QCOM
Quillen College of Medicine, East Tennessee State University


Baseline Variability

-Fluctuations in the FHR of 2 cycles per min or greater

-Variability is visually quantitated as the amplitude of peak-to-trough in beats per min
  -Absent: amplitude range undetectable
  -Minimal: amplitude range detectable but 5 beats per min or fewer
  -Moderate (normal): amplitude range 6-25 beats per min
  -Marked: amplitude range greater than 25 beats per min

(NICHD)


In the normal fetus, there is an interplay between the sympathetic (acceleration) and parasympathetic (deceleration) nervous sytems in the control of heart rate.  These systems exert their control via the cerebral cortex, the medulla oblongata, the sympathetic ganglia and the vagus nerve. The interaction between these systems results in a difference in the beat-to-beat intervals resulting in variability of the fetal heart rate tracing. In the term fetus, moderate variability is considered normal as it indicates a normally functioning central nervous system.  Conditions that alter the integrity of this neuro-cardiac axis, such as hypoxemia, result in loss of heart rate variability. Variability therefore, is the single most important indicator of an adequately oxygenated fetus.  Below are examples of moderate variability.

 
Moderate Variabilty Video 1
Moderate Variability Video 2

Decreased or absent variability therefore represents some dysfunction in one or both of these systems, or in increased and dominant tone of one system over the other, such as during sleep cycles or due to the effects of drugs.

Causes of decreased variability include:

  • Hypoxemia/acidosis
  • Fetal sleep cycles
  • Drugs (Analgesics, barbiturates, tranquilizers, phenothiazines, para-sympatholytics, anesthetics)
  • Prematurity
  • Arrhythmias
  • Fetal tachycardia
  • Preexisting neurological abnormality
  • Congenital anomalies

Below is an example of minimal variability.  Please click to enlarge.

 
 
Minimal Variability Video 1
Minimal Variability Video 2
Minimal Variability Video 3
Minimal Variability Video 4
 
 
Below is an example of absent variability.  Please click to enlarge.

Absent Variability Video 1
Absent Variability Video 2

Marked variability in the baseline FHR is present when the amplitude exceeds 25 BPM.This pattern (sometimes called a saltatory pattern) suggests acute hypoxia or mechanical compression of the umbilical cord and is often seen during the second stage of labor. When coupled with decelerations, this pattern is considered non reassuring and should warn the physician to search for, and correct, potential causes of hypoxia. Causes of marked (increased or saltatory) variability include:

  • Fetal stimulation
  • Mild, transient hypoxemia
  • Drugs
Below is an example of marked variability.  Please click to enlarge.

 
 
Marked Variability Video 1
Marked Variability Video 2


Two types of variability have also been described prior to the NICHD definitions. Short term variability (STV) describes the beat-to-beat fluctuations in the FHR measured from R wave to R wave by fetal ECG. Long term variability (LTV) represents the amplitude changes of the normal baseline over time, with a cyclic changes occurring in less than one minute.

The NICHD does not distinguish between LTV and STV, because they are viewed as a unit when the FHR is visually analyzed. Additionally, LTV and STV generally respond synchronously in response to changes in autonomic tone, and thus differentiation between LTV and STV generally does not provide additional information regarding the fetal oxygenation. There are some special circumstances where this not true; for example, a fetus with severe anemia demonstrates a pattern with absent STV but present LTV. This unique tracing is referred to as a sinusoidal pattern and will be discussed separately.