Alterations in temporal patterns of heart rate variability after coronary artery bypass graft surgery

Preliminary studies have indicated that autonomic nervous system dysfunction may be present in patients after cardiac surgery. The purpose of this study was to evaluate cardiac autonomic nervous system function, as assessed by analysis of heart rate variability (HRV), in adult patients undergoing uncomplicated coronary artery bypass graft surgery.

Longitudinal changes in HRV were determined perioperatively by continuous electrocardiographic monitoring in 40 adult patients undergoing elective coronary artery bypass graft surgery and were compared with HRV in two groups of control subjects: 15 patients undergoing nonthoracic major vascular surgery and 19 healthy volunteers. Exclusion criteria were diabetes, renal failure, recent or perioperative myocardial infarction, or use of inotropic drugs. HRV data during electrocardiographically documented episodes of myocardial ischemia were omitted.

There were no differences in any measurement of preoperative HRV between groups during the day, but HRV was greater at night (12:00 AM to 5:00 AM) in volunteers than in patients in either surgical group. In the hour after induction of anesthesia (before cardiopulmonary bypass), the components of HRV were decreased compared with those in the preoperative daytime but were similar in the two surgical groups. After surgery, HRV in the group undergoing nonthoracic vascular surgery remained at about the same level as that observed after induction of anesthesia, whereas in the group undergoing coronary artery bypass graft surgery, HRV was further reduced and was approximately 40-50% less than that in the vascular surgery group (P < 0.05). In the coronary artery bypass group, the reduction in HRV compared with the preoperative daytime measurements persisted on postoperative day 5.

HRV is reduced after uncomplicated coronary artery bypass graft surgery. Although we cannot exclude the effects of uncontrolled variables in this reduction of postoperative HRV, the observed changes in HRV did not appear to result from general anesthesia, perioperative stress responses, and other factors associated with the early postoperative period. These data are consistent with the supposition that cardiac autonomic nervous system function is impaired after cardiac surgery.