Volume 26, Issue 1 , Pages 26-31, February 2012
Comparison of Transthoracic and Transesophageal 2-Dimensional Speckle Tracking Echocardiography
Article Outline
- Abstract
- Materials and Methods
- Results
- Discussion
- Study Limitations
- Clinical Implications
- Conclusion
- References
- Copyright
Objectives
Two-dimensional (2D) strain imaging has been established as a reliable and reproducible technique for the assessment of left and right ventricular function using transthoracic echocardiography (TTE). However, the reproducibility of transesophageal echocardiographic (TEE) 2D strain imaging and the agreement with TTE 2D strain imaging remains unclear. In the present study, the authors studied the reproducibility of TEE 2D strain imaging parameters.
Design
A comparative, observational clinical study.
Setting
The echocardiography laboratory of the tertiary referral center.
Participants
Healthy individuals with a suspected patent foramen ovale.
Interventions
None.
Measurements and Main Results
Thirty-four patients were included in the study. None of the patients had any structural cardiovascular disease. TTE and TEE images of the subjects were recorded and analyzed offline (EchoPAC 6.1; GE Vingmed Ultrasound AS, Horten, Norway). Longitudinal strain and strain rate measurements of the 4 chambers, the apical long axis, 2 chambers, and the right ventricle were obtained for each record of TTE and TEE. The mean age of the patients in this study was 36 ± 9.2 years. Bland-Altman analysis showed that there were generally good agreements between strain and strain rate measurements on TEE and TTE. The inter- and intraobserver agreement for TEE parameters was good.
Conclusions
Transesophageal 2D strain imaging is a reproducible method to measure ventricular function and has a good agreement with TTE 2D strain imaging.
Key Words: transthoracic echocardiography , transesophageal echocardiography , 2-dimensional strain imaging , speckle tracking
IN RECENT YEARS, transesophageal echocardiography (TEE) became an important diagnostic tool in cardiac anesthesia.1, 2, 3, 4 It is essential to know the perioperative quantitative ventricular function in the management of high-risk patients. The left and right ventricular functions are evaluated conventionally by transthoracic echocardiography (TTE) such as M-mode, tissue Doppler imaging (TDI), and Doppler-based strain.5, 6 Transthoracic echocardiographic 2-dimensional (2D) strain imaging is a new robust method for the assessment of left and right ventricular function compared with Doppler-based strain and TDI.7, 8 There are a great number of articles in the literature describing the clinical application of 2D strain imaging on TTE.9, 10, 11 However, there are very limited studies related to 2D strain imaging on TEE for the evaluation of ventricular function.12, 13, 14 The purpose of this study was to test the reproducibility of TEE 2D strain imaging and the agreement of TEE 2D strain imaging with TTE 2D strain imaging.
Materials and Methods
The patients with atrial fibrillation, previous ischemic heart disease, left ventricular hypertrophy, left bundle-branch block, pericardial disease, poor image quality, significant foreshortening, and an inability to give consent were excluded from the study. The study population consisted of 34 healthy individuals (16 women and 18 men). TEE echocardiography was indicated for patients included in the study for the evaluation of suspected patent foramen ovale. The study protocol was approved by the institutional review board, and patients provided a written informed consent.
Echocardiographic examination was performed by using a GE Vivid 7 system (GE Vingmed Ultrasound, Horten, Norway) with a phased-array 3.5-MHz probe for TTE and a 5.0-MHz probe for TEE. Transthoracic echocardiographic data acquisition was performed at a depth of 16 cm in the parasternal and apical views (standard parasternal short-axis from the midventricular level, apical long-axis, 2-chamber, and 4-chamber images). The left ventricular end-diastolic (LVEDD) and left ventricular end-systolic diameters (LVESD) were measured in the parasternal long-axis view, and left ventricular ejection fraction was assessed by apical 4- and 2-chamber views (biplane) with the modified Simpson rule. TEE was performed after a 4-hour fasting period on all the patients just after TTE. Ten percent lidocaine spray was used for posterior pharyngeal anesthesia. The TEE probe was inserted with the subject lying in the left lateral position. The procedure was performed with continuous monitoring of heart rate, blood pressure, and a 1-lead electrocardiogram. Machine settings were optimized to obtain the highest frame rate. A TEE multiplane 5-MHz probe was introduced into the esophagus to obtain midesophageal 4-chamber (4C, 0°-20°), midesophageal 2-chamber (2C, 70°-90°), and midesophageal long-axis (110°-130°) images. All the recordings (both TTE and TEE) were made during apnea at end-expiration simultaneously with electrocardiography. Transesophageal echocardiographic measurements were analyzed by a 2nd observer who was blinded to the results of the TTE data. Atropine and sedation were not used. The frame rate for both TTE and TEE was between 50 and 90 frames per second, and 3 cardiac cycles were stored in the cine-loop format. All data were transferred to a workstation for further offline analysis (EchoPAC 6.1, GE Vingmed Ultrasound).
