The anatomical and hydrostatic basis of orthopnea and of right hydrothorax in cardiac failure

Abstract

When an adult of average size is recumbent, the blood flowing from the pulmonary venous bed must be lifted from 4 to 7 cm. against the force of gravity in order to fill the left ventricle; but when the body is erect, the pulmonary outflow descends from 3 to 5.5 cm. to reach the left ventricle. Thus the pulmonary venous pressure which is adequate in the erect position to maintain a given level of left ventricular filling and cardiac output is lower than that needed to maintain the same level of cardiac output, when the individual is recumbent, by the pressure represented by a column of blood 7.5 to 10 cm. high. In the same way, the pulmonary venous outflow from the right lung of an adult in the right lateral recumbent posture must be lifted 10 cm. or more to reach the left ventricle, that from the left lung, when in the left lateral position, need be lifted only 5 cm. Therefore, the average venous pressure needed to maintain the flow of blood will be much higher in the right lung than in the left, even if the individual spends equal periods lying on the two sides. In patients, like many cardiac patients, who favor the right lateral decubitus, this difference in average pressure between the venous beds in the lungs will be increased. It is suggested that these hydrostatic factors of anatomical origin are of paramount importance in causing orthopnea and the preponderance of right pleural transudation in patients with heart failure.