The use of three-dimensional images of a Computed Tomography (CT) scan to visualise heart arteries is invaluable in providing a correct diagnosis
IS it possible to scan the heart of an outpatient in a few seconds and visualise the heart arteries in three-dimensional views without inserting a tube into the heart arteries? The answer is yes. Over the past decade, advances in imaging technology has resulted in an increasing use of non-invasive Computed Tomography (CT) X-ray scan to image the heart arteries. Unlike coronary angiography (CAG), CT of the heart arteries not only allows visualisation of the lumen but also allows the wall of the artery to be visualised in any plane and detailed analysis can be performed to assess whether the narrowed segment has soft “vulnerable plaques”, hardened plaques or calcium deposits.
It is not uncommon for disease in the same heart artery to be interpreted differently when visualised by different imaging techniques. Extensive diffuse plaque and narrowing of a heart artery with a diffusely narrowed lumen which can clearly be seen on the CT images can appear as an apparently “normal” artery with a diffusely small lumen on CAG.
The usefulness of three-dimensional visualisation becomes more apparent in situations where the CAG shows areas of narrowing alternating with widened segments along an artery, presenting doctors with the dilemma of trying to determine whether the narrower segments represent segments which are narrowed by cholesterol deposits or whether the apparently narrower segments are normal, and the widened segments are actually diseased due to abnormal swelling of the arterial wall, also called “ectasia”.
The three-dimensional images from the CT which allow visualisation of the arterial walls will be invaluable in providing the correct diagnosis. Practice guidelines from professional bodies such as the American Heart Association and American College of Cardiology have expanded the indications for the use of CT and increasingly, CT is now being used in place of CAG for the assessment of heart arteries. When compared to CAG, the advantages of CT are obvious – it is a non-invasive, less costly outpatient procedure and is completed within seconds. In highly experienced centres, CT is highly accurate and the scan can be completed with low-radiation doses.
Identifying the functionally significant blockage
In trying to distinguish narrowed heart arteries which do not result in significantly impaired blood flow to the heart from those which cause significant reduction in blood flow, a new method of assessment was developed to measure and compare the pressure before and after the narrowed segment and the value derived was termed fractional flow reserve (FFR).
The pressure changes were measured using a special wire with pressure sensors that were put into the heart artery to measure the difference in pressure across the narrowed segment. In 2009, the “Fame” study was published in the New England Journal of Medicine (NEJM), using FFR to address the issue of trying to identify narrowed segments which were functionally significant as opposed to just considering the degree of physical narrowing.
The study compared those who underwent opening of the heart arteries with stents (cylindrical metallic meshes) following CAG with another group which only underwent stenting if the pressure measurements across the narrowing were abnormal.
The authors concluded that using FFR to select patients resulted in better outcomes in a follow-up after a year. The implication was that using the pressure wire resulted in better identification of narrowed segments that benefited from stenting, and hence decreased stent usage, and this benefit was coupled with better outcomes.
A subsequent study, Fame II, published in September 2012 in the NEJM compared the outcomes of those who had at least one functionally significant heart artery narrowing on FFR who were treated with stents to those who were treated with medication only. The study was stopped early as it showed that stenting provided a significantly better outcome as compared to those treated with medication only.
The implication of these studies is that those narrowed heart artery segments which were functionally significant will benefit from stents placed into the heart arteries and patients have better outcomes with stents as compared to medication only.
Is the physical or functional significance of heart artery narrowing more important? Should one then decide on the significance of a blockage of the heart artery purely by its functional significance using FFR? In a substudy of Fame published in 2010 in the Journal of the American College of Cardiology, of all the heart arteries which had a physical diameter narrowing of 50 per cent to 70 per cent on CAG, 65 per cent were deemed functionally non-significant. For those with diameter narrowing of 71 per cent to 90 per cent, 20 per cent were considered as not functionally significant. Even for those with more than 90 per cent diameter narrowing, 4 per cent were deemed as not functionally significant.
It will be very difficult to convince any cardiologist that any heart artery narrowing of 80 per cent or more is not significant. If functional significance were the only optimal deciding factor, then those whose treatment was solely determined by FFR should theoretically have excellent outcomes at the end of one year.
However, the reality was that for those whose decision to stent their narrowed arteries was based on FFR, after a one-year follow-up, more than 10 per cent of patients developed adverse events including 1.8 per cent death, 5.7 per cent heart attacks, and 6.5 per cent repeat revascularisation procedures. Simply put, FFR failed to recognise the narrowed segments that had “vulnerable plaques” which can lead to heart attacks and sudden death.
Using FFR will result in missing out on narrowed segments which are not functionally significant on FFR but which can be functionally significant during vigorous physical exertion. Compared to FFR-guided stenting, stenting procedures which are guided by CT have shown virtually no heart attacks or death at the one-year follow-up.
Understanding heart artery disease
The definition of a significantly narrowed heart artery segment varies depending on the method of assessment used. Currently, CAG is the “gold standard” used for determination of a significant narrowed heart artery although it has several limitations.
The current data shows that FFR (functional significance) is a better determinant than CAG (two-dimensional arterial lumen assessment) when trying to determine which narrowed artery was significant and required stenting. However, using functional significance by FFR has several limitations including being an invasive procedure, having added cost and a failure to recognise vulnerable plaques. Also, stenting based on FFR has been found to have a high adverse outcome rate during early follow-up (although less than CAG).
Stenting based on non-invasive CT assessment (three-dimensional heart artery evaluation) has been shown to be safe and feasible and is associated with an extremely low rate of adverse outcomes during early follow-up.
Are non-significant heart artery narrowings unimportant?
Current recommendations and appropriateness criteria for stenting-narrowed artery segments will include most with physically significant (greater than 70 per cent diameter narrowing) or functionally significant narrowing in a major heart artery. However, we do not seem to have all the answers as many women with heart attacks do not have physically or functionally significant heart-artery narrowing. In addition, a large number of heart attacks that are due to “plaque rupture” occur in heart arteries that do not have physically or functionally significant heart artery narrowing. While we are getting better at determining those significantly narrowed arteries which will benefit from stents, we have yet to devise simple non-invasive ways to identify those who have non-significant narrowings in their heart arteries but are predisposed to a heart attack.
What is certain is that if all the risk factors for heart disease such as blood pressure, cholesterol levels, sugar levels and smoking are well-controlled, the risk of a heart attack will be extremely slim. Ultimately, prevention is better than cure and definitely less costly!