Relationship between fetal heart rate patterns and a time course for evaluation of fetal well-being: "the 30 minutes rule" for decision of mechanical delivery.

OBJECTIVE: To predict acidosis in fetus showing deceleration associated with non-reassuring fetal status during delivery, we examined the relationship between duration of the deceleration and umbilical arterial pH.

MATERIALS AND METHODS: A total of 19,907 deliveries in eight facilities of the Juntendo Perinatal Care Group, 895 cases of vaginal deliveries with level 3 decelerations were selected for the subjects of this study. The cut-off point of time when the umbilical arterial pH fell below 7.20 in all cases of level 3 and for each deceleration type were examined. The explanatory variables were the pH and pO2 of umbilical arterial gas and the time from onset of the level 3 pattern to delivery. From receiver operating characteristic (ROC) analysis using these variables, the critical point indicating low Apgar score was set at an umbilical arterial pH < 7.20.

RESULTS: The cut-off point of time when the umbilical arterial pH fell below 7.2 was 33.5 min for all cases of level 3, and 604 cases of severe variable decelerations with normal baseline variability and normal baseline heart rates, the cut-off point was 33.5 min as well. For 108 cases of late decelerations, there was no significant cut-off point for either the mild or severe cases. Mild prolonged deceleration showed the cut-off point of 34.5 min.

CONCLUSIONS: We confirmed the time indices for predicting and preventing acidosis in fetuses showing decelerations. To prevent fetal acidosis, the decision related to proper timing for performing assisted delivery by considering the time course is important.