Effect of lower rib cage expansion and diaphragm shortening on the zone of apposition.

The relationship among diaphragm length (LD), the width of the zone of apposition (WZapp), and transverse chest diameter (Drc) was developed from model equations and statistical analysis. We present a theoretical model of diaphragm motion that predicts that the decrease in WZapp during inspiration is the result not only of shortening of the diaphragm muscle but also of expansion of the lower rib cage. To test our model, static lengths of costal LD, WZapp, and Drc were measured in 15 normal volunteers using posteroanterior chest X-ray films taken at four or five lung volumes spanning the vital capacity. We found a strong correlation between WZapp and LD: WZapp = 0.95 LD - 15.2 (R2 = 0.81). Expressing WZapp as a combined function of LD and Drc significantly reduced the unexplained variance in WZapp: WZapp = 0.96 LD - 0.47 Drc - 2.18 (R2 = 0.95). The coefficients for LD and Drc derived statistically are close to those predicted from our theoretical model. Repeating the analysis with LD as the dependent variable, we obtained similar results: LD = 0.85 WZapp + 17.1 (R2 = 0.81) and LD = 0.98 WZapp + 0.46 Drc + 3.48 (R2 = 0.94). We conclude that shortening of WZapp is dependent on both diaphragm shortening and rib cage expansion and that roentgenographic measurements of Drc and WZapp can be used to predict diaphragm length and length change.