Peripheral artery disease (PAD) is a common circulatory condition where narrowed arteries reduce blood flow to the limbs, most frequently affecting the legs and feet. This narrowing occurs due to the buildup of plaque, a process called atherosclerosis, which restricts the delivery of oxygen-rich blood to the muscles and tissues. Identifying PAD early is important because it acts as a marker for widespread atherosclerosis, significantly increasing the risk of severe cardiovascular events. Untreated PAD can lead to serious complications in the affected limbs, including chronic non-healing ulcers, tissue death (gangrene), and potentially the need for amputation.
The Primary Screening Test: Ankle-Brachial Index
The Ankle-Brachial Index (ABI) is the most commonly utilized screening test for PAD, a simple, non-invasive procedure often performed in a doctor’s office or vascular laboratory. This test compares the systolic blood pressure measured at the ankle to the systolic blood pressure measured at the arm. The resulting ratio indicates how well blood is flowing to the lower extremities.
To perform the ABI, blood pressure cuffs are placed on both arms and both ankles. A handheld Doppler ultrasound device measures the systolic pressure in the major arteries of each limb. The ABI ratio is calculated by dividing the highest ankle systolic pressure by the highest brachial (arm) systolic pressure.
A normal ABI ratio falls between 1.0 and 1.4, suggesting no significant blockage. An ABI value of 0.90 or lower is considered diagnostic for PAD. Values between 0.91 and 0.99 are considered borderline.
The severity of the disease is categorized based on the ratio:
- Mild PAD: ABI between 0.70 and 0.90.
- Moderate PAD: ABI ranging from 0.40 to 0.70.
- Severe PAD: ABI below 0.40, indicating a high degree of blockage.
An abnormally high ABI, above 1.4, suggests that the arteries are stiff and non-compressible due to calcification, often seen in individuals with advanced age or diabetes.
Assessing Blood Flow with Doppler Ultrasound
If the ABI suggests the presence of PAD, a Duplex Ultrasound, or Doppler flow study, is often the next step to provide a more detailed assessment. Unlike the ABI, which only provides a pressure ratio, the ultrasound uses sound waves to directly visualize the blood vessels and assess the characteristics of blood flow. This non-invasive test helps pinpoint the exact location and extent of the arterial narrowing or blockage.
The ultrasound assesses blood flow by measuring the peak systolic velocity within the artery. When an artery narrows, the velocity of the blood flow speeds up through the constricted section. By comparing the velocity within the narrow segment to the velocity in a normal segment just before it, a velocity ratio can be calculated to estimate the percentage of stenosis. For instance, a velocity ratio greater than 2 suggests the vessel is at least 50% narrowed.
The analysis also involves examining the shape of the blood flow waveform. A healthy artery displays a characteristic triphasic pattern, but as PAD progresses, this changes to a biphasic or monophasic waveform.
Functional Assessment
In cases where symptoms of pain upon walking (intermittent claudication) are present but the resting ABI is normal, a functional assessment is necessary. An exercise ABI or a treadmill test with Doppler can be performed. This exercise-based test reveals a pressure drop in the ankles that only becomes evident when the muscles demand more blood flow.
Detailed Visualization for Treatment Planning
Once PAD is confirmed, advanced imaging techniques are used to create detailed anatomical maps of the arteries, particularly when a surgical or minimally invasive intervention is being considered. These visualization methods provide highly detailed, three-dimensional views that are necessary for treatment planning.
Computed Tomography Angiography (CTA) and Magnetic Resonance Angiography (MRA) are frequently used, as they are capable of depicting the entire arterial system from the aorta down to the feet. Both CTA and MRA require the injection of a contrast agent into the bloodstream to highlight the vessels. CTA uses X-rays and iodine-based contrast, while MRA uses magnetic fields, radio waves, and a gadolinium-based contrast agent to generate high-resolution images.
These non-invasive angiography techniques accurately identify the location and severity of atherosclerotic plaque and are highly reliable alternatives to the more traditional, invasive catheter angiography. Catheter angiography, also known as Digital Subtraction Angiography (DSA), involves inserting a catheter directly into an artery and injecting contrast dye. DSA is primarily reserved for cases when a procedure, such as an angioplasty, will be performed immediately after the diagnostic mapping. The detailed images from CTA and MRA guide the physician in deciding the best course of action, whether it is medical management, bypass surgery, or an endovascular procedure.