Topic 7 - Lower Limb Arterial Doppler Flashcards
The reasons for referring a patient for a lower limb arterial study usually relate to one or more of the following;
- Leg pain (rest pain or claudication)
- Ischemic tissue loss (gangrene or ulcers)
- Loss of distal pulses, often in connection with other risk factors (eg diabetes)
- Pulsatile focal mass (eg true or false aneurysm)
What are some non imaging tests for the arteries?
Ankle brachial indices
Plethysmography
How is a Ankle brachial indices performed?
- performed by applying blood pressure cuffs to the upper arms and to the lower calves above the ankles.
- A continuous wave Doppler probe monitors the radial (or brachial) pulse while the arm cuff is inflated and then deflated until the returning pulse indicates the systolic pressure.
- The same procedure is then repeated for the posterior tibial and dorsalis paedis arteries
- The patient is then exercised until they cannot continue because of claudication or until they have reached a predetermined distance.
- The arm and the higher of the pedal pressures are then repeated
How is the Ankle Brachial Index calculated?
• In normal individuals, the pedal pressure to brachial pressure is equal to 1 or higher (>0.9 given measurement variability).
• Reductions in the resting ABI can be grouped as follows:
o mild 0.7-0.9
o Severe 0.5-.7
o rest pain 0.3-0.5
o critical ischemia <0.3
What are the pitfalls of ABI?
o Calcification in the arteries will resist compression and produce an artificially high pressure (ABI) or the artery may not occlude at all thus making measurement impossible.
o ABI’s cannot specifically identify the location or degree of stenosis.
o In monitoring bypass grafts, ABI measurements are less sensitive to the development of stenosis when compared to velocity changes seen in Duplex scanning.
What is Plethysmography?
• Plethysmography generally measures the changes in limb volume or skin microcirculation and can be used to assess either the arterial or venous circulation.
Plethysmography is especially effective in detecting changes caused by blood flow. It can help your doctor determine if you have a blood clot in your arm or leg. It can also help your doctor calculate the volume of air your lungs can hold.
If you cannot make out the arterial lumen on bmode what should you do?
- In some cases, the grey-scale image of the artery cannot be clearly obtained, so defining the width of the arterial lumen using colour Doppler is an alternate technique
- this technique should only be used when the arterial wall cannot be defined on the B-mode image
How should you adjust colour setting for a lower limb Doppler?
- Colour Doppler is used to define areas of likely stenosis by identifying the colour aliasing
- The colour sensitivity and PRF (scale) should be adjusted to eliminate excessive aliasing in a normal artery
- In regions of suspected stenosis from colour Doppler screening, the spectral gate must then be passed from the non aliased colour segment above the suspected lesion, through the region of aliasing and into a non aliased artery below the lesion.
- This technique is called “walking the Doppler”
- Once this point is identified return to the site of highest velocity for measurement of the peak systolic velocity.
When performing a lower limb Doppler what is important when scanning the aorta?
- important to recognise aneurysmal dilatation of the aorta and iliac arteries during an arterial study
- if present, it must be fully documented.
How can abdominal contents obscuring arteries be overcome?
- imaging from the distal external iliac artery toward the aorta.
- by applying gentle to moderate transducer pressure to help move the bowel.
- Using other views such as a coronal view or rolling the patient on their side.
What three steps should be taken when adjusting colour mode after first beginning the exam?
o adjust the colour sensitivity for the flow which is expected in the artery (proximal arteries usually require ‘high’ flow sensitivity settings while distal arteries require ‘low’ flow settings).
o Set the colour scale (PRF) so that the bulk of the aliasing is eliminated or there is a small degree of aliasing in the centre of the artery during systole.
o Set the colour gain to reduce the tissue colour artefact (colour bleed) so the colour signal is maintained within the artery.
What are the PSV criteria for arterial stenosis in the leg?
Normal <150 cm/s <1.5:1 velocity ration
30-49% 150-200 1.5:1 - 2:1
50-75% 200-400 2:1 - 4:1
>75% >400 >4:1
How is stenosis recognised on colour doppler?
- Colour aliasing is the main tool used to identify regions of possible stenosis.
- Focal regions of excessive aliasing usually indicate regions of high velocity which can then be interrogated by spectral Doppler
- Other indicators of a stenosis are narrowing of the colour within the lumen of the artery
- or persisting colour during the diastolic phase in an artery which normally shows absent or reverse flow in diastole.
How is occlusion recognised on colour Doppler?
- can only be suggested when there is absent colour flow within a defined B-mode image of the lumen, even with the machine adjusted to detect very low flow states (low flow sensitivity, low PRF and high gain).
- In regions where the lumen is poorly imaged (eg. Calcified segments of artery) clues such as inward flowing arterial branches below the lesion (collateral arteries) may suggest the presence of an occlusion or tight stenosis.
- Outward flowing branches above the suspected occlusion cannot be interpreted as collateral vessels.
How does a near occlusion appear on colour Doppler?
trickle flow
• when the visualised lumen contains so much luminal atheroma or thrombus that only a narrow patent channel can be seen.
• Often these channels take a tortuous path through the lumen of the artery
