Low end-tidal carbon dioxide identifies high-severity pulmonary embolism and shock-level haemodynamics

End-tidal carbon dioxide tension (P _ETCO2 ) may be reduced when lung units are ventilated but under-perfused, as occurs in pulmonary embolism (PE). We sought to determine whether early P _ETCO2 measurement reflects PE severity according to European Society of Cardiology (ESC) risk categories and identifies shock-level haemodynamics among patients undergoing catheter-directed therapy (CDT).

Patients admitted with acute PE confirmed by computed tomography pulmonary angiography were enrolled in this prospective observational study. P _ETCO2 was measured using nasal capnography within 24 h of presentation or immediately prior to urgent CDT. P _ETCO2 was compared between lower and higher severity PE groups and between patients referred for CDT or not. In patients undergoing mechanical thrombectomy (CDT) with invasive haemodynamic assessment, pulmonary arterial pressures and cardiac indices were recorded. A subgroup analysis evaluated patients with cardiogenic shock defined as cardiac index ≤1.8 L·min^-1·m^-2.

62 patients were analysed. P _ETCO2 was significantly lower in the combined higher risk group compared to the combined lower risk group (mean 21.7 (95% CI 20.2-23.3) versus 30.2 (95% CI 27.8-32.6) mmHg; p<0.001). P _ETCO2 was also lower in patients referred for CDT compared to those not referred (mean 20.4 (95% CI 18.7-22.0) versus 28.2 (95% CI 26.2-30.1) mmHg; p<0.001). Among patients who underwent CDT and met haemodynamic criteria for cardiogenic shock (n=16), P _ETCO2 was the lowest (mean 18.6 (95% CI 16.1-21.1) mmHg).

P _ETCO2 can be rapidly obtained early in PE evaluation and reflects ESC-defined severity. A P _ETCO2 level <20 mmHg was associated with shock-level haemodynamics among normotensive intermediate-high-risk patients undergoing CDT. P _ETCO2 may serve as a non-invasive marker of physiological severity.