The ability of the right ventricle to compensate as the pulmonary artery is constricted appears to be determined by four major factors: (I) there occurs the well-known increased force of contraction as the right heart becomes distended; (2) the adequacy of the coronary circulation determines to a great extent the degree of pulmonary arterial constriction which can occur before failure occurs;(3) the circulatory reflexes apparently aid the compensation to a moderate extent; and (4) the greater the blood volume, the greater is the limit of compensation before right ventricular failure occurs.T H E effects on the dynamics of circulation of progressive pulmonary embolism and progressive constriction of the pulmonary artery have been investigated numerous times. 1 " 4 These previous investigations have demonstrated clearly the following basic principles of right ventricular compensation when pulmonary resistance is increased: First, even the slightest pulmonary embolism or slightest constriction of the pulmonary artery causes an increase in the pulmonary arterial pressure proximal to the constriction but little change in the pulmonary arterial pressure distal to the constriction. Second, for the first few ensuing heart beats following acute increase in pulmonary circulatory resistance, the right ventricle progressively dilates, and there occurs a concurrent increase in the right ventricular systolic pressure associated at first with only minor changes in enddiastolic pressure but followed later, as the right ventricle fails, by a pronounced rise in the end-diastolic pressure. Third, it has been pointed out in several studies that the pulmonary artery must be occluded approximately 60 per cent of its total cross-sectional area before the right ventricular end-diastolic pres-

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