An electrocardiogram with clearly defined P waves and QRS complexes is required for the accurate timing of the cardiac cycle. Closure of the aortic valve, which defines end-systole and thus the temporal relationship of deformation measures, can be determined by direct visualization from 2D echocardiographic images, spectral Doppler, or M-mode imaging of the aortic valve. If aortic valve closure is difficult to identify, some analysis programs automatically identify end-systole by calculating the average time to peak strain within each myocardial segment.
The technique for longitudinal deformation analysis is preset in some vendor-specific software, where strain analysis begins with placement of markers in a still frame of the midesophageal long-axis echocardiographic image. These markers are placed on the endocardial border adjacent to the mitral and aortic valve annulus, with another marker at the LV apex. The position of these markers is processed creating a colored pattern overlaying the LV in the “region of interest” that “tracks” myocardial motion throughout the cardiac cycle. The user visually reviews the tracking of the myocardium and, if acceptable, confirms that this pattern follows myocardial contraction accurately. If the tracking pattern does not adequately follow the endocardial border (Video 3, see Supplemental Digital Content 3, http://links.lww.com/AA/A698), endocardial markers may need repositioning. If a segment of the LV consistently tracks poorly as described above, an untrackable segment is best excluded to avoid contamination of global calculations. The user can exclude segments simply by not confirming acceptable tracking. Most vendor-specific software will still calculate global strain if one of 6 segments is not tracked.
Once users confirm acceptable tracking of myocardium, the analysis program calculates frame-to-frame displacement of the speckle pattern throughout the cardiac cycle and displays segmental and global longitudinal strain. After strain analysis is completed in the 3 chosen views, some software programs incorporate deformation parameters into a “bull’s-eye” view to provide an overall picture of global and regional LV function by using numerical and color-coded parameters (Fig. 10, lower right panel). Most strain analysis software, however, is oriented toward the transthoracic echocardiography perspective.
For research purposes, a more comprehensive analysis can be performed off-line by using advanced research software programs such as EchoPAC (GE Healthcare) or QLab (Philips Ultrasound), which provide detailed information about strain parameters and substantial opportunity to adjust preferences and data collection procedures. Another software package called Velocity Vector Imaging (VVI, Syngo Velocity Vector Imaging technology, Siemens Medical Solutions, Mountain View, CA) uses a novel method of feature-tracking that incorporates speckle-tracking with tracking of the endocardial contour; it can be used with any ultrasound image in standard Digital Imaging and Communications in Medicine (DICOM) format and can thus be used on recordings from various vendors. Additional software options are detailed in Table 3.
Normal reference values for longitudinal, circumferential, and radial strain are shown in Table 4. It is important to note that these “normal” reference values were measured by transthoracic echocardiography in healthy subjects who were awake and breathing spontaneously.18,31 Thus, these values may vary from patients who are anesthetized and whose lungs are mechanically ventilated and evaluated with TEE. Certainly, the choice of transthoracic versus transesophageal approach may affect strain measures.32,33 And since general anesthesia affects myocardial function,34,35 strain measured in anesthetized patients may differ from norm reference values. Furthermore, intraoperative events such as pericardial opening impact ventricular function and hemodynamic measures36,37 and may affect intraoperative strain. For these reasons, strain measured in the operating room may differ from published normal reference values. Unfortunately, reference values specific for anesthetized patients measured by TEE are unavailable. Thus, current options are limited to reference values acquired from transthoracic echocardiography.
Technical factors affect strain measures, including whether Doppler versus non-Doppler methods or which data postprocessing techniques or software analysis packages are used.38 Software options for strain analysis and a comparison of these software options are shown in Tables 3 and 5. It is interesting to note that despite presumed differences with software analysis and techniques, 1 recent meta-analysis reported that strain measurements were not affected by the choice of analysis package, although there may have been insufficient heterogeneity of echocardiographic equipment to thoroughly evaluate this variable.39 It is important to note that when strain is measured with TEE by using the same analysis software under similar conditions, results are highly reproducible in the operating room.33 A comparison of myocardial function and strain calculations in patients with normal, moderately decreased, and severely decreased myocardial function is shown in Video 4, see Supplemental Digital Content 4, http://links.lww.com/AA/A699.
