Objective: To evaluate the contributing roles of venous status, microvascular filtration, and calf muscle pump activity in the etiology of delayed orthostatic hypotension (OH).
Design: Unblinded within-subjects trial.
Setting: Academic clinical research center.
Participants: Convenience sample of healthy adult women (N=30) with an age range of 30 to 65 years.
Intervention: Plantar micromechanical stimulation applied at a 45-Hz frequency and a 50-microm amplitude for a duration of 30 minutes during upright sitting.
Main outcome measure: Diastolic blood pressure (DBP).
Results: White women (mean age, 51.8+/-1.3 y) were recruited and screened for delayed OH. About one quarter (9/33) of the screened subjects showed delayed OH as determined by a significant decrease in blood pressure after at least 15 minutes of quiet sitting. Air plethysmographic assessment provided no evidence of venous insufficiency (venous filling index, >2.5 mL/s; venous volume, >80 mL) or excessive microvascular filtration in the affected subjects, whereas activation of the calf muscle pump (CMP) through plantar-based micromechanical stimulation consistently resulted in a significant increase in systolic blood pressure (SBP) (DeltaSBP=22.8+/-3.9 mmHg, P=.003) and DBP (DeltaDBP=20.9+/-3.3 mmHg, P=.002).
Conclusions: About 25% of the adult women studied showed delayed OH during quiet sitting and the proximate cause appears to be neuromuscular in origin, specifically inadequate calf muscle tone, because venous and microvascular filtration status is normative in the delayed OH subpopulation and CMP stimulation reverses the hypotension.