Strain Imaging: An Everyday Tool for the Perioperative Echocardiographer

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Contents lists available at ScienceDirect

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Review Article

Strain Imaging: An Everyday Tool for the Perioperative Echocardiographer 1

Michael J. Benson, MD , Natalie Silverton, MD, Candice Morrissey, MD, Joshua Zimmerman, MD Department of Anesthesiology, University of Utah, Salt Lake City, UT

Strain analysis allows for global and regional analysis of myocardial function and has been shown to be an independent predictor of outcomes after cardiac surgery. Strain imaging offers advantages over traditional EF measurements in that it is relatively angle independent, it is less dependent upon loading conditions, it is reproducible, it does not rely on geometric assumptions, and it can detect subclinical systolic dysfunction. Limitations of strain analysis include high temporal resolution requirements, a strong dependence on image quality, and inter-vendor variability. In addition, there is a paucity of data on the intraoperative applications of strain. The ASE has defined a global longitudinal strain of
20% measured by transthoracic echocardiography to be considered normal, with less negative values considered abnormal. Presently, there are no published guidelines on the normal values of strain with transesophageal echocardiography (TEE). However, multiple studies have shown that a reduction in intraoperative strain assessed with TEE has been shown to be an independent predictor of complications during cardiac surgery. Accordingly, further incorporation of intraoperative strain analysis with TEE could aid in prognostication for patients undergoing cardiac surgery. As perioperative strain analysis continues to advance, an understanding of these concepts is imperative for perioperative echocardiographers. It is the authors’ goal to show that strain imaging can provide a reliable and objective measure that can be performed in real time to aid in decision-making and perioperative risk stratification. Published by Elsevier Inc. Key Words: strain; transesophageal echocardiography; intraoperative echocardiography

THE SCOPE of perioperative echocardiography has expanded dramatically over the last 2 decades, and advances in technology have produced new methods of quantifying cardiac function, which can play an important role in perioperative decision-making and outcomes. Along with these impressive technological advances, there remains a substantial gap between the literature evaluating transthoracic echocardiography (TTE) (most commonly performed by sonographers and interpreted by cardiologists) and that describing perioperative transesophageal echocardiography (TEE) (mostly commonly performed and interpreted at the point of care by

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. 1 Address reprint requests to Michael J. Benson, MD, Department of Anesthesiology, University of Utah, 30 N 1900 E, RM 3C444 SOM, Salt Lake City, UT 84132. E-mail address: [email protected] (M.J. Benson). https://doi.org/10.1053/j.jvca.2019.11.035 1053-0770/Published by Elsevier Inc.

anesthesiologists). It is the responsibility of perioperative echocardiographers to bridge this gap. The first step is to remain up to date on new techniques and technology. Strain echocardiography is a perfect example of such an emerging technique. Until recently, strain was perceived as an investigational technology; however, it has become increasingly clear that strain analysis is applicable in the perioperative setting. In this article, the authors will review the basic principles of strain imaging, review the literature on perioperative strain imaging, and briefly describe how to perform left and right ventricular strain measurements. Although the focus will be on intraoperative strain, the authors also will discuss studies describing the value of strain in other settings. The authors will conclude with a brief look into the future of perioperative strain echocardiography. It is the authors’ goal to show that strain analysis is applicable in the perioperative setting and can provide a reliable and objective measure of both clinical

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Fig 1. Components of myocardial deformation. The myocardium shortens in the longitudinal direction from the base to apex; it thickens in the radial direction from epicardium to endocardium, and shortens circumferentially.

and subclinical ventricular dysfunction. The authors also hope to show that it can be performed in real time in order to aid in decision-making and perioperative risk stratification.

circumferentially9 (Fig 1). Using these concepts, the equation for strain can be described as follows:

What Is Strain

Strain can be measured in each of these 3 directions of myocardial deformation. During contraction, the shortening of the myocardium in the longitudinal and circumferential dimensions corresponds to a negative strain value, and the thickening in the radial dimension corresponds to a positive strain value.2 Strain can be reported regionally using the 17-segment model, or globally by the average of the all the segments—for example, global longitudinal strain (GLS) is the average the longitudinal strains of all 17 segments.10 In addition to strain, strain rate (SR) can be calculated. SR is defined as the change in strain per unit time.3 Average peak systolic longitudinal SR in healthy patients measured by TTE is
1.10 § 0.01/s.11 SR peaks at mid-systole and is described by the following equation:

