Metabolic Manipulation and Therapeutic Hypothermia

Hypothermia has multiple physiological effects, including decreasing metabolic rate and oxygen consumption (VO). There are few human data about the magnitude of change in VO with decreases in core temperature. We aimed to quantify to magnitude of reduction in resting VO as we reduced core temperature in lightly sedated healthy individuals. After informed consent and physical screening, we cooled participants by rapidly infusing 20 mL/kg of cold (4°C) saline intravenously and placing surface cooling pads on the torso. We attempted to suppress shivering using a 1 mcg/kg intravenous bolus of dexmedetomidine followed by titrated infusion at 1.0 to 1.5 μg/(kg·h). We measured resting metabolic rate VO through indirect calorimetry at baseline (37°C) and at 36°C, 35°C, 34°C, and 33°C. Nine participants had mean age 30 (standard deviation 10) years and 7 (78%) were male. Baseline VO was 3.36 mL/(kg·min) (interquartile range 2.98-3.76) mL/(kg·min). VO was associated with core temperature and declined with each degree decrease in core temperature, unless shivering occurred. Over the entire range from 37°C to 33°C, median VO declined 0.7 mL/(kg·min) (20.8%) in the absence of shivering. The largest average decrease in VO per degree Celsius was by 0.46 mL/(kg·min) (13.7%) and occurred between 37°C and 36°C in the absence of shivering. After a participant developed shivering, core body temperature did not decrease further, and VO increased. In lightly sedated humans, metabolic rate decreases around 5.2% for each 1°C decrease in core temperature from 37°C to 33°C. Because the largest decrease in metabolic rate occurs between 37°C and 36°C, subclinical shivering or other homeostatic reflexes may be present at lower temperatures.