By Daniel Simon
Daniel Simon reviews a new system that simulates fractional flow reserve (FFR) measurements using computed tomography (FFRCT, HeartFlow). He explains why he believes this system, which recently received FDA approval, has the potential to reduce the number of unnecessary coronary tests that are performed.
Despite advances in technologies for the management of cardiovascular disease, there is still a significant unmet clinical need in the non-invasive diagnosis of coronary artery disease. Existing diagnostic modalities (including everything from stress echocardiograms, nuclear stress tests and regular stress tests to CT angiograms) provide insufficient predictive values to definitively determine the degree of coronary artery disease. The limitations of these tests may mean that unnecessary subsequent tests and procedures are performed. For example, a study published last year found that of more than 385,000 patients from over 1,100 US hospitals, only 45% of patients with stable angina who underwent elective diagnostic angiograms on the basis of guideline-recommended non-invasive tests actually had obstructive coronary artery disease.1 Patients should not have to undergo needless tests or procedures (or be exposed to the accompanying radiation with these tests), and healthcare providers should not have to meet the costs of such procedures. However, the use of a test that calculates FFR from coronary CT angiograms (FFRCT, HeartFlow) may be able to limit the number of “needless” tests and procedures that are performed.
FFRCT is designed to provide information on both the extent of a coronary blockage and whether that blockage is impacting blood flow to the heart—ie., the two pieces of information that are vital to individual patient management. While anatomical information can already be determined—non-invasively—with CT angiogram, functional data could previously only be obtained with invasive FFR procedures done in conjunction with angiography. The ability to get same information from a single non-invasive test has the potential to transform how coronary artery disease is identified and managed.
The test works by building a digital 3D model of the patient’s cardiovascular anatomy using a standard CT scan. Millions of complex equations are then performed to simulate blood flow within this personalised coronary model to provide mathematically computed FFR values. These values represent pressure differences around a narrowed artery, which allows clinicians to identify, on a vessel-by-vessel basis, the extent to which blood flow to the heart has been reduced. Clinicians can the use these values to determine the best way to manage the patient.
Early clinical experience at University Hospitals Case Medical Center has been very promising, as FFR CT has helped us rule out significant coronary artery disease in patients and, therefore, prevent unnecessary cardiac catheterisations. It has also helped us to avoid missing or misdirecting patients based on the results of other, less accurate non-invasive tests. The NXT (Analysis of coronary blood flow using CT angiography: next steps) study,2 which used FFR as the reference standard, showed that FFRCT has a greater ability to determine which lesions may impede blood flow compared with CT angiography alone—86% vs. 65% diagnostic accuracy, respectively. This difference was primarily due to a significantly increased specificity with FFRCT (86% vs. 60% for CT angiography alone).
Invasive angiography was performed with 71% accuracy in the study. As with any new—and potentially disruptive technology—further clinical studies of the FFRCT are needed.
The ongoing PLATFORM (Prospective longitudinal trial of FFRCT: outcome and resource impacts) study is comparing clinical outcomes, resource use, and quality of life using FFRCT-guided evaluation versus standard practice evaluation in 580 patients with suspected coronary artery disease. Results of the study are anticipated to be released later this year. If they demonstrate significant superiority of the system over current standards of care, FFRCT could prove to be a “game-changer.”
1 Patel M. Am Heart J 2014; 167 :846–52.
2 Nørgaard B. J Am Coll Cardiol 2014; 63: 1145–55.