Diaphragm thickening during inspiration.

Ultrasound has been used to measure diaphragm thickness (Tdi) 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 Tdi. We first established the accuracy and reproducibility of 2-D ultrasound by measuring Tdi with a 7.5-MHz transducer in 26 cadavers. We found that Tdi measured by ultrasound correlated significantly with that measured by ruler (R2 = 0.89), with the slope of this relationship approximating a line of identity (y = 0.89x + 0.04 mm). The relationship between lung volume and Tdi 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 Tdi vs. lung volume demonstrated that the diaphragm thickened as lung volume increased, with a more rapid rate of thickening at the higher lung volumes [Tdi = 1.74 vital capacity (VC)2 + 0.26 VC + 2.7 mm] (R2 = 0. 99; P < 0.001) where lung volume is expressed as a fraction of VC. The mean increase in Tdi between RV and TLC for the group was 54% (range 42-78%). We conclude that 2-D ultrasound can accurately measure Tdi 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.