Changes in superficial blood distribution in thigh muscle during LBNP assessed by NIRS

INTRODUCTION: The present study was designed to determine how superficial blood distributed in the lower limb muscle during graded lower body negative pressure (LBNP). METHODS: Near-infrared spectroscopy (NIRS) was used to evaluate the blood volume change in the thigh muscles of seven volunteers during 35 min graded LBNP (rest, -10, -20, -30, -40, -50 mm Hg, and recovery). RESULTS: Deoxygenated and total hemoglobin (Hb) increased in proportion to the magnitude of LBNP applied to the thigh muscles. Oxygenated Hb rose significantly at -10 mm Hg LBNP, although the increase leveled off during subsequent increments of LBNP. Systolic pressure significantly decreased from 120 mm Hg at rest, to a value of 108 at -50 mm Hg LBNP. In contrast, mean and diastolic pressures were well maintained during graded LBNP. The increased total and deoxygenated Hb might indicate that blood was held in venous space, and the magnitude of rise in blood volume corresponded to the change in LBNP. On the other hand, oxygenated Hb change seems to reflect mainly blood accumulated in arterial space by interacting between mechanical stretch induced by LBNP and vasoconstriction caused by augmented sympathetic nerve activity. CONCLUSION: From these results, blood distribution in thigh muscles was different and was affected by the strength of LBNP. The data assessing oxygenation sites of Hb were found to be useful as indices of estimating superficial blood pooling in the muscle during LBNP.