Key Points
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Perioperative applications of transesophageal echocardiography (TEE) include monitoring, diagnosis, and procedural guidance. The role of TEE in cardiac surgery, noncardiac surgery, interventional procedures, and critical care continues to evolve.
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In noncardiac surgery, intraoperative TEE serves as a routine monitor or as a rescue tool in life-threatening emergencies.
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TEE aids in decision making during cardiac surgery, and three-dimensional (3-D) TEE adds incremental value in intraoperative assessments.
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TEE is integral to structural heart interventions and is a key component of multimodality imaging.
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Echocardiography plays a vital role in the diagnosis and management of shock.
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Applications of transthoracic echocardiography (TTE) in the perioperative setting are expanding.
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Focused cardiac ultrasound is performed and interpreted at the point of care, and addresses specific questions relevant to the clinical context.
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Critical care societies have proposed distinct competencies and guidelines for critical care echocardiography beyond those included in perioperative TEE guidelines. The skill set of the intensivist performing echocardiography overlaps with that of the intraoperative echocardiographer and includes other unique components.
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Basic knowledge of echocardiography is an expectation of anesthesiology training.
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Simulation is an effective training tool when integrated within a multimodal, curriculum-based approach to echocardiography education.
Acknowledgment
The editors and publisher would like to thank Drs. Georges Desjardins, Daniel P. Vezina, Ken B. Johnson and Michael K. Cahalan for contributing a chapter on this topic in the prior edition of this work. It has served as the foundation for the current chapter.
Introduction
Echocardiography is an invaluable tool used throughout the perioperative period. Image acquisition and interpretation occur at the point of care, enabling real-time integration of the findings in the context of the patient’s clinical condition. Transesophageal echocardiography (TEE) has broad functionality in the perioperative environment. TEE serves as an intraoperative monitor, provides detailed structural and functional information, facilitates diagnosis of pathology, and guides percutaneous interventions. Applications of TEE have expanded within cardiac surgery, and into the realms of noncardiac surgery, catheter-based procedures, and critical care. As the use of clinical ultrasound grows in modern anesthetic practice, perioperative clinicians are also beginning to embrace transthoracic echocardiography (TTE). Critical care echocardiography shares many similarities with intraoperative echocardiography, but is simultaneously evolving under the larger umbrella of critical care ultrasonography. This chapter provides a historical synopsis and summary of imaging techniques and echocardiographic views and discusses applications of TEE and TTE in perioperative care.
History of Perioperative Echocardiography
Intraoperative Transesophageal Echocardiography
Decades of evolution and advancement have led to the current state of perioperative TEE ( Fig. 37.1 ). Clinical echocardiography first emerged in the 1950s with the use of motion mode (M-mode) ultrasound to record signals from cardiac structures. Nearly two decades later, the first intraoperative echocardiography was performed with epicardial imaging. The first practical clinical application of TEE was reported in 1976. Soon after, a group reported the first use of intraoperative TEE and described its utility in monitoring left ventricular (LV) function during cardiac surgery. In the 1980s, the mounting of two-dimensional (2-D) transducers onto modified flexible gastroscopes created more versatile probes, and pioneering anesthesiologists and cardiologists began to demonstrate the usefulness of TEE as an intraoperative monitor. Further advancements such as transducers with combined 2-D and Doppler capabilities, biplane imaging, multiplane imaging, and pediatric probes unlocked the diagnostic potential of intraoperative TEE. Real-time three-dimensional imaging (RT-3D) entered the scene in the mid-2000s, and since then its intraoperative use has grown exponentially. Advances in probe technology and improved computing power continue to further the practicality of three-dimensional (3-D) image acquisition.
Critical Care Echocardiography
Prior to the introduction of TEE in 1976, significant advances in 2-D transthoracic imaging occurred, leading to better visualization of cardiac structures. Production of commercially available instruments soon followed. In the intensive care units (ICUs), some clinicians incorporated echocardiography in the evaluation of cardiac function in acute respiratory distress syndrome and sepsis. TTE was not widely disseminated in ICUs, however, as the intensivist’s tool of choice at the time was the pulmonary artery catheter. During the late 1990s to 2000s, the use of pulmonary artery catheters declined, and TTE simultaneously gained more traction in the ICUs. With the miniaturization of equipment and emphasis on point-of-care ultrasound, use of echocardiography as a hemodynamic monitor and diagnostic tool in the ICU continues to increase.
Principles of Ultrasound
While a thorough discussion of ultrasound physics is beyond the scope of this chapter, a basic understanding of fundamental concepts is requisite for any echocardiographer. Sound is a mechanical, longitudinal wave of vibrations propagated through a medium. Several parameters describe sound waves including frequency, wavelength, amplitude, and propagation speed. Ultrasound waves are sound waves with frequencies above the audible human range (20-20,000 hertz [Hz]). Echocardiography typically uses frequencies between 2 and 12 megahertz (MHz). Ultrasound transducers use piezoelectric elements to convert ultrasound energy into electrical energy and vice versa. Transducers function both as transmitters and receivers of ultrasound signals. 2-D echocardiography uses a phased-array transducer, which has a row of electrically interconnected piezoelectric elements.
As ultrasound energy propagates through tissue, interactions between the wave and tissue result in absorption, divergence, reflection, and scattering of the energy. Reflection of ultrasound energy at tissue interfaces forms the basis of image generation. These sound-tissue interactions reduce the intensity of the ultrasound signal, and this attenuation limits the depth of imaging. Transmitted energy at tissue interfaces often undergoes refraction and changes directions, contributing to imaging artifacts.
