|Year : 2023 | Volume
| Issue : 2 | Page : 161-167
Echocardiographic Assessment of Left Ventricular Myocardial Work
Department of Cardiology, CARE CHL Hospitals, Indore, Madhya Pradesh, India
|Date of Submission||12-May-2023|
|Date of Decision||06-Jul-2023|
|Date of Acceptance||10-Jul-2023|
|Date of Web Publication||30-Aug-2023|
3, Sainath Colony Sector-B, Tilak Nagar, Indore - 452 018, Madhya Pradesh
Source of Support: None, Conflict of Interest: None
Prognosis in cardiac disorders is determined by left ventricular (LV) function and hence, its precise estimation is of utmost importance. Myocardial work is a new advanced technique based on computation by incorporating systolic blood pressure into strain analysis. The aim of this review article is to provide an overview and additive value of this technique for the assessment of LV systolic function. The article includes the evolution of this technique from invasive to noninvasive mode, the method of acquiring and measuring it, normal reference values, its role in various cardiac conditions as described in the current literature, and potential limitations.
Keywords: Myocardial function, myocardial work, speckle-tracking echocardiography, strain
|How to cite this article:|
Karande A. Echocardiographic Assessment of Left Ventricular Myocardial Work. J Indian Acad Echocardiogr Cardiovasc Imaging 2023;7:161-7
|How to cite this URL:|
Karande A. Echocardiographic Assessment of Left Ventricular Myocardial Work. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2023 [cited 2023 Sep 27];7:161-7. Available from: https://jiaecho.org/text.asp?2023/7/2/161/384773
| Introduction|| |
In various cardiovascular diseases, the assessment of cardiac function is important for determining the risk and guiding management. Echocardiography is the most commonly used technique to measure cardiac function. Left ventricular ejection fraction (LVEF) is the simplest to measure and the most widely used parameter to assess left ventricular (LV) systolic function. However, the estimation of LVEF has many limitations such as it is semi-quantitative and has significant interobserver and intraobserver variability. It is also not possible to detect subtle changes in contractility on serial examinations. In the last decade, strain imaging has emerged as a comprehensive technique for the characterization of myocardial mechanics with precision. Although the clinical utility of strain imaging in the management of cardiac diseases is validated in many studies, its main limitation is load dependency., Besides contractility, global longitudinal strain (GLS) also depends on afterload and this may give erroneous results. A meta-analysis of 24 studies was done by Yingchoncharoen et al. and they concluded that blood pressure (BP) and afterload could affect strain. After this observation, a new noninvasive parameter called myocardial work (MW) has evolved which measures the LV pressure–strain relationship. MW is a novel echocardiographic tool which incorporates LV afterload into the analysis, thereby reducing the load dependency of strain. Measurement of MW is not a new concept as Suga in 1979 published a study wherein they evaluated MW by pressure–volume relationship measured invasively. However, as it is an invasive method, it cannot be used in routine clinical practice. A noninvasive method to measure MW was introduced by Russell et al. in 2012. Work is a product of LV pressure and stroke volume. In the absence of aortic stenosis, BP can be used in place of LV pressure and GLS can be used as a surrogate of stroke volume as both are closely related. LV pressure curves are combined with GLS to construct pressure-stain loop (PSL). The area within this loop represents the MW. MW thus estimated has already been validated in patients undergoing cardiac resynchronization therapy (CRT), heart failure (HF) patients, and patients with coronary artery disease (CAD).,,, The aim of this review is to discuss the current status of the assessment of MW by echocardiography.
