RELATIVE BURST AMPLITUDE IN HUMAN MUSCLE SYMPATHETIC-NERVE ACTIVITY -A SENSITIVE INDICATOR OF ALTERED SYMPATHETIC TRAFFIC

Microneurographically recorded sympathetic outflow to the human muscle vascular bed is traditionally quantified by identifying pulse-synchro nous bursts of impulses in a mean voltage neurogram and expressing the m in terms of bursts per minute (burst frequency) or bursts per 100 he art beats (burst incidence). As both these measures show large inter-i ndividual differences in resting healthy subjects, a problem arises wh en comparing sympathetic traffic in cross-sectional studies, making mo derate differences in muscle sympathetic nerve activity (MSA) between groups difficult to identify. Absolute measures of the strength of the sympathetic discharges (burst amplitude or area) can also be evaluate d. However, as they critically depend on the proximity of the microele ctrode to the recorded fibres, such measures cannot be used for interi ndividual comparisons. The aim of the present study was to evaluate th e use of relative burst amplitude spectra for quantification of MSA, d escribing the proportion of small vs large bursts in a neurogram. We r ecorded MSA in 18 patients with mild to moderate congestive heart fail ure (CHF) (New York Heart Association functional classes I-IIIA) and 1 8 matched healthy controls. Sympathetic activity was expressed as burs t frequency, burst incidence and burst amplitude spectra. When compari ng the traditional burst counts between the groups (presented as the m edian and 25th-75th percentiles) there was a tendency towards higher M SA in CHF patients, but the difference was not significant (42 (34-52) vs 53 (41-63) bursts/min, 62 (51-78) vs 69 (52-84) bursts/100 heart b eats, both ns). Relative burst amplitude spectra, on the other hand, w ere clearly shifted to the right in the CHF group compared to the cont rol group (median burst amplitudes 42 (34-45) vs 30 (28-35), P = 0.000 2). Relative burst amplitude spectra thus appear to provide a more sen sitive indicator of altered MSA than traditional burst counts. The rig ht-ward shift of these spectra may suggest that sympatho-excitation oc curs early in the development of CHF. (C) 1998 Lippincott-Raven Publis hers.