Strain is a measure of deformation that might be defined by lengthening or shortening. Strain and strain rate imaging provide the measurement of regional myocardial deformation. Although strain measures deformation, the strain rate measures the rate of deformation.15 Strain and strain rate measures have been shown to provide complementary information about the clinical assessment of cardiac function.9, 16 Strain and strain rate can be calculated by several methods such as TDI, Doppler strain, and non-Doppler 2D strain imaging.
Strain (S) is represented as follows: strain (S): (l-lo)/lo: Δl/lo, where l is the instantaneous length, lo is the initial length, and Δl is the change in length. The strain rate can then be represented as follows: strain rate: S/Δt, where S is the strain and t the time. The unit of strain rate is s−1.
The formula given earlier is called Lagrangian strain. In a human heart, the unstressed original length is difficult to measure, so end-diastolic length is most often used. However, the deformation also can be expressed not only relative to the original length but also relative to the length at a previous moment in time (dt). In this definition, the reference value is not constant over time but changes during the deformation process, and the strain is called instantaneous Eulerian strain.10
The reflected ultrasound beam from the tissue is the result of interference by numerous reflected wavelets from the heterogenous region. The interference pattern (resulting in bright and dark pixels in a B-mode image) remains relatively constant for any small region in the myocardium. This unique pattern (fingerprint) is called speckle. In the speckle tracking technique, a defined region (kernel) is tracked, following a search algorithm based on the optical flow method, trying to recognize the most similar speckle pattern from 1 frame to another.10 The algorithm searches for an area with the smallest difference in the total sum of pixel values, which is the smallest sum of absolute differences. The technique is angle independent because it is based on the displacement of speckles, defined in respect to the wall rather than the ultrasound beam, as Doppler techniques do, and has been validated by ultrasonomicrometry.11
For 2D strain-imaging analysis, standard grayscale 2D images were acquired in the 4C, apical long-axis, and 2C views as well as the right ventricle for both TTE and TEE. All of the images were recorded with a frame rate of between 50 and 90 frames per second to allow for reliable operation of the software (EchoPac 6.1).17 A region of interest was traced on the endocardial cavity interface by a point-and-click approach from an end-systolic single frame. After that, an automated tracking algorithm followed the endocardium from this single frame throughout the cardiac cycle. Further adjustment of the region of interest was performed to ensure that all of the myocardial regions were included. Next, speckles, equally distributed in the region of interest, could be followed throughout the entire cardiac cycle.18 Results were reported as the peak longitudinal strain during systole for each view, and the global longitudinal strain was calculated by averaging the 3 apical views (Fig 1). The following parameters were analyzed in each view: the peak longitudinal systolic strain rate (Sr-sm), the early diastolic strain rate, and the late diastolic strain rate (Sr-am) (Fig 2).
Fig 1.
Transesophageal echocardiographic 2D peak longitudinal strain images for (A) the right ventricle, (B) the apical 4 chamber, (C) the apical 2 chamber, and (D) the apical long axis.
Fig 2.
Transesophageal echocardiographic 2D strain rate images for the right ventricle.
The data are expressed as mean ± standard deviation. The data obtained from TEE and TTE were compared. To compare continuous variable, the Student t test or the Mann-Whitney U test was used. Bland-Altman analysis and the intraclass correlation coefficient were used to compare agreement and reproducibility between the 2 measurement techniques (Table 1).19 The first 20 patients were reanalyzed for interobserver agreement and after 4 weeks for intraobserver agreement.