| How to Measure Myocardial Work?|| |
The measurement of MW is done by constructing PSL areas by combining LV pressure curves and GLS [Figure 1]. It is based on speckle-tracking echocardiography and is an extension of the conventional LV GLS assessment. Currently, the option for MW assessment is available only with EchoPAC- the proprietary strain analysis software provided by General Electrics Healthcare (Horten, Norway). The equipment software constructs an empiric reference pressure curve which is adjusted according to the duration of ejection and isovolumic phases. The MW is estimated as follows:
|Figure 1: Myocardial work is measured from the pressure strain loop (PSL) areas that are constructed from the left ventricular (LV) pressure curves combined with strain. The area within the PSL represents work performed by the left ventricle. The loop works in a counter clockwise rotation, showing the correlation between cardiac event times and the relationship between GLS and estimated LV systolic pressure (cuff blood pressure). AVC: Aortic valve closure; AVO: Aortic valve opening; LVP: LV pressure; MVC: Mitral valve closure; MVO: Mitral valve opening|
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- With the patient in the left lateral position, all three apical (four-chamber, two-chamber and long-axis) views are captured. To maintain a high level of image quality, the heart rate variability should be minimum and the frame rate must be between 60 and 80 frames/s
- By tracking all the segments properly, GLS analysis is completed and the bull's eye plot is displayed on the screen [Figure 2]a
- Now, the MW tablet is seen on the right side of the screen and after clicking it, MW analysis is initiated. First of all, the proper alignment of electrical and mechanical events is required. This can be performed in one of the following two ways-
- Doppler profiles of mitral inflow [Figure 2]b and aortic outflow [Figure 2]c can be used to set valvular event timings (mitral valve closure, aortic valve opening, aortic valve closure, and mitral valve opening). The option for Doppler event timing is available in the measurement package of the software and this must be done before initiating the MW analysis tool
- The valvular event timings can also be set in the apical long-axis view by scrolling the image frame-by-frame for identifying mitral and aotric valve opening and closing events, which could be corroborated with the electrocardiogram [Figure 2]d.
|Figure 2: Methodology of myocardial work estimation by echocardiography. (a) Longitudinal strain is evaluated in the three standard apical views; (b and c) Valvular events are measured by pulsed-wave Doppler and continuous-wave Doppler; (d) The event timings are confirmed in apical long-axis view. Systolic blood pressure and diastolic blood pressure are then entered into the software (red arrow); (e) Bull's eye of the global work index; (f) Bull's eye of global work efficiency. On the same screen, values of all the myocardial work components and bars indicating the relationship between constructive and wasted work are displayed; (g) By choosing a specific segment, we can compare the global and segmental pressure strain loop curves at the same time|
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- BP is recorded at the time of image acquisition for GLS with the patient in the same position. These BP readings are now entered into the software [[Figure 2]d red arrow]. This is crucial as the systolic blood pressure (SBP) measured is used in place of LV systolic pressure (LVSP) in the MW index (MWI) calculations
- After finishing the last two steps, the user can “approve” the analysis and the machine automatically displays the bull's eye of the global work index (GWI) [Figure 2]e and global work efficiency (GWE) [Figure 2]f of the LV using the 17 segments model. The value of all the MW components and bars indicating the relationship between constructive work and wasted work is also displayed along with the bull's eye on the same screen. By choosing a specific segment, we can compare the global and segmental PSL curves at the same time [Figure 2]g.
The bull's eye plots of GWI and GWE are very similar to that of GLS where the segmental and global values are displayed. The GWI values are also shown by colors with red indicating areas of high work, green representing areas of normal work, and blue indicating areas of negative work [Figure 2]e. Similarly, GWE bull's eye plot is also obtained with numerical values and colored shading. Green represents areas of high and red indicates lower efficiency [Figure 2]f.
| Myocardial Work and Its Components|| |
- GWI: Total work performed by the LV from mitral valve closure to mitral valve opening, including the isovolumetric phases. The area within the PSL represents the GWI [Figure 1]
- Global constructive work (GCW): Work performed by the LV segments which is productive and contributes to the overall LV performance. It includes the shortening of the segments during systole and the lengthening of the segments during isovolumetric relaxation (IVR)
- Global wasted work (GWW): Nonproductive work performed by the dyssynchronous LV segments during systole. It includes lengthening of the segments during systole and shortening during IVR. Significant wasted work is seen in conditions such as bundle branch block, myocardial ischemia, and some other myocardial diseases
- GWE: It is estimated by dividing constructive work by the sum of constructive and wasted work.
