Cerebrovascular reactivity to CO2 in patients with hepatic or septic encephalopathy.

The authors studied the effects of hypocapnic-hyperventilation on cerebral blood flow (CBF) (study 1) and on cerebral oxygenation (study 2) during mechanical ventilation in 8 patients, 4 with hepatic (HE) and 4 with septic encephalopathy (SE). In study 1, a positive linear relationship between CBF(y) and PaCO2 (x) was observed (y = 2.44x - 55.5, r = 0.6276, P less than 0.01, n = 18). In the study 2, hypocapnic-hyperventilation produced a reduction in CBF below the level required to meet the demand in 4 of 8 patients. A good linear relationship was observed between CBF/CMRO2 (CMRO2 = cerebral oxygen consumption, y) and jugular venous PO2 (PjVO2, x) (y = 0.99x - 15.53, r = 0.8962, P less than 0.01, n = 18). It is concluded that cerebrovascular reactivity to CO2 was preserved in these patients, therefore, intentional or inadvertant hyperventilation may produce cerebral ischemia. Moreover, JPVO2 may be useful in monitoring cerebral oxygenation in such patients.