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 (POST₁, 10-15 min and POST₂, 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 CO₂ 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 POST₁ (-16%; P = 0.034) and significant reductions in excursion velocity at POST₁ (-32%; P = 0.022) and POST₂ (-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 POST₁ (-48%; P = 0.009) and POST₂ (-42%; P = 0.008). Neither task significantly altered the contractile responses to CO₂. In conclusion, subcostal ultrasonography coupled with phrenic nerve stimulation is a promising method for quantifying contractile fatigue of the human diaphragm.