Effect of differing acid-base regulation on cerebral blood flow autoregulation during cardiopulmonary bypass

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Abstract
Cerebral dysfunction following cardiopulmonary bypass may be aggravatedby altered autoregulation of cerebral blood flow. We have used trans-cranial Doppler to measure middle cerebral artery blood flow velocityduring cardiopulmonary bypass managed by either pH-stat or alpha-statacid-base protocols. Fourteen patients were studied, 7 in each group.During bypass at 28 degrees C, patients underwent incremental alterationsin mean arterial pressure from 20-90 mmHg, maintaining systemic perfusionflow at 1.75 L/min per m2. The cerebral extraction ratio of oxygen wasmeasured to indicate matching of cerebral blood flow to demand. The pH-statgroup showed a pressure passive cerebral circulation with significant (r =0.999, P less than 0.05) increase in blood flow velocity with increasingarterial pressure. This also occurred in alpha-stat group during thepressure range of 20-50 mmHg (r = 0.951, P less than 0.05). During thepressure range of 50-90 mmHg in alpha-stat group the change in flowvelocity (0.16 cm/sec per mmHg) was significantly (P less than 0.05) lessthan that in pH-stat group (0.58 cm/second per mmHg). The cerebralextraction ratio of oxygen was less depressed in the alpha-stat group thanin the pH-stat group, indicating more appropriate matching of cerebralblood flow and tissue demand. These results suggest that, during alpha-statmanaged cardiopulmonary bypass, cerebral blood flow velocity is lesssubject to wide pressure alteration than pH-stat.

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