Factors affecting the arterial to end-tidal carbon dioxide gradient in ventilated neonates.

Objective . To determine factors which influenced the relationship between blood carbon dioxide (pCO 2 ) and end-tidal carbon dioxide (EtCO 2 ) values in ventilated, newborn infants. Furthermore, to assess whether pCO 2 levels could be predicted from continuous EtCO 2 monitoring. Approach . An observational study of routinely monitored newborn infants requiring mechanical ventilation in the first 28 d after birth was undertaken. Infants received standard clinical care. Daily pCO 2 and EtCO 2 levels were recorded and the difference (gradient: ∆P-EtCO 2 ) between the pairs were calculated. Ventilatory settings corresponding to the time of each blood gas assessment were noted. End-tidal capnography monitoring was performed using the Microstream sidestream Filterline H set capnograph. Main results . A total of 4697 blood gas results from one hundred and fifty infants were analysed. The infants had a median gestational age of 33.3 (range 22.3-42.0) weeks and birth weight of 1880 (395-5520) grams. Overall, there was moderate correlation between pCO 2 and EtCO 2 levels ( r = 0.65, p < 0.001). The ∆P-EtCO 2 for infants born less than 32 weeks of gestation was significantly higher (1.4 kPa) compared to infants born at greater than 32 weeks of gestation (0.8 kPa) ( p < 0.001). In infants born at less than 32 completed weeks of gestation, pCO 2 levels were independently associated with EtCO 2 , day after birth, birthweight and fraction of inspired oxygen (FiO 2 ) (model r 2  = 0.52, p < 0.001). Significance . The results of end-tidal capnography monitoring have the potential to predict blood carbon dioxide values within the neonatal population.