User's Manual

11 Monitoring FHR and FMP Using Ultrasound
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11Monitoring FHR and FMP
Using Ultrasound
To monitor a single FHR externally, you use an ultrasound transducer attached to a belt around the
mother's abdomen. The ultrasound transducer directs a low-energy ultrasound beam towards the fetal
heart and detects the reflected signal. Your monitor can also detect fetal movements and print the fetal
movement profile (
FMP) on the trace. Monitoring using ultrasound is recommended from the
25th week of gestation for non-stress testing or routine fetal monitoring.
WARNING
Performing ultrasound imaging or Doppler flow measurements together with ultrasound fetal
monitoring may cause false FHR readings, and the trace recording may deteriorate.
Technical Description
Fetal monitors use the ultrasound Doppler method for externally monitoring the fetal heart rate. Using
the Doppler method, the transducer (in transmitter mode) sends sound waves into the body which are
then reflected by different tissues. These reflections (Doppler echoes) are picked up by the transducer
(in listening mode). These Doppler echoes are amplified and sent to the monitor’s speaker through
which the fetal heart signal can be heard. In parallel the Doppler echoes are processed through an auto
correlation algorithm to determine the fetal heart rate (FHR). The FHR is displayed on the monitor’s
numeric display and on the recorded trace.
Properly representing the fetal heart rate using a device that derives heartbeats from motion is a
formidable task and the limitations of the technology will be discussed shortly. Basic fetal cardiac
physiology may contribute to difficulties in obtaining a reliable ultrasound signal.
A heart rate pattern of a fetus is capable of extraordinary variation, ranging from a stable pattern with
minimal variation while the fetus is “asleep” to robust accelerations of 40-60 bpm above baseline rate
over a few seconds, or exaggerated variability when the fetus is active. Decelerations of the rate
60-80 bpm below baseline may develop even more abruptly than the accelerations. The Beat-to-Beat
arrhythmias of the FHR may further exaggerate how much the the FHR variability at the bottom of
the variable decelerations, or in the presence of fetal breathing movements, which also tend to lower
the fetal heart rate. The recognition of these normal variations in fetal heart rate patterns will greatly
assist in the separation of genuine fetal information from the artifact.
Limitations of the Technology
All tissues moving towards or away from the transducer generate Doppler echoes. Therefore, the
resulting signal that is provided to the monitor’s speaker, and for further fetal heart signal processing,
can contain components of the beating fetal heart wall or valves, fetal movements, fetal breathing or
hiccup, maternal movements such as breathing or position changes, and pulsating maternal arteries.
The fetal heart signal processing uses an auto correlation algorithm to obtain periodic events such as
heart beats. If the signal is erratic such as from a fetal arrhythmia, the ultrasound device may have