Cardiac computed tomography (CT), and in particular computed tomographic coronary angiography (CTCA), stand poised to revolutionize the way in which cardiac pathology is diagnosed, and change the way in which decisions are made with regards to invasive treatment of cardiac disease. In particular, the diagnosis of coronary disease (the number one killer of patients in modern societies) will be strongly influenced in the coming years by this type of technology. However, there are many areas in cardiology and cardiovascular disease that will be influenced by this new technology.
For most cardiac patients, the central question relates to the presence or absence of coronary artery disease (CAD) (i.e. coronary atherosclerosis). For patients who have chest pain, the main question is whether or not that pain is a sign that there are significant flow-limiting blockages in the coronary arteries. For asymptomatic patients, the question is usually one of estimation of future risk of developing blockages or having a myocardial infarction (MI). Traditionally, it has been relatively easy to triage the patients with chest pain through a combination of clinical history, the physical exam, and utilizing stress testing. Strong suspicion for coronary disease based on symptoms would lead to invasive angiography. Likewise, a ‘high-risk’ stress test would indicate that angiography would be helpful. In patients with moderate pre-test probability, or in patients in whom a stress test is equivocal, CTCA provides an alternative route to choose from, rather than simply moving forward with invasive testing.
In the asymptomatic patient, or low-risk patient, the estimation of future risk is much more difficult to define. Patients who are (by definition) asymptomatic tend to have more ‘false-positive’ stress tests. In a patient with risk factors for CAD, a positive stress test tends to lead to unnecessary invasive cardiac catheterizations. In this situation, it may be advantageous to forego stress testing in favor of CTCA in order to define the presence or absence of coronary plaque. In the asymptomatic patient, a negative CT coronary angiogram would reassure the patient, while the discovery of plaque allows the physician to concentrate on risk-factor modification with medications and does not usually lead to further testing (see Figure 1). This particular aspect of CTCA makes it a likely candidate as the ‘test of choice’ in patients whose occupation means it is important to know whether they have high-risk anatomy.1
Although not yet universally available, the adoption and utilization of CCTA technology by cardiovascular providers has been tremendously swift. Currently, the main use for CCTA has been in the area of evaluation of low-to- moderate probability patients for the presence or absence of coronary disease. In particular, the test has been helpful in the triage of those patients who have weakly positive or equivocal stress tests towards invasive testing or medical management.
Current Practice
There is no universal agreement on the mainstream use of cardiac CT. Recently, appropriateness criteria were created to give guidance as to the reliable and generally well-accepted uses of cardiac and coronary CT based on current clinical data.2 Strongly agreed upon uses for this type of technology include the evaluation of coronary artery anomalies, and in the further triage of patients with low-to-moderate risk for flow limiting coronary disease in whom traditional stress testing is equivocal.3,4 CT has good potential application in the evaluation of the larger structures of the heart, and in defining abnormal masses or congenital abnormalities.5 There is growing evidence that this may be an acceptable test to evaluate for cardiac valvular disease as well as coronary atherosclerosis.6,7
Atrial Septal Defects
An atrial septal defect (ASD) is a large communication between the top two chambers of the heart. These are usually discovered shortly after birth, and are surgically corrected. If missed, they can lead to pressure and volume overload of the right side of the heart, and may cause heart failure or even death if the pressures in the lungs become irreversibly elevated. Closure of the hole can be effected with either surgical repair or with percutaneous closure, using a nitinol device that ‘plugs’ the hole. Closure utilizing either manner results in normalization in cardiac size and function.8 In the past, evaluation and management of this issue has been performed with echocardiography. There is growing evidence that utilization of CT is at least as good at identifying these conditions, and can also be used to follow patients post-closure to track the reduction in right-sided chamber volumes.9,10
Pre-surgical Clearance
Another potential area in which CTCA may be beneficial is in the preoperative evaluation of patients who are to undergo cardiac surgery. Traditionally, in patients who need a cardiac operation for something other than coronary disease, a cardiac catheterization is mandatory. This test is performed in order to identify or exclude those patients in whom a concomitant bypass is needed. For example, if a patient needed a mitral valve repair, cardiac catheterization would be performed prior to that surgery in order that coronary disease might be identified or excluded.
There are many patients who are of a very low probability to have significant flow limiting disease. In patients who are very young, especially in women, there is an extremely low probability that coronary disease of any significance might be found. However, it is still standard of care for an invasive test to be performed. With CTCA, there is now another way in which patients who have a low probability of having coronary disease can be evaluated prior to cardiac surgery. This has been shown in cases of atrial myxoma removal as well as valvular surgery.11,12
Pulmonary Vein Ablation
In the electrophysiology (EP) arena, there is a great deal of interest in cardiac CT imaging. The treatment of cardiac arrhythmias with catheter-based ablation rather than medical management alone has met with great success. In particular, atrial fibrillation (AF) is now an arrhythmia that may be cured, rather than just suppressed with medication. The discovery that a large percentage of AF can be traced back to its source in the pulmonary veins has allowed neutralization of these rhythms by electrically isolating the veins from the rest of the atrium.
The difficulty in this type of procedure (as in many arrhythmia treatments) has been in visualizing the heart during the procedure. For AF ablations, the variability of the the location, number, and size of the pulmonary veins has been difficult to ascertain. Traditional echocardiography and fluoroscopy has not been as successful in allowing full disclosure of the true anatomy of the left atrium, including such things as anomalous or accessory pulmonary veins. Utilizing CT imaging, the left atrium can be visualized by the physician, and the images can now be incorporated into 3-D mapping systems.13–16 Cardiac CT imaging may also be useful in other EP procedures such as left-sided pacemaker lead placement.17
Conclusion
CTCA and cardiac CT is a promising new technology that is already having an impact on the way in which coronary disease is discovered and followed. This modality is exciting in the way it might reduce the risk of cardiac evaluations. Although the focus so far has been on the ability of CTCA to replace invasive cardiac catheterization in selected patients, it also shows promise in other areas of cardiac evaluation and in situations other than those patients with chest pain or equivocal stress tests.