Active core rewarming in neurologic, hypothermic patients: effects on oxygen-related variables.

Abstract

To determine in hypothermic patients if a) the decrease in oxygen consumption (VO2) is exclusively dependent on the decrease in metabolic rate, or b) as a consequence of the greater hemoglobin affinity for oxygen, hypothermic tissues have impaired oxygen extraction. Clinical, prospective study; sequential measurements of oxygen-related variables during active core rewarming. Intensive care unit of a university hospital. Twelve patients (44 +/- 16 yrs of age) admitted to the intensive care unit with a core temperature of < 34 degrees C due to severe neurologic damage. Rewarming (with heated enemas, gastric infusions, and heated blankets) to increase body temperature at a rate of approximately 1 degree C/hr. Measurements of oxygen-related variables were performed at a baseline of 31.0 +/- 1.1 degrees C, and repeated at each 1 degree C increase to reach a core temperature of approximately 35 degrees C. Oxygen-related variables of rewarmed patients were allocated into two groups, above or below the observed mean core temperature of 33.1 degrees C recorded for all measurements (n = 45). Comparison of the low core temperature group (31.1 +/- 1.4 degrees C; n = 20) with the high core temperature group (34.7 +/- 0.9 degrees C; n = 25) showed that the group with the lower core temperatures had a significant increase in VO2 index (67 +/- 22 vs. 103 +/- 38 mL/min/m2 [p < .001]), oxygen delivery index (183 +/- 73 vs. 290 +/- 123 mL/min/m2 [p < .001]), and the PO2 value at which hemoglobin was half-saturated with oxygen ([P50] 23 +/- 5.7 vs. 27.7 +/- 5.7 torr [3.0 +/- 0.7 vs. 3.6 +/- 0.7 kPa] [p < .02]). An increase in metabolic acidosis could be observed in the lower temperature group: arterial pH 7.47 +/- 0.15 vs. 7.34 +/- 0.13 (p < .01); base deficit -3.7 +/- 6.7 vs. -8.2 +/- 4.9 mEq/L (p < .02). The oxygen extraction ratio remained unchanged: 0.39 +/- 0.10 vs. 0.38 +/- 0.10 (NS). These data show that VO2 was reduced to half of normal values during hypothermia. Active core rewarming produced an average 4.5% increase in VO2 per 1 degree C that was characterized by the wide variation observed in this metabolic response between different patients and for individual cases. Despite the rightward shift of P50 observed during rewarming (mainly due to the Bohr effect), no change was reflected on the oxygen extraction ratio.