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Probe Placement

Probe Placement

The probe is placed into the oesophagus, either orally or nasally and is similar to placing a nasogastric tube. The ODM monitor uses Doppler ultrasound technology to determine directly a patient’s central vascular blood flow and fluid status during the intraoperative period.

Easy to use and quick to focus, the device generates a low frequency ultrasound signal which is highly sensitive to changes in flow and measures them immediately.

Once in position the clinician is able to measure real time blood velocities within the descending aorta.

For more information, visit Deltex e-Learning or access the Training Workbook for a deeper understanding of how to use this technology.

Inserting the Probe Orally

Instructions for use:
  • Lubricate tip of probe with plenty of water-based gel. Do not use oil-based products.
  • The technique is similar to that of placing an orogastric tube into the oesophagus.
  • Do not use force.
  • There are 3 depth markers on the adult probe. They are set at 35, 40 and 45cm from the tip. Using these depth markers and assessing the signal, will ensure the tip of the probe is at approximately T5/T6 area of the descending aorta. This is the region where the oesophagus and descending aorta will be closest.
  • For oral use in an adult, the incisors should be between maker 1 and 2. It may be acceptable for the incisors to be just outside of these markers by approximately 1 or 2 cm if the patient is particularly short or tall. Also consider the length of the torso if a signal cannot be found.  Start at the deepest marker to find the optimal focus.
  • It can be used together with a nasogastric tube, a temperature probe or even a transoesophageal echocardiogram (TOE) probe.
  • ODM probes have been used successfully with an LMA. It may be helpful to place the probe before the LMA is placed. Ensure the seal of the LMA is lubricated well and when inflated, it should easily accommodate the probe.
Considerations:
  • Abnormalities or surgery of the mouth, pharynx, aorta and oesophagus.
  • Base of skull fractures.
  • Coagulopathies.
  • Remove probe if patient having an MRI.
  • Remove probe or disconnect if cardioversion required.
The following may make locating an optimal signal difficult:
  • Coartation of the aorta.
  • Use of intra aortic balloon pump.
  • Anatomical abnormalities.
Securing the probe:
  • Many users do not secure the probe since peristalsis in the oesophagus may cause the probe to move internally.
  • If the user wishes to secure the probe with tape or a nasal clip, then ensure the probe can be manipulated to check focus.

Inserting the Probe Nasally

Instructions for use:

The probe can be used nasally in sedated, anaesthetised, awake and waking adult patients.

It should not be used nasally in paediatric patients.

  • Lubricate tip of probe with plenty of water-based gel. Do not use oil-based products.
  • Insert nasally into the oesophagus via nose and then the oropharynx. The technique is similar to that of placing a nasogastric tube into the oesophagus.
  • Do not use force.
  • Having the tip of the probe in the direction of the top of the head may aid insertion into and around the oropharynx.
  • In awake patients, the nasal mucosa may be anaesthetised if required and the posterior oropharynx can be sprayed with aerosol-based local anesthetic. Refer to local polices for further reference.
  • Having the patient swallow water may aid the movement of the probe down the oesophagus.
  • If necessary, small amounts of sedation may useful for increasing patient tolerance.

There are 3 depth markers on the adult probe. They are set at 35, 40 and 45cm from the tip. Using these depth markers and assessing the signal, will ensure the tip of the probe is at approximately T5/T6 area of the descending aorta. This is the region where the oesophagus and descending aorta will be closest.

For nasal use, the external nares in an adult should be between maker 2 and 3. It may be acceptable for them to be just outside of these markers by approximately 1 or 2 cm if the patient is particularly short or tall. Also consider the length of the torso if a signal cannot be found. Start at the deepest maker to find the optimal focus.

It can be used together with a nasogastric tube, a temperature probe or even a transoesophageal echocardiogram (TOE) probe.

ODM probes have been used successfully with an LMA. It may be helpful to place the probe before the LMA is placed. Ensure the seal of the LMA is lubricated well and when inflated, it should easily accommodate the probe.

Considerations:
  • Abnormalities or surgery of the mouth, pharynx, aorta and oesophagus.
  • Base of skull fractures.
  • Coagulopathies.
  • Remove probe if patient having an MRI.
  • Remove probe or disconnect if cardioversion required.
The following may make locating an optimal signal difficult:
  • Coartation of the aorta.
  • Use of intra aortic balloon pump.
  • Anatomical abnormalities.
  • Securing the probe
Securing the probe:
  • Many users do not secure the probe since peristalsis in the oesophagus may cause the probe to move internally.
  • If the user wishes to secure the probe with tape or a nasal clip, then ensure the probe can be manipulated to check focus.

 Focussing the Probe

How to get the optimal focus
  • It is useful to place the probe as early as possible for instance immediately after intubation so that a settling in period can be achieved where the probe sits comfortably against the oesophageal wall and the patient’s secretions will aid signal acquisition.
  • Insert the probe to deepest marker of where it was placed, i.e. 3rd marker if placed nasally, 2nd marker if placed orally to start with in an adult.
  • Turn up the sound on the monitor.
  • Choose this depth and rotate slowly all way round without letting go.
  • If no signal is seen at this depth, pull out slightly (by approximately 1 cm at a time) and rotate again slowly without letting go.
  • Repeat these changes of depth followed by rotations until a signal is found.
  • Do not pull and rotate at the same time as this will cause a spiral effect and the appropriate area may be missed.
  • Ensure the edge of the nose or the incisors is as near to the appropriate depth markers. If the patient is very tall or very short, it might be acceptable for the depth markers to be outside of nose or incisors by a small amount.
The qualities of a good signal:
  • The loudest, sharpest ‘whipcrack’ sound
  • The tallest waveform
  • The brightest waveform
  • A well-defined triangle with a black centre surrounded by red with white in the trailing edge. This indicates an equal distribution of red blood cell velocities at a given point in time and the probe is facing the centre of the aorta. If the waveform has no black centre, this is called spectral dispersion and may indicate that the probe is not facing the centre of the aorta.
  • The incisors or edge of nostrils will be as close to the appropriate depth marker as possible.

Find a signal that meets the above requirements and it may be useful to recheck at different depths once more to ensure the optimal signal has not been missed.

Check the gain manually or use autogain. This will help to adjust the signal quality.

When in run mode, check the cycle time. 5 is the default but can be increased for arrhythmias or decreased if there is noise interference.

Ensure the green follower line is set tightly against the waveform and the white arrows are placed on the triangle.

To refocus:
  • Ensure the probe is at the same depth when the optimal signal was found
  • Turn up the volume
  • Rotate slowly without letting go to find the appropriate signal again before evaluating the parameters.