Validation of near-infrared spectroscopy in humans

Abstract

Near-infrared (NIR) spectroscopy is a noninvasive technique that uses the differential absorption properties of hemoglobin to evaluate skeletal muscle oxygenation. Oxygenated and deoxygenated hemoglobin absorb light equally at 800 nm, whereas at 760 nm absorption is primarily from deoxygenated hemoglobin. Therefore, monitoring these two wavelengths provides an index of deoxygenation. To investigate whether venous oxygen saturation and absorption between 760 and 800 nm (760-800 nm absorption) are correlated, both were measured during forearm exercise. Significant correlations were observed in all subjects (r = 0.92 +/- 0.07; P < 0.05). The contribution of skin flow to the changes in 760-800 nm absorption was investigated by simultaneous measurement of skin flow by laser flow Doppler and NIR recordings during hot water immersion. Changes in skin flow but not 760-800 nm absorption were noted. Intra-arterial infusions of nitroprusside and norepinephrine were performed to study the effect of alteration of muscle perfusion on 760-800 nm absorption. Limb flow was measured with venous plethysmography. Percent oxygenation increased with nitroprusside and decreased with norepinephrine. Finally, the contribution of myoglobin to the 760-800 nm absorption was assessed by using 1H-magnetic resonance spectroscopy. At peak exercise, percent NIR deoxygenation during exercise was 80 +/- 7%, but only one subject exhibited a small deoxygenated myoglobin signal. In conclusion, 760-800 nm absorption is 1) closely correlated with venous oxygen saturation, 2) minimally affected by skin blood flow, 3) altered by changes in limb perfusion, and 4) primarily derived from deoxygenated hemoglobin and not myoglobin.

Estimated contribution of hemoglobin and myoglobin to near infrared spectroscopy

“…Using multiple wavelengths also allows for a more accurate correction of the path length variations that occur in tissue due to scattering. Wavelengths in the vicinity of 690-850 nm have commonly been used for measuring changes only in oxy- [Hb+Mb] and deoxy-[Hb+Mb] (Mancini et al, 1994, Chance et al, 1992, Grassi et al, 1999, Ferreira et al, 2005). Mancini et al (1994) attempted to validate NIRS use in humans using a continuous wave dual frequency (760-800nm) spectrometer.…”

“…On the one hand, it has often been assumed that both the NIRS signal at rest, and changes in the signal during exercise, are derived primarily from Hb (>90%), while Mb is thought to contribute minimally (<10%) to the signal (Mancini et al, 1994, Seiyama et al, 1987. In contrast, other studies suggest that Mb may be the predominant contributor to the NIRS signal , Richardson et al, 1995.…”

Quadriceps muscle deoxygenation during functional electrical stimulation in adults with spinal cord injury

“…Further evidence that correlates the NIRS response with femoral blood oxygenation levels 10,31 and alterations in cytochrome oxidase activity (an enzymatic marker for aerobic metabolism) 32 are needed to con®rm these ®ndings. It should be pointed out that the absolute amount of oxygen consumed by these muscles cannot be quanti®ed by NIRS because the path length of the signal is not known.…”

“…It should be pointed out that the absolute amount of oxygen consumed by these muscles cannot be quanti®ed by NIRS because the path length of the signal is not known. 10,11 Di culties associated with the scattering of photons once they penetrate the muscle tissue and the changing geometry of the muscle during exercise make this evaluation di cult.…”

“…10,11 NIRS is based on the di erential absorption properties of hemoglobin and myoglobin in the near infra-red range, ie at wave lengths between 700 and 1000 nM. At 760 nM hemoglobin is in the deoxygenated form, whereas at 850 nM it is in the oxygenated state.…”