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J Thorac Cardiovasc Surg 1997;113:619-620
© 1997 Mosby, Inc.
LETTERS TO THE EDITOR |
Research Director
Cardiac Surgical Research/Cardiothoracic Surgery
The Rayne Institute
St. Thomas' Hospital
London SE1 7EH, United Kingdom
Reply to the Editor:
My coauthors and I would like to thank Professor Murkin for his comments relating to our article.
1 He neatly combines complimentary comments with a slight admonition relating to our unfortunate error in failing to cite his excellent study, 2 published in this Journal in August 1995, in which he addresses the issue of cognitive dysfunction and the effects of acid-base regulation during cardiopulmonary bypass (CPB) in patients undergoing coronary artery bypass grafting (CABG). In our defense, we can only apologize for this omission and state that our original manuscript was submitted to the Journal at the beginning of August 1995; invariably, our copy of the Journal does not arrive in the United Kingdom until 1 to 2 months later. Additionally, Professor Murkin cites our work in his study (reference 18), attributing it to The Annals of Thoracic Surgery as "in press"; this, however, is erroneous because we had never submitted the study to this journal although it had been submitted some 18 months previously to Anesthesiology!
We agree that our results essentially confirm those described in Professor Murkin's article. We also observed a significant reduction in cognitive dysfunction in the alpha-stat managed group of patients (20% vs 49% in the pH-stat group) at 6 weeks after the operation; these data compare well with the respective values (27% vs 44%) in the study by Murkin and colleagues at 2 months after the operation. However, differences in cognitive dysfunction between the acid-base management regimens in Murkin's study were not observed when all patients were taken into account; the above differences were observed only in those patients with CPB durations in excess of 90 minutes. In our study, all patients were included; the number of patients with CPB durations greater than 90 minutes in our study groups was 10 of 35 (29%) in the pH-stat group and 7 of 35 (20%) in the alpha-stat group. Further analysis of this subgroup of patients showed that 7 of the 10 pH-stat patients (70%) had cognitive dysfunction in two or more tests (which was the same as for the whole group at this level) whereas only 1 of the 7 alpha-stat patients (14%) had cognitive dysfunction at the same level (compared with 46% for the whole group). This discrepancy further emphasizes the advantage bestowed by the use of alpha-stat acid-base management of CPB on reducing cognitive dysfunction compared with pH-stat management.
Professor Murkin further points out that there appears to be a considerable difference in the ages of the patients between the two studies. His statement is based on the upper limit of the confidence interval for our patient population being less than the mean value for his population. Further analysis does not support this contention, however. We concede that the use of 95% confidence intervals for mean values of parameters such as age, CPB duration, and crossclamp duration may not be ideal. In Table III of Murkin's study, it is not stated whether the mean values are given with standard deviation or standard error; we assume it to be standard deviation because this was used in Table V. On this basis, the mean age (± standard deviation) of our patients was 56.9 ± 5.7 years for the alpha-stat group and 57.6 ± 7.8 years for the pH-stat group. These ages would not appear to be significantly younger than those in Murkin's study. In addition, our aortic crossclamp durations were 39.7 ± 10.8 minutes and 40.3 ± 14.7 minutes and CPB durations were 75.1 ± 17.3 minutes and 83.4 ± 38.7 minutes for the alpha-stat and pH-stat groups, respectively. Consequently, although the aortic crossclamp durations were similar, the mean CPB durations of our patients do appear to have been shorter.
Professor Murkin professes to be puzzled with our finding that "there was a significantly greater reduction in CMRO2 [cerebral metabolic rate for oxygen] in the pH-stat group during hypothermia." We admit that it is not immediately obvious from Fig. 3 that these values are significantly different. However, the Results section states that CMRO2 decreases by 49% and by 63% of the prebypass value in the alpha-stat and pH-stat groups, respectively. The actual values were 0.561 and 0.432 ml oxygen 100 gm–1 · –1, respectively, and the p value for the difference between these values was 0.0064. Thus we stand by our statement that the value of CMRO2 for the pH-stat group of patients, during the hypothermic phase of CPB, was reduced significantly more than in the alpha-stat group.
Professor Murkin also comments on the apparently significantly higher values of cerebral blood flow (CBF) and CMRO2 obtained in our study compared with those of a previous study 3 conducted by his group. He points out that the difference observed between these studies could be accounted for by the differences in anesthetics administered during the operative procedure. We agree that anesthetic agents are among the factors that can influence cerebral hemodynamics. We also consider that the depth of anesthesia is important and would respectfully point out that we acknowledged such in the Discussion of our article, citing Professor Murkin's study 3 as an example of where this might be the case. However, it is the changes in CBF and CMRO2 that are of interest; the initial baseline values are, essentially, irrelevant (so long as they fit within the accepted normal values). In addition, we should point out that, in our study, continuous on-line arterial blood monitoring was practiced, which enabled precise control of acid-base status in both the alpha-stat and pH-stat groups. This precise control was not practiced by Murkin and colleagues.
We thank Professor Murkin for his overall congratulations regarding our study, which we readily reciprocate. Now that there are two relatively large trials demonstrating the benefits of alpha-stat acid-base management during CPB on reducing cerebral dysfunction in patients having CABG alone, we hope that other centers that currently do not use the alpha-stat management regimen during CPB may be persuaded of the benefits.
12/8/78753
References
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