Cardiac Ultrasound

Anatomy and Physiology

The heart sits obliquely behind and slightly to the left of the sternum, with the atria more superiorly positioned in the direction of the right shoulder, and the ventricles lie anterior and inferior, ending approximately at the left nipple. The right ventricle is the heart's most anterior chamber, typically at the fourth intercostal space. The right atrium sends blood to the right ventricle in a right-to-left and superior-to-inferior direction via the tricuspid valve. The left atrium simultaneously sends blood to the left ventricle in parallel via the mitral valve. The ascending aorta takes off anteriorly from the left ventricle, heading behind the sternum to the right before it courses superiorly and to the patient’s left.

Indications

Following are some important indications of doing a cardiac ultrasound:

1. Chest pain

• Left ventricle wall motion abnormalities, or failure of certain parts of the left ventricle wall to contract, suggest acute coronary syndrome in the appropriate clinical setting.
• Right ventricle dilation (the right ventricle as large or larger than the left ventricle) suggests right heart strain. If there is associated right ventricle free wall thickness greater than 5 mm or tricuspid regurgitation with velocities higher than 4 m/s, this supports chronic right heart strain. However, a right ventricle or right atrial thrombus (the echogenic mass that has movement independent of the ventricle/atrium) and or a McConnell sign (right ventricle hypokinesis with apical sparing) in the setting of chest pain or shortness of breath suggests a pulmonary embolism.
• Visualization of an intimal flap in the ascending aorta (highly specific) or dilation of the aortic outflow tract greater than 4 cm (leading edge to leading edge) with or without a pericardial effusion suggests a Stanford type A aortic dissection.[1][2][3]

2. Shortness of breath

• A pericardial effusion explains undifferentiated dyspnea in up to 13% of cases.[4]
• A depressed left ventricle ejection fraction of less than 50% (less than 30% fractional shortening or an E-point septal separation of greater than 7 mm) supports congestive heart failure or myocarditis.[5]
• Acute coronary syndrome
• Pulmonary embolism
• Endocarditis[6]
• Valvular heart disease[7][8][7]

3. Hypotension

• Massive or submassive pulmonary embolism[9]
• Cardiogenic shock
• Pericardial effusion with tamponade physiology
• Aortic dissection
• Hyperdynamic squeeze may suggest sepsis or hemorrhagic shock

4. Penetrating trauma or significant blunt trauma to the chest

• Pericardial effusion with or without tamponade physiology
• Left ventricular wall motion abnormalities may be appreciated in the setting of a cardiac contusion[10][11]

5. Cardiac arrest

• Organized cardiac activity seen on ultrasound following PEA arrest is associated with survival compared to disorganized activity.
• Detection of reversible causes such as pericardial effusion or pulmonary embolism can be identified.
• Cardiac ultrasound may offer guidance/feedback to medical providers delivering chest compressions regarding the quality of compressions. Transesophageal echocardiography is preferred for this indication as it can better visualize the heart during cardiopulmonary resuscitation (CPR) to see if the ventricles are adequately compressed.[12]

Contraindications

Parasternal cardiac ultrasound should not be done during CPR, but other views are appropriate if feasible. Parasternal views can occur during the pulse check, and towels should be nearby to clean off the gel immediately before CPR resumes. Care should be taken not to scan over a wound or incision to avoid contamination and infection.[13]

Equipment

Cardiac ultrasound should be performed with a low-frequency probe that has a small footprint that can fit between the ribs (a phased array is ideal), using a cardiac setting. Ultrasound is defined as a frequency greater than 20,000 Hertz (Hz).

Personnel

A trained provider can perform a cardiac ultrasound.[14] Upon residency graduation, Emergency physicians are required to correctly perform and interpret a minimum of 25 to 50 cardiac ultrasound exams. Many nurse practitioners and physician assistants are also skilled in this examination.

Preparation

The patient should be lying supine on a stretcher with the head of the bed approximately 30 degrees upright. For males, the chest should be completely exposed. For females, the hospital gown can be gathered at the breast level, and towels can be tucked around the gown edges to keep it dry from the ultrasound gel. Alternatively, a towel can be draped over this area if the patient is not wearing a gown. For dominant right-hand operators, the ultrasound machine should be positioned at the patient’s anatomic right, plugged in (if applicable), and turned on. The lights should be dimmed if possible.

Technique or Treatment

There are historically two conventions used when performing a cardiac ultrasound.[15][1] The first was established by cardiologists in the 1960s, where the operator stands at the patient’s anatomic left, the indicator on the probe is directed either to the patient’s anatomic left or anatomic right, depending on the view, and the indicator on the screen is on the right side. When emergency physicians adopted ultrasound, it was for multiple applications from head to toe and included procedural guidance. Having a convention where the indicator on the screen is always to the left side, and the indicator on the probe is to the operator’s left (and up to 90 degrees clockwise from that position) keeps orientation consistent for the operator to perform and interpret, and is more practical for procedures. For this reason, the technique will be described with the latter convention. A low-frequency phased array probe is best for a cardiac ultrasound. There are four key views:

Subxiphoid

The phased array probe is placed inferior to the xiphoid process, indenting the skin one to two centimeters, with the indicator directed to the patient’s anatomical right and the footprint of the probe directed up towards the patient’s heart (handle of the probe parallel with the patient’s skin). Structures closest to the probe appear at the top of the screen, and structures further away appear towards the bottom of the screen, so in this view, the top of the screen is both anterior and inferior, and the bottom of the screen is more posterior and superior. For this reason, the left lobe of the liver is often visualized at the top of the screen, followed by the right atrium and right ventricle, and finally, the left atrium and left ventricle near the bottom of the screen. Adjust the depth to see the entire heart and interrogate for a pericardial effusion. Pericardial effusions are anechoic (black), and usually, encircle the heart when clinically significant. This view is often easy to obtain in thin patients, as well as patients with chronic obstructive pulmonary disease. It is more difficult in obese patients. Rotating the probe clockwise 90 degrees (so the indicator is pointing cephalad) allows visualization of the inferior vena cava emptying into the right atrium.

