Lower limb-localized vascular phenomena explain initial orthostatic hypotension upon standing from squat

Area of Science:

• Physiology
• Cardiovascular Research
• Human Physiology

Background:

• Initial orthostatic hypotension, a transient blood pressure drop upon standing, has unclear causes.
• Local vascular phenomena in leg muscles during the effort of standing are hypothesized to contribute.

Purpose of the Study:

• To test the hypothesis that initial orthostatic hypotension is caused by local vascular changes in leg muscles during the effort of standing.
• To investigate the relationship between leg vascular conductance and blood pressure changes upon rising from a squat.

Main Methods:

• Seventeen healthy young adults performed active rises from various squat durations (10s, 1min, 5min).
• Beat-by-beat arterial blood pressure, cardiac output, and femoral artery blood flow were measured using Finometer and Doppler ultrasound.
• Vascular conductance and predicted hypotension were calculated and compared to observed values.

Main Results:

• Mean arterial blood pressure significantly decreased in all squat conditions despite increased cardiac output.
• Increased leg vascular conductance upon standing was observed and correlated with the magnitude of blood pressure fall.
• The calculated hypotension based on leg vascular conductance closely matched the observed hypotension.

Conclusions:

• Rapid leg muscle dilation induced by the effort of standing is the primary cause of initial orthostatic hypotension.
• This vasodilation, potentially combined with an elevated arteriovenous pressure gradient, exceeds compensatory cardiac output responses.
• Local vascular phenomena in the legs are the established cause of transient blood pressure drops upon standing.