ZONE OF APPOSITION IN THE PASSIVE DIAPHRAGM OF THE DOG

We determined the regional area of the diaphragmatic zone of appositio n (ZAP) as well as the regional craniocaudal extent of the ZAP (ZAP(ht )) of the passive diaphragm in six paralyzed anesthetized beagle dogs (8-12 kg) at residual lung volume (RV), functional residual capacity ( FRC), FRC + 0.25 and FRC + 0.5 inspiratory capacity, and total lung ca pacity (TLC) in prone and supine postures. To identify the caudal boun dary of the ZAP, 17 lead markers (1 mm) were sutured to the abdominal side of the costal and crural diaphragms around the diaphragm insertio n on the chest wall. Two weeks later, the dogs' caudal thoraces were s canned by the use of the dynamic spatial reconstructor (DSR), a protot ype fast volumetric X-ray computer tomographic scanner, developed at t he Mayo Clinic. The three-dimensional spatial coordinates of the marke rs were identified (+/-1.4 mm), and the cranial boundary of the ZAP wa s determined from 30-40 1.4-mm-thick sagittal and coronal slices in ea ch DSR image. We interpolated the DSR data to find the position of the cranial and caudal boundaries of the ZAP every 5 degrees around the t horax and computed the distribution of regional variation of area of t he ZAP and ZAP(ht) as well as the total area of ZAP. The ZAP(ht) and a rea of ZAP increased as lung volume decreased and were largest near th e lateral extremes of the rib cage. We measured the surface area of th e rib cage cephaled to the ZAP (A(L)) in both postures in another six beagle dogs (12-16 kg) of similar stature, scanned previously in the D SR. We estimated the entire rib cage surface area (A(rc) = A(ZAP) + A( L)) The A(ZAP) as a percentage of A(rc) increased more than threefold as lung volume decreased from TLC to RV, from similar to 9 to 29% of A (rc).