The units of all MW components except GWE are same, i.e., mmHg%. GWE is expressed in % as it is an estimated value obtained by dividing GCW by the sum of GCW and GWW.
| What are the Normal Values?|| |
As MW estimation is a recent development, only few studies are available describing the normal values. The Normal Reference Ranges for Echocardiography (NORRE) study was the first study which published the normal values in 226 normal subjects. It was a large, multicenter, prospective study which reported normal reference values according to age and sex [Table 1]. The normal values from this study are used in most echocardiography laboratories worldwide. The lowest values of MW indices in women and men were 1310 mmHg % and 1270 mmHg % for GWI, 1544 mmHg % and 1650 mmHg % for GCW, and 91% and 90% for GWE, respectively. The maximum value for GWW was 239 mmHg % in women and 238 mmHg % in men. Women had significantly higher values of GWE and lower values of GWW. Interestingly, no strong correlation of MW was found with age and gender. Although GWI and GCW increased with age, in females, it was mainly due to increased BP. Limited ethnic representation, lack of adjustment for body mass index, and the small sample size were the main limitations of this study.
The STAges A/B and Determinants of Progression (STAAB) cohort study published normal values of MW components according to age and sex and compared them with conventional echocardiography parameters, such as EF and GLS. This study found that all values except GWI were independent of sex and stable till the age of 45 years. There was modest increase in GWI and GCW till the age of 45 but not in older subjects. GWW was found to increase with age associated with a decrease in GWE. There was only a weak correlation of MW indices with EF and GLS. This suggests that MW is a more accurate method of evaluating LV performance.
Galli et al. also reported normal values of MW components according to age, gender, and LV territory in healthy controls. They showed statistically significant differences in men and women, particularly GWI and GCW were found to be higher in females. There was no significant difference between age-specific subgroups. They also reported that segmental GWI increased from LV base to apex.
| What are the Clinical Applications?|| |
As MW is not affected by the loading conditions, it has been found to be useful in various clinical settings. MW indices provide incremental information over and above the LVEF and strain.
Currently, CRT is indicated in symptomatic HF patients with LVEF <35% and wide QRS complex. However, in about 30% of cases, the patient is not benefited after CRT. Several echocardiographic parameters were devised to predict the CRT response without success. Prediction of the therapeutic response to CRT is the first and the most promising application of MW indices. In a recent study, MW was assessed to identify patients who might respond to CRT, and wasted work in the septum together with the wall motion score index was found to be a strong predictor of response to CRT. In a large multicenter prospective study, the lateral wall – septal work difference identified CRT responders with good accuracy. The accuracy was further improved if the work difference was combined with septal viability by cardiac magnetic resonance imaging (CMR). In addition, the acute redistribution of regional MW between the septum and lateral wall of LV was found to be an important determinant of reverse remodeling after CRT. This suggested that the main aim of CRT should be the treatment of loading imbalance. In another study, GCW was found to predict CRT response at 6-month follow-up and was significantly associated with remodeling in ischemic and nonischemic patients. However, due to the lack of robust evidence supporting utility of these MW indices, currently they are not recommended as independent parameters for selecting patients for CRT.
In patients of HF with reduced EF (HFrEF), GCW was shown to increase significantly at 6-month follow-up after treatment with sacubitril/valsartan and after 12 months, GWE improved significantly. A cutoff value of GCW <910 mmHg % identified patients at high risk of major adverse cardiac events (MACE). In another study, GWI of <500 mmHg % along with N-terminal pro-B-type natriuretic peptide (NT-proBNP) was found to be a strong predictor of poor prognosis. In diabetic patients with reduced LVEF (<50%), a greater improvement in GWI, GCW, and GWW was seen by combining glucagon-like peptide-1 receptor agonists and the sodium-glucose cotransporter-2 inhibitors than insulin treatment or each medicine alone after 12 months of therapy. In a recent study, GWI <750 mmHg % was associated with significantly high risk of HF hospitalization and all-cause mortality.
In patients of HF with preserved EF receiving spironolactone, the exertional increase in GCW was able to predict an increase in exercise capacity. In patients of HF having normal EF and GLS, the values of GWW at rest and exercise were found to be elevated compared to controls which was indicative of subclinical LV dysfunction. In addition, a close correlation was detected between GWE and pulmonary congestion, exercise capacity along with reduced contractile response during exercise.
Hence, in patients of HF with subclinical as well as evident LV dysfunction, MW serves as an additive tool in the characterization of myocardial performance. However, the prognostic value of MW in HF patients is poor and further studies are required to make it useful in routine practice.
