Cerebral arteriovenous oxygen difference as an estimate of cerebral blood flow in comatose patients

The hypothesis that cerebral arteriovenous difference of oxygen content (AVDO₂) can be used to predict cerebral blood flow (CBF) was tested in patients who were comatose due to head injury, subarachnoid hemorrhage, or cerebrovascular disease. In 51 patients CBF was measured daily for 3 to 5 days, and in 49 patients CBF was measured every 8 hours for 5 to 10 days after injury. In the latter group of patients, when a low CBF (≤0.2 ml/gm/min) or an increased level of cerebral lactate production (CMRL) (≤-0.06 μmol/gm/min) was encountered, therapy was instituted to increase CBF, and measurements of CBF, AVDO₂, and arteriovenous difference of lactate content (AVDL) were repeated. When data from all patients were analyzed, including those with cerebral ischemia and those without, AVDO₂ had only a modest correlation with CBF (r = -0.24 in 578 measurements, p<0.01). When patients with ischemia, indicated by an increased CMRL, were excluded from the analysis, CBF and AVDO₂ had a much improved correlation (r = -0.74 in 313 measurements, p<0.01). Most patients with a very low CBF would have been misclassified as having a normal or increased CBF based on the AVDO₂ alone. However, when measurements of AVDO₂ were supplemented with AVDL, four distinct CBF patterns could be distinguished. Patients with an ischemia/infarction pattern typically had a lactate-oxygen index (LOI = -AVDL/AVDO₂) of 0.08 or greater and a variable AVDO₂. The three nonischemic CBF patterns had an LOI of less than 0.08, and could be classified according to the AVDO₂. Patients with a normal CBF (mean 0.42 ± 0.12 ml/gm/min) had an AVDO₂ between 1.3 and 3.0 μmol/ml. A CBF pattern of hyperemia (mean 0.53 ± 0.18 ml/gm/min) was characterized by an AVDO₂ of less than 1.3 μmol/ml. A compensated hypoperfusion CBF pattern (mean 0.23 ± 0.07 ml/gm/min) was identified by an AVDO₂ of more than 3.0 μmol/min. These studies suggest that reliable estimates of CBF may be made from AVDO₂ and AVDL measurements, which can be easily obtained in the intensive care unit.