Post-Treatment Partial Pressure of Carbon Dioxide for Prognostic Stratification and Non-Invasive Ventilation Guidance in Elderly Patients with Community-Acquired Pneumonia and Heart Failure: The "Subgroup with Impaired Cardiac Response".

Background: Prognostic stratification in elderly patients with co-existing community-acquired pneumonia (CAP) and heart failure (HF) is challenging due to the limitations of static admission scores. Objective : To evaluate the prognostic value of the 72-hour post-admission partial pressure of carbon dioxide (PCO2_post) and its utility in guiding non-invasive ventilation (NIV) therapy. Methods: We analyzed 100 elderly patients with CAP and HF. Six-month mortality was the primary endpoint. Propensity score matching (PSM) was used to explore mechanistic differences between risk strata. Results: PCO2_post was a potent exploratory predictor of 6-month mortality (AUC=0.968), comparable to the Pneumonia Severity Index (AUC=0.960). A threshold of 47-48 mmHg stratified patients into two distinct risk groups (HR 23.55, P < 0.001). NIV significantly reduced HF events and accelerated symptom resolution only in the low-risk group (PCO2_post < 47mmHg). PSM analysis revealed that high-risk patients (PCO2_post mmHg ≥ 47) exhibited a "subgroup with impaired cardiac response", characterized by paradoxical BNP increases (median +1722.9 pg/mL), persistent inflammation (higher CRP), and slow clinical recovery. Conclusion: PCO2_post is a pragmatic dynamic biomarker that identifies a specific subgroup with impaired cardiac response who derive limited benefit from NIV, suggesting the need for intensified clinical monitoring and consideration of alternative therapeutic approaches.