountainous outdoor site at 2385 m elevation, with an average barometric pressure of 573 mm Hg.

Participants Six male and 2 female volunteers (mean age, 34.6 years; range, 28-39 years).

Main Outcome Measures Burial time, SpO2, partial pressure of end-tidal CO2 (ETCO2), partial pressure of inspiratory CO2(PICO2), respiratory rate, and heart rate at baseline (in open atmosphere) and during snow burial while breathing with the device and without the device but with a 500-cm3 air pocket in the snow.

Results Mean burial time was 58 minutes (range, 45-60 minutes) with the device and 10 minutes (range, 5-14 minutes) without it (P=.001). A mean baseline SpO2 of 96% (range, 90%-99%) decreased to 90% (range, 77%-96%) in those buried with the device (P=.01) and to 84% (range, 79%-92%) in the control burials (P=.02). Only 1 subject buried with the device, but 6 control subjects buried without the device, decreased SpO2 to less than 88% (P=.005). A mean baseline ETCO2 of 32 mm Hg (range, 27-38 mm Hg) increased to 45 mm Hg (range, 32-53 mm Hg) in the burials with the device (P=.02) and to 54 mm Hg (range, 44-63 mm Hg) in the control burials (P=.02). A mean baseline PICO2 of 2 mm Hg (range, 0-3 mm Hg) increased to 32 mm Hg (range, 20-44 mm Hg) in the burials with the device (P=.01) and to 44 mm Hg (range, 37-50 mm Hg) in the control burials (P=.02). Respiratory and heart rates did not change in burials with the device but significantly increased in control burials.

Conclusions In our study, although hypercapnia developed, breathing with the device during snow burial considerably extended duration of adequate oxygenation compared with breathing with an air pocket in the snow. Further study will be needed to determine whether the device improves survival during avalanche burial.

Figures in this Article

Avalanches annually claim about 100 lives in Europe and 40 in North America.1,2 Approximately 75% of avalanche deaths are due to asphyxiation; up to 25%, to trauma; and very few, to hypothermia.3,4 Several factors may contribute to asphyxia during avalanche burial. First, the airway may become obstructed with snow. Second, less air is available in snow for breathing after an avalanche. Snow may be 80% to 90% air in the undisturbed snowpack, but less than 60% air in avalanche debris.4,5 Third, water in humidified exhaled air condenses and freezes on the snow around the mouth and face forming a thin "mask" of ice that impairs diffusion of air.4 Fourth, the weight of snow may cause chest restriction and impair breathing.6

Time to extrication is the major determinant of survival during avalanche burial. Falk and colleagues7 report probability of survival as 92% for persons extricated within 15 minutes, but only 30% at 35 minutes. They suggest that survival after 35 minutes is dependent on an air pocket in the snow for breathing. Survival at 90 minutes is 27%, but decreases to only 3% at 130 minutes, representing avalanche burial victims with an air pocket succumbing to late asphyxiation.

Even though an air pocket appears to be critical for prolonging survival, we are aware of no prior studies characterizing the physiology of breathing with an air pocket after avalanche burial. Preliminary studies from our group8 show that a larger air pocket or an artificial air pocket device that separates inspired from expired air (AvaLung; Black Diamond Equipment Ltd, Salt Lake City, Utah)9 may provide adequate oxygenation for a longer time after burial.

The artificial air-pocket device allows a person to breathe air contained in snow (Figure 1). A mouthpiece is connected to a single circuit of respiratory tubing used for inspiration and expiration. A plastic mesh 500-cm3 air pocket is connected to the side of the respiratory tubing by two 1-way inspiratory valves. Expired air passes from the mouthpiece through the respiratory tubing circuit to an expiratory 1-way valve and then exits around the back away from the air pocket. The device is built into a vest that is worn over all other clothing.

Figure 1. Schematic Drawing of the AvaLung as It Is Positioned in a Vest Worn Over All Other Clothing
Graphic Jump Location
Air from the snowpack enters the plastic mesh air pocket (E) and on inspiration flows into the respiratory tubing circuit via two 1-way valves (D) to mouthpiece (A). The capnometer sampling tubing (B) and emergency oxygen tubing (C) are shown where they insert in-line with the respiratory tubing just below the mouthpiece. These attachments were part of our experimental setup but are not required for actual function of the device during avalanche burial. Expired air passes from the mouthpiece through the respiratory tubing to 1-way expiratory valve (F) and then exits out the back of the device (G). The inset panel shows the back view of the vest with the position of the expiratory tubing as it sits between layers of mesh fabric. The expiratory tubing angles up to the shoulder rather than continuing to the midline of the back to avoid interfering with a backpack.

