Objective: Several indexes are used to quantify the severity of hypoxemia, including the arterial to alveolar oxygen ratio (a/APO(2)), the alveolar-arterial difference P(a-a)o(2), the ratio Pao(2)/Fio(2), and the oxygenation index (OI = mean airway pressure x Pao(2)/Fio(2)). This study was carried out to test how stable these indexes are when small changes in Fio(2) are made in ventilated neonates.
Design: Open prospective clinical study.
Setting: Level III neonatal intensive care unit of a teaching hospital.
Patients: Forty studies were performed in 31 clinically stable ventilated neonates (median birth weight, 1450 g; median gestation, 30.6 wks), monitored by transcutaneous Pao(2)-oxygen saturation (Sao(2)).
Interventions: If hyper- or hypoxemia without derangements of Paco(2) or pH were detected in a blood sample taken from an indwelling arterial catheter, Fio(2) 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 Fio(2) change were analyzed.
Measurements and main results: Median baseline P(a-a)o(2) was 211.7 torr, median a/APO(2) was 0.24, median Pao(2)/Fio(2) was 161 torr, and median OI was 6.14. After the Fio(2) 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)o(2), a/APO(2), Pao(2)/Fio(2), and OI, respectively. Changes in the value of each index were correlated to changes in Fio(2), indicating a dependency on Fio(2). When data were analyzed as "high Fio(2)" (approximate Sao(2) 95%) vs. "low Fio(2)" (approximate Sao(2) 90%), differences were statistically significant for all indexes except for a/APO(2).
Conclusions: All the indexes tested showed a dependency on the value of Fio(2): increasing Fio(2) spuriously made neonates appear less hypoxemic. The a/APO(2) appeared to perform better than other indexes in this study, with a lower variability and a lower oxygen dependency.