Estimates of regional work in the canine left ventricle

The non-invasive assessment of myocardial work by pressure-strain analysis: clinical applications

Pressure–volume (PV) analysis is the most comprehensive way to describe cardiac function, giving insights into cardiac mechanics and energetics. However, PV analysis still remains a highly invasive and time-consuming method, preventing it from integration into clinical practice. Most of the echocardiographic parameters currently used in the clinical routine to characterize left ventricular (LV) systolic function, such as LV ejection fraction and LV global longitudinal strain, do not take the pressure developed within the LV into account and therefore fall too short in describing LV function as a hydraulic pump. Recently, LV pressure-strain analysis has been introduced as a new technique to assess myocardial work in a non-invasive fashion. This new method showed new insights in comparison to invasive measurements and was validated in different cardiac pathologies, e.g., for the detection of coronary artery disease, cardiac resynchronization therapy (CRT)-response prediction, and different forms of heart failure. Non-invasively assessed myocardial work may play a major role in guiding therapies and estimating prognosis. However, its incremental prognostic validity in comparison to common echocardiographic parameters remains unclear. This review aims to provide an overview of pressure-strain analysis, including its current application in the clinical arena, as well as potential fields of exploitation.

Computing work in the ischemic heart

This study analyzes the effect of a poorly perfused (ischemic) region on active tension and strain at a point in a model of the cardiac ventricles. The cellular active mechanics and electrophysiology were calculated using a coupled electromechanical cell model. The deformation of the heart was calculated combining an anisotropic constitutive law with the active tension generated by the cell models. An ischemic cell model was developed by adapting the Noble 98-HMT cell model parameters. The ischemic model exhibits a raised resting potential, shortened action potential duration and reduced amplitude. Ischemic cells were introduced into the heart model in the anterior wall of the left ventricle. The model was solved with and in the absence of the ischemic region. The stress and strains at a point located in the ischemic region were calculated. The ischemic cell model produced negligible amounts of active tension and this reduced the work load of ischemic cells by several orders of magnitude. The ischemic region had a significant effect on pump function in both the left and right ventricles.

End–diastolic myofiber stress and ejection strain increase with ventricular volume overload

BACKGROUND

Myocardial stress and strain are considered primary mechanical stimuli for hypertrophic remodeling. Their values and significance in the intact beating heart during chronic overload remain poorly characterized.

METHODS AND RESULTS

Left-ventricular (LV) dimensions (echocardiography) and pressure (invasive) were simultaneously recorded in anesthetized dogs at sinus rhythm (SR), acute and 1, 2, 6, 12 weeks of atrioventricular block (AVB), leading to structural, electrical and contractile remodeling. Mechanical load of the myocardium was quantified as myofiber stress (sigma(f)), being force along myofiber orientation per cross-sectional area, and natural myofiber strain (e(f)), being change in natural logarithm of myofiber length (l) divided by its reference length: e(f) = ln(l/l(ref)). Time courses of sigma(f) and e(f) were calculated from LV pressure and dimensions, using a validated mathematical model of cardiac mechanics. End-diastolic sigmaf increased from 2.0 +/- 0.1 kPa at SR to 3.4 +/- 0.3 kPa at acute AVB, remaining elevated for > 6 weeks. Systolic sigma(f) was not affected by AVB. Ejection strain rose instantly upon AVB, reaching a maximum at 2 weeks: 0.24 +/- 0.02 vs. 0.10 +/- 0.01 at SR. The increase of myofiber stroke work (sigma(f)-e(f) loop area) from 3.1 +/- 0.3 at SR to 6.0 +/- 0.5 kJ/m(3)/beat at 1 week AVB was attributed mainly to an increase of strain during ejection. Stroke work and ejection strain remained elevated up to 12 weeks. The rate of LV-mass increase was maximal (2.2 +/- 0.4 g/day) at 1 week AVB.

CONCLUSIONS

Serial mechanical phenotyping is feasible in the intact anesthetized dog with chronic ventricular overload. Our new approach yields values of mechanical load that are comparable to those found in isolated myocardium by others. In chronic AVB, both end-diastolic myofiber stress and ejection strain are increased. Early increases of both parameters coincide with peak hypertrophic growth, suggesting their important role for mechanotransduction. Peak systolic sigmaf is likely not important for hypertrophy in this model, since it does not change throughout the experiment.

Regional myocardial mechanics: integrative computational models of flow-function relations

Many cardiac disorders result in regionally altered myocardial mechanics. Although myocardial strain distributions can be measured experimentally and clinically, regional wall stresses must be computed from computational models. Combining these approaches can provide insight into the structural basis of regional dysfunction under conditions such as acute myocardial infarction and ischemia-reperfusion. Recently, 3-dimensional computational models have helped to elucidate the structural basis of the functional border zone adjacent to acutely ischemic myocardium. They have also shown that heterogeneous dysfunction in ischemic-reperfused stunned myocardium does not necessarily imply heterogeneous myofilament injury. Now that computational models are able to reproduce many complex features of the 3-dimensional patterns of regional myocardial deformation observed experimentally, we suggest possible roles for such integrative models in clinical diagnosis.

Length-tension relationships of sub-epicardial and sub-endocardial single ventricular myocytes from rat and ferret hearts

In vivo the sub-epicardial myocardium (EPI) and sub-endocardial myocardium (ENDO) operate over different ranges of sarcomere length (SL). However, it has not been previously shown whether EPI and ENDO work upon different ranges of the same or differing length-tension curves. We have compared the SL-tension relationship of intact, single ventricular EPI and ENDO myocytes from rat and ferret hearts. Cells were attached to carbon fibres of known compliance in order to stretch them and to record force at rest (passive tension) and during contractions (active tension). In both species, ENDO cells were significantly stiffer (i.e. had steeper SL-passive tension relationships) than EPI cells. Ferret ENDO cells had significantly steeper SL-active tension relationships than EPI cells; rat cells tended to behave similarly but no significant regional differences in active properties were observed. There were no inter-species differences in the active and passive properties of EPI cells, but ferret ENDO cells displayed significantly steeper passive and active SL-tension relationships than rat ENDO. We conclude that in vivo, ferret EPI and ENDO myocytes will function over different ranges of different SL-tension curves. There is a close relationship between SL and active tension (the Frank-Starling law of the heart), and our observations suggest that regional differences in the response to ventricular dilation will depend on both the change in SL and differing regional slopes of the SL-active tension curves.

Heterogeneities in myocardial flow and metabolism: exacerbation with abnormal excitation

Because regional myocardial blood flows are remarkably heterogeneous-with a 6- to 10-fold range of flows in normal hearts-and because the spatial profiles of the flows are stable over long periods and over a range of conditions, the relation between flows and other physiologic functions has been explored. Local fatty acid uptake and oxygen consumption are almost linearly related to the flows. Coronary network structure and hydrodynamic resistances give suitable flow heterogeneity but are thought to be a response to local needs rather than being causative. Presumably the cause is the need for adenosine triphosphate (ATP) synthesis locally, and therefore flows, substrate delivery, and oxygen utilization are driven primarily by local rates of ATP hydrolysis, mainly by contractile proteins. This hypothesis is by no means fully tested. Data on pacing dog hearts from different sites, on patients with left bundle branch block, and on unloading transplanted rat hearts, all point in the same direction: unloading ventricular muscle leads to diminished flow and exaggeratedly diminished glucose uptake. The mechanism is likely to be that discovered by Taegtmeyer and colleagues, namely, the expression of fetal genes in regions where the muscle is unloaded and particular metabolic enzymes and transporters are downregulated.