Strain is a dimensionless measurement that describes the deformation of a structure. Myocardial strain analysis is a measurement of global and regional myocardial function. It is the fractional displacement of segments of the myocardium—a unitless measurement expressed as a percentage difference in length at 2 time points (end systole and end diastole).1 Early strain analysis utilized tissue Doppler imaging to measure myocardial velocities at a specific point over time. This technique is not used widely now because of limitations that exist with this method,1 including the requirement of high frame rates to minimize signal noise and significant angle dependence on the ultrasound beam being in the direction of myocardial motion.2-5 Currently, speckle-tracking echocardiography (STE) has been used to measure strain. With this technology, unique ultrasound patterns of acoustic reflections or “speckles” are tracked throughout the cardiac cycle. Strain is determined by measuring the displacement of these “speckles” in relation to each other, which circumvents the angle dependence required by tissue Doppler imaging.4,6 STE can be used to track speckles throughout the myocardium or along individual lines and layers depending on the software being employed. The left ventricular (LV) myocardium consists of circumferential and longitudinal fibers within the different layers of the myocardium. Myofibers are oriented in both a right-handed helix and left-handed helix.7 Systolic function is determined by the sum of contraction of these fibers in addition to preload, afterload, and wall stress.8 Myocardial contraction is a complex phenomenon; it can be simplified into 3 dimensions of motion. During contraction, the myocardium shortens in the longitudinal direction from the base to apex; it thickens in the radial direction from epicardium to endocardium, and shortens

Strain ¼ ðLength in Systole

Length in DiastoleÞ=Length in Diastole

Strain rate ¼ change in strain=time As stated previously, systolic function is the result of a complex interplay between myocardial fibers and loading conditions. It is well known that traditional EF is significantly affected by loading conditions.12 Strain is less dependent on these loading conditions; however, there is some reliance on loading conditions. For example, in the normal range of LV cavity size, strain is relatively stable. However, when the LV is under filled, radial strain is increased and longitudinal strain is decreased. SR measurements, however, are even less dependent on loading conditions, including in conditions outside of normal.2 For example, Ferferieva et al. (2011)13 have shown that radial and circumferential strain are significantly influenced by changes in afterload, whereas SR is less influenced by changes in preload and afterload, and therefore may be a more robust measurement of intrinsic myocardial contractile function. It is apparent that there are multiple applications available for strain analysis, including the methods described earlier in

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addition to atrial strain, diastolic strain, and layer-specific strain. However, currently GLS has been the most studied. In addition the software for GLS is the most widely available and the only measurement that is described in the current ASE guidelines.10 The ASE has defined a GLS of
20% measured by TTE to be considered normal, whereas less negative values indicate worsening myocardial function. For these reasons, the authors will focus their discussion on this measurement.

parameters.22 It was therefore concluded that GLS may be safely used in routine clinical practice. However, the ASE still recommends that GLS measurements should preferably be interpreted relative to previous examinations with the same vendor equipment and software or versus vendor-specific reference values, when possible.10