Table 1. Quantitative Classifications of Intraclass Correlation Coefficient (ICC) as a Degree of Agreement
| ICC Value | Degree of Agreement |
|---|---|
| 0 | None |
| 0.00 < ICC value ≤ 0.40 | Poor |
| Fair | |
| 0.40 < ICC value ≤ 0.75 | Moderate |
| 0.40 < ICC value ≤ 0.75 | Substantial |
| 0.75 < ICC value ≤ 1.00 | Almost perfect |
Results
Table 2 shows the basal clinical parameters of the patients. The mean age of patients included in the study was 36 ± 9.2 years; 18 were men and 16 were women. Strain and strain rate measurements for TTE and TEE are given in Table 3. There were significant differences for apical 4C-S, apical 4C-Sr-sm, and 4C-Sr-am between TEE and TTE strain values (Table 3). The systematic error was only documented in the intertest (between TTE and TEE) agreement analysis of apical 4C-S, 4C-Sr-sm, and 4C-Sr-am. A high mean difference was assessed with apical 4C-S and apical 4C-Sr when Bland-Altman analyses were examined. These data show that it has lower agreement in comparison to those other parameters (Fig 3 and Table 4). A total of 578 segments were analyzed (17 segments for each patient). A total of 8% of segments were excluded from the study because of no analysis, either manually and/or automatically. The most common segments excluded from the analysis were basal inferior, basal lateral, and apical segments. Moreover, intraobserver and interobserver TTE values had low mean difference and no systematic error. The reproducibility and agreement values are shown in Table 5, Table 6.
Table 2. Basic Characteristics of Patients
| Age (y, mean ± standard deviation) | 36±9.2 |
| Male (%) | 52.9 |
| Smoking (%) | 29.4 |
| Body mass index (kg/m2, mean ± standard deviation) | 24.8±3.5 |
| Body surface area (m2, mean ± standard deviation) | 1.78±0.14 |
| SBP (mmHg, mean ± standard deviation) | 117±23 |
| DBP (mmHg, mean ± standard deviation) | 73±12 |
| HR (per minute, mean ± standard deviation) | 80±12 |
| Simpson EF (%, mean ± standard deviation) | 63±2.9 |
Table 3. Strain and Strain Rate Values for TTE and TEE
| TTE | TEE | p Value | |
|---|---|---|---|
| 4C-S (%) | −19.6±2.52 | −22.2±2.40 | 0.001⁎ |
| LAX-S (%) | −22.2±3.61 | −21.8±2.55 | 0.43 |
| 2C-S (%) | −21.5±2.77 | −22.5±2.39 | 0.17 |
| GS (%) | −21.1±2.32 | −22.2±1.65 | 0.06 |
| RV-S (%) | −22.2±2.90 | −23.0±2.96 | 0.21 |
| 4C-Sr-sm (1/s) | −1.15±0.15 | −1.44±0.36 | 0.001⁎ |
| 4C-Sr-em (1/s) | 1.33±0.48 | 1.61±0.68 | 0.26 |
| 4C-Sr-am (1/s) | 0.92±0.39 | 1.22±0.42 | 0.005⁎ |
| LAX-Sr-sm (1/s) | −1.32±0.24 | −1.40±0.22 | 0.35 |
| LAX-Sr-em (1/s) | 1.35±0.67 | 1.21±0.57 | 0.61 |
| LAX-Sr-am (1/s) | 1.12±0.40 | 1.39±0.66 | 0.06 |
| 2C-Sr-sm (1/s) | −1.28±0.25 | −1.31±0.25 | 0.18 |
| 2C-Sr-em (1/s) | 1.32±0.36 | 1.34±0.32 | 0.55 |
| 2C-Sr-am (1/s) | 0.92±0.33 | 1.08±0.43 | 0.63 |
| RV-Sr-sm (1/s) | −1.37±0.35 | −1.49±0.36 | 0.21 |
| RV-Sr-em (1/s) | 1.42±0.44 | 1.40±0.46 | 0.17 |
| RV-Sr-am (1/s) | 1.14±0.25 | 1.30±0.32 | 0.61 |
⁎ Significantly different (p < 0.05) between TTE and TEE. |
Fig 3.
A Bland-Altman plot showing the degree of agreement between transthoracic echocardiographic and transesophageal echocardiographic values of (A) the apical 2 chamber, (B) the apical 4 chamber, (C) the apical long axis, (D) the left ventricular global strain, and (E) the right ventricle. (Color version of figure is available online.)
