HOW TO FOCUS
Example of a good waveform
See video above.
- Use plenty of water-based gel on the tip of the probe.
- Place the probe orally or nasally, as preferred, in sedated or anaesthetised patients (nasally in awake patients).
- Insert the probe with the bevel facing upwards (i.e., towards the roof of the mouth). This will mean the probe crystals are more likely to be facing in the right direction (i.e., backwards) when once the probe is at the correct depth.
- Insert the probe to the deepest marker depending on placement i.e., 2nd marker (or slightly deeper) if placed orally, 3rd (or slightly deeper) if placed nasally. Occasionally the probe may be just outside of the appropriate depth marker in very tall or very short patients, but should not be excessively far away from them.
How to Obtain the Optimal Signal
- Once at the depth, slowly rotate the probe all the way around without letting go (or until a signal is found).
- If no signal is found, withdraw the probe slightly by approximately 1 cm and rotate again slowly in the opposite direction.
- Do not pull and rotate at the same time as the signal may be missed due to spiralling the probe.If a signal cannot be obtained after 5 minutes, leave the probe for 5 minutes. First, the insertion/movement may result in a loss of gel from the probe tip and leaving time for mucous to accumulate around the tip will likely improve the transmission of the ultrasound. Second, a lot of movement can result in torsion building up in the spring. Leaving this to relax allows the probe to settle against the oesophageal wall and should enable easier focussing later.
- The best signal will have the loudest, sharpest ‘whipcrack’ sound, accompanied by the tallest and brightest waveform (as near to the appropriate depth markers as possible) above the line. See top right.
- The waveform colour is important. It should be orange/white on the outside edges (particularly the downstroke), with a darkish centre. See top right.
- In Run Mode, ensure the green line lies as close to the waveform edge as possible. If this begins to look ‘ragged’ the probe may not be fully focused. See ‘inadequate signals’ below.
- Ensure the white triangles fall at the base of the flow waveform and not consistently wide of this base since this could indicate the probe is not optimally focused.
Other Hints and Tips
- If the probe will not remain in focus, do not worry. Simply return to the previous depth of the optimised signal and rotate to find it again, ensuring it is optimised (based on the description above).
- Even if the signal appears satisfactory, the user should check it is optimised before taking a new measurement.
- If the quality of the waveform fades with ventilation, a small change in depth or rotation may resolve it. If not, try increasing the gain slightly to pick up all waveforms.
- Only use the filter if valve noises cannot be resolved by adjusting the probe. These are seen as vertical lines close to the waveform edge and with a very distinctive sound.
- Use the appropriate gain. Too much will result in a very white waveform, too little will result in a very faint waveform. Obtain the best signal first and then adjust the gain.
- Ensure you are measuring at the correct area to obtain the correct waveform! See below for examples of waveforms from other vessels.
- If an NG tube is suspected of being in the way, either insert water into the NG tube as ultrasound travels through liquid better, or remove both a little and replace the Doppler probe towards the patient’s left and potentially they may change position.
Inadequate Doppler Waveforms
- The waveform should not have spectral dispersion, i.e., it should not be filled in completely with red. If this is seen, it is likely to represent incorrect focus, but also ensure the gain is not too high.
- Ensure optimal focus as above.
Signals From Other Vessels
If the probe is not at the correct depth, flow velocity waveforms for alterantive vessels (other than the descending aorta) may be identified using the ODM+.
A intracardiac signal will sound like a ‘galloping horse’ and is likely to have flow above and below the line, due to the different flow directions in the heart.
A venous signal will sound slower and ‘whooshy’. The flow profile is likely to be below the line as it represents flow in the opposite direction (i.e. flow travelling towards the probe rather than away from it, as in the descending aorta).
The coeliac axis may be located when the probe is inserted too deeply. The sound may be less sharp and the waveform will be wide with significant diastolic flow that tapers down.
A flow profile from pulmonary artery may be located if the probe is not deep enough and orientated forwards. It may sound similar to the descending aorta but flow will reside below the line.