Analyses of coupling between cardiac and muscle pump and cardiovascular responses during standing

Abstract

The importance of the skeletal muscle pump in reducing the accumulation of blood in the veins during orthostatic stress is well recognized. We hypothesized that the increase in venous return elicited by the muscle pump requires coupling of heartbeats to muscle contraction. To address this, we simulated the mechanical muscle pump effect using rhythmic lower-leg cuff inflation (RCI) with 120 mmHg cuff pressure at three different cycles. Time series of R-R interval (RRI) and beat-to-beat blood pressure (BP) were acquired in 13 healthy subjects without or with RCI during quiet standing. The coupling strength was quantified by phase coherence (&lambda) between cuff-pressure induced heart rate variability (HRV) and cuff-pressure rhythm. Shortening of RRI during standing was suppressed by RCI and increase in &lambda was associated with the increase in parasympathetic activity estimated from HRV analysis, suggesting that coupling may play a role in an increase in venous return.