Prolonged QT Interval in Hospitalized Internal Medicine Patients: A Retrospective Cohort Study of Determinants and Reversibility

Introduction QT interval prolongation is a common electrocardiographic abnormality in hospitalized patients, associated with malignant ventricular arrhythmias and sudden cardiac death. Its determinants in internal medicine, and the reliability of correction formulas used to identify it, remain insufficiently characterized. Methods We conducted a retrospective, single-center study including 277 patients hospitalized with QTc prolongation — defined by Bazett correction (QTcB >450 ms in men; ≥460 ms in women) — between January 2016 and December 2023. QTc was independently calculated using Bazett (QTcB) and Framingham (QTcF) formulas, and agreement between the two was assessed using Bland-Altman analysis. Multivariable logistic regression (complete-case analysis) explored predictors of QTcF-confirmed prolongation, and reversibility was assessed by repeat ECG performed after correction of the underlying reversible cause, or within 72 hours when no such cause was identified. Results Mean age was 59.2 ±17.2 years, with a slight female predominance (54.2%). Only 78 patients (28.1%) had QTcF-confirmed prolongation despite all meeting QTcB inclusion criteria. Bland-Altman analysis showed a mean bias of +31.7 ms between QTcB and QTcF (SD 18.9; 95% limits of agreement: −5.3 to +68.7 ms), strongly correlated with heart rate (r=0.92, p<0.001): the overcorrection increased from +3.6 ms at heart rate <70 bpm to +64.4 ms at >110 bpm. QTcB normalized on the follow-up ECG in 68.2% of patients. In a complete-case multivariable model (n=96 with available serum bicarbonate), no variable reached conventional statistical significance; serum bicarbonate (OR=1.121 per mmol/L; 95% CI: 0.991–1.268; p=0.069) and hypertension (OR=2.522; 95% CI: 0.875–7.275; p=0.087) showed borderline associations with QTcF-confirmed prolongation. In an exploratory analysis of the diabetic subgroup (n=148), peripheral neuropathy (DN4 ≥4) was associated with a 15.8 ms higher mean QTcB (age-adjusted 95% CI: 4.7–27.0 ms; p=0.006). Conclusion Bazett's formula substantially and heart-rate-dependently overestimates QT prolongation compared with the Framingham formula, a discrepancy now precisely quantified. QT prolongation in hospitalized internal medicine patients is predominantly acquired and reversible. Diabetic peripheral neuropathy was associated with higher QTcB values in an exploratory analysis. Serum bicarbonate showed only a non-significant trend toward association with QTcF-confirmed prolongation after complete-case multivariable adjustment; this hypothesis warrants confirmation in prospective studies with complete acid-base characterization before any clinical application.