Because myocardial deformation reflects the interaction between myocardial loading conditions and contractility, changes in loading conditions influence myocardial deformation. SR is highly correlated with LV end-systolic pressure-volume relationship and the rate of rise of LV pressure (dP/dt) and thus is a robust noninvasive measure of LV contractility.40–43 However, dP/dt and other measures of myocardial contractility are subject to changes in contractile state, preload, and afterload44; thus, strain and SR may also be affected.
Changes in loading conditions affect all components of myocardial deformation. In animals, longitudinal strain and SR were reduced when afterload was increased, whereas increased preload increased strain and SR by the Frank Starling mechanism.45 Radial and circumferential strain are sensitive to changes in afterload, while SR is a more robust measure of contractility, because it is less influenced by alterations in preload and afterload.46 Strain is inversely related to heart rate in some42 but not all animal models.47 Heart rate has less effect on Doppler SR than strain.42 Because acute changes in load occur during surgery, serial echocardiographic examinations performed intraoperatively should take changes in heart rate, preload, and afterload into consideration.
Deformation is useful for assessment of right ventricular (RV) function. Because afterload is lower and compliance is higher in the RV, RV velocities are consistently greater than the LV. Because longitudinal shortening provides the largest contribution to RV performance, RV function can be largely assessed by using longitudinal strain and SR. Global RV strain and SR in healthy subjects are −29.5% ± 5.5% and −2.1 ± 0.4 s−1.48 An RV ejection fraction of ≥50% is typically accompanied by systolic strain at the basal RV free wall of −25%, and a SR of −4 s−1 measured with TDI strain.49
Subclinical RV dysfunction can be identified by strain analysis. After mitral valve surgery, for example, reduced RV longitudinal strain is evident in patients with normal 3D RV ejection fractions.50 Furthermore, asymptomatic patients with diabetes demonstrate subclinical RV dysfunction with reduced RV systolic strain, SR, and early diastolic SR.51 RV strain and SR abnormalities are also seen with amyloidosis, congenital heart disease, and arrhythmogenic RV cardiomyopathy. In patients with chronic heart failure, RV strain <−21% is associated with acute heart failure and death.48 Pulmonary hypertension significantly reduces RV strain while also impacting LV strain and torsion.52
Speckle-tracking provides a noninvasive alternative to sonomicrometry and tagged MRI for evaluation of the complex 3D contractile motion of the LV, dictated by the spiral structure of the myocardial fibers. The subendocardium consists of myocardial fibers oriented in a right-handed helix evolving gradually into a left-handed helix in the subepicardium.10–12 Subendocardial fibers are nearly longitudinally oriented (an angle of approximately 80° with respect to the circumferential direction of the heart); the midmyocardial fibers are parallel to the circumferential direction (at approximately 0°), and subepicardial fibers are at −60°.10–12 This ventricular structure enables a twisting or “wringing” motion during systole (Fig. 11).
During isovolumic contraction, the apex briefly rotates in a clockwise direction but quickly reverses into a counterclockwise direction during ejection when viewed from the apex. Concurrently, the base rotates in a clockwise direction around the LV long-axis.53 This twisting motion of the LV causes thickening and longitudinal shortening of the myocardium, while concurrent circumferential shortening causes LV ejection. Untwist, the subsequent recoil of twist, occurs during diastole when restoring forces are released, causing diastolic suction and facilitation of early LV filling. Most untwisting occurs during isovolumic relaxation and is completed during early diastole.54 The terms, LV rotation, twist, and torsion, describe the complex 3D myocardial motion and are sometimes used interchangeably. For the purpose of this discussion, LV rotation measures degrees of rotation viewed from the apex,53 and LV twist is calculated as rotation of the apex relative to the base or, in other words, the absolute apex-to-base difference in rotation is measured in degrees55 (Table 1). Torsion refers to the apex-to-base gradient in the rotation angle of the LV long axis: the apex-to-base twist angle is divided by the distance between measured locations of the base and apex and is thus calculated in degrees per centimeter.11,12 Both TDI56 and speckle-tracking echocardiography57 allow calculation of twist and torsion from LV short-axis views.
