Invasive Studies for CAD
Invasive Studies for the Assessment of CAD
Invasive studies for the assessment of CAD - those which use an instrument or device inserted into the vascular system - include catheter coronary angiography, intravascular ultrasound, and angioscopy.
Catheter coronary angiography, also called coronary arterial lumenography, cardiac catheterization, coronary arteriography, and sometimes just "cath," is a procedure in which a small catheter (a long, thin, flexible tube) is inserted through the artery in the upper leg (the femoral artery) and threaded through the arterial system back to the heart at the origins of the coronary arteries from the aorta (the large vessel that exits the heart and supplies blood to the remainder of the body). The catheter can be turned, rotated, and manipulated by the operator to engage the coronary arteries. Intravenous contrast, often referred to as "dye," can then be injected into the coronary artery while x-rays are being generated to create a picture of the internal aspect of the coronary artery. (Figures 10 and 11)
A normal coronary artery has a smooth internal surface with a constant internal diameter which gradually tapers as the vessel gets smaller. On catheter coronary angiography, CAD will appear as irregularity of the internal surface of the coronary artery, or, if more severe, the coronary artery will show narrowing at sites of CAD (Figure 12A), or, in the most severe disease, frank occlusion of the coronary artery. The degree of narrowing is often quantified as "percent luminal stenosis" - this refers to how narrow the abnormal segment is compared to adjacent normal-appearing segments. For example, if an area of CAD visually appears to narrow the coronary artery diameter by about 50% compared to the adjacent normal parts of the artery, the vessel would be considered to have a "50% stenosis."
Rather than using a mere visual assessment of the severity of CAD, computerized quantitative coronary angiography may provide more precise quantitative analysis of CAD in the entire coronary arterial system. This technique is widely used in studies assessing drug treatment of atherosclerosis to assess for plaque progression or regression.
Catheter coronary angiography also offers the ability to treat CAD, in addition to diagnosing and quantifying it. Small balloons may be inflated in areas of narrowing caused by CAD to open these narrowed segments - this procedure is known as angioplasty. (Figure 12B), Also, small metal devices, known as stents, may be deployed in areas of coronary artery narrowing to keep the narrowed areas open over time. (Figure 12C),
Despite the fact that catheter coronary angiography is a very powerful tool and has long been considered the "gold standard" for diagnosing CAD, it does have limitations. The visual assessment of the severity of a given stenosis is not highly accurate - different readers may have different impressions of how severe a stenosis is. This "interobserver variability" may be as high as 30-60%. Additionally, when CAD is present diffusely throughout the coronary arterial tree, visual assessments of the severity of CAD, which rely on comparing the affected segment to an adjacent "normal" segment, become inaccurate. As one can see, comparing the area of narrowing to an area that is assumed to be normal, but is in fact diseased, leads to an underestimation of the severity of the narrowing - this could potentially lead to incorrect treatment decisions. This situation is occasionally encountered in diabetic patients.
Catheter coronary angiography provides an image of the internal diameter of the coronary artery - this image is often referred to as a luminogram because the internal aspect of a hollow structure such as a vessel is referred to as the lumen. In fact, early in the course of the development of CAD, the affected coronary artery often dilates as a compensatory response - this situation is referred to as positive remodeling. The effect of this process is that the affected coronary artery maintains its internal diameter for some time even in the presence of progressive CAD - at least for awhile. So, if one takes a picture of the internal surface of a coronary artery with plaque undergoing positive remodeling, the diameter of the vessel will appear normal and it will be incorrectly concluded that the vessel is unaffected by CAD. This error is the result of the fact that catheter coronary angiography only depicts the inside of the vessel, not the vessel wall itself, which is where CAD begins. This is particularly important because evidence suggests that coronary artery plaques that exhibit positive remodeling are the plaques that tend to produce unstable angina and MI. Intravascular ultrasound does have the ability to detect CAD in the vessel wall before the internal diameter of the vessel becomes narrowed, as does coronary CT angiography and MR.
Finally, because it is an invasive technique, catheter coronary angiography is associated with certain "major" complications in about 1 in every 1000 patients undergoing the procedure. These complications include death, MI, stroke, aortic or coronary vascular damage, cardiac rupture, air embolus (air entering the catheter and then the blood stream, which can cause stroke), arrhythmia, and peripheral arterial injury. Minor complications related to catheter coronary angiography are relatively common and include hematoma at the catheter entry site into the femoral artery, transient angina, fainting reactions, and allergies to contrast agents and drugs used during the procedure.
