Handbook of Clinical Anesthesia
With its ability to provide comprehensive evaluation of myocardial, valvular, and hemodynamic performance, echocardiography is the first imaging technique to enter the mainstream of intraoperative patient monitoring (Table 28-1). (Perrino AC, Popescu WM, Skubas NJ: Perioperative echocardiography. In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams & Wilkins, 2009, pp 715–750). In conjunction with the National Board of Echocardiography, the American Society of Anesthesiologists is establishing a second certification pathway in basic perioperative echocardiography.
- Principles and Technology of Echocardiography
Echocardiography generates dynamic images of the heart from the reflections of sound waves.
- Physics of Sound.In clinical echocardiography, a mechanical vibrator, known as the transducer, is placed in contact with the esophagus (transesophageal echocardiography [TEE]), the skin (transthoracic echocardiography), or the heart (epicardial echocardiography) to create tissue vibrations.
- Properties of Sound Transmission in Tissue.Echocardiographic imaging relies on the transmission and subsequent reflection of ultrasound energy back to the transducer.
- Transducers.Ultrasound transducers use piezoelectric crystals to create a brief pulse of ultrasound.
- Beam Shape.The ultrasound transducer emits a three-dimensional ultrasound beam similar to a movie
projection. (The beam is narrow in the near field and then diverges into the far field zone.)
Table 28-1 Application of Intraoperative Echocardiography
- Resolution.Three parameters are evaluated when assessing the resolution of an ultrasound system: the resolution of objects lying along the axis of the ultrasound beam (axial resolution), the resolution of objects horizontal to the beam's orientation (lateral resolution), and the resolution of objects lying vertical to the beam's orientation (elevational resolution).
- Signal Processing.To convert echoes into images, the returning ultrasound pulses are received, electronically processed, and displayed.
- Image Display.Ultrasonic imaging is based on the amplitude and time delay of the reflected signals.
- With B-mode echocardiography, the amplitude of the returning echoes from a single pulse determines the display brightness of the representative pixels.
- Motion-mode (M-mode) echocardiography provides a one-dimensional, single-beam view through the heart but updates the B-mode images at a very high rate, providing dynamic real-time imaging. M-mode echocardiography remains the best technique for examining the timing of cardiac events.
- Two-dimensional (2-D) echocardiography is a modification of B-mode echocardiography and is the mainstay of the echocardiographic examination.
- Spatial versus Dynamic Image Quality.These selections determine whether sector size, spatial resolution, or dynamic motion is best displayed. A common approach is to focus each part of the examination on a given structure of interest and select the imaging plane that best delineates the structure in the near field. In situations in which the maximal frame rate is desired, M-mode echocardiography is chosen.
- Two-Dimensional and Three Dimensional Transesophageal Echocardiography
Two-Dimensional and Three Dimensional Transesophageal Echocardiography is the favored approach to intraoperative echocardiography. In the operating room, TEE is useful because the probe does not interfere with the operative field and can be left in situ, providing continuous, real-time hemodynamic information used to diagnose and manage critical cardiac events. TEE is also useful in situations in which the transthoracic examination is limited by various factors (e.g., obesity, emphysema, surgical dressings, prosthetic valves) and for examining cardiac structures that are not well visualized with TEE (e.g., left atrial appendage). However, the diagnostic capability of TEE depends on image acquisition and interpretation.
- Probe Insertion.The TEE probe is inserted in the anesthetized patient in a manner similar to insertion of an orogastric tube. For improved image quality, the stomach is emptied of gastric contents and air before probe insertion. The TEE probe is advanced beyond the larynx and the cricopharyngeal muscle (around 25 to 30 cm from the teeth) until a loss of resistance is appreciated. At this point, the TEE probe lies in the upper esophagus, and the first cardiovascular images are seen.
- TEE Safety.When performed by qualified operators, TEE has a low incidence of complications. Various studies have suggested an association between swallowing dysfunction after cardiac surgery and the use of intraoperative TEE. (Postoperative swallowing dysfunction is associated with pulmonary complications.)
- Contraindication to TEE Probe Placement(Table 28-2). To maintain the safety profile of TEE, each patient should be evaluated before the procedure for signs, symptoms, and history of esophageal pathology. The most feared complication of TEE is esophageal or gastric perforation.
- Probe Manipulation.Image acquisition is dependent on precise manipulation of the TEE probe. By advancing the shaft of the probe, the probe position can be moved from the upper esophagus to the midesophagus and into the stomach. The shaft can also be manually rotated to the left or right. By using the large knob on the probe handle, the head of the probe can be anteflexed (turning the knob clockwise) and retroflexed (turning the knob counterclockwise). The smaller knob, located on top of
the large knob, is used to tilt the head of the probe to the right or left. Using the electronic switch on the probe handle, the operator can rotate the ultrasound beam from 0 degrees (transverse plane) to 180 degrees in 1-degree increments.
Table 28-2 Contraindications to Transesophageal Echocardiography Probe Placement
- Orientation.An understanding of the basic rules of imaging orientation is essential to echocardiographic interpretation.
