Life-threatening arrhythmia and conduction disturbances are rare and occur years after radiation exposure. They are different from the frequent, asymptomatic, nonspecific and transient repolarization abnormalities seen shortly after irradiation . Serious abnormalities reported after radiotherapy include atrioventricular nodal bradycardia, all levels of heart block - including complete heart block [103, 104] - and sick sinus syndrome [66,104].In their case series report and review of the literature, Orzan et al.  reported that infranodal blocks were more common than nodal blocks, and that right bundle branch block was one of the most common abnormalities. Many, but not all, of the reports reviewed by Orzan, however, involved survivors who had received RT with techniques and doses no longer in use. Unfortunately, only a few prospective studies have reported the incidence of conduction abnormalities. In a study of 134 survivors of childhood cancer treated with anthracyclines and/or chest irradiation,ventric-ular tachycardia was significantly greater in those treated with chest radiotherapy than in a group of historical controls, at a mean 5 years after therapy . This increased rate was not seen in those treated with anthracycline without radiotherapy. The frequency of QTc >0.44 seconds was 12.5% in those treated with chest irradiation alone and 18.9% in those treated with irradiation and anthracyclines.
Persistent tachycardia at a fixed rate and loss of circadian variability in heart rate may be common in survivors of childhood HD treated with chest irradiation . This picture is similar to cardiac transplant patients who have a denervated heart. It suggests that there may be autonomic nervous system dysfunction in HD survivors treated with mediasti-nal radiotherapy, which could lead to the decreased perception of anginal chest pain observed in some of these patients . Symptoms from conduction abnormalities are uncommon but range from palpitations to syncope to sudden death. However, conduction delays, which produce symptoms, rarely occur without some other injury to the heart associated with irradiation .
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