Benefits

Traditional LVEF assesses only radial systolic function and does not capture other aspects of myocardial performance. In addition, LVEF may be normal in cardiac disease processes such as aortic insufficiency or mitral regurgitation, in which LV cavity size changes proportional to changes in stroke volume. Strain measurement of cardiac deformation avoids these limitations and those mentioned earlier. STE, therefore, has emerged as a more sensitive indicator than EF for detecting LV systolic dysfunction.14,23 Much of the advantage of myocardial strain appears to be in detecting systolic dysfunction before a reduction in LVEF. Strain has been shown to provide early diagnostic and more accurate prognostic information in a variety of clinical settings outside of the operating room. Routine serial strain is currently recommended when monitoring for early cardiac dysfunction in the presence of cardiotoxic cancer chemotherapy.14 Reduction of EF is often a late clinical indicator and may signal toxicity at a time when intervention is no longer possible. Strain has been shown to identify preclinical changes in systolic function 3 to 6 months before changes in EF in these patients.15,24-27 Strain analysis also has been shown to detect subclinical myocardial dysfunction after Tetralogy of Fallot (TOF) repair and may help guide clinical management in these patients.28 In a recent review, Singh et al. (2019) showed that a patient with any major valvular lesion (mitral regurgitation, mitral stenosis, aortic stenosis, aortic insufficiency) and preserved systolic function has reduced strain measurements compared to healthy controls.15 In patients with heart failure, mildly decreased EF (>45%) does not predict outcome29; however, strain has been shown to predict outcomes such as cardiac events and mortality in heart failure with preserved EF30 in addition to chronic31-34 and acute systolic heart failure.35 GLS also has been shown to be useful in providing early risk assessment after myocardial infarction.36-38 In heart transplant medicine, GLS has been shown to be an important noninvasive method for identifying early dysfunction and recovery in acute cellular rejection.39 In valvular disease, longitudinal strain is associated with symptoms40 and risk of cardiac events in conservatively managed severe aortic stenosis.41 Preoperative GLS also has been shown to predict outcomes in patients undergoing valve surgery. In patients symptomatic from aortic stenosis with >40% EF undergoing surgical aortic valve replacement, preoperative GLS was a predictor of major adverse cardiac events.42 In addition, it is well documented that it is difficult to predict LV failure after mitral valve surgery in the setting of severe mitral regurgitation because of the difficulty in accurately assessing LV systolic function at baseline. Because

As a measure of myocardial function, strain analysis with STE has the advantages of being relatively angle independent, less dependent on loading conditions than traditional twodimensional ejection fraction (EF), and does not rely on geometric assumptions. Of clinical importance, STE has the ability to detect regional dysfunction and to predict systolic dysfunction before changes in EF, which has been utilized in patients undergoing potentially cardiotoxic chemotherapy.5,14,15 In addition, current software used for measuring STE is user friendly and highly automated, making it easily reproducible and superior to the traditional measurements of EF and wall scoring.4 Although it has been argued that strain may have significant intra- and interobserver variability, multiple studies have demonstrated this variability with traditional 2D EF (measured by the modified Simpson’s Biplane method) is greater.16,17 Thavendiranathan et al. (2013)16 showed that in patients with stable GLS, standard 2D EF using the modified Simpson’s Biplane method had 11% interobserver variability. Variability is even greater when using the widely accepted “visual estimation” method of EF.18,19 Even with the gold standard, three-dimensional (3D) EF, intra- and interobserver variability can range from 5% to 10%.16,20 In contrast, it is documented that the intra- and interobsever variation in repeat GLS measurements ranges from only 2% to 5%.21 This limited variability associated with strain analysis may prove to help play a role in better predicting perioperative outcomes, especially in patients with presereved EF. Limitations Limitations of strain analysis with STE include dependence on a high temporal resolution, a strong dependence on image quality (spatial resolution), intervendor variability, and a paucity of data for intraoperative TEE.4,5 A high temporal resolution is required to maintain tracking of the “speckle” motion between frames; this is also why image quality plays an important role in strain analysis with STE. In 2015, the EACVI and ASE convened a task force to address the intervendor variability of strain measurements. In this study, 62 volunteers were studied using strain systems from 7 different manufacturers. Each volunteer was examined by the same sonographer on all machines; inter- and intraobserver variability of GLS was determined. Although there was significant variability that existed, they found that the errors in GLS were lower than for traditional 2D left ventricular ejection fraction (LVEF) and most other conventional echocardiographic