- The ultrasound beam is always directed perpendicular to the probe face. An easy way to understand this orientation is to place your right hand in front of your chest with the palm facing down, the thumb oriented left, and the fingers oriented anterior right. The scan lines that generate the TEE image start at your fingers and sweep toward your thumb.
- Consequently, the right anatomical structures are displayed on the left side of the monitor (similar to chest radiography orientation) (Fig. 28-1).
- Goals of the Two-Dimensional Examination.A comprehensive evaluation is preferred with each cardiac chamber and valve imaged in at least two orthogonal planes. However, in an emergency situation, such examination may not be possible. In these cases, most echocardiographers focus the TEE examination to views that are most likely to provide a diagnosis, including the transgastric short axis view of the left ventricle (LV) for diagnosing hypovolemia, coronary ischemia, and acute heart failure (Table 28-3).
- Three-Dimensional Echocardiography.This technology is capable of acquiring full volumes of the left ventricle, visualizing heart valves in three dimensions, and assessing the synchrony of LV contraction.
Figure 28-1. Orientation of the examiner's hand for an imaging plane of 0 degrees. The imaging plane is projected like a wedge anteriorly through the heart. The image is created by multiple scan lines traveling back and forth from the patient's left (green line) to the patient's right (red line). The resulting image is displayed on the monitor as a sector with the green edge (green line) on the right side of the monitor and the red edge (red line) on the left.
III. Doppler Echocardiography and Hemodynamics
- Doppler Echocardiography and Hemodynamics.2-D echocardiography captures high-fidelity motion images of cardiac structures but not blood flow. Blood flow indices, such as blood velocities, stroke volume, and pressure gradients, are the domain of Doppler echocardiography. The combination of 2-D images and quantitative Doppler measurements create a uniquely powerful diagnostic tool.
- The motion of an object causes a sound wave to be compressed in the direction of the motion and expanded in the direction opposite to the motion. This alteration in frequency is known as the Doppler effect.
- By monitoring the frequency pattern of reflections of red blood cells, Doppler echocardiography can determine the speed, direction, and timing of blood flow.
Table 28-3 Main Uses of Various Transesophageal Echocardiography Views
- Doppler Techniques.Two Doppler techniques, pulsed-wave (PW) and continuous-wave (CW) Doppler are commonly used to evaluate blood flow.
- PW Doppleroffers the ability to sample blood flow from a particular location. Doppler data are frequently presented as a velocity–time plot known as the spectral display.
- CW Doppleravoids the maximal velocity limitation of PW systems, and blood flows with very high velocities are recorded accurately (determining the high-velocity jet of aortic stenosis).
- Color-Flow Doppler(CFD) provides a dramatic display of both blood flow and cardiac anatomy by combining 2-D echocardiography and PW Doppler methods (Fig. 28-2). Red hues indicate flow toward the transducer, and blue hues indicate flow away from the transducer.
- The ability to provide a real-time, integrated display of flow and structural information makes CFD useful for assessing valvular function, aortic dissection, and congenital heart abnormalities.
- CFD is susceptible to alias artifacts.
Figure 28-2. Color flow Doppler of the aortic valve (AV) in the midesophageal long-axis (ME AV LAX) view. Aortic insufficiency (AI) is graded using the relative ratio of the AI jet thickness to the diameter of left ventricular outflow tract (LVOT).
- Hemodynamic Assessment.The ability of Doppler echocardiography to quantitatively measure blood velocity yields a wealth of information on the hemodynamic state (stroke volume, chamber pressures, valvular disease, pulmonary vascular resistance, systolic and diastolic ventricular function, anatomic defects).
- Echocardiographic Evaluation of Systolic Function
- Evaluation of global and regional left ventricular (LV) systolic function is a primary component of every echocardiographic examination (Figs. 28-3 and 28-4). Modalities used are 2-D and M-mode echocardiography, which image the LV walls and cavity, and Doppler echocardiography, which measures the velocity of blood flow and moving tissue.
Figure 28-3. Two-dimensional evaluation of left ventricular (LV) global and regional function. Regional and global evaluation of the LV using the transgastric short-axis view at the midpapillary level. Measurements are performed at end-diastole (ED) and end-systole (ES). Top panels: Measurement of diameters (D), areas (A), and wall thickness. Wall thickness is measured at ED in the anteroseptal and inferolateral wall segments. Bottom panel: Diameter and wall thickness measured using M-mode echocardiography with the cursor crossing the middle of inferior (top) and anterior (bottom) segments. The percent change of wall thickness of the midanterior wall segment can be used to grade its regional function. In this example, wall motion score (WMS) is 1 (normal) because the segment thickens more than 30%.
Figure 28-4. Quantitation of left ventricular (LV) systolic function. The midesophageal LV (ME) four-chamber (ME 4C) and two-chamber (ME 2C) views are obtained. The images are examined in end-diastole (ED) and end-systole (ES). The LV endocardium is traced. This automatically defines the LV area (A) and long axis (L). The system software will calculate LV volumes using either the method of discs (MOD) or the area-plane method (A-L). EDV = end-diastolic volume; EF = ejection fraction; ESV= end-systolic volume; SV = stroke volume.
