Insight into variable fetal heart rate decelerations from a mathematical model.

During labor and delivery, variable decelerations in the fetal heart rate (FHR) are commonly seen on the cardiotocogram (CTG) that is used to monitor fetal welfare. These decelerations are often induced by umbilical cord compression from uterine contractions. Via changes in oxygenation and blood pressure, umbilical cord compression activates the chemo- and baroreceptor reflex, and thus affects FHR. Since the relation between the CTG and fetal oxygenation is complex, assessment of fetal welfare from the CTG is difficult. We investigated umbilical cord compression-induced variable decelerations with a mathematical model. For this purpose, we extended our model for decelerations originating from caput compression and reduced uterine blood flow with the possibility to induce umbilical venous, arterial and total cord occlusion. Model response during total occlusion is evaluated for varying contractions (duration and amplitude) and sensitivity of the umbilical resistance to the uterine pressure. A clinical scenario is used to simulate a labor CTG with variable decelerations. Simulation results show that fetal mean arterial pressure increases during umbilical cord occlusion, while fetal oxygenation drops. There is a clear relation between these signals and the resulting FHR. The extent of umbilical compression and thus FHR deceleration is positively related to increased contraction duration and amplitude, and increased sensitivity of the umbilical resistance to uterine pressure. No relation is found between contraction interval and FHR response, which can probably be ascribed to the lack of catecholamines in the model. The simulation model provides insight into the complex relation between uterine pressure, umbilical cord compression, fetal oxygenation, blood pressure and heart rate. The model can be used for individual learning, and incorporated in a simulation mannequin, be used to enhance obstetric team training.