When you are unable to determine a baseline rate and variability occurs between consecutive
There are several ways to verify the source and/or accuracy of the recorded fetal heart rate pattern.
Verification of the FHR with:
An obstetric stethoscope
A fetal scalp electrode
Verification of the maternal heart rate:
Using pulse oximetry - for a maternal heart rate pattern displayed simultaneously with the FHR
(Cross-Channel Verification (CCV) feature)
Using Maternal ECG - for a maternal heart rate pattern displayed simultaneously with the FHR
Manual determination of the maternal pulse
Whenever possible measure the maternal pulse rate to make use of the monitor's Cross-Channel
Verification (CCV) feature, especially during the second stage of labor, or when the maternal pulse is
elevated over 100 bpm. The Avalon fetal monitor provides a Toco MP or CL Toco
for maternal pulse detection and the creation of a maternal heart rate pattern plotted on the same
recorder as the FHR pattern. In case of difficulties deriving a stable maternal pulse reading using the
Toco MP or CL Toco
When either of these parameters is utilized, the monitor will automatically and continuously perform a
CCV of the maternal heart rate pattern against the FHR pattern displayed on the monitor. If the
patterns and rates are similar, the CCV provides an alarm that both rates are probably from the same
source (i.e., they both represent the maternal heart rate pattern and the fetus is not being monitored).
Repositioning the ultrasound transducer will usually correct this, but it may be necessary to apply a
fetal scalp electrode. Advising the mother to temporarily cease pushing during contractions may help
to more rapidly resolve any uncertainty in this situation.
Doubling: The auto correlation algorithm can display a doubled fetal or maternal heart rate if the
duration of diastole and systole are similar to each other, and if the heart rate is below 120 bpm.
Doubling, usually brief, is accompanied by an abrupt switch of the trace to double the baseline value.
Halving: With fetal tachycardia (above 180 bpm) and some interference from breathing or maternal
arteries the auto correlation algorithm may only recognize every second beat resulting in a halved rate
for a limited time. If the actual FHR is above the maximum limit of the monitor (240 bpm), the
algorithm will also half-count. Halving is accompanied by an abrupt switch of the trace to exactly half
the prior baseline value. This switch may simulate a FHR deceleration and be referred to by clinicians
as a "false deceleration."
Switching to maternal heart rate (also referred to as "Maternal Insertion"): The fetal heart can
move partly or fully out of the ultrasound beam and the auto correlation algorithm may then pick up
and display the maternal heart rate. Depending on the signal mix in the ultrasound signal, switching to
the maternal heart rate may mimic several conditions with the potential for erroneous interpretation
and response as follows:
The switch to the maternal heart rate may simulate a FHR deceleration (i.e., a decrease of
the fetal heart rate, and be referred to by clinicians as a "false deceleration").
11 Monitoring FHR and FMP Using Ultrasound
MP transducer, use SpO
or MECG instead.