Low cerebral blood flow after cardiac arrest is not associated with anaerobic cerebral metabolism

Aim of the study Estimation of cerebral anaerobic metabolism in survivors and non-survivors after cardiac arrest. Methods We performed an observational study in twenty comatose patients after cardiac arrest and 19 healthy control subjects. We measured mean flow velocity in the middle cerebral artery (MFV_MCA) by transcranial Doppler. Arterial and jugular blood samples were used for calculation of the jugular venous-to-arterial CO₂/arterial to-jugular venous O₂ content difference ratio. Results After cardiac arrest, MFV_MCA increased from 26.0[18.6–40.4] cm/sec on admission to 63.9[48.3–73.1] cm/sec after 72 h (p < 0.0001), with no significant differences between survivors and non-survivors (p = 0.4853). The MFV_MCA in controls was 59.1[52.8–69.0] cm/sec. The oxygen extraction fraction (O₂EF) was 38.9[24.4–47.7]% on admission and decreased significantly to 17.3[12.1–26.2]% at 72 h (p < 0.0001). The decrease in O₂EF was more pronounced in non-survivors (p = 0.0173). O₂EF in the control group was 35.4[32.4–38.7]%. The jugular bulb-arterial CO₂ to arterial-jugular bulb O₂ content difference ratio was >1 at all time points after cardiac arrest and did not change during admission, with no differences between survivors and non-survivors. Values in cardiac arrest patients were similar to those in normal subjects. Conclusions In this study, low CBF after cardiac arrest is not associated with anaerobic metabolism. Hypoperfusion appears to be the consequence of a decrease of neuronal functioning and metabolic needs. Alternatively, hypoperfusion may decrease cerebral metabolism. Subsequently, metabolism increases in survivors, consistent with resumption of neuronal activity, whereas in non-survivors lasting low metabolism reflects irreversible neuronal damage.