Background and Indications for Examination
Ultrasound of the heart, or echocardiography (echo), is among the most challenging sonographic examinations to master. However, with goal-directed training clinicians can quickly and accurately answer some of the most critical questions using point-of-care echo:
- Is the heart pumping (and how well?)
- Is there significant fluid around the heart (pericardial effusion)?
Beyond these basic questions there is a wealth of information that echo can provide in critical care and emergency patients. The challenge is defining what can reliably be diagnosed based on the sonographer’s experience. Understanding limitations and obtaining consultant-performed echo when available and appropriate is essential.
Echo should be used liberally in patients in whom a pericardial effusion is suspected. This includes patients presenting with chest pain, shortness of breath, tachycardia, hypotension, or syncope. Echo can expedite life-saving care in penetrating chest trauma. In a code or near-code situation, echo can rapidly identify some of the reversible causes of pulseless electrical activity (PEA). The finding of an enlarged heart on chest x-ray is an excellent indication for point-of-care echo, as this tends to be heart failure or pericardial effusion—both of which are readily diagnosed with echo but can be difficult to differentiate by physical exam and respond to very different treatments.
Echo may be easily integrated into algorithms for point-of-care sonography that include multiple goal-directed examinations, each designed to answer specific binary questions that can directly affect patient management. The extended focused assessment by sonography for trauma (FAST) exam includes goal-directed examinations of the peritoneum, pelvis, pleural cavities, as well as echo. Other indications where point-of-care echo should be incorporated into a diagnostic algorithm include unexplained hypotension (Chap. 25) and unexplained dyspnea (Chap. 26).
This section focuses on transthoracic echo (TTE). TTE is readily performed as a point-of-care examination at the bedside. While the anatomy and pathology remain the same, transesophageal echo (TEE) may provide superior images and diagnostic sensitivity, although it is more invasive, more technically challenging, and requires special equipment, sedation, and attention to airway management. Point-of-care TEE may be very helpful in a critically ill patient who is already endotracheally intubated.
Normal Ultrasound Anatomy
The heart is best thought of as a cone, with a base and apex (Figs. 6-1 and 6-2a and b). The base, which contains the valves and atria, is positioned posteriorly and toward the right shoulder. The apex, where the ventricles come to a point, is directed obliquely and anteriorly toward the left hip. Thus, the “long axis” of the heart is a line or plane directed from the right shoulder to the left hip. The “short axis” is perpendicular to the long axis, a line or a plane directed from the left shoulder to the right hip. Recalling geometry, when an ultrasound plane cuts through a cone along its axis a triangle results (long-axis view) (Fig. 6-3a), while when a plane cuts through a cone perpendicular to this axis it generates a circle (short-axis or “donut” view of the heart) (Fig. 6-3b).
Cardiac anatomy, coronal plane. Inferior vena cava (IVC) courses through posterior to the liver to enter the right atrium (RA). Blood passes through the tricuspid valve (TV) to enter the right ventricle (RV). After leaving RV through the pulmonary valve (not shown) and coursing through the lungs, blood re-enters the heart via the pulmonary veins (pulm V.) draining into the left atrium (LA). Blood enters the LV through the mitral valve (not completely shown), which is tethered to the papillary muscles (PM) by the chordae tendinae (CT). Blood exits the LV through the aorta (Ao) to enter the systemic circulation. Apex is toward the left hip, while base includes the valves and atria. (Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist.)
Probe positions on the chest. Arrows show indicator direction for a general EM/critical care orientation, with indicator corresponding to left of screen as it is viewed. (a) Long axis goes from base to apex (right shoulder to left hip). Short axis is perpendicular (left shoulder to right hip). (b) Parasternal long axis (PSLA) is shown in the third intercostal space. (rotate PSLA 180° to obtain cardiology PSLA view: “fourth and long”). Parasternal short axis (PSSA) is obtained by rotating probe 90° counterclockwise. Subxiphoid (SX) is shown using the liver as a window. Apical four chamber (A4C) at apex. (Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist.)
Basic cardiac windows. (a) Shows the parasternal long-axis (PSLA) window. (b) Shows the parasternal short-axis (PSSA) window, note papillary muscles. (c) Shows apical four-chamber view (A4C). (d) Shows a subxiphoid (SX) view. (Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist.)
