Our study determined the average excursion of the diaphragms during tidal breathing in a standing position in a health screening center cohort using dynamic chest radiography (“dynamic X-ray phrenicography”). These findings are important because they provide reference values of diaphragmatic motion during tidal breathing useful for the diagnosis of diseases related to respiratory kinetics. Our study also suggests that dynamic X-ray phrenicography is a useful method for the quantitative evaluation of diaphragmatic motion with a radiation dose comparable to conventional posteroanterior chest radiography (22).
Our study demonstrated that the average excursions of the bilateral pmsf during tidal breathing (right: 11.0 mm, 95% CI 10.4 to 11.6 mm; left: 14.9 mm, 95% CI 14.2 to 15.5 mm) were numerically less than those during forced breathing in previous studies using other modalities 2; 7 ; 8. Using fluoroscopy, Alexander reported that the average right excursion was 27.5 mm and the average left excursion was 31.5 mm during forced breathing in the standing position in 127 patients (2). Using ultrasound, Harris et al. reported that the average right diaphragm excursion was 48 mm during forced breathing in the supine position in 53 healthy adults (7). Using MR fluoroscopy, Gierada et al. reported that the average right excursion was 44 mm and the average left excursion was 42 mm during forced breathing in the supine position in 10 healthy volunteers (8). The difference in diaphragmatic excursion during tidal breathing versus forced breathing is unsurprising.
Our study showed that the excursion and peak motion speed of the left diaphragm are significantly greater and faster than those of the right. With regard to the excursion, the results of our study are consistent with those of previous reports using fluoroscopy in a standing position 2 ; 3. However, in the previous studies evaluating diaphragmatic motion in the supine position, the asymmetric diaphragmatic motion was not mentioned 7 ; 8. The asymmetric excursion of the bilateral diaphragm may be more apparent in the standing position, but may not be detectable or may disappear in the supine position. Although we cannot explain the reason for the asymmetry in diaphragmatic motion, we speculate that the presence of the liver may limit the excursion of the right diaphragm. Regarding the motion speed, to the best of our knowledge this study is the first to evaluate it. The faster motion speed of the left diaphragm compared to that of the right diaphragm would be related to the greater excursion of the left diaphragm.
We found that higher BMI and higher tidal volume were independently associated with the increased excursions of the bilateral diaphragm by both univariate and multivariate analyses, although the strength of these associations was weak. We cannot explain the exact reason for the correlation between BMI and the excursion of the diaphragm. However, a previous study showed that BMI is associated with peak oxygen consumption (23), and the increased oxygen consumption in an obese participant may affect diaphragmatic movement. Another possible reason is that lower thoracic compliance due to higher BMI may cause increased movement of the diaphragm for compensation. Regarding the correlation between tidal volume and excursion of the diaphragm, given that diaphragmatic muscle serves as the most important respiratory muscle, the result is to be expected. Considering our results, the excursion evaluated by dynamic X-ray phrenicography could potentially predict tidal volume.