The accurate display of an image depends on imaging resolution, components of which include spatial resolution, temporal resolution, and contrast resolution ( Table 37.1 ). Spatial resolution can be described according to the three beam dimensions: axial, lateral, and elevational. Image formation involves a tradeoff between spatial resolution and depth of penetration. Higher frequency (shorter wavelength) transducers provide superior axial resolution, but limited penetration depth due to attenuation. Lower frequencies (longer wavelengths) penetrate more deeply at the expense of axial resolution. Higher frequencies are better for imaging superficial structures; lower frequencies are better for deeper structures.
| Description | Practical points | |
|---|---|---|
| Spatial resolution | The ability to discriminate between two closely-spaced objects The minimum distance necessary between two structures so that they are displayed as separate objects | Determines image details Factors that improve spatial resolution often come at the expense of decreased temporal resolution and vice versa |
| Axial | Differentiates between objects along the length of the ultrasound beam | Higher frequency transducers have shorter wavelengths and better axial resolution Axial resolution is superior to lateral resolution |
| Lateral | Discriminates side-to-side (or horizontal) relative to the direction of the beam | Determined by the beam width Best at the focal point of the beam |
| Elevational | Discriminates between objects vertical to the plane of the ultrasound beam | Determined by the beam height Best at the focal point of the beam |
| Temporal resolution | The ability to accurately display a structure’s motion with respect to time | The higher the frame rate, the better the temporal resolution Factors that decrease the time to scan the sector (e.g., decreasing the imaging depth or creating a narrower imaging sector) improve temporal resolution |
| Contrast resolution | The ability to resolve subtle differences in echogenicity, which are displayed as different shades of gray | Improved by harmonic imaging, use of contrast agents, B-color maps, and post-processing controls |
Formation of ultrasound images relies on several assumptions: (1) ultrasound energy propagates in a straight line; (2) all returning echoes originate from the central beam, which is extremely thin; (3) echoes return to the transducer after one reflection; (4) attenuation is constant; and (5) the speed of sound is constant, therefore the depth of a reflector is proportional to the round-trip transit time. Violations of these assumptions result in imaging artifacts. Spectral and color flow Doppler (CFD) and 3-D images are also susceptible to imaging artifacts.
Ultrasound Modalities
Key ultrasound modalities used in echocardiography include motion mode (M-mode), 2-D imaging, Doppler (spectral and color flow), and Doppler tissue imaging (DTI). Table 37.2 highlights important aspects of these techniques. Additional techniques may be applied in specific contexts and are introduced in the next section.
| Modality | Description/Features |
|---|---|
| Brightness Mode (B-mode) |
|
| Motion Mode (M-mode) |
|
| Two-Dimensional (2D) |
|
| Doppler |
|
| Pulsed-Wave Doppler (PWD) |
|
| Continuous Wave Doppler (CWD) |
|
| Color Flow Doppler (CFD) |
|
| Doppler Tissue Imaging (DTI) |
|
Strain and Strain-Rate Imaging
Strain and strain-rate imaging techniques (also called cardiac mechanics or myocardial deformation) are used to quantify global and regional ventricular function. Strain is the relative change in shape or size of an object as a result of an applied force. In the context of the myocardium, it is the fractional change in the length of a segment relative to its baseline and expressed as a percentage. Positive strain indicates lengthening or thickening and negative strain represents shortening or thinning. Three axes are relevant to myocardial deformation: longitudinal, circumferential, and radial. Normal deformation patterns during systole are longitudinal shortening (negative strain), circumferential shortening (negative strain), and radial thickening (positive strain).
Methods for strain analysis include DTI and the more commonly used speckle tracking echocardiography (STE). DTI measures velocities at two points in the myocardium and derives the strain and strain rate. STE, in contrast, tracks unique acoustic speckles in the myocardium over a series of frames and derives strain and strain rate from the change in distance. Duncan and colleagues have written an excellent review on perioperative strain and strain-rate imaging.
Harmonic Imaging
Harmonic imaging is a processing technique used to improve the quality of 2-D images. A sound wave propagates nonlinearly through tissue, distorting the wave’s shape. This produces harmonic frequencies, which are integer multiples of the fundamental frequency originally transmitted by transducer. Signals return to the transducer at both the fundamental frequency and harmonic frequencies. Harmonic imaging typically creates images from the second harmonic frequency and filters the fundamental frequency signals. Harmonic imaging increases contrast resolution, improves signal-to-noise ratio, and decreases some artifacts. Disadvantages include slightly decreased spatial resolution and thickened appearance of some cardiac structures.
Contrast Echocardiography
Contrast echocardiography can be used to improve diagnostic evaluation and enhance suboptimal images. Two methods of contrast echocardiography include injection of agitated saline or injection of commercially available contrast agents. Agitated saline can be used to identify the presence of intracardiac right-to-left shunt, because microbubbles created by agitation do not cross the pulmonary circulation. Commercially available contrast agents use encapsulated microbubbles of high-density gas and are able to traverse the pulmonary circulation, allowing for opacification of the left side of the heart. Perioperative applications of contrast echocardiography include improved endocardial border delineation for assessment of function, assessment of regional wall motion, and exclusion of intracardiac thrombus or mass.
Three-Dimensional Image Acquisition
3-D imaging provides information that complements the findings from the 2-D examination. Transducers with 3-D capabilities contain thousands of piezoelectric elements in a rectangular (or matrix) array configuration and create pyramidal-shaped images. The optimal 3-D image involves tradeoffs between temporal resolution, spatial resolution, and sector size. 3-D imaging is susceptible to the same artifacts as 2-D imaging and additional artifacts unique to 3-D image construction. We encourage the interested reader to refer to published 3-D echocardiography guidelines as well as a practical overview focused on intraoperative 3-D image acquisition.