In this study, we characterized the physiology of breathing after snow burial with and without an artificial air-pocket device. We hypothesized that the device would maintain adequate oxygenation and ventilation longer than an air pocket alone during burial in snow of similar density to avalanche debris. To test this hypothesis, we studied 8 subjects breathing with the device during snow burial for up to 60 minutes. As controls, we studied 7 of the same subjects breathing with only an air pocket in the snow during burial.

This study took place at 2385 m elevation (average barometric pressure, 573 mm Hg) in the Wasatch Mountains, Utah, from December 1998 to March 1999. The experimental set-up simulated avalanche debris and consisted of a large mound of snow compacted with body weight. Snow density was determined in multiple sites using a 250-cm3 wedge density cutter (Snowmetrics, Ft Collins, Colo) that measured the weight of water per cubic meter. Snow density is reported as a percentage (ie, 300 kg/m3 is 30% density snow or 70% air). Snow densities for the study burials were 30% to 40% for 4 subjects (moderate-density group) and 55% to 60% for 4 subjects (high-density group). Reported density of avalanche debris ranges from 30% for a midwinter dry snow avalanche to 60% or higher for a springtime wet snow avalanche.5 A shoulder-width trench was dug into 1 end of the snow mound, and a sitting platform was created for the subject so that the subject's head would be 30 cm and the device, 100 cm, under the surface (Figure 2).

Figure 2. Subject Burial With Device
Graphic Jump Location
The left panel shows the compacted snow mound with a subject sitting in the trench in burial position. The right panel shows the subject buried to the neck prior to full burial. The device mouthpiece with capnometer tubing, emergency oxygen tubing, and physiologic monitoring cables is visible.

Subjects were paid volunteers, 6 men and 2 women, mean age 34.6 years (range, 28-39 years). All subjects except 1 lived at 1500- to 2500-m elevation. Subject 7, who declined the control burial, was a healthy, active sea-level resident who had spent 3 nights at 2500 m prior to burial with the device. All other subjects were studied first during a burial with the device and then during a control burial. No subject smoked cigarettes. Two subjects had asthma: subject 3, atopic asthma and subject 5, exercise-induced asthma. Both were being treated with β-agonist inhalers, used their inhalers prior to the study, and experienced no symptoms of asthma. Subject 6 had a history of hypothyroidism but was not being treated. Subject characteristics are listed in Table 1. The LDS Hospital Research and Human Rights Committee approved this study, and written informed consent was obtained from the volunteers.

Physiologic parameters that were measured at baseline with subjects breathing ambient air and continuously monitored during the burial studies included partial pressure of end-tidal carbon dioxide (ETCO2) and partial pressure of inspiratory CO2 (PICO2) in millimeters of mercury, respiratory rate (RR), oxygen saturation as measured by pulse oximetry (SpO2), surface 3-lead electrocardiogram, heart rate, and skin surface temperature measured by a probe in the axilla. Monitoring of ETCO2, PICO2, SpO2, and RR was done with a capnometer (NPB-75; Mallinckrodt, St Louis, Mo) attached in-line with an airway adapter (Microsteam CO2 Accessory Filterline ICU; Spegas Industries, Jerusalem, Israel) to the device mouthpiece. A portable monitor (NPB-4000; Mallinckrodt) was used to track electrocardiogram, heart rate, SpO2, and body surface temperature. Respiratory rate was also measured with the portable monitor using transthoracic impedance. A digital pulse oximeter probe was attached to 1 finger on each hand. Physiologic parameters were observed continuously and recorded every minute.

During burial with the device, subjects wore the artificial air-pocket vest over multiple layers of clothing. A warm hood with a face mask and goggles covered the head. Snow was compacted by hand as subjects sat in the snow mound trench and were buried completely. Subjects were in communication with the surface team via intercom. Time 0 of burial was noted when the subject's head was completely buried. Study burial was terminated after 60 minutes, at the subject's request, or when SpO2 fell below 84%. An emergency oxygen backup line was attached to the device mouthpiece and could deliver 15 L/min of 100% oxygen to increase inspired partial pressure of oxygen and flush CO2.

