Cardiovascular changes in conscious dogs during spontaneous deep breaths.

Cardiovascular functions were evaluated beat by beat during 29 spontaneous deep breaths in three conscious dogs. When pleural pressure was significantly lower than during quiet breathing, stroke volume was reduced, heart rate elevated, as well as transmural pressure in the pulmonary artery (PPA-Ppl) and in the thoracic aorta (PAO-Ppl); the left ventricular filling pressure (LVEDP-Ppl) did not decrease. The authors suggest that these findings are not consistent with the classical hypothesis which explains the decrease in left ventricular output primarily by a reduction in the venous return to the left heart. The results indicate that the decrease in pleural pressure is responsible for an increase in the afterload on the left heart and suggest that this is the predominate factor in the reduction of the stroke volume without decrease in left ventricular filling pressure.