Myocardial work by echocardiography: a novel method ready for clinical testing

This editorial refers to ‘A new approach to assess myocardial work by non-invasive left ventricular pressure–strain relations in hypertension and dilated cardiomyopathy’ by J. Chan et al., pp. 31–39. Assessment of ventricular systolic function is an essential part of all echocardiographic examinations. Due to its deep-roots in cardiology, left ventricular (LV) ejection fraction (LVEF) is still the most commonly used parameter of systolic function despite its many limitations.1 More recently, LV global longitudinal strain (GLS) has been gaining ground. Although GLS may detect more subtle abnormalities in systolic function where LVEF is normal,2 it also suffers from being load-dependent. Left ventricular pressure–volume analysis incorporates LV load and as shown in experimental studies, is a powerful method to quantify LV function. Furthermore, the pressure–volume loop area reflects myocardial O2-consumption.3Figure 1 illustrates how pressure–volume relations may be used to calculate LV work. Similar to pressure–volume relations, LV pressure–strain relations may be used to assess regional LV function and segmental work (Figure 1). Due to its invasive nature, LV pressure–volume or pressure–strain analysis were never implemented in clinical routine. The left panel shows schematically LV pressure–volume relations: the pressure–volume loop area indicated by the pressure-volume area (PVA) represents external myocardial work (EW), and the triangular area to the left represents potential energy (PE). P, pressure; V, volume; V0, unstressed LV volume. Reproduced with permission from Suga.3 The right panel shows LV pressure–strain loops from a patient with cardiomyopathy and compares LV pressure measured by high-fidelity micromanometer to LV pressure estimated by echocardiography. The area of the LV pressure–strain loop reflects segmental work. Reproduced with permission from Russell et al.4 Our group has recently developed a non-invasive method for assessing regional myocardial work by LV pressure–strain loop analysis.4 The method has been included in echocardiographic software, making myocardial work calculations commercially available. External investigators recently evaluated the method and showed excellent agreement between measured and estimated pressure–strain loops. Figure 1, right panel illustrates correspondence between the non-invasive pressure estimate and directly measured pressure in a representative patient. Myocardial work incorporates LV pressure, and therefore, provides incremental information to LVEF and strain which are sensitive to LV afterload. This is illustrated in Figure 2 which is from a study where afterload was acutely elevated.5 In this experiment, strain was reduced in response to increased afterload, which may lead to the false conclusion of decreased contractility, when this was in fact a result of a change in afterload. Furthermore, since the pressure–strain loop area reflects myocardial metabolic demand and oxygen consumption, the work method provides insight into myocardial energetics.4 Myocardial strain (A) and LV pressure–strain loops (B) from a canine experiment before (yellow) and during aortic constriction (red). Peak LV pressure increased by about 30 mmHg. This was associated with a substantial decrease in longitudinal strain, whereas work was unchanged as shown in the right panel. Modified and reproduced with permission from Boe et al.4 Chan et al.6 applied myocardial work by LV pressure–strain analysis in a cohort of patients referred to coronary angiography. The patients were subdivided into three main groups: controls, hypertensives, and patients with cardiomyopathy. Although the groups are unbalanced, their study shows interesting results, particularly in hypertensive patients with a systolic blood pressure >160 mmHg. In this group, the global work index was significantly higher when compared with controls despite GLS and LVEF being normal and relatively unchanged. The findings confirm the inability of the traditional methods to detect increased performance and load imposed on the myocardium. In a normal heart, the different parts of the ventricle contract and relax in a synchronized fashion and all segments contribute efficiently to stroke volume. The work contributed from each normally contracting segment is positive and is named ‘constructive work’. In hearts with dyssynchronous contractions as seen during bundle branch blocks, ischaemia, and other myocardial diseases, there may be systolic lengthening during LV ejection. In the latter case, other segments generate the work needed to cause systolic lengthening, and this represents ‘wasted work’ since it does not contribute to LV ejection. Not only systolic lengthening, but also myocardial shortening after aortic valve closure (post-systolic shortening) represents wasted work since it does not contribute to LV ejection. ‘Myocardial work efficiency’ is calculated as the ratio between constructive work and the sum of wasted and constructive work, reported in percentage or just as the ratio with 1 as the maximum. As shown by Russell et al.4 and confirmed in the study by Chan et al.,6 in a normal heart there is very little wasted work, and therefore, highwork efficiency. Wasted work reduces LV efficiency and adds additional mechanical burden to the intact myocardium and may be a factor that contributes to remodelling in the failing ventricle. Chan et al.6 found that work efficiency was reduced in cardiomyopathy patients along with GLS and LVEF. Reduced efficiency reflects impairment of ventricular performance and may provide additional information in failing ventricles. The clinical consequence of measuring myocardial work efficiency remains to be determined. It holds the potential of improving the accuracy in selecting patients for cardiac resynchronization therapy or revascularization of patients with infarcted ventricles. Since work by definition equals force times length, the use of pressure and strain does not provide a direct measure of work, but rather an index. This becomes a problem when comparing work by pressure–strain analysis in ventricles with different sizes as was done in the study of Chan et al.6 In a dilated ventricle there is higher wall stress at any given LV pressure than in a smaller ventricle, and therefore, work is relatively underestimated in the dilated ventricle. The pressure estimate by echocardiography and arterial cuff pressure does not reproduce the details of true pressure, but as shown in validation studies it performs very well for calculation of work.7 A more direct measure of work may be obtained by incorporating LV dimensions and local radii of curvature which should be feasible with 3D imaging. The authors should be commemorated for using an echocardiography-based method to calculate end-systolic wall stress, which was more than doubled in the cardiomyopathy cohort compared with controls. The non-invasive method of calculating myocardial work has currently been applied in research regarding response to cardiac resynchronization therapy8 and acute coronary syndrome.5 Chan et al.6 shows the application of the method in a broader patient group, where myocardial work assessment may be beneficial, especially for hypertensive patients. The method may be used in situations to elucidate if hypertension or attenuated contractility is the cause of reduced contractions.5 Patients with acute hypertensive heart failure have reduced LVEF and GLS, however, due to the high blood pressure, the shortening indices may underestimate systolic function. This may be another group of patients where myocardial work assessment could be useful to determine response to therapy. Future developments of the work method should attempt to include LV geometry, wall thickness, and local radii in order to measure wall stress rather than pressure. This may be feasible by incorporating the method into 3D echocardiographic software. The introduction of the non-invasive work calculation has added a new dimension in the clinical evaluation of myocardial function. In every-day cardiology, we have always been aware of load-dependency, but unable to objectively account for it in our measurements. The work method represents a means to incorporate loading conditions in the assessment of ventricular function. If this is in fact a paradigm shift remains to be seen, but basic haemodynamic principles can now be implemented into clinical practice in a new way. Conflict of interest: O.A.S. is co-inventor, but has no longer ownership of the patent ‘Method for myocardial segment work analysis’, which was used to calculate myocardial work in the clinical study. The other authors have nothing to disclose.