Normal value for twist in healthy volunteers is 7.7° ± 3.5°. These values increase with age, likely because of less opposition to apical rotation.11,12 Thus, LV twist is higher in healthy subjects older than 60 years of age compared with those younger than 40 years old (10.8° ± 4.9° vs 6.7° ± 2.9°, respectively).55 Apical wall motion abnormalities, however, significantly impair LV twist. Delay of LV untwisting may partially explain diastolic dysfunction in patients with LV hypertrophy54,58 and age-related diastolic abnormalities.55 Torsion, in contrast, is normally about 3° and does not change significantly with age.59 LV twist and untwist have a profound impact on LV systolic and diastolic mechanics and may allow detection of systolic and diastolic abnormalities in surgical patients; however, perioperative application of this technique requires further investigation.
Myocardial deformation analysis objectively quantifies alterations in LV function; thus, subtle changes in myocardial function during the perioperative period can be detected with strain analysis. Myocardial deformation analysis can differentiate between regional dysfunction, such as coronary artery occlusion, and global myocardial dysfunction, such as ischemia-reperfusion injury. Because systolic wall motion abnormalities occur within seconds of coronary occlusion,60 alterations in deformation appear quickly after onset of ischemia in affected myocardial segments. Thus, new regional wall motion abnormalities caused by an acute coronary bypass graft occlusion can be identified by an acute reduction in strain in the affected coronary artery territory. In addition, strain measured with speckle-tracking echocardiography can distinguish between true myocardial contraction and passive myocardial segmental motion in patients suspected of having a myocardial infarction. An example of an acute transmural myocardial infarction causing severe hypokinesis and akinesia in affected myocardial segments is shown in Fig. 12.
In contrast to regional alterations in strain caused by coronary artery occlusion, ischemia and reperfusion injury or myocardial stunning demonstrate global alterations in myocardial deformation, which involve reductions in myocardial strain in multiple myocardial segments without directly corresponding to a specific coronary artery territory. This distinction between regional and global alterations in myocardial deformation is important, because regional ischemia may prompt antiischemic or thrombolytic therapy or possibly revascularization, whereas global myocardial dysfunction occurring with hypotension or hemodynamic instability suggests a need for generalized hemodynamic support. Strain measurements may thus guide anesthetic management and determine therapeutic intervention.
Myocardial deformation imaging can detect subtle myocardial pathology, and small decrements in myocardial performance before overt disease is apparent. In the following sections, alterations in LV mechanics that characterize various cardiac pathologies are discussed (Table 6).
Patients with coronary artery disease demonstrate abnormal myocardial deformation. Attenuated longitudinal strain measurements provide an early indication of subendocardial ischemia,61–63 though nonischemic myocardial regions may compensate for impaired systolic function with increased shortening.60,64 In patients with coronary artery disease, longitudinal segmental strain cutoffs of −14.1% and −6.65% detected ischemic and infarcted myocardial segments, respectively.65 Patients with recent anterior wall myocardial infarction demonstrate reduced radial and longitudinal strain, while greater reduction in circumferential strain is seen if LVEF is reduced.65,66
Deformation analysis may demonstrate distinct findings suggestive of asynchronous myocardial contraction during ischemia. For example, postsystolic shortening, characterized by the occurrence of peak strain after end-systole, is highly sensitive, although nonspecific, for ischemia (Fig. 13).16,60,67–70 Prestretch, demonstrated by early systolic lengthening before later systolic shortening, may occur with regional ischemia16 though prestretch may be a normal finding related to slight conduction delays.11,12 Since visual recognition of asynchronous myocardial contraction is unreliable,71 examination of strain curves may permit early detection of asynchronicity in the operating room.
