Electrocardiography

The resting ECG is the most frequently performed investigation in evaluating patients with cardiovascular disease (see Chapter 4). Electrocardiography is a highly versatile diagnostic test, providing information on a broad spectrum of clinical conditions, ranging from metabolic disturbances (e.g., hypo- and hyperkalemia) and pharmacologic toxicity to ischemic heart disease (e.g., acute myocardial infarction [MI], unstable angina), arrhythmia, and pericardial disease (see Chapter 4). With such versatility, this simple-to-perform test is cost effective.

In the investigation of suspected or known arrhythmias, Holter electrocardiographic monitoring augments the resting ECG by allowing correlation of patient symptoms to the rhythm disturbance and the subsequent monitoring of the patient’s response to treatment. This can be in the form of a continuous 24- to 72-hour monitor, a patient-activated event monitor worn for 1 to 4 weeks, or a subcutaneous Reveal device (up to 2 years). Continuous ST-segment monitoring also collects prognostic data on patients who have had a coronary event.

Exercise ECG is a relatively inexpensive investigation used in the diagnosis and management of coronary artery disease (CAD). However, with a sensitivity of approximately 67% and a specificity of 84% for the detection of significant CAD in an optimal setting (and much lower accuracy reported in other settings), the main value of exercise ECG lies in excluding CAD in patients who have a moderate or low pre-test probability of significant coronary stenoses. The risk of false-negative results in patients with a high pre-test probability of CAD is relatively high; these patients should be referred for a more sensitive test such as coronary angiography.

Biochemical Markers

Serum troponin T and I are highly sensitive and specific markers for myocardial injury that are widely accepted as the standard biomarkers for the diagnosis of MI. Elevated troponin levels predict mortality in acute coronary syndromes as well as other diseases, including heart failure, renal failure, and sepsis. In acute MI, serum troponin levels rise after 2 to 3 hours, become detectable in the bloodstream at 6 to 12 hours, and remain elevated for up to 14 days. Caution interpreting positive troponin results should be used, however, because of the wide range of nonischemic cardiac and noncardiac conditions that can cause elevated serum concentrations. These conditions are numerous and include tachyarrythmia, myocarditis, direct current cardioversion, renal failure, sepsis, pulmonary embolism, and stroke. The other main caution in interpretation is an understanding of the details of the test used locally; the many commercially available assays have different upper limits of normal.

Brain natriuretic peptide and its co-secreted N-terminal fragment are useful in the diagnosis of acute heart failure in an emergency department and management of chronic heart failure in a primary care setting. They may be useful to establish prognosis in heart failure, in that both markers are typically higher in patients with worse outcomes. Although they are highly sensitive and therefore have very few false-negative results, they unfortunately lack the specificity needed to exclude false-positive results and are often therefore used as a “rule out” test for heart failure. Serum levels are elevated in patients with renal failure, because both peptides are renally excreted. Results should be interpreted in the clinical context, and positive results should invariably be followed by functional imaging such as an echocardiogram to formally assess cardiac function.

Echocardiography

Echocardiography provides a versatile and cost-effective method for assessing cardiac anatomy and function (see Chapter 6). The greatest values of echocardiography are the capacity for simultaneous assessment of valvular, pericardial, myocardial, and extracardiac abnormalities. Because complex image processing is not needed, the results of the study are immediately available to the experienced echocardiographer. In addition, it is possible to perform echocardiography on critically ill patients who cannot be moved, or in other circumstances when a portable test is preferable. For these reasons, echocardiography is the preferred screening imaging test for further assessing suspected myocardial dysfunction. Moreover, the use of Doppler echocardiography (Doppler) to measure flow allows the measurement of peak velocity across valves, the mapping of regurgitant jets, the estimation of pulmonary artery pressures, and the detection of shunts (e.g., ventricular and atrial septal defects). The severity of valvular heart disease and its contribution to the clinical presentation can be determined immediately. For patients with chest pain, congestive heart failure, or arrhythmias, echocardiography provides a rapid means of determining underlying cardiovascular function.

Transesophageal echocardiography adds to the sensitivity of transthoracic echocardiography, because views of the heart are not impeded by artifact related to the lungs or chest wall (Figs. 3-1 and 3-2). In addition, transesophageal echocardiography allows visualization of structures that are usually not well seen by transthoracic echocardiography (e.g., the left atrial appendage). The development of transesophageal echocardiography has also been an important advance in the management of patients who are undergoing cardiothoracic surgery, providing information on left ventricular (LV) function and the success of valvular repair. In addition, transesophageal echocardiography may allow a more accurate determination of valvular dysfunction and assessment for bacterial endocarditis, intracardiac thromboses, or both.

Figure 3-1 Transesophageal echocardiography.

Figure 3-2 Transesophageal echocardiography: Positions.

In addition to its usefulness in assessing valvular heart disease, echocardiography provides information on regional wall motion abnormalities suggestive of myocardial ischemia or necrosis in patients with CAD. The addition of pharmacologic or exercise-induced stress to detect inducible ischemia provides increased sensitivity and specificity compared with ECG exercise testing (Fig. 3-3, upper panel). In 21 studies, the sensitivity of exercise stress echocardiography averaged 84% (range 71% to 97%) and the specificity averaged 86% (range 64% to 100%). The use of echocardiography can be limited by technical considerations, including an inability to obtain diagnostic images in some patients (an estimated 15%). Stress echocardiography is indicated for individuals who have an intermediate prior probability of CAD and for individuals with abnormal ECGs or who are prescribed medications that can cause ECG abnormalities with stress (such as digoxin). In either of these cases the predictive value of exercise ECG is substantially reduced, justifying the use of an imaging technique during stress.

Figure 3-3 Exercise and contrast echocardiography.

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