A new method of quantifying human muscle sympathetic nerve activity for frequency domain analysis

We present a new method for quantitative analysis of muscle sympathetic nerve activity (MSNA), expressed as muscle sympathetic burst area (MSBA). This technique is likely to be useful for inter-individual comparisons and for frequency domain analysis of MSNA. After standardization of MSNA so that the burst of highest amplitude in the integrated MSNA trace was 1000 units, MSNA was assessed by measuring the area of each burst in the integrated MSNA trace, with baroreflex latency of about 1.3 sec while triggered by consecutive R-waves of the ECG. We examined the relationship between MSBA and burst rate (burst number/min) or plasma norepinephrine levels at rest in 50 healthy subjects, aged 23-82 years. MSBA showed positive correlations with burst rate (r = 0.91, n = 50) and plasma levels of norepinephrine (r = 0.64, n = 22). During head-up tilting in 6 subjects, MSBA showed linear correlations with sine values of tilt angles and with plasma norepinephrine levels. These results suggest that MSBA is a useful index of MSNA for evaluating both intra-individual and inter-individual variations of MSNA.