10 Monitoring FHR and FMP Using Ultrasound
96
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 autocorrelation 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
trouble tracking the abrupt changes and may misrepresent the true FHR pattern. Signals such as those
from moving fetal limbs are usually very strong, thereby masking the fetal heart signal. During
prolonged movements where the fetal heart signal is masked, the FHR appears blank on the numeric
display and as a gap on the recorded trace. Fetal position changes, maternal position changes, or
uterine contractions can move the fetal heart partly or fully out of the ultrasound beam resulting in
signal loss or even picking up Doppler echoes from pulsating maternal arteries. In these cases a
maternal heart rate or sometimes even a rate resulting from the mixture of fetal and maternal signals
may be displayed on the monitor’s numeric display and on the recorded trace.
In contrast to the timely well-defined R-peak of an ECG signal obtained with a fetal scalp electrode,
the ultrasound Doppler signal from a fetal heart consists of multiple components from atria (diastole),
ventricles (systole), valves, and pulsating arteries. These components vary depending on fetal and
transducer position and angle, and are further modulated by factors such as fetal or maternal breathing.
These effects may produce what is called “artifact”. Optimal transducer positioning therefore is key to
minimizing these effects and thereby minimizing artifact.
Misidentification of MHR as FHR
FHR detection by the monitor may not always indicate that the fetus is alive. Confirm fetal life before
monitoring, and continue to confirm that the fetus is the signal source for the recorded heart rate (see
“Confirm Fetal Life Before Using the Monitor” on page 10).
Here are some examples where the MHR can be misidentified as the FHR.
• When using an ultrasound transducer:
– It is possible to pick up maternal signal sources, such as the maternal heart, aorta, or other
large vessels.
– Misidentification may occur when the maternal heart rate (MHR) is higher than normal
(especially when it is over 100 bpm).
NOTE
When an ultrasound transducer is connected to the monitor but not applied to the patient, the
measurement may generate unexpected intermittent FHR readings.
• When Fetal Movement Profile (FMP) is enabled:
The FMP annotations on a fetal trace alone may not always indicate that the fetus is alive. For
example, FMP annotations in the absence of fetal life may be a result of:
– Movement of the deceased fetus during or following maternal movement.
– Movement of the deceased fetus during or following manual palpation of fetal position
(especially if the pressure applied is too forceful).
– Movement of the ultrasound transducer.
To Next Page
Loading...