The study setup for control burials was identical to burials with the device except that subjects breathed through the device mouthpiece that opened into a 500-cm3 volume air pocket created in the snow. The capnometer and emergency oxygen backup line were attached to the device mouthpiece. Control burial was terminated at the subject's request or when SpO2 fell below 84%.

Baseline measurements were compared with burial study data using a Friedman analysis of variance and a Wilcoxon matched pairs test. Measurements at the end of the device and control burials were compared by Mann-Whitney U test. The number of subjects with adequate oxygenation (SpO2 ≥88%) at the end of device and control burials were compared using a χ2 test. Data from the moderate- and high-density snow groups were compared using a Mann-Whitney U test. Statistica (StatSoft, 1999 edition, Tulsa, Okla) was used for all statistical analysis. P<.05 was considered statistically significant. Data are reported as mean and range.

Subjects remained buried longer with the device vs an air pocket in the snow without the device (P=.001) (Table 2). Six subjects completed the protocol and remained buried the full 60 minutes. Subject 1 requested removal at 45 minutes because he was cold and had started to shiver. This subject was the first studied and wore less insulating clothing. The investigators terminated the burial with the device at 56 minutes for subject 6 because of hypoxia. Subject 6 also had occasional premature ventricular beats during the last minute of burial. All other subjects had normal sinus rhythm throughout the study. Burial times for the control study ranged from 5 to 14 minutes. All subjects terminated the control burial because of dyspnea; 4 subjects reported a sensation of vertigo, and 3 subjects reported headache.

Table Graphic Jump LocationTable 2. Baseline and End Point Data From Device and Control Burials*

Data for each subject at baseline and at the end of device and control burials are shown in Table 2. Data for ETCO2, PICO2, and SpO2 at baseline and throughout the device and control burials are shown in Figure 3. Mean baseline SpO2 of 96% (90%-99%) significantly decreased to 90% (77%-96%) in burials with the device (P=.01), and to 84% (79%-92%) in control burials (P=.02). In device burials, SpO2decreased below 88% in only 1 subject, while in the shorter control burials, SpO2 in 6 subjects dropped below 88% (P=.005).

Figure 3. Mean Values and Ranges for SpO2, PICO2 at Baseline and Throughout Device and Control Burials and Throughout Device and Control Burials
Graphic Jump Location
Mean values and ranges are shown for physiologic parameters at baseline with subjects breathing ambient air, during device burials, and during control burials. X-axis represents time relative to full burial in minutes for all panels. Top, Oxygen saturation by pulse oximeter (SpO2); middle, end-tidal carbon dioxide (ETCO2); and bottom, partial pressure of inspiratory carbon dioxide (PICO2). Some mean data points at the end of the control burial are missing or do not have ranges because of subject dropout (burial times of 5-14 minutes) and because in the subject who remained buried for 14 minutes, the capnometer failed to record data when PICO2 approached ETCO2. Compared with baseline measurements, SpO2 percentage decreased during control (P<.05) and device burials (P<.003), ETCO2 increased during control (P<.002) and device burials (P<.001), and PICO2 increased during control (P<.03) and device burials (P<.001). Statistical comparisons of baseline and device or control burial data were done with a Friedman analysis of variance.

Mean baseline ETCO2 of 32 mm Hg (27-38 mm Hg) significantly increased to 45 mm Hg (32-53 mm Hg) in device burials (P=.02), and to 54 mm Hg (44-63 mm Hg) in control burials (P=.02). Mean baseline PICO2 of 2 mm Hg (0-3 mm Hg) significantly increased to 32 mm Hg (20-44 mm Hg) in the device burials (P=.01), and to 44 mm Hg (37-50 mm Hg) in the control burials (P=.02). Mean baseline RR of 16/min (7-22/min) did not change during device burials but significantly increased to 28/min (19-41/min) in the control burials (P=.03). Mean baseline heart rate of 71/min (63-90/min) did not change in the device burials but significantly increased to 97/min (74-126/min) in the control burials (P=.02). P values indicate statistical comparison of baseline and device or control burial end point data using a Wilcoxon matched-pairs test.

Body surface temperature significantly decreased only during device burials (Table 2). Most subjects felt cold and were starting to shiver at the end of the burial, with subjective reports ranging from "not cold" in subject 5 to shivering and "very cold" in subjects 7 and 8.

