Understanding metabolic responses to forearm arterial occlusion measured with two-channel broadband near-infrared spectroscopy.

Significance: Broadband near-infrared spectroscopy (bbNIRS) is useful for the quantification of cerebral metabolism. However, its usefulness has not been explored for broad biomedical applications.

Aim: We aimed to quantify the dynamic responses of oxidized cytochrome c oxidase ( $\Delta \left[\mathrm{oxCCO}\right]$ ) within the mitochondria to arterial occlusion and the dynamic correlations between hemodynamic ( $\Delta \left[\mathrm{HbO}\right]$ ) and $\Delta \left[\mathrm{oxCCO}\right]$ responses during and after occlusion in forearm tissues.

Approach: We recruited 14 healthy participants with two-channel bbNIRS measurements in response to a 5-min forearm arterial occlusion. The bbNIRS system consisted of one shared white-light source and two spectrometers. The modified Beer-Lambert law was applied to determine the occlusion-induced changes in $\Delta \left[\mathrm{oxCCO}\right]$ and $\Delta \left[\mathrm{HbO}\right]$ in the shallow- and deep-tissue layers.

Results: During the 5-min occlusion, dynamic responses in hemodynamics exhibited the expected changes, but $\Delta \left[\mathrm{oxCCO}\right]$ remained constant, as observed in the 1- and 3-cm channels. A linear correlation between $\Delta \left[\mathrm{HbO}\right]$ and $\Delta \left[\mathrm{oxCCO}\right]$ was observed only during the recovery phase, with a stronger correlation in deeper tissues. The observation of a constant $\Delta \left[\mathrm{oxCCO}\right]$ during the cuff period was consistent with two previous reports. The interpretation of this observation is based on the literature that the oxygen metabolism of the skeletal muscle during arterial occlusion remains unchanged before all oxy-hemoglobin (and oxy-myoglobin) resources are completely depleted. Because a 5-min arterial occlusion is not adequate to exhaust all oxygen supply in the vascular bed of the forearm, the local oxygen supply to the muscle mitochondria maintains redox metabolism uninterrupted by occlusion.

Conclusions: We provide a better understanding of the mitochondrial responses to forearm arterial occlusion and demonstrate the usefulness of bbNIRS.