Pulmonary embolism (PE) can reduce blood flow in the lungs and impair gas exchange, leading to lower end-tidal carbon dioxide (EtCO₂) levels. This prospective observational study aims to evaluate whether EtCO₂ can help identify high-risk and intermediate-high-risk patients and monitor early treatment response, especially during thrombolytic therapy. A total of 120 participants were included: high-risk PE, intermediate-high-risk PE patients who received thrombolysis, intermediate-high-risk PE patients treated only with anticoagulation, and healthy controls.

The study measured EtCO₂ along with oxygen saturation, heart rate, respiratory rate, perfusion index, and radiological obstruction scores. By comparing these parameters at diagnosis and during the first 24 hours, the study seeks to determine whether EtCO₂ can serve as a simple, noninvasive marker of disease severity and early hemodynamic improvement in patients with acute PE.

Inclusion Criteria:

Adults aged 18 years or older Diagnosis of acute pulmonary embolism confirmed by CT pulmonary angiography (for PE groups) Classified as high-risk or intermediate-high-risk pulmonary embolism according to ESC/ERS 2019 guidelines Ability to provide informed consent For healthy control group: no history of cardiopulmonary disease, coagulation disorders, or acute medical illness

Exclusion Criteria:

Known chronic respiratory diseases significantly affecting EtCO₂ measurement (e.g., advanced COPD, severe asthma, chronic respiratory failure) Active infection, sepsis, or acute complications of malignancy Prior thrombolytic therapy for pulmonary embolism Anatomical or technical limitations preventing reliable nasal capnography Pregnancy Individuals younger than 18 years Inability or refusal to provide informed consent

This study included adult patients diagnosed with acute pulmonary embolism who were classified as high-risk or intermediate-high-risk based on hemodynamic status, right ventricular dysfunction, and cardiac biomarker elevation. Participants were consecutively enrolled at a tertiary university medical center. The study also included a healthy control group composed of adults without cardiopulmonary disease or significant medical comorbidities. A total of 120 individuals were studied across four cohorts representing different clinical risk categories and routine treatment pathways.