The vascular laboratory

Introduction

The vascular laboratory specializes in several tests that look for diseases that may affect blood flow in the arteries – the blood vessels that take blood to the tissues, and veins – the blood vessels that bring blood back to the heart. Commonly performed procedures include the ankle–brachial index (ABI), pulse volume recordings (PVRs), and segmental blood pressure analyses. These tests use inflatable blood pressure cuffs to assess arterial blood flow in the legs and arms. Another commonly used test in the vascular laboratory is Doppler ultrasound, which evaluates blood vessels and their surrounding structures using high-frequency, inaudible sound waves. Vascular ultrasound is safe and non-invasive, and it does not require the use of radiation or intravenous contrast dye. In the vascular laboratory, ultrasound can be used to evaluate the arteries and veins in the neck (carotid ultrasound), abdomen (abdominal aortic ultrasound), kidneys (renal artery ultrasound), arms, and legs (upper and lower extremity ultrasound). Most ultrasound procedures involve the use of a clear jelly placed on the skin under the scanning probe to assist with getting images.

For most procedures in the vascular laboratory, patients can take all their usual medications and eat prior to arrival. However, patients may be asked not to eat for a certain amount of time prior to studies that evaluate the abdomen, such as abdominal aortic and renal or mesenteric artery ultrasounds. Eating or drinking prior to these studies may lead to excessive bowel gas that interferes with getting images. Nevertheless, each test is different, and patients should confirm specific dietary and medication restrictions with their physician or the ultrasound staff beforehand. Patients should also plan to wear loose, comfortable clothing on the day of the procedure, since they may be asked to change into a gown.

Your physician has ordered the following test(s) (see checked box, □):

□ Indirect physiologic testing: ankle–brachial index, pulse volume recordings & segmental blood pressure analyses

Indirect physiologic testing helps determine whether a patient has peripheral artery disease (PAD). PAD is a term used to describe blockages in the arteries that supply blood to the legs or arms. Indirect physiologic testing includes the ABI, PVRs, and segmental limb pressure (SLP) analyses. All three tests often are performed at once. The advantage of indirect physiologic testing is that it is simple to perform, and testing time is short, often requiring less than 45 minutes for a complete study of both legs or both arms. One disadvantage is that indirect testing does not give actual pictures of the blood vessels.

An ABI study gives a general evaluation of blood flow to the legs. Patients with PAD have decreased circulation to the limbs due to narrowed or blocked arteries, and this test helps determine the severity of the disease. Patients at risk for PAD are those with a history of coronary artery disease (the blockages that may lead to heart attacks), tobacco use, or diabetes mellitus, as well as those with other risk factors (such as high cholesterol, hypertension). The test is also recommended for anyone who complains of having pain or discomfort in his or her legs while walking that resolves with rest, which is called claudication. In this test, the patient lies flat while blood pressures in the arms and legs are obtained using cuffs and an ultrasound probe.

Under normal circumstances, the blood pressure at the ankle should be similar to or slightly higher than the pressure in the arms, and therefore the ABI ratio (i.e. ankle blood pressure/arm blood pressure) should be equal to or greater than 1.0. If there is a significant narrowing or blockage within the arteries that take blood to the legs, the ankle blood pressure will be lower than that of the arm, and the ABI will be less than 1.0.

PVRs and segmental pressure analyses are tests in which a series of blood pressure cuffs and an ultrasound probe are used to obtain even more specific information about blood flow in the arteries of the legs (Figure 1). This test not only determines whether PAD is present, but also helps pinpoint the general location of the blockages. As with an ABI, this procedure is performed with the patient lying flat. Inflatable blood pressure cuffs are placed over the thighs, calves, ankles, and feet, along with blood pressure cuffs on both arms. Then a Doppler ultrasound probe is placed on arteries in the arm and the leg to detect the pulse. The distance between two adjacent cuffs forms a segment within the leg that is being analyzed, so cuffs are used to cover the entire length of the leg. The cuffs are inflated and then slowly deflated, and the Doppler probe is used to detect pressures in each segment. The patient may feel pressure on the leg as the cuff is being inflated, but the test usually is not painful. A substantial drop in pressures between cuffs suggests the presence of a blockage within that segment.

Figure 1.

Ankle–brachial index and pulse volume recording (ABI/PVR) testing in a vascular laboratory. Multiple blood pressure cuffs are placed on the legs and arms and the technologist listens to the pulse with a Doppler probe. Illustration by Michael Weinreich, reproduced with permission.

Once the procedure is complete, a vascular specialist will review the findings to determine if there is evidence of mild, moderate, or severe PAD, and where blockages may be present. Sometimes, findings from these studies may lead to additional testing.

□ Exercise ankle–brachial index

Sometimes it may be necessary to undergo exercise ankle–brachial index stress testing to differentiate muscle pain caused by poor circulation (claudication) from pain caused by spinal nerve compression, arthritis, or other non-vascular causes. Walking or running on a treadmill can help diagnose PAD that may not have been seen on results obtained while at rest. Patients should inform their physician of medical conditions that may prevent them from participating in this test, such as advanced heart or lung disease, leg pain at rest, or ulcers (sores) on the feet or legs. However, this test is not as rigorous as a cardiac stress test, and the risk of cardiac complications is small.

Patients should wear loose clothing and a good pair of walking shoes on the day of the test. Prior to treadmill exercise, the vascular technologist obtains an ABI at rest. Then the patient walks on the treadmill for up to 5 minutes, or until pain or discomfort prevents further walking. Most protocols set the treadmill at 1.5 to 2.0 mph at a 10% or 12% uphill grade. The technologist asks the patient to describe when symptoms begin, where they are located, and how severe they are. After walking, the patient is brought promptly to the exam bed, and the technologist obtains repeat blood pressures with the Doppler to calculate the ABI after exercise, which will be compared to the ABI at rest. Some laboratories obtain further post-exercise measurements (i.e. PVR or Doppler waveforms). The entire test takes about 45 minutes to complete.

