RESUMO
Blood flow in large veins is dependent on arterial-atrial pressure gradients and pumping mechanisms in concert with valve recruitment. Classic descriptions of muscle and respiratory pumps describe venous transmural pressure changes that cause flow. Not often considered is the transmission of pulsatile energy from arteries to veins directly adjacent to each other. Recently, an ex vivo study demonstrated a novel arterial pump effect in venoarterial bundles when valves were active in managing venous flow. We sought to show in vivo evidence of this arterial pump mechanism in 16 healthy young adults. Venous blood flow was measured in the venoarterial bundled deep femoral vein (DFV) and the greater saphenous vein (GSV), which is not bundled with an artery. Veins were studied through randomized body positions of -6° head-down tilt (HDT), supine, 20° head-up tilt (HUT), and 40° HUT, with the assumption that greater HUT postures increased valve dependence to observe the arterial pump effect. Between 20° and 40° HUT conditions, bundled DFV blood flow did not change (68 ± 36 vs. 71 ± 56 mL·min-1; Padj > 0.99), whereas nonbundled GSV blood flow decreased (6.1 ± 4.8 vs. 3.5 ± 3.9 mL·min-1; P = 0.01). Diameters between 20° and 40° HUT conditions increased in DFV (0.90 ± 0.16 vs. 1.04 ± 0.19 cm; P < 0.01), but not in GSV (0.33 ± 0.10 vs. 0.32 ± 0.08 cm; P = 0.60). These data support previous ex vivo observations that when venous pressure gradients rely on valve recruitment, presence of an adjacent artery may protect against further decreases in blood flow. The arterial pump mechanism is an underappreciated contributor to venous return and warrants further investigation.NEW & NOTEWORTHY Venous return mechanisms have classically considered muscle and respiratory pumps; however, recent ex vivo evidence suggests that pulsatile energy imparted from arteries to adjacent bundled veins can increase venous flow under certain driving pressures. We tested this concept in humans by manipulating hydrostatic pressures and measuring flow in bundled and nonbundled veins. The bundled vein exhibited flow preservation at the highest hydrostatic pressure. We suggest a novel conservation of energy mechanism within the circulatory system.