Changes of phasic pleural pressure in awake dogs during exercise: potential effects on cardiac output.

Eighty experiments were performed with nine awake dogs to study the changes of phasic-pleural pressure with exercise. The increased minute volume with exercise was obtained by more frequent pleural pressure swings and by a substantial extension of the pressure swings in both directions. The cyclic changes of stroke volume following the pressure swings support the hypothesis that alterations of pleural pressure affect the stroke volume and can act, if necessary, as a secondary pump for the circulation. Mean pleural pressure during exercise fell by 2.5 cm H2O from the rest value of 12.1 cm H2O. The absolute right atrial pressure during exercise (-2.69 mm Hg) was not different from that at rest (-2.39 mm Hg). However, the transmural right atrial pressure of 7.6 mm Hg during exercise was higher than the pressure of 6.2 mm Hg at rest because during exercise the right atrium was perfused by 38% higher blood flow than that at rest. The phasic pattern of right atrial pressure shows that there is good reason to assume that during inspiration the extrathoracic veins are collapsed at their entrance into the chest, but this collapse is removed during expiration. There is no reason to assume an effective, sustained collapse of extrathoracic veins. Rather we can visualize a rhythmical change of flow in extrathoracic veins from transient limitation to transient acceleration.