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Model studies of the contribution of ventricular interdependence to the transient changes in ventricular function with respiratory efforts.

The effects of variations in intrathoracic pressure on left ventricular function were studied using a mathematical model of the circulation. The variations in intrathoracic pressure directly affect the left ventricular afterload, and indirectly alter left ventricular filling by changing the right ventricular volume. The decrease in intrathoracic pressure with sustained inspiratory efforts increased the left ventricular afterload and thus reduced the left ventricular stroke volume. Secondary to the reduction in stroke volume, the left ventricular end systolic and end diastolic volumes increased. Decreasing intrathoracic pressure also increased the systemic venous return, thereby increasing right ventricular volume. The model predicted that an enlarged right ventricular volume would, through diastolic ventricular interdependence, immediately reduce the left ventricular end diastolic volume (which in turn reduced the left ventricular stroke volume), while through systolic ventricular interdependence it would increase left ventricular stroke volume and reduce left ventricular end systolic volume. An increased right ventricular volume would also increase right ventricular stroke volume and after a delay of a few heart beats raise left ventricular end diastolic and stroke volumes. The net effect of respiratory variations in intrathoracic pressure on left ventricular function would be a combination of these effects. Thus on sustained inspiration the left ventricular stroke volume initially decreased (left ventricular afterload and diastolic interdependence secondary to the increase in right ventricular volume partly counteracted by the effects of systolic interdependence), followed by an increase as the increased venous return reached the left ventricle. The model indicated that the response to a forced expiratory effort was not simply the opposite of the inspiratory response, since the increase in intrathoracic pressure during a forced expiration is accompanied by increases in abdominal pressure. On sustained expiration the left ventricular stroke volume initially increased, with no significant initial change in end diastolic volume. The cardiovascular response to respiration is complex, and model studies can help to isolate and identify the various components involved.