RESPIRATORY ULTRASONOGRAPHY OF HUMAN PARASTERNAL INTERCOSTAL MUSCLE IN-VIVO

The parasternal intercostal muscle (PS) is phasically active during in spiration, 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 transduc er to obtain ultrasonograms of the second right and left interspace in the sagittal plane, 2-3 cm lateral to the sternum, in 4 seated subjec ts (3M, 1F), during tidal breathing and at residual volume (RV), funct ional residual capacity (FRC) and total lung capacity (TLC). Images we re recorded on videotape and off-line, digitized, transferred to a wor kstation, and traced manually to outline the external and pleural bord ers 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 d istance (Lies), PS thickness (Tps), and motion of the midpoint of the muscle relative to the midpoint of the reference rectangle (Mps). We a lso calculated the average radius of curvature of the external and ple ural PS borders (Re, Rp) over the mid 50% of Lies, 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 decreas ed 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). Lies and Tps did not change ( p > 0.19). We conclude that, during inspiration, the PS moves ventrall y and straightens, and lung volume, neural activation and pleural pres sure influence PS shape and motion. The findings support an intercosta l stabilizing function of the PS and suggest a novel mechanism by whic h the PS may contribute to the inspiratory fall in pleural pressure. ( C) 1998 World Federation for Ultrasound in Medicine & Biology.