The Tocodynamometer, often simply called a Toco monitor, is a common external device used during pregnancy and labor to observe uterine activity. This non-invasive tool helps healthcare providers track the timing and pattern of a person’s contractions without needing to enter the uterus. The information it provides is displayed as a wavy line, which is an integral part of electronic fetal monitoring. Understanding this graphic output is important for tracking labor progression and ensuring the well-being of both the parent and the fetus.
The Purpose and Placement of the Tocodynamometer
The Toco monitor itself is a pressure-sensitive disc, or transducer, secured to the maternal abdomen with an elastic belt. Its purpose is to detect and record the changes in the shape and tension of the abdominal wall that occur when the uterus tightens during a contraction. As the uterine muscle shortens, it pushes against the abdominal wall, and the Toco converts this external pressure change into an electrical signal that is then graphed.
Proper placement is essential for gathering the most reliable data on uterine activity. The transducer is typically positioned over the uterine fundus, the top, dome-shaped part of the uterus. Contractions physiologically begin in this area and are strongest there, making the fundus the ideal location for the Toco to register the maximum external pressure change. By tracking these changes, the Toco reliably provides data on the frequency and duration of contractions.
Decoding the Contraction Waveform
The reading generated by the Toco monitor appears as a series of hills or waves on a graph, each representing a single uterine contraction. Analyzing the pattern of these waves allows clinicians to assess the progression and characteristics of labor.
The lowest point of the line, which occurs when the uterus is relaxed between contractions, is known as the resting tone or baseline. When a contraction begins, the line rises gradually from this baseline, peaking at its highest point, called the acme. The height of this peak is the monitor’s numerical representation of the pressure change it detects.
The duration of the contraction is measured from the point where the line first begins to rise until it returns to the resting tone. This duration is typically measured in seconds, with contractions in active labor often lasting between 30 and 70 seconds. The frequency of contractions is calculated by measuring the time interval from the beginning of one contraction to the beginning of the next contraction, expressed in minutes.
Why Toco Readings Do Not Measure Contraction Intensity
While the Toco monitor provides a numerical value for the peak of a contraction, this number is a relative measure of external pressure, not a direct measurement of true uterine strength, or intensity. The device only measures the change in abdominal tension at the site of placement, which is an indirect assessment of the force generated by the uterine muscle.
The numerical reading is significantly influenced by external factors that have nothing to do with the actual strength of the muscle contraction. These variables include the tightness of the securing belt, the exact position of the transducer on the abdomen, and the patient’s body size. For instance, a small shift in the monitor’s position or a slight tightening of the belt can cause the numerical value to increase without any change in the actual intensity of the contraction itself.
The only way to accurately and directly measure contraction intensity is through an invasive procedure using an Intrauterine Pressure Catheter (IUPC). This catheter is inserted into the uterus, past the cervix, and measures the pressure of the amniotic fluid in millimeters of mercury (mmHg). Because the Toco is external, its primary and most reliable function remains the accurate tracking of contraction frequency and duration, which are independent of these external variables.
Recognizing Abnormal Uterine Activity Patterns
The pattern of uterine activity on the Toco strip is closely monitored because certain patterns can indicate potential issues for the fetus or the parent. These abnormal patterns are important indicators that require prompt assessment by healthcare staff to ensure appropriate intervention.
- Uterine Tachysystole: This is defined as having more than five contractions within a ten-minute period, averaged over a 30-minute window. This excessive frequency can compromise fetal oxygen supply by reducing the necessary time for the placenta to refill with oxygenated blood between contractions.
- Prolonged Contraction: This is generally defined as one lasting longer than 90 to 120 seconds. Such an extended tightening reduces the resting time for the uterus, which can also decrease the oxygen available to the fetus.
- Inadequate Resting Tone: This occurs when the tracing baseline remains elevated and the uterus does not fully relax between contractions. This signals ongoing high pressure within the uterus.