Influence of leg position and environmental temperature on segmental volume expansion during venous occlusion plethysmography

Blood flow determinations by venous occlusion plethysmography applying the strain-gauge technique are frequently used. A problem with the strain-gauge technique is that the relationship between venous volume and transmural pressure is not linear and, furthermore, changes with the sympathetic tone. The present study tests the hypothesis that these factors lead to a redistribution of venous blood, which may impair the accuracy of the technique. The relative volume expansion rates of four leg segments were studied with the leg in different positions and at disparate temperatures, thereby inducing varying venous pressures and sympathetic tone (n=6). With elevated leg and relaxed veins (at 50 °C), the distal thigh showed a relatively low expansion rate (25.8±4.5 ml·min^-1·l^-1), whereas values in the calf segments were higher (34.5–39.0 ml·min^-1·l^-1). With lower initial transmural pressure, calf segments can increase their volume much more during occlusion compared with the distal thigh. In a higher transmural pressure region (lowered leg), the difference in compliance between limb segments is less. In this case, compliance and volume expansion rate was higher in the distal thigh (14.2, 13.5 and 22.2 ml·min^-1·l^-1 at 10, 20 and 50 °C respectively) than in the calf segments (for the distal calf: 6.4, 7.7 and 16.2 ml·min^-1·l^-1 respectively). There was a significant interaction (P<0.001) between temperature and leg position, indicating a higher degree of sympathetic vasoactivity in the calf. It is concluded that blood flow determination by strain-gauge plethysmography is less accurate, due to a potential redistribution of the venous blood. Therefore possible influences of variations in sympathetic tone and venous pressure must be considered even in intra-individual comparisons, especially in interventional studies.