Abstract
Completely avalanche-buried patients are frequently exposed to a combination of hypoxia and hypercapnia with a risk of normothermic cardiac arrest. Patients with a long burial time and an air pocket are exposed to a combination of hypoxia, hypercapnia, and hypothermia which may lead to the development of the “triple H syndrome”. This specific combination has several pathophysiological implications, particularly on the cardiovascular system and oxygen transport (oxygen supply and oxygen consumption). To examine the effects on hemoglobin oxygen affinity, we investigated venous blood samples from 15 female and 15 male healthy subjects. In a factorial design of four different carbon dioxide partial pressure (PCO2) levels (20, 40, 60, and 80 mmHg) and five different temperature levels (13.7°C, 23°C, 30°C, 37°C, and 42°C), 30 unbuffered whole blood samples were analyzed in a newly developed in vitro method for high-throughput oxygen dissociation curve (ODC) measurements. P50s, Hill coefficients, CO2-Bohr coefficients, and temperature coefficients were analyzed using a linear mixed model (LMM). Mean P50 at baseline (37°C, 40 mmHg PCO2) was 27.1 ± 2.6 mmHg. Both CO2-Bohr (p < 0.001) and temperature coefficients (p < 0.001) had a significant effect on P50. The absolute CO2 effect was still pronounced at normothermic and febrile temperatures, whereas at low temperatures, the relative CO2 effect (expressed by CO2-Bohr coefficient; p < 0.001, interaction) was increased. The larger impact of PCO2 on oxygen affinity at low temperature may be caused by the competition of 2,3-BPG with PCO2 and the exothermic binding characteristic of 2,3-BPG. In a model of an avalanche burial, based on published data of CO2 levels and cooling rates, we calculated the resulting P50 for this specific condition based on the here-reported PCO2 and temperature effect on ODC. Depending on the degree of hypercapnia and hypothermia, a potentially beneficial increase in hemoglobin oxygen affinity in the hypoxic condition might ensue.
Highlights
A completely avalanche-buried patient who is still able to breathe into an air pocket may cool down during a long burial time and suffer from cardiovascular and respiratory changes that are not exclusively explainable by the effect of decreased core temperature
Despite hypercapnia can speed up the cooling rate [6], the combination of hypothermia with hypoxia and hypercapnia may result in a worse neurological outcome in completely avalanche-buried patients compared to patients suffering solely from hypothermic cardiac arrest [7,8,9]
P50, oxygen partial pressure at which 50% of hemoglobin is saturated with oxygen; PCO2, carbon dioxide partial pressure. * An asterisk between two factors indicates the effect of interaction of the two factors. - A dash indicates that the factor was not inserted in the linear mixed model (LMM)
Summary
A completely avalanche-buried patient who is still able to breathe into an air pocket (any space in front of mouth and nose) may cool down during a long burial time and suffer from cardiovascular and respiratory changes that are not exclusively explainable by the effect of decreased core temperature. Despite hypercapnia can speed up the cooling rate [6], the combination of hypothermia with hypoxia and hypercapnia may result in a worse neurological outcome in completely avalanche-buried patients compared to patients suffering solely from hypothermic cardiac arrest [7,8,9]. Such outcome is determined by the temporal sequence of the events, as a decrease in oxygen supply and CO2 removal usually proceed the development of hypothermia
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