Work of Breathing into Snow in the Presence versus Absence of an Artificial Air Pocket Affects Hypoxia and Hypercapnia of a Victim Covered with Avalanche Snow: A Randomized Double Blind Crossover Study

Abstract

Presence of an air pocket and its size play an important role in survival of victims buried in the avalanche snow. Even small air pockets facilitate breathing. We hypothesize that the size of the air pocket significantly affects the airflow resistance and work of breathing. The aims of the study are (1) to investigate the effect of the presence of an air pocket on gas exchange and work of breathing in subjects breathing into the simulated avalanche snow and (2) to test whether it is possible to breathe with no air pocket. The prospective interventional double-blinded study involved 12 male volunteers, from which 10 completed the whole protocol. Each volunteer underwent two phases of the experiment in a random order: phase "AP"--breathing into the snow with a one-liter air pocket, and phase "NP"--breathing into the snow with no air pocket. Physiological parameters, fractions of oxygen and carbon dioxide in the airways and work of breathing expressed as pressure-time product were recorded continuously. The main finding of the study is that it is possible to breath in the avalanche snow even with no air pocket (0 L volume), but breathing under this condition is associated with significantly increased work of breathing. The significant differences were initially observed for end-tidal values of the respiratory gases (EtO2 and EtCO2) and peripheral oxygen saturation (SpO2) between AP and NP phases, whereas significant differences in inspiratory fractions occurred much later (for FIO2) or never (for FICO2). The limiting factor in no air pocket conditions is excessive increase in work of breathing that induces increase in metabolism accompanied by higher oxygen consumption and carbon dioxide production. The presence of even a small air pocket reduces significantly the work of breathing.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. Flow diagram of the study with enrollment, allocation and analysis of the participants.

Fig 2. Scheme of the breathing circuit and its installation in the snow.

(Illustration: B. Kracmar).

Fig 3. Time to breathing experiment termination for two different conditions: (1) one liter air pocket and (2) no air pocket.

The term “completed” means that the subject terminated the experiment upon his own request. The term “interrupted” means that the experiment was interrupted due to other reasons, e.g. the order by the anesthesiologist, critical physiological parameters determined by the life function monitor, or any technical problem.

Fig 4. The difference between SpO2 during NP and AP phases.

The symbol * represents statistically significant differences at p ≤ 0.05.

Fig 5. Inspiratory (FIO2) and end-tidal (EtO2) fractions of oxygen in the breathing gas during NP and AP phases.

The symbol * represents statistically significant differences in FIO2 between NP and AP groups; the symbol # represents statistically significant differences in EtO2 between NP and AP phases; p ≤ 0.05.

Fig 6. The courses of inspiratory (FICO2) and end-tidal (EtCO2) fractions of carbon dioxide in the breathing gas during NP and AP phases.

The symbol * represents statistically significant differences in EtCO2 between the NP and AP groups; p ≤ 0.05.

Fig 7. The difference in breathing effort expressed as imposed Pressure-Time Product (iPTP) caused by the flow resistance of snow between no air pocket and one liter air pocket.

The symbol * represents the statistical significant differences described by p ≤ 0.05.

Fig 8. Airflow resistance of snow to breathing expressed as an overpressure in the breathing circuit developed at a constant flow rate of 60 L/min for no air pocket (NP) and one liter air pocket (AP) before and after completing the breathing experiment.