The pressure/volume relationship of the calf: a measurement of vein compliance?

OBJECTIVES: The role of compliance changes in the patho-physiology of venous disease is not well known mainly because of difficulty to measure compliance of veins in situ. This study suggests a method to determine the calf pressure/volume relationship by utilizing venous occlusion plethysmography combined with dorsal vein pressure.
DESIGN: Comparison between two techniques of measuring calf pressure/volume relationship using air plethysmography with validation against popliteal vein diameter changes detected by duplex ultrasound. SETTING Vascular laboratory. MATERIALS: In 6 normal and 6 radiographically confirmed post-thrombotic lower limbs, the calf pressure/volume relationship was determined. The dorsal vein pressure was continuously recorded. Simultaneously calf volume changes were obtained by an air plethysmograph during venous occlusion plethysmography (outflow slope coefficient) and fractionated tilting of the subject from erect to supine position (volume at 40 mmHg). During the tilt, sagittal diameter of the popliteal vein was measured (% change/mmHg = distensibility).
RESULTS: The outflow pressure/volume slope coefficient correlated significantly with the volume at 40 mmHg during tilt maneuver (r = 0.92) and the popliteal vein distensibility (r = 0.86). Variations in arterial inflow, venous outflow obstruction, or reflux did not affect the occlusion plethysmographic method. Plethysmographic changes related directly to venous volume changes, i.e. vein expansion suggesting that the pressure/volume relationship described vein compliance.
CONCLUSION: The result show a direct relationship between the pressure/volume curve of the calf and deep vein distention. Shifts of the pressure/volume curve are likely to be mainly caused by vein wall changes, but other factors (e.g. condition of surrounding tissue, reduced venous volume) may also contribute and this needs further investigation.