Coronary artery disease
MW has been shown to have better accuracy and sensitivity compared to EF and GLS in detecting CAD. The contractile pattern of ischemic myocardium strongly depends on loading conditions and with acute increase in afterload, hypokinetic segments become dyskinetic. This limitation is successfully overcome by MW, and in both acute and chronic settings, it provides diagnostic and prognostic information. In a previous study, GWI, GCW, and GWE were significantly reduced in patients of obstructive CAD with normal EF while GWW slightly increased. A GWI cutoff value of <1810 mmHg % detected CAD with positive predictive value of 95%. In non-ST elevation, myocardial infarction (MI) patients, regional MWI can identify acute coronary occlusion and was found to be superior to GLS and LVEF. MWI of <1700 mmHg % in 4 adjacent segments has shown higher sensitivity, specificity, and negative predictive value compared to functional risk area measurement with strain. GWI has also been shown to be superior to LVEF in identifying acute coronary occlusion. In ST elevation MI patients, GCW has been shown to be of incremental value over LVEF and GLS in predicting segmental and global LV recovery as well as complications such as HF and LV apical thrombus.
Hypertension and diabetes mellitus
MW appears to be more sensitive in hypertensive patients. Increased values of GWI and GCW were detected in patients of hypertension which indicates enhanced contractility of LV as it has to pump against an increased pressure. Another study also confirmed these findings and it also detected additional impact of type 2 diabetes mellitus on GCW in hypertensive patients. Loncaric et al. analyzed the distribution of MW and detected an apex-to-base gradient indicating an impairment in basal segments and compensation by apical segments. Large multicenter studies are needed to define the clinical utility of MW indices and its prognostic impact on cardiovascular outcomes in these settings.
Valvular heart disease
Chronic aortic regurgitation (AR) is associated with volume overload which leads to LV remodeling. In this situation, MW may be a reliable tool to individuate incipient LV systolic dysfunction. D'Andrea et al. in a study of asymptomatic severe AR patients subjected to physical effort showed significant correlation of baseline GLS and MWE with functional capacity, LV filling pressure as well as pulmonary congestion during exercise. Since the technique of MW involves the estimation of SBP with a cuff manometer which is assumed to be equal to systolic LV pressure, its evaluation is not feasible in patients of AS and hypertrophic obstructive cardiomyopathy (HOCM). In these conditions, SBP does not represent LVSP. In AS, LVEF remains preserved until late stages while GLS has load dependence, hence estimation of MW may be useful. Recently, Jain et al. have proposed the use of sum of aortic valve mean gradient and systolic BP as an estimate of LV peak pressure in patients of AS undergoing transcatheter aortic valve replacement (TAVR). A good correlation was detected between LV pressure measured invasively and the one estimated noninvasively using the above method. They observed a significant reduction in GWI and GCW after TAVR that was attributed to an acute reduction in afterload. However, larger studies are required to validate this method for use in routine practice.
MW has also been evaluated in patients of HFrEF with severe functional mitral regurgitation treated with MitraClip(R). Only the GCW measured at baseline was the statistically significant predictor of reverse remodeling 1 year after the clip procedure. A cutoff value of GCW >846 mmHg % was associated with 10% LV end-systolic volume reduction. Again, the sample size was very small in this study, so MW cannot be recommended for daily practice in such patients.
MW can be useful in evaluating cardiomyopathy patients. As the MW measurement algorithm does not consider geometric variation, we expect that calculated work may not be accurate in remodeled thick ventricles. However, Hiemstra et al. evaluated the additive value of MW indices in different phenotypes of hypertrophic cardiomyopathy (HCM) and found a good correlation with survival., In patients of nonobstructive HCM, GWI, GCW, and GWE were significantly impaired while GWW was increased., In another study, a good correlation between GCW and LV diastolic dysfunction, maximum LV wall thickness as well as QRS duration was detected. When late gadolinium enhancement CMR was used to assess LV fibrosis, GCW emerged as the only predictor of fibrosis. A cutoff value of <1730 mmHg % was associated with poor long-term prognosis. However, further large trials are required to study the additive value of MW in these cardiomyopathy patients.
In patients of ischemic as well as nonischemic dilated cardiomyopathy, a significant impairment of GWI, GCW, and GWE was detected while GWW was increased indicating impaired contractile function of the left ventricle. In ischemic dilated cardiomyopathy patients, 6-min walking distance increased 6 months after therapy along with significant improvement in GWI but the LVEF and GLS remained same. This suggests that MW is more sensitive in detecting functional improvement and the effectiveness of therapy. In patients of cardiac amyloidosis with normal LVEF, MW indices have shown a good correlation with known prognostic parameters such as NT-proBNP, estimated glomerular filtration rate, and troponins. A combination of GWI and apical to base segmental work ratio was a strong predictor of MACE.
