Air plethysmography

Photoplethysmography and air plethysmography are simple tests designed to evaluate calf muscle dysfunction and degree of venous reflux. These techniques are used to observe the change in blood volume within the lower limb before and after exercise. Application of a tourniquet to restrict blood flow within the superficial system allows the deep venous system to be assessed for a potential obstruction. 

Plethysmography is a volume-measuring method, for example, of blood or air volumes in the lungs. Figure 10.19 illustrates a basic one-compartment model of a blood vessel segment of length L. As a one-compartment model, the electrical contribution of the vessel wall has been neglected. The resistance R of the blood volume is R = pL/A, where A is the cross-sectional area and p is the resistivity of the blood. The volume v = LA and A = v/L is put into the equation so that v = pLVR. 

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