Circulatory arrest: pH-stat implications

During hypothermia, pH-stat (reporting of blood gases at body temperature) corrects for the effects of temperature on pH in order to keep the pH static (7.4). This often results in the addition of CO₂ to the CPB circuit. In contrast, alpha-stat (reporting of blood gases at 37 degrees) attempts to preserve electrical neutrality especially of the alpha imidazole ring of histidine (a major buffer during hypothermia). pH-stat samples analyzed at 37 degrees will be acidotic and hypercarbic. The implications of acidosis and hypercarbia include increased cerebral blood flow, rightward shift of the oxy-hemoglobin dissociation curve, and decreased cerebral metabolic rate of oxygen consumption (CMRO₂). Although controversial, pH-stat may be associated with improved neurodevelopment outcomes in children. Important points:

• Increased cerebral blood flow may be harmful in adults whose post-bypass morbidity is often cognitive dysfunction resulting from vascular microemboli. It may also result in increased intracranial pressure and steal phenomenon in patients with cerebrovascular disease.
• Rightward shift of the oxyhemoglobin dissociation curve promotes oxygen delivery to tissues (which is especially useful in low flow states or circulatory arrest) as well as more homogenous brain cooling.
• Decreased CMRO₂ helps balance oxygen supply and demand during circulatory arrest. There is no blood flow, so it is preferable to minimize CMRO₂.