Montevideo Units Cannot Be Accurately Measured from External Tocodynamometry
Montevideo Units (MVUs) cannot be accurately measured from an external tocodynamometer because external tocodynamometry does not provide reliable measurements of contraction intensity (mmHg), which is essential for calculating MVUs.
Understanding External Tocodynamometry vs. Internal Monitoring
External tocodynamometry is widely used in clinical practice for monitoring uterine contractions, but it has significant limitations:
External tocodynamometers can reliably detect the frequency and duration of contractions but cannot accurately measure the amplitude or intensity of contractions 1
The intensity of contractions detected externally is significantly less than internally measured contractions, with a mean difference of 19.7 ± 15.9 mmHg (p < 0.001) 2
Montevideo Units are calculated by multiplying the intensity (mmHg) of contractions by their frequency per 10 minutes, requiring precise measurement of contraction strength 1
Why Internal Monitoring Is Required for MVUs
To accurately calculate Montevideo Units, internal tocodynamometry is necessary:
Intrauterine pressure catheters are considered the most sensitive and specific technique for measuring uterine activity and have become the standard by which other methods are judged 1
Only internal tocodynamometry provides quantitative measurements of contraction intensity in mmHg, which is essential for calculating MVUs 3
External monitoring can detect the presence of contractions with high sensitivity (90.8%) and specificity (98.1%), but cannot adequately measure their intensity 2
Limitations of External Tocodynamometry
Several factors limit the usefulness of external tocodynamometry for quantitative measurements:
External monitors require frequent adjustment during labor and might not work well in patients who are obese 1
The predictive value of external monitoring is 97.3% for detecting uterine contractions and 93.6% for recording the absence of uterine contractions, but this does not extend to intensity measurement 2
External tocodynamometers measure the relative tension of the abdominal wall rather than actual intrauterine pressure 4
Alternative Approaches
While MVUs cannot be accurately measured externally, other approaches to monitoring uterine activity include:
Electrohysterography, a noninvasive technology that detects uterine electrical activity using electrodes placed on the mother's abdominal wall, is at least as reliable and accurate as tocodynamometry for detecting contractions but still doesn't provide MVU measurements 1
Beltless external tocodynamometer systems have shown improved data usability (94% vs 45% for belted systems) but still cannot measure contraction intensity accurately 5
Clinical Implications
The inability to measure MVUs externally has important clinical implications:
When precise quantification of uterine activity is required, such as in cases of suspected hyperstimulation during oxytocin administration, internal monitoring may be necessary 3
For routine monitoring of contraction frequency and pattern, external tocodynamometry remains appropriate and avoids the risks associated with internal monitoring 1
The American College of Obstetricians and Gynecologists (ACOG) notes that home uterine activity monitoring using external tocodynamometers is not recommended for predicting preterm birth, partly due to these measurement limitations 6
In conclusion, while external tocodynamometry is valuable for detecting the presence, frequency, and duration of contractions, the calculation of Montevideo Units requires accurate measurement of contraction intensity in mmHg, which can only be reliably obtained through internal tocodynamometry.