Once the baseline is determined, the next step in a systematic interpretation of a fetal heart rate is to assess variability. FHR variability is defined as fluctuations in the baseline fetal heart rate that are irregular in amplitude and frequency. Variability is measured in beats per minute, from peak to trough.
Variability is the visual representation of the fetal heart rate change that occurs with every heartbeat. These slight corrections in the heart rate maximize fetal cardiac output and fetal tissue oxygenation via optimized blood flow distribution. The minute, beat-to-beat changes in heart rate are referred to as short-term variability, while the wider, undulating patterns are known as long-term variability. As these two oscillations occur simultaneously and are assessed as a whole, no distinction is therefore made between short and long term variability.
Many factors contribute to fetal heart rate variability, including:
- Cardiac pacemakers (SA node, AV node) and the cardiac conduction system
- Autonomic innervations (parasympathetic and sympathetic)
- Catecholamines
- Extrinsic factors, such as medication
- Local factors, such as calcium and potassium
Variability can be defined as:
- Absent: amplitude range is undetectable to the unaided eye
- Minimal: amplitude range that is detectable but ≤5 bpm
- Moderate: amplitude range of 6-25 bpm
- Marked: amplitude range > 25 bpm
Moderate FHR variability indicates that the fetal heart rate is not currently affected by an interruption of fetal oxygenation to the point of metabolic acidemia; it therefore reliably predicts the absence of fetal acidemia at the time it is observed. Moderate variability provides evidence of adequate oxygenation and indicates normal neurologic function at the time it is observed, though it cannot rule out a preexisting neurologic injury.
Decreased FHR variability, such as in the case of minimal or absent variability, may occur for a number of reasons, including:
- Fetal sleep cycle
- Fetal tachycardia
- Medications (narcotics, barbiturates, phenothiazines, tranquilizers, general anesthetics, atropine)
- Prematurity
- Congenital anomalies
- Fetal anemia
- Fetal cardiac arrhythmia
- Infection
- Preexisting neurologic injury
- Fetal metabolic acidemia
It is through the lens of this last mechanism that fetal well-being is most often assessed when interpreting fetal heart rate variability. Prolonged decreased variability may indicate recurrent or sustained interruption of fetal oxygenation, leading to metabolic acidemia and reflected as decreased minute-to-minute fetal heart rate regulation. While periods of minimal variability are common and expected, persistent minimal and absent variability are not and indicate the need for evaluation and intervention.
The clinical significance of marked variability remains unclear. It may represent a normal physiological variant or could be an early indicator of fetal hypoxemia. Therefore, continuous observation and assessment of the FHR tracing’s evolution are crucial. During periods of marked variability, the baseline is considered indeterminate.
