Human calf microvascular compliance measured by near-infrared spectroscopy

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Abstract
The purpose of this study is to develop a new method for the measurement in humans of the compliance of the microvascular superficial venous system of the lower limb by near-infrared spectroscopy (NIRS). This method is complementary to strain-gauge plethysmography, which does not allow compliance between deep and superficial venous or between venous and arterial compartments to be distinguished. In practice, hydrostatic pressure (P) changes were induced in a calf region of interest by head-up tilt of the subject from α = −10 to 75°. For P ≤ 24 mmHg, the measured compliance [0.086 ± 0.005 (SD) ml ⋅ l−1 ⋅ mmHg−1] based on NIRS data of total, deoxygenated, and oxygenated hemoglobin, reflects essentially that of the superficial venous system. For P ≥ 24 mmHg, no distinction can be made between arterial and venous volumes changes. However, by following the changes in oxy- and deoxyhemoglobin in the P range −16 to 100 mmHg, it appears to be possible to assess the characteristics of the vasomotor response of the arteriolar system.