起立時の循環系調節に及ぼす筋ポンプと呼吸ポンプの相互作用

抄録

The skeletal muscle pump and the respiratory pump play important adjunctive roles in preventing excessive orthostatic venous pooling by promoting venous return, but the nature of their interactions remains unclear. This study sought to examine the interaction between muscle pump and respiratory pump functions and cardiovascular and autonomic responses to orthostatic stress. Thirteen healthy volunteers (11 males and 2 females, aged 22 ± 0.8 years) participated in the study. After 3 min of sitting (SIT) followed by 3 min of quiet standing (QS), the subject performed repetitive heel up (HUP) maneuvers at 10 cycles/min for 6 min while standing. HUP was initiated either at the onset of inspiration (Insp) or at the onset of expiration (Exp). Respiratory flow, R-R interval (RRI), and beat-to-beat blood pressure (BP) were determined using a hot-wire flowmeter, electrocardiography, and a Finapres device, respectively. From the continuous BP measurement, beat-by-beat stroke volume (SV) was estimated using the pulse-contour method and cardiac output (CO) was calculated as SV multiplied by heart rate. Using spectral analysis, RRI variability was analyzed for the low- (LF) and high-frequency (HF) bands. The following three periods were analyzed separately : 2 min of steady state SIT (0-2 min from the start of recording), 2 min of QS (1-3 min from the start of standing), and the final 2 min of HUP. Reductions in SV and CO during QS were suppressed by HUP (p < 0.01) in both Exp and Insp conditions. Systolic BP was increased during HUP compared with QS under Exp condition (p < 0.01). HUP in the Insp condition led to an increase in HF power (p < 0.01) and a decrease in LF-to-HF ratio (p < 0.01) compared to Exp, indicating a shift of the sympathovagal balance toward parasympathetic activation. In contrast, the sympathovagal balance shifted towards sympathetic predominance in the Exp condition. These results suggest that cardiovascular and autonomic responses are influenced by the timing of HUP with respect to breathing during standing.