Imaging Modes
Two-Dimensional Multiplane Acquisition
Matrix array (3-D) transducers have multiplane imaging capabilities and can simultaneously display two or more live 2-D imaging planes. In biplane imaging, the first image serves as the reference view, and the second image is obtained by rotating the scanning plane around the longitudinal axis of the reference view. The secondary image can also be adjusted by tilting the imaging plane in the elevational or lateral axis. Multiplane imaging allows simultaneous display of multiple rotatable imaging planes ( Fig. 37.2 ). Box 37.1 lists common perioperative applications of multiplane imaging.
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Simultaneous visualization of segmental wall motion
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Characterization of the mechanism and origin of valvular regurgitation
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Procedural guidance during percutaneous closure of atrial septal defect
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Guidance of transseptal puncture for transcatheter left atrial appendage and mitral valve procedures
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Evaluation of the left atrial appendage morphology and exclusion of thrombus
Real-Time Three-Dimensional Imaging
3-D images can be displayed live or in “real time.” RT-3D imaging acquires data over a single heartbeat; some authors refer to this as 4-D imaging. The proprietary nomenclature varies, but there are three main RT-3D modes of differing sector size:
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Narrow sector: this mode displays a pyramidal volume with the best temporal and spatial resolution of the live modes. The major limitation is that it often does not capture an entire structure of interest ( Fig. 37.3 ).
Fig. 37.3
Real-time three-dimensional imaging using narrow sector (top) and wide sector (bottom) modes. (A) Narrow sector imaging displays a narrow, pyramidal volume. (B) Wide sector imaging displays a defined region of interest selected from a larger pyramidal volume. (C) Narrow sector image of the mitral valve after cropping and rotating. Only a portion of the mitral valve structure is visualized. (D) Wide sector image of the mitral valve after cropping and rotating. The entire mitral valve structure is visualized, but at the expense of decreased spatial and temporal resolution.
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Wide sector: this mode “zooms in” on a selected region of interest. There is a decrease in temporal and spatial resolution compared to narrow sector imaging. Images are easy to acquire. This mode is ideal for real-time image manipulation (see Fig. 37.3 ).
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Full-volume: full-volume mode has the largest imaging sector. Live full-volume images have reduced temporal and spatial resolution. Ideally, acquisition of full-volume images occurs over multiple beats. Live full-volume mode has utility when multiple-beat acquisition is not feasible.
Gated Acquisition
Gated acquisition divides the imaging volume into multiple narrow subvolumes that are acquired over a specified number of heartbeats. “Stitching” the subvolumes together creates the final image. In order to obtain the subvolumes at the same time in the cardiac cycle, image acquisition is gated to the R-wave of the electrocardiogram (ECG). Gated acquisition requires normal cardiac rhythm and the absence of electrical interference and respiratory variation.
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Wide sector: multiple-beat gated acquisition using wide sector mode significantly improves the temporal resolution of the “zoomed in” view. Spatial resolution marginally improves.
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Full-volume: this mode has the largest sector size with optimal spatial resolution and high temporal resolution ( Fig. 37.4 ; Video 37.1).
Fig. 37.4
(A) Schematic of multibeat gated full-volume image acquisition. Subvolume acquisition is gated to the R-wave of the electrocardiogram. In this example, subvolume acquisition occurs over five consecutive heartbeats. The individual subvolumes are then synchronized and “stitched” together to create a larger full-volume three-dimensional image. (B) Creation of the three-dimensional (3D) full-volume image from narrow subvolumes.
Modified from Desjardins G. Perioperative echocardiography. In: Miller R, ed. Miller’s Anesthesia . 8th ed. Philadelphia, PA: Elsevier/Saunders; 2015:1396–1428.
Color Flow Doppler
CFD may be incorporated with any of the modalities, but leads to a reduction in temporal resolution. Optimal CFD images are obtained using R-wave gated multiple beat acquisitions.
Quantitative Analysis
Multiplanar Reformatting
Multiplanar reformatting enables alignment of orthogonal planes to accurately measure linear dimensions and areas (e.g., planimetry of stenotic orifice, annular area).
Advanced Applications
Quantitative applications exist for 3-D imaging and eliminate some of the geometric assumptions required in 2-D imaging calculations. Example analyses include calculation of ventricular ejection fraction (EF), analysis of mitral valve structure, and dynamic annular measurements. The degree of automation and need for post-processing manipulation vary. Some analyses are performed exclusively offline, making them useful in research, but less applicable to intraoperative decision making.
Indications and Practice Guidelines
Perioperative Transesophageal Echocardiography
The American Society of Anesthesiology (ASA) and Society of Cardiovascular Anesthesiologists (SCA) issued perioperative TEE practice guidelines in 1996, categorizing indications for TEE in various clinical settings. An updated version in 2010 recommended the indications for perioperative TEE based on literature review and expert opinion. In the absence of contraindications, TEE should be used in all adult open heart and thoracic aortic surgical procedures. It should be considered in coronary artery bypass graft surgeries to confirm diagnostic information, detect pathology, influence the anesthetic or surgical plan, and assess surgical results. It is recommended that TEE be considered on a case-by-case basis in small children undergoing cardiac surgery due to unique risks in this population. For catheter-based intracardiac procedures, the 2010 guidelines state TEE may be used. Separate guidelines focus specifically on the role of echocardiography in interventional and catheter-based procedures. In noncardiac surgery, TEE is indicated in the setting of persistent unexplained life-threatening circulatory instability. If the surgical procedure or the patient’s known or suspected pathology could result in severe hemodynamic, pulmonary, or neurologic compromise, TEE may be used. TEE is recommended in critical care if the diagnostic information is expected to alter management and cannot be obtained by other modalities expediently. European guidelines offer similar recommendations for the use of perioperative TEE, while the American Heart Association (AHA) and American College of Cardiology (ACC) make more limited recommendations regarding specific indications for intraoperative TEE.