Table 4. Bland-Altman Analysis for Transthoracic Echocardiographic and Transesophageal Echocardiographic Parameters
| Variables | Mean Differences | 95% Limits of Agreement |
|---|---|---|
| TTE-GS and TEE-GS | −1.1 | −5to2.9 |
| TTE-4C-S and TEE-4C-S | 2.6 | −3.9to9 |
| TTE-LAX-S and TEE-LAX-S | −0.5 | −5.5to6.3 |
| TTE-2C-S and TEE-2C-S | 1 | −3.9to6 |
| TTE-RV-S and TEE-RV-S | 0.8 | −6.1to7.7 |
| TTE-4C-Sr-sm and TEE-4C-Sr-sm | 0.29 | −0.51to1.09 |
| TTE-4C-Sr-em and TEE-4C-Sr-em | −0.28 | −1.45to0.89 |
| TTE-4C-Sr-am and TEE-4C-Sr-am | −0.30 | −1.27to0.67 |
| TTE- LAX-Sr-sm and TEE- LAX-Sr-sm | 0.08 | −0.40to0.57 |
| TTE- LAX-Sr-em and TEE- LAX-Sr-em | 0.13 | −0.98to1.24 |
| TTE- LAX-Sr-am and TEE- LAX-Sr-am | −0.27 | −1.59to1.05 |
| TTE-2C-Sr-sm and TEE-2C-Sr-sm | 0.03 | −0.37to0.42 |
| TTE-2C-Sr-em and TEE-2C-Sr-em | −0.03 | −1to0.94 |
| TTE-2C-Sr-am and TEE-2C-Sr-am | −0.16 | −0.76to0.45 |
| TTE- RV-Sr-sm and TEE- RV-Sr-sm | 0.12 | −0.52to0.77 |
| TTE- RV-Sr-em and TEE- RV-Sr-em | 0.02 | −0.84to0.88 |
| TTE- RV-Sr-am and TEE- RV-Sr-am | −0.15 | −0.93to0.62 |
Table 5. Interobserver Variability for Transesophageal Echocardiographic 2D Strain Measurements
| Mean Difference | Limit of Agreement %95 | |
|---|---|---|
| 4C-S | −0.01 | −2.4to2.3 |
| LAX-S | −0.2 | −2.3to1.9 |
| 2C-S | −0.3 | −2.3to1.8 |
| RV-S | −1.2 | −3.5to1.1 |
| 4C-Sr-sm | −0.06 | −0.37to0.25 |
| 4C-Sr-em | 0.13 | −0.43to0.69 |
| 4C-Sr-am | 0.05 | −0.34to0.43 |
| LAX-Sr-sm | −0.03 | −0.25to0.20 |
| LAX-Sr-em | 0.05 | −0.36to0.47 |
| LAX-Sr-am | −0.05 | −0.66to0.57 |
| 2C-Sr-sm | −0.04 | −0.30to0.22 |
| 2C-Sr-em | 0.12 | −0,18to0.42 |
| 2C-Sr-am | 0.07 | −0.35to0.45 |
| RV-Sr-sm | −0.22 | −0.51to0.06 |
| RV-Sr-em | 0.03 | −0.42to0.48 |
| RV-Sr-am | 0.09 | −0.26to0.44 |
Table 6. Intraobserver Variability for Transesophageal Echocardiographic 2D Strain Measurements
| Mean Difference | Limit of Agreement %95 | Intraclass Correlation Coefficient | |
|---|---|---|---|
| 4C-S | −0.0 | −1.8to1.7 | 0.95(0.88-0.95) |
| LAX-S | −0.3 | −2.3to1.6 | 0.92(0.82-0.97) |
| 2C-S | 0.36 | −1.1to1.82 | 0.96(0.90-0.98) |
| RV-S | −0.8 | −2.7to1.1 | 0.94(0.87-0.97) |
| 4C-Sr-sm | −0.06 | −0.36to0.23 | 0.92(0.81-0.96) |
| 4C-Sr-em | 0.07 | −0.35to0.48 | 0.95(0.89-0.98) |
| 4C-Sr-am | 0.07 | −0.19to0.34 | 0.94(0.87-0.97) |
| LAX-Sr-sm | −0.04 | −0.20to0.12 | 0.92(0.81-0.96) |
| LAX-Sr-em | −0.02 | −0.24to0.27 | 0.97(0.93-0.97) |
| LAX-Sr-am | −0.03 | −0.57to0.50 | 0.92(0.82-0.97) |
| 2C-Sr-sm | −0.06 | −0.24to0.11 | 0.93(0.85-0.97) |
| 2C-Sr-em | −0.10 | −0.16to0.36 | 0.92(0.80-0.96) |
| 2C-Sr-am | 0.09 | −0.30to0.48 | 0.90(0.77-0.96) |
| RV-Sr-sm | −0.12 | −0.32to0.07 | 0.95(0.88-0.98) |
| RV-Sr-em | 0.06 | −0.20to0.32 | 0.94(0.87-0.97) |
| RV-Sr-am | 0.08 | −0.15to0.30 | 0.95(−0.88-0.98) |
Discussion
In this study, 2D strain and strain rate values of right and left ventricular function with TTE and TEE were analyzed in 34 healthy individuals. The authors showed that TEE strain imaging has a good reproducibility and good agreement with TTE 2D strain imaging. However, agreement for the measurements of apical 4C-S and 4C-Sr were lower than the other measurements. As a result, the authors proposed that TEE 2D strain imaging could be used in the perioperative period.
