During pregnancy and labor, providers monitor both the baby’s heart rate and the mother’s uterine contractions using external sensors. One of these sensors is the Tocodynamometer, or Toco for short, which tracks uterine activity. Electronic fetal monitoring (EFM) uses two separate transducers placed on the maternal abdomen to gather this data. The information collected by the Toco is printed on a continuous strip of paper or displayed on a screen, providing a visual representation of how the uterus is behaving.
Defining the Tocodynamometer (Toco)
The Tocodynamometer is a non-invasive, external sensor used to detect and record changes in abdominal pressure caused by the tightening of the uterus. It is a small, disc-shaped pressure transducer secured to the mother’s abdomen using an elastic belt. This device is typically positioned over the uterine fundus, the top part of the uterus where contractions are often felt most strongly. Correct placement is usually determined by palpating the abdomen during a contraction to find the firmest area.
The Toco is easily distinguishable from the Doppler ultrasound transducer, which uses sound waves to pick up the fetal heart rate. The Toco is a simple pressure-sensitive button. When the uterus contracts, it pushes against the maternal abdominal wall, depressing the plunger inside the sensor. This mechanical pressure change is converted into an electrical signal displayed as a waveform on the monitor.
How the Toco Measures Uterine Activity
The primary function of the Toco is to measure the timing aspects of uterine contractions, providing data on their frequency and duration. When a contraction begins, the pressure exerted on the sensor increases, causing the tracing line on the monitor to rise. As the contraction subsides, the pressure lessens, and the tracing returns to its baseline level. This creates a characteristic “hill” shape on the printed strip, with each hill representing one contraction.
The frequency of contractions is measured by counting the number of peaks that occur within a 10-minute window, averaged over a 30-minute period. The duration is measured from the point where the tracing line begins to rise until it returns to the resting baseline. This timing information is important for assessing the progress of labor and ensuring the contraction pattern is efficient. A consistent pattern of increasing frequency and duration often indicates true labor is progressing, allowing clinicians to make informed decisions about care.
Why the Toco Cannot Measure Contraction Strength
Despite the Toco’s ability to record changes in abdominal pressure, it cannot accurately measure the true physiological strength or intensity of a uterine contraction. Because the Toco is an external device, the pressure it detects is influenced by factors external to the uterus. For instance, a patient’s body size, the amount of adipose tissue on the abdomen, and slight movements can significantly affect the reading.
The tightness of the elastic belt holding the sensor in place also directly impacts the Toco number, making the readings arbitrary and relative to each patient. Therefore, the number displayed on the monitor, typically ranging from 0 to 100, is not an absolute measure of contraction strength in millimeters of mercury (mmHg). To obtain a true, quantitative measurement of intensity, an internal method is required, such as the Intrauterine Pressure Catheter (IUPC). The IUPC is a small tube inserted into the uterus after the amniotic sac has ruptured, allowing it to directly measure the pressure inside the uterine cavity in mmHg.