An ankle-brachial index (ABI) machine consists of a pump, blood pressure cuffs, Doppler pen, photoplethysmography (PPG) sensors, a remote control, and a printer—all in one machine. Before we get into the steps involved in performing an automated ABI test, let’s get a bit familiar with the ABI machine.
Getting to know the ABI machine
Hoses and blood pressure cuffs
The hoses on the ABI machine are color-coded and hook up to the cuff attachments. For example, the brachial pressure hoses may be white, the ankle hoses may be orange, and the toes hoses may be black. These colors can be different with different machines.
For a basic ABI machine, there is a set of four cuffs. A blood pressure cuff is placed on each upper arm and each ankle (just above the malleolus) during an ABI test. Typically, a 10 cm cuff is used on each extremity. But, a 12 cm by 40 cm cuff may be needed if the patient is obese.
The remote control
The remote control for the ABI machine has three main parts:
The sound control buttons
Sometimes, the machines start without the volume on, so the volume will need to be turned up when starting the ABI machine. However, there is often an unpleasant static sound that occurs between obtaining pulses and pressures, so it’s helpful to use the mute button.
The mute button helps to avoid extraneous static noise that may occur from the pen movement or from a lack of gel. Always hit the mute button between recordings and then unmute when actively listening for a pulse. It’s also a good idea to warn your patient that the Doppler microphone can be noisy.
The cuff control button
The cuff inflation button controls the cuff pressure. When you stop pushing this button, it stops inflating the cuff and starts slowly deflating. Like a manual ABI, the pressure is pumped to 20 mmHg above the last arterial beat, and then the cuff is allowed to deflate until a return beat is heard.
Press the freeze button once you hear the first three arterial beats. Then, use the arrows at the bottom of the remote to scroll back to the pressure reading of the first returning arterial beat. Hit the save button to save the data.
Check out this video from our Ultrasound Masterclass: Arteries of the Legs Course to see a demonstration of how to listen for a return arterial beat:
The waveform control buttons
The buttons used to adjust waveforms are to the right of the volume buttons on the remote. The auto button will optimize your waveforms.
For the best analysis, manually adjust the baseline (e.g., position) and scale (e.g., size) of the waveform to take up about 3/4 of the screen height in the center of the screen. Most protocols optimize the dorsalis pedis artery (DPA) waveform and then do not change the waveform’s size thereafter so the vessels’ waveform amplitude can be accurately compared.
Once an accurate waveform (that is 3/4 the size of the screen) is obtained, press the freeze button and then use the scroll buttons at the bottom of the remote to find the optimal waveform and save it. The machine will then prompt which waveform to obtain next.
How to use the ABI machine to record pressures and capture waveforms
Now that the details of how to use the machine are clear, let’s review the flow of the arterial evaluation. To begin, let’s go through the steps involved in performing automated ABI testing.
Start by obtaining brachial pressures
- Place the blood pressure cuff on the right upper arm.
- Place the Doppler probe at the antecubital fossa (to find the right brachial artery) or the thumb side of the wrist (to find the radial artery).
- Brace your ring finger or pinky on the patient’s arm or bed to steady your hand and avoid artifacts. Keep in mind that the upper extremity waveforms are never recorded for an ABI test, so don’t worry about optimizing them. Just try to make the Doppler pen line up with the vessel to get the clearest signal.
- To capture brachial pressures, press and hold the inflate button until the pressure reaches 20 mmHg above the last arterial beat. Then, release the inflate button and allow the pressure to slowly deflate.
- Press freeze when you hear the first three arterial beats return.
- Use the arrow buttons at the bottom of the remote to scroll the recording back to the pressure reading of the first returned arterial beat.
- Press the green save button to save the data.
- Repeat these steps to obtain the left brachial pressure.
Obtain Doppler waveforms and pressures for the ankles
- Wrap the blood pressure cuff just above the malleolus on the right ankle.
- Position the Doppler pen on the appropriate artery. Most protocols start with the DPA pulse and pressure recordings before proceeding to the posterior tibial artery (PTA).
- Find the right DPA pressure using the same technique for brachial pressures (steps 4–7).
- To capture the pulse waveforms, start by adjusting the baseline (e.g., position button) and scale (e.g., size button) of the waveform so it is in the center and takes up about 3/4 of the screen height.
- Press the freeze button once an accurate waveform is obtained.
- Scroll to the optimal waveform and press save.
- Repeat steps 2–7 to record the right PTA pressure and pulse waveforms.
- Repeat this process for the left ankle.
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