In addition to describing indications for perioperative TEE, the 1996 ASA/SCA guidelines also distinguish between basic and advanced levels of training. In 2002, an American Society of Echocardiography (ASE)/SCA Joint Task Force authored guidelines for training in perioperative echocardiography, defined as TEE, epicardial echocardiography, or epiaortic ultrasonography performed in surgical patients immediately before, during, or after surgery. These guidelines do not include TTE. Expectations for cognitive and technical skills of both basic and advanced training are outlined with recommendations for the minimum numbers of echocardiographic examinations performed. Further discussion of training and certification occurs at the end of the chapter.
Critical Care
Critical care societies have further defined competencies and applications of echocardiography in critical care beyond that established by the ASA/SCA guidelines. Several internationally based expert panels have proposed training objectives and competency-based training standards in consensus statements. Similar to the distinction between basic and advanced training in perioperative echocardiography, these statements distinguish between basic and advanced critical care echocardiography, or a basic and expert skill set. The Society of Critical Care Medicine has provided evidence-based recommendations for the use of bedside echocardiography (TEE and TTE) in the evaluation of the critically ill patient.
Transesophageal Echocardiography Examination
Contraindications and Complications
TEE is a safe and relatively minimally invasive procedure, although echocardiographers should be aware of potential complications resulting from probe insertion and manipulation. Table 37.3 lists absolute and relative contraindications to TEE. The risks and benefits must be carefully considered in patients with oral, esophageal, or gastric disease. Hilberath and associates provide an excellent summary on the safety of TEE, with reviewed studies reporting an overall complication rate in adults of 0.18% to 2.8% and major complication rate of 0.2% to 1.2%. A 2017 retrospective study reviewed recent data regarding complications in a contemporary cardiac surgery cohort. Of the 7948 patients that underwent TEE, 111 (1.4%) were identified as having complications likely attributable to TEE (no other cause identified). Incidence of significant dysphagia was 0.3% and esophageal or gastric complications 0.9%. Independent risk factors associated with complications were age, body mass index, previous stroke, procedure other than isolated coronary artery bypass grafting (CABG), cardiopulmonary bypass time, and return to the operating room for any reason.
| Absolute Contraindications | Relative Contraindications |
|---|---|
Esophageal pathology
| History of:
|
| Active upper gastrointestinal bleeding | Recent upper gastrointestinal bleeding |
| History of esophagectomy | Active peptic ulcer disease, esophagitis |
| Perforated viscus | Esophageal varices |
| Barrett’s esophagus | |
| Symptomatic hiatal hernia | |
Restricted cervical spine mobility
| |
| Coagulopathy or thrombocytopenia |
Transesophageal Echocardiography Image Acquisition
Probe Manipulation and Imaging Planes
TEE probe manipulation includes withdrawal/advancement, turning left/right, and angle adjustment. The large knob on the handle controls anteflexion/retroflexion and the smaller knob allows flexion to the left and right. TEE image acquisition occurs at four different levels: upper esophageal (UE), midesophageal (ME), transgastric (TG), and deep transgastric (DTG) ( Fig. 37.5 ).
Basic Examination
The basic perioperative TEE examination serves as an intraoperative monitoring tool used to identify cardiac causes of hemodynamic or respiratory instability. The basic perioperative examination includes 11 views suited to evaluating hemodynamic instability. A suggested examination sequence progresses through the 11 views as follows: ME four-chamber (ME 4C), ME two-chamber (ME 2C), ME long axis (ME LAX), ME ascending aorta long-axis, ME ascending aorta short-axis, ME aortic valve short-axis (ME AV SAX), ME right ventricular inflow-outflow (ME RV Inflow-Outflow), ME bicaval, TG midpapillary short-axis (TG SAX), descending aorta short-axis, and descending aorta long-axis.
Comprehensive Examination
A comprehensive TEE examination enables practitioners with appropriate training to fully utilize the diagnostic capabilities of TEE. Twenty-eight views (including the 11 basic views) are recommended for comprehensive assessment ( Fig. 37.6 ). Technical aspects of image acquisition have been extensively reviewed elsewhere. Echocardiographers should complete their examinations systematically to avoid missing key findings. As a result of individual patient anatomy and the time constraints of the intraoperative environment, not all views are obtained in every examination.
Epicardial and Epiaortic Imaging
While epicardial echocardiography ushered in the era of intraoperative echocardiography, its use was supplanted by TEE. In the setting of contraindications to TEE or the need for better visualization of anterior structures, epicardial echocardiography remains an alternative modality.
Epiaortic ultrasound (EAU) serves as an imaging adjunct, allowing visualization of the ascending aorta and aortic arch, regions that are unable to be visualized on TEE. Detection of ascending aorta atheroma is superior with EAU compared to that of TEE or manual palpation. EAU impacts surgical strategy, but data as to whether this translates into improved clinical outcomes are limited and heterogeneous. Examination guidelines and example imaging views are available.
Transthoracic Echocardiography Examination
Transthoracic Echocardiography Image Acquisition
Three elements describe transthoracic sonographic views: (1) transducer position or window, (2) imaging plane, and (3) structures imaged. When feasible, patients should turn to their left side with the left hand resting behind the head. The main windows are parasternal, apical, subcostal, and suprasternal. Imaging motions include translation (sliding), tilting, angulation, and rotation.
Focused Cardiac Ultrasound Versus Limited Examination
With increasing use of ultrasonography at the point of care, it is important for the clinician to recognize the distinction between focused cardiac ultrasound (FoCUS, FCU) and limited TTE examination. Guidelines endorsed by several societies describe the key differences between the two examinations. FoCUS is a simplified, point-of-care ultrasound examination that complements the physical examination ( Box 37.2 ). FoCUS addresses specific questions within a given clinical context. Relevant abnormalities are characterized as present or absent, and questions are answered in a yes/no format. In contrast, limited TTE is broad in scope. Interpretation includes normal, pathological, and incidental findings, and quantitative techniques may be employed. Fewer images are acquired in a limited examination compared to a comprehensive examination. Limited TTE requires a practitioner with advanced training and expertise.
