Integrated regional work equals total left ventricular work in regionally ischemic canine heart

One step closer to myocardial physiology: From PV loop analysis to state-of-the-art myocardial imaging

Recent advances in cardiac imaging have revitalized the assessment of fundamental physiological concepts. In the field of cardiac physiology, invasive measurements with pressure-volume (PV) loops have served as the gold standard methodology for the characterization of left ventricular (LV) function. From PV loop data, fundamental aspects of LV chamber function are derived such as work, efficiency, stiffness and contractility. However, the parametrization of these aspects is limited because of the need for invasive procedures. Through the utilization of recent advances in echocardiography, magnetic resonance imaging and positron emission tomography, it has become increasingly feasible to quantify these fundamental aspects of LV function non-invasively. Importantly, state-of-the-art imaging technology enables direct assessment of myocardial performance, thereby extending functional assessment from the net function of the LV chamber, as is done with PV loops, to the myocardium itself. With a strong coupling to underlying myocardial physiology, imaging measurements of myocardial work, efficiency, stiffness and contractility could represent the next generation of functional parameters. The purpose of this review is to discuss how the new imaging parameters of myocardial work, efficiency, stiffness and contractility can bring cardiac physiologists, researchers and clinicians alike one step closer to underlying myocardial physiology.

Left ventricular myocardial work in the culprit vessel territory and impact on left ventricular remodelling in patients with ST-segment elevation myocardial infarction after primary percutaneous coronary intervention

AIMS

Adverse left ventricular (LV) remodelling after ST-segment elevation myocardial infarction (STEMI) is associated with poor outcome. Global and regional LV myocardial work (LVMW) derived from speckle tracking echocardiographic strain data in combination with non-invasive blood pressure recordings could provide information for prediction of LV remodelling after STEMI. The aim of the study was to assess the predictive value of global and regional LVMW for LV remodelling before discharge in patients with STEMI.

METHODS AND RESULTS

Three-hundred and fifty STEMI patients treated with primary percutaneous coronary intervention (PCI) were included [265 men (76%), mean age: 61 ± 10 years]. Clinical variables, conventional echocardiographic parameters, global and regional measures of myocardial work index (MWI), and myocardial work efficiency were recorded before discharge. The primary endpoint was early LV remodelling defined as increase in LV end-diastolic volume (LVEDV) ≥20% at 3 months after STEMI. Eighty-seven patients (25%) showed early LV remodelling. The global and regional LVMW in the culprit territory were significantly lower in patients with early LV remodelling. Peak troponin I (OR 1.109, 95% CI 1.046-1.177; P = 0.001), LVEDV (OR 0.972, 95% CI 0.959-0.984; P < 0.001) and regional MWI in the culprit vessel territory (OR 0.602, 95% CI 0.383-0.945; P = 0.027) were independently associated with early LV remodelling.

CONCLUSION

In STEMI patients treated with primary PCI and optimal medical therapy, the regional cardiac work index in the culprit vessel territory before discharge is independently associated with early adverse LV remodelling.

Application of three-dimensional speckle tracking echocardiography to assess left ventricular regional work using wall tension-regional area loop

Three-dimensional (3-D) speckle tracking echocardiography allows us to track a change in regional endocardial surface area. The change of regional area during a cardiac cycle should be useful for assessing left ventricular regional work. We investigated the feasibility of assessing regional work, calculated as the area within the wall tension-regional area (T-A) loop using 3-D echocardiography. Three-dimensional full-volume images were acquired using 3-D echocardiography (Artida, Toshiba) at baseline and during brief occlusion of the left circumflex coronary artery in eight dogs. Wall tension was calculated according to Laplace's law for a spherical model. Area change ratio (in %) determined by area tracking was transformed into a change of regional area (in cm2) by a custom software. We calculated the area within the T-A loop (TAA) in the area under transient ischemia (risk area) and the remote area as regional work and validated the T-A loop method by comparing the global integral of TAA with the total work assessed by the pressure-volume loop. During coronary occlusion, regional work for the risk area significantly decreased (baseline vs. occlusion, 26.8 ± 10.7 vs. 18.4 ± 7.8 mmHg·cm3; P < 0.05), whereas that for the remote area did not change. The global integral of TAA closely correlated with the total work assessed by the pressure-volume loop ( r = 0.91, P < 0.0001). The wall T-A loop reflected regional dysfunction caused by myocardial ischemia. This analysis using 3-D speckle tracking echocardiography might be useful to quantify left ventricular regional work.

A novel clinical method for quantification of regional left ventricular pressure-strain loop area: a non-invasive index of myocardial work

Aims

Left ventricular (LV) pressure–strain loop area reflects regional myocardial work and metabolic demand, but the clinical use of this index is limited by the need for invasive pressure. In this study, we introduce a non-invasive method to measure LV pressure–strain loop area.

Methods and results

Left ventricular pressure was estimated by utilizing the profile of an empiric, normalized reference curve which was adjusted according to the duration of LV isovolumic and ejection phases, as defined by timing of aortic and mitral valve events by echocardiography. Absolute LV systolic pressure was set equal to arterial pressure measured invasively in dogs (n = 12) and non-invasively in patients (n = 18). In six patients, myocardial glucose metabolism was measured by positron emission tomography (PET). First, we studied anaesthetized dogs and observed an excellent correlation (r = 0.96) and a good agreement between estimated LV pressure–strain loop area and loop area by LV micromanometer and sonomicrometry. Secondly, we validated the method in patients with various cardiac disorders, including LV dyssynchrony, and confirmed an excellent correlation (r = 0.99) and a good agreement between pressure–strain loop areas using non-invasive and invasive LV pressure. Non-invasive pressure–strain loop area reflected work when incorporating changes in local LV geometry (r = 0.97) and showed a strong correlation with regional myocardial glucose metabolism by PET (r = 0.81).