During burials with the device, PICO2 was significantly higher in the high-density snow group within 5 minutes (P=.006), and both PICO2 and ETCO2 were higher in the high-density snow group throughout the burials (P<.001) (Figure 4). The SpO2 was significantly lower in the high-density snow group (P<.001), but the difference was not clinically important because all subjects except subject 6 maintained adequate oxygenation (SpO2>88%) throughout burial with the device.

Figure 4. Mean Values and Ranges for ETCO2 and PICO2 by Snow-Density Group
Graphic Jump Location
ETCO2indicates end-tidal partial pressure of carbon dioxide; PICO2, partial pressure of inspiratory carbon dioxide. Moderate-density snow is 30% to 40% snow; high-density snow, 55% to 60% snow.

The artificial air-pocket device allowed subjects to maintain adequate oxygenation for up to 60 minutes during snow burial while a similarly sized air pocket in the snow resulted in hypoxemia within 5 to 14 minutes. The device also increased the time required for subjects to reach a clinically significant degree of hypercapnia. In burials with the device, PICO2 increased up to 44 mm Hg during 60 minutes, equivalent to a fraction of inspired CO2 (FICO2) of 8%. In the control burials, the same increase in FICO2 and hypercapnia occurred within 5 minutes.

Worsening hypoxemia occurs with progressive hypercapnia because CO2 displaces oxygen in the alveoli according to the alveolar gas equation. The artificial air-pocket device maintains adequate oxygenation because it diverts expired CO2 away from the air pocket and delays the increase in FICO2 that results in hypercapnia. The limit to breathing with the device during snow burial, therefore, is related to the rise in FICO2. At the barometric pressure of our study site, when the increase in FICO2 is sufficient to cause an ETCO2 of greater than 65 mm Hg, cerebral oxygenation will be severely compromised by a partial pressure of oxygen in the alveoli of 25 to 30 mm Hg.10 Unconsciousness followed by death from asphyxiation will ensue. The device considerably extends the time required to reach that critical degree of hypoxemia. Extrapolation of the best-fit linear equation for the mean ETCO2 data suggest that an ETCO2 of 65 mm Hg may occur after about 145 minutes (y=36 + 0.2x) when breathing with the device during snow burial vs only 15 minutes (y=36 + 2.0x) without the device. Hypercapnia causes CO2 narcosis when arterial CO2 levels rise to greater than 75 mm Hg.10 During avalanche burial, a critical degree of arterial hypoxemia will occur before hypercapnia is severe enough to cause CO2 narcosis.

Higher-density snow may result in more rapid asphyxia during avalanche burial because our results suggest that diffusion of CO2 in snow is inversely related to snow density. Subjects buried in higher-density snow had higher PICO2 and ETCO2 measurements because CO2 was diffusing more slowly away from the subject vs subjects buried in moderate-density snow.

Respiratory rates did not significantly change during burial with the device, but all subjects reported a subjective sensation of breathing with an increased tidal volume. It is known that in response to hypercapnia, initial increases in minute ventilation occur with an increase in tidal volume rather than RR.11 Subjects also reported increased resistance to expiration while breathing with the device as a result of positive end-expiratory pressure (PEEP). When a subject is breathing at rest with the device, PEEP is 2.5 cm H2O; during hyperventilation, PEEP increases to approximately 5 cm H2O. The increasing PEEP with increasing flow may have caused subjects to adopt a more efficient pattern of breathing with a higher tidal volume and lower RR. PEEP also may have increased functional residual capacity and improved oxygenation.

In both burials, the degree of dyspnea reported by subjects was out of proportion to the level of hypoxemia, suggesting that hypercapnia contributed to subjective dyspnea. Hypercapnia is known to contribute to the sensation of dyspnea.12 All subjects had mountaineering experience at or above 4000 m and reported that dyspnea during this study was worse than that experienced acutely at an altitude where SpO2 would be comparable to our end point values.

This study was limited to testing the device with subjects buried in the sitting position. We have performed 2 additional studies with subjects using the device while buried in a left-lateral decubitus position and in an elbow-knee prone position (data not reported). Subjects maintained an SpO2 greater than 90% during a 60-minute burial. Burial in the supine position may not be as well tolerated. In the preliminary study conducted by our group,9 1 subject in the supine position remained buried for only 12 minutes because condensation dripping over the nose and mouth made it difficult to breathe even though SpO2 remained normal.

