Doppler Pressure Evaluation of Infrainguinal Occlusive Disease

Physiology of Arterial Obstruction

The principle of continuity dictates that in a closed fluid system (such as a segment of the superficial femoral artery with no branches) when the tube narrows the flow speed must increase, converting pressure (potential) energy into velocity (kinetic) energy. For minimal narrowing, all of this kinetic energy is recovered downstream when the flow speed decreases, with no loss distally in overall blood pressure. As the narrowing reaches hemodynamic significance, viscous and flow turbulence effects prevent complete energy recovery, and there is a measurable pressure drop.

The higher the volume flow, the less narrowing is required for a hemodynamically significant lesion, which is why systolic pressure is profoundly affected by stenoses with little impact on diastolic pressures. The degree of narrowing that achieves hemodynamic significance is termed a critical stenosis. This varies for different blood vessels and flow conditions. At rest in the lower extremities it typically requires at least the equivalent of a 60% diameter reduction (approximate 84% cross-sectional area reduction) to reach a critical stenosis. However, during periods of increased flow, such as during exercise, a much lower degree of stenosis results in a pressure reduction. With a fivefold increase in volume flow through the superficial femoral artery induced by exercise, the equivalent of a 40% diameter reduction (approximate 60% area reduction) can achieve hemodynamic significance.

The physiologic response to a significant arterial obstruction is a reduction in distal peripheral resistance through arteriolar dilation. The relationship governing flow is pressure equals volume flow times peripheral resistance; if pressure and resistance decrease by the same relative amount, volume flow in the affected limb can remain constant. When peripheral resistance can no longer adapt enough to a pressure drop caused by arterial stenosis, ischemia results, with its associated symptoms.

Technique

To noninvasively measure systolic blood pressure in the lower extremities, a blood pressure cuff is placed over the limb segment of interest and inflated to a pressure 30 to 40 mm Hg above the systolic pressure, then slowly deflated (2–4 mm Hg/sec) while monitoring blood flow at a site distal to the cuff. Typically a continuous wave Doppler flowmeter is used for this purpose, monitoring one of the lower leg tibioperoneal arteries at the level of the ankle. The cuff pressure noted when the return of arterial flow is detected is taken to be the systolic pressure at the site of the occluding cuff. Ideally the inflatable cuff bladder is appropriately sized to the limb segment being measured. Current American Heart Association guidelines call for the bladder width to be 20% greater than the limb’s diameter (40% of the circumference) with the bladder length approximately two times its width. If the cuff is too narrow, pressure within the bladder will not be effectively transmitted to the tissue and the resulting systolic pressure measurement will be artifactually elevated.