Distension of central great vein decreases sympathetic outflow in humans

Abstract

Classic canine studies suggested that central great vein distension evoked a reflex tachycardia (“Bainbridge reflex”). Our recent reports suggest that distension of human forearm veins evokes reflex increases in muscle sympathetic nerve activity (MSNA) and blood pressure (BP). It is unknown whether these responses are evoked by the same reflex system. We hypothesized that distension in inferior vena cava (IVC) in humans would not evoke tachycardia, but lead to a decrease in MSNA. IVC distension (echocardiography) was induced with 2‐min head‐down tilt (HDT, ~13°, N=13) and 2‐min lower body positive pressure (LBPP, 40 mmHg, N=10) in 2 visits. HDT induced IVC distension (20.6±1.3 to 23.1±1.4 mm, P<0.001), and a small increase in mean BP (Finometer, 82.4±2.4 to 84.7±2.7 mmHg, P=0.005), and a significant decrease in MSNA (14.5±2.0 to 9.3±2.0 bursts/min, P<0.001, N=10). HR did not change significantly (61.6±3.4 to 61.1±3.3 beats/min. P=0.37) during the HDT. LBPP also induced IVC distension (Δ2.4±0.2 mm, P<0.001), a small increase in BP (Δ2.0±0.7 mmHg, P<0.01), no increase in HR (P=0.34). These data suggest that central venous distension does not lead to tachycardia but a decrease in systemic sympathetic outflow in humans. Thus, central venous distension and peripheral venous distension evoke high dissimilar autonomic responses. These reflexes (peripheral vs. central venous distension) are not evoked by the same reflex mechanism. Supported by P01 HL096570 and UL1 TR000127.