Optimal pH-management during operations requiring hypothermic circulatory arrest : an experimental study employing pH- and/or α-stat strategies during cardiopulmonary bypass

Abstract

Cessation of the blood circulation for some time during surgery of the aortic arch and repair of congenital heart defects is normally required to allow a bloodless operation field. Hypothermia is the most important mechanism for end-organ protection, particularly the brain, during such operations. Cardiopulmonary bypass is used for core cooling before total hypothermic circulatory arrest (HCA) or selective cerebral perfusion (SCP) are initiated. During hypothermic cardiopulmonary bypass, pH can be managed according to either pH- or alpha-stat principles. In the present work, the optimal pH management strategy for operations requiring HCA or SCP was explored.

An experimental porcine model was used. Firstly, outcome was evaluated in a HCA model using either the α- or pH-stat perfusion strategy (I). Secondly, we sought to determine which acid-base management is more effective in attenuating ischemic brain injury during combined HCA and embolization conditions (II). In the third study, the impact of propofol anesthesia and α-stat perfusion strategy on outcome was explored (III). Finally, the acute effects of perfusion strategies in a SCP porcine were compared (IV). Hemodynamics, temperature, EEG (I-III), brain microdialysis, intracranial pressure (I-III), brain tissue oxygen partial pressure (I-III), and intravital microscopy (IV) were monitored intraoperatively. In the chronic studies, survival, postoperative neurologic recovery and brain histopathologic examination were evaluated (I-III).

pH-stat strategy was associated with superior outcome compared to the α-stat strategy during a 75-minute period of deep HCA (I). In addition, despite the pH-stat strategy-related cerebral vasodilatation, this method provided better neuroprotection in a setting of cerebral particle embolization prior to a 25-minute period of deep HCA (II). Propofol anesthesia combined with α-stat perfusion strategy was observed to deteriorate the brain injury during HCA evaluated by key brain microdialysis parameters (III). Finally, when employing moderately hypothermic SCP, the differences between pH- and α-stat strategies in cerebral metabolism and microcirculation were minimal.

These findings are clinically relevant since α-stat perfusion strategy is still the most commonly used acid-base perfusion strategy during hypothermic cardiopulmonary bypass in adults, and propofol one of the most used anesthetics in clinical practice. It is also noteworthy that the pH-stat strategy is not currently used in adults because of the perceived increased risk of atherosclerotic embolization. However, the advantage of pH-stat strategy over α-stat strategy could not be observed when employing SCP.