pH-Stat management is more appropriate during deep hypothermia, especially when circulatory arrest is added

Abstract

during cooling and rewarming with proper pH management, since patients do not go from 38° to 18°C or vice versa instantaneously.The better postoperative neurologic outcome following deep hypothermic perfusion in infants with pH-stat than with alpha-stat management is known [9], and probably results from less hypoxia-induced excitotoxicity and minimization of NO generation than with alpha-stat strategies.With the use of eucapnic or slightly hypercapnic ventilation (expired [CO 2 ] of 5.1% to 5.7% providing pH of 7.23-7.1, PaCO 2 of 50-60 mmHg, and PaO 2 of 230-260 mmHg, uncorrected for temperature, equivalent to normoxic pH-stat hypothermic cardiopulmonary bypass), spinal cord function in rabbits after 1 h of ischemia was consistently preserved by surface-induced hypothermia to only 29.5°C [10], which is considered too high a temperature if 1 h of ischemia is to be protected with alpha-stat hypothermia [11].Mild acidosis decreases Ca 2ϩ influx, glutamate neurotoxicity, and neuronal injury from deprivation of oxygen and glucose by reducing the activation of NMDA receptors [12][13][14][15]. Alkalosis sensitizes neurons to ischemia and exacerbates excitotoxicity, potentiating reperfusion injury [15,16], which is the scenario with alpha-stat DHCA.Eucapnic ventilation can usually be maintained with minimal changes in respiratory rate down to temperatures of 32°-33°C. The alpha-stat strategy might be preferable for temperatures Ͼ32°C, at which most coronary bypass operations are performed, but the pH-stat strategy is probably more physiologic for temperatures Ͻ32°C [10], regardless of whether conditions are normoxic or hyperoxic [17,18], particularly if circulatory arrest is being contemplated.