A model approach to assess diaphragmatic volume displacement

Diaphragmatic volume displacement (Vdi) is calculated from two models using measurements obtained from anteroposterior fluoroscopic images of supine anesthetized dogs. In model 1, diaphragmatic descent was treated as if it were a "piston in a cylinder." In contrast, model 2 incorporated thoracic configuration as well as inspiratory changes in rib cage diameter and diaphragm shape. In one dog, a computerized tomography reconstruction of Vdi was compared with Vdi calculated using the models. Vdi calculated from model 2 lay within 11% of the computerized tomographic value, whereas Vdi based on model 1 was 30% larger. In seven animals, radiopaque markers were sewn to the right costal diaphragm. Digitized fluoroscopic images were used to measure intermarker distance, an index of muscle shortening. For four tidal breaths per dog, in model 2 Vdi averaged 49 +/- 18% of tidal volume and was weakly correlated with costal diaphragm muscle shortening (R = 0.74). It is concluded that Vdi can be estimated from linear dimensions in the coronal plane, provided that inspiratory changes in rib cage diameter and diaphragmatic shape change are taken into account.