Left Ventricular Strain

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strain has been shown to detect subclinical dysfunction and is a measurement of intrinsic systolic function that is less load dependent that traditional LVEF, strain analysis may be very helpful in these settings. Preoperative LV GLS is a predictor of long-term LV systolic function in patients undergoing mitral valve repair, whereas LVEF is not.43 In similar patients, preoperative LV GLS also predicts worsening heart failure, reoperation, and all-cause mortality.44 Intraoperative LV Strain Whereas cardiac deformation analysis has been shown to be useful in a variety of clinical settings outside of the operating room, the applicability in the perioperative setting is still being examined. It is known that assessment of LV function is an important component of risk assessment in cardiac45 and major noncardiac surgery.46,47 Given what is known about the utility of strain for detecting early LV dysfunction, it is a natural extension to apply this tool intraoperatively. One of the limitations of intraoperative STE is that most of the work to date in strain is with transthoracic echocardiography (TTE) in the awake, spontaneously breathing patient. Intraoperative strain is performed with TEE in a mechanically ventilated patient under general anesthesia. This has physiological implications owing to changes in loading conditions and technological implications owing to changes in views and limitations in angles of insonance. Although there are limits of the comparability of intraoperative TEE deformation analysis and strain outside of the operating room,48 intraoperative myocardial strain is an emerging field and has been found to be feasible and reproducible with, at least, a moderate correlation to TTE values (r = 0.5-0.6).48,49 A recent prospective study of 100 patients undergoing aortic valve replacement (AVR) for aortic stenosis found that reduced intraoperative TEE-derived GLS and SR were associated with prolonged hospitalization, and LVEF was not.50 In patients with low EF receiving isolated on-pump coronary artery bypass graft, intraoperative TEE-derived LV GLS was found to be a good predictor of poor postoperative outcomes including prolonged inotropic support, prolonged mechanical ventilation, use of an intra-aortic balloon pump, or death.51 Similarly, another recently published retrospective study of 275 patients found a prebypass GLS of >
17% to be an independent predictor of low cardiac output syndrome after on-pump cardiac surgery as defined as inotropic or mechanical circulatory support for greater than 24 hours postoperatively.52 In a recent prospective study of patients with normal EF undergoing AVR, intraoperative TEE-derived GLS provided incremental predictive value for the development of postoperative atrial fibrillation.53 There is limited data on the effects of inotropic support and pacing on strain values, but as these are common practices after CPB, they certainly should be taken into consideration when assessing intraoperative strain with TEE. As would be expected, inotropic support has been shown to improve strain values; postbypass LV strain measured by TEE was increased by 17% to 18% with the addition of milrinone and 25% to 30% with the addition of levosimendan.54 However, these

improvements in strain value may not improve outcomes.55 Ventricular pacing has been shown to worsen strain values by TTE, but no data exist for TEE.56 In summary, there are few studies exploring intraoperative myocardial strain with TEE; however, those that have been done have shown that TEE-derived strain in the patient under general anesthesia is feasible and has prognostic value for postoperative outcomes and risk stratification. More research needs to be done to further explore the application of strain analysis in this setting. Right Ventricular Strain The real-time assessment of right ventricular (RV) function is essential for perioperative decision-making and RV dysfunction is associated with poor outcomes after cardiac surgery.57-60 The complex shape of the RV, however, makes traditional echocardiographic means of quantifying ventricular function problematic. This is particularly true with TEE, as the probe position behind the left atrium renders angle-dependent measures of RV function, such as tricuspid annular plane systolic excursion (TAPSE) and tricuspid annular systolic velocity (S’), inaccurate.61 Several studies have suggested that TAPSE may be a particularly poor measure of RV function after cardiac surgery because of changes in the longitudinal motion of the RV after cardiotomy. These changes occur within minutes of opening the pericardium62 and TAPSE remains depressed for up to 12 months even in the setting of a normal three-dimensional RV ejection fraction (3D RVEF), fractional area change, or cardiac index.63,64 Without objective tools to accurately assess RV function during cardiac surgery, the intraoperative echocardiographer is left with subjective assessment or “eyeballing,” a practice that has been shown to have significant inter-rater variability and is highly influenced by operator experience.65,66 Speckle tracking strain echocardiography has emerged as a tool to assess RV systolic function in addition to LV systolic function. When this software is applied to the lateral wall of the right ventricle and the interventricular septum in a 4-chamber view, it is termed right ventricular global longitudinal strain (RV GLS) (Fig 2). When it is applied to the RV lateral wall only, it is called RV free wall strain (FWS) (Fig 2). The most current guidelines suggest that RV FWS >
20% (ie, more positive than
20%) suggests dysfunction.10 Focardi et al. compared RV strain and traditional echocardiographic measures of RV function to a reference standard of cardiac magnetic resonance 3D RVEF and found that RV FWS had the highest diagnostic accuracy for predicting RV dysfunction.67 Traditional measures of RV function such as TAPSE and S’ correlated moderately with 3D RVEF. Fractional area change, RV GLS, and RV FWS, on the other hand, were highly correlated with 3D RVEF. Right ventricular strain also has been shown to be an independent predictor of outcome in patients with systolic heart failure and those with inferior wall myocardial infarction.68-70 In cardiac surgery, RV strain is a more sensitive predictor of mortality than fractional area change.71 Abnormal RV strain also has been associated with clinical RV failure after left

