16.2: Basic Terms of Fetal Heart Rate and Contraction Patterns
By the end of this section, you will be able to:
- Explain the basic terminology used to describe the FHR baseline
- Explain the basic terminology used to describe FHR baseline variability
- Explain the basic terminology used to describe periodic changes in the FHR
- Explain the basic terminology used to interpret contraction patterns
The fetal heart rate (FHR) is the heart rate and rhythm of the fetus. A uterine contraction (UC) is the tightening and shortening of the uterine muscles. Both can be monitored during labor and birth. When the fetal monitor is used for continuous monitoring, it is important for the labor and delivery nurse to be aware of fetal heart rate baseline, variability, accelerations , and decelerations. Monitoring is used both antenatally and during the process of labor and birth. Nurses can use identifiable patterns of the FHR as assessment cues of fetal well-being and to determine appropriate interventions to ensure a positive outcome.
AWHONN offers several levels of fetal monitoring courses , including introduction, intermediate, advanced instructor workshops. These courses also qualify as continuing education hours.
Fetal Heart Rate Baseline
The FHR baseline is the average beats per minute in a 10-minute segment, excluding periodic changes or marked variability. The baseline is documented in increments of 5 beats per minute (bpm). The normal FHR baseline ranges from 110 to 160 bpm. Figure 16.2 illustrates a normal FHR baseline. According to the American College of Obstetricians and Gynecologists ([ACOG], 2009), the FHR baseline is controlled by the sympathetic and parasympathetic nervous systems. The sympathetic nervous system increases the FHR, while the parasympathetic nervous system decreases the FHR. Fetal hypoxia and hypercapnia can activate chemoreceptors that also influence the FHR.
This site provides an in-depth explanation of basic fetal heart rate patterns to help the nurse understand baseline and common findings.
Tachycardia
An FHR baseline greater than 160 bpm for 10 minutes is called tachycardia . Figure 16.3 shows a monitor tracing that indicates FHR tachycardia. Tachycardia can be caused by multiple factors. A common cause of FHR tachycardia is the presence of fever in the pregnant person. Tachycardia should not be considered a sign of fetal distress in the absence of FHR decelerations . Other risk factors for tachycardia are listed in Table 16.1. Many of these can be addressed, and the FHR will return to normal.
| Patient | Risk Factors |
|---|---|
| Pregnant person |
|
| Fetus |
|
Bradycardia
Fetal bradycardia is defined as an FHR baseline less than 110 beats for 10 minutes. Figure 16.4 shows a monitor tracing that indicates FHR bradycardia. Bradycardia can be caused by multiple factors. The nurse must determine if the decrease in FHR is a benign episode of bradycardia or a pathologic prolonged deceleration (possibly lasting several minutes) that may need further intervention. Initial nursing actions for fetal bradycardia include repositioning the laboring person, administering intravenous (IV) fluid bolus, then notifying the health-care provider. See Table 16.2 for risk factors for fetal bradycardia.
| Patient | Risk Factors |
|---|---|
| Pregnant person |
|
| Fetus |
|
Terbutaline
The nurse caring for laboring persons at times administers terbutaline to relax the uterus, as in cases of fetal bradycardia related to tachysystole. Terbutaline can be used in antepartum and intrapartum situations. Tachycardia is a common side effect, and the nurse will assess the pulse prior to administration. If the pulse is greater than 120 bpm, terbutaline is not administered. The FHR will also increase while terbutaline is being used.
- Generic Name: terbutaline
- Trade Name: none
- Class: tocolytic
- Mechanism of Action: selectively stimulates beta-2 adrenergic receptors, relaxing smooth muscle
- Indications: bronchospasm, tocolysis (preterm labor, tachysystole, prolonged deceleration)
- Contraindications: hypersensitivity to drug, ischemic heart disease, hypertension, arrhythmia, diabetes mellitus, seizure disorder, hyperthyroidism,
- Route: subcutaneous injection
- Dose: 0.25 mg every 20 to 30 minutes, with maximum of 1 mg per 4 hours
- Black Box Warning: Injectable terbutaline is not approved for prolonged tocolysis greater than 48 to 72 hours. Serious adverse effects include increased heart rate, transient hyperglycemia, hypokalemia, cardiac arrythmias, pulmonary edema, myocardial infarction, and death reported after use in pregnant persons; increased fetal heart rate and neonatal hypoglycemia may also occur.
