Interplay between diaphragm thickening, force generation and lung volume change: a physiological study in health and diaphragm weakness

INTRODUCTION: Diaphragm ultrasound is increasingly used to assess diaphragm function. Diaphragm thickening (DTF) is widely interpreted as a surrogate of diaphragm force generation. However, the independent contributions of force generation and lung volume change to diaphragm thickening remain unclear. This study aimed to determine the interplay between DTF, force generation and lung volume change. METHODS: Healthy participants and patients with a congenital myopathy performed unloaded concentric contractions, isometric contractions and loaded concentric contractions. Mouth pressure (Pmo), volume and DTF were recorded and transdiaphragmatic pressure (Pdi) was measured in healthy participants. Leakage during isometric efforts was modelled to assess its impact on lung volume change. RESULTS: 20 healthy participants and 21 patients completed unloaded concentric contractions and isometric contractions. During isometric contractions, Pmo and Pdi increased without an increase in DTF in either group (5.2%; -13.5-23.9% to 5.0%; -12.6-22.4%, p=1.000 and -8.3%; -26.6-10.0% to -15.8%; -33.0-1.3%, p=0.999, respectively). However, DTF increased during unloaded concentric contractions (21.9%; 1.2-42.6% to 105.4%; 88.1-122.7%, p<0.001 and 7.9%; -12.1-27.9% to 84.5%; 67.6-101.5%, p<0.001, respectively). In an additional cohort (21 healthy participants and 21 patients) DTF increased with increasing lung volume (16.5%; 4.2-28.7%; to 134.9%; 123.1-146.7%, p<0.001 and 7.4%; -7.0-21.8% to 49.5%; 36.0-63.1%, p<0.001, respectively), but did not differ across inspiratory loading up to 50% of maximal inspiratory pressure. Modelling demonstrated that leakage during isometric efforts may induce lung volume changes sufficient to increase DTF. DISCUSSION: In physiological breathing, diaphragm contraction results in diaphragm thickening and lung volume increase. Diaphragm thickening relates to lung volume change associated with muscle shortening during muscle contraction rather than force generation per se developed by the contraction.