Mechanism of venous valve closure and role of the valve in circulation: a new concept.

Abstract

The purpose of this study was to investigate the blood flow changes and venous wall movements that occur in the perivalvular area during venous flow, to learn how these physiologic events influence the movements of the valve cusps, and to learn how the movements of the valve cusps influence the venous flow. Twenty healthy volunteers (10 male, 10 female, age 18 to 52) were subjects of this study. Each volunteer was examined in semi-recumbent and standing positions at rest and during active foot movements. Ultrasound examinations were performed in the B-flow mode supplemented by B-mode and pulsed-wave Doppler scanning. Four phases of the valve cycle are described. During the opening phase (0.27 +/- 0.05 s), the cusps move from the closed position toward the sinus wall. After reaching a certain point, the valves cease opening and enter the equilibrium phase. During this phase (0.65 +/- 0.08 s), the leading edges remain suspended in the flowing stream and undergo self-excited oscillations with an amplitude of 0.01 to 0.16 cm. During the closing phase (0.41 +/- 0.07 sec), the leaflets move synchronously toward the center of the vein. The subsequent closed phase has a duration of 0.45 +/- 0.05 seconds when the cusps remain closed. During the equilibrium phase, flow separation occurs at the leading edge of the cusp with reattachment at the wall of sinus. At this point, flow splits into two streams at each valve cusp. Part of the flow is directed into the sinus pocket behind the valve cusp, forming a vortex along the valve cusp before re-emerging in the main stream in the vein. When the valve is maximally open, the two cusps create a narrowing of the lumen about 35% smaller than the vein distal to the valve. In this narrowed area flow accelerates, forming a proximally directed jet. The valve cusps undergo the four phases constituting the valve cycle. The local hemodynamic events, such as flow separation and reattachment, and vortical flow in the sinus play important roles in the valve operation. In addition to prevention of retrograde flow, the valve acts as a venous flow modulator. The vortical stream behind the valve cusps participates in the operation of the valve, and prevents stasis inside the valve pocket. The central jet possibly facilitates outflow.