Ultrasonographic evaluation of diaphragm fatigue in healthy humans.

Assessment of diaphragm function and fatigue typically relies on the measurement of transdiaphragmatic pressure (P(di)). Although P(di) serves as an index of diaphragm force output, it provides limited information regarding the ability of the muscle to shorten and generate power. We asked whether ultrasonography, combined with P(di), could be used to quantify changes in diaphragm function attributable to fatigue. Eight healthy men [mean (SD) age, 23 (7) years] completed two tasks on separate occasions: (i) 2 min of maximal isocapnic ventilation (MIV); or (ii) 3 × 5 min of maximal inspiratory resistive loading (IRL). Diaphragm function was evaluated before (PRE) and after each task (POST(1), 10-15 min and POST(2), 30-35 min) using synchronous recordings of P(di) and subcostal ultrasound traces of the right crural hemidiaphragm during anterolateral magnetic stimulation of the phrenic nerves and progressive CO(2) rebreathing. Fatigue was quantified as pre- to post-loading changes in twitch P(di), excursion velocity (excursion/time) and power (P(di) × velocity). Both tasks resulted in significant reductions in twitch P(di) (P < 0.05). There were no effects of MIV on ultrasound-derived measures. In contrast, IRL elicited a significant reduction in twitch excursion at POST(1) (-16%; P = 0.034) and significant reductions in excursion velocity at POST(1) (-32%; P = 0.022) and POST(2) (-28%; P = 0.013). These reductions in excursion velocity, alongside the concurrent reductions in twitch P(di), resulted in significant reductions in diaphragm power at POST(1) (-48%; P = 0.009) and POST(2) (-42%; P = 0.008). Neither task significantly altered the contractile responses to CO(2). In conclusion, subcostal ultrasonography coupled with phrenic nerve stimulation is a promising method for quantifying contractile fatigue of the human diaphragm.