An artificial air-pocket device provides a significant advantage for breathing during snow burial because expired CO2 is diverted away from the air pocket, aspiration of snow is prevented, and an ice mask impermeable to air does not form. However, during an actual avalanche burial a major issue will be the ability to place the mouthpiece in the mouth. Stiff respiratory tubing that allows the mouthpiece to remain in a ready position facilitates this. Once buried, snow density, clothing, and individual physiologic differences will influence survival. If the device prolongs survival with longer extrication times, then hypothermia may become a more important factor because core body temperature will drop at a rate of about 3°C/h.13

Wearing an artificial air-pocket device does not replace conservative judgment and appropriate safety precautions when traveling in avalanche terrain. For persons who are buried in an avalanche, however, our preliminary tests suggest that such a device may reduce probability of asphyxiation before a rescue.

Atkins D. Colorado Avalanche Information Center: US & world avalanche accident stats [serial online]. Available at: http://caic.state.co.us/USgraphs.html. Accessed June 14, 1999.
Page CE, Atkins D, Shockley LW, Yaron M. Avalanche deaths in the United States, a 45-year analysis.  Wilderness Environ Med.1999;10:146-151.
Grossman MD, Saffle JR, Thomas F, Tremper B. Avalanche trauma.  J Trauma.1989;29:1705-1709.
Williams K, Armstrong BR, Armstrong RL. Avalanches. In: Auerbach PS, ed. Wilderness Medicine: Management of Wilderness and Environmental Emergencies. 3rd ed. St Louis, Mo: Mosby; 1995:616-643.
McClung D, Schaerer P. The Avalanche Handbook1st ed. Seattle, Wash: Mountaineers; 1993:115.
Stalsberg H, Albretsen C, Gilbert M.  et al.  Mechanism of death in avalanche victims.  Virchows Archiv A Pathol Anat.1989;414:415-422.
Falk M, Brugger H, Adler-Kastner L. Avalanche survival chances [letter].  Nature.1994;368:21.
Radwin MI, Keyes L, Radwin DL. Avalanche air space physiology. In: Proceedings of the 1998 International Snow Science Workshop, Sunriver, Ore, Sept 27–Oct 1, 1998.
Margid J, Beidleman N, Harmston C, Crowley CT, Hatton C, Crowley TJ. O2 and CO2 levels with the Black Diamond AvaLung during human snow burials lasting up to one hour. In: Proceedings of the 1998 International Snow Science Workshop, Sunriver, Ore, Sept 27–Oct 1, 1998.
Nunn JF. Applied Respiratory Physiology3rd ed. London, England: Butterworths; 1987:226, 475.
Berger AJ. Control of breathing. In: Murray JF, Nadel JA, eds. Textbook of Respiratory Medicine. 2nd ed. Philadelphia, Pa: WB Saunders Co; 1988:199-218.
De Vito EL, Roncoroni AJ, Berizzo EA, Pessolano F. Effects of spontaneous and hypercapnic hyperventilation on inspiratory effort sensation in normal subjects.  Am J Respir Crit Care Med.1998;158:107-110.
Locher T, Walpoth B, Pfluger D, Althaus U. Akzidentelle Hypothermie in der Schwiez (1980-1987): Kasuistik und prognostische Faktoren.  Schweiz Med Wochenschr.1991;121:1020-1028.

