Continuous measurements of respiratory muscle blood flow and oxygen consumption using noninvasive frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy.

Prior studies of muscle blood flow and muscle-specific oxygen consumption have required invasive injection of dye and magnetic resonance imaging, respectively. Such measures have limited utility for continuous monitoring of the respiratory muscles. Frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy (FD-NIRS & DCS) can provide continuous surrogate measures of blood flow index (BF) and metabolic rate of oxygen consumption (MRO). This study aimed to validate sternocleidomastoid FD-NIRS & DCS outcomes against electromyography (EMG) and mouth pressure (Pm) during incremental inspiratory threshold loading (ITL). Six female and six male healthy adults (means ± SD; 30 ± 7 yr, maximum inspiratory pressure 118 ± 61 cmHO) performed incremental ITL starting at low loads (8 ± 2 cmHO) followed by 50-g increments every 2 min until task failure. FD-NIRS & DCS continuously measured sternocleidomastoid oxygenated and deoxygenated hemoglobin + myoglobin (oxy/deoxy[Hb + Mb]), tissue saturation of oxygen (StO), BF, and MRO. Ventilatory parameters including inspiratory Pm were also evaluated. Pm increased during incremental ITL ( < 0.05), reaching -47[-74 to -34] cmHO (median [IQR: 25%-75%]) at task failure. Ventilatory parameters were constant throughout ITL (all > 0.05). Sternocleidomastoid BF and MRO increased from the start of the ITL (both < 0.05). Deoxy[Hb + Mb] increased close to task failure, concomitantly with a constant increase in MRO, and decreased StO. Sternocleidomastoid deoxy[Hb + Mb], BF, StO, and MRO obtained during ITL via FD-NIRS & DCS correlated with sternocleidomastoid EMG (all < 0.05). In healthy adults, FD-NIRS & DCS can provide continuous surrogate measures of respiratory BF and MRO. Increasing sternocleidomastoid oxygen consumption near task failure was associated with increased oxygen extraction and reduced tissue saturation. This study introduces a novel approach, frequency-domain near-infrared spectroscopy and diffuse correlation spectroscopy (FD-NIRS & DCS), for noninvasive continuous monitoring of respiratory muscle blood flow and metabolic rate of oxygen consumption. Unlike prior methods involving invasive dye injection and magnetic resonance imaging, FD-NIRS & DCS offers the advantage of continuous measurement without the need for invasive procedures. It holds promise for advancing muscle physiology understanding and opens avenues for real-time monitoring of respiratory muscles.