Contribution of live heartworms harboring in pulmonary arteries to pulmonary hypertension in dogs with dirofilariasis

To investigate whether adult heartworms harboring in the pulmonary arteries contribute to pulmonary hypertension, we determined the cardio-pulmonary values immediately before and after removal of heartworms from the pulmonary arteries and before and after insertion of live worms in their place. In 10 heartworm-infected dogs, 8 to 46 worms were removed. The mean pulmonary arterial pressure fell significantly from 24.5 +/- 7.9 mmHg to 16.3 +/- 4.9 mmHg (p less than 0.01) immediately after removal. The right cardiac output decreased in 7 of the 10 cases. The total pulmonary resistance and right ventricular stroke work index also decreased. At 24 hours after removal, live heartworms were put back into the pulmonary arteries of their host dog. The mean pulmonary arterial pressure elevated significantly (p less than 0.01) immediately after insertion. The right cardiac output further decreased in 7 of the 10 dogs, and the total pulmonary resistance and right ventricular stroke work index increased. Separate from this, 12 to 42 heartworms were transplanted into the pulmonary arteries of 5 heartworm-free dogs. Immediately after transplantation, the pulmonary arterial pressure did not show any significant change. However, the stroke volume decreased, and the total pulmonary resistance increased. These facts suggest a contribution of live heartworms to the pulmonary hypertension, although there is a complicated interaction among the presence of heartworms, the pulmonary lesions and the pulmonary hypertension.