The effects of sympathetic stimulation induced by mental stress on signal-averaged electrocardiogram

We evaluated the influence of sympathetic stimulation, induced by mental stress test, on signal-averaged ECG in 30 healthy subjects and in 30 patients with previous myocardial infarction. Both patient and control groups underwent three consecutive signal-averaged ECG: under basal conditions, during a continuous mental stress test, and 5 min after the mental stress test. The signal-averaged ECG parameters, analyzed by mean of a bandpass filter of 25-250 and 40-250 Hz, were: the filtered QRS duration, the duration of the low-amplitude high frequency signals in the terminal portions of the QRS complex below 40 microV (HFLA), and the root mean square voltage in the last 40 ms of the filtered QRS (RMS). The mathematical stress test was considered effective when the heart rate increased by at least 15 beats/min, remaining stable for the total duration of the signal-averaged ECG acquisition. A significant reduction of the filtered QRS duration and HFLA with a simultaneous increase of RMS were found in control group, while no significant changes occurred in patient group. These results indicate that the sympathetic nervous system may affect the results of signal-averaged ECG, probably by influences on intraventricular conduction, in healthy subjects. On the contrary, myocardial alterations in ischemic heart disease induce modification of signal-averaged ECG not influenced by autonomic nervous system activity.