Transthoracic Echocardiography

Clinical Correlation

Point-of-care cardiac ultrasound is indicated for a wide variety of clinical presentations, including cardiac trauma, cardiac arrest, tachycardia, hypotension, shortness of breath, chest pain, and syncope. It requires a minimum of two views of the heart and may also require visualization of the inferior vena cava (IVC) and pulmonary ultrasound to assess for intravascular volume status, associated lung abnormalities, and correlation with the cardiac ultrasound findings. Cardiac sonography is truly a time-sensitive assessment of the undifferentiated symptomatic patient. Some professional societies have published guidelines outlining basic and advanced/expert point-of-care cardiac sonography skills. The basic evaluation usually includes an assessment for pericardial effusion, global cardiac function, relative chamber size, and patient volume status. Other pathologic processes may be demonstrated with more advanced echocardiography skills, but comprehensive echocardiography with cardiology consultation is often recommended, as diagnoses such as wall motion abnormalities, valvular disease, endocarditis, and aortic disease require additional training in comprehensive echocardiography techniques.

Normal Anatomy

Deoxygenated blood flows from the right atrium (RA) across the tricuspid valve to the right ventricle (RV) and then to the lungs via the pulmonary artery. Oxygenated blood returns to the left atrium (LA) via the pulmonary veins, flowing across the mitral valve (MV) into the left ventricle (LV), and finally exiting the heart via the aortic outflow tract through the aortic valve. Parts of the ascending and descending aorta can be seen in basic echocardiography, whereas the aortic arch can be seen with advanced cardiac ultrasound evaluation ( Fig. 9.1 ).

The IVC is a thin-walled, compliant vessel that delivers deoxygenated blood from the lower body back to the heart. The IVC runs inferior to superior in the retroperitoneal space to the right of the spine and crosses the diaphragm to empty into the RA.

Imaging

Bedside Echocardiography

Four cardiac views are used in point-of-care basic cardiac ultrasound: subxiphoid (SX), parasternal long axis (PSL), parasternal short axis (PSS), and apical four-chamber (AP4). All views are obtained using the low-frequency (3–5 MHz) phased array transducer. Patients are typically positioned supine; however, the PSL, PSS, and AP4 views are often more easily obtained with the patient in the left lateral decubitus position (see Fig. 9.1 ). The transducer’s indicator icon may be oriented on the left or right side of the screen, depending on the conventions followed by the specialty/institution. The following basic cardiac ultrasound descriptions are written with the screen’s indicator dot on the left side of the screen. (If the screen indicator on the right side is preferred, rotate the transducer 180 degrees from the positions described; Fig. 9.2 .)

Subxiphoid

Place the transducer on the abdomen just inferior to the xiphoid process in plane with the abdominal wall. The transducer indicator faces the patient’s right, and the transducer footprint should be slightly directed toward the patient’s left shoulder. Visualize the heart deep to the xiphoid process by applying pressure while flattening the transducer along the patient’s abdomen. The liver is used as an acoustic window. Adjust the screen depth to include the posterior pericardium. This view can be improved by asking the patient to take and hold a deep breath, as inspiration will lower the diaphragm, causing the heart to move closer to the transducer ( Fig. 9.3 ). This view is traditionally used to evaluate for pericardial effusion; however, other pathologies can also be seen.

Parasternal long axis

Place the transducer perpendicular to the chest wall at the left third or fourth intercostal space adjacent to the sternum, with the transducer indicator pointing toward the patient’s left hip. This window is used to visualize the RV, LV, LA, aortic outflow tract, and descending aorta. Adjust the screen depth to include the descending aorta in the inferior aspect of the image ( Fig. 9.4 ). This view is valuable in assessing left ventricular systolic function and to differentiate pericardial effusion from a left-sided pleural effusion.

Parasternal short axis

Rotate the transducer 90 degrees from the PSL view. The indicator should be pointing toward the patient’s right hip. The LV can be visualized in cross-section from the level of the MV (“fishmouth” view), to the chordae tendineae and papillary muscles view, and to the LV’s apex by sweeping the transducer from its base to its apex ( Fig. 9.5 ). This view allows for assessment of global LV systolic function (LVSF) when visualizing the LV at the level of the papillary muscles and for RV strain.

Apical four-chamber

Place the transducer at the apex of the heart overlying the point of maximal impulse (PMI), typically located in the midclavicular line at approximately the fifth intercostal space. Direct the indicator toward the patient’s right, and angle the transducer toward the patient’s right shoulder while flattening it in plane with the long axis of the left ventricle ( Fig. 9.6 ). This view is useful to compare ventricular chamber sizes.

Inferior Vena Cava Ultrasound

The IVC is thin-walled and compliant and changes diameter according to patient volume status and with variations in intrathoracic pressure during the respiratory cycle. It also reflects changes when there are cardiac flow changes. Sonographic IVC evaluation involves assessment of IVC diameter, as well as collapsibility, during respiration. Position the patient supine and select the low-frequency (3–5 MHz) phased array transducer. Obtain an SX cardiac view, then rotate the transducer 90 degrees so that the indicator is directed caudally. With this technique, the IVC can be visualized in the longitudinal plane as it enters the RA. Once the IVC is identified, measure the diameter 2 to 4 cm from where it enters the RA in both expiration and inspiration. Respiratory variation can also be measured in M-mode. With M-mode, the cursor is placed across the IVC. The IVC size variation will be plotted along the time axis. Inspiration causes IVC collapse, as negative intrathoracic pressure draws blood into the thoracic cavity, increasing venous return to the heart ( Fig. 9.7 ). The longitudinal IVC can also be visualized when placing the probe in the right midaxillary line, with the indicator directed cephalad and the transducer angled posteriorly. This view of the IVC, however, has not been proven to correlate with intravascular volume status.

Pathology

Pericardial Effusion

A pericardial effusion appears as an anechoic stripe between the pericardium and the myocardium. Small effusions are seen in dependent areas (posterior and inferior), whereas large effusions may also be seen anteriorly ( Fig. 9.8 ). The epicardial fat pad must be distinguished from a pericardial effusion. Epicardial fat is typically seen anteriorly and appears echogenic. In comparison, a pericardial effusion is visualized posteriorly or inferiorly and appears anechoic. A pleural effusion may also appear as an anechoic area posterior to the heart and is often mistaken for a pericardial effusion. In the PSL view, pleural effusions can be distinguished from pericardial effusions by their location posterior to the descending aorta ( Fig. 9.9 ). Obtain images in at least two cardiac windows for a more accurate evaluation of a pericardial effusion.

Cardiac Tamponade

Cardiac tamponade is a clinical diagnosis involving pericardial fluid, hypotension, tachycardia, pulsus paradoxus, and distended neck veins. Sonographic signs of tamponade include pericardial fluid, delayed RV diastolic expansion, and RV diastolic collapse due to increased pericardial pressures on the RV free wall ( Fig. 9.10 ). A dilated IVC with reduced respiratory variation should also be seen in cardiac tamponade.