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Fig 2. Calculation of right ventricular strain (RV strain). (A) RV global longitudinal strain includes the lateral wall of the right ventricle and the interventricular septum in a 4-chamber view. (B) RV free wall strain is applied to the RV lateral wall only.

ventricular assist device placement.72,73 All of the aforementioned studies, however, have used TTE, and the literature surrounding intraoperative RV strain with TEE is less robust. Intraoperative RV Strain Tousignant et al. found that intraoperative RV strain measurements with TEE were feasible. They compared TEE RV strain to virtually simultaneous TTE RV strain measurements and found that RV GLS was not significantly different between the 2 different modalities (TEE:
20.4%, TTE:
20.1%). Lateral wall measurements (essentially RV FWS) were significantly higher, however, with TEE than with TTE. This difference may be because the lateral wall of the RV was more clearly visualized with TEE.74 Duncan et al. measured intraoperative strain and SR using TEE in patients undergoing AVR for aortic stenosis. They found that while LV strain was unchanged and LV SR improved immediately after AVR, RV strain worsened (
21.8% to
17.3%) and RV SR was unchanged.75 These data suggest that changes in ventricular function can be documented intraoperatively during cardiac surgery using speckle-tracking strain software and that although LV function may immediately improve after AVR, RV function may be reduced. More recently, Ting et al. used TEE to quantify RV systolic function intraoperatively immediately before and after cardiac surgery. They found that a postbypass RV GLS of >
13.5% was a better predictor of prolonged postoperative inotrope requirement (>24 hours) than more traditional measures of RV systolic function, such as TAPSE, tricuspid annular peak systolic velocity (S’), fractional area change, or RV myocardial performance index.76 RV strain with intraoperative TEE also has been used to evaluate RV function immediately before and after left ventricular assist device (LVAD) placement. Although it was shown that intraoperative RV strain measurements were feasible and reproducible, none of the measures of intraoperative

RV systolic function were associated with subsequent clinical RV failure after LVAD (>14 days of inotropes or RV assist device placement).55 It was concluded that the assessment of RV function can be confounded by the intraoperative use of inotropes, as these agents may temporarily improve function without improving intermediate outcome. There are a number of limitations to the use of RV strain for the intraoperative assessment of RV function. The first and foremost is that the majority of the software used to measure speckletracking strain is designed for the LV, especially that which is available on most echocardiography machines. Until recently, real-time dedicated RV strain software packages have been limited. Some vendors require the use LV specific software that has to be applied to the RV. Offline RV specific speckle-tracking strain software is available, but requires transferring images to an offline computer, which often precludes real-time intraoperative assessment. However, it has been shown that intraoperative RV strain measurements of RV GLS and RF FWS using LV-specific strain software correlate well with offline RV strain measurements using RV-specific strain software.77 Another limitation of RV FWS and RV GLS is that these measurements are confined to a 4-chamber view and therefore are still a regional measure of RV function when compared with 3D RVEF. Finally, as mentioned earlier, although there is a growing body of literature surrounding the use of speckle-tracking strain for the assessment of RV function, there are only a handful of intraoperative TEE based RV strain studies. As the TTE literature shows the prognostic value of RV strain measurements, specifically in cardiac surgery patients,71-73,78 future research on intraoperative RV assessment should focus on validating these measurements with TEE as well. Conclusion As strain analysis has continued to evolve, there is promising data on a new application of strain, 3D strain, which may be even more accurate and less time-intensive than 2D strain.79

