Carbon dioxide affects cerebral autoregulation after cardiac arrest

Learning Objectives: Cerebral autoregulation (CAR), which ensures constant brain perfusion for a broad range of arterial pressures, can be altered in postanoxic brain injury. Recent studies showed an association between early hypocapnia and increased mortality in patients resuscitated from cardiac arrest (CA). Thus, we aimed at investigating CAR at different PaCO2 levels during targeted temperature management (TTM) after CA. Methods: This prospective interventional study included adult (> 18 years) patients treated with TTM at 33°C after CA. Exclusion criteria were arrhythmias; ECMO; basal PaCO2 50 mmHg. We used transcranial Doppler (TCD) to assess flow velocity in left middle cerebral artery (FV). The Person correlation coefficient between invasive mean arterial pressure (MAP) and FV defined the “Mxa” index, which ranges from -1 (best CAR) to +1 (worst CAR). Impaired CAR was defined by Mxa > 0.3. After the first TCD at baseline PaCO2 values, ventilator settings were modified to obtain a change of > 7 mmHg, keeping everything else constant. The probe was kept in place throughout the examination by a dedicated helmet. Results: We enrolled 12 patients (8 male, 65 [52–71] years, 8 cardiac origin; no-flow and low flow were 0 [0–7] (n = 9) and 22 [10–36] minutes respectively. Hypothermic phase (34 [33.8–34.3]°C) lasted 24 [19–26] hours. All patients were sedated and 6 patients were treated with norepinephrine (13 [5–37] mcg/min). Baseline MAP was 72 [70–87] mmHg. PaCO2 was 37 [34–40] (low-CO2) and 45 [43–47] (high-CO2) mmHg, respectively (p < 0.01). There was no significant difference in hemodynamic and respiratory variables. Mxa was 0.19 [0.05–0.52] during low-PCO2 and 0.58 [0.19–0.73] during high PCO2 (p < 0.01). CAR was impaired in 5 (42%) and 8 (67%) patients in the two CO2 groups, respectively (p = 0.2). Conclusions: In postanoxic brain damage, cerebral autoregulation seems to be less efficient at higher than at lower PaCO2 levels.