Respiration During Snow Burial Using an Artificial Air Pocket

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.

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.

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).

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.

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).

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.

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.