Electron Beam Angiography White Paper

A NON-INVASIVE APPROACH FOR ANATOMICALLY DEFINING CORONARY HEART DISEASE

By C. Michael Wright, MD FACC

Electron Beam Angiography | Indications for Electron Beam Angiography
Sample Electron Beam Angiography Images Obtained at LifeScore® |Summary

1. Refining the Non-Invasive Model for Diagnosing and Managing Coronary Atherosclerosis

LifeScore®'s program HeartScore is designed to identify coronary atherosclerosis by quantifying coronary artery calcium. Patients are scanned using the electron beam CT. This rapid, painless, and remarkably accurate test is used to assess the risk for developing symptomatic coronary heart disease. No other non-invasive test can visualize coronary plaque, and no other test can provide an accurate quantification of plaque. Once plaque has been identified, recommendations may include lifestyle changes, medications, nutritional supplements, and, if the amount of plaque is significant, further testing.

Until now, patients who required an anatomic assessment of their coronary arteries had no choice but to undergo cardiac catheterization. In this procedure, a plastic tube, or catheter, is introduced into the femoral artery, advanced up the aorta, and placed into the opening of the coronary arteries. Then, while contrast is injected, a radiographic film is acquired. Following removal of the catheter, pressure is applied to the femoral artery until bleeding stops, and the patient remains hospitalized for several hours in a supine position with the leg extended. This invasive procedure provides essential information to determine whether a patient is a candidate for angioplasty or bypass surgery.

Cardiac catheterization is costly and not without risk. In cases where the likelihood of severe coronary artery disease is high, the cost and risk is justified. But in many cases, the likelihood of finding significant coronary narrowings is in the middle probability range, often defined as a likelihood between 30 and 70%. In these cases, where up to 50% of the tests may reveal narrowings that are best treated without surgery or angioplasty, a non-invasive anatomic assessment of the coronary arteries is an appropriate first step. Such an approach will significantly reduce the cost and risk to the patient, while still providing the clinician with all the information needed to determine an optimal treatment plan.

The advantage of such a non-invasive approach is that it offers patients for the first time the ability to obtain a complete coronary assessment with minimal risk. As discussed below, there are many situations where non-invasive angiography will be preferable to cardiac catheterization.

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2. Electron Beam Angiography

In November, 1999, the Food and Drug Administration approved the Imatron electron beam CT (EBCT) scanner for performing non-invasive coronary angiography. Numerous studies have appeared in the medical literature over the past several years demonstrating the usefulness of EBCT for contrast-enhanced analysis of the coronary vasculature , , , , , These studies have demonstrated that electron beam angiography (EBA), while not indicated as a replacement for cardiac catheterization, can be considered an alternative in certain defined clinical situations. Overall, the studies indicate that EBA is 80-85% sensitive and 90-95% specific for detecting significant coronary stenoses in the proximal 2/3 of the coronary vasculature. The sensitivity and specificity is even higher for detecting stenoses in coronary bypass grafts.

The technique for EBA at LifeScore® is as follows. A 20 gauge peripheral access catheter is placed in the right or left antecubital vein and attached to a normal saline infusion. An initial injection of 12cc of Optiray 350 is used to determine the circulation time to the coronary arteries by generating a time-density curve. Subsequently, 135cc of the contrast agent is injected at a rate of 3.5ml/second. At the pre-calculated time, 40-45 coaxial images are obtained at a speed of 100ms per image. Each image is triggered by an electrocardiographic signal generated at 40% of the R-R interval. The patient is instructed to inspire and cease respiratory activity during image acquisition. Images are transmitted to an AccuImage workstation for subsequent analysis. As a first step, the images are edited to remove surrounding tissues. Then, further editing is done to eliminate the atrial appendages and the pulmonary artery, because these structures can interfere with visualization of the left main artery and the most proximal segments of the left anterior descending artery, the circumflex artery, and the right coronary artery. Then, the images are first examined using the three dimensional volume rendering technique (VRT). This technique displays the surface of the heart and can be manipulated by rotation around a vertical axis or a horizontal axis, or any combination of the two. Next, the images are examined using maximum intensity projections (MIP). These projections allow analysis of slabs perpendicular to three dimensional constructions in the sagittal, coronal and transaxial planes. By using these two techniques, the arteries can be examined from multiple perspectives, thus diminishing the likelihood of inaccurate interpretations.

The entire study takes approximately 20 minutes. Patients are instructed to only drink clear liquids for 4 hours prior to the study, and are instructed to drink plenty of fluids afterwards.

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a non-ionic contrast agent is used to minimize the likelihood of adverse reactions

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3. Indications for Electron Beam Angiography

At LifeScore®, a specific protocol is followed to determine which patients are candidates for EBA. The following diagram illustrates the protocol for patients who have undergone coronary calcium scoring:

Patients may also be referred from other physician's offices. Indications for EBA in clinical practice include:

· Patients post coronary bypass surgery with new-onset symptoms.
· Patients post angioplasty with new-onset symptoms.
· Patients with symptoms suggestive of coronary heart disease and mild, moderate, or equivocal results on stress testing.

In general, EBA is an excellent alternative to cardiac catheterization when the pre-test likelihood of significant ischemia is in the moderate range (30-70% likelihood).

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4. Sample Electron Beam Angiography Images Obtained at LifeScore® 

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5. Summary

Over 1 million cardiac catheterizations are performed each year in the US. Up to 30% are normal or mild to moderately abnormal, requiring no further intervention. The cost of a cardiac catheterization ranges from $3000 to $6000, and requires a short hospital stay (6-8 hours). Mortality and morbidity from the procedure are 0.15% and 1.5%, respectively. Electron beam angiography is a new non-invasive coronary angiogram which, in defined situations, can provide critical information without subjecting the patient to an invasive procedure. LifeScore® is the first location in San Diego to offer EBA. EBA is an integral component of LifeScore®'s non-invasive approach to the diagnosis and management of coronary atherosclerosis, and the prevention of acute coronary syndromes.