Evaluating Near Infrared Spectroscopy Signals From Skeletal Muscle

Abstract

Near Infrared Spectroscopy (NIRS) uses the relative absorption of light at 850 and 760nm to determine skeletal muscle oxygen saturation. There are currently disagreements in the literature over how to report oxygen saturation and the relative contributions of hemoglobin and myoglobin. PURPOSE: 1) Compare the separate 850nm (HbO2) and 760nm (HHb)signals during rest, ischemia, and reperfusion, and 2) test whether electrical stimulation to increase metabolic rate changes oxygen saturation values during 5-6 minutes of complete ischemia. METHOD: Ten participants ages = 20-29 years were measured. NIRS measurements were made in the forearm flexor muscles at rest, after 30 seconds of 6 Hz electrical stimulation, during and after 5-6 minutes of complete ischemia produced by arterial occlusion preceded by 30 seconds of 6 Hz electrical stimulation. After 5-6 minutes of ischemia 30 seconds of 6 Hz electrical stimulation was performed. RESULTS: Six Hz stimulation for 30 s increased metabolic rate for HbO2 and HHb; 6.2+3.1 and 5.5+2.7 fold over rest, respectively. Six Hz stimulation during cuff ischemia did not change either the HHb or HbO2 values (p>0.05). There was significantly greater (p<0.05) pulsitility associated with heart rate in the HbO2 (0.78+0.41% range) compared to HHb (0.16+0.09% range). Pulse size increased during reperfusion; HbO2 (2.37+0.38% range) compared to HHb (0.96+0.32% range). CONCLUSIONS: The difference in the magnitude of pulsitility between HbO2 and HHb signals suggests different anatomical locations for these signals, further suggesting that using various ratios of HbO2 and HHb signals should be performed with caution. Increasing metabolic rate with electrical stimulation during complete ischemia did not further change the NIRS signals, suggesting complete ischemia is obtained with 5-6 minutes of complete cuff occlusion. Keywords: Skeletal Muscle, Near Infrared Spectroscopy (NIRS), Oxygen consumption, Hemoglobin, Myoglobin