Huntleigh SonicaidTeam, Operator'S Manual, Page: 41 / 113

Sonicaid Team Operator’s Manual
41
False recording of low baseline FHR
When monitoring a low baseline FHR using Doppler ultrasound, the heart rate may
be falsely reported. This effect is known as
double-counting , and is characteristic of
ultrasound fetal monitoring.
In normal circumstances the atrium and ventricle beat almost simultaneously. The
ultrasound reflected from these two chambers is used by fetal monitors to calculate
the FHR. When the FHR is low, at 70-80bpm, there is a longer time interval between
the atrial and ventricular contractions. A fetal monitor may take the reflection from
each chamber as a separate beat and therefore falsely calculate the FHR.
It can also happen, though very rarely, that the monitor double counts signals which
are maternal in origin.
The Sonicaid FM800’s heart rate detection system separates movements of the heart
away from the transducer from those towards the transducer. This helps to correct
some instances of double-counting, but does not entirely prevent it.
How to minimise the chances of double counting occurring
1 Always palpate the abdomen and listen to the fetal heart with a Pinard stetho-
scope or hand-held Doppler unit before applying the ultrasound transducers. This
helps to verify the fetal heart and to locate the area where best signal quality can
be expected.
2 Palpate the maternal pulse for one minute simultaneously and record it on the
printed output.
3 Recording a signal for maternal ECG will help to identify any cross-correlation
between maternal and fetal heart rates.
4 Listen to the fetal heart rate using the FM800 Audio signal. The sound should be
like a galloping horse, not a swishing sound from maternal vessels.