Fetal Monitoring Equipment and Techniques
Core concepts
Electronic fetal monitoring uses Doppler ultrasound for heart rate and tocodynamometry or an intrauterine pressure catheter for contractions. External ultrasound detects motion of cardiac structures; it is affected by maternal obesity, fetal position, and movement artifact. The tocodynamometer detects tension changes and estimates frequency and duration but not intensity. An intrauterine pressure catheter measures actual pressure and is used when contraction intensity or adequacy is in question.
Fetal heart rate detection can also use fetal scalp electrode, which gives a direct fetal electrocardiogram signal and is more reliable when the external signal is poor. Each method has limits and risks: scalp electrodes require ruptured membranes and adequate dilation.
Interpretation starts with verifying the signal. Maternal heart rate can be mistaken for fetal heart rate, especially with maternal tachycardia or when the ultrasound transducer picks up maternal vessels. Always confirm by palpating the maternal pulse and comparing rates.
Traps
Questions frequently ask which device measures intensity (intrauterine catheter) versus frequency only (external tocodynamometer). Another trap is assuming a good-looking tracing is valid when the rate equals the maternal pulse. Also know that ultrasound does not measure fetal oxygen saturation, and that a non-reassuring pattern must be confirmed as fetal before acting.
Rule of thumb
If the fetal heart rate is within a few beats of the maternal pulse, or if it does not change when you move the transducer, suspect recording of the maternal heart rate.
Worked example
A tracing shows a baseline of 120 beats per minute with minimal variability in a patient whose pulse is 118. Palpate the maternal pulse during the tracing; if it matches, reposition and reacquire the fetal signal before interpreting the strip.