AUTONOMIC EFFECTS ON NOISE RECORDED DURING SIGNAL-AVERAGED ELECTROCARDIOGRAPHY

The purpose of this study was to assess the effects of autonomic stimu lation and blockade on noise levels and to compare the noise measureme nts in the ST and TP segments of the signal-averaged ECG. Five-minute electrocardiographic data were recorded in 14 normal volunteers (8 mal es and 6 females; mean age 28.5 +/- 5.0 years) on two separate days (d ay 1-baseline, epinephrine infusion, isoproterenol infusion, beta-bloc kade, and combined adrenergic and parasympathetic blockade; day 2-base line, phenylephrine infusion, parasympathetic blockade, and during phe nylephrine infusion following atropine). Signal averaging was done off -line on 100 beats and noise was measured in both the ST and TP segmen ts as the standard deviation of voltage in the segment of interest. Fo r all conditions tested, the mean noise level measured in the ST segme nt (0.46 +/- 0.16 mu V) was significantly less than that measured in t he TP segment (0.52 +/- 0.24 mu V; P = 0.0003), but there was good cor relation between the noise measured in the ST and the TP segment (R-2 = 0.62, P < 0.0001). Noise increased with isoproterenol infusion and d ecreased following adrenergic blockade. In addition, day 2 baseline no ise was less than baseline noise on day 1. Finally, neither parasympat hetic stimulation or blockade nor alpha-adrenergic stimulation signifi cantly affected signal-averaged electrocardiography (SAECG) noise leve ls. Thus, the data support the notion that enhanced sympathetic tone i ncreases 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 t urn impact on the reproducibility or technical aspects of the SAECG. I n addition, because noise in She ST and TP segments are highly correla ted and the noise measured in the ST segment is less than thai in the TP segment, uniform adoption of noise measurement in the ST segment se ems most appropriate.