Respiratory ultrasonography of human parasternal intercostal muscle in vivo.

The parasternal intercostal muscle (PS) is phasically active during inspiration, but its mechanical function in humans is poorly understood. The aim of this study was to describe PS motion ultrasonographically during respiration. We used a 7.5-MHz curvilinear phased array transducer to obtain ultrasonograms of the second right and left interspace in the sagittal plane, 2-3 cm lateral to the sternum, in 4 seated subjects (3M, 1F), during tidal breathing and at residual volume (RV), functional residual capacity (FRC) and total lung capacity (TLC). Images were recorded on videotape and off-line, digitized, transferred to a workstation, and traced manually to outline the external and pleural borders of the PS muscle in relation to a rectangle bounded by the second and third ribs. To assess PS shape and motion, we measured inter-rib distance (Lics), PS thickness (Tps), and motion of the midpoint of the muscle relative to the midpoint of the reference rectangle (Mps). We also calculated the average radius of curvature of the external and pleural PS borders (Re, Rp) over the mid 50% of Lics, and 1/Re and 1/Rp. During tidal breathing, Mps moved ventrally by 0.42 +/- 0.06 mm (p = 0.001) against the pleural pressure gradient, and 1/Re and 1/Rp decreased by 1.1 x 10(-2) +/- 1.6 x 10(-3) mm-1 and 8.4 x 10(-3) +/- 1.4 x 10(-3) mm-1, respectively (p < 0.001). Lics and Tps did not change (p > 0.19). We conclude that, during inspiration, the PS moves ventrally and straightens, and lung volume, neural activation and pleural pressure influence PS shape and motion. The findings support an intercostal stabilizing function of the PS and suggest a novel mechanism by which the PS may contribute to the inspiratory fall in pleural pressure.