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*Measured in lower part of internal carotid artery fRatio of peak systolic velocity in internal carotid artery stenosis relative to proximal measurement in common carotid artery

*Measured in lower part of internal carotid artery fRatio of peak systolic velocity in internal carotid artery stenosis relative to proximal measurement in common carotid artery

By combining the pulsed Doppler system with real time B mode ultrasound imaging of vessels, it is possible to examine Doppler flow patterns in a precisely defined area within the vessel lumen. This combination of real time B mode sound imaging with pulsed Doppler ultrasonography is called duplex scanning. The addition of colour frequency mapping (so called colour duplex or triplex scanners) makes the identification of arterial stenoses even easier and reduces the scanning time.

Investigations of arterial disease

Ankle brachial pressure index

Under normal conditions, systolic blood pressure in the legs is equal to or slightly greater than the systolic pressure in the upper limbs. In the presence of an arterial stenosis, a reduction in pressure occurs distal to the lesion. The ankle brachial pressure index, which is calculated from the ratio of ankle to brachial systolic pressure, is a sensitive marker of arterial insufficiency.

The highest pressure measured in any ankle artery is used as the numerator in the calculation of the index; a value > 1.0 is normal and a value < 0.9 is abnormal. Patients with claudication tend to have ankle brachial pressure indexes in the range 0.5-0.9, whereas those with critical ischaemia usually have an index of < 0.5. The index also has prognostic significance because of the association with arterial disease elsewhere, especially coronary heart disease.

Diabetic limbs

Systolic blood pressure in the lower limbs cannot be measured reliably when the vessels are calcified and incompressible—for example, in patients with diabetes—as this can result in falsely high ankle pressures. An alternative approach is to use either the pole test or measurement of toe pressures. Normal toe systolic pressure ranges from 90-100 mm Hg and is 80-90% of brachial systolic pressure. A toe systolic pressure < 30 mm Hg indicates critical ischaemia.

Spectral analysis of blood velocity in a stenosis, and unaffected area of proximal superficial femoral artery. The velocity increases from 150 to 300 m/s across the stenosis

Colour duplex scanning of blood flow through stenosis of superficial femoral artery. Colour assignment (red or blue) depends on direction of blood flow and colour saturation reflects velocity of blood flow. Less saturation indicates regions of higher blood flow and deeper colours indicate slower flow; the absence of flow is coded as black

Colour duplex scanning of blood flow through stenosis of superficial femoral artery. Colour assignment (red or blue) depends on direction of blood flow and colour saturation reflects velocity of blood flow. Less saturation indicates regions of higher blood flow and deeper colours indicate slower flow; the absence of flow is coded as black

Patient survival according to measurements of ankle brachial pressure index (adapted from McKenna et al, Atherosclerosis 1991;87:119-28)

Pole test for measurement of ankle pressures in patients with calcified vessels: the Doppler probe is placed over a patent pedal artery and the foot raised against a pole that is calibrated in mm Hg. The point at which the pedal signal disappears is taken as the ankle pressure

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