Mechanical effects of inspiration on heart functions: a review.

Left ventricular stroke volume decreases during inspiratory efforts whether lung volume is actually increasing as in normal inspiration or whether it remains constant as in a Mueller Maneuver (M.M.). Explanations have included phase lag between lung volume changes and left ventricular volume changes during inspiration as well as increased capacity of pulmonary vessels due to inflation. The capacitance changes could not be used to explain stroke volume (SV) fall in Mueller Maneuvers where lung volume is constant. The increased negative pleural pressure may be responsible for increases in right heart volume due to increased venous return. This mechanism has also been suggested as one of the causes of the fall in left ventricular stroke volume by ventricular interdependence due to changes in left ventricular pressure/volume (P/V) relations, i.e., compliance. In all these explanations a decrease in left ventricular diastolic filling and decreased diastolic pressure is assumed. The fact that decreases in pleural pressure may also act like an increase in left ventricular afterload and impede outflow, thereby decreasing stroke volume, has not been generally considered. We found that left ventricular stroke volume can still decrease without a decrease in left ventricular filling and even when increases in venous return have been prevented (right-heart bypass, i.e., constant pulmonary inflow). Thus the fall in stroke volume during inspiratory effort appears to be caused by a variety of factors, among which increased left ventricular afterload must be considered of primary importance. In addition, decreased left ventricular diastolic compliance due to increased right ventricular (RV) volume also has to be considered (interdependence).