Abstract
To the Editor:
We appreciate the comments of Dr Muller. He makes a number of important points. We did not measure oxygen consumption or electromyography in our subjects, and, therefore, we did not obtain objective data to document shivering heat production. We agree that objective measurement of shivering would have been optimal. We did record subjective onset of shivering (not reported in the manuscript), and all subjects experienced onset of vigorous shivering prior to, or during, extrication from snow burial. Mild shivering occurred in all subjects prior to extrications. We have measured oxygen consumption in the field in a previous study of cooling during snow burial, 1 and it is a technical challenge that in our experimental setup requires fixed inspired carbon dioxide and oxygen concentrations. In the study under discussion here, we chose to use the AvaLung breathing device that results in a rising carbon dioxide, and falling oxygen concentration during the course of the 1-hour snow burial. This is a better simulation of the changing inspired gas concentrations during avalanche burial.2,3
In regard to the question pertaining to the title of our paper and measurement of shivering thermogenesis: the title of our paper refers to the method of rewarming used, spontaneous endogenous rewarming, which is rewarming by shivering. Since our subjects remained at rest during rewarming, the method of rewarming was predominately by shivering with adequate insulation provided from the hypothermia wrap. Given the specific limitations we discuss in the paper, we are able to make conclusions based on our data about the ability to rewarm mildly hypothermic avalanche burial victims using the method in our study.
We disagree with Dr Muller regarding the likely mechanism for afterdrop in our study. The rapid increase in cooling rate during the afterdrop period observed in our study is due to a predominant convective mechanism; core blood perfuses cold peripheral tissue, cools off, and returns to the core, causing a continued core cooling, or afterdrop. Anything that increases perfusion of cold tissue increases the afterdrop. In our subjects, the increased activity during extrication contributed to increased blood flow to the extremities and a convective afterdrop mechanism. The conductive mechanism for afterdrop, due to equilibration of the cooler periphery and warmer core, always occurs; but it is a slower process that cannot account for the rapid increase in cooling rate seen in our study. It is well established that the convective mechanism occurs much faster than the conductive mechanism 4 –7 and is the operative mechanism for afterdrop in settings of increased physical activity, as occurred during extrication of our subjects.
We disagree that further study is required before widespread guidelines for rewarming avalanche victims can be implemented. We do not claim that the method of rewarming we applied is the most optimal, and we agree that the lack of a control group makes it impossible for us to claim that our rewarming method is the best. Our objective was to demonstrate that successful rewarming can occur by simply placing an extricated avalanche burial victim in a hypothermia wrap in a manner that would occur in a field situation. We tested a commonly used insulation system and demonstrated that it can effectively rewarm subjects after extrication from snow burial. Further studies are required to determine the optimal system, and to characterize individual differences in rewarming rates under different circumstances.