November 2009
Heart disease prevention is a high priority for the pilot and executive. Traditionally, cardiac
risk stratification into low (<10%), medium (10%-20%) and high (>20%) risk was based on the Framingham Risk Score (FRS). In 1948 researchers enrolled 5000 patients from Framingham Massachusetts, a small town perfectly suited for a long-term study and began one of the longest longitudinal observational cardiovascular risk factor studies in history.
The Framingham Study revealed the relative and accumulative effects of high blood pressure, high cholesterol, smoking, obesity, diabetes and physical inactivity on developing heart disease such as sudden cardiac death, heart attack, heart failure and angina. Unfortunately, as medical science progressed, it became clear that while the Framingham Risk Score was a good predictor of heart disease, it was not perfect and some patients with apparent high risk did not develop heart disease, and many with minimal risk factors did.
As medical science progressed, so then did the identification of new risk factors that were not originally included in the Framingham Risk Score. These include High Sensitivity C-Reactive Protein, Metabolic Syndrome, Coronary Artery Calcium Scores and a growing list of genetic markers to name a few.
Since the majority of heart attacks and sudden cardiac death occur in individuals with intermediate risk, and since this is a very large group within each Framingham Cohort, considerable effort is being focused on more accurately stratifying risk within this group. There is considerable evidence that aggressive risk factor reduction reduces adverse outcomes but not everyone needs aggressive treatment, and there are controversial issues regarding cost, efficacy and side effects associated with treatment of patients who are actually not at great risk.
Recently, researchers have discovered that coronary artery disease involving the gradually clogging of arteries with plaque, detected by exercise stress testing (GXT), CT angiography (CTA), Coronary Artery Calcium Score (CAC) and traditional Angiography do not predict all patients at risk of serious heart disease, particularly those who are younger, in their 40’ to 60’s. In fact, patients can have an essentially normal exercise stress test and have serious underlying heart disease that can be missed even by angiography.
The name for this kind of heart disease is “vulnerable plaque” and it is a combination of cholesterol and other substances that accumulate in the inner lining of a coronary artery, covered by a thin fibrous sheath of endovascular tissue known as the intima. In susceptible individuals, while the amount of plaque is insufficient to cause any obstruction in flow (hence the failure of traditional testing to identify those with this condition), an inflammatory process results in the plaque rupturing, releasing the cholesterol “goo” into the blood vessel and causing a sudden occlusion, leading to the clinical syndromes of myocardial infarction (heart attack), acute coronary syndrome (unstable angina) and sudden death.
Identifying individuals with vulnerable plaque in their coronary arteries has proven challenging, to say the least. In the recent Asteroid Trial, Dr. Nissen from the Cleveland Clinic used a coronary artery angiocatheter to image the lumen and wall of coronary arteries and show the reduction of plaque in a group of patients with minimal coronary artery disease over a 2-year period of treatment with Crestor. But this catheter is both expensive (about $40,000) and invasive and therefore could not be adopted for routine screening.
A very exciting recent discovery is that the Carotid Intima Media Thickness (CIMT) a test that images the thickness of the inner wall of the artery feeding blood to your brain cab be used as a surrogate marker for Coronary Artery Disease. This non invasive test does not expose the patient to any radiation and uses a sophisticated ultrasound probe that images the layers of the carotid artery in the neck.
In this CIMT image, the intima layer is shown as the distance between the pink and yellow lines, and is 0.81 mm near and 0.64 mm far wall. Depending on age, gender and race there are specific standards with respect to how thick this layer should be. Importantly, when the thickness is greater than the 75 percentile of the age group, or greater than 0.99 mm, then there is a 150% to 490% increase in the risk of stroke and heart disease. And this risk stratification is independent of the Framingham Risk Score.
What is more intriguing is that regression of CIMT with treatment is associated with reduction of cardiovascular and cerebrovascular risk. That means that one can monitor response to treatment interventions such as smoking cessation, optimizing nutrition and fitness, utilization of supplements and alternative agents and prescription of pharmaceutical agents. Several recent landmark trials such as the PLAC-II study, the METEOR trial and the ENHANCE trials used CIMT to measure outcomes that were also associated with reduced clinical risk, confirming the power of CIMT to predict true future risk.
The fact that most heart attacks occur in individuals with intermediate FRS confirms that traditional risk factor stratification is limited. A more sophisticated and accurate determination of individual vascular risk includes family history of CAD, insulin resistance, central obesity, metabolic syndrome, HS-CRP, physical inactivity and CIMT.
Using CIMT to calculate the vascular age of a subject, rather than the chronological age, more than 1/3rd of patients with initially intermediate risk were reclassified as high risk and about 1/6th were reclassified as low risk. This substantially increased or decreased treatment for almost half of the patients in the intermediate risk group.
Given the limitations of traditional risk factor screening, utilization of advanced detection of subclinical atherosclerosis with CIMT or CAC scores would result in a 10% reduction in the risk of cardiovascular death and the 25% reduction in heart attack. The SHAPE task force in the US for example now recommends that all asymptomatic men aged 45-75 years and women aged 55-75 years be screened with CIMT or CAC in addition to traditional risk factors and managed according to a modified risk score.
When deciding which screening test to use, CIMT or CAC, the literature is currently unclear. The MESA trial suggested that CAC was better at defining cardiovascular risk in adults aged 45-84 years, however there were fewer ethnic differences in CIMT scores in this trial. A study published in the Mayo Clinic in 2009 however suggested that CIMT was more sensitive than CAC for determining risk in young to middle-aged patients.
At Affinity Health, we have included CIMT screening as part of our Executive Health Assessment (EHA), and combine this with a comprehensive cardiovascular screening protocol to identify individuals at higher risk of heart attack and stroke to ensure effective countermeasures. For pilots, we offer this additional screening test for $150. It is currently not a benefit of Provincial Health Insurance Plans.
Dr. Randy Knipping BSc MD CCFP
Medical Director
Affinity Health