Hyperfunction with normal inotropic state of the hypertrophied left ventricle

Computational modeling of cardiac growth and remodeling in pressure overloaded hearts-Linking microstructure to organ phenotype

Cardiac growth and remodeling (G&R) refers to structural changes in myocardial tissue in response to chronic alterations in loading conditions. One such condition is pressure overload where elevated wall stresses stimulate the growth in cardiomyocyte thickness, associated with a phenotype of concentric hypertrophy at the organ scale, and promote fibrosis. The initial hypertrophic response can be considered adaptive and beneficial by favoring myocyte survival, but over time if pressure overload conditions persist, maladaptive mechanisms favoring cell death and fibrosis start to dominate, ultimately mediating the transition towards an overt heart failure phenotype. The underlying mechanisms linking biological factors at the myocyte level to biomechanical factors at the systemic and organ level remain poorly understood. Computational models of G&R show high promise as a unique framework for providing a quantitative link between myocardial stresses and strains at the organ scale to biological regulatory processes at the cellular level which govern the hypertrophic response. However, microstructurally motivated, rigorously validated computational models of G&R are still in their infancy. This article provides an overview of the current state-of-the-art of computational models to study cardiac G&R. The microstructure and mechanosensing/mechanotransduction within cells of the myocardium is discussed and quantitative data from previous experimental and clinical studies is summarized. We conclude with a discussion of major challenges and possible directions of future research that can advance the current state of cardiac G&R computational modeling. STATEMENT OF SIGNIFICANCE: The mechanistic links between organ-scale biomechanics and biological factors at the cellular size scale remain poorly understood as these are largely elusive to investigations using experimental methodology alone. Computational G&R models show high promise to establish quantitative links which allow more mechanistic insight into adaptation mechanisms and may be used as a tool for stratifying the state and predict the progression of disease in the clinic. This review provides a comprehensive overview of research in this domain including a summary of experimental data. Thus, this study may serve as a basis for the further development of more advanced G&R models which are suitable for making clinical predictions on disease progression or for testing hypotheses on pathogenic mechanisms using in-silico models.

Intermittent pressure overload triggers hypertrophy-independent cardiac dysfunction and vascular rarefaction

For over a century, there has been intense debate as to the reason why some cardiac stresses are pathological and others are physiological. One long-standing theory is that physiological overloads such as exercise are intermittent, while pathological overloads such as hypertension are chronic. In this study, we hypothesized that the nature of the stress on the heart, rather than its duration, is the key determinant of the maladaptive phenotype. To test this, we applied intermittent pressure overload on the hearts of mice and tested the roles of duration and nature of the stress on the development of cardiac failure. Despite a mild hypertrophic response, preserved systolic function, and a favorable fetal gene expression profile, hearts exposed to intermittent pressure overload displayed pathological features. Importantly, intermittent pressure overload caused diastolic dysfunction, altered beta-adrenergic receptor (betaAR) function, and vascular rarefaction before the development of cardiac hypertrophy, which were largely normalized by preventing the recruitment of PI3K by betaAR kinase 1 to ligand-activated receptors. Thus stress-induced activation of pathogenic signaling pathways, not the duration of stress or the hypertrophic growth per se, is the molecular trigger of cardiac dysfunction.

Heart size-independent analysis of myocardial function in murine pressure overload hypertrophy

Pressure overload cardiac hypertrophy may be a compensatory mechanism to normalize systolic wall stress and preserve left ventricular (LV) function. To test this concept, we developed a novel in vivo method to measure myocardial stress (sigma)-strain (epsilon) relations in normal and hypertrophied mice. LV volume was measured using two pairs of miniature omnidirectional piezoelectric crystals implanted orthogonally in the endocardium and one crystal placed on the anterior free wall to measure instantaneous wall thickness. Highly linear sigma-epsilon relations were obtained in control (n = 7) and hypertrophied mice produced by 7 days of transverse aortic constriction (TAC; n = 13). Administration of dobutamine in control mice significantly increased the load-independent measure of LV contractility, systolic myocardial stiffness. In TAC mice, systolic myocardial stiffness was significantly greater than in control mice (3,156 +/- 1,433 vs. 1,435 +/- 467 g/cm(2), P < 0.01), indicating enhanced myocardial contractility with pressure overload. However, despite the increased systolic performance, both active (time constant of LV pressure decay) and passive (diastolic myocardial stiffness constant) diastolic properties were markedly abnormal in TAC mice compared with control mice. These data suggest that the development of cardiac hypertrophy is associated with a heightened contractile state, perhaps as an early compensatory response to pressure overload.

