To the Editor:
We have read with great interest the observational cohort study “Postoperative Hypotension after Noncardiac Surgery and the Association with Myocardial Injury,” by Liem et al.1 In this study the authors examined postoperative hypotension after noncardiac surgery as a risk factor for myocardial injury by defining multiple mean arterial pressure (MAP) thresholds and different characterizations of blood pressure exposures. We commend the authors for further emphasizing the association between postoperative hypotension and myocardial injury and stressing the potential benefit of postoperative continuous blood pressure monitoring. May we ask the authors to provide some additional details that will help address some concerns and will better put their findings into clinical perspective? First, the secondary outcome of 30-day all-cause mortality was not compared between patients with versus patients without myocardial injury. May we kindly ask the authors to provide baseline characteristics including 30-day all-cause mortality stratified for myocardial injury and no myocardial injury? Second, the authors concluded that postoperative duration under a MAP threshold of 75 mmHg was associated with increased risk of myocardial injury. We are concerned that the corresponding figure 3 may lead some readers to falsely interpret the results, because the association between duration under a MAP threshold of 75 mmHg and myocardial injury was only significant for a duration of more than 635 min. Additionally, for a duration of more than 635 min under a MAP threshold of 75 mmHg, CIs are gradually increasing. Moreover, when comparing duration under MAP for five different thresholds, duration under a threshold of 75 mmHg did not remain significant. Please consider highlighting alternative thresholds that might be better supported by your data. Third, previous studies have additionally adjusted for use of cardiovascular medications before surgery (i.e., angiotensin-converting enzyme inhibitor or angiotensin-receptor blocker, calcium channel blocker, β-blocker, statin, diuretics, aspirin, oral anticoagulants).2–4 We are concerned that not adjusting for preoperative cardiovascular medication may have led to an overestimation of the association between hypotension and injury or death. Please provide a sensitivity analysis adjusting for those important confounders. This will help clinicians to further understand the impact of postoperative hypotension on myocardial injury.
Competing Interests
Dr. Mueller reports grants and nonfinancial support from the Swiss National Science Foundation (Bern, Switzerland), the Swiss Heart Foundation (Bern, Switzerland), the University Hospital Basel (Basel, Switzerland), Basel University (Basel, Switzerland), the Foundation for Cardiovascular Research Basel (Basel, Switzerland), Abbott (Baar, Switzerland), Beckman Coulter (Nyon, Switzerland), Idorsia (Basel, Switzerland), Novartis (Basel, Switzerland), Ortho Diagnostics (Zug, Switzerland), Quidel (San Diego, California), Roche (Basel, Switzerland), Siemens (Zuerich, Switzerland), Singulex (Basel, Switzerland), and BRAHMS (Hennigsdorf, Germany) outside the submitted work. The other authors declare no competing interests.