Many times when attempting to detect the fetal heart, the maternal vascular sounds are
heard instead of (or in some cases, in addition to) the fetal sounds. These maternal sounds
can come from one of the major arteries, the placenta or the umbilical cord. The maternal
vascular sounds are typically higher in frequency at a lower rate. The heart rate calculation
will display either the maternal rate or the fetal rate, whichever portion of the signal is
stronger.
If the fetal heart sounds cannot be located using the procedure as described above, a
second exam should be performed using another commercially available fetal monitor as a
repeated test.
5 and 8 MHz Vascular
Peripheral arterial sounds are typically higher in frequency. For the best sounds, angle
the probe approximately 45 degrees from the skin surface over the general location of
the vessel. Slowly move the probe side to side and vary the angle of the probe until
the vascular sounds are heard. Changing the angle of the probe has an effect on the
frequency of the sound. The steeper the probe angle is, the higher the frequency of the
sound.
Peripheral venous sounds are not typically periodic and vary greatly depending on patient
movement and breathing. These sounds are more like the wind at the ocean and vary in
pitch as the patient moves or breathes.
Obtaining Doppler Signals
Caution
• Doppler examinations should be performed only by trained individuals.
For any Doppler examination, it is essential that an adequate supply of coupling gel is used
to transmit the ultrasound energy from the probe to the surface of the skin. Re-apply more
gel if it starts to dry out or spread so thinly that an air gap occurs between the probe and
the skin. It is not necessary to cover the entire surface of the probe, only the probe face.
Applying too much gel makes the unit difficult to clean and does not aid in the performance
of the probe.
Volume Slider
The audio level can be adjusted using the Volume Slider. Moving the slider up will increase
the volume, while moving it down will decrease it.
Signal Quality Indicator (TR250 only)
An inadequate signal can produce erroneous rates from the heart rate calculation. The
signal level that is being obtained is shown on the Signal Quality Indicator bars. This
indicator provides a visual aid in obtaining a strong audio signal by showing the pulsatile
nature of the signal. A large difference between the highest and lowest signal bars that are
lit confirms that the quality of the signal is good and thus ensures the heart rate calculation
is operating at peak performance.
The heart rate can be verified manually by counting the audible beats for 20 seconds and
multiplying by 3, or for 15 seconds and then multiplying by 4. Counting for less than 15
seconds is not recommended due to a decrease in accuracy with the small sample size.
Obstetrical
Fetal heart sounds are quite different from peripheral vascular blood flow sounds. Fetal
sounds are typically much lower in frequency and much higher in rate. For early term fetal
detection, start the probe at the pubic bone and slowly move along the midline – rocking
the probe slowly from side to side until a heart beat is heard. For mid to late term fetal
detection the best chance of finding the heart sounds are to start on the fundus and move
toward the navel and from one side of the abdomen to the other, slowly rocking the probe
until the heart beat is heard. The fetal heart reminds many people of a galloping horse and
can vary in tone from a distant swishing sound to a hard clopping sound depending on the
position of the baby and probe.
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