In athletes performing long-term exercise, MW can detect myocardial changes in the presence of normal LVEF. A large study showed that in athletes, GWE and GWW at rest were preserved despite reduction in GLS due to LV adaptation. In athletes, functional capacity and pulmonary congestion at peak exercise can be predicted with GWE in a better way than LVEF. Sengupta et al. evaluated MW indices in South Asian recreational athletes who completed half-marathon and showed that GWE and GWW did not change significantly after competition but GWI increased in a subgroup of athletes with higher BNP and higher heart rate. This can be an early manifestation of myocardial stress and may be a precursor of myocardial fatigue.
During stress echocardiography, loading conditions change with exercise and MW estimation can be valuable. During normal exercise, GWI increases, GLS also increases but GWE remains unchanged. With exercise-induced ischemia, segmental MWI increases in affected segments, GLS decreases, and GWE also decreases. In normal subjects, constructive work increases after exercise with a little change in wasted work. However, in patients with ischemia, constructive work does not change significantly after exercise while wasted work increases. In other studies, good correlation was detected between GWI measured at peak stress and functional capacity., Another study showed an increase in GWI, GCW, and GWW in patients with uncontrolled hypertension (SBP >180 mmHg) after exercise without any significant changes in GWE and GLS. Cardiac amyloidosis patients have significantly reduced baseline GWI compared with healthy controls. The GWI increases in controls after exercise but not in amyloidosis patients. GWE decreased in the cardiac amyloidosis group after exercise while remaining stable in the control group. The segmental analysis revealed that in subjects with cardiac amyloidosis, after exercise, there was an increase in MWI in apical segments with no remarkable increase in basal or mid segments, while in controls, MWI increased in all segments. Further large studies are needed to evaluate MW during stress echocardiography.
| What are the Limitations and Future?|| |
Although it is feasible to analyze MW in most patients, as with all modalities, MW estimation also has certain limitations. MW involves strain analysis and hence, many of the limitations of strain imaging are also applicable to MW assessment. The frame rates and image quality should be adequate for accurate tracking.
The protocol for the estimation of MW involves SBP (LV pressure) as the sole measure of afterload. It does not consider the afterload that comes from arterial stiffness, vascular resistance, and reflection waves. It should also include aortic stiffness for complete assessment of afterload.
The current method for MW estimation also does not take into account geometric variations. The actual determinant of afterload is wall stress which, besides LV pressure, also depends on wall thickness, LV curvature, and radius. The wall stress is directly proportional to LV pressure and radius while inversely proportional to LV thickness. Hence, LV hypertrophy and dilatation should affect the afterload. Accordingly, at present, only the normal ventricles without hypertrophy and dilatation should be suitable candidates for MW estimation but fortunately, the current method has also been found to be useful in remodeled ventricles. In the future, it would become possible to eliminate the false results caused by geometric assumptions by using a four-dimensional application that would include LV mass, work produced from circumferential direction, and area strain.
The current method of estimating MW assumes that the LV end-diastolic pressure is low and does not affect the PSL area. Hence, it can yield false results if filling pressures are high. In the future, preload should be considered in the MW methodology. At present, it is difficult to measure MW in the presence of atrial fibrillation as the preload changes from beat to beat.
Aortic valve stenosis and HOCM are the two conditions where SBP is not equivalent to LVSP. Jain et al. have proposed a corrected method for measuring MW in AS patients described above.
MW estimation requires proprietary software which is currently available only with a single vendor platform making it accessible to only limited number of patients.
| Conclusion|| |
The noninvasive assessment of MW incorporates afterload into strain and minimizes load dependency of LVEF and GLS. There are many potential indications for this parameter as a few studies have demonstrated its usefulness in assessing myocardial contractility under various loading conditions as well as remodeling. However, the current method of estimating MW has several limitations because of which it is recommended only as a complementary tool to EF and GLS for clinical purposes or as a research tool. Larger multi-centric studies are required to establish its prognostic implications and use in clinical decision-making.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]