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.

Walk test

Exercise testing will assess the functional limitations of arterial stenoses and differentiate occlusive arterial disease from other causes of exercise induced lower limb symptoms—for example, neurogenic claudication secondary to spinal stenosis. A limited inflow of blood in a limb with occlusive arterial disease results
in a fall in ankle systolic blood pressure during exercise induced peripheral vasodilatation. The walk test is performed by exercising the patient for 5 minutes, ideally on a treadmill, but walking the patient in the
surgery or marking time on the spot are adequate. The ankle brachial pressure index is measured before and after exercise. A pressure drop of 20% or more indicates significant arterial disease. If there is no drop in ankle systolic pressure after a 5 minute brisk walk, the patient does not have occlusive arterial disease proximal to the ankle in that limb.

Duplex scanning

Duplex ultrasonography has a sensitivity of 80% and a specificity of 90 100% for detecting femoral and popliteal disease compared with angiography, but it is less reliable for assessing the severity of stenoses in the tibial and peroneal arteries. Duplex scanning is especially useful for assessing the carotid arteries and for surveillance of infrainguinal bypass grafts where sites of stenosis can be identified before complete graft occlusion occurs and before there is a fall in ankle brachial pressure index. The normal velocity within a graft conduit is 50 120 cm/s. As with native arteries, a twofold increase in peak systolic velocity indicates a stenosis of 50% or more. A peak velocity < 45 cm/s occurs in grafts at high risk of failure.

Identification of distal vessels for arterial bypass grafting

In critically ischaemic limbs, where occlusive disease tends to be present at multiple levels, arteriography often fails to show patent calf or pedal vessels as potential outflows for femorodistal bypass grafting. Alternative non invasive approaches have been developed for preoperative assessment, including pulse generated run off and dependent Doppler assessment.

Transcranial Doppler ultrasonography

Lower frequency Doppler probes (1 2 MHz) can be used to obtain information about blood flow in arteries comprising the circle of Willis and its principal branches. Mean flow velocities > 80 cm/s in the middle cerebral artery, or > 70 cm/s in the posterior and basilar arteries, indicate a serious stenosis. Transcranial Doppler scanning has several applications but is especially useful for intraoperative and postoperative monitoring of patients having carotid endarterectomy.

Helical or spiral computed tomography 

Spiral computed tomography is a new, minimally invasive technique for vascular imaging that is made possible by combining two recent advances: slip ring computed tomography (which allows the x ray tube detector apparatus to rotate continuously) and computerised three dimensional reconstruction. A helical scan can cover the entire region of interest (for example, the abdominal aorta from the diaphragm to the iliac bifurcation) in one 30 40 second exposure, usually in a single breath hold. This minimises motion artefact and allows all the scan data to be collected during the first pass of an intravenous bolus of contrast through the arterial tree—that is during the time of maximal arterial opacification. A large number of finely spaced slices from one scan can then be reconstructed to produce high quality two or three dimensional images of the contrast enhanced vessels.

Magnetic resonance angiography

Magnetic resonance angiography has developed rapidly over the past five years. It has the advantage of imaging a moving column of blood and does not require ionising radiation or iodinated contrast, but the technique has obvious drawbacks in terms of cost efficiency and accessibility to scanners. A variety of
imaging sequences are used depending on the vessels being studied and the field strength of the machine. The most commonly used techniques include time of flight, two and three dimensional angiography and phase contrast. Use of a magnetic resonance imaging scanner with a high field strength (which allows rapid acquisition of data) and a carefully timed bolus of gadolinium contrast enables high quality angiographic images to be obtained in a single breath hold. Magnetic resonance angiography is well established for examining the cerebral vessels and the car