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Goal-directed
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Problem-oriented
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Limited in scope
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Simplified
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Time sensitive and repeatable
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Qualitative or semiquantitative
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Performed at the point of care
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Usually performed by clinicians
FoCUS, Focused Cardiac Ultrasound.
Abnormalities identified by FoCUS require follow-up with formal comprehensive echocardiography. If FoCUS does not reveal pathology but there is clinical suggestion of cardiac disease, formal comprehensive echocardiography should also be pursued. Many FoCUS protocols exist, most including five key views: parasternal long-axis (PLAX), parasternal short-axis (PSAX), apical four-chamber (A4C), subcostal four-chamber (SC4), and subcostal inferior vena cava (SIVC) ( Fig. 37.7 through 37.10 ; Video 37.2). Protocols often incorporate lung ultrasound. For those interested in details of image acquisition, we refer the reader to a practical introductory primer.
Comprehensive Examination
A minority of anesthesiologists have the necessary training required to perform and/or formally interpret comprehensive TTE examinations. Standard imaging views occur at the parasternal, apical, and subcostal windows, plus the suprasternal notch. In addition to the views included in FoCUS, a comprehensive TTE examination includes the PLAX right ventricular outflow, PLAX right ventricular inflow, PSAX apical, PSAX mitral (basal), PSAX aortic, apical right ventricle-focused, apical five-chamber (A5C), apical two-chamber (A2C), apical three-chamber (A3C), and suprasternal notch long axis views.
Qualitative Assessment
Ventricular Size and Function
Left Ventricle
Qualitative visual assessment of LV systolic function is the mainstay of perioperative echocardiography. Visual estimation of EF by TTE correlates well with quantitative methods and real-time intraoperative TEE interpretation can reasonably estimate function compared to formal offline methods. Wall thickening is symmetric and vigorous when LV systolic function is normal (Video 37.3). The key TEE views for global assessment of LV function include the TG SAX ( Fig. 37.11 ), ME 4C, ME 2C, and ME LAX views ( Fig. 37.12 ). Analogous TTE views are PSAX, A4C, A2C, and PLAX views. The TG SAX and PSAX views display distributions of all three coronary arteries.
Assessment of regional function focuses on the thickening and shortening of individual segments. Translational motion, tethering, or dyssynchrony due to conduction delay or pacing can make interpretation challenging. A 17-segment model divides the heart into six basal segments (from the mitral valve annulus to the tips of the papillary muscles), six midpapillary level segments (from the tips to the base of the papillary muscles), four apical (or distal) segments (from the base of the papillary muscles to the LV apex), and the apical cap. The apical cap is difficult to visualize with TEE due to foreshortening. Segmental thickening should be assessed in the context of coronary distribution, recognizing variability among individuals. The left anterior descending artery (LAD) consistently supplies the anterior and anteroseptal segments and apical cap ( Fig. 37.13 ). The right coronary artery (RCA) supplies the right ventricle (RV) and the basal inferior, basal inferoseptal, and mid-inferior and possibly the mid-inferoseptal segments, the latter of which may be supplied by the LAD. The left circumflex typically supplies the lateral segments, although contributions from the LAD to the anterolateral segments or RCA to the inferolateral segments are not uncommon. Each segment should be evaluated as normal/hyperkinetic, hypokinetic (reduced thickening), akinetic (minimal or absent thickening), or dyskinetic (thinning, stretching, or aneurysmal). Echocardiography is more sensitive than the ECG in detecting ischemic changes.
Right Ventricle
The complex geometry of the RV makes assessment more challenging. Qualitatively, the RV should appear approximately two-thirds the size of the left ventricle, assessed in ME 4C and A4C views. Typically the right ventricle does not extend to the LV apex. RV hypertrophy suggests chronic pulmonary hypertension or the presence of cardiomyopathies.
The interventricular septum provides additional information about RV pathology. Normally, the septum curves toward the RV, and the LV cavity appears circular in short axis. In states of RV volume overload, the septum shifts away from the RV and flattens resulting in a D-shaped LV cavity mainly in mid-to-late diastole (Video 37.4) . RV pressure overload leads to leftward septal shift and septal flattening throughout the cardiac cycle, with the changes most pronounced at end-systole (Video 37.5) .
Valvular Function
Qualitative assessment of valvular function begins with sequential assessment of the valve in multiple imaging planes, noting the motion of the leaflets, presence of leaflet thickening or calcification, vegetations or masses, and additional structural abnormalities ( Fig. 37.14 ). Malcoaptation and annular dilation help define the etiology of dysfunction in regurgitant lesions. 3-D imaging modalities clarify the location of structure abnormalities.
CFD provides visual information about flow through valves. Aliasing or turbulence of forward flow through a valve should prompt evaluation for additional signs of stenosis. Further characterization of stenotic lesions is primarily quantitative in nature. Color Doppler imaging also detects the presence of valvular regurgitation. The three color Doppler components of a regurgitant jet are proximal flow convergence, vena contracta, and jet area ( Figure. 37.15 ). As blood approaches the regurgitant orifice, it accelerates, forming a series of hemispheric shells of increasing velocity and decreasing area. This flow convergence region provides qualitative information about regurgitation severity and is also utilized in quantitative assessments. The vena contracta is the narrowest portion of a regurgitant jet at or immediately downstream of the regurgitant orifice. The jet area provides information about the mechanism of regurgitation. For example eccentric regurgitation often arises from structural abnormalities while centrally originating regurgitation may be secondary in nature. The size of the jet area is subject to both technical and hemodynamic limitations, so visual estimation of jet area alone is not a reliable means of assessing regurgitation severity.
