PH STRATEGIES AND CEREBRAL ENERGETICS BEFORE AND AFTER CIRCULATORY ARREST

The pH-stat strategy compared with the alpha-stat strategy provides mo re rapid recovery of brain high-energy phosphate stores and intracellu lar pH after 1 hour of hypothermic circulatory arrest in pigs. Possibl e mechanisms for this difference are (1) improved oxygen delivery and homogeneity of brain cooling before deep hypothermic circulatory arres t and (2) greater cerebral blood flow and reduced reperfusion injury o wing to extracellular acidosis during the rewarming phase. To identify which of these mechanisms is predominant, we studied 49 4-week-old pi glets undergoing 1 hour of deep hypothermic circulatory arrest. Four g roups were defined according to cooling/rewarming strategy: alpha/alph a, alpha/pH, pH/alpha, and pH/pH. In 24 animals cerebral high-energy p hosphate levels and intracellular pH were measured by magnetic resonan ce spectroscopy (alpha/alpha group 7, alpha/pH group 5, pH/alpha group 7, pH/pH group 5). In 25 animals cerebral blood flow was measured by labeled microspheres, cerebral metabolic rate by oxygen and glucose ex traction, and the redox state of cytochrome aa, and hemoglobin oxygena tion by near infrared spectroscopy (alpha/alpha group 7, alpha/pH grou p 5, pH/alpha group 7, pH/pH group 6). Cerebral blood flow was greater with pH-stat than alpha-stat during cooling (56.3% +/- 3.7% versus 32 .9% +/- 2.1% of normothermic baseline values, p < 0.001). Cytochrome a a(3) values became more reduced during cooling with alpha-stat than wi th pH-stat (p = 0.049). Recovery of adenosine triphosphate levels in t he initial 45 minutes of reperfusion was more rapid in group pH/pH com pared with that in the other groups (p = 0.029). Recovery of cerebral intracellular pH in the initial 30 minutes was faster in group pH/pH c ompared with that in group alpha/alpha (p = 0.026). Intracellular pH b ecame more acidic during early reperfusion only in group alpha/alpha, whereas it showed continuous recovery in the other groups. This study suggests that there are mechanisms in effect during both the cooling a nd rewarming phases before and after deep hypothermic circulatory arre st that could contribute to an improved cerebral outcome with pH-stat relative to more alkaline strategies.