Reproducibility of conventional and power spectral measurements of cardiovascular sympathetic activation in normal subjects.

1. Normal subjects (n = 5; age 20-42 years; mean resting blood pressure (+/- 1 s.d.) 116 +/- 21/61 +/- 11 mmHg) underwent cardiovascular reflex testing five times each. On every occasion systolic blood pressure (SBP) responses to sustained handgrip (GRIP) and cold pressor (COLD) tests were measured and continuous non-invasive SBP and heart period (RRINT) data were analysed in the frequency domain using fast Fourier transforms. Power spectral (PS) density estimates of high frequency/total power (HF%; a measured of vagal activity), low frequency/HF ratio (LF/HF; a measure mainly of cardiovascular sympathetic activity for heart period) and low frequency/total power (LF%; a proposed measure of sympathetic activity for SBP) at rest, during and 2 min after the end of stimuli were calculated. 2. The data from the rest and recovery periods did not differ and showed that cardiovascular recovery to baseline measures following sympathetic stimulation occurred within 2 min. 3. There was a significant rise in SBP with GRIP and COLD. The LF/HF(RRINT) rose significantly with GRIP, but not with COLD. The LF%(SBP) did not change significantly with GRIP or COLD. 4. The SBP and PS analyses showed low intra-individual reproducibility of responses to reflex tests, with coefficients of variation for PS measures at rest of 25-41% and on sympathetic stimulation of up to 80%. 5. The high variability of these observations indicates that PS methods may not be suitable for the analysis of transient cardiovascular reflexes