Utility of arterial to end-tidal carbon dioxide gradient as a severity index in critical care

Background: The arterial to end-tidal carbon dioxide gradient (P [a-Et] CO₂) reveals the ventilation-perfusion (V/Q) status of critically ill patients. V/Q mismatch has several causes and affects the clinical outcomes of critically ill patients. We investigated the relationship between P (a-Et) CO₂ and the clinical outcomes in critically ill patients. Methods: Critically ill patients (n = 1,978) on mechanical ventilation and capnography in the intensive care units of two institutions were enrolled and categorized into three groups: P (a-Et) CO₂ ≤10 mmHg (low group), 10 mmHg < P (a-Et) CO₂ ≤ 20 mmHg (medium group), and 20 mmHg < P (a-Et) CO₂ (high group). Results: The Acute Physiology and Chronic Health Evaluation II score was 29.5 ± 8.1, 31.3 ± 8.2, and 33.3 ± 8.7 in the low, medium, and high groups, respectively (p < 0.001). Overall mortality rates were 25.5 %, 35.6 %, and 52.8 % in the low, medium, and high groups, respectively (p < 0.001). The odds ratio was 1.456 (95 % confidence interval [CI]: 1.117–1.897, p = 0.002) and 2.320 (95 % CI: 1.635–3.293, p < 0.001) for the medium and high groups, respectively, with the low group as a reference. The area under the receiver operating characteristic curve of P (a-Et) CO₂ for overall mortality was 0.604 (p < 0.001). Conclusions: P (a-Et) CO₂ is a simple, easily accessible indicator that potentially impacts patient care and outcomes as an independent marker for assessing disease severity and predicting mortality, especially in non-respiratory critical care scenarios.