RELATIONSHIP BETWEEN REPEATED-MEASURES OF HEMODYNAMICS, MUSCLE SYMPATHETIC-NERVE ACTIVITY, AND THEIR SPECTRAL OSCILLATIONS

Background We determined the intraclass correlation coefficients (ICC) of repeated measures of the mean levels and variability of RR and mus cle sympathetic nerve activity (MSNA) in 7 normal subjects. We examine d whether spontaneous fluctuations in RR and MSNA over repeated measur ements were mirrored by changes in spectral components of RR and MSNA. Methods and Results Twenty-minute recordings of respiration, RR, bloo d pressure (BP), and MSNA were performed at day 1, 1 week, 1 month, an d 3 months and divided into two 10-minute periods for the analysis of short-term reliability. Comparison between these recordings also deter mined the long-term reliability. Linear regressions examined the relat ionship between changes in these measurements and changes in spectral components of RR and MSNA. AU analyses were carried out blinded to sub ject and session. Short-term ICC of RR, BP, MSNA and of the variabilit ies of RR and MSNA (in % of total power) ranged between .98 and .70 an d indicated a good short-term reliability. The long-term reliability o f RR variability was comparable to MSNA variability (range of ICC, .34 to .52). Spontaneous decreases in RR during the repeated recordings w ere accompanied by increases in sympathetic drive, as evidenced by inc reases in the ratio of low-frequency to high-frequency variability (LF /HF ratio) of RR interval (r=-.43, P<.01) and by increases in MSNA (r= -.36, P=.01). The changes in the LF/HF ratio of RR were mirrored by pa rallel changes in the LF/HF ratio of MSNA (r=+.30, P<.05). Spontaneous decreases in BP were accompanied by increases in the LF/HF ratio of M SNA (r=-.52 P=.0001). Conclusions Heart rate, MSNA, and their variabil ity are stable in the short-term, but less so over the long term. Spon taneous changes in repeated measurements of RR interval and blood pres sure over the long term are accompanied by parallel changes in the nor malized spectral components of RR and MSNA variability. Thus even over an extended period, there is a synchrony between changes in absolute cardiovascular measures and changes in their spectral components.