71 ALVEOLAR-ARTERIAL OXYGEN DIFFERENCE AND RATIO IN VENTILATED NEONATES

Abstract

Both the alveolar-arterial oxygen difference (AaDO2) and the arterial/alveolar ratio (a/AR) are widely used as indices of gas exchange, although data on their performance in neonates are lacking. This study was carried out to test 1) how stable these two indices are, and 2) how accurate is the prediction (based on a/AR) of changes in PaO2 when FiO2 is changed. 26 studies were done in 20 clinically stable ventilated neonates (median birth weight and gestational age: 1640 g and 30 weeks respectively). An arterial blood sample was taken from an indwelling catheter and AaDO2 and a/AR were calculated. The predicted PaO2 (based on a/AR) was compared with the actual PaO2 in a second sample obtained 26-80 minutes (median 40) after a change in FiO2 (mean 15%); AaDO2 and a/AR were calculated again.Results: mean baseline AaDO2 was 30.7 kPa (range 7.1-58.1); mean a/AR was 0.29 (0.085-0.69). Changes in AaDO2 were highly correlated with changes in FiO2 (r=0.94, P<0.0001, slope = 0.56 kPa/percent FiO2). Changes in a/AR were slightly correlated with changes in FiO2 (r=0.366, P<0.1). The mean difference between predicted and measured values of PaO2 after changes in FiO2 was 0.14 kPa (SD 3.15), yielding 95% confidence limits of −6.02 to 6.32 kPa.conclusions: 1) AaDO2 is highly dependent on FiO2, and should not be used to quantify gas exchange; a/AR is also (though to a lesser degree) sensitive to changes in FiO2; 2) the prediction of Pa02 based on a/AR calculation is accurate on average, but in individual cases the confidence limits are wide.