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Instructions For Use
EN
Appendix 7 Limitations of Ultrasonic
Monitoring
A7.1 How Does Ultrasound Work
When the ultrasound waves strike an object, they bounce back and create an echo. If
the object moves toward the sound source, the frequency of the echo increases. If the
object moves away from the sound source, the frequency of the echo decreases. This
is called “Doppler Effect”. In the 1960's, the ultrasonic technique was first applied to
medical diagnostic imaging.
The ultrasound process involves placing a small device called a transducer, against
the skin of the patient near the region of interest. The ultrasound transducer combines
functions of emitting and receiving ultrasounds in one device. This transducer produces
a stream of inaudible, high frequency sound waves which penetrate into the body and
bounce off the organs inside. It detects sound waves as they bounce off or echo back
from the internal structures and contours of the organs. The movement of the organs
produces the Doppler Effect, and this movement can be measured and described by
measuring the echo.
In fetal monitoring, the ultrasound transducer produces a stream of sound waves
which penetrate into the maternal abdomen and bounce off the fetal heart. Then the
transducer receives the echoes and transfers them to the monitor, which turns the
signal into fetal heart beating sound and fetal heart rate trace.
Therefore, placement of the transducer is critical to ultrasound fetal heart monitoring.
A7.2 Artifacts in Fetal Heart Monitoring
(1)
How does artifact happen?
The transducer detects sound waves as they bounce off or echo back from the fetal
heart. However, the sound waves bouncing off from maternal blood vessels may be
detected by the transducer and then be processed by the monitor as well. As a result,
artifacts may be produced.
The artifacts, if not correctly interpreted, may cause the physicians to perform
unnecessary interventions, or to fail to detect the fetal distress and the need for
interventions.
The most common artifacts are doubling and halving.
(2) Doubling:
When the FHR drops to 120 bpm or lower, the diastole and systole become far apart,
thereby the monitor may mistake these two movements of a single heartbeat for two
separate heartbeats. As a result, a heart rate trace that is double the actual heart
rate is produced. This often happens during severe decelerations and bradycardia,
representing an abrupt switch of the trace to double the actual heart rate.
