Effect of body orientation on regional lung expansion in dog and sloth

Recent studies (E.A. Hoffman, J. Appl. Physiol. 59: 468-480, 1985) using fast multisliced X-ray computed tomography have demonstrated a ventral-dorsal gradient of fractional lung air content (3.29% air/cm lung height) in supine dogs and an essentially uniform ventral-dorsal air content distribution in the prone dogs mean = 66 +/- 0.6% (SE) air content]. Since the prone orientation is the dog's normal body posture, we sought to study an animal whose normal body posture was "opposite" to that of the dog. Four two-toed sloths were scanned in the Dynamic Spatial Reconstructor in the prone and supine postures. A supine fractional air content gradient was demonstrated with a regression equation of y = 2.09x + 74.3 (r = 0.92), where y is percent air content and x is vertical height in the lung, and ventral-dorsal air content distribution in the prone posture was uniform with a mean of 85 +/- 0.4% (SE) air content. The low functional residual capacity lung density in the sloth was attributable to unusually large alveoli. The mean heart volume-to-body weight ratio in the dogs was 16.4 +/- 0.6 (SE) ml/kg and that in the sloth was 7.3 +/- 0.4 (SE) ml/kg. Mean lung volume-to-body weight ratios for dogs and sloths were 57 +/- 7 (SE) and 89 +/- 6 ml/kg, respectively. Of particular interest was the fact that large changes in prone vs. supine rib cage and diaphragm geometry previously found in dogs did not occur in sloths, though significant alterations of ventral and dorsal lung geometry prone vs. supine were demonstrated, and lung shape changes in both dog and sloth are attributable to shifts in the intrathoracic position of mediastinal structures.