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-synchronous bursts of impulses in a mean voltage neurogram and expressing them in terms of bursts per minute (burst frequency) or bursts per 100 heart beats (burst incidence). As both these measures show large inter-individual differences in resting healthy subjects, a problem arises when comparing sympathetic traffic in cross-sectional studies, making moderate 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 evaluated. However, as they critically depend on the proximity of the microelectrode to the recorded fibres, such measures cannot be used for interindividual comparisons. The aim of the present study was to evaluate the use of relative burst amplitude spectra for quantification of MSA, describing the proportion of small vs large bursts in a neurogram. We recorded MSA in 18 patients with mild to moderate congestive heart failure (CHF) (New York Heart Association functional classes I-IIIA) and 18 matched healthy controls. Sympathetic activity was expressed as burst frequency, burst incidence and burst amplitude spectra. When comparing the traditional burst counts between the groups (presented as the median and 25th-75th percentiles) there was a tendency towards higher MSA 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 beats, both ns). Relative burst amplitude spectra, on the other hand, were clearly shifted to the right in the CHF group compared to the control group (median burst amplitudes 42 (34-45) vs 30 (28-35), P = 0.0002). Relative burst amplitude spectra thus appear to provide a more sensitive indicator of altered MSA than traditional burst counts. The right-ward shift of these spectra may suggest that sympatho-excitation occurs early in the development of CHF.