DIURNAL-VARIATIONS OF NEUROCARDIAC RHYTHMS IN ACUTE MYOCARDIAL-INFARCTION

To determine the diurnal pattern of cardiac autonomic tone in acute myocardial infarction (AMI), this study examined the power spectrum of heart rate (HR) variability in 24 patients during a single 24-hour segment within 4 days of AMI. Patients were nonrandomly allocated to a group (n = 14) without autonomic drugs and to a group (n = 10) already receiving beta blockers at the time of AMI. With use of autoregressive modelling, the power spectrum of HR variability was computed from continuous 1-hour electrocardiographic segments recorded at equally spaced intervals; 7 to 8 A.M., 3 to 4 P.M., and 11 to 12 P.M. All patients were supine, awake and pain free during recordings. There were no differences in HR, HR variance or the low-frequency peak power (0.06 to 0.1 Hz) from one temporal sequence to another. For the patients not taking beta blockers, the high-frequency peak power (0.2 to 0.36 Hz) or vagal component increased significantly from 3 P.M. to 11 P.M. (28 +/- 11 to 45 +/- 20 beats/min2.Hz-1, p < 0.01). There was a significant decrease in the low- to high-frequency peak power and area ratios from 3 P.M. to 11 P.M. All power spectral parameters in the patients taking beta blockers remained unchanged over 24 hours. There was significantly heightened vagal modulation of sinus node activity in those receiving beta blockers, especially at 7 A.M. and 3 P.M. The data suggest that under steady-state wakeful conditions in the early recovery phase after an AMI, vagal tone is more pronounced during the late evening hours with a possible shift to relative sympathetic dominance during early morning and midafternoon hours. Beta blockade appears to dampen any cyclic variation of neurocardiac rhythms as measured by the HR autospectrum.