Ask the Clinical Instructor: A Q&A column for those new to the Cath Lab
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.
It should be noted that IVUS and pressure wire procedures require that the patient receive anticoagulants prior to the beginning of the procedure. Since a catheter or a wire is being placed in the coronary artery, these medications must be given to prevent life-threatening clotting on the devices. At our facility, we try to utilize bivalirudin (Angiomax, The Medicines Company, Parsippany, NJ) as much as possible, because of the short half-life of the medication. If the procedure results in a negative finding, then the sheath can be removed within 2 hours.
A FFR of 1.0 is normal, but coronary arteries generally show a FFR > 0.94. As a general rule, FFR findings and their recommended course of treatment can be seen in Table 1.1
FFR can also be used to determine an appropriate course of treatment for a patient. A recent case at our facility demonstrates the importance that FFR can have in the determination of treatment. The patient presented with continuing chest pain on exertion. The RCA is totally occluded (probably chronic) and a large posterior descending artery (PDA)/posterolateral artery (PLA) system is noticed on angiography of the left arteries. The collaterals to the PL/PD appear to come from the LAD and circumflex. The chronic total occlusion (CTO) appears moderate in length once the collateral and main vessels are visualized, with a calcific channel outlined (Figures 5-7).
The LAD shows “lumpidy-bumps,” but nothing more than a 70-80% (remember, it is subjective) lesion. There are smaller lesions throughout the artery. The circumflex has a lesion that could be repaired with angioplasty.
The discussion that took place in this case was whether or not to fix the circumflex and bring the patient back later in an attempt to open the CTO in the RCA. Further review of the LAD angiograms aroused suspicion about the lesions present. If the lesions were not significant, the angioplasty would proceed; however, if the lesions were significant, the patient would be sent to surgery for at least a three-artery bypass.
Since IVUS would clearly show a calcium burden, which was somewhat visible on the angiograms, the PressureWire (Radi Medical Systems, Wilmington, MA) is utilized to determine the physiological status of the artery. It is also possible that the IVUS catheter may not completely reach the lesions or be able to pass through the tortuosity in the LAD.
Even upon placement of the wire, before adenosine administration, a gradient was present across the lesion (Figure 8).
After the administration of adenosine, the gradient increases to a maximum of 0.63. The decision was made to send the patient to surgery, since the LAD had physiologically significant disease as demonstrated by FFR (Figures 9 and 10).
It is best to simply remember that IVUS looks at structure, and FFR looks at physiology. While sometimes structure is all you need to know in order to intervene on a lesion, sometimes it is not enough. We want to make sure that before we place a foreign body in the patient (stent) that the patient really needs it. Sometimes knowing the physiologic status of the vessel is important to make the appropriate decision for the patient. Most importantly, the physician now has documentation to show the basis for their decision, instead of relying on angiography alone.
If you have any questions about this article, or you or your lab have a question about a particular topic, please send an email to firstname.lastname@example.org.
Next month, a question concerning vasovagal response will be addressed.