The COVID-19 pandemic has profoundly disrupted healthcare systems globally, raising concerns about its impact on non-COVID-19 patients requiring immediate and intensive cares. This paper investigates the effects of the pandemic on the quality of care for Acute Myocardial Infarction (AMI) patients in Lombardy, Italy. Taking advantage of rich administrative data (i.e. hospital discharges data, emergency call and mortality registry) and leveraging the national lockdown as an exogenous shock in a quasi-experimental framework, we estimate the causal effects of COVID-19 on in-hospital and out-of-hospital mortality rates and on changes in ambulance response time. Our results reveal a 63% increase in daily out-of-hospital deaths during the pandemic and significant delays, of average 11 min, in ambulance response times. In-hospital mortality remained stable, suggesting that delays in ambulance transport did not directly affect outcomes for patients who reached hospitals.