Conclusions

The novel non-invasive method for regional LV pressure–strain loop area corresponded well with invasive measurements and with directly measured myocardial work and it reflected myocardial metabolism. This method for assessment of regional work may be of clinical interest for several patients groups, including LV dyssynchrony and ischaemia.

Regional myocardial work by strain Doppler echocardiography and LV pressure: a new method for quantifying myocardial function

There is a need for better methods to quantify regional myocardial function. In the present study, we investigated the feasibility of quantifying regional function in terms of a segmental myocardial work index as derived from strain Doppler echocardiography (SDE) and invasive pressure. In 10 anesthetized dogs, we measured left ventricular (LV) pressure by micromanometer and myocardial longitudinal strains by SDE and sonomicrometry. The regional myocardial work index (RMWI) was calculated as the area of the pressure-strain loop. As a reference method for strain, we used sonomicrometry. By convention, the loop area was assigned a positive sign when the pressure-strain coordinates rotated counterclockwise. Measurements were done at baseline and during volume loading and left anterior descending coronary artery (LAD) occlusion, respectively. There was a good correlation between RMWI calculated from strain by SDE and strain by sonomicrometry ( y = 0.73 x + 0.21, r = 0.82, P < 0.01). Volume loading caused an increase in RMWI from 1.3 ± 0.2 to 2.2 ± 0.1 kJ/m3 ( P < 0.05) by SDE and from 1.5 ± 0.3 to 2.7 ± 0.3 kJ/m3 ( P = 0.066) by sonomicrometry. Short-term ischemia (1 min) caused a decrease in RMWI from 1.3 ± 0.2 to 0.3 ± 0.04 kJ/m3 ( P < 0.05) and from 1.3 ± 0.3 to 0.5 ± 0.2 kJ/m3 ( P < 0.05) by SDE and sonomicrometry, respectively. In the nonischemic ventricle and during short-term ischemia, the pressure-strain loops rotated counterclockwise, consistent with actively contracting segments. Long-term ischemia (3 h), however, caused the pressure-strain loop to rotate clockwise, consistent with entirely passive segments, and the loop areas became negative, −0.2 ± 0.1 and −0.1 ± 0.03 kJ/m3 ( P < 0.05) by SDE and sonomicrometry, respectively. A RMWI can be estimated by SDE in combination with LV pressure. Furthermore, the orientation of the loop can be used to assess whether the segment is active or passive.

Analysis of effect of two concurrent ischaemic zones on left ventricular function

Left ventricular (LV) function due to two concurrent ischaemic zones (IZs) is investigated using a cardiovascular system model. The model comprises a three-compartment LV, the venous return and the arterial system. Haemodynamic responses of the LV to changes in the IZ size and myocardial contraction timings are explored. Results show that the greater the degree of asynschonisation is between the normal zone and the IZ, and the larger the ischaemic size, the more severe the LV dysfunction. Pre-load augmentation improves LV function. Model-predicted features are consistent with reported observations associated with myocardial ischaemia. The extent of the usefulness and limitations of this model is also discussed.

Cardiac efficiency

Efficiency is defined as the ratio of the energy delivered by a system to the energy supplied to it. Depending on the particular question being addressed, there exist a plethora of definitions of efficiency in medical texts, thus hampering their comparison. If only the ventricular work seen by the arterial system is under investigation, pressure-volume work will serve as a useful numerator. If, on the other hand, external and internal work together, i.e. the total mechanical work, is of interest, the pressure-volume area might be employed. Total myocardial oxygen consumption (MVO2) will be a useful denominator in the case of aerobic energy production. The MVO2 for the unloaded contraction must be assessed if, as in other energy transfer systems, net efficiency is to be addressed. If even smaller steps in the chain of energy transfer are to be investigated MVO2 for the arrested heart must be assessed. With an appropriate therapy, hemodynamic determinants can be varied, to improve cardiac efficiency. Nonetheless, measurement of all variables necessary for the calculation of efficiency remains a challenge, in particular in the clinical setting. Separation of the direct effects of drugs on efficiency is even more difficult, since hemodynamic conditions can hardly be controlled throughout the observation period, and changes in efficiency might be secondary to changes in hemodynamics. Whether the heart by itself employs mechanisms to improve its efficiency is still a matter of discussion: there is evidence that when oxygen supply decreases, the heart can switch from one substrate to a less costly one, or possibly can improve efficiency through better use of oxygen. Moreover, the heart seems to "sense" an even more decreased oxygen supply and reduce function in response. Myocardial stunning could be regarded as a protective mechanism as well, with function remaining depressed and the oxygen supply being normal or close to normal. One may conclude from the decreased efficiency that the excess oxygen consumption is used up for repair processes. The improved efficiency found in hypertrophied hearts represents another adaptive process. The underlying mechanism is unclear: a shift towards isomyosin V3 or some undefined shift in metabolic pathway is discussed. It is also still a moot question towards which objective the efficiency of the heart is adjusted. It has been described that under physiologic conditions, the efficiency of both the left and the right ventricle ought to be maximized.(ABSTRACT TRUNCATED AT 400 WORDS)