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Hypotension doesn’t add to kidney risks from cardiopulmonary bypass in anemic patients

NEW YORK (Reuters Health) – The co-occurrence of hypotension and anemia during cardiopulmonary bypass (CPB) does not increase the risk of acute kidney injury (AKI) over that of anemia alone, researchers say.

An earlier study had suggested that low blood pressure on the heart-lung machine might influence the anemia-associated risk of AKI after heart surgery, said Dr. Mark Stafford-Smith from Duke University School of Medicine in Durham, North Carolina, who led the new research.

“What was notable also about the issue of low blood pressure on the heart-lung machine was that ours and other previous investigations had not noted this as a factor in predicting acute kidney injury (as long as the flow rate was maintained),” Dr. Stafford-Smith told Reuters Health. “Hence, our study tried to validate this surprising finding, which if true would raise questions about how we currently care for our patients.”

His team compared AKI rates in patients with anemia and hypotension with those in patients with anemia alone in a retrospective study of nearly 4,000 patients undergoing on-pump nonemergent coronary artery bypass graft (CABG) and valve surgery.

AKI rates did not differ significantly between patients with low hematocrit/low blood pressure and patients with low hematocrit only (41.6% vs. 44.3%, respectively; p=0.39), the teams reports in The Annals of Thoracic Surgery, online November 22.

There were, however, strong associations between anemia and postoperative AKI, so that patients in the low hematocrit-only group had a significantly greater AKI risk than did either the low blood pressure-only group (35.4%) or the normal hematocrit/normal blood pressure group (33.4%).

“This reassures us that our current practice with regards to anemia and blood pressure management while on the heart-lung machine does not appear to be of concern, as would have been the case had we been able to reproduce the findings of Haase and colleagues,” which suggested that hypotension might add to the anemia-associated risk, Dr. Stafford-Smith said. “Of note, the findings of Haase and colleagues were not in error, just likely happened by chance alone.”

Dr. Rinaldo Bellomo from Austin Health in Melbourne, Australia, was a coauthor of the aforementioned report by Haase et al. (Nephrol Dial Transplant 2012;27:153-60). “I think a lot of these associations may be related to practice style and variables entered in the model and granularity of data collection and data quality,” he told Reuters Health by email.

“We need randomized controlled trials,” Dr. Bellomo said. “This kind of surgery is really common, and we should be able to randomize patients to 2 different hematocrit targets to begin with.”

Dr. Won Ho Kim from Samsung Medical Center in Seoul, South Korea, who recently published a clinical risk score to predict AKI after aortic surgery, said: “I think (these results) should not be interpreted that hypotension during the entire surgery period is not associated with postoperative AKI. A recent meta-analysis of the influence of perioperative hemodynamic optimization on postoperative renal function has revealed that perioperative hemodynamic optimization decreased the risk of renal impairment.”

“During the cardiopulmonary bypass period, more attention should be paid on anemia rather than hypotension in these patients based on (this study),” Dr. Kim, who was not involved in the new work, told Reuters Health by email. “However, as the authors commented in Discussion, retrospective design and missing risk factor including pump flow rate still necessitate further studies to change our clinical practice.”


Ann Thorac Surg 2013.