Biventricular systolic function in young lambs subject to chronic systemic right ventricular pressure overload

In various clinical situations of congenital heart disease, the right ventricle (RV) is subject to a chronic systemic pressure overload which affects biventricular function and may progress to the development of RV failure. Young lambs (2-3 wk old) underwent adjustable pulmonary artery banding (PAB) at systemic (aortic) level for 8 wk. Biventricular function was determined by using load-independent indexes of global ventricular contractile performance by the end-systolic pressure-volume relationship (ESPVR) using the conductance catheter at baseline and during dobutamine infusion. PAB resulted in a significant fivefold increase in RV end-systolic pressure (12-64 mmHg) and a doubling of the RV-to-left ventricular (LV) wall thickness ratio (P < 0.01). RV global contractile performance increased significantly, as indicated by an increased slope of the ESPVR. Compared with age-matched control lambs, cardiac output decreased from 2.6 to 1.6 l/min (P < 0.05) whereas heart rates were equal. In contrast with RV volume, LV volume decreased significantly after PAB (P < 0.01), whereas the LV-ESPVR slope was unchanged. In the PAB group, the RV, but not the LV, showed a reduced response to dobutamine. We concluded that chronic RV pressure overload for 8 wk results in diminished pump function despite compensatory increased RV global contractile performance.

Disruption of the myocardial extracellular matrix leads to cardiac dysfunction

MMP activity with disruption of structural collagen has been implicated in the pathophysiology of dilated cardiomyopathy. To examine the role of this enzyme in cardiac function, a transgenic mouse was created that constitutively expressed human collagenase (MMP-1) in the heart. At 6 months of age, these animals demonstrated compensatory myocyte hypertrophy with an increase in the cardiac collagen concentration due to elevated transcription of type III collagen. Chronic myocardial expression of MMP-1 produced loss of cardiac interstitial collagen coincident with a marked deterioration of systolic and diastolic function at 12 months of age. This is the first animal model demonstrating that direct disruption of the extracellular matrix in the heart reproduces the changes observed in the progression of human heart failure.

Left ventricular wall stress normalization in chronic pressure-overloaded heart: a mathematical model study

It is generally accepted that the left ventricle (LV) hypertrophies (LVH) to normalize systolic wall stress (sigma(s)) in chronic pressure overload. However, LV filling pressure (P(v)) may be elevated as well, supporting the alternative hypothesis of end-diastolic wall stress (sigma(d)) normalization in LVH. We used an LV time-varying elastance model coupled to an arterial four-element lumped-parameter model to study ventricular-arterial interaction in hypertension-induced LVH. We assessed model parameters for normotensive controls and applied arterial changes as observed in hypertensive patients with LVH (resistance +40%, compliance -25%) and assumed 1) no cardiac adaptation, 2) normalization of sigma(s) by LVH, and 3) normalization of sigma(s) by LVH and increase in P(v), such that sigma(d) is normalized as well. In patients, systolic and diastolic blood pressures increase by approximately 40%, cardiac output (CO) is constant, and wall thickness increases by 30-55%. In scenarios 1 and 2, blood pressure increased by only 10% while CO dropped by 20%. In scenario 2, LV wall thickness increased by only 10%. The predictions of scenario 3 were in qualitative and quantitative agreement with in vivo human data. LVH thus contributes to the elevated blood pressure in hypertension, and cardiac adaptations include an increase in P(v), normalization of sigma(s), and preservation of CO in the presence of an impaired diastolic function.

Effect of myocardial hypertrophy on systolic and diastolic function in children: insights from the force-frequency and relaxation-frequency relationships

OBJECTIVE

The objective of this study was to evaluate the effect of myocardial hypertrophy on systolic and diastolic properties of the left ventricle in children.

BACKGROUND

In children with myocardial hypertrophy, ejection phase indices are invariably increased. However, indices of force-generation, e.g., end-systolic elastance and invasive indices of diastolic properties, have been studied infrequently in children with myocardial hypertrophy.

METHODS

We studied 10 children with congenital aortic stenosis or coarctation of aorta and nine control patients. Systolic properties were assessed from shortening fraction, end-systolic fiber elastance (Ef(es)) measured at resting heart rates, and force-frequency relationship measured at heart rates increasing from 110 to 160 beats per minute. Diastolic properties were assessed from time constant of relaxation (tau) at matched heart rates, chamber stiffness constant, myocardial stiffness constant, and relaxation-frequency relationship measured at gradually increasing heart rates.

