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Appendix 7: Limitations of Ultrasonic Monitoring
Auscultation should be applied when sudden changes in baseline are detected.
If the amniotic membrane rupture and cervical dilatation are sufficient, consider using a spiral
electrode to obtain precise FHR with direct fetal ECG as the signal source.
(4) Erratic Traces / Drop out
When the fetal heart moves partially out of the ultrasound wave path, the transducer receives
mixed or weak signals, and thereby the monitor presents erratic traces. When the fetal heart
moves fully out of the path, inadequate consecutive and periodic signals are received, and no
trace is represented.
Erratic traces and transitory episodes of drop out are common, especially when the fetus or/and
mother move(s). If they exist for an extended period, it indicates that the transducer is not aimed
at the fetus. Repositioning of the transducer is needed.
A7.3 Audio Output and Screen Reading
In most instances, the audio output from the monitor speaker corresponds to the readings
presented on the monitor screen. But occasionally the fetal heart sound may differ from the trace
and numeric.
When the fetal heart moves partially out of the ultrasound wave path, the transducer receives
weaker FHR signal and other stronger signals (usually maternal heart/pulse rate). After the
signals are transmitted to the monitor, the audio system and the video system of the monitor
process the signals separately. On one hand, the audio circuit filters the low-frequency signals and
gives audio output of the high-frequency signals, so fetal heart sound is heard. On the other hand,
the autocorrelation algorithm computes the stronger signal source and thereby the maternal
heart/pulse rate is displayed. As a result, the audio output differs from the screen reading.
If this situation occurs, it can be corrected by repositioning the transducer.
In summary, the abnormalities listed above (artifacts, sound and reading differences) are caused
by the limitations of ultrasonic monitoring technique. Fortunately they rarely occur. But a good
understanding of how to detect them and what countermeasures should be taken will help obtain
better fetal monitoring effect.
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