Electronic fetal monitoring is a widely used technology during labor and delivery to continuously assess the condition of the fetus by tracking the heart rate and the mother’s uterine contractions. The resulting printout, known as a Fetal Heart Rate (FHR) strip or cardiotocography (CTG), provides a continuous visual record of these two physiological parameters. Healthcare providers use the patterns on this strip to evaluate fetal well-being, looking for signs of adequate oxygenation and a healthy nervous system response. Interpretation involves analyzing the strip’s physical layout, the baseline heart rate and its variability, and any transient changes. The goal is to identify changes that may signal fetal distress, allowing for timely intervention.
Understanding the Physical Layout of the FHR Strip
The FHR strip is essentially a continuous sheet of paper divided into a grid system that plots two separate recordings simultaneously. The upper section of the strip displays the Fetal Heart Rate, measured in beats per minute (BPM), while the lower section charts the mother’s uterine activity, which indicates contractions. Each track is represented by a distinct line, allowing for a side-by-side comparison of the baby’s heart rate response to the force of the uterus.
The vertical axis of the upper track is scaled to represent the heart rate, with each small vertical box typically representing 10 BPM. The lower track’s vertical axis measures uterine pressure, often in millimeters of mercury (mmHg) if an internal monitor is used, or the relative strength of the contraction with an external monitor. The horizontal axis across both tracks represents time, which is crucial for determining the duration and timing of events. Each small square horizontally represents six seconds, and a thicker, large square typically represents one minute, allowing for precise timing of heart rate changes relative to contractions.
Defining the Fetal Heart Rate Baseline and Variability
The Fetal Heart Rate baseline is defined as the mean FHR rounded to the nearest five BPM during a ten-minute window, excluding any temporary accelerations or decelerations. A normal baseline heart rate for a term fetus ranges between 110 and 160 BPM. A sustained rate below 110 BPM is termed bradycardia, while a rate above 160 BPM is called tachycardia, and both warrant further clinical investigation.
FHR variability refers to the subtle, irregular fluctuations in the baseline heart rate that are a direct result of the interplay between the fetal sympathetic and parasympathetic nervous systems. This fluctuation is measured as the amplitude range from the peak to the trough of the heart rate line. The presence of moderate variability, defined as an amplitude range of 6 to 25 BPM, is a reassuring sign. It indicates that the fetal nervous system is well-oxygenated and healthy, reflecting the baby’s ability to adapt to the stresses of labor.
Variability that is minimal (detectable but 5 BPM or less) or absent (undetectable) is often a non-reassuring finding. It can suggest fetal sleep, maternal medications, or, more concerningly, a lack of oxygen reaching the fetal brain. Conversely, marked variability (amplitude greater than 25 BPM) is also outside the normal range and can sometimes signal an early response to a lack of oxygen or be associated with fetal activity. A reduction in this beat-to-beat irregularity is considered a reliable indicator of potential fetal compromise.
Analyzing Transient Events: Accelerations and Decelerations
Beyond the baseline, the FHR strip must be analyzed for transient changes. An acceleration is an abrupt, temporary increase in the FHR, defined as an increase of at least 15 BPM above the baseline, lasting for at least 15 seconds. The presence of accelerations is a reassuring sign, demonstrating a responsive nervous system and adequate oxygen reserves.
A deceleration is a temporary decrease in the FHR below the baseline, and these are categorized by their shape and their timing relative to a uterine contraction. An early deceleration is a gradual drop that mirrors the shape of the contraction, with the lowest point (nadir) of the heart rate decrease occurring at the same time as the peak of the contraction. This pattern is typically benign and is thought to be caused by head compression, which triggers a reflexive slowing of the heart rate via the vagus nerve.
Variable decelerations appear as an abrupt, jagged drop and recovery in the heart rate, often resembling a “V,” “W,” or “U” shape, and their timing is inconsistent with the peak of the contraction. These are most commonly caused by umbilical cord compression, which temporarily restricts blood flow. A late deceleration is a gradual, symmetrical drop that begins after the contraction has peaked, with the nadir occurring after the peak of the contraction. This pattern is concerning because it suggests uteroplacental insufficiency, meaning there is a delayed and insufficient blood flow from the placenta to the fetus during the contraction.
Categorizing the Tracing for Clinical Assessment
Healthcare providers synthesize all the characteristics of the FHR strip into a standardized, three-tiered categorization system. This system provides a framework for clinical assessment and guides management decisions. Category I tracings are considered normal and are strongly predictive of a normal acid-base status in the fetus. They must include a normal baseline rate and moderate variability, with no late or variable decelerations.
Category III tracings are abnormal and are predictive of an abnormal fetal acid-base status, requiring immediate evaluation and intervention. These tracings are characterized by absent variability accompanied by recurrent late or recurrent variable decelerations, an extremely slow baseline heart rate, or a sinusoidal pattern. Category II tracings are considered indeterminate, encompassing all patterns that do not fit into Category I or Category III. This broad category includes findings such as minimal or marked variability, or intermittent decelerations, and requires continued surveillance to determine fetal well-being.