RESULTS

Ef(es) remained unchanged by myocardial hypertrophy, however, tau was prolonged (tauL: 27.3+/-2.3 vs. 21.8+/-2.2 ms, p < 0.001; and tauD: 43.2+/-3.1 vs. 34.3+/-3.3 ms, p < 0.001). Both chamber and myocardial stiffness constants remained unchanged. Incremental increases in heart rate produced incremental improvement in both contraction and relaxation. Slopes of force-frequency and relaxation-frequency relationships remained unchanged in the experimental group. However, the relaxation-frequency relationship manifested a parallel shift upward.

CONCLUSIONS

In conscious, sedated children with myocardial hypertrophy, systolic function assessed by an index of force generation remains unchanged. However, relaxation is prolonged but passive diastolic properties remain unaffected. The combined effect of hypertrophy and heart rate does not alter the force-frequency and relaxation-frequency relationships.

The ionic mechanism of reperfusion-induced early afterdepolarizations in feline left ventricular hypertrophy

Left ventricular hypertrophy (LVH) potentiates reperfusion-associated ventricular fibrillation. To study the mechanism responsible, patch-clamp techniques were used to evaluate transmembrane ionic currents during "reperfusion" after a CN(-)-induced metabolic surrogate for ischemia in isolated myocytes from a feline model of experimental LVH. Reperfusion caused the generation of early afterdepolarizations (EADs) from an average take-off potential of -33 mV in LVH cells but not in cells from normal hearts. 10 min after initiating reperfusion of normal cells, action potential duration (APD) at 50% repolarization (APD50) lengthened from 198 +/- 41 to 233 +/- 57 ms whereas in LVH cells APD50 lengthened from 262 +/- 84 to 349 +/- 131 ms (P < 0.05). Among the LVH cells, APD50 lengthening was significantly greater in the cells that had developed EADs. During reperfusion, steady state outward current in the voltage range of the action potential plateau (between -20 and +20 mV) was reduced from the control values in LVH cells but not in normal cells. Reperfusion-related reduction of steady state outward current in LVH cells was abolished under experimental conditions in which L-type Ca2+ current was isolated from other classes of currents whereas it was still observed under the condition in which pure K+ currents could be recorded. Thus, reduction of steady state outward current due to the reduction of outward K+ current over the action potential plateau voltage range appears to be responsible for an excessive prolongation of APD, leading to the development of EADs.

Enhanced left ventricular contractility in autonomic failure: assessment using pressure-volume relations

Severe autonomic failure is usually characterized by both supine hypertension and orthostatic hypotension. Inadequate preload reserve, insufficient arterial resistance and abnormal cardiac performance have been postulated to contribute to the hypotension. To clarify these mechanisms, left ventricular performance and contractility were assessed using radionuclide ventriculography and systolic pressure-volume relations when supine and with graded head-up tilt in 11 patients with autonomic failure. Results were compared with those of 12 normal subjects, using phenylephrine infusion for pharmacologic afterload augmentation after autonomic blockade with atropine and propranolol. In a subset of four patients with autonomic failure, systolic pressure-volume relations were similar by both the tilt and phenylephrine methods. In autonomic failure, end-diastolic volume, end-systolic volume and stroke volume decreased with progressive degrees of tilt (p less than or equal to 0.007 for each). The supine radionuclide ejection fraction and cardiac output were similar to those of normal subjects (69% versus 68% and 5.4 versus 4.9 liters/min, respectively, p = NS). However, the slopes of the pressure-volume relations and the supine pressure/volume ratio in autonomic failure were much greater than normal (8.8 versus 2.5, and 6.3 versus 3.6 mm Hg/ml, respectively, p less than or equal to 0.04 for both). The baseline total peripheral resistance was greater than normal (24.9 versus 17.4 mm Hg.min-1/liter, p = 0.01), but the resistance at maximal tilt failed to increase (20.8 +/- 6.1 units). Plasma norepinephrine concentrations were lower than normal. Thus, patients with autonomic failure had hypercontractile left ventricular performance when assessed by pressure-volume relations, and their hearts were well matched to the elevated peripheral resistance.(ABSTRACT TRUNCATED AT 250 WORDS)

Systolic and diastolic dysfunction during atrial pacing in conscious dogs with left ventricular hypertrophy

