Changes in TcPCO2 regarding pulmonary mechanics due to pneumotachometer dead space in ventilated newborns

The aim of this study was to analyze the effect of added dead space on PaCO2 after application of a pneumotachometer during the measurement of pulmonary mechanics. The study was based on 24 observations of TcPCO2 changes during the measurement of pulmonary function in 9 newborns subjected to mechanical ventilation. All newborns remained stable during the 23 minutes of the test. The introduction of a low dead space pneumotachometer (1.7 mL) for 10 minutes led to an increase in TcPCO2 of 5.40 +/- 2.66 mm Hg, from 39.76 +/- 8.69 to 45.17 +/- 9.22. Pulmonary mechanics indexes that correlated with the percentage of CO2 increase were peak inspiratory flow and expiratory time/time constant. When the pneumotachometer was removed, TcPCO2 fell but remained 0.99 +/- 2.13 mm Hg above basal TcPCO2. Final TcPCO2 tended to relate negatively with the minute volume. We conclude that this transient increase in PaCO2 should be born in mind in neonates with a high basal level and can be prevented by maintaining a long expiratory time and a high minute volume.