Doppler Ultrasonography

Physical Principles
The ABI

Physical Principles
Frequency of sound changes when reflected from a moving structure
- proportional to velocity of the structure.
Ft-Fr=(2xFtxVxcosA)/C
- where Ft and Fr are sound frequencies
- V is velocity of blood
- A is angle waves hit blood
- C is velocity of sound constant
Thus better signal if higher frequency
But attenuation of sound is greater at higher frequencies
- so lower frequencies used for deeper vessels.
- air has high attenuation, so use gel on the skin.
A probe has 2 active piezoelective crystals
- one transmits, one receives
- the apparatus arranges things to get a sound we can hear
Sound may be generated from a non-palpable vessel
- this may be confusing as it is unclear if it is from a vein or artery underneath.

The ABI

Ankle blood pressure
Place cuff around lower third of recumbent patient's calf.
Apply gel over artery.
Adjust probe angle and position until optimum signal.
Inflate cuff until no signal
Deflate until audible signal.
--> this is the systolic pressure.
Compare with brachial arteries on both sides.

Disadvantages
1.  If calcified a falsely high reading is obtained.
- this problem is common in diabetics.
- can be impossible to occlude the artery even with pressure >300mmHg
- can use alternative
2.  Painful ulcers may exist around the ankles.
3.  Brachial pressures may be falsely low in pts with occlusive upper limb disease.

Ankle-brachial index (ABI)
Expressing the result as an index allows comparison between pts, or from day-to-day in one pt, as it removes the effect of BP variation.
- ankle:brachial.
The level of ABI is proportional to clinical severity of disease.
- used to confirm severity
- to monitor progress (expect variability of 0.1-0.15units)
- to determine effectiveness of treatment.

Severity Correlations:
(Guide)
Normal: ABI >0.9
Claudication: ABI 0.4-0.9
Critical Ischaemia / rest pain: ABI <0.5
Gangrene: ABI <0.2

Exercising ABI
Increase sensitivity by measuring before and after exercise.
- exercise, then measure as soon as possible.
- within 30-45 seconds is ok.
If calf pressure has fallen, measure again every 30-60s until back to resting level.
- level to which pressure falls with exercise and duration of the fall are proportional to severity of the disease.
Not necessary to do this in all cases
- helpful in pts with resting ABI higher than expected with severity of symptoms.
- ie if ABI does not fall with exercise, vascular disease is not their problem.