Objective <p>This study aimed to investigate the prevalence, characteristics, and associated clinical factors of arrhythmias—particularly atrial fibrillation (AF) and high-grade premature ventricular contractions (PVCs)—as detected by 24-hour Holter ECG monitoring in hospitalized elderly patients with diagnosed heart failure with ejection fraction ≤ 50%.</p> Methods <p>A cross-sectional study was conducted between May 2023 to March 2024 at the Cardiovascular Institute, 108 Central Military Hospital, Hanoi, Vietnam. A total of 72 hospitalized patients aged ≥ 65 years with confirmed heart failure (LVEF &lt; 50%) were enrolled. Clinical characteristics, laboratory data, echocardiographic parameters, and 24-hour Holter monitoring results were collected. Multivariable logistic regression with stepwise forward selection (p-entry &lt; 0.2) was used to identify predictors of AF and high-grade PVCs (Lown grade III–V).</p> Results <p>The mean age was 73.7 ± 8.9 years, and 66.7% were male. AF was detected in 19.4% of patients, while a high PVC burden was common, with a median (25th -75th ) of 1955 (54-38514) and 58.3% falling into Lown grades III–V. Multivariable analysis identified increased body mass index (OR = 1.71, 95% CI: 1.18–2.48, <i>p</i> = 0.005) and elevated systolic pulmonary artery pressure (sPAP) (OR = 1.06, 95% CI: 1.00–1.13, <i>p</i> = 0.049) as independent predictors of AF. For high-grade PVCs, significant predictors included elevated sPAP (OR = 1.06, 95% CI: 1.01–1.12, <i>p</i> = 0.024), lower creatinine (OR = 0.99, 95% CI: 0.98–1.00, <i>p</i> = 0.014).</p> Conclusion <p>AF and high-grade PVCs are common in hospitalized elderly patients with diagnosed heart failure and are associated with distinct clinical and laboratory parameters. Elevated sPAP was a shared predictor of both arrhythmias. These findings support the use of integrated echocardiographic and Holter monitoring for risk stratification and management in this population.</p>