Diaphragm thickening during inspiration

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-di with 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 = 0.89x + 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.