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3D analysis is independent of geometric assumptions that can limit traditional 2D analysis. In addition, because it captures the entire myocardium in one image loop, the 3D analysis can detect motion out of the plane of the 2D image and more accurately describe the motion of the obliquely oriented helices of muscle fibers in the myocardium.79,80 Because this is a relatively recent application of strain with limited availability, there are few studies on its applications. It has been shown that preoperative 3D TTE strain is an independent predictor of intensive care unit length of stay and 1-year event-free survival in cardiac surgery patients81,82 and that different types of cardiac surgical procedures are associated with different changes in regional myocardial strain postoperatively.82 However, at this point, 3D strain imaging requires specialized offline software, so it is not yet applicable to the intraoperative use of strain. As is evidenced earlier, the application of perioperative strain analysis is still in its infancy. The authors have shown that perioperative strain analysis is feasible and reproducible and that it can aid in decision making and perioperative risk stratification and prognostication. The authors know for sure that TTE values of strain moderately correlate with TEE values, that a GLS of <
20% is normal on TTE, and that less negative values indicate worsening myocardial function. Although this information is currently being extrapolated to guide intraoperative strain analysis, further studies need to be done to answer many questions, including validating “normal values” for TEE, exploring future applications, and determining specific numerical cutoffs for normal values and concerning values in the perioperative setting. As the field of perioperative echocardiography continues to advance, it is incumbent on practitioners to engage in the process of continuing improvement in echocardiography practices and to contribute to this growing body of literature that pertains to the care of patients with cardiovascular disease.

Step 1: Acquire the Software Most echocardiography machine vendors have on-cart speckle-tracking strain software available as an add-on application. For Phillips Qlab, the application is called Chamber Motion Quantification (CMQ) and is designed for the LV. For GE, the application is called Automated Function Imaging (AFI) and also is designed specifically for the LV. The Siemens application Velocity Vector Imaging (VVI) is designed for both the LV and RV strain calculations. Often these applications must be purchased separately. Step 2: Image Acquisition In order to calculate LV GLS, 3 specific images must be acquired (Fig 3). A midesophageal 4-chamber view will be used to calculate the longitudinal strain of the inferoseptal and anterolateral walls of the LV. A midesophageal 2-chamber view will be used to calculate the longitudinal strain of the anterior and inferior walls of the LV. A midesophageal longaxis view will be used to calculate the longitudinal strain of the anteroseptal and inferolateral walls. In order to calculate LV global circumferential strain, 3 transgastric LV short-axis views are obtained at the basal, mid, and apical levels. For RV strain, an RV-focused midesophageal 4-chamber view is acquired. This is simply a standard midesophageal chamber view turned slightly to the right such that the entire lateral wall is visualized from tricuspid annulus to ventricular “apex” (Fig 2). When acquiring 2D images for speckle-tracking strain analysis, a minimum frame rate of 40 to 50 frames/s is required for accurate assessment because low frame rates will cause the software to lose track of speckle motion between frames. It follows then that 2D images should be free from color flow Doppler boxes as this significantly reduces frame rate. Step 3: Open the Software

Intraoperative Strain Measurements: Try This at Home The fast-paced decision-making required of intraoperative echocardiographers can lead to a reliance on qualitative “eyeball” assessments, particularly when measurement software is cumbersome. The subjective assessment of ventricular function, however, has been shown to have poor diagnostic accuracy and significant inter-rater variability.65,66,83 Simple to use software that allows for the timely quantification of ventricular function is therefore important. Speckletracking strain software is as easy to use and arguably as timely as the measurements of EF either in 2D or 3D. But as with most functions, speed comes with repeated practice. The purpose of this section is to familiarize the reader with the general steps of speckle-tracking strain measurements in order to encourage the everyday intraoperative echocardiographer to try this software at home. There are several different speckle-tracking strain software programs available in the operating room. This guide will take the reader through the general steps of image acquisition and processing (Video 1 and 2) (Figs. 4 and 5).

Once the images are acquired, they can be selected as a group and then imported into the software application. Often this is done by highlighting or selecting all of the clips at once, then simply opening the software application. For efficiency, it is important to import multiple clips at once because not all views can be used for defining the systolic period and because for the LV, the global assessment is based on more than one image. Step 4: Define Systole Systolic strain is defined as the percent change in myocardial length during systolic ejection. Because of a phenomenon called post-systolic shortening, this value is often less than “peak strain,” which is the percent change in length of the myocardium over the entire cardiac cycle. This distinction is significant because the presence of post-systolic shortening has been found to be an important indicator of myocardial ischemia.84 When calculating LV strain, the midesophageal long-axis image can be used to demarcate both the end of diastole (closure of the mitral valve) and the end of systole (closure of the aortic

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Fig 3. Calculation of left ventricular global longitudinal strain (LV GLS). Three images must be acquired: (A) A midesophageal 2-chamber view. (B) A midesophageal 4-chamber view. (C) A midesophageal long-axis view. (D) Display of GLS.

