Perfusion pressure and coronary bypass

Abstract

From their prospective study of 248 patients undergoing first-time coronary bypass, Gold and associates1Gold JP Charlson ME Williams-Russo P Szatrowski TP Peterson JC Pirraglia PA et al.Improvement of outcomes after coronary artery bypass: a randomized trial comparing intraoperative high versus low mean arterial pressure.J Thorac Cardiovasc Surg. 1995; 110: 1302-1314Abstract Full Text Full Text PDF PubMed Scopus (325) Google Scholar concluded that higher perfusion pressures during cardiopulmonary bypass improved all outcomes of operation. The authors would have us believe that increasing mean perfusion pressure from 52 to 70 mm Hg ( Table III a: mean pressure recorded at all flows) for 90 minutes of cardiopulmonary bypass reduced permanent cardiac complications, reduced neurologic complications, and eliminated two deaths (one from multisystem failure and one from lung cancer) at 6 months after the operations. The incidences of cardiac complications, neurologic complications, and death in the high-pressure compared with the low-pressure groups were not significantly different at the conventional p < 0.05 level. When these outcomes were selectively pooled, however (congestive heart failure, minor neurologic deficits, cognitive outcomes, and deterioration in quality of life were arbitrairily omitted), a significant difference in “overall” outcome of morbidity and mortality was claimed (p < 0.026). As a justification for pooling outcomes, the authors invoked a common mechanism, namely, that the low-pressure group was “below the autoregulatory limits of the coronary and cerebral circulations.” Autoregulation of the cerebral circulation provides constancy of cerebral blood flow through a wide range of perfusion pressures, including 52 mm Hg. No such pressure-related autoregulatory phenomenon occurs in the coronary circulation. Furthermore, the coronary circulation was not perfused at all during half of the bypass period because the aorta was crossclamped, and no data on the prevalence of postbypass and postoperative hypotension or hypertension were included, even though these events are not rare. To ascribe all, albeit not significant, differences in postoperative cardiac complications for 6 postoperative months to 45 minutes of perfusion at 18 mm Hg higher pressure during partial aortic occlusion bypass is difficult to accept. In a separate publication2Barbut D Hinton RB Szatrowski TP Hartman GS Bruefach M Williams-Russo P et al.Cerebral emboli detected during bypass surgery are associated with clamp removal.Stroke. 1994; 25: 2398-2402Crossref PubMed Scopus (238) Google Scholar 1 year earlier these same authors reported studies of cerebral embolization by transcranial Doppler sonography in 20 patients undergoing coronary bypass who appear to be a subset of the 248 patients in the present study. They reported that postoperative neurocognitive deterioration after coronary bypass was directly related to the embolic load to the brain, which was greatest at unclamping an atherosclerotic aorta during the operation. One patient among the three with the highest embolic load had a stroke during the operation. The present study did not include data on cerebral embolic load or on grading of atheromatous plaque of the aorta. Because the nonsignificant difference in neurologic outcome between the high- and low-pressure groups was the largest contributor to the difference in “overall” outcome, differences in the presence of aortic atheromatous disease between the two groups could completely explain the observed differences in outcome, without invoking differences in perfusion pressure. As the authors themselves state, “Clear relationships between the severity of aortic disease, the number of ES [embolic signals] at cross clamp removal and neuropsychological outcome must be demonstrated before modification of surgical techniques are indicated.”2Barbut D Hinton RB Szatrowski TP Hartman GS Bruefach M Williams-Russo P et al.Cerebral emboli detected during bypass surgery are associated with clamp removal.Stroke. 1994; 25: 2398-2402Crossref PubMed Scopus (238) Google Scholar This caveat would and should equally apply to management of perfusion pressure during cardiac operations as well.