- Education: Educate the pregnant person on common side effects of anxiety, restlessness, and increased pulse. Educate the person to call for chest pain, difficulty in breathing, and fast, pounding, irregular heartbeat.
(Vallerand & Sanoski, 2022)
Fetal Heart Rate Variability
FHR variability is the beat-to-beat fluctuations in the FHR baseline . These fluctuations are caused by the “push and pull” of the sympathetic and parasympathetic nervous systems (ACOG, 2009). These fluctuations reflect an intact central nervous system (CNS) with normal cardiac responses. The variability is classified as absent, minimal, moderate, or marked. When the fetus is well oxygenated and has a normal acid-base balance, the FHR variability is typically moderate. When the fetus is hypoxic, the variability is minimal, absent, or marked.
Moderate FHR Variability
Fluctuations between 6 and 25 bpm are considered moderate FHR variability . This is considered normal FHR variability. In most cases, moderate FHR variability is associated with a normal umbilical cord pH. Figure 16.5 shows a monitor tracing that indicates moderate FHR variability.
Minimal FHR Variability
FHR fluctuations of 5 bpm or fewer are considered minimal FHR variability . Minimal variability is associated with fetal acidemia but cannot be the only measure of fetal well-being . Minimal variability can be caused by many factors, such as fetal sleep cycles, fetal anomalies of the CNS, and medications administered to the pregnant patient. Preterm gestation and magnesium sulfate treatment are associated with minimal FHR variability. Figure 16.6 shows a monitor tracing that indicates minimal FHR variability.
Absent FHR Variability
A lack of FHR baseline fluctuation is considered absent FHR variability . Absent variability is most often a sign of a severely compromised fetus. Figure 16.7 illustrates absent FHR variability. Fetal compromise can be the result of poor oxygenation leading to metabolic acidosis. Fetal acidemia depresses the CNS, causing a loss of FHR variability. Acidemia of the pregnant person can also cause absent FHR variability. Changes in the FHR variability from moderate to minimal or to absent may indicate fetal stress and require further investigation by the nurse and notification of the health-care provider.
Marked FHR Variability
A FHR baseline fluctuation greater than 25 bpm is considered marked FHR variability . Because of the extreme fluctuations, the FHR baseline is undeterminable. Marked FHR variability suggests hypoxia and is usually seen during the second stage of labor. Marked variability requires further assessment to determine the cause and notification of the health-care provider. Figure 16.8 shows a monitor tracing that indicates marked variability.
Sinusoidal
The sinusoidal FHR pattern appears wave-like with regular frequency (3 to 5 per minute) and amplitude. The wave pattern is not variability; variability is actually absent. The FHR is usually within the normal range, but the appearance of the uniform wave is apparent. Sinusoidal pattern is linked with fetal compromise, such as fetal anemia due to loss of blood, and medications administered to the pregnant person. See Figure 16.9 for an example of a sinusoidal pattern.
Periodic Changes in the Fetal Heart Rate
Both accelerations and decelerations of the FHR in relation to the FHR baseline are considered periodic changes in the FHR . Periodic changes are described as abrupt or gradual. Accelerations are usually associated with fetal movement, are not related to uterine contraction activity, and occur independently. Decelerations are classified as early, late, variable, and prolonged. Decelerations can be associated with fetal hypoxia and require nursing interventions.
Accelerations
Abrupt increases in the FHR above the baseline with an onset-to-peak of less than 30 seconds are called accelerations . They are identified by an FHR peak of at least 15 bpm above the baseline with a duration of at least 15 seconds but less than 2 minutes. Before 32 weeks, accelerations of the FHR are expected to peak at 10 bpm above the baseline with a duration of at least 10 seconds but less than 2 minutes. Figure 16.10 illustrates a monitor tracing with FHR accelerations. Accelerations occur with fetal movement, uterine contractions, fetal scalp stimulation, and acoustic stimulation. Accelerations are a reassuring indication of fetal well-being .
