A neonatal in-vitro study on the effect of the inflation pressure on end-tidal carbon dioxide levels

Background: Describing the association of the peak inflation pressure (PIP) with end-tidal carbon dioxide (ETCO2) is a prerequisite for the development of closed loop ventilation in neonatal intensive care. We aimed to develop an in-vitro system to study this relationship.Methods: A ventilator was connected to a test lung, supplied with a stable CO2 concentration from a cylinder. The PIP was altered and the change in ETCO2 per unit of PIP was calculated in three models mimicking respiratory distress syndrome (RDS), bronchopulmonary dysplasia (BPD) and viral bronchiolitis.Results: The median (IQR) change in ETCO2 per unit of PIP was 0.23(0.13-0.38) kPa/cmH2O, using 138 paired measurements of PIP and ETCO2. The median (IQR) change in ETCO2 per unit of PIP, was higher when starting at an ETCO2 > 6 kPa [0.43(0.33-0.58) kPa/cmH2O] compared to starting at an ETCO2 < 6 kPa [0.14(0.08-0.20) kPa/cmH2O, p < 0.001]. The median (IQR) change in ETCO2 per unit of PIP, was larger in the model of RDS [0.33(0.13-0.51) kPa/cmH2O] compared to the BPD [0.23(0.13-0.33) kPa/cmH2O, p = 0.043] and the bronchiolitis models [0.15(0.10-0.31) kPa/cmH2O, p = 0.017].Conclusions: The change in ETCO2 in response to increasing PIP was larger for higher ETCO2 values and in a model simulating neonatal RDS, compared to BPD and bronchiolitis.