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Anesthesiology:
doi: 10.1097/ALN.0b013e3181acfc5b
Correspondence

Hypothermia Should Also Have Been Considered to Be a Predictor of Adverse Perioperative Cardiac Events

Kheterpal, Sachin M.D., M.B.A.*; Tremper, Kevin K. Ph.D., M.D.

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In Reply:— We thank Dr. Roth for his interest in our article and insightful commentary. Previous literature has demonstrated an association between intraoperative hypothermia and cardiac adverse events. However, we did not evaluate this clinical element in our analysis for several reasons. First, previous data regarding hypothermia and cardiac adverse events is limited to high-risk patients who had a preexisting diagnosis of coronary artery disease or several known risk factors for coronary artery disease undergoing high-risk thoracic, intraperitoneal, or vascular procedures.1 Although our dataset included some high-risk patients, only 9.6% had a previous cardiac intervention and only 22% were undergoing high-risk surgery.2 As a result, the studied population was dissimilar to previous work, and we were skeptical of being able to identify an association between hypothermia and cardiac adverse events in this more representative population. Second, although our studied dataset was large, we were only able to observe 83 events. As a result, we had to limit the number of independent variables evaluated in the logistic regression full-model fit to reduce the impact of model overfitting.3 Hypothermia was one of several independent variables that we were unable to assess because of this statistical analysis constraint.
Finally, the absence of a consistent way to separate “hypothermic” versus “normothermic” groups in an observational dataset presented the final challenge. There are several ways to define hypothermia. First, we could evaluate median temperatures within 10-min epochs, similar to the presented hypotension analysis. Second, some may advocate that a single temperature measurement below 36°C would qualify as “hypothermic.” Third, others may suggest that we employ the absence of active warming to be consistent with prospective, controlled studies.
We agree that intraoperative hypothermia should be evaluated in future studies. We look forward to conducting large, multicenter observational dataset analyses that may offer us the statistical power necessary to do so.
Sachin Kheterpal, M.D., M.B.A.*
Kevin K. Tremper, Ph.D., M.D.
*University of Michigan Medical School, Ann Arbor, Michigan. sachinkh@med.umich.edu
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References

1. Frank SM, Fleisher LA, Breslow MJ, Higgins MS, Olson KF, Kelly S, Beattie C: Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events: A randomized clinical trial. JAMA 1997; 277:1127–34

2. Kheterpal S, O'Reilly M, Englesbe MJ, Rosenberg AL, Shanks AM, Zhang L, Rothman ED, Campbell DA, Tremper KK: Preoperative and intraoperative predictors of cardiac adverse events after general, vascular, and urological surgery. Anesthesiology 2009; 110:58–66

3. Harrell FE Jr, Lee KL, Matchar DB, Reichert TA: Regression models for prognostic prediction: Advantages, problems, and suggested solutions. Cancer Treat Rep 1985; 69:1071–7

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