The three-tiered guidelines for FHR or FHT interpretation are given in Table 2-3.
Baseline rate: lasts for at least 2 minutes during a 10-minute section rounded to the nearest 5 beats/min.
Normal rate: 110 to 160 beats/min
Bradycardia: A baseline FHR <110 beats/min. Causes of bradycardia include fetal head compression, hypoxemia, and maternal hypothermia. The clinical picture is as important as the heart rate in interpreting fetal bradycardia.
Tachycardia: A baseline FHR >160 beats/min. The most common cause is maternal fever or infection. Other less common causes of fetal tachycardia include fetal arrhythmias or maternal administration of parasympatholytic or sympathomimetic drugs.
Variability: fluctuations in the FHR. It is most reliable when measured with an FSE.
Absent: absent variability
Minimal: detectable variability of <5 beats/min
Moderate: variability of 6 to 25 beats/min
Marked: variability of >25 beats/min
Accelerations: For GA >32 weeks, an acceleration is an increase in FHR of at least 15 beats/min that lasts for at least 15 seconds. For GA <32 weeks, an acceleration is an increase in FHR >10 beats/min for 10 seconds.
An FHT is reactive if it shows two accelerations within 10 minutes.
A sinusoidal FHT is a persistent smooth undulating pattern with a frequency of 3 to 5 cycles/min. It is concerning and requires immediate evaluation. Fetal anemia; analgesic drugs such as morphine, meperidine, alphaprodine, and butorphanol; and chronic fetal distress should be considered.
Decelerations: a decrease in FHR below the baseline. In some instances, the pattern of deceleration of the FHR can be used to identify the cause.
Variable decelerations may start before, during, or after the uterine contraction starts (hence, the designation “variable”). They usually show an abrupt onset to nadir in <30 seconds and return, which gives them a characteristic V shape. The decrease is >15 beats/min lasting >15 seconds but <2 minutes. Variable decelerations are commonly caused by umbilical cord compression.
Early decelerations are shallow and symmetric and reach their nadir at the peak of the contraction. They are caused by vagus nerve–mediated response to fetal head compression.
Late decelerations are U-shaped decelerations of gradual onset to nadir in >30 seconds and gradual return, reach their nadir after the peak of the contraction, and do not return to the baseline until after the contraction is over. They may result from uteroplacental insufficiency and relative fetal hypoxia. Recurrent late decelerations can be an ominous sign.
Prolonged deceleration: a deceleration that lasts longer than 2 minutes but <10 minutes
Recurrent decelerations: occur with >50% of uterine contractions in any 20-minute span
Intermittent decelerations: occur with <50% of uterine contractions in any 20-minute span.
Category I FHT must have a baseline FHR between 110 and 160 beats/min and moderate variability, and accelerations may be present or absent, with no late or variable decelerations.
Category II FHT are those that cannot be classified as category I or III.
Category III FHT have concerning findings such as absent variability with recurrent variable or late decelerations, bradycardia, or sinusoidal pattern. Consideration for delivery should be given.
Nonreassuring FHR patterns do not necessarily predict adverse events, and although electronic fetal heart monitoring has resulted in increased cesarean deliveries, there has not been a decrease in long-term adverse neurologic outcomes such as cerebral palsy. Nevertheless, the known relationships between fetal hypoxemia/acidemia and abnormal heart rate patterns make FHT interpretation a critical part of labor management.
Oxygen: Maternal supplemental oxygen often results in improved fetal oxygenation, assuming adequate placental exchange and circulation.
Maternal position: Left lateral positioning releases vena cava compression by the gravid uterus, promoting increased venous return, increased cardiac output, increased BP, and improved uterine blood flow.
Discontinue oxytocin until the FHR and uterine activity become normal.
Vibroacoustic stimulation or fetal scalp stimulation: Fetal stimuli may be used to induce accelerations when the FHR lacks variability for a long period of time. Heart rate acceleration in response to these stimuli indicates the absence of acidosis and correlates with a mean pH value of about 7.30. Conversely, a 50% chance of acidosis exists in a fetus that fails to respond to vibroacoustic stimulation in the setting of a nonreassuring heart rate pattern.
Amniotomy: If the FHR cannot be monitored adequately externally, an amniotomy should be performed to place internal monitors, unless these are contraindicated by the clinical situation.
Fetal scalp electrode: Direct application of an FSE records the fetal electrocardiogram waveform and may allow closer evaluation of the FHR. An FSE may be contraindicated in cases of fetal coagulopathy or maternal infections such as human immunodeficiency virus or hepatitis B or C.
Intrauterine pressure catheter and amnioinfusion: A catheter is inserted into the chorioamnionic sac and attached to a pressure gauge. Pressure readings provide quantitative data on the strength and duration of contractions. Amnioinfusion of room temperature normal saline can be used to replace amniotic fluid volume to relieve recurrent variable decelerations in patients with oligohydramnios. Care should be used to avoid overdistention of the uterus.
Tocolytic agents:β-Adrenergic agonists (eg, terbutaline, 0.25 mg subcutaneously or 0.125-0.25 mg intravenously) can be administered to decrease uterine activity in the presence of uterine tachysystole. Potential side effects of β-adrenergic agonists include elevated serum glucose levels and increased maternal and FHRs.
Management of maternal hypotension: Maternal hypotension, as a complication of the sympathetic blockade associated with epidural anesthesia or from compression of the vena cava, can lead to decreased placental perfusion and FHR decelerations. Intravenous fluid bolus, left uterine displacement, and ephedrine or phenylephrine administration may be appropriate.