The contraction monitoring paper is a record produced by an electronic fetal monitor (Cardiotocography or CTG). This printout continuously tracks the activity of the uterus and the corresponding fetal heart rate, especially during labor or non-stress tests. The recorded information helps medical professionals assess the progress of labor and the well-being of the fetus. While only healthcare providers offer a complete clinical interpretation, understanding the basic elements of the tracing offers patients knowledge regarding their care.
Decoding the Monitoring Paper Layout
The monitoring paper is divided into two distinct horizontal tracks, presented one above the other. The upper track is dedicated to charting the fetal heart rate (FHR), which is measured in beats per minute. The lower track is the tocogram, which records uterine activity or contractions, and is the focus for understanding labor progression.
The paper’s grid system uses intersecting lines to represent two different measurements, much like a standard graph. The vertical lines, which run the length of the paper, represent the passage of time on the X-axis. The standard scaling marks a bold vertical line every minute, with five lighter lines in between, each representing a 10-second interval.
The horizontal lines represent the amplitude or strength of the measurement on the Y-axis. On the lower track, the upward movement of the line indicates increasing uterine tension. This two-dimensional grid provides a continuous, synchronized record where any point on the contraction line can be directly correlated with the precise time it occurred.
Measuring Contraction Frequency and Duration
Contraction frequency is a measure of how often the uterus is contracting, calculated by measuring the time interval between successive contractions. To determine this, you must use the time grid (X-axis) to find the difference from the peak of one contraction to the peak of the very next contraction. For instance, if the peak of one contraction is at the 10-minute mark and the next is at the 13-minute mark, the frequency is three minutes apart.
The duration of a single contraction is the total time the uterus spends tightening, measured from the moment the uterine activity line begins to rise above its resting baseline. It ends when the line fully returns to that same baseline. By counting the number of 10-second intervals across the width of the contraction peak, you can determine how many seconds a specific contraction lasted. Both frequency and duration are crucial metrics for assessing the effectiveness and progression of labor.
Understanding Contraction Intensity Measurements
Interpreting the vertical height of the contraction line, which represents intensity, depends entirely on the method of monitoring being used. The most common method, external monitoring, uses a pressure-sensitive device called a tocodynamometer, or Toco, placed on the mother’s abdomen. The Toco measures the tension of the abdominal wall, which is an indirect signal of the change in intrauterine pressure.
External Monitoring
Because the external monitor is affected by factors like the placement of the belt, the patient’s body mass, or movement, the height of the peak represents the relative strength of one contraction compared to others on the same trace. A taller peak only indicates a stronger contraction than the preceding one, but it does not provide an objective, absolute measurement of the actual pressure inside the uterus. The numerical scale on the Y-axis of the external trace is arbitrary and cannot be read as a direct pressure measurement.
Internal Monitoring
A significantly more precise method is internal monitoring, which is achieved using an Intrauterine Pressure Catheter (IUPC) inserted into the uterus after the membranes have ruptured. The IUPC directly measures the fluid pressure within the amniotic cavity, providing an objective, quantifiable reading of the contraction intensity. The Y-axis on a paper from an internal monitor is calibrated in millimeters of mercury (mmHg).
With an IUPC, you can read the numerical value of the contraction peak directly from the mmHg scale. This provides an absolute measurement, allowing professionals to determine the true strength of the contraction. For example, a contraction peak reaching the 60 mark on the Y-axis directly corresponds to a pressure of 60 mmHg. The IUPC also objectively measures the baseline tone, which is the resting pressure in the uterus between contractions.