Effect of changes in inspired oxygen tension on indexes of oxygenation in ventilated neonates

Abstract

Several indexes are used to quantify the severity of hypoxemia, including the arterial to alveolar oxygen ratio (a/APO2), the alveolar-arterial difference P(a−a)o2, the ratio Pao2/Fio2, and the oxygenation index (OI = mean airway pressure × Pao2/Fio2). This study was carried out to test how stable these indexes are when small changes in Fio2 are made in ventilated neonates. Open prospective clinical study. Level III neonatal intensive care unit of a teaching hospital. Forty studies were performed in 31 clinically stable ventilated neonates (median birth weight, 1450 g; median gestation, 30.6 wks), monitored by transcutaneous Pao2-oxygen saturation (Sao2). If hyper- or hypoxemia without derangements of Paco2 or pH were detected in a blood sample taken from an indwelling arterial catheter, Fio2 was changed (median change, 0.05; range, −0.3 to 0.25) and another arterial blood sample was obtained 26–83 mins (median, 42) after. The indexes were calculated in the two blood samples, and for each index the changes between baseline and the value after Fio2 change were analyzed. Median baseline P(a−a)o2 was 211.7 torr, median a/APO2 was 0.24, median Pao2/Fio2 was 161 torr, and median OI was 6.14. After the Fio2 change, the coefficients of variation (sd/mean) were calculated, and they were 27.5%, 23.8%, 24.5%, and 31.6% for P(a−a)o2, a/APO2, Pao2/Fio2, and OI, respectively. Changes in the value of each index were correlated to changes in Fio2, indicating a dependency on Fio2. When data were analyzed as “high Fio2” (approximate Sao2 95%) vs. “low Fio2” (approximate Sao2 90%), differences were statistically significant for all indexes except for a/APO2. All the indexes tested showed a dependency on the value of Fio2: increasing Fio2 spuriously made neonates appear less hypoxemic. The a/APO2 appeared to perform better than other indexes in this study, with a lower variability and a lower oxygen dependency.