Continuous measurements of respiratory muscle blood flow and oxygen consumption using non-invasive 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 in patients. Frequency domain near-infrared spectroscopy and diffuse correlation spectroscopy (FD-NIRS & DCS) can provide continuous surrogate measures of blood flow index (BFi) and metabolic rate of oxygen consumption (MRO2). This study aimed to validate sternocleidomastoid (SCM) FD-NIRS & DCS outcomes against electromyography (EMG) and mouth pressure (Pm) during incremental inspiratory threshold loading (ITL). Six females and six male healthy adults (30±7 years, maximum inspiratory pressure 118±61 cmH2O) performed ITL with 50g increments every two minutes until task failure. FD-NIRS & DCS continuously measured SCM oxygenated and deoxygenated hemoglobin+myoglobin (oxy/deoxy[Hb+Mb]), tissue saturation of oxygen (StO2), BFi and MRO2. Ventilatory parameters (e.g., Pm) were also evaluated. Pm increased during incremental ITL (P<0.05), reaching -47[−74, -34] cmH2O at task failure. Ventilatory parameters were constant throughout ITL (P>0.05). SCM BFi and MRO2 increased from the start of the ITL (P<0.05). Deoxy[Hb+Mb] increased close to task failure, concomitantly with a plateau in BFi, a constant increase in MRO2, and decreased StO2. SCM deoxy[Hb+Mb], BFi, StO2 and MRO2 correlated with sternocleidomastoid EMG (P<0.05 for all). Increasing SCM oxygen consumption near task failure was associated with increased oxygen extraction while delivery plateaued and reduced StO2