Cardiac Ultrasonography

Two-dimensional echocardiography has become an invaluable tool in the evaluation of cardiac anatomy and function. Echocardiography is capable of evaluating myocardial wall motion, valve function, the great vessels, and pericardial fluid collections. The examination of these structures can be technically demanding, requiring specialized transducers and specialized training. Applications of bedside ultrasound by emergency physicians is therefore limited to critically time-dependent diagnoses that can be recognized by emergency physicians with a modicum of training and the standard ultrasound equipment available in the ED. The evaluation of cardiac trauma, pulseless electrical activity (PEA), and pericardial tamponade are such applications. The sonographic findings of interest in such settings are pericardial fluid collections and myocardial wall activity.

The use of ultrasound in the evaluation of potential cardiac trauma is discussed earlier, in the section on the FAST examination. Both blunt and penetrating trauma can result in hemopericardium, which can be rapidly diagnosed with ultrasound ( Fig.,295:7). Electromechanical dissociation is a cause of PEA that has an extremely poor prognosis. An echocardiogram demonstrating a flaccid, inactive heart in a patient with PEA suggests very little chance of survival. In contrast, a hyperdynamic heart with small right-heart dimensions suggests hypovolemia, a readily treatable condition. Cardiac tamponade can be very difficult to diagnose at the bedside without the assistance of ultrasound. Echocardiography provides a means of rapidly determining whether a pericardial fluid collection is present, without which there can be no tamponade.

A number of windows used to sonographically evaluate the cardiac structures have been described. The subcostal view is the most useful to emergency physicians. This view can be obtained while other procedures are being performed with a 3.5-MHz transducer. Other views are best obtained using specialized transducer heads with small footprints (the surface area of the portion of the transducer that comes into contact with the patient) in order to image between the ribs. The subcostal view, obtained by placing the transducer in the area of the xiphoid process aiming toward the left shoulder, provides visualization of all four cardiac chambers. As always, structures that are closest to the transducer appear at the top of the image monitor; the liver, therefore, appears uppermost on the monitor, with the right atrium adjacent to it. The left atrium and ventricle appear closest to the bottom of the screen.

Other views used to evaluate cardiac structure include the left parasternal short-axis (LPSA), the left parasternal long-axis (LPLA), and the apical views. The parasternal window is obtained by placing the transducer between the second and fourth intercostal spaces adjacent to the sternum. In the LPLA, the ultrasound beam is directed in a plane parallel to a line drawn from the right shoulder to the left hip. This view images the aortic valve, proximal ascending aorta, and left ventricle well. The short-axis view (LPSA) is obtained by rotating the transducer 90°, so that the beam is parallel to a line drawn from the left shoulder to the right hip. Here the left ventricle appears as a round, thick-walled chamber, and the right ventricle appears more anteriorly in a crescent shape. The LPSA view is best used to image the mitral valve, papillary muscles, and aortic valve. The apical view is obtained by placing the transducer over the point of maximal cardiac impulse on the precordium (the apex) with the beam directed to the right shoulder. This view allows for the assessment of chamber size and the identification of aneurysms and intracardiac masses. The emergency physician can generally obtain the information necessary for the bedside indications mentioned above using only the subcostal view. The other views described may provide added information if the examiner has the appropriate equipment and training.

SONOGRAPHIC CONSIDERATIONS The echocardiographic evaluation of cardiac tamponade requires little training and can be rapidly performed at the bedside. The subcostal view should visualize any pericardial fluid collection large enough to result in tamponade. The pericardium is a dense, fibrous, echogenic sac that surrounds the heart. The pericardial space contains less than 50 mL of pericardial fluid under normal circumstances. Pericardial effusions appear as echo-free areas within the pericardial space. Small pericardial effusions (<100 mL) usually occupy a dependent position in the pericardial sac, while large effusions (>300 mL) are present both anteriorly and posteriorly. Whether a pericardial fluid collection affects cardiac function depends on such variables as the amount of fluid present, the rate of formation, and the underlying condition of the pericardium (diseased or not). Intrapericardial pressure rises abruptly after 80 to 200 mL of fluid has collected rapidly in the previously normal pericardial space. Early sonographic signs of increased intrapericardial pressure include right atrial and right ventricular collapse in diastole, but these findings are not always appreciable during the ED sonographic examination.

The interpretation of an echocardiographic finding of pericardial fluid collection must incorporate the clinical status of the patient. While the absence of a pericardial fluid collection excludes the diagnosis of cardiac tamponade, the mere presence of a pericardial fluid collection in an unstable patient is not diagnostic of pericardial tamponade. When pericardiocentesis is deemed necessary, sonographic localization of the heart will assist in determining the best approach for the procedure.

The patient with PEA (i.e., electrical cardiac activity without a palpable pulse) can be suffering from a wide variety of conditions, some fatal, some easily treated. Ultrasound greatly assists in the rapid diagnosis and treatment of such patients. The sonographic examination is straightforward. Cardiopulmonary resuscitation should be stopped briefly during the ultrasound examination. Any of the standard cardiac windows can be utilized, but the subcostal view has the advantages of easy imaging with standard ultrasound transducers and lack of interference with other procedures. Once visualized, the ventricles and valves are examined for evidence of activity. A patient in true electromechanical dissociation has no demonstrable cardiac activity. Several treatable causes of PEA can be diagnosed with a sonogram and appropriate treatment instituted. The ventricles of a hypovolemic patient contract vigorously, while the right-heart dimensions are diminished. Cardiac tamponade, another treatable cause of PEA, can also be easily diagnosed with echocardiography, as discussed above.

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