J Thorac Cardiovasc Surg 2002;123:395-396
© 2002 The American Association for Thoracic Surgery
Reply
C. Dean Kurth, MD,
Margaret Priestley, MD
Brain Research Laboratory
Department of Anesthesiology and Critical Care Medicine
The Children's Hospital of Philadelphia
Department of Anesthesia and Pediatrics
University of Pennsylvania School of Medicine
Philadelphia, PA 19104
Reply to the Editor:
We thank Miyamoto and Miyamoto
1 and Wong for their comments on pH-stat versus alpha-stat blood gas management during deep hypothermic cardiopulmonary bypass (CPB). The central question, "Which is better?," has been debated for many years. 2
The purpose of our study was to characterize neurologic injury in a piglet model of infant cardiac surgery using deep hypothermic CPB and total circulatory arrest (DHCA) with alpha-stat blood gas management strategy. 3 Miyamoto and Miyamoto 4 suggest that the neuronal injury resulted from the alpha-stat CPB management and not DHCA. Their rationale is that deep hypothermia and alkalosis each increase oxyhemoglobin affinity, the combination of which prevented oxygen unloading to the point of tissue hypoxia. They suggest that neurologic injury would not have occurred if pH-stat management had been used, because the acidosis with pH-stat counterbalances the effect of deep hypothermia to prevent tissue hypoxia. Miyamoto and Miyamoto also cite the use of a pH-stat strategy in hibernating animals and calculations from a hypothermic rabbit spinal cord ischemia study to support their suggestion.
The experimental data, however, do not support Miyamoto and Miyamoto's suggestion. First, in our study, clinical and histologic neuronal injury was not observed after deep hypothermic CPB; neuronal injury occurred only after DHCA. 3 Second, during deep hypothermic CPB with alpha-stat management, there is no evidence of tissue hypoxia, because brain adenosine triphosphate concentrations are preserved and brain tissue PO2 is within normal limits. 5,6 Third, hemoglobin readily unloads oxygen at deep hypothermia despite the effect of alpha-stat on binding. 5,7 Fourth, many hibernating animals use the alpha-stat strategy, during which brain tissue adenosine triphosphate concentrations remain normal, and there is no neuronal injury on recovery from hibernation. 2,8 Wong makes additional arguments against alpha-stat management causing neuronal injury from brain tissue hypoxia.
As to "which is better" for heart surgery with DHCA in infants, several studies show that pH-stat improves outcome compared with alpha-stat. In a piglet model, Priestley and associates 9 observed better neurologic performance and less histologic damage with pH-stat than alpha-stat management. Other piglet studies found better cerebral blood flow, metabolism, and oxygenation with pH-stat management. 5,7 In clinical studies, the results are not as strong. 10,11
pH-Stat management appears to confer neurologic protection against ischemia by several mechanisms. 5,7,9 It increases cerebral oxygen supply during CPB cooling and slows cerebral deoxygenation during arrest. It improves brain-cooling efficiency to augment the protection provided by hypothermia. It may also inhibit lactate buildup and glutamate excitotoxicity during arrest and early reperfusion. Finally, it improves cerebral oxygen supply during reperfusion, thereby preventing secondary injury.
In summary, we believe alpha-stat management is not detrimental for deep hypothermic CPB, whereas pH-stat management is neuroprotective for DHCA. Thus, for operations in which continuous deep hypothermic CPB is used, either the alpha-stat or pH-stat strategy may be used. For operations performed with DHCA or low-flow CPB (where ischemia is a possibility), the pH-stat strategy is preferable.
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Miyamoto TA, Miyamoto KJ. Alkalosis induced by alpha-stat management: cause of neuronal injury after deep hypothermic perfusion. J Thorac Cardiovasc Surg. 2001;121:817-8.[Free Full Text]
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