Gas analysis Archives - Revista Brasileira de Ginecologia e Obstetrícia

PURPOSE: to evaluate Doppler velocimetry of the ductus venosus as a noninvasive test of abnormal pH and gas analysis in preterm fetuses with "brain sparing reflex". METHODS: a cross-sectional study was performed. The studied population consisted of 48 pregnant women between the 25th and the 33rd week of gestation, whose fetuses presented brain sparing reflex (umbilical/cerebral ratio >1). The time elapsed between Doppler velocimetry and the birth (cesarean section under peridural anesthesia) was of up to 5 h. The following parameters were studied: S/A ratio of the ductus venosus, pH and base excess (BE) of fetal blood sample (collected from the umbilical vein immediately after birth). The S/A ratio of the ductus venosus was considered abnormal when superior to 3.6. The fetuses were classified according to the gas analysis result. They were considered abnormal when pH <7.26 and BE £ 6 mMol/L. Fisher's test was used for statistical analysis and considered significant when p £ 0.05. RESULTS: there was a significant correlation between umbilical blood gas analysis in preterm fetuses with brain sparing reflex and ductus venosus S/A ratio (p = 0.0000082; Fisher test). Ductus venosus Doppler velocimetry identified 10 of 14 fetuses with abnormal gas analysis. On the other hand, 32 of 34 fetuses with normal gas analysis were correctly identified. The sensitivity of the ductus venosus S/A ratio for the diagnosis of abnormal blood gas analysis was 71%, specificity 94%, false-negative rate 8%, false-positive rate 4%, positive predictive value 83% and negative predictive value 89%. Pretest likelihood, post-test posterior probability following a positive test result (post-test likelihood) and post-test posterior probability following a negative test result (post-test likelihood) were 31, 84 and 10%, respectively. CONCLUSION: the analysis of the ductus venosus S/A ratio is adequate for the diagnosis of abnormal blood gas analysis in preterm fetuses presenting brain sparing reflex.