Venous tone evaluation by elastic modulus and therapeutic implications.

The venous system plays an important role in the determination of cardiac output and venous return. Venous tone is a property of the venous system which reflects the venous resistance and pressure. An increased venous tone caused by venoconstriction leads to an increased net capillary filtration by increasing the venous resistance and venous pressure. Use of a phlebotonic agent to increase the venous tone would prove useful in treating venodilation associated with worsening venous insufficiency. Various methods have been used to investigate the venous tone and these include: 1. isolated vein segment technique; 2. determination of the mean circulatory filling pressure; 3. constant cardiac output-reservoir techniques; 4. venous occlusion plethysmography to assess pressure-volume relationship. These methods have their inherent limitations and their suitability for determining the effects of drugs on the venous tone is sometimes in doubt. Venous occlusion plethysmography using strain gauge plethysmography and measurement of venous pressures with the aid of a needle inserted into a vein on the dorsum of the foot have been used to assess the function of the vein wall by quantifying the elastic modulus <>. The elastic modulus is calculated from the change in pressure and the corresponding change in volume which are measured from the linear high pressure part of the pressure volume curve. Results obtained using this method show a clear difference in elasticity between normal limbs (K = 17.24) and limbs with varicose veins (K = 1.26) and deep venous incompetence (K = 1.21). The limbs of normal individuals that are at a high risk of developing varicose veins have an abnormal K(K = 1.33). This method is however invasive and we have developed a non-invasive method of calculating the elastic modulus <> using air plethysmography. This method has shown a good reproducibility in 3 healthy volunteers examined and this technique can be used to study the effect of a phlebotonic agent on the venous tone in patients at high risk of developing venous insufficiency.