PURPOSE:Cardiac rehabilitation (CR) could be an important opportunity for optimization of guideline-directed medical therapies (GDMT) for patients with heart failure with reduced ejection fraction (HFrEF). We set out to describe GDMT prescribing among enrollees in CR with HFrEF. METHODS:We queried the electronic health record of an academic medical center and identified patients with HFrEF who attended CR between 2016 and 2023. We defined CR cycles as at least 6 CR sessions with fewer than 6 months between sessions. Using a validated algorithm, we generated medication optimization scores (MOS, 0%-100% optimized) at the first and final CR session. The algorithm's input included GDMT, New York Heart Association classification, systolic blood pressure, heart rate, creatinine, potassium, allergies, and race. Wilcoxon Signed-Rank analysis was used to compare MOS. RESULTS:A total of 172 CR cycles were completed by 152 patients (64% male, 78% White, 67.5 ± 12.1 years old). Mean sessions per CR cycle was 26.4 ± 10.6. At the end of CR, 85 (49%) patients were on a beta-blocker, 84 (49%) a renin-angiotensin-aldosterone inhibitor, and 31 (18%) a mineralocorticoid receptor antagonist. Accounting for contraindications, patients were eligible for initiation or uptitration of at least 1 GDMT class after 84% of cycles. Median MOS at the start of CR was 39% (IQR: 14, 57) and 35% (IQR: 14, 57) at the end of CR ( P = .90). CONCLUSION:GDMT utilization among patients with HFrEF in CR is suboptimal. There is a substantial opportunity to develop and validate strategies to improve GDMT prescribing during CR.