Introduction: Underuse and under-dosing of guideline-directed medical therapy (GDMT) for HFrEF in US outpatient practice is well documented. Yet changes to GDMT and clinician-reported reasons for not making medication changes are not well described.Methods: US cardiologists were recruited from the Schlesinger Group, a nationwide panel of physicians who participate in web-based surveys. The cardiologists conducted anonymized chart reviews of their own randomly selected patients meeting eligibility criteria. Eligible patients had HFrEF<45% without end-stage kidney disease, and an outpatient cardiology visit between 2017-2019. For each index outpatient visit, cardiologists abstracted data on medication changes and reported their primary rationale for not making a change in each specific therapy.Results: Overall, 238 cardiologists participated and reviewed outpatient visits for 458 patients under their care (median age 64 years, 65% male). Before the index visit, proportions of patients not prescribed beta-blocker, MRA, ARNI, and SGLT2i were 17%, 58%, 72%, and 86%, respectively. Across medications, proportions of patients with medication initiation ranged from 5-11%, medication discontinuation from 0-11%, with variable rates of dose changes (Figure, Panel A). Among patients receiving medication, for each therapy, the most common clinician-reported reason for no medication change was patient clinical stability (43-57%). Other reasons for no medication change included already being at target dose (29-44% of cases), and perceived risk of intolerance (6-11%) (Figure, Panel B).Conclusions: We observed relatively few medication changes made among these HFrEF patients. Patient stability and perceived risk of intolerance were frequently cited by cardiologists as reasons for not making guideline-directed medication changes, suggesting clinical inertia as a major barrier to closing gaps in GDMT.