Baseline and 24-Hour Changes in the Frontal QRS-T Angle During Non-Invasive Ventilation and Their Association with Clinical Outcomes in Acute Hypercapnic Respiratory Failure

Background/Objectives: Acute hypercapnic respiratory failure is associated with significant morbidity and mortality despite advances in non-invasive mechanical ventilation (NIMV) therapy. The frontal QRS-T angle is a simple electrocardiographic marker reflecting ventricular depolarization–repolarization heterogeneity and has been associated with adverse cardiovascular outcomes. However, the relationship between dynamic changes in frontal QRS-T angle and NIMV treatment response in acute hypercapnic respiratory failure remains unclear. This study aimed to evaluate the association between frontal QRS-T angle changes after NIMV treatment and clinical outcomes in patients with acute hypercapnic respiratory failure. Methods: This retrospective observational cohort study included 186 adult patients hospitalized with acute hypercapnic respiratory failure and treated with bilevel positive airway pressure between January 2024 and December 2025. Frontal QRS-T angle values were obtained from standard 12-lead electrocardiograms recorded before NIMV initiation (T0) and at the 24th hour of treatment (T24). Clinical, laboratory, arterial blood gas, and electrocardiographic parameters were analyzed. Receiver operating characteristic (ROC) analysis, Kaplan–Meier survival analysis, correlation analysis, and multivariable logistic regression analysis were performed to evaluate the prognostic significance of frontal QRS-T angle. Results: Successful NIMV treatment was achieved in 143 patients (76.9%), whereas 43 patients (23.1%) experienced NIMV failure. Patients with failed NIMV treatment had significantly higher baseline frontal QRS-T angle values compared with the successful group [128° (94–156) vs. 71° (48–101), p < 0.001]. Frontal QRS-T angle significantly decreased after treatment in the successful NIMV group but not in the failed group. A significant positive correlation was observed between ΔPaCO2 and Δfrontal QRS-T angle (r = 0.624, p < 0.001). ROC analysis demonstrated that baseline frontal QRS-T angle predicted NIMV failure with an AUC of 0.842 (95% CI: 0.773–0.898) and in-hospital mortality with an AUC of 0.811 (95% CI: 0.735–0.873). Kaplan–Meier analysis showed significantly lower survival rates in patients with frontal QRS-T angle >90° (log-rank p < 0.001). Multivariable logistic regression analysis identified frontal QRS-T angle >90° as an independent predictor of in-hospital mortality (OR: 3.617, 95% CI: 1.484–8.813, p = 0.005). Conclusions: Baseline frontal QRS-T angle was associated with NIMV failure and in-hospital mortality, while its 24-h change was associated with physiological response among patients with paired T0–T24 measurements. These findings are hypothesis-generating and do not establish the utility of frontal QRS-T angle monitoring for immediate escalation or intubation decisions.