Cardiovascular changes during mild therapeutic hypothermia and rewarming in infants with hypoxic-ischemic encephalopathy

Background. Clinical trials of mild cooling to 35 degrees C or below in inf ants with early hypoxic-ischemic encephalopathy are under way. The objectiv e of this study was to systematically document cardiovascular changes assoc iated with mild therapeutic hypothermia and rewarming in such infants. Patients and Methods. Nine infants with gestational ages of 36 to 42 weeks, with 10-minute Apgar scores of 5 or less, clinical encephalopathy, and an abnormal electroencephalogram before 6 hours were cooled by surface cooling the trunk (n = 3) or by applying a cap perfused with cooled water (n = 6) for a median of 72 hours. The target core temperature was 34.0 degrees C to 35.0 degrees C for head-cooled infants and 33.0 degrees C to 34.0 degrees C for surface-cooled infants. Maintenance heating and rewarming were provid ed by an overhead heater. Results. Mean arterial blood pressure increased by a median of 10 mm Hg dur ing cooling and fell by a median of 8 mm Hg on rewarming. Heart rate decrea sed by a median of 34 beats/minute on cooling and increased by a median of 32 beats/minute on rewarming. A large increase in the output of the overhea d heater decreased mean arterial blood pressure in 5 infants. Anticonvulsan t drugs, sedatives, or intercurrent hypoxemia also produced falls in temper ature. The inspired oxygen fraction had to be increased by a median of .14 to maintain oxygenation during cooling with 2 infants requiring 100% oxygen , an effect probably attributable to pulmonary hypertension, which was reve rsible with rewarming. Conclusions. Therapeutic cooling produces changes in heart rate and blood p ressure that are not hazardous, but the combination of inadvertent overcool ing and inappropriately rapid rewarming, together with sedative drugs that can impair normal thermoregulatory vasoconstriction, can cause hypotension in posthypoxic newborn infants. Infants who already require 50% oxygen shou ld be cooled cautiously because pulmonary hypertension may develop. Knowled ge of these cardiovascular changes, careful monitoring, anticipation, and c orrection should help to avoid potential adverse effects in the upcoming cl inical trials.