Atrial and ventricular dilation suggest chronicity of severe regurgitation; normal chamber size would be unusual in the presence of chronic severe regurgitation. Density and shape of the continuous-wave Doppler (CWD) recording of the regurgitant jet provide additional qualitative information.
An important limitation of intraoperative assessment of valvular regurgitation is the hemodynamic consequences of general anesthesia, resulting in reduced preload, afterload, and contractility. General anesthesia commonly reduces the severity of MR. Pharmacologic increase of afterload may provide more representative measurements, but may also lead to significant overestimation of regurgitation severity.
Quantitative Assessment
Ventricular Size and Function
Left Ventricle
Most indices of systolic function used in routine evaluation are load-dependent, necessitating interpretation in context and serial assessments due to changing intraoperative conditions. Global systolic function is assessed by measuring the difference between an end-diastolic and end-systolic value of a parameter and dividing by the end-diastolic value. Fractional shortening uses length measurements and fractional area change (FAC) uses area measurements. Both approaches have limitations in the setting of regional wall motion abnormalities (RWMAs). Ejection fraction (EF) is the difference between the end-diastolic volume (EDV) and end-systolic volume (ESV), divided by EDV:
EF=(EDV−ESV)EDV
The recommended method for 2-D calculation of EF is the biplane method of disks (modified Simpson rule). This requires tracing the endocardial border in two perpendicular views and assumes the LV volume consists of a stack of elliptical disks. The upper limit of normal for EDV is 74 mL/m 2 for males and 61 mL/m 2 for females; ESV is 31 mL/m 2 for males and 24 mL/m 2 for females. Table 37.4 presents updated reference ranges for LVEF. Assessment of EF using 3-D echocardiography ( Fig. 37.16 ) is more accurate than 2-D echocardiography. In the presence of significant valvular regurgitation, myocardial contractility may be depressed and forward cardiac output low despite a normal EF.
| Systolic Function | Male | Female |
|---|---|---|
| Normal | 52%–72% | 54%–74% |
| Mild dysfunction | 41%–51% | 41%–53% |
| Moderate dysfunction | 30%–40% | 30%–40% |
| Severe dysfunction | <30% | <30% |
Additional indices of LV systolic function include DTI of the mitral annular systolic velocity (s′), myocardial performance index (MPI), and the change in LV pressure over time (dP/dt). Pulsed-wave DTI of the myocardium at the level of the mitral annulus yields a myocardial velocity profile ( Fig. 37.17 ). The systolic velocity wave (s′) reflects the apically-directed motion of the myocardium and correlates with EF. Normal s′ reference ranges vary based on sex, age, and mitral annulus measurement size. MPI assesses both systolic and diastolic performance and is calculated by the sum of isovolumetric contraction and relaxation times divided by ejection time. MPI identifies impaired global function and has prognostic value, however it is not routinely part of the perioperative examination. The change in LV pressure over time (dP/dt) derived from the CWD signal of MR provides information about LV contractility. Normal LV dP/dt is greater than 1200 mm mmHg/s; dP/dt less than 800 mmHg/s is consistent with severe dysfunction. Global longitudinal strain is emerging as a technique with utility in identifying systolic dysfunction prior to decrement in EF. Normal values vary across vendor platforms, but in general, a value of −20% (or more negative) would be expected in normal systolic function ( Fig. 37.18 ).
Right Ventricle
Parameters for assessment of RV function include FAC, volumetric assessments (EF), tricuspid annular plane systolic excursion (TAPSE), DTI of the tricuspid annular systolic velocity (s′), and MPI. RV FAC is obtained by tracing the endocardial border in diastole and systole, from the annulus along the free wall (excluding trabeculations) to the apex, from the apex to the annulus along the interventricular septum, and then returning to the annular starting point. RV FAC is calculated as:
RVFAC=RVEDA−RVESARVEDA∗100
RV FAC is expressed as a percentage and has a lower limit of normal of 35%. Estimation of RV EF using 2-D echocardiography requires geometric assumptions, and guidelines do not recommend its use. When expertise is available 3-D RV EF is recommended, with less than 45% usually representing abnormal function.
TAPSE is commonly used in TTE for evaluating RV longitudinal function, measured by aligning the M-mode cursor parallel to the tricuspid annular motion in the A4C view. TAPSE less than 17 mm is consistent with RV systolic dysfunction. Parallel alignment of the tricuspid annulus with the M-mode cursor is difficult in TEE, and ME 4C measurements of TAPSE correlate poorly with TTE measurements. Alternative methods for measuring tricuspid annular motion with TEE have been explored. Anatomical M-mode measurements in ME 4C and DTG 4-chamber 0-degree views demonstrate agreement with TTE TAPSE. TEE speckle tracking of tricuspid annular motion has also been demonstrated to correlate well with TTE TAPSE ; whether this will have practical perioperative applications requires further investigation. Reduction in TAPSE following cardiac surgery is well described. Whether reduced TAPSE reflects a true decrement in RV systolic function or rather geometric alteration from pericardiotomy remains controversial.
An s′ of less than 9.5 cm/s is consistent with RV dysfunction, but like TAPSE is an index of longitudinal excursion and may be reduced after cardiac surgery. A right ventricular myocardial performance index (MPI) of greater than 0.43 by pulsed-wave Doppler (PWD) or greater than 0.54 by DTI reflects abnormal RV function. RV strain and strain rate are emerging as prognostic parameters in the cardiology literature, but reference values are vendor specific and measurements are not yet routinely part of the perioperative examination.
