Protecting the Brain

Perfusion Notes:

(Keeping it Brief…)

Hypothermia even when mild (34°-36°C) is established to reduce cerebral metabolism and allows the brain to tolerate longer periods of hypoperfusion or ischemia.

Embolization of air or particulate matter represents one of the most significant hazards for cerebral insult to the patient undergoing CPB.  It is important to note that during the most critical moments associated with embolization during CPB (aortic cannulation, aortic cross-clamping, aortic cross-clamp removal, and weaning) the brain may be relatively normothermic and therefore less protected.

Approaches to cerebral preservation include pharmacologic avenues such as barbiturate therapy which has been studied most frequently.  Thiopental may be beneficial due to the temporary effect it has on gas emboli, whereas calcium-entry blocking agents, free radical scavengers, and exitotoxin therapy are modalities that need further evaluation.  Other rationales that have demonstrated no consistent pattern for ensuring against or promoting cerebral dysfunction, include: avoidance of glucose containing prime, maintenance of an arbitrarily set mean arterial pressure during bypass, and duration of bypass.

Alpha-stat or pH-stat blood gas management as a means to effect cerebral protection, is predicated on temperature correction (pH-stat) or non-correction (alpha-stat).  Hypothermia results in an inversely proportional shift of arterial pH of 0.147 points per 1°C on the pH scale.  As core temperature drops the pH rises, and places the patient in an alkalotic state.

Temperature correction of arterial blood gas factors out this shift in pH.  It is essential that the anesthesia team be aware of which system is being used for the operation at hand.  Alpha-stat represents alkalotic management of cerebral perfusion while pH-stat relies on hypercarbia to manage cerebral blood flow.

The essential difference is that alpha-stat management represents cerebral blood flow that is not drive-dependent on mean arterial pressure, whereas pH-stat management demonstrates a direct relationship between mean arterial pressure and cerebral blood flow.

With pH-stat management: Turning down CPB gas sweep rate or adding CO₂ to the oxygenator…

• Counteracts increased gas solubility
• Requires an increased total body CO₂ content to maintain neutrality at cooler temperatures
• pH-stat causes cerebral vasodilatation above metabolic demands (loss of autoregulation) and possibly faster, more homogenous cooling
• Proponents argue that pH-stat also improves oxygen delivery by counteracting the pH- and hypothermia- associated leftward shift in the oxyhemoglobin dissociation curve. (To see referenced article click here )

With α-stat (not temperature correcting) : Requires that neutrality is maintained only at 37°C, and permits the hypothermic alkaline drift.

• Additional CO₂ is not needed and cerebral autoregulation is maintained
• Cellular transmembrane pH gradients:  More Normal
• Protein functioning, and enzyme activity: More normal when the pH is allowed to drift alkaline in parallel with the temperature
• A  relatively alkaline pH is beneficial before the ischemic insult of circulatory arrest.  (To see referenced article click here )
  

References:

Rogers AT, Newman SP, Stump DA, Prough DS. Neurologic effects of     cardiopulmonary bypass. In Gravlee GP, Davis RF, Utley JR, eds.     Cardiopulmonary Bypass: Principles and Practice. Baltimore, MD. Williams &     Wilkens; 1993:542-576.

Nussmeier NA. Neuropsychiatric complications of cardiac surgery. Journal of Cardiothoracic and Vascular Anesthesia. 1994;8;(Suppl 1):13-18.