Study of Cardiopulmonary Exercise Testing in Women Who Have HER2-Positive Breast Cancer With Mild Cardiotoxicity

Inclusion Criteria: * Age ≥ 18 years * Female * Diagnosed with a HER2-positive solid tumor (stage I-IV) * Left ventricular dysfunction prior to the start of HER2-targeted therapy, defined by a LVEF \< 53% (or lower limit of normal), or 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 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: * 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