Long-term alterations of heart rate dynamics after coronary artery bypass graft surgery.

We tested the hypothesis that there may be long-term alterations in overall heart rate (HR) variability and in fractal HR behavior after coronary artery bypass graft (CABG) surgery. Reduced HR variability predicts morbidity in various patient populations. Continuous 24-h electrocardiograph recordings were performed in 25 elective CABG surgery patients 1 wk before the operation and 6 wk and 6 mo after. Seventeen of the patients also had recordings 12 mo after CABG. Time and frequency domain measures of HR variability were assessed, along with measurement of short-term fractal scaling exponent (alpha1), approximate entropy, and power-law relationship of relative risk interval variability (beta-slope). The high, low, very low, and ultra low frequency powers decreased significantly after the operation and remained at a significantly decreased level 6 wk and 6 and 12 mo after the operation than before (P = 0.01, P < 0.001, P < 0.001, and P < 0.001 for overall difference between the time points, respectively). The fractal scaling exponent alpha1 was at significantly more decreased 6 wk after (P < 0.05) CABG than before surgery but recovered to the preoperative level 6 mo after the operation. Long-term fractal organization (beta-slope) remained stable, but the overall complexity (approximate entropy) decreased toward more predictable HR dynamics during the study period (P < 0.01 after 1 yr). The predictive value of temporary and persistent long-term changes of the HR dynamics after CABG surgery for long-term outcome is not clear.

Relation of heart rate dynamics to the occurrence of myocardial ischemia after coronary artery bypass grafting.

Postoperative myocardial ischemia is a common finding after coronary artery bypass grafting (CABG) and is associated with an adverse short-term clinical outcome. The reasons and pathophysiologic background for the occurrence of ischemia after CABG are not well established. We tested the hypothesis that altered heart rate (HR) behavior precedes the onset of myocardial ischemic episodes in patients after CABG. Time-domain HR variability measurements, along with analysis of Poincaré plots and fractal scaling analysis were assessed in 40 CABG patients from 48-hour postoperative Holter recordings. Twenty patients experienced 195 ischemic episodes during the postoperative course. In the univariate analysis of HR variability measurements of the first postoperative day (POD), the increased ratio between the short-term (SD1) and long-term (SD2) HR variability analyzed from the Poincaré plot and the decreased short- and intermediate-term fractal scaling exponents alpha(1) and alpha(2) were significantly associated with ischemia during the study period (p <0.01, p <0.05, and p <0.05, respectively). In the multivariate model, the increased SD1/SD2 ratio of the first POD was the most powerful independent predictor of all possible confounding variables for the occurrence of postoperative ischemia (corresponding to a change of 0.15 U; odds ratio 2.2 and 95% confidence interval 1.2 to 5.7; p <0.01). Altered HR dynamics have been associated with myocardial ischemic episodes in patients after CABG, suggesting that the autonomic nervous system has an important role in the pathogenesis of myocardial ischemia in the postoperative phase of CABG.

The breakdown of fractal heart rate dynamics predicts prolonged postoperative myocardial ischemia.

UNLABELLED: Patients with myocardial ischemia after noncardiac surgery have a three- to ninefold increased risk of adverse cardiac events. In this study we tested the hypothesis that altered preoperative heart rate variability (HRV) predicts postoperative prolonged myocardial ischemia (>10 min) in elderly surgical patients. Thirty-two patients, age 60 yr or older, admitted to hospital for surgical repair of a traumatic hip fracture with preoperative night and daytime Holter recordings were included. Holter monitoring was initiated at arrival at hospital and continued until the third postoperative morning. Conventional HRV measures along with analysis of short-term fractal scaling exponent (alpha(1)) of RR intervals were assessed for night (from 2 AM to 5 AM) and day (7 AM to 12 AM) periods in each patient. Preoperative alpha(1) was significantly lower (i.e., increased randomness in HRV) during the nighttime compared with daytime (mean +/- SEM; 0.92 +/- 0.08 versus 1.03 +/- 0.06; P = 0.002) in patients with postoperative myocardial ischemia. Patients without ischemia had no such difference. In stepwise multivariate logistic regression analysis, increased preoperative night-day difference of alpha(1) was the only independent predictor of postoperative prolonged ischemia. The odds ratio for an increase of 0.16 U in night-day difference of alpha(1) (corresponding to interquartile range) was 7.7 (95% confidence interval, 1.9-51.4; P = 0.0018). Breakdown of fractal-like heart rate dynamics is predictive for postoperative prolonged myocardial ischemia in elderly patients having emergency surgery for traumatic hip fracture. IMPLICATIONS: Night and daytime Holter recordings before surgical repair of traumatic hip fracture were analyzed with linear and nonlinear heart rate variability methods. Preoperatively increased randomness in heart rate variability was predictive for postoperative, silent prolonged myocardial ischemia. Prolonged myocardial ischemia increases the risk for adverse cardiac events.