Blood enters the heart from the inferior vena cava (IVC) as it drains into the right atrium, then passes through the tricuspid valve to enter the right ventricle (RV). The RV is more anterior (most frequently injured in stab or gunshot wounds), and can also be seen adjacent to the liver. The hepatic veins can be found in the liver and followed as they enter the IVC and ultimately the heart. The RV is a low-pressure, high-compliance chamber that can be thought of as wrapping around the conical and muscular left ventricle (LV) in a crescent shape.
Blood drains into the left atrium (LA) from the pulmonary veins (challenging to see on TTE). The LA is the most posterior structure of the heart and lies just anterior to the esophagus, providing a window for TEE. The mitral valve has an anterior and posterior leaflet and is attached to two papillary muscles via the chordae tendinae. The papillary muscles may be prominent on echo and should not be mistaken for clot.
Blood leaves the LV through the three cusps of the aortic valve, which is located in the center of the heart, superior to the right atrium. The aortic root is best seen using a parasternal window. After arching out of view (unless a suprasternal window is available), the descending aorta may be seen posterior to the heart just anterior to the thoracic vertebral bodies on a parasternal view.
Probe Selection and Technical Considerations
Perhaps more than any other exam, cardiac images may vary significantly depending on machine quality and probe selection/settings. While basic cardiac views (ie, looking for large effusion or cardiac motion in a FAST exam or code situation) may be accomplished with a curvilinear probe, much better images will generally be obtained using a phased-array probe. This is a small footprint probe, typically with a flat square face and a frequency from 2 to 5 MHz. The beam is electronically steered, and the image will often appear to generate from a point at the top of the screen. This narrow point of origination allows an image to be more easily obtained between ribs. Broadband imaging and the use of tissue harmonics will improve image quality.
It is important to use the cardiac setting as this will have a higher dynamic range (providing more contrast to see the endocardium) and appropriate persistence and frame rate (high-frame rate is important with a moving structure).
Due to longstanding imaging conventions, cardiologists use a probe orientation that is reversed from general abdominal and obstetrical scanning. This means that when a cardiac preset is chosen, the indicator will typically switch to the right side of the screen (rather than the correspondence with the left side of the screen as it is seen in general ultrasound). The indicator direction is often signified by a machine logo or dot on the screen. Gel can also be placed on the probe to determine the side of the indicator.
The different conventions in image orientation have caused confusion when providers performing point-of-care ultrasound do exams that incorporate echo with other imaging. Cardiologists and echo sonographers typically scan from the left side of the bed, using their left hand, while general ultrasound imaging is done from the right side of the bed. In addition to reversing the indicator/screen orientation, cardiologists also switch the indicator direction for most views (ie, directed to patient’s left rather than patient’s right). In practice, of the three primary windows that we emphasize in this text (parasternal, subxiphoid, and apical), only the parasternal long-axis (PSLA) view actually appears “reversed” on the screen when cardiologists perform it; that is, the apex of the heart in a cardiology PSLA is seen on the left side of the screen as it is viewed.
There are several potential solutions to this. When performing a point-of-care echo, the provider may adopt the complete cardiology convention, performing the exam from the left side of the bed with the left hand and the indicator reversed. Alternately, a general imaging orientation can be used, scanning from the right side of the bed. In order to develop consistent scanning patterns and hand–eye coordination for point-of-care ultrasound in general, it is the recommendation of this text that when possible ultrasound be performed consistently from the right side of the bed, using the right hand. Directing the indicator to the patient’s right shoulder in a PSLA view will also provide an image orientation that is consistent with other point-of-care ultrasound orientation and echo and cause less confusion. However, if an image similar to that seen in cardiology texts is desired, the indicator may be directed to the patient’s left hip in the PSLA view. This is the so-called “fourth and long” position: indicator is pointed to the 4 o’clock position for a long axis. The same structures will be imaged either way (see “current issues with emergency cardiac ultrasound probe and image conventions” in Additional Reading).
Imaging Tips and Protocol
There are three primary windows to the heart for TTE. These include the parasternal (long axis and short axis), apical four-chamber (A4C) view, and the subcostal (four-chamber and sagittal).
The PSLA view (EM/CC orientation) is obtained by placing the probe in the third or fourth intercostal space just to the left of the sternum, with the indicator directed to the right shoulder. The thumb should be placed on the indicator with the probe held similar to a pencil. Structures visualized should include the RV (anteriorly), the LA, mitral valve, LV cavity, LV outflow tract, and descending aorta (see Figs. 6-3a and 6-4). The parasternal view may be helped by having the patient turn on their left side, exhaling if possible. The PSLA is an excellent view for assessment of the LA, LV, and aortic outflow and is the most reliably obtainable window.