Diastolic Function
Many complex interactions contribute to ventricular filling during diastole, but the echocardiographer should have a basic understanding of diastolic physiology and the progression of LV filling abnormalities along a continuum of impaired relaxation and decreased compliance. Myocardial relaxation primarily affects LV filling during early diastole (isovolumetric relaxation and early rapid filling), whereas the effects of ventricular compliance predominate in late diastole (diastasis and atrial contraction).
Transmitral PWD flow patterns reflect changes in LV filling and are used to characterize the stages of diastolic dysfunction. Two waveforms correspond to early rapid filling (E wave) and atrial contraction (A wave), respectively. ( Fig. 37.19 ). The E:A ratio changes with progressive diastolic dysfunction ( Fig. 37.20 ), and traditionally, other parameters such as isovolumetric relaxation time, deceleration time, pulmonary venous flow profiles, and propagation velocities have provided supportive information when grading diastolic dysfunction. Under normal conditions, most ventricular filling occurs during early diastole (resulting from relaxation and suction forces), and the E:A is ratio greater than 1. Grade 1 diastolic dysfunction manifests as impaired relaxation with decreased early diastolic filling and a compensatory increase in late diastolic filling (E < A). Relaxation remains impaired in more advanced degrees of diastolic dysfunction. In grade 2 diastolic dysfunction (pseudonormal pattern) , decreasing LV compliance leads to a rise in LV end-diastolic pressure (LVEDP). Ultimately an increase in left atrial pressure (LAP) increases driving pressures across the mitral valve, increasing early filling, and “normalizing” the E:A ratio. With progression to a restrictive pattern (grade 3 dysfunction), markedly decreased LV compliance and significantly elevated LVEDP and LAP result in very elevated peak E-wave velocity (E:A > 2) and rapid equilibration of LV and LAPs.
DTI is an important technique for evaluating diastolic function. DTI of mitral annular myocardial velocities produces two diastolic waveforms: e′ (early diastolic tissue velocity) and a′ (late diastolic tissue velocity), as shown in Fig. 37.17 . Preload conditions have less effect on e′ than on the transmitral E wave. DTI e′ is reduced when impaired relaxation is present, making this a useful parameter for assessing whether diastolic dysfunction is present or absent. Much has been published about site of measurement (septal vs. lateral vs. averaged) and corresponding normal thresholds. A lateral e′ velocity of 10 cm/s or greater virtually excludes diastolic dysfunction (pericardial constriction being an exception). The E:e′ ratio provides information about LV filling pressures: usually filling pressures are normal when the average E:e′ is less than 8, whereas average E:e′ greater than 14 usually reflects elevated filling pressures. However, many factors limit the interpretation of e′, including mitral stenosis, significant mitral annular calcification, mitral valve prosthesis or ring, left bundle branch block, ventricular pacing, and significant MR.
Updated ASE guidelines address the perceived complexities of diastolic function assessment and propose a simplified approach to evaluation. In patients with preserved EF, four parameters should be assessed initially: e′, E:A ratio, left atrial volume index, and peak tricuspid jet annular velocity. These updates reflect an appreciation that increased LA size serves as a marker of chronically elevated LVEDP. In patients with reduced EF, characterization of diastolic dysfunction begins with transmitral E:A ratio. These updated guidelines may not be generalizable to the perioperative period, and moreover, TEE cannot reliably assess LA size. There are several algorithms relevant to the perioperative period that can be referenced for a more in-depth review of diastolic dysfunction.
Valvular Function
Stenosis
Aortic and mitral stenosis are the most common stenotic lesions in the perioperative setting. Fundamental to the quantitative evaluation of valvular stenosis is Doppler echocardiography. Two key concepts underlying Doppler hemodynamic measurements are the continuity principle and the pressure-velocity relationship (see Hemodynamic measurements section). Recommended parameters for the assessment of aortic stenosis include peak aortic velocity, mean transvalvular gradient, and valve area calculation by continuity equation ( Table 37.5 ). The peak gradient derived using echocardiography estimates the instantaneous peak pressure difference across the valve, whereas conventional catheterization data report the difference between peak LV and peak aortic pressures (peak-to-peak gradient). Dimensionless index and 3-D planimetry corroborate findings obtained by recommended methods. Discordant grading of aortic stenosis by valve area and mean gradient requires additional evaluation of LV function, stroke volume (SV), and assessment of flow reserve. Recommended parameters for assessing mitral stenosis include gradient measurements, planimetry, and pressure half-time. The continuity equation and the proximal isovelocity surface area (PISA) method may be used for additional information.
| Severity of Stenosis | |||
|---|---|---|---|
| Parameter | Mild | Moderate | Severe |
| Peak velocity (m/s) | 2.6–2.9 | 3.0–4.0 | ≥4.0 |
| Mean gradient (mm Hg) | <20 | 20–40 | ≥40 |
| AVA (cm 2 ) | >1.5 | 1.0–1.5 | <1.0 |
| Indexed AVA (cm /m 2 ) ∗ | >0.85 | 0.6–0.85 | <0.6 |
∗ In patients with small body size, aortic valve area (AVA) should be indexed to body surface area. A small valve area in a small patient may only represent moderate stenosis. Indexing to very large body surface area is controversial.
Limitations exist with quantitative methods. Geometric assumptions made when using the continuity equation may lead to an underestimation of calculated valve area. Loading conditions impact flow, and as a result, impact peak velocities and calculated pressure gradients. Under general anesthesia, gradients may underestimate the severity of stenosis. In high-flow states, elevated gradients may reflect high cardiac output.
