Noninvasive Vascular Laboratory Testing and Diagnostic Imaging
Physiologic testing
Measure the ABI with the patient supine after ten minutes of rest, using a cuff about 20% wider than the limb, and record the higher of the dorsalis pedis and posterior tibial pressures. Values of 0.90 or less are abnormal, 0.91–1.0 borderline, and above 1.3 suggest medial calcinosis from diabetes or renal failure; in those patients use toe pressures (normal toe-brachial index above 0.7; toe pressure under 30 mmHg predicts poor healing) or transcutaneous oximetry. Segmental pressures and pulse volume recordings localize disease. Exercise ABI, by treadmill or toe raises, unmasks iliac disease when resting studies are normal.
Duplex ultrasound
Carotid criteria are velocity-based and laboratory-specific: peak systolic velocity above 125 cm/s suggests 50% or greater internal carotid stenosis, and above 230 cm/s with an internal carotid to common carotid ratio above 4.0 suggests 70% or greater. Near-occlusion paradoxically shows low velocities with a visible string sign - a classic trap. For DVT, rely on compression, augmentation, and phasicity rather than color alone. Graft surveillance uses peak systolic velocity, a velocity ratio above 2.5, and low-flow states.
Cross-sectional imaging
CT angiography is the workhorse for planning, showing calcium, thrombus, and access diameter. MR angiography avoids iodinated contrast and radiation but overestimates stenosis and is degraded by stents. Catheter angiography remains the reference standard and permits pressure measurement: a resting gradient above 10 mmHg, or above 20 mmHg after hyperemia, is significant.
Rule of thumb: numbers are only as good as the technique - a normal ABI in a diabetic smoker with rest pain means measure the toe.