Human calf microvascular compliance measured by near-infrared spectroscopy

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 comple mentary to strain-gauge plethysmography, which does not allow compliance be tween deep and superficial venous or between venous and arterial compartmen ts 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 al pha = -10 to 75 degrees. For P less than or equal to 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 tha t of the superficial venous system. For P greater than or equal to 24 mmHg, no distinction can be made between arterial and venous volumes changes. Ho wever, 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.