This Editorial View accompanies the following article: Kurth CD, O'Rourke MM, O'Hara IB: Comparison of pH-stat and alpha-stat cardiopulmonary bypass on cerebral oxygenation and blood flow in relation to hypothermic circulatory arrest in piglets. Anesthesiology 1998; 89:110–8.
IT has been slightly more than 10 years since Murkin et al. reported that pH-stat management results in greater cerebral blood flow (CBF) during cardiopulmonary bypass (CPB) than does [Greek small letter alpha]-stat management. After this landmark study, numerous animal and clinical studies have addressed the question of which technique might be best for the brain during cardiac surgery. It appears we may now have the answer-it depends. In children undergoing deep hypothermic circulatory arrest (DHCA), pH-stat should be used. In adults undergoing routine cardiac surgery, [Greek small letter alpha]-stat should be used. Why the difference?
Two clinical studies, one retrospective and a recent prospective randomized trial, find better neurologic outcome with pH-stat management in children undergoing DHCA. In this issue of Anesthesiology, Kurth et al, provide important new insight into two mechanisms by which pH-stat management may be helping the brain during DHCA. First, their work shows that pH-stat management increases the rate of brain cooling, probably on the basis of greater CBF with pH-stat. Although the effect is fairly small, it is nonetheless important because clinical studies suggest brain cooling often may be incomplete before DHCA when [Greek small letter alpha]-stat management is used. [5,6]Thus pH-stat management may provide extra brain protection during DHCA simply by increasing the likelihood that the desired level of cerebral hypothermia is actually clinically achieved. The other important finding of this experiment is that the rate of brain oxygen depletion during DHCA is considerably slower with pH-stat than with [Greek small letter alpha]-stat management. pH-stat management was found to substantively prolong the interval between onset of arrest and exhaustion of brain oxygen stores. At profound hypothermia (17 [degree sign]C), pH-stat management reduces brain oxygen consumption an additional 30–40% below that achieved with [Greek small letter alpha]-stat management. [7,8]This supplemental reduction of brain oxygen consumption may underlie Kurth's observation of a slower rate of oxygen depletion with use of pH-stat management. Prevention of cellular oxygen exhaustion may greatly increase the brain's tolerance to DHCA.
In contrast, in adults undergoing continuous moderately hypothermic CPB, three randomized prospective studies indicate postoperative neurologic or neuropsychological outcome is slightly, but consistently, better with [Greek small letter alpha]-stat. [9–11]The mechanism is not known, but it is presumed that the lesser CBF of [Greek small letter alpha]-stat management results in fewer cerebral emboli during CPB. In adult cardiac surgery patients, postoperative neurologic outcome is related, at least in part, to the number of cerebral emboli that occur during surgery. [12–15]In contrast to DHCA, global impairment of brain oxygenation during continuous CPB does not appear to be a major mechanism of neurologic injury in adults. [16,17]With use of membrane oxygenators or arterial line filters, the rate of cerebral embolization during CPB often is pretty low. Therefore, with short bypass runs, the number of emboli originating from the CPB circuit may be sufficiently small so that acid-base management may not significantly affect neurologic outcome. Consistent with this hypothesis, Murkin et al. reported that [Greek small letter alpha]-stat management was not associated with better neurologic outcome until CPB durations exceeded 90 min. Overall in adults undergoing continuous CPB, the weight of current evidence favors use of [Greek small letter alpha]-stat.
During the past 10 years, integration of laboratory and clinical investigations have lead to mechanistic insight and improved neurologic outcomes after pediatric and adult cardiac surgery. Once mysterious, mechanisms of brain injury during cardiac surgery are becoming more clear. Because the brain can be injured in more than one way during cardiac surgery, our approach to prevention and treatment will depend on the nature of the injury.
Bradley J. Hindman, M.D.
Department of Anesthesia; The University of Iowa; College of Medicine; Iowa City, Iowa 52242