Cerebral blood flow is determined by arterial pressure and not cardiopulmonary bypass flow rate

Background.: During cardiopulmonary bypass, global hypoperfusion of the brain has been shown to result in ischemic insult and subsequent neurologic injury. Furthermore, outcome after focal cerebral ischemia depends on collateral circulation, which is determined by the parameters of global perfusion. We therefore measured cerebral blood flow during independent manipulations of arterial blood pressure and pump flow rate to determine which of these hemodynamic parameters regulates cerebral perfusion during cardiopulmonary bypass. Methods.: Seven anesthesized baboons were placed on cardiopulmonary bypass and cooled to 28°C. Pump flow rate and arterial blood pressure were altered in varied sequence to each of four conditions: (1) full flow (2.23 ± 0.06 L·min^-1·m^-2, mean ± standard deviation) at high pressure (61 ± 2 mm Hg), (2) full flow (2.23 ± 0.06 L·min^-1·m^-2) at low pressure (24 ± 3 mm Hg), (3) low flow (0.75 L·min^-1·m^-2) at high pressure (62 ± 2 mm Hg), and (4) low flow (0.75 L·min^-1·m^-2 at low pressure (23 ± 3 mm Hg). During each of these hemodynamic conditions cerebral blood flow was measured by washout of intracarotid xenon¹³³. Results.: Cerebral blood flow was greater at high blood pressure than at low pressure during cardiopulmonary bypass both at low flow (34 ± 8.3 versus 14.1 ± 3.7 mL·min^-1·100 g^-1) and full flow (27.6 ± 9.9 versus 16.8 ± 3.7 mL·min^-1·100 g^-1) (p < 0.01). At comparable mean arterial blood pressures alteration of pump flow rate produced no changes in cerebral blood flow. Conclusions.: These results indicate that cerebral blood flow during moderately hypothermic cardiopulmonary bypass is regulated by arterial blood pressure and not pump flow rate.