Fetal Heart Rate Monitoring:
Intrapartum fetal heart rate (FHR) monitoring can detect fetal hypoxia, umbilical cord compression, tachycardia, and acidosis. Fetal heart rate monitoring can significantly reduce the risk of newborn seizures (relative risk 0.5); however, the risk of cesarean section is increased (relative risk 1.21).
Uterine contractions decrease placental blood flow and result in intermittent episodes.
The fetus normally tolerates contractions without difficulty, but if the frequency, duration, or strength of contractions becomes excessive, fetal hypoxemia may result.
Continuous FHR and contraction monitoring may be accomplished externally or internally. Internal FHR monitoring is accomplished with a spiral wire attached to the fetal scalp or other presenting part. Uterine contractions are monitored externally or internally.
Fetal Heart Rate Patterns:
The FHR at term ranges from 120-160 bpm. The initial response of the FHR to intermittent hypoxia is deceleration, but baseline tachycardia may develop if the hypoxia is prolonged and severe.
Tachycardia may also be associated with maternal fever, intra-amniotic infection, and congenital heart disease.
Variability. Decreasing fetal heart rate variability is a fetal response to hypoxia. Fetal sleep cycles or medications may decrease Monitoring the FHR variability. The development of decreased variability in the absence of decelerations is unlikely.
Accelerations. Accelerations are common periodic changes in labor. They are usually associated with fetal movement. These changes are reassuring and almost always confirm that the fetus is not hyopoxic.
Variable Decelerations. Variable decelerations are characterized by slowing of the FHR with an abrupt onset and return. They are frequently followed by small accelerations of the FHR. They vary in depth, duration, and shape. Variable decelerations coincide with cord compression, and they usually coincide with the timing of the uterine contractions. Variable decelerations are the most common decelerations seen in labor, and they are caused by umbilical cord compression. They are generally associated with a favorable outcome. Persistent, deep, and long lasting variable decelerations are nonreassuring.
Persistent variable decelerations to less than 70 bpm, lasting more than 60 seconds are concerning. Variable decelerations with persistently slow return to baseline are considered nonreassuring, as these reflect persistent hypoxia. Nonreassuring variable decelerations are associated with tachycardia, absence of accelerations, and loss of variability.
Late Decelerations. Late decelerations are U-shaped with a gradual onset and gradual return. They are usually shallow (10-30 beats per minute), and they reach their nadir after the peak of the contraction. Late decelerations occur when uterine contractions cause decreased fetal oxygenation. In milder cases, they can be a result of CNS hypoxia. In more severe cases, they may be the result of direct myocardial depression. Late decelerations generally become deeper as the degree of hypoxia becomes more severe. Occasional or intermittent late decelerations are not uncommon during labor.