Ischemic heart disease affects other variables of myocardial deformation. For example, patients with myocardial ischemia have reduced longitudinal and circumferential SR at rest and during dobutamine stress echocardiography.65,72 Peak twist and untwist decreases after anterior wall myocardial infarction corresponding to the severity of LV dysfunction.66 When LV deformation and twist mechanics are significantly affected by ischemia, myocardial performance worsens.
Because a delay in LV relaxation often precedes systolic wall motion abnormalities,73 indices of diastolic function may provide earlier indications of ischemia.64 For example, early diastolic SR was significantly reduced in ischemic myocardial segments.62 And substantial delays in early LV relaxation during exercise in patients with stable effort angina, measured as a radial strain diastolic index, provided a sensitive method for detection of myocardial ischemia.74
Patients with valvular disease experience ventricular remodeling as a consequence of chronic volume or pressure overload where structural and histopathologic changes to the myocardium lead to a progressive decline in LV function. If prolonged, LV remodeling becomes irreversible. Altered strain patterns can identify subclinical decrements in LV function, which may improve timing for surgical intervention before irreversible myocardial dysfunction occurs.75,76
LVEF is often preserved or increased in patients with mitral regurgitation because of compensatory changes in preload and afterload. However, despite normal LVEF, asymptomatic patients with mitral regurgitation often demonstrate reduced longitudinal and radial strain.77 Longitudinal SR may be more attenuated than circumferential or radial SR.78 Furthermore, LV untwisting is delayed in patients with mitral regurgitation and may provide early signs of LV dysfunction.79
Aortic regurgitation induces LV remodeling and a significant increase in LV end-diastolic volume, which can mask onset of clinical LV dysfunction. However, strain analysis detects reduced longitudinal strain in young athletes with bicuspid aortic valves and mild aortic insufficiency,83 though others reported normal mechanics in patients with aortic regurgitation.84 Outer circumferential and radial strain may actually increase in early stages to preserve LVEF and compensate for reduced inner circumferential and radial strain,85 but in later stages of disease, radial and longitudinal function decline.86 Longitudinal and radial peak systolic SR are reduced with advanced aortic regurgitation and are inversely correlated with LV end-systolic and end-diastolic volumes.86
Preoperative detection of abnormal strain in patients with aortic regurgitation may improve timing of surgical intervention, resulting in improved myocardial function and postoperative outcomes.76 Reduced preoperative systolic myocardial strain increases risk of heart failure, dilated LV, and impaired LV function after aortic valve replacement surgery.87 Decreased preoperative radial SR <1.82 s-1 was highly sensitive and specific for detecting postoperative LVEF <50%.88 Thus, detection of abnormal LV mechanics may improve preoperative risk stratification and provide an earlier opportunity for clinical intervention in attempts to improve postoperative outcomes.
Aortic stenosis results in progressive LV hypertrophy in response to chronically increased afterload, but LVEF is preserved until late stages of disease. LV systolic longitudinal strain and SR may nonetheless be attenuated early because of interstitial fibrosis, and the presence of abnormal strain may predict worse outcomes. Patients with asymptomatic aortic stenosis demonstrate impaired global longitudinal strain, especially in basal segments, and basal longitudinal strain worse than −13% increased risk of rehospitalization, aortic valve surgery, and death.89 With progression of aortic stenosis and interstitial myocardial fibrosis, deformation analysis demonstrates reduced longitudinal, circumferential, and radial strain, with reduced SR.90,91 Twist mechanics in patients with aortic stenosis are also affected, resulting in increased apical rotation and greater LV torsion, perhaps in compensation for increased intracavitary pressure.92 Fortunately, strain improves in all dimensions after aortic valve replacement.90,93
Myocardial deformation is abnormal in hypertensive heart disease because of chronically increased afterload, LV hypertrophy, and progressive myocardial fibrosis. Longitudinal strain is reduced with hypertensive heart disease,94–96 which correlates with markers of myocardial collagen turnover and interstitial fibrosis.97 Circumferential strain, however, remains normal or even increases in early stages.94–96 However, when concentric LV hypertrophy develops, strain and SR decrease in all directions.96,98,99 Diastolic dysfunction is evident with LV hypertrophy by reduced early diastolic peak relaxation rate.94 Twist or torsion may decrease but occasionally experiences a compensatory increase.95,97,98 Thus, subclinical abnormalities are evident with myocardial deformation analysis.
