Capillary filtration in the lower limb ‐ a confounder in the study of venous compliance?

Objective Recent studies have suggested gender differences in venous compliance of the lower limb. Capillary filtration could however be a confounder in the compliance calculations. The present study was designed to evaluate the effect of net capillary filtration in calculating venous compliance. Design and methods 12 females and 16 males (22.9±0.5 and 23.2±0.5 years) were included. Lower body negative pressure (LBNP) of 15, 30 and 60 cm H 2 0 was used to create a defined transmural pressure gradient in the lower limb. Plethysmographic technique was used on the calf to assess venous capacitance as well as net capillary filtration. Venous compliance was calculated with aid of a quadratic regression equation [(Δ limb volume) = β 0 + β 1 * (transmural pressure) + β 2 * (transmural pressure) 2 ]. Results A greater venous capacitance was seen in males than in females (p<0.05), and β 1 and β 2 was higher in males than in females indicating a higher compliance in males, p<0,05. Net capillary filtration however, was greater in females than in males, p<0.05. However, when the total volume increase (capacitance plus filtration) was put in the equation, the gender difference in venous compliance disappeared. Conclusion Males have a higher venous compliance in the lower limb than females. The net capillary filtration in females however is higher, masking the gender differences in compliance, emphasizing the importance to separate venous capacitance from net capillary filtration when evaluating venous compliance.