Perioperative TEE provides critical information in patients undergoing cardiac surgery such as coronary artery or valve surgery as well as noncardiac surgery. The assessment of perioperative quantitative myocardial function is especially important in the management of high-risk patients.1, 2, 4, 20, 21 In recent years, 2D strain imaging increasingly is being used in the assessment of global ventricular function.7, 8, 9, 18, 22 Because it is angle independent, reproducible, and able to provide quantitative data, the utility of 2D strain imaging in the evaluation of ventricular function is further increased. Although transthoracic 2D strain imaging has been studied extensively in the assessment of the ventricular function, transesophageal 2D strain applications are very limited.12, 13, 14 This study is somewhat different from the TEE 2D strain imaging studies. Maclaren et al14 showed that TDI of radial cardiac motion appears to be the most feasible technique of measuring myocardial velocity, strain, and strain rate during cardiac surgery. Their study included 19 patients who underwent coronary artery bypass graft (CABG) surgery, and both TEE 2D strain and TEE TDI values were recorded. This study is important in respect to the feasibility of transesophageal 2D strain imaging.14 In another study, Kukucka et al13 speculated that strain calculation from TEE images were feasible. Their study also included patients undergoing CABG surgery, and only TEE measurements were studied. Tousignant et al12 performed a study in which 21 patients underwent CABG surgery, and TTE and TEE values were obtained just for the right ventricle. In this study, the global right ventricular strain value was similar using both methods (20.1% v 20.4%). Similar results were obtained in the present study as well (22.3% v 23.2%).12 From this point of view, the inclusion of healthy individuals and the recording of both ventricles make the present study different from the others. In the present study, the authors found notable mean differences in 4CS, 4CSr-sm, and 4CSr-am between the TTE and TEE measurements (lower in TTE). This is likely caused by apical foreshortening in the apical 4- and 2-chamber views. The authors did not encounter any study in the literature that compared TTE and TEE 2D strain values of ventricular function. As a result, this study may be the first of its kind to compare TEE and TTE 2D strain imaging.
Study Limitations
This study was performed on a limited number of subjects; 2D strain imaging is dependent on good 2D image quality. For this reason, poor 2D image quality results in a poor success rate. Foreshortening was seen in the apical 4C and 2C views. In order to overcome this limitation, the authors performed the retroflexion maneuver. One of the limitations of the present study is the settings in which TEE was performed. The patients were all in the left lateral position and spontaneously ventilating in contrast to the intraoperative settings. Another limitation that is worth mentioning is the time to obtain TEE 2D strain measurements during the perioperative period. The authors did not use an automated function imaging program and did not measure the time for calculating TEE 2D strain measurements. Therefore, larger studies are needed to incorporate TEE 2D strain imaging in clinical practice.
Clinical Implications
In this study, the authors used transesophageal 2D strain imaging and hypothesized that it could be part of the TEE evaluation for the assessment of the left and right ventricular function during the perioperative period. Evaluating ventricular function with TEE routinely does not appear to be a practical and proper approach in all patients. However, 2D strain imaging might be used during TEE in the assessment of ventricular function quantitatively as part of the echocardiographic evaluation in the following situations: (1) for the evaluation of ventricular function perioperatively, and (2) for patients with a poor TTE windows (eg, obesity, after cardiac surgery). Although TEE 2D strain imaging for the intraoperative assessment of ventricular function may be time consuming because of the requirement of off-line analysis with the workstation, the automated functioning imaging program, which is the computer-based program that gives similar data about the global and 4C, long-axis, and 2C strain imaging in a short time may be useful. The automated functioning imaging program is available in all Vivid-7 echocardiographic machines. In daily practice, the authors have used this application. In comparative studies, automated functioning imaging software was shown to be feasible and applicable compared to offline workstation analysis in which data were obtained in a very short time (<1 minute).21, 22 The results of this study may contribute to partially filling the gap to give insight on the potential application of TEE 2D strain imaging to evaluate the function of ventricles during the perioperative period, but further larger analyses are needed.
