An Early Warning Marker in Acute Respiratory Failure: The Prognostic Significance of the PaCO2–ETCO2 Gap During Noninvasive Ventilation

TL;DR

This study shows that changes in the PaCO2–ETCO2 gap during noninvasive ventilation can predict worsening outcomes in patients with acute respiratory failure.

Contribution

The study demonstrates that the delta PaCO2–ETCO2 gap is a novel early warning marker for deterioration in acute respiratory failure patients.

Findings

An increasing PaCO2–ETCO2 gap during NIMV is linked to higher risks of intubation, ICU admission, and mortality.

A delta gap > 2.90 mmHg predicted intubation with high sensitivity and specificity.

The delta gap also predicted ICU admission and mortality with significant accuracy.

Abstract

Background and Objectives: Acute respiratory failure (ARF) has a heterogeneous course in the emergency department (ED), and early prediction of noninvasive mechanical ventilation (NIMV) failure is difficult. The PaCO2–ETCO2 gap reflects ventilation–perfusion mismatch and increased physiologic dead space; however, the prognostic value of its short-term change during NIMV is unclear. This study evaluated baseline, post-treatment, and delta (post–pre) PaCO2–ETCO2 gap values for predicting intubation, intensive care unit (ICU) admission, and mortality in ED patients with ARF receiving NIMV. Materials and Methods: This prospective observational study enrolled adults (≥18 years) treated with NIMV in a tertiary ED. Exclusion criteria included GCS < 15, intoxication, pneumothorax, trauma, pregnancy, gastrointestinal bleeding, need for immediate intubation/CPR, or incomplete data. ETCO2 was recorded within the first 3 min of NIMV and at 30 min; concurrent arterial blood gases provided PaCO2. The PaCO2–ETCO2 gap was calculated at both time points and as delta. Outcomes were intubation, ICU admission, and mortality. ROC analyses determined discriminatory performance and cutoffs using the Youden index. Results: Thirty-four patients were included (50% female; mean age 73.26 ± 10.07 years). Intubation occurred in 9 (26.5%), ICU admission in 20 (58.8%), and mortality in 10 (29.4%). The post-treatment gap and delta gap were significantly higher in intubated patients (p = 0.007 and p = 0.001). For predicting intubation, post-treatment gap > 10.90 mmHg yielded AUC 0.807 (p = 0.007; sensitivity 77.8%, specificity 76.0), while delta gap > 2.90 mmHg yielded AUC 0.982 (p = 0.001; sensitivity 88.9%, specificity 92.0). Delta gap also predicted ICU admission (cutoff > 0.65 mmHg; AUC 0.746, p = 0.016) and mortality (cutoff > 2.90 mmHg; AUC 0.865, p = 0.001). Conclusions: In ED ARF patients receiving NIMV, an increasing PaCO2–ETCO2 gap—especially the delta gap—was associated with higher risks of intubation, ICU admission, and mortality, supporting serial CO2 gap monitoring as a practical early warning marker of deterioration.

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