The first step in interpreting a fetal heart rate (FHR) tracing is to determine the baseline fetal heart rate.
A baseline fetal heart rate:
- Is defined as the mean FHR rounded to increments of 5 bpm during 10-minute segment
- Excludes accelerations, decelerations, periods of marked variability
- Is defined as a single number, not as a range, as other fetal heart rate components are based on the degree of variation from the baseline rate.
- May be interpreted during contractions
- Must be for a minimum of 2 minutes (not necessarily continuous) in any 10-minute segment; otherwise the baseline is defined as “indeterminate”.
In a case where a baseline is deemed “indeterminate”, it may be necessary to refer to the previous 10-minute segment in order to determine the baseline.
A normal FHR baseline is between 110 to 160 bpm.
Baseline fetal heart rate is regulated by many factors, including intrinsic cardiac pacemakers and conduction pathways, autonomic innervation, humoral factors such as catecholamines, extrinsic factors such as medication, and local factors, such as calcium and potassium. Fluctuations in pO2, pCO2 and blood pressure are detected by chemoreceptors and baroreceptors in the fetal aortic arch and carotid arteries, and autonomic input regulates the fetal heart rate in response.
→ Sympathetic innervation, plasma catecholamines = ↑ FHR
→ parasympathetic innervation = ↓ FHR
What does a baseline FHR tell us?
While the primary objective of FHR monitoring is to assess fetal oxygenation, certain maternal and fetal conditions can alter the heart rate tracing even when fetal oxygen levels are normal. It is critical to account for these factors when an abnormal tracing appears in order to accurately assess fetal well-being and decide on the appropriate intervention. The most common influences include:
Maternal: fever, infection, medications, hypothyroidism
Fetal: prematurity, anemia, fetal heart block, fetal tachyarrhythmia, congenital anomaly, preexisting neurologic injury, sleep cycles
Tachycardia is defined as a fetal heart rate greater than 160 bpm.
The most common causes of fetal tachycardia are maternal fever, infection and anemia.
Other potential causes include:
- Medication or drugs (sympathomimetics, parasympatholytics, caffeine, theophylline, cocaine, methamphetamines)
- Maternal hyperthyroidism
- Arrhythmias (sinus tachycardia, supraventricular tachycardia (SVT), atrial fibrillation, atrial flutter, and ventricular arrhythmia)
Fetal tachycardia is not a direct indication of compromised fetal oxygenation, as there are many possible causes of fetal tachycardia. However, it can be noted that when a fetus is experiencing metabolic acidemia, the blunting of the parasympathetic cardiac stimulation can cause the fetal heart rate to rise. Therefore, metabolic acidemia should always be considered in the presence of tachycardia.
Bradycardia is defined as a fetal heart rate less than 110 bpm.
Potential causes include:
- Medications (included sympatholytics)
- Cardiac conduction abnormalities
- Heart block
- Heterotaxy syndrome
- Structural cardiac defects
- Viral infections, (i.e. cytomegalovirus)
- Sjogren’s antibodies
- Fetal heart failure
- Maternal hypoglycemia
- Maternal hypothermia
- Interruption of fetal oxygenation
Bradycardia should not be confused with the term prolonged deceleration, during which there is a periodic or episodic interruption of the baseline to a lower bpm for at least 2 minutes and up to 10 minutes. Decelerations can and often do reflect an interruption in fetal oxygenation, whereas a true bradycardia baseline is not specifically related to fetal oxygenation.
