Posturally induced leg vasoconstrictive responses: relationship to standing duration, impedance and volume changes.

Shifting legs to a gravity-dependent position provokes a physiological vasoconstrictive response that forms the basis of several diagnostic tests based on initial (< 5 min) blood perfusion decreases. However, it is not known if responses are maintained over longer duration and if they depend on the volume shifted to the limb during the manoeuvre. These issues were investigated by measurements of blood perfusion changes on foot and ankle (laser Doppler) and below-knee volume and impedance changes induced by 30 min of standing in 10 healthy volunteers. Initial perfusion decreases were 66.4% +/- 2.6% and 49.3% +/- 3.8% for ankle and foot dorsum, respectively, and were fully maintained during sustained standing without evidence of 'vasodilator escape'. Response magnitudes were not dependent on leg volume changes using geometric or impedance measures. A close correlation (r2 = 0.78) between impedance and volume changes suggests the former as a useful way of assessing dynamic limb volume changes. Sustained vasoconstrictive responses make it unlikely that extending the duration of such tests would offer more, diagnostic information than is currently available.