Image Acquisition

The emergency department environment often makes the echocardiogram more challenging than in the controlled environment of the echocardiography lab. Transthoracic echocardiographic (TTE) examination of patients with chest pain should focus on the evaluation of biventricular function and the presence of regional wall motion abnormalities (lack of normal wall thickening, particularly if in a vascular territory). It may also include assessment of myocardial blood flow using echocardiographic contrast (if available). The study should also serve to screen for nonischemic causes of cardiac chest pain such as aortic stenosis, hypertrophic cardiomyopathy, pericardial effusion, and aortic dissection (although sensitivity for the last diagnosis is limited, and a negative TTE is not sufficient to rule it out).

The standard two-dimensional (2D)-TTE views necessary for assessment of wall thickening are the parasternal long- and short-axis views and the apical four-chamber, two-chamber and three-chamber views (Fig. 13-1 demonstrates the apical two-chamber and short-axis views). If adequate image quality cannot be achieved from the parasternal or apical views, subcostal views can be extremely helpful. Off-axis or foreshortened views make the interpretation of regional wall motion abnormalities difficult and increase the likelihood of error. Images should be analyzed in accordance with the American Society of Echocardiography guidelines.⁹

Figure 13-1 Still frames from the echocardiogram of a patient with acute chest pain, showing akinesis (arrows) of the basal inferior and inferolateral segments, in the apical two-chamber view (A, diastole; B, systole) and the basal parasternal short axis (C, diastole; D, systole).

The use of echocardiographic contrast agents can be invaluable in defining endocardial borders for wall motion analysis (see Chapter 3). Commercially available intravascular echocardiographic contrast agents contain gas-filled microbubbles, which are small enough to pass through the pulmonary circulation, resulting in opacification of the LV. This results in enhancement of endocardial borders (Fig. 13-2).

Figure 13-2 Improvement in LV endocardial border delineation, particularly at the apex, with the use of intravenous echocardiographic contrast and second harmonic imaging (A, without contrast; B, with contrast).

A well-trained, experienced sonographer or echocardiographer is essential in the emergency setting. The sonographer should acquire necessary images in an efficient manner with attention to proper patient positioning, transducer selection, and technical settings. The technical settings may vary depending on the patient characteristics, such as the use of lower-frequency imaging in obese patients. Second harmonic imaging increases the signal-to-noise ratio, resulting in clearer imaging. The use of echocardiographic contrast requires changes in technical settings for endocardial definition versus myocardial perfusion, particularly in mechanical index.

The advancement of echocardiographic technology has substantially reduced the number of patients with inadequate echocardiographic images. In an early study by Horowitz and colleagues on the use of echocardiography in patients with acute chest pain, adequate images were obtained in 81% of patients.¹⁰ A recent study evaluated patients with technically difficult imaging and found that the addition of contrast agent improved the percentage of myocardial segments visualized from 68% to 99%. This improvement had a direct impact on patient care.¹¹

Stress Echocardiography

Echocardiographic imaging in conjunction with exercise or pharmacologic stress to assess for ischemia is safe and effective in detecting inducible wall-motion abnormalities. Exercise stress can be performed using a standard treadmill protocol (i.e., Bruce) or a supine bicycle protocol. The advantage of bicycle protocols is the ability to image wall motion throughout exercise, as the patient exercises on an echocardiographic table that can be tilted into a left lateral position. The supine bicycle protocol increases the energy expenditure, typically in 30-watt increments every 3 minutes. This allows for image comparison at low, medium, and high workload. Additional Doppler information including pulmonary artery systolic pressure estimate, transaortic stroke volume, and valvular gradients can be obtained.

Treadmill studies image patients at baseline and just after peak exercise only. For maximum sensitivity, it is essential that the echocardiographic imaging begin immediately after exercise terminates, before significant heart rate decrease. This stress modality is primarily for assessment of global and regional systolic function (see Chapter 15).

For patients unable to exercise adequately, pharmacologic stress testing can be performed (see Chapter 16). Dobutamine is the agent most studied and, therefore, most commonly used in North America. Dobutamine stress echocardiography (DSE) has demonstrated excellent negative predictive value for the presence of obstructive CAD (96% at 6 months).¹² Baseline images are obtained, followed by the infusion of dobutamine at incremental levels (usually 5, 10, 20, 30, 40 mcg/kg/min) of 3 minutes’ duration. The test is terminated when the patient’s heart rate reaches 85% of maximum predicted heart rate, or there is evidence of myocardial ischemia, hypotension, significant arrhythmia, significant LV outflow tract obstruction, or drug-related side effects. Patients who do not reach target heart rate despite 40 mcg/kg/min of dobutamine can perform handgrip maneuvers or be given intravenous atropine in doses of 0.1 to 0.25 mg up to a total of 1 mg.