valve).85 Two of the major strain software applications, Phillips CMQ and GE AFI, automatically use the R wave on the electrocardiogram signal as the beginning of systole, but end systole must be manually set by marking the closure of the aortic valve. For RV strain, the midesophageal 4-chamber view used to calculate RV FWS and RV GLS cannot be used to accurately define end systole. Accordingly, a second image, either a midesophageal long-axis or RV inflow-outflow view, must be used to mark the closure of either the aortic valve or the pulmonic

valve. Alternately, spectral Doppler data can be used to determine the aortic valve opening time and this data can be entered manually. Step 5: Identify The Region of Interest The region of interest (ROI) is the specific myocardial segments selected to be processed with strain analysis. With Phillips CMQ and GE AFI, the ROI is generated by manually marking

Fig 4. Video still image of Video 1: How to perform left ventricular strain.

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Fig 5. Video still image of Video 2: How to perform right ventricular strain.

points on the endocardium on either side of the mitral valve for the LV or the tricuspid valve for the RV. A third point at the ventricular apex is then marked (LV apex for LV strain, RV “apex” for RV strain). Siemens VVI is similar only instead of manually identifying 3 distinct points; the endocardial border is manually traced. The software then automatically generates the ROI, which can be manually adjusted to improve tracking. Care must be taken to identify the myocardial boarders, as inclusion of the pericardium will falsely lower calculated strain values. Table 1 Take Home Points for Cardiac Anesthesiologists  Strain analysis is a measurement of myocardial function that is angle independent, less dependent on loading conditions than traditional EF, and does not rely on geometric assumptions.  Strain can detect subclinical myocardial dysfunction.  GLS <
20% is normal (and less negative values indicate worsening myocardial function).  TTE values correlate at least moderately with TEE values.  Perioperative strain analysis is feasible and reproducible.  Perioperative strain analysis may aid in decision-making, risk stratification, and prognostication in patients undergoing cardiac surgery.  Current software is readily available and highly automated on TEE machines. Abbreviations: 3D, three dimensional; EF, ejection fraction; GLS, global longitudinal strain; TEE, transesophageal echocardiography; TTE, transthoracic echocardiography. Table 2 Gaps and Future Questions  Validated “normal values” for both LV and RV TEE strain.  Validated numerical cutoffs associated with prognosis after cardiac surgery.  Intraoperative 3D TEE strain “normal values.”  More readily available RV-focused software on TEE machines. Abbreviations: LV, left ventricular; RV, right ventricular; TEE, transesophageal echocardiography.

Step 6: Display Your Results For LV strain, global and regional strain and SR can be displayed and reported. For Phillips CMQ and GE AFI, the LV 4chamber model can be used to calculate RV GLS. Caution must be taken as the labels demarking the lateral and septal walls may be reversed when using the LV software to calculate RV strain, depending on the sequence in which the endocardial borders were marked. After RV GLS is calculated, the septal segments can be removed to generate RV FWS (Fig 2). It must be noted, however, that more recent versions of Phillips CMQ software use the length of line method for calculating strain. With this software, removing multiple segments invalidates the calculation; therefore the assessment of RV FWS is not possible.

Step 7: Application of Results There are currently no published guidelines or recommendations for perioperative strain analysis. Although no recommendations will be made here, a clinical scenario of application will be considered. It is known that perioperative strain analysis is feasible, reproducible, and that it can aid in perioperative decision-making. Given what is known about TTE values, their correlation with TEE, and that less negative values indicate worsening myocardial function, consider a patient undergoing mitral valve repair for severe mitral regurgitation with normalappearing systolic function preoperatively. A low intraoperative GLS may push the anesthesiologist or intensivist to initiate inotropic support earlier than they may otherwise have given the possibility of subclinical dysfunction. Similarly, borderline RV function or elevated pulmonary artery pressures with low RV FWS or GLS may push one to initiate inotropic or inodilator support earlier. Again, the application of

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perioperative strain analysis is still in its infancy, but at this point it may be an additional tool for perioperative physicians to employ, as long as its limitations are recognized. Conflict of Interest

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The authors have no conflicts of interest. Supplementary materials

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Supplementary material associated with this article can be found in the online version at doi:10.1053/j.jvca.2019.11.035.

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