Relationship of brain blood flow and oxygen consumption to perfusion flow rate during profoundly hypothermic...

A study was made of the relation of brain blood flow and oxygen consumption to changes in perfusion flow rate during cardiopulmonary bypass at 20 degrees C in nine cynomolgus monkeys. Four perfusion flow rates varying from 0.25 to 1.75 L X min-1 X m-2 were randomly instituted, each for a 10 minute period. At the end of each period, brain arteriovenous oxygen content difference was measured and 15 mu radioactive microspheres were injected into the arterial perfusion line. The brain was then removed and section into anatomic regions and radioactivity was counted. Regional and total brain blood flows were calculated, as was whole brain oxygen consumption. Brain perfusion continued in all areas at all perfusion flow rates. Whole brain blood flow decreased (p less than 0.0001) as perfusion flow rate was reduced (45 +/- 6.5, 41 +/- 7.9, and 23 +/- 2.8 ml X min-1 X 100 gm-1 at 1.5, 1.0, and 0.5 L X min-1 X m-2, respectively). The proportion of the total perfusion delivered to the brain increased (p = 0.003) with decreasing perfusion flow rates (5.4% +/- 0.78%, 7.1% +/- 1.24%, and 8.2% +/- 1.11% at 1.5, 1.0, and 0.5 L X min-1 X m-2, respectively). Brain blood flow resistance remained unchanged (p = 0.4) while that of the remaining body increased (p less than 0.0001). There was a greater reduction of blood flow in the cortical white matter (p = 0.01) than in other regions of the brain. Brain oxygen consumption was the same (p = 0.5) at all perfusion flow rates, related to an increasing percent oxygen extraction with decreasing perfusion flow rate (p less than 0.0001). The data indicate that all areas of the brain remain perfused, even at low perfusion flow rates, during profoundly hypothermic cardiopulmonary bypass, and that brain oxygen consumption is maintained in part by increased oxygen extraction and in part by redistribution of the perfusate from the remaining body to the brain.