Immediate effect of expiratory loading on left ventricular stroke volume

While the steady-state effects of positive pleural pressure on the circulation have been extensively studied, less is known about the immediate effects of positive intrathoracic pressure on cardiac dynamics. Therefore, we performed electrocardiographically gated radionuclide ventriculography with a respiratory gating technique in nine healthy subjects during quiet breathing and during expiration against a 24 cm H₂O expiratory threshold load. During expiration, respiratory loading caused an increase in stroke counts by 29.4% (p<.001) due to an increase in end-diastolic counts of 26.1% (p<.001). End-systolic counts also rose 18.8% (p<.05). The ejection fraction did not change significantly. These findings indicate that the increase in left ventricular stroke volume that occurs during the first 1 or 2 beats of a loaded expiration is due to an increase in left ventricular filling and not to augmentation of left ventricular ejection. This immediate increase in pulmonary venous return may reflect increased distensibility of the left ventricle due to decreased filling of the right ventricle.