Diaphragm thickening during inspiration

Abstract

Cohn, David, Joshua O. Benditt, Scott Eveloff, and F. Dennis McCool. Diaphragm thickening during inspiration. J. Appl. Physiol. 83(1): 291–296, 1997.—Ultrasound has been used to measure diaphragm thickness ( T di) in the area where the diaphragm abuts the rib cage (zone of apposition). However, the degree of diaphragm thickening during inspiration reported as obtained by one-dimensional M-mode ultrasound was greater than that predicted by using other radiographic techniques. Because two-dimensional (2-D) ultrasound provides greater anatomic definition of the diaphragm and neighboring structures, we used this technique to reevaluate the relationship between lung volume and T di. We first established the accuracy and reproducibility of 2-D ultrasound by measuring T diwith a 7.5-MHz transducer in 26 cadavers. We found that T di measured by ultrasound correlated significantly with that measured by ruler ( R 2 = 0.89), with the slope of this relationship approximating a line of identity ( y = 0.89 x + 0.04 mm). The relationship between lung volume and T di was then studied in nine subjects by obtaining diaphragm images at the five target lung volumes [25% increments from residual volume (RV) to total lung capacity (TLC)]. Plots of T di vs. lung volume demonstrated that the diaphragm thickened as lung volume increased, with a more rapid rate of thickening at the higher lung volumes [ T di = 1.74 vital capacity (VC)2 + 0.26 VC + 2.7 mm] ( R 2= 0.99; P < 0.001) where lung volume is expressed as a fraction of VC. The mean increase in T di between RV and TLC for the group was 54% (range 42–78%). We conclude that 2-D ultrasound can accurately measure T di and that the average thickening of the diaphragm when a subject is inhaling from RV to TLC using this technique is in the range of what would be predicted from a 35% shortening of the diaphragm.