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

The relationship among diaphragm length (L(D)), the width of the zone of apposition (W(Zapp)), and transverse chest diameter (D(rc)) was developed from model equations and statistical analysis. We present a theoretical model of diaphragm motion that predicts that the decrease in W(Zapp) 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 L(D), W(Zapp), and D(rc) 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 W(Zapp) and L(D): W(Zapp) = 0.95 L(D) - 15.2 (R² = 0.81). Expressing W(Zapp) as a combined function of L(D) and D(rc) significantly reduced the unexplained variance of W(Zapp): W(Zapp) = 0.96 L(D) - 0.47 D(rc) - 2.18 (R² = 0.95). The coefficients for L(D) and D(rc) derived statistically are close to those predicted from our theoretical model. Repeating the analysis with L(D) as the dependent variable, we obtained similar results: L(D) = 0.85 W(Zapp) + 17.1 (R² = 0.81) and L(D) = 0.98 W(Zapp) + 0.46 D(rc) + 3.48 (R² = 0.94). We conclude that shortening of W(Zapp) is dependent on both diaphragm shortening and rib cage expansion of that roentgenographic measurements of D(rc) and W(Zapp) can be used to predict diaphragm length and length change.