Are oxygen indices effective for predicting pathological intrapulmonary shunt in mechanically ventilated children?

RESUMO

Background: This study was undertaken to evaluate whether oxygen indices accurately predict pathological intrapulmonary shun (Qsp/Qt), and to evaluate the sensitivity and specificity of the FiO2-required formula to obtain a desired arterial oxygen tension (PaO2) in mechanically ventilated children. Methods. A prospective, hospital-based, comparative study was conducted on 50 mechanically ventilated children at the Intensive Care units of the national Institute Of pediatrics (INP) in Mexico City. Blood gas data were prospectively collected from 50 critically ill, mechanically ventilated children, 50 taken before and 40 after FiO2, change. Assessment of Qsp/Qt, P(A-a)O2, PaO2/FiO2, PaO2/PAO2, and P(A-a)O2/PaO2 was carried out before and after FiO2 change. Results. In first blood gas data, 31 patients were hypoxemic (PaO2 < 90 Torr), were normal, and 9 were hyperoxemic (PaO2 > 100 Torr). Forty patients required required FiO2 modifications that were carried out according to Maxwell's formula. Five children showed persisrent oxygen disturbance after FiO2 changes. P(A-a)O2, PaO2/PAO2, and P(A-a)O2/PaO2 had sensitivities of 0.66, 0.71, 0.98, and 0.93, respectively, and specificities of 0.79, 0.91, 0.29, and 0.64, respectively, to detect pathological Qsp/Qt. All oxygen indices changed significantly after FiO2 modifications compared from initials; Qsp/Qt also showed significant change after FiO2 change. Pearson product-moments showed lineal correction between each index, and Qsp/Qt demonstrated their significant correlation (p <0.01). Correlation of Qsp/Qt and PaO2/FiO2 and PaO2/PAO2 was significantly higher in younger children (< 13 years) p <0.05. The FiO2-required formula to obtain a desired PaO2 had a sensitivity of 0.93 and a specificity of 0.75. Conclusions. The oxygen indices showed sufficient efficacy to detected pathological intra-pulmonary shunt, and to have a statistically significant lineal correlation that permits its use during the clinical evaluation of oxygen transport data in most mechanically ventilated children, which is consistent with other reports on adult populations. However, one limitation for its use in clinical assessment, as reported in previous studies, would be that all indices in the present study are FiO2-dependent; therefore, when the FiO2 varies, the use is misleading