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

Background: End-tidal carbon dioxide pressure (PetCO(2)) is unreliable for monitoring PaCO(2) in several conditions because of the unpredictable value of the PaCO(2)–PetCO(2) 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 PCO(2) was measured (expiratory time: 4 s) after this manoeuvre (PACO(2)) in comparison with the PetCO(2) measured by the monitor. We analysed 67 Δ(a-et)CO(2), Δ(a-A)CO(2) 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 PaCO(2)–PetCO(2) gradients from 11 ± 5.7 and 5.7 ± 3.4 mm Hg (p < 0.001) to PaCO(2)–PACO(2) 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, PACO(2) showed the lowest dispersion (−7 to +6 mm Hg) and better correlation with PaCO(2) (R(2) = 0.92). The PACO(2) had a sensitivity of 0.81 and a specificity of 0.93 for identifying hypercapnic patients (PaCO(2) > 50 mm Hg). Conclusions: Measuring end-tidal PCO(2) after prolonged inspiratory time reduced the PaCO(2)–PetCO(2) gradient to the point of obtaining values close to PaCO(2). This measure identified hypercapnic patients in both intensive care and during anaesthesia.