Noninvasive Assessment of Neuromechanical Coupling and Mechanical Efficiency of Parasternal Intercostal Muscle during Inspiratory Threshold Loading

This study aims to investigate noninvasive indices of neuromechanical coupling (NMC) and mechanical efficiency (MEff) of parasternal intercostal muscles. Gold standard assessment of diaphragm NMC requires using invasive techniques, limiting the utility of this procedure. Noninvasive NMC indices of parasternal intercostal muscles can be calculated using surface mechanomyography (sMMG_para) and electromyography (sEMG_para). However, the use of sMMG_para as an inspiratory muscle mechanical output measure, and the relationships between sMMG_para, sEMG_para, and simultaneous invasive and noninvasive pressure measurements have not previously been evaluated. sEMG_para, sMMG_para, and both invasive and noninvasive measurements of pressures were recorded in twelve healthy subjects during an inspiratory loading protocol. The ratios of sMMG_para to sEMG_para, which provided muscle-specific noninvasive NMC indices of parasternal intercostal muscles, showed nonsignificant changes with increasing load, since the relationships between sMMG_para and sEMG_para were linear (R² = 0.85 (0.75-0.9)). The ratios of mouth pressure (P_mo) to sEMG_para and sMMG_para were also proposed as noninvasive indices of parasternal intercostal muscle NMC and MEff, respectively. These indices, similar to the analogous indices calculated using invasive transdiaphragmatic and esophageal pressures, showed nonsignificant changes during threshold loading, since the relationships between P_mo and both sEMG_para (R² = 0.84 (0.77-0.93)) and sMMG_para (R² = 0.89 (0.85-0.91)) were linear. The proposed noninvasive NMC and MEff indices of parasternal intercostal muscles may be of potential clinical value, particularly for the regular assessment of patients with disordered respiratory mechanics using noninvasive wearable and wireless devices.