End-Tidal Carbon Dioxide Pressure Measurement after Prolonged Inspiratory Time Gives a Good Estimation of the Arterial Carbon Dioxide Pressure in Mechanically Ventilated Patients

Abstract

Background: End-tidal carbon dioxide pressure (PetCO₂) is unreliable for monitoring PaCO₂ in several conditions because of the unpredictable value of the PaCO₂–PetCO₂ gradient. We hypothesised that increasing both the end-inspiratory pause and the expiratory time would reduce this gradient in patients ventilated for COVID-19 with Acute Respiratory Distress Syndrome and in patients anaesthetised for surgery. Methods: On the occasion of an arterial blood gas sample, an extension in inspiratory pause was carried out either by recruitment manoeuvre or by extending the end-inspiratory pause to 10 s. The end-expired PCO₂ was measured (expiratory time: 4 s) after this manoeuvre (PACO₂) in comparison with the PetCO₂ measured by the monitor. We analysed 67 Δ(a-et)CO₂, Δ(a-A)CO₂ pairs for 7 patients in the COVID group and for 27 patients in the anaesthesia group. Results are expressed as mean ± standard deviation. Results: Prolongation of the inspiratory pause significantly reduced PaCO₂–PetCO₂ gradients from 11 ± 5.7 and 5.7 ± 3.4 mm Hg (p < 0.001) to PaCO₂–PACO₂ gradients of −1.2 ± 3.3 (p = 0.043) and −1.9 ± 3.3 mm Hg (p < 0.003) in the COVID and anaesthesia groups, respectively. In the COVID group, PACO₂ showed the lowest dispersion (−7 to +6 mm Hg) and better correlation with PaCO₂ (R² = 0.92). The PACO₂ had a sensitivity of 0.81 and a specificity of 0.93 for identifying hypercapnic patients (PaCO₂ > 50 mm Hg). Conclusions: Measuring end-tidal PCO₂ after prolonged inspiratory time reduced the PaCO₂–PetCO₂ gradient to the point of obtaining values close to PaCO₂. This measure identified hypercapnic patients in both intensive care and during anaesthesia.