METHODS

Animal preparation. Healthy l-to 3-year-old dogs weighing 14-23 kg were used for experimental procedures. The animals were fed a cereal type dog food (Ken-L-Ration, Quaker Oats Company), and their weight did not change appre-ciably during the course of the experiment except for an increase associated with the development of ascites. Progressive pulmonary artery stenosis was produced in 11 dogs by means of a rubber Jacobson cuff surgically implanted around the main pulmonary artery (6). With the animal under methoxyflurane anesthesia, the pulmonary artery was progressively narrowed at biweekly intervals by percutaneous injection of fluid into a connecting bulb located subcutaneously. Right ventricular pressure was monitored during this procedure using a E-gauge poly-ethylene catheter attached to a Statham P23AA transducer and displayed on a direct-writing oscillograph. The right ventricle progressively increased in size, as determined by a qualitative increase in heart size and wall thickness on thoracic radiographs and cineangiocardiograms, the appearance of a right ventricular hypertrophy(RVH) pattern on the electrocardiogram(13), and increased right ventricular mass determined at necropsy. The experiment was terminated 55-80 days following surgery, when the dogs were in advanced congestive heart failure as determined by the presence of the folIowing signs: venous distention, palpable liver enlargement, ascites, an observed decrease in exercise tolerance, and an elevation of right ventricular end-diastolic pressure above 7 mm Hg. The control group consisted of nine normal dogs without thoracic surgery and four dogs that had undergone a sham thoracotomy and pulmonary artery dissection 6-10 weeks previously. Since no functional or biochemical differences were found between the unoperated and sham-operated animals, they were considered as one group. The third group consisted of eight dogs with spontaneously acquired RVH and CHF secondary to infestation with the canine heartworm Dirofdaria immitis. Animals in this group had signs of cardiopulmonary insufficiency for from 2 months to over 2 years. The diagnosis was confirmed by the demonstration of typically dilated pulmonary arteries by cineangiocardiogram, a right ventricular hypertrophy pattern on the electrocardiogram(13), elevated tiean pulmonary artery and right ventricular systolic and enddiastolic pressure, presence of D. immitis microfilaria in the peripheral blood, presence of adult parasites in the right ventricle and pulmonary artery at necropsy, and micro-