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CTA, FFR-CT changing management of suspected angina, CAD

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Testing strategies are rapidly evolving to determine which patients with chest pain have CAD, and which of those have high enough risk to warrant revascularization. Driving reexamination of historical approaches to evaluate such patients are advances in coronary CT angiography technology, including new capabilities to determine coronary fractional flow reserve from CT images (FFR-CT).

Cardiology Today spoke with Markus D. Scherer, MD, FACC, FSCCT, director of cardiac CT and structural heart imaging at Carolinas HealthCare System’s Sanger Heart & Vascular Institute in Charlotte, North Carolina, about how these developments are leading to changes in chest pain evaluation protocols, in efforts to improve patient outcomes while reducing overall health care costs.

Question: What is your protocol for testing patients with suspected angina?
Answer: When we evaluate patients with chest pain, we begin with clinical assessment — cardiac risk factors, the presenting history, findings on the ECG — to estimate the likelihood that the root cause is related to obstructive CAD. When our pre-test suspicion for angina/CAD is low, we may decide that no additional testing is warranted.

However, many, if not most, patients referred to the cardiologist for chest pain evaluation have either intermediate or high pre-test likelihood for CAD. The traditional approach for these patients has been to undergo some form of exercise stress testing. Typically, nuclear or echocardiographic imaging is used in conjunction with the exercise stress test to improve the diagnostic accuracy of the study. If the patient cannot exercise sufficiently, then a pharmacologic stress test is performed. For those patients judged to have a high probability of angina-related severe CAD, cardiac catheterization without stress testing is often our referral path.

While this has been our general testing strategy, this paradigm is changing with a newer noninvasive test in our tool chest, namely coronary CTA.

Q: What is the role of coronary CTA and how has that evolved in recent years?
A: Coronary CTA is a test that allows clinicians directly detect if CAD is present or absent. If CAD is present, one can estimate the degree of stenosis and determine how many vessels are involved, similar to what has been done with invasive coronary angiography. In contrast, stress testing indirectly evaluates CAD. These tests determine inducible ischemia or reduced blood flow, but do not afford visual inspection of the coronary arteries for presence of CAD. Because stress tests are indirect in their assessment, even when abnormal their results do not prove that patient has CAD. Moreover, a normal stress test does not rule out the presence of underlying CAD.

We know from the National Catheterization Laboratory Data Registry that of the many U.S. patients undergoing cardiac catheterization for evaluation of suspected CAD, less than half will exhibit obstructive CAD. And, many of those patients will have had stress tests, including nuclear stress tests, prior to being sent to the cath lab. This demonstrates problem with the traditional management scheme. So, we have recognized that the standard paradigm of stress testing to detect CAD has significant limitations.

Coronary CTA is a sensitive test with a high negative predictive value to exclude obstructive CAD. With current-generation technology, it can be performed in most patients with a much lower radiation dose than a typical nuclear stress test. But while it is very powerful in excluding significant CAD, the positive predictive value of the study — meaning that a stenosis seen on the scan is indeed physiologically important — has been its weak link.

Q: What is the role of FFR - CT and how has that evolved in recent years?
A: FFR is simply the relationship of the pressure in the artery downstream to a stenosis relative to the pressure upstream, when assessed under conditions of maximal blood flow. If that ratio is at or below 0.8, then the stenosis is considered functionally significant. We have outcomes data showing that patients do better when you use FFR in the cath lab to guide whether to place a stent rather than using the visual estimation of the stenosis alone to make that decision.

The field of computational fluid dynamics — the same basic science that aids us in determining optimal aircraft design — has now been applied to the physics of blood flow through arteries to calculate FFR. This approach, known as FFR-CT, noninvasively estimates the FFR from the static coronary CTA images. Remember, for determining FFR in the cath lab, you have to give adenosine in order to create the condition of maximal blood flow. However, with FFR-CT, the modeling and computations are based on predicable relationships of blood flow and myocardial mass. So, we don’t have to give the patient any extra medication. If the patient has a moderate- to high-grade stenosis and we’re uncertain as to the clinical significant of this finding, we can just take the standard coronary CTA dataset and send it for FFR-CT analysis.

Note that if you have a normal coronary CTA, there is no need to send the test for FFR-CT analysis because there is no lesion to analyze. If you have only a mild lesion — say, less than 50% stenosis — then it is unlikely that it is hemodynamically significant. We commonly would not send such studies for FFR-CT analysis unless the lesion were to be located in a worrisome location, like the left main artery or the proximal left anterior descending artery, or if it had some particularly concerning plaque morphology.

The biggest impact for FFR-CT is with intermediate lesions (50% to 69% stenosis) where the physiologic significance is unclear. Outcome studies have shown us that visual assessment alone is inadequate in determining the significance of the lesion. While you could certainly go back and do a stress test, with FFR-CT we can just analyze the coronary CTA dataset. If the FFR-CT value is less than 0.8 — in other words, there is more than a 20% drop in the estimated pressure after a blockage — then we say the blockage is likely of clinical significance. We will typically send these patients on to cardiac catheterization or aggressively advance their medical program. However, if the value is > 0.8, we can say with confidence the blockage is not significant and manage the patient medically. This is how were using FFR-CT now.