□ Carotid artery ultrasound

A carotid artery duplex ultrasound is used to evaluate blood flow through the carotid and vertebral arteries – the blood vessels that carry blood to the brain. Narrowing or blockages of these vessels can increase the risk of stroke. Patients at higher risk of stroke are those who have had a previous stroke or transient ischemic attack (TIA), also known as a ‘mini-stroke’, or patients with medical conditions that increase the risk of stroke (such as diabetes mellitus, high blood pressure, or elevated cholesterol). Some patients are referred for carotid ultrasound because their doctor heard a sound over the neck arteries called a bruit. A bruit is a whooshing sound that can be heard with a stethoscope and may mean there is a narrowing or blockage in a neck artery.

On the day of the test, patients should wear loose clothes that allow for full exposure of the neck area and should avoid wearing necklaces. Long hair should be tied back. During the procedure, an ultrasound probe is placed over the carotid artery on the side of the neck (Figure 2), with jelly on the skin to allow for obtaining images. The probe emits high-frequency sound waves, which are used to evaluate blood flow through the arteries (Figure 3). The procedure takes approximately 30 minutes to complete.

Figure 2.

Carotid artery ultrasound in the vascular laboratory. Illustration by Michael Weinreich, reproduced with permission.

Figure 3.

Color Doppler image of a carotid artery. The left panel depicts blood flow through a normal carotid artery. The right panel depicts turbulent blood flow through a vessel with a blockage due to plaque (red arrow).

If narrowing of the carotid artery is detected, treatment options include lifestyle changes such as eating a healthier diet, medications (such as aspirin and a cholesterol-lowering medication), and in some cases surgery (carotid endarterectomy) or stent placement. Depending on the presence and severity of plaque in your carotid arteries, your physician may recommend that you return for surveillance imaging in the future.

□ Abdominal aortic ultrasound

An abdominal aortic ultrasound is a test used to examine the aorta (the largest artery in the body) as it passes through the abdomen. The test evaluates the size of the aorta and can be used to detect an abdominal aortic aneurysm (also known as an AAA), which is an abnormal enlargement in the vessel wall. This test is recommended at least once for men aged 65 to 75 who have smoked more than 100 cigarettes in their lifetime. It is also recommended for those who have not smoked but are at increased risk of having an aneurysm, such as those with a first-degree family member (parent, sibling, or child) with an aortic aneurysm or those with genetic conditions that are known to cause aneurysms.

The procedure is performed by placing an ultrasound probe over different parts of the abdomen. The sound waves emitted by the probe allow the technologist and physician to see if there are any abnormalities in the walls of the aorta. Generally, the procedure takes about 30 minutes.

If an abnormality in the abdominal aorta is found, treatment options include lifestyle modifications, such as changing diet and quitting smoking, improving blood pressure control, and periodic surveillance ultrasound examinations of the aorta. If the aneurysm is excessively large, surgery or endovascular (stent graft) repair may be necessary.

□ Venous duplex ultrasound

A venous duplex is a study in which ultrasound is used to evaluate the veins in the legs or arms, and occasionally the abdomen. This test is ordered when patients have discomfort or swelling in the legs or arms, often to look for blood clots called deep vein thrombosis (DVT). DVTs can occur in some people with genetic predispositions, after surgery or when being sedentary for an extended period (e.g. a long airplane flight or hospitalization), or in some cancers.

The procedure is performed by placing an ultrasound probe over different areas of the arms and legs and pushing on the skin with the probe to collapse the vein. Normal veins collapse with mild pressure, whereas veins with DVT do not collapse. Patients with pain at the site being examined may have some discomfort, though typically the test is not painful. The sound waves emitted by the probe can detect if a blood vessel is filled with a blood clot. The procedure generally takes about 30 minutes.

□ Renal and mesenteric artery ultrasound

Ultrasound can be used to detect blood flow to the kidneys, the intestines and other abdominal organs. Diminished blood flow to the kidney can lead to kidney disease or high blood pressure. A renal artery ultrasound may be ordered for patients with unexplained high blood pressure, worsening kidney function, or other concern for decreased blood flow to the kidney. Diminished blood flow to the intestines or other abdominal organs can lead to abdominal pain. A mesenteric artery ultrasound may be ordered for patients experiencing abdominal discomfort to evaluate blood flow to the abdominal organs.

The procedure is performed by placing an ultrasound probe over the kidneys, which are on each side of the abdomen. The sound waves emitted by the probe can detect blood flow to the kidneys. For evaluation of the mesenteric arteries, the ultrasound probe is placed over the abdomen. The procedure takes about 60 minutes.

Treatment options for decreased blood flow to the kidneys include lifestyle modifications, such as dietary changes and smoking cessation, and taking high blood pressure medications. If the high blood pressure does not respond to several medications or if kidney function gets progressively worse, an invasive procedure to prop open the arteries of the kidney with a metal scaffold called a stent may be considered.

Summary

Vascular laboratory procedures are common medical tests that can help diagnose and treat a variety of vascular conditions, including PAD, carotid artery plaque and narrowing, DVT, abdominal aortic aneurysms, and narrowing of the renal and mesenteric arteries.

We recommend that when patients need a vascular test, they select a location that is accredited by the Intersocietal Accreditation Commission (IAC), which oversees vascular labs (www.intersocietal.org). Additionally, the American College of Radiology has an Ultrasound Accreditation Program to assure quality control of practice locations (www.acraccreditation.org/Modalities/Ultrasound).