Rewarming, Ultraprofound Hypothermia and Cardiopulmonary Bypass

Abstract

Rewarming, a key event in resuscitation from accidental, experimental and clinical hypothermia, is sometimes followed by neurologic, cardiac, and respiratory sequelae and may lead to death. The rate of rewarming has been implicated but not quantified as etiologic in these sequelae. Under anesthesia fifteen dogs were cannulated and connected to an extracorporeal circuit for oxygenation, core cooling and rewarming. They were subjected to ultra-profound hypothermia with a core (esophageal) temperature as low as 1.3°C, cardiac arrest, blood substitution, and continuous low flow perfusion. After 2-3 hours of cardiac arrest, rewarming began. Mechanical activity of the heart was seen between 10° and 28°C and respiration resumed at 29°C. The rewarming rates of the 15 dogs were retrospectively studied. They were placed into three categories (G) based on the outcome. G-I (N = 2): no neurological complications, G-II (N=8): transient neurological problems, and G-III (N=5): death, mainly from cardiovascular and respiratory complications confirmed at death by autopsy. Heat gain by each animal was recorded as a function of time for all experiments. The time it took each dog to reach 35°C was determined and a mean was calculated (rewarming rate). Normal body temperature for a dog is 37.8°C. Statistical analysis (ANOV A) was performed ex post facto to determine the relationship between rewarming rate and outcome. Our data contradicts the notion that slow core rewarming from nadir to normal temperature offers better outcome. During rewarming metabolic needs change at different temperatures and our laboratory observations appear to provide an explanation for complications that may occur upon rewarming from profound hypothermia.