Noninvasive Cardiac Diagnostic Procedures
Noninvasive diagnostic testing for patients with CHD includes electrocardiography, serum biomarker measurements, echocardiography, various types of stress testing, and imaging studies. Assessment of cardiac risk using various measures (see Chapter 4 in this volume) will identify those patients most likely to benefit from testing.
The ECG may appear normal between episodes of ischemia in patients with angina. During angina, the ST segments often become elevated or depressed by up to 5 mm. T waves may be inverted, they may become tall and peaked, or inverted T waves may normalize. These ECG findings, when associated with characteristic anginal pain, are virtually diagnostic of IHD. However, absence of ECG changes does not definitively exclude myocardial ischemia.
During myocardial infarction, QT-interval prolongation and peaked T waves may appear. The ST segments may be depressed or elevated (see Fig 5-2). ST-segment elevation may persist for several days to weeks before returning to normal. T-wave inversion appears in the leads corresponding to the site of the infarct. Q waves or a reduction in the QRS amplitude appears with the onset of myocardial necrosis. Q waves are typically absent in a subendocardial (nontransmural) infarction. Tachycardia and ventricular arrhythmias are most common within the first few hours after the onset of infarction. Bradyarrhythmias, such as heart block, are more common with inferior infarction; ventricular tachycardia and fibrillation are more common with anteroseptal infarction.
Serum biomarker testing
Cardiac enzymes are released into the bloodstream when myocardial necrosis occurs and, therefore, are valuable in differentiating MI from unstable angina and noncardiac causes of chest pain. With the advent of assays for cardiac-specific troponins, serum biomarker testing has also proved useful in identifying patients with ACS who are at greatest risk for adverse outcomes.
Cardiac-specific troponins are accepted as the most sensitive and specific biochemical cardiac markers in ACS. Cardiac isoforms of troponins (troponins T and I) are important regulatory elements in myocardial cells and, unlike creatine kinase MB (CK-MB), are not normally present in the serum of healthy individuals. Moreover, unlike CK-MB levels, troponin levels are not elevated in patients with skeletal muscle injury. Troponins T and I have been shown to be more cardiac-specific and cardiac-sensitive than CK-MB, allowing for more accurate diagnosis of cardiac injury. Troponin levels remain elevated from 3 hours to 14 days after MI. Mildly elevated troponin levels may be found in patients with NSTEMI who otherwise would be considered to have unstable angina. Troponin levels are elevated after MI, but they may also be elevated in patients with myocarditis, stress cardiomyopathy, and chronic kidney disease.
In addition to being diagnostically valuable, troponin levels provide prognostic information. Patients with an ACS who present with elevated troponin levels have an increased risk of CHF, cardiogenic shock, death, recurrent nonfatal infarction, and need for revascularization with PCI or coronary artery bypass grafting (CABG, discussed later in this chapter). Finally, a quantitative relationship has been demonstrated between the peak amount of troponin measured and the risk of death in patients who present with ACS. Patients who are at greatest risk for adverse outcomes can be identified in the emergency department, allowing for more appropriate medical decisions and therapeutic triage.
Serum myoglobin is the first marker to rise following myocardial damage, and levels can be elevated between 1 and 20 hours after infarction. Although myoglobin might appear to be ideal for early detection of MI, its performance is not consistent and its specificity for cardiac events is poor. The cardiac-specific troponins remain the optimal biomarker test in the setting of ACS.
Echocardiography employs 1- and 2-dimensional ultrasound and color flow Doppler techniques to image the ventricles and atria, the heart valves, left ventricular contraction and wall-motion abnormalities, left ventricular ejection fraction, and the pericardium. Patients with IHD, particularly following infarction, commonly have regional wall-motion abnormalities that correspond to the areas of myocardial injury. Other, less frequent complications of infarction, such as mitral regurgitation from papillary muscle injury, ventricular septal defect, ventricular aneurysm, ventricular thrombus, and pericardial effusion, can also be detected with echocardiography. Color flow Doppler imaging provides information on the flow of blood across abnormal valves, pressure differences within the chambers, intracardiac shunts, and cardiac output. However, cardiac biomarkers are far more sensitive and specific than echocardiography in detecting cardiac injury.
Stress echocardiography (exercise echocardiography) is useful for imaging cardiac-valve and wall-motion abnormalities and ventricular dysfunction induced by ischemia during exercise or after pharmacologic challenge. Predischarge exercise stress echocardiography provides useful prognostic information following acute MI.
Exercise stress testing
Patients with angina may have normal findings on clinical examination, electrocardiography, and echocardiography between episodes of ischemia. Standardized exercise tests have been developed to induce myocardial ischemia under controlled conditions. The ECG, heart rate, blood pressure, and general physical status of the patient are monitored during the procedure. The endpoint in angina patients is a symptom or sign of cardiac ischemia, such as chest pain, dyspnea, ST-segment depression, arrhythmia, or hypotension. A modified exercise stress test can also be performed in patients with a recent MI to help determine functional status and prognosis. Stress testing is useful both in establishing the diagnosis of ischemic heart disease and in assessing its severity.
Radionuclide scintigraphy and scans
Radionuclide techniques can be used to increase the sensitivity of exercise testing. Left ventricular dysfunction can result from necrotic tissue, myocardial hibernation after injury, or myocardial stunning. Approximately 20%–40% of patients with left ventricular dysfunction on echo or stress testing still have viable myocardial tissue, which may improve with reperfusion. Several agents are available for injection during testing, including thallium-201, technetium-99m (Tc99m) sestamibi, and technetium-99m tetrofosmin.
Thallium accumulates in healthy myocardium and reveals a perfusion defect in areas of myocardial ischemia. Thallium scans have a high sensitivity and specificity for CHD. Reversible thallium or Tc99m sestamibi defects are those that are present during exercise but resolve during rest. This correlates with myocardial ischemia. In contrast, a fixed thallium or Tc99m defect is present during both exercise and rest and represents a region of prior infarction or nonviable tissue. For patients unable to exercise vigorously enough to reach the required heart rates during the exercise stress test, a thallium scan or echocardiogram in conjunction with a pharmacologic stress test may provide information similar to that of an exercise examination. Tomographic imaging of myocardial perfusion is possible with thallium-201 or Tc99m via a technique called single-photon emission computed tomography (SPECT), which provides better imaging of infarcts, enhanced detection of multivessel disease, and fewer artifacts.
Other imaging technologies that may add clinically useful information include the following:
Positron emission tomography, which differentiates metabolically active myocardium from scar tissue.
Coronary CT angiography, which is useful in evaluating occlusive vascular disease and ruling out atherosclerotic disease.
Electron beam CT; this quantifies coronary artery calcification, which correlates with atherosclerosis and is highly sensitive but not specific. It may be an alternative to angiography in some patients.
Cardiac MRI, which provides excellent imaging, and perfusion testing with gadolinium. MRI may be contraindicated in some patients with ICDs or pacemakers, but it can be safely used in the presence of coronary stents. Cardiac CT and MRI are also useful in assessing congenital or acquired coronary abnormalities.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.