[Circadian fluctuations of the signal-averaged electrocardiography].
Circadian periodicity for the time of onset of acute myocardial infarction has been shown; the early morning peak of infarction coincides with the onset of other related phenomena, including sudden cardiac death, ventricular arrhythmias, thrombotic stroke, etc. Late potentials detected by the signal-averaged ECG are considered to be independent markers of vulnerability to ventricular arrhythmias. The signal-averaged ECG enables the amplifying and recording of small bioelectric signals of cardiac origin, while eliminating extraneous electrical "noise." To determine whether late potentials are themselves subject to circadian influence, 31 patients (age range 41-79) who had had an old or recent myocardial infarction underwent late potential assessment by the signal-averaged ECG. 4 indices were studied: duration of late LPD potentials (LPD), total QRS duration (TQRS), and root mean square voltage of the last 40 msec, and of the last 50 msec (RMS 40 and RMS 50). These indices were assessed 3 times, during the early morning hours, at noon and during the evening. Morning LPD differed significantly from noon and evening LPD and the morning RMS 40 similarly differed from noon and evening values. TQRS and RMS 50, even though remaining in the normal range, also showed a tendency to abnormal values during morning hours. These findings could possibly be related to the early morning incidence peaks of severe ventricular arrhythmia and sudden cardiac death, since abnormal late potentials constitute the physiopathological basis for certain ventricular arrhythmias.