Impact of Hypothermia on Oxygenation Variables and Metabolism in Survivors of Out-of-Hospital Cardiac Arrest Undergoing Targeted Temperature Management at 33°C Versus 36°C

Targeted temperature management (TTM) exerts substantial impact on hemodynamic function in out-of-hospital cardiac arrest (OHCA) patients. Whole-body oxygen consumption (VO2) and delivery (DO2) have not previously been investigated in a clinical setting during TTM at different levels of temperature after OHCA. A substudy of 151 patients randomized at a single center in the TTM-trial, where patients were randomly assigned TTM at 33°C (TTM33) or 36°C (TTM36) for 24 hours. We calculated VO2 according to the principle of Fick (VO2 = cardiac output*arteriovenous oxygen content difference). DO2 was calculated as cardiac output*arterial oxygen content. Cardiac output was measured by pulmonary artery catheter with thermodilution. Arteriovenous oxygen content difference was calculated from arterial and mixed venous oxygen saturation and hemoglobin. Oxygen extraction ratio = VO2/DO2. At 24 hours, the VO2 was 169 ± 59 mL O2 per minute in TTM33 and 217 ± 53 mL O2 per minute in TTM36 (p < 0.0001). During 24 hours of TTM, the overall difference was 53 mL O2 minute (95% confidence interval [CI]: 31-74, pgroup < 0.0001). After rewarming at 36 and 48 hours, there was no difference in VO2 between the groups. DO2 was overall 277 mL O2 per minute (95% CI: 175-379, pgroup < 0.0001) higher in the TTM36-group during TTM. Oxygen extraction ratio during TTM was not significantly different between the two groups (2% [95% CI: -0.1 to 5, pgroup = 0.09]). VO2 during the first 36 hours after OHCA correlated significantly with temperature, and VO2 was 19 mL O2 per minute lower per degree reduction in temperature (95% CI: 15-22), p < 0.0001. TTM at 33°C compared to 36°C after OHCA is associated with significantly lower VO2 and DO2, however, oxygen extraction ratio was not significantly different. For each degree lower body temperature, the VO2 fell by 19 mL O2 per minute.