Cardiac ultrasound, or echocardiography, is a noninvasive diagnostic modality that can provide detailed hemodynamic information in a short period at the patient bedside. It was first adopted by cardiologists for diagnostic purposes in the 1960s, and later by emergency physicians as one of several point-of-care ultrasound applications spanning from head to toe. As a result of these two temporally and specialist independent adaptations, there are two different conventions used to perform a cardiac ultrasound exam, which will be discussed further under “Technique.”
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 anterior and inferior ending approximately at the left nipple. The right ventricle (RV) is the most anterior chamber of the heart 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 (LV) 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.
1. Chest pain:
2. Shortness of breath
4. Penetrating trauma or significant blunt trauma to the chest
5. Cardiac arrest
Parasternal cardiac ultrasound should not be done during CPR, but other views are appropriate if feasible. Parasternal views can take place 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.
Cardiac ultrasound should be performed with a low-frequency probe that has a small footprint that can fit between the ribs (phased array is ideal), using a cardiac setting.
A trained provider can perform a cardiac ultrasound. Emergency physicians are required to correctly perform and interpret a minimum of 25 to 50 cardiac ultrasound exams upon residency graduation.
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 level of the breast and towels 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 isn’t 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.
There are historically two conventions used when performing a cardiac ultrasound. 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 spanning 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.
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 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 space, with the handle of the probe perpendicular to the chest wall, an indicator to the patient’s right shoulder. The footprint of the probe is aligned with the long axis of the patient’s heart in this view. 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 is helpful to confirm the presence of a pericardial effusion, evaluate the aortic outflow tract, and assess the left ventricle function. A circumferential pericardial effusion should track anterior to the descending thoracic aorta in this view (whereas a left pleural effusion will track posterior to the descending thoracic aorta). A dilated aortic outflow tract greater than 4cm measured from leading edge to leading edge, especially in the presence of a pericardial effusion, may indicate a type A aortic dissection in the appropriate clinical context. Left ventricular function can be assessed by how well the LV myocardium comes together, where a fractional shortening [(LV end-diastolic diameter – LV end-systolic diameter) / LV end-diastolic diameter] of about 30% to 45% correlates with a good squeeze. An indirect way to assess LV function is via the E-point septal separation (EPSS), which is 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 7 mm or greater indicates depressed left the ventricular function. Diastolic dysfunction may cause falsely lower this distance, and mitral valve dysfunction may falsely increase this distance.
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 position of the probe is unchanged (still approximately the 4 intercostal space, just lateral to the sternum, with the handle of the probe 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 the majority of the screen displays the circular left ventricle, and anteriorly (top left of the screen) is the RV, which should appear as a crescent shape. This view is ideal for assessing the left ventricle function, but can also confirm the presence of a pericardial effusion, or signs of right heart strain such as paradoxical septal motion and RV enlargement. The overall function of the left ventricle is best assessed concentrically at the level of the papillary muscles. The operator can also look for wall motion abnormalities and correlate with the ECG in this view. The walls of the LV 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 will 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, and 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, but also to confirm the presence of a pericardial effusion. The normal RV to LV ratio is 0.6 to 1, but to keep the assessment of the RV specific for right heart strain as well as rapid and easy to perform, the binary assessment is whether the RV is smaller than the LV (no significant right heart strain), or if it is as big or bigger than the LV (evidence of right heart strain). This is particularly helpful in the setting of acute onset chest pain or shortness of breath when a pulmonary embolism is in the differential diagnosis. A patient with no significant past medical history who has a large RV on cardiac ultrasound has a pulmonary embolism until proven otherwise. Even more specific for pulmonary embolism is a McConnell sign, where the RV is large and hypokinetic with apical sparing (the apex is not hypokinetic like the rest of the RV). These ultrasound findings for an otherwise healthy patient younger than the age of 65 yours may benefit from thrombolytics.
Cardiac ultrasound, like most diagnostic ultrasound applications, is associated with little if any risk. There may be some associated discomfort when acquiring certain views, such as the apical four-chamber view, with the pressure of the probe pressing against the ribs.
Cardiac ultrasound is a non-invasive, rapid, inexpensive application that expedites diagnosis and management of imminently life-threatening disease, 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.