Chest wall motion during spontaneous breathing and mechanical ventilation in dogs

We measured the volume change of the thoracic cavity (ΔVth) and the volumes displaced by the diaphragm (ΔVdi) and rib cage (ΔVcr) in six pentobarbital-anesthetized dogs lying supine. A high-speed X-ray scanner (dynamic spatial reconstructor) provided three-dimensional images of the thorax during spontaneous breathing and during mechanical ventilation with paralysis. Tidal volume (VT) was measured by integrating gas flow. Changes in thoracic liquid volume (ΔVliq, presumably caused by changes in thoracic blood volume) were calculated as ΔVth - VT. Absolute volume displaced by the rib cage was not significantly different during the two modes of ventilation. During spontaneous breathing, thoracic blood volume increased during inspiration; ΔVliq was 12.3 ± 4.1% of ΔVth. During mechanical ventilation, ΔVliq was nearly zero. Configuration of the relaxed chest wall was similar during muscular relaxation induced by either pharmacological paralysis or hyperventilation. Expiratory muscle activity produced 50 ± 11% of the ΔVth during spontaneous breathing. We conclude that at constant VT the volume displaced by the rib cage is remarkably similar during the transition from spontaneous breathing to mechanical ventilation, while both diaphragmatic volume displacement and changes in intrathoracic blood volume decrease by a similar amount.