Ivabradine in the Management of Cardiac Autonomic Dysfunction Associated With Thoracic Radiation Therapy.

Inclusion Criteria: * Survivors of mediastinal lymphoma (either Non-Hodgkin's Lymphoma or Hodgkin's Lymphoma) with no active malignancy * Prior mediastinal or mantle radiation ≥ 5 years prior to enrollment in the study * Age 18-80 years. * Participants must have normal organ function as defined below: * AST(SGOT)/ALT(SGPT) ≤2.5 × institutional upper limit of normal * creatinine clearance ≥ 15 mL/min/1.73 m2 for participants with creatinine levels above institutional normal. * Normal sinus rhythm with resting heart rate ≥ 80 bpm on screening EKG * Based on findings in animals, ivabradine may cause fetal harm when administered to a pregnant woman. For this reason, women of child-bearing potential must agree to use adequate contraception. * Ability to understand and the willingness to sign a written informed consent document Exclusion Criteria: * Participants who are receiving any other investigational agents. * History of allergic reaction to ivabradine. * Participants receiving any medications or substances that are inhibitors or inducers of cytochrome P450 3A4 are ineligible. * Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, acute coronary syndrome, symptomatic known coronary artery disease, severe valvular heart disease, active malignancy, or psychiatric illness/social situations that would limit compliance with study requirements. * Pregnant women are excluded from this study because ivabradine is an agent with the potential for teratogenic effects. Because there is an unknown but potential risk for adverse events in nursing infants secondary to treatment of the mother with ivabradine, breastfeeding should be discontinued if the mother is treated with ivabradine. * HIV-positive participants on combination antiretroviral therapy. * Patients with systolic blood pressure \< 90 mm Hg. * Patients with sick-sinus syndrome, sino-atrial block, third degree heart block, atrial fibrillation, and those with permanent pacemakers. * Patients with other established indications for ivabradine: stable, symptomatic chronic HF with a left ventricular ejection fraction ≤ 35% and in sinus rhythm with a resting HR ≥ 70 bpm, who are taking maximally tolerated doses of beta-blockers or have contraindications to beta-blocker use. * Patients with severe hepatic dysfunction (Child Pugh Class C). * Patients who have had chemotherapy or radiotherapy within 4 weeks (6 weeks for nitrosureas or mitomycin C) prior to entering the study or those who have not recovered from adverse events due to agents administered more than 4 weeks earlier.