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Ann Thorac Surg 2006;81:1548-1549
© 2006 The Society of Thoracic Surgeons
a Division of Cardiovascular Surgery, St. Vincent Mercy Medical Center, 2213 Cherry St ACC Bldg, Suite 309, Toledo, OH 43608
b Departments of Medicine and Surgery, Medical University of Ohio, 3000 Arlington Ave, Toledo, OH 43614
(Email: robert_habib{at}mhsnr.org).
We read with great interest the article by Dr Weerasinghe and colleagues [1]. The authors' main conclusion was that compared with on-pump coronary bypass surgery (ONCAB), off-pump (OPCAB) coronary bypass surgery may reduce the risk of both "minor" and "major" renal adverse outcomes (RAO). Their discussion of the potential mechanisms for these findings was limited to: (1) the potential adverse role of increased complement activation in ONCAB, (2) the renoprotective role of comparatively higher mean arterial pressures with OPCAB, and (3) a potential renoprotective role of pulsatile versus nonpulsatile flow. We wish to make several comments regarding this article.
The Society of Thoracic Surgeons (STS) defines postoperative acute renal failure (ARF) as the coincidence of postoperative creatinine (Cr) > 2 mg/dL and postoperative Cr> 2 times preoperative levels. This approach rightly differentiates between patients with similar postoperative Cr (eg, 2.4 mg/dL) whose baseline renal function (eg, preoperative Cr = 1 [140% increase; ARF per STS] versus preoperative Cr = 1.8 mg/dL [33% increase; no ARF per STS]) is substantially different. The major RAO definition used by Weerasinghe and colleagues [1] does not incorporate a similar restriction. Also, the distributions of preoperative Cr values for the OPCAB versus ONCAB groups were significantly different (refer to Table 2; Weerasinghe and colleagues [1]). These two facts make it unclear if (and how) the employed multivariate analysis adjusts for the previously described varying renal decrement yet similar postoperative Cr values. We suggest that a more appropriate approach might have been to define minor and major RAO based on referenced Cr values, such as %
Cr (or percent peak postoperative change in serum creatinine relative to preoperative values) as used by others [2, 3].
According to the authors, the results of the propensity-adjusted multivariate analysis (refer to Fig 1; Weerasinghe and colleagues [1]) indicated that ONCAB is systematically associated with greater probability of both minor and major RAO irrespective of preoperative Cr levels; please note here that authors' Figure 1 seems to be mislabeled with the labels "minor" and "major" being interchanged. The probability of minor RAO should be greater than major RAO. Here it is noteworthy that these modeling results for major RAO are the reverse of the observed (unadjusted) trend of 5.3% of ONCAB (77 of 1,224) compared with 10.0% of OPCAB (82 of 817) patients. This, coupled with the largely similar risk factors in Table 2 (based on which the propensity score was calculated), leads us to reasonably question if these findings are, at least partly, an over-modeling artifact. The authors could try alleviating this concern as follows. Although the propensity scores for the two groups were not provided (or compared) in the article, it is fair to assume that there was substantial overlap for the two groups given the similarity of risk factors. This means it is possible to have a large majority of OPCAB patients greedily (or 1 to 1) matched to ONCAB counterparts. Renal outcomes of the matched sub-cohorts can then be compared.
Our last point relates to the potential causes of increased renal injury with ONCAB. A critical difference between the two operations is the necessarily greater degree of hemodilution in ONCAB due to the mixing of the patients' blood with the crystalloid used for pump prime and cardioplegia. More hemodilution is in turn correlated with increased frequency of intraoperative and postoperative transfusions. These two factors were not considered (nor discussed) by the authors as potential explanatory variables of their results [1]. Briefly, Ranucci and associates [4] first described a correlation between increasing on-pump hemodilutional anemia and worse postoperative renal outcomes in 1994. Since then, several other articles [2, 3, 5, 6] have confirmed this association. In a recent article [3], our group reported that both %Cr and ARF exhibited sigmoidal dose-dependent associations to the lowest on-pump hematocrit. Moreover, we showed that these relations are: (1) modulated by the duration of cardiopulmonary bypass (longer is worse), (2) worse in patients with relatively elevated pre-bypass Cr (
1.2 mg/dL), and (3) exacerbated rather than mitigated by the early use of intraoperative packed red blood cell transfusions used to reverse the on-pump anemia. These results do not contradict the authors' findings of less renal injury by avoiding cardiopulmonary bypass. However, they do suggest that by incorporating the relevant hematocrit and transfusion data into their analysis, the findings reached by Dr Weerasinghe and colleagues [1], and their interpretation, might differ appreciably.
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A. Weerasinghe, T. Athanasiou, and P. P. Tekkis Reply. Ann. Thorac. Surg., April 1, 2006; 81(4): 1549 - 1550. [Full Text] [PDF]
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