EFFECT OF AUTONOMIC BLOCKADE ON POWER SPECTRUM OF HEART-RATE-VARIABILITY DURING EXERCISE

To validate power spectral analysis of heart rate variability (HRV) as an autonomic indicator during exercise, ten males performed four iden tical progressive cycling tests during infusions of saline, esmolol (b eta(1)-blocker), glycopyrrolate (muscarinic blocker), or both drugs. P ower spectra were constructed from the recorded electrocardiogram by F ourier algorithm and integrated for low-frequency power (LF) and high- frequency power (HF). Four different LF bands (0.004-0.1, 0.004-0.15, 0.05-0.1, and 0.05-0.15 Hz) and two different HF bands (0.1-1.0 and 0. 15-1.0 Hz) were evaluated. The parasympathetic index, HF, decreased ex ponentially with workload and was attenuated by glycopyrrolate and com bined treatments with both HF frequency bands measured. Whereas some s ympathetic indexes (LF/total power and LF/HF) did reflect expected inc reases in sympathetic nerve activity associated with progressive incre ases in work intensity, none of the measured increases responded appro priately to autonomic blockade. It is concluded that HRV is a valid te chnique for noninvasive measurement of parasympathetic tone during exe rcise, but its validity as a measure of sympathetic tone during exerci se is equivocal.