Postoperative myocardial injury is correlated with increasing resting preoperative heart rate in noncardiac surgery patients, according to a new study. Interestingly, the researchers concluded that the relationship may be J-shaped instead of linear, demonstrating that abnormally low heart rates may be potentially dangerous, too.
“One of the theories about myocardial injury is that heart rate, being one of the key determinants of myocardial oxygen supply and demand, is a crucial component,” said Karim Ladha, MD, assistant professor of anesthesia at the University of Toronto. “A recent study [Br J Anaesth 2016;117:172-811] showed that preoperative heart rate was indeed associated with myocardial injury after noncardiac surgery. However, the problem with this analysis was that it defined preoperative heart rate [as] immediately before the induction of anesthesia, meaning it is completely susceptible to the patients either being sedated or anxious. Our goal was to replicate that study using ambulatory vital sign data.”
Dr. Ladha explained that a clearer understanding of this relationship may help determine appropriate targets for perioperative heart rate reduction with negative chronotropic drugs, such as beta blockers.
Elective Noncardiac Surgery
The researchers reviewed the records of 41,138 patients undergoing elective noncardiac surgery at the multiple-site health care system between 2008 and 2014. Data on preoperative heart rate were obtained during the outpatient preoperative clinic visit and categorized into less than 60, 60-69, 70-79, 80-89 and 90 or more beats per minute.
“We excluded patients with heart rates less than 40 or greater than 140, because we felt that these were likely errors and beyond the normal physiologic range,” Dr. Ladha said. For purposes of the analysis, myocardial injury was defined as a peak postoperative troponin greater than 0.03 ng/mL or an ICD-10 code indicating a post-admission myocardial infarction.
The investigators also performed multivariable logistic regression modeling to determine the association between heart rate and myocardial injury after adjustment for various factors. Covariates included age, sex, ASA physical status, surgical procedure, comorbidities (including coronary artery disease, heart disease, diabetes, anemia and smoking), and preoperative medications (including beta blockers, non-dihydropyridine calcium channel blockers, aspirin, angiotensin-converting enzyme inhibitors and statins). Sensitivity analyses were also performed to determine the robustness of the results.
As Dr. Ladha reported at the 2017 annual meeting of the Canadian Anesthesiologists’ Society (abstract 286509), 4,856 patients (11.8%) experienced a myocardial injury. Although the researchers found no clinically significant unadjusted difference in median heart rates between patients with and without myocardial injury, patients with preoperative heart rates of 90 or more beats per minute had a significantly higher adjusted odds of myocardial injury than those with heart rates of less than 60 beats per minute (odds ratio, 1.21; 95% CI, 1.06-1.39; P=0.005). This result was consistent across all sensitivity analyses.
“We also performed fractional polynomial analysis, which accounts for any nonlinear association between the exposure and the odds of the outcome,” Dr. Ladha said. “We found a J shape to the curve, suggesting that a very low heart rate may be too much of a good thing.”
Dr. Ladha explained that the analysis, like all retrospective analyses, had limitations. “Only 15% of patients had their troponins measured and that may bias the results. And when we look at a single heart rate, we’re really looking at a static measure of a patient’s physiology. Whether that’s actually a good measure of prediction is difficult to say, but we likely need more complicated models that include the interaction of several measures to predict outcomes.”
Future research, he added, can confirm these findings, particularly with studies that prospectively monitor heart rate for longer periods during the preoperative period.
Session co-moderator Lucie Filteau, MD, assistant professor of anesthesiology at the University of Ottawa, in Ontario, commented, “Your rate of myocardial injury seemed high in a population that wasn’t particularly at risk. Do you have any ideas as to why that might be?”
“Our rate was approximately 11%, which is fairly consistent with the VISION study [JAMA 2012;307:2295-2304] and other trials where troponin was measured in all patients,” Dr. Ladha replied. “I think part of it is the institution. Being a tertiary center, we care for sicker patients and have a high proportion of vascular surgery patients, who likely have some underlyin g coronary artery disease.
“Believe it or not, though, some studies quote the myocardial injury rate at 20%,” he added. “One of the theories is that this is an underrecognized problem; and since we don’t measure troponin on everybody, we just don’t know that it exists.”
Photo courtesy of: Anesthesiology News
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