Regurgitation
Recall that the vena contracta is the narrowest portion of a regurgitant jet at or downstream of the regurgitant orifice. The vena contracta width is a semiquantitative parameter for grading regurgitation severity, with cutoff values differing between valves. Pulsed-wave interrogation of flow patterns provides additional semiquantitative information. Pulmonary vein systolic flow reversal is specific for severe mitral regurgitation (MR) and hepatic vein systolic flow reversal is specific for severe tricuspid regurgitation . ∗ Holodiastolic flow reversal in the descending aorta with velocities sustained above 20 cm/s is specific for severe aortic insufficiency.
Quantitative methods for grading regurgitation include calculation of regurgitant volume, regurgitant fraction, and effective regurgitant orifice area (EROA). Approaches include the stroke volume method, ventricular volumetric method, and the proximal isovelocity surface area (PISA) method. The radius of the previously mentioned flow convergence region is included in the PISA calculation. The details of these approaches are beyond the scope of this chapter. 3-D echocardiography allows for measurement of vena contracta area, which may be advantageous in the case of multiple jets or noncircular regurgitant orifices.
Hemodynamic Measurements
Cardiac Output
Doppler-based methods can be used to estimate SV and cardiac output ( Fig. 37.21 ). Volumetric flow or SV can be calculated as a cylindrical volume as follows:
SV(cm3)=CSA(cm2)∗VTI(cm),
CSA LVOT = π ( LVOT Diameter 2 ) 2
. In this example, <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='CSALVOT=3.14∗(2.1cm1)2=3.46cm2′>CSALVOT=3.14∗(2.1cm1)2=3.46cm2CSALVOT=3.14∗(2.1cm1)2=3.46cm2
CSA LVOT = 3.14 ∗ ( 2.1 cm 1 ) 2 = 3.46 cm 2
. (B) Velocity of blood flow through the LVOT is measured using pulsed-wave Doppler in the deep transgastric view. The velocity and diameter measurements should be performed at the same anatomic location. (C) The spectral envelope is traced to determine the area under the curve, which is equal to the velocity time integral (VTI). This represents the distance the column of blood travels with each beat and is sometimes referred to as “stroke distance.” Stroke volume (SV) is the product of the cross-sectional area of the LVOT and the LVOT VTI, or <SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax style="POSITION: relative" data-mathml='SV=CSALVOT∗LVOTVTI’>SV=CSALVOT∗LVOTVTISV=CSALVOT∗LVOTVTI
SV = CSA LVOT ∗ LVOT VTI
. (D) CO is derived by multiplying the stroke volume (SV) by heart rate (HR) , or <SPAN role=presentation tabIndex=0 id=MathJax-Element-8-Frame class=MathJax style="POSITION: relative" data-mathml='CO=3.46cm2∗17.6cm∗76bpm=4628mLmin’>CO=3.46??2∗17.6??∗76bpm=4628mLminCO=3.46cm2∗17.6cm∗76bpm=4628mLmin
CO = 3.46 c m 2 ∗ 17.6 c m ∗ 76 bpm = 4628 mL min
. In the example provided, the machine is configured to automatically calculate stroke volume and cardiac output as displayed in panel C.
Cardiac output calculation is typically performed at the LV outflow tract (LVOT). The diameter is measured in the ME AV LAX view using TEE and the parasternal LAX view using TTE. The LVOT VTI is obtained in the Deep TG LAX using TEE and the A5C using TTE. Potential sources of error include inaccurate measurement of the LVOT diameter (with a squaring of any error when calculating the cross-sectional area), poor Doppler alignment with the blood flow, and measurement of the area and velocity measurements at different anatomic locations.
Many studies in surgical and ICU patients have compared Doppler-derived cardiac output (CO) to CO by thermodilution. Some authors interpret the literature to indicate Doppler-derived measurements have good agreement with CO by thermodilution. Other authors, however, highlight the methodological limitations and heterogeneity of studies, and argue echocardiographic and thermodilution-derived CO cannot be used interchangeably. Stronger evidence supports the use of echocardiographic-derived CO for assessing trends over time.
Continuity Principle
The continuity principle states that flow rate proximal to a restricted orifice equals the flow rate through the orifice. Applying this principle to pulsatile intracardiac flow, the stroke volumes proximal to (SV 1 ) and at a restricted orifice (SV 2 ) are the same.
SV1=SV2
CSA1∗VTI1=CSA2∗VTI2
Common clinical applications of this equation include calculation of stenotic valve areas, prosthetic valve areas, and regurgitant orifice areas.
Bernoulli Equation
Pressure cannot be measured directly using echocardiography, however applications of the Bernoulli principle allow for estimation of gradients from velocity information ( Fig. 37.22 ). This technique is commonly applied to quantify the severity of stenosis. The modified Bernoulli equation estimates the transvalvular pressure gradient (Δ P ) as follows:
ΔP=4(V22−V12)
ΔP=4V2
If V 1 exceeds 1.5 m/s it should be included in the calculation.
Intracardiac Pressure Estimates
Right Atrial Pressure
In spontaneously breathing individuals, right atrial pressure (RAP) can be estimated from the inferior vena cava (IVC) diameter and collapsibility of the IVC during inspiration (“sniff” maneuver). The IVC collapsibility index (cIVC) is defined as:
cIVC=(Dmax−Dmin)Dmax∗100%
| RAP | Estimated RAP (Range) (mm Hg) | IVC Diameter (cm) | cIVC |
|---|---|---|---|
| Normal | 3 (0–5) | ≤2.1 | >50% |
| Intermediate | 8 (5–10) | ≤2.1 | <50% |
| Intermediate | 8 (5–10) | >2.1 | >50% |
| Elevated | 15 (10–20) | >2.1 | <50% |
Chamber Pressures
Velocity measurements of valvular regurgitation or shunt can be used to estimate chamber pressures using the simplified Bernoulli equation. The pressure gradient (Δ P ) reflects the pressure difference between the chamber where blood flow originates and blood flow is received. This is expressed as:
POC−PRC=4V2
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