Stress Testing and the Detection of Heart Disease
There was a story published last week in a respected cardiology journal that on cursory review may seriously mislead the lay public. The topic was the under diagnosis of coronary heart disease by stress imaging testing using radioactive markers, using coronary artery calcification scores as the gold standard. In almost 1200 patients at some risk for, but without the diagnosis of, coronary artery disease, stress imaging testing by treadmill or bicycle was compared to coronary calcium scanning done within six months of each other. These researchers found that 56 per cent of patients passed their exercise tests but had significantly positive coronary calcium scores indicating diseased coronary arteries.
Let's review what this study is really saying, and what it is not saying. First, we will briefly review what coronary calcium indicates. Then we will look at what exercise testing indicates. And then we will look at the gaps between these diagnostic modalities and explain how they work together to help doctors and patients get to a true assessment of a patient's coronary status.
There is increasing evidence that not only is the presence of calcium in arteries and indicator of underlying atherothrombosis, or colloquially hardening of the arteries, but that the absolute amount of calcium may correlate with the extent of disease. That is, the higher the calcium score, or burden, the greater the likelihood of significant obstruction of the artery. The calcium does not cause the disease; it is simply a marker of disease, much like calcium in the joints or tendons indicates chronic injury, but did not cause the original injury.
There are different types of stress testing that each has its role in sorting out the status of a patient's coronary arteries. The most basic is the routine treadmill or bicycle exercise test in which a patient's symptoms, electrocardiogram, blood pressure, heart rate and rhythm are monitored with increasing levels of standardized exercise. If the arteries are having a hard time supplying heart muscle with enough oxygen and nutrients with increasing demand, the patient may develop symptoms, electrocardiographic changes, or instability of blood pressure or heart rhythm.
Routine stress tests are limited in their ability to detect significant disease, and their accuracy is highly dependent on the population of patients being tested. In high-risk patients, the diagnostic accuracy is quite good. In healthy populations, the accuracy is considerably less. Consequently, enhanced exercise tests involving imaging have been developed that significantly improve the diagnostic accuracy in most risk categories. Stress echocardiography, which looks at the function of heart muscle undergoing exercise, and stress nuclear imaging, which looks at blood distribution of the heart undergoing exercise have excellent track records in detecting about 90 per cent of significantly obstructed coronary arteries.
The key word here is significantly . Blood flow is not compromised in a linear fashion. A 50 per cent obstructed blood vessel does not block 50 per cent of flow. A marathoner can do quite well even if all of his blood vessels had a 50 per cent obstruction in each artery. In fact, blood flow is not compromised until at least 75 per cent or more of a vessel is obstructed.
Coronary calcium may be present with NO obstruction at all. Indeed, I have had patients referred to me with high calcium scores who eventually had coronary angiograms that showed no obstruction in any coronary artery. Conversely, I have a patient with a calcium score of ZERO who had a cardiac arrest and extensive underlying coronary obstructions, but survived to tell the tale.
Pitting one diagnostic test against another is a bit like the straw man analogy. It's a bit artificial and used to tout one test over another.
Just remember that the presence of coronary calcium is de facto evidence of some coronary artery disease and patients indeed should be treated aggressively with lifestyle changes, diet, exercise, weight management, risk factor assessment, and appropriate medications as needed.
Stress testing is looking for significant coronary obstruction that may require angiograms, angioplasty or surgery. As I have described in prior columns, stress testing will not necessarily detect an impending heart attack in a patient, since the mechanism there is the rupture of a plaque that may be less than 50 per cent obstructed before it ruptures. Stabilization of high-risk plaques to avoid the rupture that triggers an acute heart attack involves risk factor assessment, lifestyle and, if necessary, pharmacologic interventions.
Dr. Loh is medical director of the Ventura Heart Institute in Thousand Oaks, CA. His e-mail address is firstname.lastname@example.org . Previous Second Opinion columns are accessible at www.venturaheart.com.