Echocardiography is a complex field. US is a dynamic tool that is well suited to imaging an organ in motion such as the heart. Much information about cardiac function and flow can be gained from complicated statistical calculation packages, advanced Doppler, and M-Mode measurements. This section focuses on the primary views most relevant to the EP with the hope of providing a foundation for more in-depth study of advanced cardiac US techniques in the future.
The probe orientation indicator in echocardiography is conventionally set to the right side of the screen. In Radiology, the probe indicator is set to the left side of the screen. Some controversy exists in EM as to which side of the screen to have the probe indicator for cardiac US. The approach preferred by the present authors is to set the orientation indicator on the left side of the screen, consistent with its location for other emergency US indications. It is impractical to switch the location of the indicator in the middle of a FAST exam or when performing multiple studies in a hypotensive patient. To obtain the conventional echocardiographic views, the probe must be rotated 180° from the conventional emergency US position. The left–right reversal of the indicator and the rotation of the probe result in the same image displayed on the screen. Most modern machines have a cardiac preset that automatically adjusts a number of factors to maximize cardiac imaging. It is important for the sonographer to be aware that this preset places the orientation indicator on the right side of the image. Left–right inversion is easily achieved on most machines.
The subxiphoid view is probably the most commonly used view by EPs. It provides visualization of the four heart chambers and allows for a superior evaluation for pericardial fluid. It can be performed without interruption of cardiopulmonary resuscitation or the insertion of chest tubes and subclavian central venous lines. The subxiphoid view is often the easiest view to incorporate into the FAST exam. This view can be limited in patients with a protuberant abdomen, abdominal pain, abdominal injuries, free air below the diaphragm, and/or nausea.
Place the probe in the subxiphoid space. Angle the probe cephalad into the patient's chest with the marker aimed to the patient's right (Figure 29-11A). Hold the probe with your fingers out from the underside or topside to allow for a shallow angle (approximately 20°) between the probe and the patient. The shallow angling of the transducer risks breaking contact between the footprint and skin. Apply an adequate amount of gel and firm pressure into the subxiphoid space to maintain contact.
The subxiphoid view. A. Patient and probe positioning. B. Diagram of the US image. C. The US image. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
To obtain a complete view of the heart, the US beam depth must be increased beyond that typically used for most other cardiac and abdominal imaging. To grasp the spatial orientation of this view, bear in mind that the probe is aimed from the inferior aspect of the heart. The US beam first traverses the left lobe of the liver, and hence, liver tissue is seen at the top of the image (Figures 29-11B & C). The plane of the beam then slices through the right-sided chambers, the septum, and then the left-sided chambers (Figures 29-11B & C). The echogenic pericardium is seen surrounding the myocardium.
Adjustments in a number of planes may improve the quality of the image. First, try to increase or decrease the steepness of the angle of the probe. While the heart lies more to the patient's left, so does the stomach, which contains air that scatters the US beam. The liver, on the other hand, serves as a good “acoustic window” or a transmitter of the beam allowing for better image acquisition. Taking advantage of imaging through the liver often involves veering from just right of midline, a slight counter clockwise rotation of the probe, and angling toward the patient's left shoulder. Ask the patient to take a deep breath to bring their heart inferiorly and into the scanning plane to improve the image.
Parasternal Long Axis View
The parasternal long axis view may not be familiar to most EPs. It often is easier to obtain, provides clearer images, and is better tolerated by the patient. Place the probe to the left of the sternum, along the long axis of the heart (Figure 29-12A). Hold the probe perpendicular to the chest wall with the marker aimed toward the apex of the heart or the PMI. This is roughly toward the patient's left hip. Unfortunately, shadowing from bone and scattering from air in the lung surrounds the heart. Place the probe between two ribs and just lateral to the sternum, but not over lung tissue. The third or fourth intercostal space affords the best view.
The parasternal long axis view. A. Patient and probe positioning. B. Diagram of the US image. C. The US image. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
The right ventricle sits underneath the probe and is visualized in the near field of the screen (Figures 29-12B & C). This view also provides an excellent glimpse into the left side of the heart. The left ventricle lies beneath the right ventricle. The interventricular septum is well visualized and extends to the apex of the heart on the left side of the screen. The base of the heart can be visualized on the right side of the screen (Figure 29-12C). The left ventricle empties into the aortic outflow tract, with the aortic valve and root usually visible. The left atrium lies deep, and the mitral valve can be seen opening into the left ventricle (Figures 29-12B & C). To visualize the apex of the heart in the center of the screen, slide the probe in the direction of the marker (toward the PMI) and in the opposite direction to the center of the base of the heart. The descending aorta may be visualized in a transverse slice along the underside of the heart and is an important landmark in distinguishing pericardial from pleural fluid. Pericardial fluid collects posteriorly and will appear as a black stripe separating the myocardium from the pericardium and descending aorta (Figure 29-13). Pleural fluid, on the other hand, will reside outside of the bright pericardium and tapers to a stop at the descending aorta.
