Air plethysmography

Air plethysmography indirectly measures changes in leg volume by quantifying air displacement within a polyurethane cuff wrapped around the calf.
(A) The test begins with the patient supine. The limb being evaluated is elevated to drain the venous system. The cuff is placed on the lower leg and inflated to a low pressure to calibrate the equipment.
(B) The patient is asked to stand, which fills the veins. The maximum volume achieved is the functional venous volume (VV). The time needed to fill 90 percent of the functional venous volume (90%VV) is the venous filling time (VFT90). The venous filling index (VFI) is 90%VV/VFT90.
(C) With the limb filled with blood, the patient is asked to dorsiflex at the ankle (toe-up) which ejects blood from the leg (measured as the ejection volume [EV]). The ejection fraction (EF) is the ratio of ejection volume to the functional venous volume (EV/VV x 100%).
(D) Next, the patient is asked to perform a series of consecutive dorsiflexion movements (toe-ups), which empties blood from the leg allowing the calculation of the residual volume (RV). The residual volume fraction is the ratio of the residual volume to the functional venous volume (RV/VV x 100%).
(E) With the limb dependent and resting, the leg refills with blood.