Combined multi-metric assessment of diaphragm contractile function in healthy humans: Feasibility, validity and...

The combined use of subcostal ultrasonography and respiratory manometry represents a novel, integrative method for quantifying diaphragm contractile function (force, velocity and power). We evaluated the technical feasibility, construct validity and within-day test-retest reliability of this method during non-volitional, volitional and reflexive respiratory perturbations in healthy adults. Two independent cohorts were studied. In Experiment 1 (n = 10), diaphragm excursion (subcostal ultrasonography) and transdiaphragmatic pressure (P_di, manometry) were measured during unilateral magnetic phrenic nerve stimulation (non-potentiated and potentiated twitches, paired stimuli at 10-100 Hz) and maximal sniffs. In Experiment 2 (n = 8), the same measurements were obtained during progressive CO₂ rebreathing. All protocols were repeated after 20 min of rest. Diaphragm velocity and power were calculated as excursion/time and P_di × velocity, respectively. Ultrasound analysis was successful in >95% of cases. Potentiated twitches elicited greater P_di, excursion and power than non-potentiated twitches, with responses increasing at higher stimulation frequencies. Reliability improved with potentiation and high-frequency stimulation and was moderate to excellent for peak responses during sniffs and CO₂ rebreathing (ICC_{3, k} = 0.70-0.94) but poor for slope-based measures (ICC_{3, k} ≤ 0.20). During CO₂ rebreathing, excursion and velocity correlated strongly with inspiratory tidal volume (r = 0.83, P < 0.001) and mean inspiratory flow (r = 0.69, P < 0.001), respectively. These findings demonstrate that subcostal ultrasonography combined with manometry is a feasible, valid and reliable method for assessing diaphragm contractile function across non-volitional, volitional and reflexive perturbations. With further refinement, this integrated method has translational potential for mechanistic research and clinical application.