Pro and Con

I N 1976, A SURVEY of 26 cardiovascular centers was compiled for the Association of Cardiac Anesthesiologists (Garman JK: Unpublished data, 1976). That survey showed that there were wide differences in opinion concerning the flow rates and blood pressures used during cardiopulmonary bypass (CPB). Acceptable blood pressures varied from 30 to 120 mm Hg. Acceptable bypass flows varied from 1.2 to 3.5 L/min/m’ (or 30 to 90 mL/kg/min). Obviously, when opinions vary as widely as this, the data on which decisions are made must not be definitive. Unfortunately, this is a very difficult subject to study because many variables confound the results. Why is this controversy important? Many cardiovascular surgical and anesthesia teams prefer to use a low-flow, low-pressure bypass technique. There are very good reasons for choosing this technique. It is very important to understand these reasons in order to rationalize the choice of a technique as controversial as this one. The reasons are: (1) less trauma to blood elements; (2) less stress on pump tubing and connections; (3) slower rewarming of the heart due to less bronchial flow entering the heart; (4) a clearer operative field for the same reason; (5) less trauma to the aorta at the site of the cross-clamp; and (6) the ability to use smaller venous and arterial cannulae. Given these advantages, many cardiovascular teams have decided that the risk-benefit ratio of using a low-flow, low-pressure technique is acceptable. Surgeons used to the advantages of this technique find it difficult to tolerate the bloodier operative field, the necessity for more vigorous cooling measures, and the resultant longer bypass times found with higher flows and pressures. Is this a safe technique? Many studies have demonstrated no increased incidence of neurological dysfunction in groups of patients having cardiac surgery with low-pressure, lowflow bypass techniques. Kolkka and Hilberman’ studied 204 patients and showed that low pressures during bypass did not correlate with cerebral injury. The average mean arterial pressure (MAP) in this group was 49 mm Hg with an average CPB flow of 42 mL/kg/min. Ellis et al2 used preoperative and postoperative standard psychometric testing on 30 patients undergoing cardiac surgery with a low-flow, low-pressure bypass technique and showed a zero incidence of irreversible cerebral dysfunction at 6 months postoperation. The average MAP during CPB in these patients was 60 mm Hg. The average CPB flow was 39 mL/kg/min. Both of these studies can easily be criticized for experimental design flaws. However, they do establish that there is little difference in outcomes in these patient groups when compared with the rate of complications in high-flow, high-pressure bypass patient groups. Even more interesting are some studies using prostacyclin (a prostaglandin used to prevent platelet aggregation during bypass). The major side effect of this agent is profound vasodilatation. When used during bypass, perfusion pressures consistently run below 30 mm Hg unless supported with vasopressors. In two studies, Aren et a13,“ concluded that although CPB with hypothermia prolonged central brain conduction time, the hypotension produced by prostacyclin did not further impair conduction when compared with a nonhypotensive group. They also concluded that the hypotensive group did not have an increased risk of postoperative cerebral damage. Fish et al’ studied 100 patients with detailed neuropsychiatric examinations and CT scans before and after CPB. Patients were randomized into groups receiving prostacyclin or saline. Perfusion pressures in the prostacyclin group were lower than the control group. There were no differences in the groups regarding perioperative cognitive changes. If it is decided to use this technique, how can its safety be maximized? It is important to understand the effect of some easily manipulated variables on cerebral autoregulation. Several studies have examined the effect of using different methods of blood gas management during CPB. Murkin et al” randomized 38 patients undergoing CPB into two groups: one group managed with the pH-stat method that requires CO2 to be added to maintain normal blood gases as measured in a temperature-corrected analyzer, and the second group managed with the a-stat method with no CO, added and blood gases maintained normal without temper-