Early Decelerations
Gradual FHR declines that then return to baseline, mirroring the uterine contraction, are called early decelerations . Fetal head compression activates the vagal nerve, causing a deceleration in the FHR. The degree of deceleration is proportional to the strength of the uterine contraction. Decelerations are commonly seen during active labor as the fetal head descends into the pelvis. Early decelerations are considered normal. Figure 16.11 illustrates early FHR decelerations .
Late Decelerations
Decelerations in the FHR below the baseline that start during a contraction and continue after completion of the contraction are called late decelerations . Figure 16.12 illustrates late FHR decelerations. Late decelerations are nonreassuring and almost always indicate fetal hypoxia . They are associated with uteroplacental insufficiency , a lack of oxygenated blood coming from the uterus to the placenta to the fetus. During a contraction, the decrease in oxygen to the fetus causes a deceleration that begins late in the contraction. The nadir, or lowest point, of the deceleration is after the peak of the contraction. Another common cause of late decelerations is tachysystole (contractions occurring too frequently or lasting longer than 2 minutes), often seen with the use of oxytocin (Pitocin) . Spinal or epidural anesthesia causes hypotension leading to hypoperfusion to the placenta, producing late decelerations. Other causes of late decelerations are hypertension , postmature placenta, placental abruption, and anemia. Late decelerations, like early decelerations, are often repetitive, forming a pattern.
Variable Decelerations
Abrupt decelerations of the FHR of at least 15 bpm below the baseline that last at least 15 to 30 seconds are called variable decelerations . They do not typically appear in a pattern and vary in onset, depth, and duration. Variable decelerations are caused by compression of the umbilical cord. Cord compression can be caused by a fetal body part squeezing the cord against the wall of the uterus during a uterine contraction or movement related to the pregnant person or fetus. Because of this, the nurse will notice that variable decelerations may or may not occur in conjunction with uterine contractions. Variable decelerations may lead to fetal hypoxia , especially when they are repetitive for a prolonged period. Figure 16.13 shows a monitor tracing with an example of variable decelerations.
Prolonged Decelerations
Isolated, sporadic decelerations of at least 15 bpm from the FHR baseline that last 2 to 10 minutes from onset to return to baseline are called prolonged decelerations . The causes of prolonged decelerations are uterine hyperactivity, cord compression, hypotension, placental abruption, seizure, or impending birth. These decelerations are concerning due to the risk for fetal hypoxia. The extent of hypoxia relates to the depth and duration of the deceleration; the deeper and longer the deceleration, the greater the risk of fetal hypoxia. After the prolonged deceleration returns to baseline, the FHR tracing may show decreased variability and tachycardia . These are signs of a significant hypoxic event. Figure 16.14 shows a monitor tracing with an example of a prolonged deceleration.
Basic Terminology Used to Interpret Contraction Patterns
Uterine contractions are monitored for frequency, intensity, and duration. As discussed in Chapter 15 Process of Labor and Birth, the frequency is how many minutes from the start of one contraction to the start of the next contraction. Intensity is the strength of the contraction. Duration is the number of seconds from the start to the end of a contraction.
As shown in Figure 16.15, the monitor graph is divided by darker or more prominent vertical lines to represent 1 minute of time and less prominent vertical lines showing 10-second intervals. The upper set of horizontal lines represents the fetal heart rate in beats per minute (bpm), usually in intervals of 10 bpm. The lower set of horizontal lines represents the intensity of the contractions in millimeters (mm) of mercury (Hg), in intervals of 5 to 10 mm Hg.
Monitoring a Nonviable Fetus
Trisomy 18 is a genetic disorder caused by having three copies of chromosome 18 instead of two. This disorder, also known as Edwards syndrome, occurs once in every 3,315 births (Mai et al., 2019).
Fetuses with trisomy 18 are usually growth restricted and small for gestational age. Many have heart defects and other organ abnormalities. Most fetuses with trisomy 18 die prior to birth. If born alive, most neonates with this disorder die within the first month of life (Trisomy 18 Foundation, 2023).
Discussion Questions
In view of this prognosis,
- what is the best way to monitor the FHR during labor?
- what should the pregnant person know to make an informed decision regarding monitoring?
- how can the nurse provide support to the family making these decisions?