Figures

Figure 1. Schematic Drawing of the AvaLung as It Is Positioned in a Vest Worn Over All Other Clothing
Graphic Jump Location
Air from the snowpack enters the plastic mesh air pocket (E) and on inspiration flows into the respiratory tubing circuit via two 1-way valves (D) to mouthpiece (A). The capnometer sampling tubing (B) and emergency oxygen tubing (C) are shown where they insert in-line with the respiratory tubing just below the mouthpiece. These attachments were part of our experimental setup but are not required for actual function of the device during avalanche burial. Expired air passes from the mouthpiece through the respiratory tubing to 1-way expiratory valve (F) and then exits out the back of the device (G). The inset panel shows the back view of the vest with the position of the expiratory tubing as it sits between layers of mesh fabric. The expiratory tubing angles up to the shoulder rather than continuing to the midline of the back to avoid interfering with a backpack.
Figure 2. Subject Burial With Device
Graphic Jump Location
The left panel shows the compacted snow mound with a subject sitting in the trench in burial position. The right panel shows the subject buried to the neck prior to full burial. The device mouthpiece with capnometer tubing, emergency oxygen tubing, and physiologic monitoring cables is visible.
Figure 3. Mean Values and Ranges for SpO2, PICO2 at Baseline and Throughout Device and Control Burials and Throughout Device and Control Burials
Graphic Jump Location
Mean values and ranges are shown for physiologic parameters at baseline with subjects breathing ambient air, during device burials, and during control burials. X-axis represents time relative to full burial in minutes for all panels. Top, Oxygen saturation by pulse oximeter (SpO2); middle, end-tidal carbon dioxide (ETCO2); and bottom, partial pressure of inspiratory carbon dioxide (PICO2). Some mean data points at the end of the control burial are missing or do not have ranges because of subject dropout (burial times of 5-14 minutes) and because in the subject who remained buried for 14 minutes, the capnometer failed to record data when PICO2 approached ETCO2. Compared with baseline measurements, SpO2 percentage decreased during control (P<.05) and device burials (P<.003), ETCO2 increased during control (P<.002) and device burials (P<.001), and PICO2 increased during control (P<.03) and device burials (P<.001). Statistical comparisons of baseline and device or control burial data were done with a Friedman analysis of variance.
Figure 4. Mean Values and Ranges for ETCO2 and PICO2 by Snow-Density Group
Graphic Jump Location
ETCO2indicates end-tidal partial pressure of carbon dioxide; PICO2, partial pressure of inspiratory carbon dioxide. Moderate-density snow is 30% to 40% snow; high-density snow, 55% to 60% snow.

Tables

Table Graphic Jump LocationTable 2. Baseline and End Point Data From Device and Control Burials*

References

Atkins D. Colorado Avalanche Information Center: US & world avalanche accident stats [serial online]. Available at: http://caic.state.co.us/USgraphs.html. Accessed June 14, 1999.
Page CE, Atkins D, Shockley LW, Yaron M. Avalanche deaths in the United States, a 45-year analysis.  Wilderness Environ Med.1999;10:146-151.
Grossman MD, Saffle JR, Thomas F, Tremper B. Avalanche trauma.  J Trauma.1989;29:1705-1709.
Williams K, Armstrong BR, Armstrong RL. Avalanches. In: Auerbach PS, ed. Wilderness Medicine: Management of Wilderness and Environmental Emergencies. 3rd ed. St Louis, Mo: Mosby; 1995:616-643.
McClung D, Schaerer P. The Avalanche Handbook1st ed. Seattle, Wash: Mountaineers; 1993:115.
Stalsberg H, Albretsen C, Gilbert M.  et al.  Mechanism of death in avalanche victims.  Virchows Archiv A Pathol Anat.1989;414:415-422.
Falk M, Brugger H, Adler-Kastner L. Avalanche survival chances [letter].  Nature.1994;368:21.
Radwin MI, Keyes L, Radwin DL. Avalanche air space physiology. In: Proceedings of the 1998 International Snow Science Workshop, Sunriver, Ore, Sept 27–Oct 1, 1998.
Margid J, Beidleman N, Harmston C, Crowley CT, Hatton C, Crowley TJ. O2 and CO2 levels with the Black Diamond AvaLung during human snow burials lasting up to one hour. In: Proceedings of the 1998 International Snow Science Workshop, Sunriver, Ore, Sept 27–Oct 1, 1998.
Nunn JF. Applied Respiratory Physiology3rd ed. London, England: Butterworths; 1987:226, 475.
Berger AJ. Control of breathing. In: Murray JF, Nadel JA, eds. Textbook of Respiratory Medicine. 2nd ed. Philadelphia, Pa: WB Saunders Co; 1988:199-218.
De Vito EL, Roncoroni AJ, Berizzo EA, Pessolano F. Effects of spontaneous and hypercapnic hyperventilation on inspiratory effort sensation in normal subjects.  Am J Respir Crit Care Med.1998;158:107-110.
Locher T, Walpoth B, Pfluger D, Althaus U. Akzidentelle Hypothermie in der Schwiez (1980-1987): Kasuistik und prognostische Faktoren.  Schweiz Med Wochenschr.1991;121:1020-1028.

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