Does the regional oxygen uptake measured by near infrared spectroscopy reflect the phase II pulmonary oxygen uptake at the onset of exercise?

It has been hypothesized that the signals of near infrared spectroscopy (NIRS) would reflect muscle O ₂ uptake (mV̇O ₂). Although it is not definite that NIRS signals accurately reflect mV̇O ₂, there is every possibility that NIRS signals at least reflect regional O ₂ uptake (rV̇O ₂). The phase II kinetics of pulmonary oxygen uptake (pV̇O ₂) is regarded as reflecting mV̇O ₂ at the onset of exercise. To examine whether the rV̇O ₂ on-kinetics measured by NIRS reflects the mV̇O ₂ on-kinetics at the onset of exercise, we compared the rV̇O ₂ as measured by NIRS with the phase II kinetics of pV̇O ₂ at the onset of exercise. Twelve healthy male subjects cycled a Monark ergometer at three different intensities: below the ventilatory threshold (VT) level (below-VT), on the VT level (on-VT), and above the VT level (above-VT), for 6 minutes on three separate occasions. The rV̇O ₂ was calculated from the concentration of oxyhemoglobin and deoxyhemoglobin, as measured by NIRS every 3 seconds. The pV̇O ₂ was determined by the breath-by-breath method. A significant relationship between the amount of increases of pV̇O ₂ and rV̇O ₂ from rest to the end of exercise among all levels of exercise intensity was found (r=0.935, P<0.001). The time constants of rV̇O ₂ (rV̇O ₂-Tc: below-VT: 6.514±2.159s, on-VT: 7.760±2.035 s, above-VT: 9.532±2.342 s) were significantly faster than the time constants of pV̇O ₂ (pV̇O ₂-Tc: below-VT: 23.8±4.4 s, on-VT: 25.9±5.1 s, above-VT: 26.3±5.7 s) (P<0.001). There was no significant relationship between rV̇O ₂-Tc and pV̇O ₂-Tc for each intensity (P>0.05). We conclude that the rV̇O ₂ on-kinetics measured by NIRS does not necessarily reflect the mV̇O ₂ kinetics at the onset of exercise.