Limb capillary filtration coefficient in human subjects: the importance of the site of measurement

Capillary filtration coefficient is a critical determinant of fluid flux across the microvascular wall. Changes in capillary filtration coefficient have been described in a number of disease processes. Measurement is typically made by venous occlusion plethysmography using either the upper or lower limb, but a variety of measurement protocols have been used and the importance of the site of measurement remains unclear. In this study, forearm and calf capillary filtration coefficient were measured in healthy volunteers, either simultaneously (group A; n = 11) or sequentially in random order (group B; n = 11) using venous occlusion plethysmography, with the subject supine and the limb at heart level. In both studies capillary filtration coefficient was significantly higher when measured at the forearm than at the calf (group A: 6.1 +/- 1.0 versus 3.7 +/- 1.1 x 10(-3) ml min(-1) mmHg(-1) 100 ml(-1) (mean +/- SD), p < 0.01; group B: 5.1 +/- 1.2 versus 3.2 +/- 1.1 x 10(-3) ml min(-1) mmHg(-1) 100 ml(-1), p < 0.01). Isovolumetric venous pressure (the maximum pressure at which there is neither net filtration nor absorption at the microvascular wall) was similar in upper and lower limbs in both groups of subjects. We conclude that limb capillary filtration coefficient is dependent on the site of measurement. Caution is required when comparing data recorded at different sites even if corrected for the volume of soft tissue under study.