Comprehensive and Detailed Explanation From Exact Extract NCC-Recommended Sources
NCC-recommended fetal assessment texts emphasize that external Doppler ultrasound may undercount very rapid fetal arrhythmias such as fetal supraventricular tachycardia (SVT). Because Doppler detects mechanical motion rather than electrical activity, the device may record only every other cardiac contraction, a phenomenon known as “half-counting.”
Menihan’s Electronic Fetal Monitoring explains that with SVT—often exceeding 200 to 260 bpm—the monitor “may display a fetal heart rate at approximately half the true atrial rate.” AWHONN teaching materials affirm that rapid, regular tachyarrhythmias may appear deceptively slower on the external monitor due to Doppler under-sampling. Simpson & Creehan note that half-counting is a recognized technical limitation and may cause clinicians to miss true tachyarrhythmias if internal monitoring is not applied.
In contrast, artifact displays irregular, inconsistent, and non-physiologic deflections. Matching the maternal pulse suggests maternal heart rate misinterpretation, not SVT.
Miller’s Pocket Guide also highlights that half-counting is “commonly seen in fetal SVT when using external Doppler due to failure to detect each rapid contraction.”
Therefore, fetal SVT most commonly appears as half the actual rate on an external fetal monitor.
[References:, AWHONN – Fetal Heart Monitoring Principles & PracticesMenihan – Electronic Fetal MonitoringSimpson & Creehan – Perinatal NursingCreasy & Resnik – Maternal-Fetal MedicineMiller’s Pocket Guide]