Q: To what extent have these technologies been able to improve outcomes?
A: As far as mortality, there have been some well-designed trials so far, both for stable outpatients and those in the ED. For the stable outpatient population, the large-scale PROMISE trial showed at 2 years no difference in survival or clinical outcomes between coronary CTA and functional stress testing. But keep in mind that the PROMISE trial involved a low-risk population of stable outpatients, and it is difficult to show an impact of any diagnostic imaging test on survival when you have a low-event-rate in the patient population to begin with. What the PROMISE trial did show was that coronary CTA offered a practical alternative to conventional stress testing for CAD evaluation. For those patients ultimately referred to invasive angiography, fewer patients undergoing coronary CTA had nonobstructive coronary disease as compared to conventional stress testing. This means that the stress test route was more likely to have false-positive results than coronary CTA.

There was another major trial, the SCOT-HEART trial in the United Kingdom, which showed a trend toward reduced incidence of MI when using a coronary CTA approach as compared to the standard of care. It also clearly showed a significant impact on increasing the physician’s diagnostic confidence in whether the patient’s chest pain was due to CAD. Coronary CTA gave physicians more confidence on how to manage patients and often changed treatment strategies.

Also, the recently conducted PLATFORM trial compared the usual strategy of clinical decision-making to one informed by coronary CTA with provisional FFR-CT. The trial showed equally safe but more costeffective outcomes when using a coronary CTA/FFR-CT strategy as opposed to directly taking patients to the cath lab. In PLATFORM, using the FFR-CT-guided strategy, cardiac catheterizations were canceled more than 60% of the time with no compromise in clinical outcome — now out to 2 years.

One thing to keep in mind when reviewing the literature is that the coronary CTA technology has evolved so rapidly in the past few years that some of the conclusions from these trials may be already outdated, especially with comparing outcomes like radiation doses. The studies were performed in the 2010-2013 timeframe, and many sites had scanners installed before 2010. Our CT technology has improved dramatically, so we feel some of these data are already out of date, even though it doesn’t seem like these trials are that old. Expectations are higher now with improvements in CT technology. If you have access to a current-generation scanner and you perform guideline-directed coronary CTA with good technologists and good physicians, improved diagnostic accuracy should follow.

Whether adopting coronary CTA as a first-line test (instead of stress testing) will translate into a mortality benefit is unclear. The initial outcomes data in the original coronary CTA trials were only for 1 to 2 years. We know that certain medical therapies, like statin use, have an impact on mortality, but these outcomes are only seen over a longer time horizon. If we can identify more patients who have CAD, which coronary CTA will detect and stress tests may miss, we may have a more significant impact on our patient populations by getting patients with nonobstructive but significant CAD on statin and other preventive therapies. If we can do this efficiently with coronary CTA, and accomplish this with even less radiation using newer-generation scanners, then that’s an even bigger win.

Q: To what extent have these technologies advance d diagnostic confidence?
A: After a coronary CTA, we know whether a patient has CAD and, if so, the extent and the morphology of the plaque. All these features help estimate a patient’s risk and guide treatment strategies. In reference to plaque characterization, a purely calcified plaque, although potentially associated with angina, is unlikely to cause an MI. Other types of plaques with outward bulging —which we call “positive remodeling”— or less dense, noncalcific plaques confer higher risk and are more likely to be associated with future MI. Gaining such information may help guide and individualize preventive treatment and follow-up strategies in ways we simply can’t from a stress test data. So, one of the ways coronary CTA technology has improved our confidence is that we are providing the best and most appropriate care for our patients.

Coming back to the issue of CT technology, major advances have occurred over the past 10 years. There has been dramatic improvement in spatial resolution — which is the ability to see smaller structures more clearly — and in temporal resolution — which is the ability to see movement more clearly. That’s important when it comes to coronary arteries because, unlike with CT scanning of other areas of the body, the heart is always beating and moving, so we need to have ways to freeze the motion of the heart. By having higher temporal resolution scanners, we can now do this. This has improved our ability to scan patients with higher heart rates than we could in the past. The spatial resolution improvements have allowed us to be more precise about our estimation of stenoses. We also have had improvements in the tubes that generate the X-rays and in the detectors receiving them, so that the images are clearer. Consequently, we’re able to perform scans in a broader range of patients. For example, we can scan patients with obesity or with high heart rates more confidently.

Again, the other major enhancement has been the ability to perform FFR-CT. Now we have the added confidence of FFR-CT as an adjunct to the coronary CTA for any intermediate- to high-grade stenosis of unclear significance. This helps us to determine if the patient needs cardiac catheterization or medical therapy.

Q: H ow else has this process benefited patients?
A: We are getting more detailed information with coronary CTA and FFR-CT than we can obtain with conventional stress testing. This allows for a more individualized or patientcentered approach to care. We gain a better idea of a patient’s overall risk for an adverse event. We can determine how likely what we see on imaging is related to a patient’s symptoms. From all of this, we expect improvements not only in physician confidence, but also in patient confidence and satisfaction with the whole management approach.

As far as where coronary CTA and FFR-CT stand, it is important to point out that we will have even more data in the near future. Specifically, we have the ADVANCE registry, which will be completed within the next year. It is an international registry of more than 5,000 patients, and it will allow us to see the impact FFR-CT has on outcomes for patients.

Putting all of this together, coronary CTA with FFR-CT not only holds promise for us to take better care of patients on an individual level, but through its ability to characterize CAD burden it also provides a means for us to better manage our patient population as a whole. – by Erik Swain

Disclosure: reports he was an unpaid consultant for HeartFlow at time of this interview.