Relationship between venous pressure and tissue volume during venous congestion plethysmography in man

1. Venous congestion strain-gauge plethysmography enables the non-invasive assessment of arterial blood flow, fluid capacity (K-f), venous pressure (P-v) and isovolumetric venous pressure (P-vi) in man. One of the major assumptions of this technique, that of cuff pressure (P-cuff) applied to the limb equals P-v at the level of the strain gauge, was tested in this study. 2. In nine health male volunteers (mean age, 29.3 +/- 1.2 years) the saphenous vein was cannulated with an 18-gauge catheter proximal to the medial malleolus. The subjects were supine and P-v was continuously measured during the application of small step (8-10 mmHg) increases in congestion P-cuff (up to 70 mmHg). P-cuff, changes in limb circumference and P-v were recorded by computer for off-line analysis. Since the determination of K-f is influenced by changes in plasma oncotic pressure, venous blood samples were obtained at the start of the study, when P-cuff was raised to 30 mmHg and again to 65 mmHg and 4 min after deflation of the cuff. 3. The relationship between P-v and P-cuff was linear over the range of 10-70 mmHg (n=9, 69 measurements, slope 0.91, r=0.97, P << 0.001). The non-invasively measured calf P-v, based on the intercept of the relationship between the vascular compliance component (V-a) and P-cuff, was 8.0 +/- 0.4 mmHg, which was not significantly different from the corrected invasively measured P-v value of 8.8 +/- 0.3 mmHg (P=0.08). 4. Venous blood lactate and haemoglobin concentrations, as well as colloid osmotic pressure, total protein and albumin concentrations were unchanged throughout the protocol, whereas significant decreases in PO-2 and blood glucose concentration were observed when P-cuff reached 65 mmHg. Assuming a constant oxygen consumption, this may suggest a reduction in tissue perfusion. 5. This study demonstrates the close correlation between P-cuff and P-v in the saphenous vein. Since the small congestion P-cuff step protocol does not cause significant increase in plasma oncotic pressure, we conclude that P-v, as well as K-f, can be accurately determined with this venous congestion plethysmography protocol.