Hypertrophic cardiomyopathy is characterized by myocardial fiber disarray and eventual LV systolic and diastolic dysfunction. Measurement of myocardial defor mation in patients with early disease detects global subclinical systolic dysfunction with reduced longitudinal, radial, and circumferential strain corresponding to the degree of ventricular fibrosis100,101 and associated with functional status.102 However, paradoxical systolic lengthening sometimes occurs.103 Longitudinal systolic and diastolic SR are also reduced compared with subjects without disease.104 In contrast, overall LV twist remains near normal, although subtle rotation abnormalities may be present.102 LV untwisting is slowed, and the increase in LV untwisting rate associated with exercise is blunted.54
Dilated cardiomyopathy demonstrates diminished systolic strain and SR in all directions.94,105,106 Interestingly, global longitudinal strain is a better predictor of arrhythmic events than LVEF in cardiomyopathy patients.107 Torsional and diastolic deformation variables including peak relaxation rate are also reduced,94,105 and LV rotation is abnormal.105
Patients with heart failure and preserved ejection fraction, termed diastolic heart failure, have abnormalities of both systolic and diastolic function at rest that worsen with exercise as measured by stress echocardiography.108 Despite the fact that patients with early diastolic heart failure may maintain normal LVEF, they typically have attenuated longitudinal systolic strain compensated by preserved LV twist and circumferential strain.109 A decrease in circumferential strain reflects more advanced disease and is associated with worse outcomes. Indeed, abnormal global circumferential strain predicted rehospitalization and cardiac death.9 Systolic heart failure demonstrates reduced circumferential strain and LV twist109 consistent with late impairment of LV function.110 Abnormal longitudinal strain predicts mortality in patients with heart failure more accurately than LVEF,7 and peak LV twist and untwisting rate are decreased in patients with heart failure.111
The use of myocardial deformation analysis has clinically important perioperative value. Because TDI strain is compromised by its angle-dependence, a significant limitation especially when using the transesophageal approach, strain measured with speckle-tracking echocardiography has greater potential for intraoperative use. Strain measured with speckle-tracking echocardiography is angle-independent and can measure 2 axes simultaneously. Though some software analyses programs loaded on the echocardiographic workstation currently provide only assessment of longitudinal deformation, more options for measurement of radial and circumferential strain may become available in the future.
Speckle-tracking echocardiography is currently limited by the lack of standardization among vendors.112–114 Fortunately, there is currently a joint effort among the American Society of Echocardiography, European Asso ciation of Cardiovascular Imaging and industry to standardize methodology for speckle-tracking echocardiography. Standardization would allow clinicians to comparably interpret results generated by equipment from various vendors.
Three-dimensional speckle-tracking technology has been introduced for transthoracic echocardiography115 but is not yet available for TEE. This technology remains limited by a low frame rate and poor temporal resolution. It is likely, though, that 3D speckle-tracking will bypass limitations of out-of-plane motion inherent in 2D imaging. Especially at acceptable frame rates of 18 or 25 frames/s, 3D strain analysis appears to adequately estimate myocardial strain.116 Three-dimensional speckle-tracking may provide an opportunity to evaluate motion of all myocardial segments in a single analysis step, thereby significantly reducing analysis time.117
In summary, measurement of myocardial deformation provides important quantitative information on global and regional myocardial function. It is thus likely that echocardiographic evaluation of strain and SR will increasingly be incorporated into clinical practice. That said, the technique is relatively new, and more research will be required to identify the diagnostic accuracy of different strain and SR variables and their reproducibility in various disease states. Future studies will also determine the extent to which strain and SR measurements can enhance patient management and improve postoperative outcomes.
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