Endocardial definition is essential for reliable wall motion assessment. Images during stress decrease in quality because of the cardiac movement and hyperventilation. The use of second harmonic imaging has increased the sensitivity for the diagnosis of CAD from 64% to 92% in patients with poor image quality. Specificity is unchanged with second harmonic imaging.¹³ The use of echocardiographic contrast agents has also increased the sensitivity of stress echocardiography for the diagnosis of CAD. Rainbird and colleagues¹⁴ analyzed 300 consecutive patients undergoing DSE. The percentage of wall segments visualized increased from 94.4% to 99.8% with the use of LV opacification during peak exercise (P less than 0.01). There was no decrease in segment visualization at peak exercise, as was the case if an echocardiographic contrast agent was not used.

This technique has been safely used when performed by specially trained nurses and sonographers, with images electronically transferred to the cardiologist for interpretation.^15,16

Image Interpretation

Accurate interpretation of regional wall motion also requires an experienced echocardiographer. Wall thickening and systolic endocardial motion are evaluated for each LV segment, leading to its classification as hyperkinetic, normal, hypokinetic, akinetic, or dyskinetic.

The American Society of Echocardiography recommends a 16-segment model of regional wall motion evaluation (Fig. 13-3).⁹ This model is consistent with the models used in nuclear and magnetic resonance imaging. Each segment is assigned a score based on visual assessment of contractility: normal = 1, hypokinesis = 2, akinesis = 3, dyskinesis = 4, and aneurysmal = 5. The wall motion score index, a semiquantitative measure of regional wall motion abnormality, is calculated by averaging wall motion scores for visualized segments. An LV with normal systolic function has a wall motion score index of 1, whereas a dysfunctional ventricle will have an increasing wall motion score index proportional to the severity. This score correlates with myocardial infarct size on pathologic studies¹⁷ and to perfusion defect size on single photon emission computed tomographic (SPECT) imaging.¹⁸ A similar 14-segment wall motion index predicts long-term survival in patients with chest pain.¹⁹

Figure 13-3 A 16-segment model for evaluation of regional wall motion as recommended by the American Society of Echocardiography. A, Anterior; AL, anterolateral; AS, anteroseptal; I, inferior; IL, inferolateral; IS, inferoseptal; PL, posterolateral; PS, posteroseptal.

(Adapted from Schiller NB, Shah PM, Crawford M, et al: Recommendations for quantification of the left ventricle by two-dimensional echocardiography. J Am Soc Echocardiogr 2:358-367, 1989.)

Evaluation of stress-induced regional wall motion abnormalities is based on detailed comparison of rest and stress images, which is facilitated by simultaneous, synchronized display of images.

Recent refinements in tissue Doppler/speckle tracking imaging and the measurement of myocardial velocity allow for quantitative analysis of regional function.^20,21 Although not routine at present, these quantitative measures have been shown to distinguish between patients with non–ST-elevation myocardial infarction and unstable angina or noncoronary chest pain.²²

Myocardial Contrast Echocardiography

MCE allows for the assessment of myocardial perfusion with the use of intravenous echocardiographic contrast agents, as well as assessment of wall thickening. Myocardial perfusion is assessed by the injection of microbubbles at the bedside, in real time, with rapid image acquisition. These inert, intravascular microbubbles are delivered to the capillary bed in territories with normal perfusion. Areas that are hypoperfused lack the echocardiographic enhancement generated by the contrast agent (perfusion defect). The difference in areas with and without adequate perfusion is highlighted by destruction of the bubbles at timely intervals, allowing for image acquisition as the areas of normal perfusion increase in signal intensity faster compared to areas with hypoperfusion. The destruction of bubbles occurs with the use of high-intensity pulses (mechanical index 1.0) synchronized to end-systole, whereas imaging occurs at a mechanical index of approximately 0.5 (Fig. 13-4).

Figure 13-4 An example of a normal and a positive stress echocardiogram using intravenous echocardiographic contrast material for assessment of myocardial perfusion.

A, Apical four-chamber view with normal myocardial perfusion septum and basal lateral wall (myocardium appearing dark); B, apical four-chamber view with an inducible perfusion defect of the septum and apex at stress (myocardium appears dark when compared to the lateral wall).

Triage Of Patients With Chest Pain

Ischemia-induced regional wall motion abnormalities develop quickly once myocardial blood flow falls below a critical level. Rest echocardiography has therefore been used in the assessment of patients with symptoms suggestive of ACS. The diagnostic accuracy has, however, varied vastly between studies, likely because of the following factors: timing in relation to symptoms, infarct size, and technical variation. A recent study of emergency department patients with symptoms suggestive of ACS reported TTE to have a negative predictive value of 97%; however, the positive predictive valve was only 24%.²³ Although normal systolic function at rest is reassuring, it is insufficient to exclude the diagnosis of ACS (Table 13-1). The evaluation of wall thickening by TTE is deemed appropriate in patients with suspected ACS but nondiagnostic initial testing.²⁴

TABLE 13-1 Chest Pain in the Emergency Department