Are Oxygen Indices Effective for Predicting Pathological Intrapulmonary Shunt in Mechanically Ventilated Children?

Abstract

Background This study was undertaken to evaluate whether oxygen indices accurately predict pathological intrapulmonary shunt (Qsp/Qt), and to evaluate the sensitivity and specificity of the FiO 2-required formula to obtain a desired arterial oxygen tension (PaO 2) 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 FiO 2 change. Assessment of Qsp/Qt, P(A-a)O 2, PaO 2/FiO 2, PaO 2/PAO 2, and P(A-a)O 2/PaO 2 was carried out before and after FiO 2 change. Results In first blood gas data, 31 patients were hypoxemic (PaO 2 <90 Torr), 10 were normal, and 9 were hyperoxemic (PaO 2 >100 Torr). Forty patients required FiO 2 modifications that were carried out according to Maxwell’s formula. Five children showed persistent oxygen disturbance after FiO 2 changes. P(A-a)O 2, PaO 2/FiO 2, PaO 2/PAO 2, and P(A-a)O 2/PaO 2 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 FiO 2 modifications compared from initials; Qsp/Qt also showed significant change after FiO 2 change. Pearson product-moment showed lineal correlation between each index, and Qsp/Qt demonstrated their significant correlation ( p <0.01). Correlation of Qsp/Qt and PaO 2/FiO 2 and PaO 2/PAO 2 was significantly higher in younger children (<13 years) p <0.05. The FiO 2-required formula to obtain a desired PaO 2 had a sensitivity of 0.93 and a specificity of 0.75. Conclusions The oxygen indices showed sufficient efficacy to detect pathological intrapulmonary 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 FiO 2-dependent; therefore, when the FiO 2 varies, the use is misleading. The FiO 2-required formula is efficient for defining the appropriated FiO 2 for the obtaining of the desired PaO 2, but will always be merely a guide that should be confirmed through blood gas analysis.