Canine cardiomyopathy after whole heart and partial lung irradiation.

The late radiation response of the heart is of concern because of many reports of heart disease following radiation therapy of thoracic tumors. This study was done because of the clinical relevance of the pathophysiology of cardiopulmonary irradiation and because the heart is a good model for late effects of vasculoconnective tissue due to its lack of acutely responding parenchymal cells. Thoracic irradiation of adult beagle dogs including the heart and one third of the lung volume produced an early response in the heart at 1 and 3 months which consisted of an increase in left ventricle and septal wall thickness, decreased left ventricle ejection fraction, increased heart rates, intraventricular conduction disturbances and a high probability for pericardial effusion at 3 months. Radiation doses were 36, 44, or 52 Gy given in 4 Gy fractions in 4 weeks. Premature atrial contractions, paroxysmal atrial tachycardia, sustained atrial tachycardia and atrial fibrillation occurred at all dose levels. Evidence suggests that both early and late responses were due, at least in part, to direct injury to the cardiac microvasculature. The later effects appeared to be enhanced by injury to the lung. The early response appeared to resolve in 6 to 9 months, after which there was thinning of the myocardium at higher doses and resolution of pericardial effusions. At 12 months, elevations in right atrial pressure, but not pulmonary wedge pressure, were suggestive of right-sided congestive heart failure. Pulmonary hypertension was also present at 12 months presumably due to partial lung irradiations, and may have exacerbated right-sided congestive heart failure. The radiation injury may continue to increase with time leading to serious deficits in cardiopulmonary function. The functional studies may aid in predicting late effects and evaluating residual injury.