Ask the Clinical Instructor: A Q&A column for those new to the Cath Lab
- 11 Nov 08
- Posted on: 11/5/08
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There are two very important tools in the cath lab that are often misunderstood. Intravascular ultrasound (IVUS) and fractional flow reserve (FFR) analysis are two totally separate tools looking at two different things. Basic comprehension of these tools may help you understand why the physician is using them. Each of these tools could be an article or a class in themselves, but here I will try to summarize why they are used in the cath lab.
While coronary angiography can clearly identify many lesions, there are also some limitations:
1. It’s only a “lumenography.” In other words, we can only see where the contrast is traveling, and cannot see the entire vessel structure.
2. With diffuse disease, we do not know what vessels may actually be causing the symptoms.
3. With borderline lesions (50-80% on angiography), we still don’t know if this type of lesion is causing symptoms and whether it needs to be fixed or not.
Let’s look at a scenario where the physician is performing an angiogram on a left anterior descending artery (LAD) and there appears to be some haziness in the mid-portion of the vessel. After numerous views and angulations, there does not appear to be any change in the haziness. (Another scenario might be in the LAD on one particular angulation where there appears to be an eccentric lesion, a lesion not well identified on any other view because of overlapping of other vessels.)
In these cases, angiography does not specifically resolve the issue of what is going on. IVUS can be used to look “inside” the vessel, to see what is going on with the structure, by looking at plaque burden. A specialized catheter with an ultrasound tip on the end can ‘peek’ into the vessel to show the structure from the inside out.
The catheter is able to get a 360˚ view of the inside of the artery. As the catheter is methodically moved in the artery (pull back), slices of ultrasound images are taken and pieced together so that a larger picture is viewable from a length perspective.
With IVUS, you should be able to clearly see the 3 layers of the artery (adventitia, external media and intima) as well as any plaque or thrombus that may be present (Figures 1a and 1b). Through the controls of the IVUS machine, diameters, lengths and areas can be measured. These processes help identify portions of the artery that are diseased, and to what extent. (This detailed process may be a subject of a future article.)
IVUS also has a specific place in interventional cardiology to assure proper stent sizing, as well as apposition. Some machines (Boston Scientific’s iLab, for example) can also allow you to place a “virtual stent” on the vessel image to help determine the appropriate size and length of a stent (Figure 2). Numerous cases of stent thrombosis have been documented as a result of the stent not being fully expanded against the vessel wall. IVUS can help assure appropriate placement and expansion. Many facilities and physicians routinely perform a “pre” and “post” IVUS assessment to assure proper sizing and apposition to the vessel wall.
There are many reports, studies and trials that attempt to link the cross-sectional areas (CSA) of the vessel to the need to fix the vessel. Some of them are promising and can be used as a basic guideline for treatment. The literature states that a CSA of < 4.0 mm2 in a major epicardial artery is considered a significant stenosis (6.0 mm2 in the left main).2-3
There is a limitation to applying this CSA theory, however. Look at the two people in Figure 3. Which one of these people would likely have the larger epicardial arteries? Granted, this is a little bit of an exaggeration, but it would be the large man, of course.
A 4.0 mm2 lumen in the large man is small in relation to his larger epicardial arteries, whereas a 4.0 mm2 lumen in the small lady, with smaller epicardial arteries, means very little stenosis (Figure 4). Simply put, the standard 4.0 mm2 rule may underestimate the severity of the stenosis in a larger person, whereas the same rule may overestimate the severity in a smaller person. This needs to be taken into consideration when trying to use IVUS to determine the physiological severity of a stenosis.
Now let’s look at the scenario where a patient comes into the cath lab for a diagnostic procedure. They come to the cath lab because they have been continuing to have chest pain on exertion even after medical management. Previous stress tests were inconclusive.
The physician takes an angiogram of the right coronary artery (RCA), and finds a lesion that appears to be 70%. (Again, using angiography, this is all subjective). Most people would consider this a “borderline” lesion. The question is whether the lesion is causing the symptoms or not. An IVUS catheter placed in the artery may show a large plaque burden and confirm 70% narrowing of the artery, but it still does not tell us whether this is compromising flow during exercise.4
Fractional flow reserve is simply a “stress test on the table.” Medication is administered, usually adenosine, to dilate the microvasculature to obtain the maximum blood flow/perfusion possible (hyperemia) distal to the lesion to simulate exercise. A pressure reading is obtained proximal and distal to the lesion, and a gradient is established to determine any flow restriction during exercise. This is accomplished by simultaneously obtaining the proximal pressure through the guiding catheter, and the distal pressure by the pressure wire.
1. Baim DS, ed. Grossman’s Cardiac Catheterization, Angiography, and Intervention. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006.
2. Brigouri C, Anzuini A, Airoldi F, et al. Intravascular ultrasound criteria for the assessment of the functional significance of intermediate coronary artery stenosis and comparison with fractional flow reserve. Am J Cardiol 2001;878:136-141.
3. Jasti V, Ivan E, Yalamanchili V, et al. Correlations between fractional flow reserve and intravascular ultrasound in patients with ambiguous left main coronary artery stenosis.
4. Libby P, Bonow RO, Mann DL, Zipes DP. Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine. 8th Edition. Philadelphia: Elsevier Science; 2007.