We employed high-speed multisliced X-ray-computed tomography to determine the relative volume contributions of rib cage (ΔV(rc)) and diaphragmatic motion (ΔV(di)) to tidal volume (VT) during spontaneous breathing in 6 anesthetized dogs lying supine. Mean values were 40 ± 6% (SE) for ΔV(rc) and 62 ± 8% of VT for ΔV(di). The difference between VT and changes in thoracic cavity volume was taken to represent a change in thoracic blood volume (2 ± 3% of VT). To estimate how much of ΔV(rc) was caused by diaphragmatic contraction and how much of ΔV(di) was caused by rib cage motion, ΔV(rc) and ΔV(di) were determined during bilateral stimulation of the C5-C6 phrenic nerve roots in the apneic dog and again during spontaneous breathing after phrenicotomy. Thoracic cavity volume (V(th)) measured during hypocapnic apnea was consistently larger than V(th) at end expiration, suggesting that relaxation of expiratory muscles contributed significantly to both ΔV(rc) and ΔV(di) during spontaneous inspiration. Phrenic nerve stimulation did not contribute to ΔV(rc), suggesting that diaphragmatic contraction had no net expanding action on the rib cage above the zone of apposition. Spontaneous breathing after phrenicotomy resulted in small and inconsistent diaphragmatic displacement (8 ± 4% of VT). We conclude that the diaphragm does not drive the rib cage to inflate the lungs and that rib cage motion does not significantly affect diaphragmatic position during spontaneous breathing in anesthetized dogs lying supine.
ASJC Scopus subject areas
- Physiology (medical)