The parasternal long axis view of a pericardial effusion. Pericardial fluid separates the myocardium from the pericardium and the poorly visualized descending aorta (white arrow).
Adjustments on the tilt of the probe may optimize the view. Place the patient in the left lateral decubitus position to bring the heart closer to the chest wall and improve the image. Slide the probe laterally, to a more cephalad intercostal space, or open its rotation with a counter-clockwise turn to better optimize the view for patients with CHF. Move the probe to a more caudal intercostal space, close the angle of the probe with a clockwise rotation, or use the subxiphoid view to better optimize the view for patients with emphysema and an inferiorly displaced heart.
Parasternal Short Axis View
The parasternal short axis view slices through the heart transversely. Place the probe similar to the parasternal long axis view, but with the probe rotated 90° to rest along the heart's short axis (Figure 29-14). Aim the marker toward the patient's right hip. The left ventricle appears as a prominent circle in the center of the screen, with the right ventricle resting atop it as a flatter or a crescent-shaped chamber (Figures 29-14B & 29-15). Tilting or sliding the probe along the heart's long axis toward the patient's left hip allows visualization of the apex of the heart. Tilting or sliding the probe toward the patient's right shoulder allows visualization of the base of the heart. The ventricles should cone down to a tip at the apex. When tilting back up from the apex through the heart, the papillary muscles (Figure 29-15) and mitral valve with its “fish-mouth” appearance (Figure 29-16) will come into view. Continue tilting the probe upward toward the base of the heart. The three leaflets of the aortic valve will be seen centrally (Figure 29-17). At this point, the scan is beyond the left ventricle, allowing the atria to be visualized as well as the right ventricle emptying into the pulmonary artery.
The parasternal short axis view. A. Patient and probe positioning. B. Diagram of the three US views depending on probe angulation. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
The parasternal short axis view at the level of the papillary muscles. A. Diagram of the US image. B. The US image. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
The parasternal short axis view at the level of the mitral valve. A. Diagram of the US image. B. The US image. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
The parasternal short axis view at the level of the base of the heart. A. Diagram of the US image. B. The US image. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
The apical four-chamber view is obtained by placing the probe at the PMI, just inferior to the left nipple and angling up through the heart (Figure 29-18). Aim the marker toward the patient's right. This study is relatively simple to perform by transitioning from the parasternal short axis view. Slide the probe down to the apex of the heart and then tilt it upward toward the base of the heart (Figure 29-18A). The image reveals a side-by-side view of the ventricles in the near field and the atria in the far field (Figures 29-18B & C). The lung adjacent to the heart impedes imaging. Place the patient in the left lateral decubitus position to alleviate scattering. The left ventricle appears larger and has thicker walls than the right ventricle. The mitral valve sits slightly lower than the tricuspid valve. Ventricular function, flow across the valves, and septal defects can be assessed with this view. This view allows for comparing chamber size and evaluating for right ventricular dilatation if there is concern for an obstructive PE.
The apical four-chamber view of the heart. A. Patient and probe positioning. B. Diagram of the US image. C. The US image. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
Subcostal Inferior Vena Cava View
Sonographic evaluation of the IVC can provide valuable hemodynamic information to the EP.14,15 This view is also referred to as the subxiphoid long axis view. Place the probe in the subxiphoid space, perpendicular to the patient's abdominal wall and with the marker aimed toward the patient's head (Figure 29-19A). The longitudinal IVC will appear posteriorly, beneath the liver and the bowel, as a long black cylinder. It is important to distinguish the IVC from the aorta. The aorta lies on the patient's left side, is more “pipe-like” in appearance, is noncompressible when pressure is applied by the US probe, and has a recognizable pulsatility. The IVC is compressible when pressure is applied by the US probe and varies in diameter with respiration (Figures 29-19C & D). It may be beneficial to start in the transverse plane in which both the IVC and the aorta are visualized (Figure 29-20). Starting with the transverse view, rotate the US probe 90°, maintaining the IVC in the center of the screen to obtain the longitudinal view (Figure 29-19). Tilt the probe cephalad to visualize the IVC entering the right atrium (Figures 29-19C & D).
