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J Thorac Cardiovasc Surg 1997;113:619
© 1997 Mosby, Inc.

LETTERS TO THE EDITOR

Alpha-stat acid-base regulation during cardiopulmonary bypass

Professor of Anaesthesia
Director, Cardiac Anaesthesia
London Health Sciences Centre
University Campus
University of Western Ontario
London, Ontario N6A 5A5, Canada

To the Editor:

I read with interest the recent study by Patel and associates, ¹ particularly their opening comments that the issue of differing acid-base regulation "has not been tested in a relatively large trial of patients having CABG [coronary artery bypass grafting] alone." This is erroneous. Their lack of referral to a previous study by my colleagues and myself ² is surprising considering that it was published in this same journal just 1 year previously. Their omission is all the more surprising because their results essentially confirm our observations that 2 months after the operation the incidence of cognitive dysfunction is reduced in patients having CABG who undergo cardiopulmonary bypass for longer than 90 minutes using alpha-stat management. It should be noted that the population they operated on were significantly younger than those we reported, the upper limit of their 95% confidence interval for age being less than the mean age of our population. Fundamentally, however, given differences in patient age and type of cognitive testing performed, the results of these two studies are quite comparable.

I am also puzzled by the authors statement that "there was a significantly greater reduction in CMRO₂ [cerebral metabolic rate for oxygen] in the pH-stat group during hypothermia." It is certainly not clear either from Fig. 3 or from the discussion on cerebral metabolism that, in fact, there was any significant difference in CMRO₂ between the alpha-stat and the pH-stat groups. What was the statistical strength of this observation? In a previous study, we ³ did not find any significant difference in CMRO₂ between groups despite cerebral blood flow (CBF) that was more than 50% lower in the alpha-stat group.

With respect to the absolute values of CBF and CMRO₂ that Dr. Patel's group measured, they do appear to be significantly higher than those same values that we had reported during cardiopulmonary bypass. ³ Although all of the factors mentioned by the authors, specifically, differences in perfusion technique and pressure management, can influence cerebral hemodynamics, it should also be borne in mind that anesthetics can have a significant influence on both CBF and CMRO₂. Accordingly, the patients that we had previously reported ³ received significantly higher doses of narcotic (fentanyl 0.1 mg/kg and a high dose of diazepam, 0.5 mg/kg), whereas their patients received 2 to 4 mg midazolam and 0.01 to 0.02 mg/kg fentanyl. I believe that when these fundamental differences in anesthetic management are taken into account, their CBF physiology measurements and those that we previously reported will be increasingly compatible.

Overall, the authors are to be congratulated for this study combining intraoperative cerebral physiology and postoperative cognitive performance.

References

1. Patel RL, Turtle MR, Chambers DJ, James DN, Newman S, Venn GE. Alpha-stat acid-base regulation during cardiopulmonary bypass improves neuropsychologic outcome in patients undergoing coronary artery bypass grafting. J Thorac Cardiovasc Surg 1996;111:1267-79.[Abstract/Free Full Text]
   
2. Murkin JM, Martzke JS, Buchan AM, Bentley C, Wong CJ. A randomized study of the influence of perfusion technique and pH management strategy in 316 patients undergoing coronary artery bypass surgery. II. Neurologic and cognitive outcomes. J Thorac Cardiovasc Surg 1995;110:349-62.[Abstract/Free Full Text]
   
3. Murkin JM, Farrar JK, Tweed WA, McKenzie FN, Guiraudon G. Cerebral autoregulation and flow/metabolism coupling during cardiopulmonary bypass: the influence of PaCO₂. Anesth Analg 1987;66:825-32.[Abstract/Free Full Text]
   

This Article Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): John M. Murkin Permission Requests Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Murkin, J. M. Search for Related Content PubMed PubMed Citation Articles by Murkin, J. M.