Autonomic effects on noise recorded during signal-averaged electrocardiography

The purpose of this study was to assess the effects of autonomic stimulation and blockade on noise levels and to compare the noise measurements in the ST and TP segments of the signal-averaged ECG. Five-minute electrocardiographic data were recorded in 14 normal volunteers (8 males and 6 females; mean age 28.5 +/- 5.0 years) on two separate days (day 1-baseline, epinephrine infusion, isoproterenol infusion, beta-blockade, and combined adrenergic and parasympathetic blockade; day 2-baseline, phenylephrine infusion, parasympathetic blockade, and during phenylephrine infusion following atropine). Signal averaging was done off-line on 100 beats and noise was measured in both the ST and TP segments as the standard deviation of voltage in the segment of interest. For all conditions tested, the mean noise level measured in the ST segment (0.46 +/- 0.16 microV) was significantly less than that measured in the TP segment (0.52 +/- 0.24 microV; P = 0.0003), but there was good correlation between the noise measured in the ST and the TP segment (R2 = 0.62, P < 0.0001). Noise increased with isoproterenol infusion and decreased following adrenergic blockade. In addition, day 2 baseline noise was less than baseline noise on day 1. Finally, neither parasympathetic stimulation or blockade nor alpha-adrenergic stimulation significantly affected signal-averaged electrocardiography (SAECG) noise levels. Thus, the data support the notion that enhanced sympathetic tone increases noise levels and beta-adrenergic blockade may decrease noise levels, likely due to effects from muscle sympathetic nerve activity. These findings are important since the target population for the SAECG are patients with myocardial infarction and congestive heart failure, conditions associated with increased sympathetic tone, which may in turn impact on the reproducibility or technical aspects of the SAECG. In addition, because noise in the ST and TP segments are highly correlated and the noise measured in the ST segment is less than that in the TP segment, uniform adoption of noise measurement in the ST segment seems most appropriate.