The subcostal IVC view. A. Patient and probe positioning. B. Diagram of the US image. C. US of the IVC during expiration. D. Diagram of the IVC during inspiration. (Used with permission from: Ma OJ, Mateer JM, Blaivas M: Emergency Ultrasound, 2nd ed. New York: McGraw-Hill, 2008.)
The transverse IVC view. The IVC is located to the right of the patient's aorta.
Measurements of the IVC proximal to its entrance into the right atrium allows for a noninvasive estimate of CVP. The negative pressure generated in the chest by inspiration draws blood cephalad and decreases the diameter of the IVC (Figures 29-19C & D). Normal dimensions for the IVC include a diameter of 1.5 to 2.0 cm and an inspiratory collapse of 50%.14,15 A smaller diameter and greater inspiratory collapse are indicative of a low CVP.14–16 A larger diameter and lesser inspiratory collapse reflect a high CVP.14–16 Obtain estimates by having the patient sniff deeply and freeze the image postinspiration. Use the cine-rewind feature on the US machine to identify images or frames that allow for the measurement of the maximal and minimal IVC diameters through the respiratory cycle (Figure 29-19). M-Mode tracing of the respiratory cycle allows for precise measures of the inspiratory and expiratory IVC diameters (Figure 29-21).
M-Mode tracing of the IVC. Respiratory variation is seen. The dark stripe represents the diameter of the IVC over time. Measurements are taken at the point of inspiratory collapse (white lines).
US Guidance for Pericardiocentesis
US-guided pericardiocentesis has proven to be safe and is the method of choice for most institutions.36 A brief description of the procedure is provided in this section. Please refer to Chapter 36 for the complete details regarding pericardiocentesis.
Emergent pericardiocentesis can be guided by US using either a static or a dynamic approach. For the static approach, visualize the effusion by US and determine the best approach for needle placement. Remove the probe from the patient and proceed with the pericardiocentesis procedure. For the dynamic approach, the heart is visualized throughout the procedure to guide needle placement. Sterile technique is required for the US probe and cord. The availability of a second ultrasonographer or an assistant for dynamic guidance is helpful, particularly if an agitated-saline injection is attempted.
The pericardiocentesis site depends on the patient's body habitus, the location of the maximal visualized effusion, and the US views obtainable. The two most common sites for needle insertion are the subxiphoid space and left anterior parasternal chest wall.
For the static approach, visualize the path of needle penetration with the corresponding US image. Measure the distance from the top of the image to the pericardial space to determine the depth of needle insertion. It is important to note that the liver is often visualized in the anticipated needle trajectory with the subxiphoid US view. Use the parasternal long axis view because the pericardium is more superficial in this view. This author prefers the parasternal long axis approach for these reasons. Proceed with the pericardiocentesis procedure.
For the dynamic approach, insert the needle directly adjacent to the transducer. Angle the probe to interface with the plane of needle insertion (Figure 29-22). Aim the US probe marker cephalad and between the ribs to provide good visualization of the intercostal space. Insert the needle over the superior edge of the rib and along the plane of the US beam. It is optional to use color Doppler in the near field to ensure the intercostal and internal mammary arteries are not punctured. Advance the needle while aspirating with the syringe as the needle is advanced. The pericardium may “tent” as the echogenic needle presses upon it (Figure 29-23A) and then enters the pericardial space (Figure 29-23B).
US probe and pericardiocentesis needle positioning.
Pericardiocentesis. A. The needle is visualized tenting (white arrow) the anterior pericardium. A hazy pericardial effusion is present. B. The needle is visualized within the pericardial fluid as a bright white point (white arrow). (Image courtesy of Jason Gookhul, MD.)
Injection of agitated saline may be attempted to confirm needle placement in the pericardial space. Agitate the saline by rapidly injecting saline back and forth from one syringe into another through two ports of a three-way stopcock: with the third port connected by sterile tubing to the pericardiocentesis needle. Once microbubbles have formed, inject the agitated saline into the pericardial space. The fluid will appear on US as a bright white scattering within the pericardial sac.
US Guidance for Cardiac Pacing
US can be used to confirm capture of transcutaneous (Chapter 31) or transvenous (Chapter 33) cardiac pacing.37,38 It is difficult to appreciate mechanical capture by simply looking for electrical changes on the ECG monitor. Transcutaneous cardiac pacing discharges often cause simultaneous jerking of the patient that masks a palpable pulse. Cardiac US evaluation during cardiac pacing allows visualization of mechanical contractions of the heart. During placement of the transvenous cardiac pacing wire, it can be visualized passing through the right atrium and tricuspid valve into the right ventricle. A subcostal view of the IVC can confirm errant passage of the wire down the IVC.