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.