Venous compliance (Vc) reflects the elastic properties of the venous wall and altered Vc affects venous return as well as hemodynamic stability. Venous occlusion plethysmography (VOP) is used to measure Vc in the limbs. However, capillary filtration from blood to tissue could be a potential confounder. We conducted a series of studies to validate VOP in lower limb. A method was developed to identify fluid filtration and to evaluate whether this is a significant confounder in the study of Vc.

Strain-gauge technique was used to study calf volume changes in 15 healthy females (22.9±3.2 years). A thigh cuff was inflated to 60 mmHg for 8 min with a subsequent linear decrease of 1 mmHg/s in cuff pressure (P). Intravenous pressure (IVP) was simultaneously measured in a foot vein. Vc was determined using the first derivate of a quadratic regression equation describing the volume-pressure relationship [Compliance=β1+2∙β2∙(P)]. The capillary filtration was subtracted from the volume curve to correct for the potential effect on Vc.

The increase in IVP showed 100% transmission and steady state was reached within 3–4 min. The following decrease of 1 mmHg/s in cuff pressure correlated well with IVP reduction (r=0.99, P<0.001). The volume increase during VOP was augmented further by approximately 60% due to capillary filtration. Without correction of capillary filtration Vc was underestimated with the most marked differences ≥30 mmHg (P<0.01).

Capillary filtration is a confounder in the study of limb Vc. The new model seems to be a valuable tool in the future studies of venous wall function as well as hemodynamic regulation.