Conclusion
In conclusion, TEE 2D strain imaging might be a reproducible technique for the assessment of ventricular function, and there is good agreement with TTE 2D strain imaging. As a result, the authors propose that TEE 2D strain imaging might be used in the quantification of ventricular function in the perioperative period.
References
- . Intraoperative transesophageal echocardiography: 5-year prospective review of impact on surgical management . Mayo Clin Proc . 2000;75:241–247
- Impact of routine use of intraoperative transesophageal echocardiography during cardiac surgery . Can J Anaesth . 2000;47:20–26
- Perioperative use of transesophageal echocardiography by anesthesiologists: Impact in noncardiac surgery and in the intensive care unit . Can J Anaesth . 2002;49:287–293
- The influence of transoesophageal echocardiography on intra-operative decision making (A European multicentre study. European Perioperative TOE Research Group) . Anaesthesia . 1998;53:767–773
- Quantification of left ventricular systolic function by tissue Doppler echocardiography: Added value of measuring pre- and postejection velocities in ischemic myocardium . Circulation . 2002;105:2071–2077
- Myocardial wall velocity assessment by pulsed Doppler tissue imaging: Characteristic findings in normal subjects . Am Heart J . 1996;132:648–656
- Assessment of myocardial mechanics using speckle tracking echocardiography: Fundamentals and clinical applications . J Am Soc Echocardiogr . 2010;23:351–369
- . Systolic dysfunction in heart failure with normal ejection fraction: Speckle-tracking echocardiography . Prog Cardiovasc Dis . 2006;49:207–214
- Two-dimensional strain imaging: A new echocardiographic advance with research and clinical applications . Int J Cardiol . 2008;123:240–248
- . Strain and strain rate deformation parameters: From tissue Doppler to 2D speckle tracking . Int J Cardiovasc Imaging . 2008;24:479–491
- New noninvasive method for assessment of left ventricular rotation: Speckle tracking echocardiography . Circulation . 2005;112:3149–3156
- Speckle tracking for the intraoperative assessment of right ventricular function: A feasibility study . J Cardiothorac Vasc Anesth . 2010;24:275–279
- The feasibility of speckle tracking for intraoperative assessment of regional myocardial function by transesophageal echocardiography . J Cardiothorac Vasc Anesth . 2009;23:462–467
- Comparative feasibility of myocardial velocity and strain measurements using 2 different methods with transesophageal echocardiography during cardiac surgery . J Cardiothorac Vasc Anesth . 2011;25:216–220
- Regional strain and strain rate measurements by cardiac ultrasound: Principles, implementation and limitations . Eur J Echocardiogr . 2000;1:154–170
- Can strain rate and strain quantify changes in regional systolic function during dobutamine infusion, B-blockade, and atrial pacing—Implications for quantitative stress echocardiography . J Am Soc Echocardiogr . 2002;15:416–424
- Novel speckle-tracking radial strain from routine black-and-white echocardiographic images to quantify dyssynchrony and predict response to cardiac resynchronization therapy . Circulation . 2006;113:960–968
- . Non-Doppler two-dimensional strain imaging by echocardiography—From technical considerations to clinical applications . J Am Soc Echocardiogr . 2007;20:234–243
- . Statistical methods for assessing agreement between two methods of clinical measurement . Lancet . 1986;1:307–310
- Intraoperative transesophageal echocardiography for detection of myocardial ischemia . Herz . 1993;18:372–378
- Relation between preoperative and intraoperative new wall motion abnormalities in vascular surgery patients: A transesophageal echocardiographic study . Anesthesiology . 2010;112:557–566
- . Non-invasive imaging: Two dimensional speckle tracking echocardiography: Basic principles . Heart . 2010;96:716–722
PII: S1053-0770(11)00442-3
doi:10.1053/j.jvca.2011.05.014
© 2012 Elsevier Inc. All rights reserved.
Volume 26, Issue 1 , Pages 26-31, February 2012
