Effect of Cooling on Oxygen Consumption in Febrile Critically III Patients

Abstract

Hyperthermic critically ill patients are commonly cooled to reduce their oxygen consumption (V̇O2). However, no previous studies in febrile humans have measured V̇O2 during cooling. We cooled 12 febrile, critically ill, mechanically ventilated patients while measuring V̇O2 and CO2 production (V̇CO2) by analysis of inspired and expired gases. All patients were mechanically ventilated for hypoxemic, hypercapneic, or shock-related respiratory failure and had a mean APACHE II score of 22.4 ± 7.7. As temperature was reduced from 39.4 ± 0.8 to 37.0 ± 0.5° C, V̇O2 decreased from 359.0 ± 65.0 to 295.1 ± 57.3 ml/min (p < 0.01) and V̇CO2 decreased from 303.6 ± 43.6 to 243.5 ± 37.3 ml/min (p < 0.01). The respiratory quotient (RQ) did not change significantly, and calculated energy expenditure decreased from 2,481 ± 426 to 1,990 ± 33 kcal/day (p < 0.01). In 7 patients with right heart catheters, cardiac output decreased from 8.4 ± 3.2 to 6.5 ± 1.8 L/min (p < 0.01) as the oxygen extraction fraction also tended to decrease from a mean of 28.2 ± 6.8 to 23.4 ± 4.7% (p = 0.12) during cooling. Accordingly, cooling the febrile patient unloads the cardiorespiratory system and, in situations of limited oxygen delivery or hypoxemic respiratory failure, may thus facilitate resuscitation and minimize the potential for hypoxic tissue injury.