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Ann Thorac Surg 2000;70:755
© 2000 The Society of Thoracic Surgeons
a Department of Cardiac Surgery, Children’s Hospital, 300 Longwood Ave, Boston, MA, 02115 USA,
e-mail: richard.jonas{at}tch.harvard.edu
Invited commentary
The report by Pearl and colleagues from Cincinnati and Kansas City describes a retrospective study of acid production during deep hypothermic circulatory arrest and relates this to the pH and oxygen strategies applied during cardiopulmonary bypass. The authors conclude that the pH stat strategy with hyperoxia results in the lowest level of acid production and therefore is the optimal strategy. They speculate that reduced acid production is related to saturation of tissues with a higher total volume of oxygen which enables ongoing aerobic metabolism during deep hypothermic circulatory arrest.
This report provides further evidence in support of use of a strategy of 100% oxygen and supplementary CO2 (pH stat strategy) for neonates and infants undergoing deep hypothermic bypass with circulatory arrest. Although the theoretical problem of increased oxygen free radical production with an hyperoxic strategy has been raised, our own laboratory studies have suggested that any disadvantage secondary to free radical production is significantly outweighed by the advantages of ongoing metabolism during hypothermic circulatory arrest [1]. Furthermore we and others have demonstrated that in a pediatric population free of atherosclerosis there is significant advantage to the pH stat strategy. In two laboratory studies we demonstrated improved intraneuronal oxygen delivery as demonstrated by near infrared spectroscopy [2] as well as improved recovery of cerebral high energy phosphates as demonstrated by magnetic resonance spectroscopy [3]. These studies confirmed that there is greater cerebral blood flow with the pH stat strategy. For this reason the pH stat strategy may not be optimal for adults in whom the principal risk for brain injury is microembolism. In a prospective randomized blinded clinical study we demonstrated that adverse events occur significantly more frequently with the alpha stat strategy relative to the pH stat strategy [4].
This article by Pearl and associates provides further support for the concept that oxygen delivery is improved by the pH stat strategy particularly when combined with a strategy of hyperoxia. The authors had a unique opportunity to compare outcomes with several combinations of CO2 and O2 management because they had fortuitously applied different pH and oxygen strategies over a two year period. It is disappointing that the authors did not take this study further in that it would have been of great interest to examine the correlation between pH and oxygen strategy and outcome variables such as inotropic support, duration of intubation, duration of intensive care unit stay, duration of hospitalization and mortality and complications in the four different groups.
In conclusion, Pearl and colleagues are to be congratulated on a useful and thoughtful analysis of bypass strategy for infants undergoing deep hypothermia with circulatory arrest. It is to be hoped that they will continue their studies including ideally a prospective randomized clinical trial which we believe will further confirm the superiority of a strategy of pH stat and hyperoxia.
References
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