THE IMPORTANCE OF THE VARIABLE CAPACITY OF THE AORTA IN AVERTING "BACK PRESSURE" IN THE RIGHT HEART AND PULMONARY ARTERY

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Abstract
An analysis of the effects of stenosis at the root of the aorta on the pulmonary circuit and right heart was made on the basis of optically recorded pressure pulses from these chambers (9 experiments reported). The following changes were noted immediately after a moderate stenosis of the aorta was made: (a) progressive rise in the pressure of the left auricle and pulmonary veins and a smaller rise in the vena cava after a short lag during which the pressure fell; (b) rise in the pulmonary arterial diastolic and systolic pressures; (c) increased pulse pressure in the pulmonary artery; (d) rise in initial pressure and in pressure amplitude in the right ventricle; (e) systolic vibrations in the left auricular pressure curve; (f) shift in the peak of the pulmonary arterial and right ventricular curve toward the latter part of ejection; (g) no significant changes in the duration of the cycle and systolic phases. The effects of changes in coronary flow which follow stenosis at the root of the aorta were evaluated by comparing the results of experiments in which the coronary return flow to the right heart was free with those of experiments where it was interrupted more than 60%. The coronary flow was interrupted either by occlusion of the sinus near its opening into the right auricle or by severing the sinus and allowing it to bleed freely. Except for the occasional absence of an increase in pulse pressure, no difference was noted in the 2 types of experiments, indicating that the changes in coronary flow play an unimportant part. The effects found are caused primarily by "back pressure" per se. The results indicate that moderate stenosis of the aorta leads to a damming back of blood in the pulmonary artery and right ventricle adequate in amt. to raise the pressure within them and augment the discharge of the latter. The additional capacity of the aorta is necessary to accommodate the excess blood and avert "back pressure." Compensatory changes follow very quickly. Evidence is given to show that they depend primarily on an increased discharge of the left ventricle which results from its increased distention with blood held back by the stenosis. The effect of aortic stenosis on the right heart varies with its degree. A greater degree of stenosis is needed to produce changes in the right heart than in the left. When the degree is excessive[long dash]the extent varying with the condition of the heart[long dash]a decrease in the pressure levels and pulse pressure eventually ensues because the right ventricle is distended beyond the physiological limit and its discharge decreased.

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