The doxorubicin should be discontinued indefinitely in this patient. The efficacy of anthracycline-based chemotherapy regimens for breast adenocarcinoma has been documented by multiple studies. However, doxorubicin, daunorubicin, and other anthracyclines have known cardiotoxicity, which is dependent on dosage, age, concomitant treatment with other cardiotoxic agents, chest radiation, and pre-existing cardiac disease. The incidence of cardiotoxicity for doxorubicin or daunorubicin is less than 1% for cumulative dosages of less than 400 mg/m² but 26% for cumulative dosages greater than or equal to 550 mg/m². Cardiotoxicity from anthracyclines usually develops within several months after initiation of chemotherapy, manifesting as either systolic or diastolic heart failure, although there can be a long latency period. Once heart failure is present, as in this patient, anthracyclines should be discontinued. In the absence of heart failure, anthracyclines also should be discontinued when new left ventricular dysfunction is detected. Routine surveillance with serial echocardiography is recommended in patients being treated with anthracyclines, with timing intervals based on the patient's baseline function, risk profile, dose, and clinical suspicion for toxicity. The maximum cumulative dose for these drugs is generally limited to 450 to 500 mg/m², but there is considerable individual variation in doses that lead to toxic responses.

This patient developed cardiotoxicity despite a low risk (1%). Therefore, neither continuing doxorubicin nor decreasing the dose is an appropriate treatment. Additionally, because cardiotoxicity is associated with this class of agents, substituting another anthracycline for the doxorubicin, such as daunorubicin, would also not be appropriate. In this patient, anthracycline therapy must be discontinued indefinitely to reverse the cardiotoxic effects and prevent them from recurring.

ACE inhibitors are indicated in patients with asymptomatic or symptomatic systolic dysfunction. There are limited data on the use of these agents to help prevent cardiotoxicity related to chemotherapy, but their use for this indication is not routinely recommended.