To determine the extent to which the hypertrophied left ventricle responds to the chronotropic stress induced by graded atrial pacing rates, we studied conscious, chronically instrumented dogs with severe compensated pressure overload left ventricular (LV) hypertrophy induced by aortic banding in puppies 8-10 weeks of age. At 1-2 years, dogs with severe LV hypertrophy (LV free wall/body wt ratio 6.8 +/- 0.6 g/kg) and sham-operated littermates (LV free wall/body wt ratio 4.0 +/- 0.3 g/kg) were instrumented with ultrasonic dimension crystals to measure LV short axis internal diameter and wall thickness, miniature LV pressure transducers, and aortic and LV catheters. During atrial pacing (240 beats/min) in eight control dogs, LV pressure did not change from 119 +/- 2 mm Hg, and mean velocity of circumferential fiber shortening (VCF) did not change from 1.25 +/- 0.09/sec. In seven dogs with LV hypertrophy, atrial pacing (240 beats/min) decreased systolic LV function; that is, LV systolic pressure decreased (p less than 0.01) by 65 +/- 12 from 254 +/- 14 mm Hg, and VCF decreased (p less than 0.01) by 0.19 +/- 0.03 from 0.97 +/- 0.15/sec. Diastolic dysfunction was also observed in the dogs with LV hypertrophy. In the control dogs during atrial pacing (240 beats/min), LV end-diastolic pressure decreased (p less than 0.01) by 8 +/- 1 from 9 +/- 1 mm Hg, end-diastolic stress decreased (p less than 0.01) by 18 +/- 2 from 22 +/- 2 g/cm2, and the radial myocardial stiffness constant did not change from 5.6 +/- 1.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile function, myosin ATPase activity and isozymes in the hypertrophied pig left ventricle after a chronic progressive pressure overload

Experimental right ventricular pressure-overload hypertrophy in small mammals is associated with early muscle dysfunction, even before the onset of overt pump failure. Experimental results are quite heterogeneous regarding muscle function of the pressure hypertrophied left ventricle. Muscle dysfunction of the right or left ventricle, when found, may be causally related to alterations of myosin ATPase activity and isozyme type. However, the effect of a gradual pressure overload, analogous to that which occurs in human aortic stenosis, on myocardial contractile function and myosin ATPase activity has not been studied in a large animal whose normal myosin isozyme pattern resembles that of man. We therefore studied pump performance, myocardial contractile function, and myosin ATPase activity and isozyme pattern in pigs with severe, gradually applied left ventricular pressure overload. Thirteen weeks after supravalvular aortic banding, 10 pigs grew more than 7-fold in body weight and were found to have an aortic stenosis area of 0.5 +/- 0.1 cm2 with a gradient of 93 +/- 12 mm Hg. Compared with nine control animals, the banded animals had a 67% increase in left ventricular mass relative to body weight without overt pump failure as measured by cardiac index and pulmonary artery wedge pressure. Left ventricular ejection performance, measured as shortening fraction, was maintained except in three animals with extreme hypertrophy, in which depressed ejection performance may have been due to an afterload mismatch, myocardial dysfunction, or both. Myocardial contractile function, determined from the end-systolic stress-diameter relationship, was normal except in two pigs in which ejection performance was depressed and left ventricular mass was more than doubled. Only the slow V3 isozyme of myosin ATPase was found in both normal and hypertrophied pig myocardium, and the ATPase activity was normal in pigs with all degrees of hypertrophy. Thus, in a large animal model of severe, gradual left ventricular pressure overload, in which myosin isozyme pattern remains apparently unaltered, moderate hypertrophy can be associated with normal myosin ATPase activity and contractile function that is normal by current methods of evaluation.

Cardiac hypertrophy: useful adaptation or pathologic process?

An extensive body of evidence supports the concept that cardiac hypertrophy and normal cardiac growth develop in response to increased hemodynamic loading and abnormal systolic and diastolic stresses at the myocardial fiber level. The pattern of hypertrophy reflects the nature of the inciting stress. Experimental studies indicate that if the stress is moderate, gradually applied, and the animal young and healthy, physiologic hypertrophy of muscle with normal contractility develops. In this circumstance, cardiac hypertrophy may be regarded as a useful adaptation to increased hemodynamic loading. When the inciting stress is severe, abruptly applied, or the animal old or debilitated, pathologic hypertrophy develops: in this circumstance, the cardiac muscle produced is abnormal and exhibits depressed contractility. Of particular clinical relevance is the intermediate situation which seems to develop in many patients with chronic left ventricular pressure-overload and perhaps also in left ventricular volume-overload. In this situation, chronic left ventricular pressure or volume overload is initially matched by adequate hypertrophy in the appropriate pattern. Eventually, in some patients, hypertrophy fails to keep pace with the hemodynamic overload so that a systolic stress imbalance occurs at the myocardial fiber level and left ventricular pump failure ensues. If this situation persists uncorrected, it is possible that the increasingly high wall stresses will convert physiologic to pathologic hypertrophy. The task of the clinician is to identify this intermediate stage and to correct the abnormal hemodynamic loading before the transition to pathologic hypertrophy becomes complete.