Inclusion Criteria:  Age ≥ 18 years Female Diagnosed with HER2-positive breast cancer (stage I-IV) Diagnosed with mild cardiotoxicity associated with HER2-targeted therapy, defined by an absolute decrease in LVEF ≥ 10% from pre-treatment to < 53% Planning to undergo additional treatment with HER2-targeted therapy (e.g. trastuzumab, pertuzumab, or ado-trastuzumab) for a minimum of 3 months (i.e. 4 cycles administered every 3 weeks). Willing and able to comply with the requirements of the protocol  Exclusion Criteria:  LVEF < 40% Symptomatic heart failure (New York Heart Association Class III or IV)  Subjects must not have any of the following absolute contraindications to cardiopulmonary exercise testing:  Acute myocardial infarction (within 30 days of any planned study procedures), Unstable angina Uncontrolled arrhythmias causing symptoms or hemodynamic compromise, Symptomatic severe aortic stenosis Recurrent syncope Active endocarditis Acute myocarditis or pericarditis Acute pulmonary embolus or pulmonary infarction (within 3 months of any planned study procedures) Thrombosis of lower extremities (within 3 months of any planned study procedures) Suspected dissecting aneurysm Uncontrolled asthma Pulmonary edema Room air desaturation at rest ≤ 85% Respiratory failure Acute noncardiopulmonary disorder that may affect exercise performance or be aggravated by exercise (i.e., infection, renal failure, thyrotoxicosis) Mental impairment leading to inability to cooperate. Enrollment onto any other interventional investigational study Inability to provide informed consent