Differences in cerebral blood flow between alpha-stat and pH-stat management are eliminated during periods of decreased systemic flow and pressure: A study during cardiopulmonary bypass in rabbits

Prior reports suggest cerebral blood flow (CBF) responses to changing bypass (systemic) flow rates may differ between alpha-stat and pH-stat management. To compare the effect of blood gas management upon CBF responses to changing systemic flow and pressure, 15 New Zealand White rabbits, anesthetized with fentanyl and diazepam, underwent nonpulsatile cardiopulmonary bypass at 25° C. One group of animals (n = 8) was randomized to alpha-stat blood gas management that maintained arterial carbon dioxide tension (Pa(CO₂) ~ 40 mmHg when measured at 37° C. A second group (n = 7) was managed with pH-stat technique, maintaining Pa(CO₂) ~ 40 mmHg when corrected to the animal's actual temperature. Bypass was initiated at a flow rate of 100 ml·kg^-1·min^-1 and, after ~20 min, control hemodynamic and CBF measurements (radioactive microspheres) were made. Thereafter, bypass flow rate was changed in random order at 15-min intervals to 50, 70, and 100 ml·kg^-1·min^-1. CBF and hemodynamic measurements were repeated at the end of each period of altered bypass flow. Groups differed significantly with respect to both pHa and Pa(CO₂). There were no significant differences between groups with respect to bypass flow rate, mean arterial pressure (MAP), central venous pressure, temperature, hematocrit, arterial oxygen tension (Pa(O₂), or bypass duration at any measurement point. MAP decreased significantly, from ~ 80 to ~ 65 mmHg with decreasing bypass flow (P = 0.0001). Over the entire range of bypass flows, CBF decreased with decreasing bypass flow (P = 0.001), and the degree of change was equivalent among regions and between groups. pH-stat animals had significantly greater global CBF values compared with alpha-stat animals only at a bypass flow of 100 ml·kg^-1·min^-1. In addition, there was a strong suggestion (P = 0.058) that CBF responses to decreased systemic flow and pressure differed between groups at this level: CBF was unchanged in the alpha-stat group, whereas CBF decreased in the pH-stat group when bypass flow decreased from 100 to 70 ml·kg^-1·min^-1. At bypass flow rates of 50 and 70 ml·kg^-1·min^-1, there were no significant differences between groups in global CBF or in the CBF response to decreased systemic flow and pressure. Differences between alpha-stat and pH-stat management in both CBF and CBF dynamics can be eliminated under certain bypass conditions in rabbits.