Comparisons Of Single And Multi-probe Near Infrared Spectroscopy Signals During Progressive Treadmill Exercise

Abstract

Over the past few years, near infrared spectroscopy (NIRS) has become a popular technique for the non-invasive determination of local oxygen saturation and blood flow in human skeletal muscle. PURPOSE The purpose of this study was to determine if any differences exist between two NIRS systems using different methods of signal sampling during progressive treadmill exercise. The Inspectra model 325 tissue spectrometer (Hutchinson Technology Inc.) measures a scaled single depth second derivative optical attenuation value which is empirically calibrated to percent hemoglobin oxygen saturation, thereby estimating percent tissue oxygen saturation (%StO2). The NIRO-300 oxygenation monitor (Hamamatsu) spatially resolves light attenuation at multiple photodiode segments (multiple send to receiver distances) to monitor the attenuation slope and thus determine a tissue oxygen index (TOI) from a light transport theoretical equation. METHODS 10 healthy subjects (age=24±1 yrs) exercised on treadmill to voluntary fatigue. Resting data were recorded for 3 min supine and 3 min sitting prior to exercise. At the completion of exercise 5 min recovery data were also recorded. %StO2 and TOI were measured continuously. Arterial saturation (%SpO2 from the finger), along with HR and RPE was also measured at the end of each stage. RESULTS As expected, HR and RPE increased with progressive stages of exercise, while measures of %SpO2 did not change throughout the test. Reviewing only a few seconds of data from each system, we saw greater fluctuation in TOI (+5%), perhaps reflecting subtle variations in muscle blood flow, where StO2 values were more stable (+1%), perhaps due to conditioning of the signal. When reviewing changes throughout an entire graded TM test, TOI had a small range from rest to maximal exercise (∼20%). StO2 values were more robust (∼60%) perhaps reflecting more accurate changes in oxygen saturation in working muscle. Both %StO2 and TOI had a significant relationship with HR, RPE and estimated oxygen consumption (rs>0.8). CONCLUSION The NIRO-300 (TOI) may provide subtle changes in tissue oxygen saturation, but the Inspectra (%StO2) may be more sensitive to overall changes in oxygen saturation than the NIRO-300. In either case, there is a strong relationship in metabolic parameters and NIRS data from both systems.