Ultrasonographic evaluation of diaphragm fatigue in healthy humans

Abstract

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.

Keywords: diaphragm; fatigue; ultrasound.

FIGURE 1

Experimental overview.

FIGURE 2

Experimental set‐up for the assessment of diaphragm excursion over time during phrenic nerve stimulation (left) and CO₂ rebreathing (right).

FIGURE 3

Representative B‐mode (top) and anatomical M‐mode (bottom) ultrasound traces of the right crural hemidiaphragm at baseline (PRE) in response to single ‘twitch’ stimulation of the phrenic nerves at relaxation volume (left panel) and during the final 15 s of CO₂ rebreathing (end‐tidal partial pressure of CO₂, 55 mmHg; inspiratory minute ventilation, 32 L/min) (right panel). B‐Mode was used initially to obtain the best diaphragmatic delineation, with the inferior vena cava (IVC) as a landmark to ensure consistency in the positioning of the ultrasound transducer. In anatomical M‐mode, diaphragm excursion (y‐axis) and excursion time (x‐axis) were measured for the determination of excursion velocity, work and power.

FIGURE 4

Individual participant (open symbols and dotted lines) and group mean (filled circles and continuous line) P _di,tw in response to phrenic nerve stimulation of increasing intensity prior to MIV (a) and IRL (b). Data are for eight participants. Abbreviations: IRL, inspiratory resistive loading; MIV, maximal isocapnic ventilation; P _di,tw, twitch transdiaphragmatic pressure.

FIGURE 5

Diaphragm pressure and shortening responses to phrenic nerve stimulation at baseline and at two time points after MIV and IRL. Individual data points are shown for eight participants. Bars and whiskers represent the mean (SD), respectively. ^* P < 0.05 versus PRE. Abbreviations: MIV, maximal isocapnic ventilation; IRL, inspiratory resistive loading; P _di,tw, twitch transdiaphragmatic pressure.