- Ejection fraction(EF) is the most frequently used estimate of LV systolic function. EF and stroke volume are affected by factors such as preload, afterload, and heart rate and thus are not always indicators of intrinsic systolic function.
- Stroke volumeis calculated as the difference between the end-diastolic volume (EDV) and the end-systolic volume (ESV), and the percentage of EF is calculated as %EF = SV/EDV = (EDV – ESV)/EDV.
- Normal values for EDV are 67 to 155 mL in men and 56 to 104 mL in women. Normal values for ESV are 22 to 58 mL in men and 19 to 49 mL in women. The normal value for EF (%) is above 55%.
- Evaluation of Left Ventricular Diastolic Function
- Echocardiographic studies have suggested that patients with diastolic dysfunction presenting for cardiac surgery may be prone to intraoperative hemodynamic instability and worse outcomes.
- Doppler echocardiography is the preferred technique for assessing diastolic performance and grading the severity of the disease process. Diastolic dysfunction is defined as the inability of the LV to fill at normal left atrium (LA) pressures and is characterized by a decrease in relaxation, LV compliance, or both.
- Diastolic dysfunction may be present in the absence of clinical symptoms of heart failure. When these symptoms occur in the presence of diastolic dysfunction, the diagnosis of diastolic heart failure is made.
- Pericardial Disease: Constrictive Pericarditis and Pericardial Tamponade
- Pericardial pathologies, such as constrictive pericarditis or pericardial tamponade, impede diastolic flow (this resembles a diastolic restrictive filling pattern).
- Pericardial effusions may be global, surrounding the entire heart, or loculated, as seen mostly after cardiac surgery (Fig. 28-5).
Figure 28-5. Echocardiographic findings in pericardial effusion. A. Transgastric short-axis (TG SAX) view showing global pericardial effusion (asterisk) surrounding both the right ventricle (RV) and left ventricle (LV). B. M-mode echocardiography demonstrating separation of the epicardium from the pericardium (asterisk) from pericardial effusion. C.Regional pericardial effusion (asterisk) compressing the left atrium (LA), seen in the midesophageal long-axis (ME LAX) view. D. M-mode echocardiography revealing systolic compression of the LA. E. After evacuation of the fluid collection, the LA size increases.
Figure 28-6. Doppler evaluation of mitral regurgitation severity. Mitral regurgitation (MR) severity is evaluated using color Doppler. A. An MR jet is imaged with color-flow Doppler (midesophageal two-chamber view). The Nyquist limit is moved upward to demonstrate flow acceleration inside the left ventricle (LV) and the neck (vena contracta) of the MR jet. B. Zoom of the proximal MR jet allows measurement of the proximal isovelocity surface area (PISA) radius and calculation of the incompetent mitral valve orifice.
- Evaluation of Valvular Heart Disease
2-D echocardiography (valve anatomy and function) and Doppler (physiologic consequences and severity of the lesion) are complementary methods in valve assessment (aortic, mitral, pulmonic) (Figs. 28-2, 28-6, and 28-7).
- Diseases of the Aorta.The evaluation of the aorta is an important part of perioperative TEE. In routine cases such as coronary artery bypass surgery, evaluation of the aorta may reveal previously unknown, significant atheromatous disease of the aorta and alter the surgical plan (off-pump bypass, alternative sites for cannulation).
- Cardiac Masses.The potential of myxomas to obstruct the inflow or outflow region of a ventricle is demonstrated with Doppler echocardiography.
- Congenital Heart Disease (CHD)
Echocardiography is the primary imaging modality for diagnostic assessment of patients with CHD (atrial septal defect, ventricular septal defect, patent ductus arteriosus, coarctation of the aorta, bicuspid aortic valve, repaired tetralogy of Fallot).
Figure 28-7. Evaluation of aortic stenosis with calculation of aortic valve area using the “double envelope” technique. The cursor of continuous-wave Doppler is placed in the middle of the blood flow traversing the stenosed aortic valve, and two envelopes are identified. The one with the slower velocity is from the left ventricular outflow tract (LVOT), and the one with the fastest is from the aortic valve. The envelopes of the velocities are traced to derive the respective velocity–time integrals (VTIs). The aortic valve area is calculated using the continuity equation. D = diameter.
- Echocardiogaphy-Assisted Procedures
In addition to its role in diagnostics, echocardiography is also used to assist during various procedures such as placement of central venous catheter, intra-aortic balloon pump catheter, coronary sinus cannula, and guidewires for other venous or arterial cannulas.
VII. Echocardiography Outside the Operating Room
Echocardiography offers rapid diagnosis by differentiating among the potential complications faced in postoperative care (hypovolemia, aortic dissection, myocardial infarction, endocarditis, pulmonary embolism).
Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
Title: Handbook of Clinical Anesthesia, 6th Edition
Copyright ©2009 Lippincott Williams & Wilkins
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