Parasternal Long Axis

The phased array probe is placed just to the anatomic left of the sternum at the four intercostal spaces, with the probe's handle perpendicular to the chest wall, an indicator of the patient’s right shoulder. In this view, the footprint of the probe is aligned with the long axis of the patient’s heart. The chamber closest to the footprint at the top of the screen (anterior aspect of the patient) is the right ventricle. The additional chambers include the left atrium, left ventricle, and aortic outflow tract. Posterior to the left atrium and left ventricle at the bottom of the screen is a cross-section of the descending thoracic aorta. This view helps confirm the presence of a pericardial effusion, evaluating the aortic outflow tract and assessing the left ventricle function. In this view, a circumferential pericardial effusion should track anterior to the descending thoracic aorta (whereas a left pleural effusion will track posterior to the descending thoracic aorta). A dilated aortic outflow tract greater than 4 cm measured from leading edge to leading edge, especially in pericardial effusion, may indicate a type A aortic dissection in the appropriate clinical context. The left ventricular function can be assessed by how well the left ventricle myocardium comes together, where a fractional shortening [(left ventricle end-diastolic diameter – left ventricle end-systolic diameter)/left ventricle end-diastolic diameter] of about 30% to 45% correlates with a good squeeze. An indirect way to assess left ventricle function is via the E-point septal separation (EPSS), the smallest distance between the anterior leaflet of the mitral valve and the interventricular septum during early diastole. The EPSS should be less than 7 mm, and a distance of 7 mm or greater indicates depressed left ventricular function. Diastolic dysfunction may falsely lower this distance, and mitral valve dysfunction may falsely increase this distance.[16]

Parasternal Short Axis

The phased array probe is rotated 90 degrees counterclockwise from the parasternal long axis position so that the indicator is pointing towards the right hip. Otherwise, the probe's position is unchanged (still approximately the 4 intercostal space, just lateral to the sternum, with the probe's handle at a perpendicular angle to the chest). The footprint of the probe is aligned with the short axis of the patient’s heart in this view, where most of the screen displays the circular left ventricle, and anteriorly (top left of the screen) is the right ventricle, which should appear as a crescent shape. This view is ideal for assessing the left ventricle function. Still, it can also confirm the presence of pericardial effusion or signs of right heart strain, such as paradoxical septal motion and right ventricle enlargement. The overall function of the left ventricle is best assessed concentrically at the level of the papillary muscles. In this view, the operator can also look for wall motion abnormalities and correlate them with the echocardiogram. The left ventricle walls in a parasternal short axis are septal, anterior, lateral, posterior, and inferior, in clockwise order.

Apical 4-Chamber

The phased array probe is positioned at the apex of the heart, usually at approximately 5 O’clock on a male patient’s left nipple or laterally beneath the left breast at the apex of the heart on a female. Palpating the pulse of maximal impulse (PMI) and placing the probe there usually result in a perfect apical four-chamber view. Similar to the subxiphoid view, the footprint of the probe should be directed up towards the patient’s heart (handle of the probe parallel with the patient’s skin), but in this view, the footprint is facing the right shoulder, and the indicator is pointing towards the right superior iliac crest. In this view, the ventricles are closest to the probe at the top of the screen, the atria are furthest away from the probe at the bottom of the screen, with the right ventricle and atrium on the left of the screen and the left ventricle and atrium on the right of the screen. This view is ideal for comparison of the right and left ventricles to look for evidence of right heart strain and confirm the presence of pericardial effusion. The normal right ventricle to left ventricle ratio is 0.6 to 1. Still, to keep the assessment of the right ventricle specific for right heart strain as well as rapid and easy to perform, the binary assessment is whether the right ventricle is smaller than the left ventricle (no significant right heart strain) or if it is as big or bigger than the left ventricle (evidence of right heart strain). This is particularly helpful in acute onset chest pain or shortness of breath when a pulmonary embolism is in the differential diagnosis.[17] A patient with no significant past medical history who has a large right ventricle on cardiac ultrasound has a pulmonary embolism until proven otherwise. A McConnell sign is even more specific for pulmonary embolism, where the right ventricle is large and hypokinetic with apical sparing (the apex is not hypokinetic like the rest of the right ventricle). These ultrasound findings for an otherwise healthy patient younger than 65 may benefit from thrombolytics.[3]

Complications

Cardiac ultrasound, like most diagnostic ultrasound applications, is associated with little, if any, risk. However, there may be some discomfort when acquiring certain views, such as the apical four-chamber view, with the pressure of the probe pressing against the ribs.

Clinical Significance

Cardiac ultrasound is a non-invasive, rapid, inexpensive application that expedites the diagnosis and management of imminently life-threatening diseases, including pericardial tamponade, acute coronary syndrome, cardiomyopathy, pulmonary embolism, and Stanford type A aortic dissection. Cardiac ultrasound can also differentiate shock states and guide resuscitative measures.[18]

Enhancing Healthcare Team Outcomes

The interprofessional team should perform cardiac ultrasound or echocardiography as needed to evaluate patients. This noninvasive diagnostic modality can quickly provide detailed hemodynamic information at the patient's bedside. Nurses should be prepared to assist the provider in point-of-care ultrasound applications from head to toe.