Effect of Beta-blocker therapy on survival in patients with severe mitral regurgitation and normal left ventricular ejection fraction

Primary Mitral Regurgitation and Heart Failure: Current Advances in Diagnosis and Management

Primary mitral regurgitation is a frequent etiology of congestive heart failure and is best treated with intervention when patients are symptomatic or when additional risk factors exist. Surgical intervention improves outcomes in appropriately selected patients. However, for those at high surgical risk, transcatheter intervention provides less invasive repair and replacement options while providing comparable outcomes to surgery. The excess mortality and high prevalence of heart failure in untreated mitral regurgitation illuminate the need for further developments in mitral valve intervention ideally fulfilled by expanding these types of procedures and eligibility to these procedures beyond only those at high surgical risk.

The current diagnosis and treatment of high-risk patients with chronic primary and secondary mitral valve regurgitation

Mitral valve regurgitation (MR) is due primarily to either primary degeneration of the mitral valve with Barlow's or fibroelastic disease or is secondary to ischemic or nonischemic cardiomyopathies. Echocardiography is essential to assess MR etiology and severity, the remodeling of cardiac chambers and to characterize longitudinal chamber changes to determine optimal therapies. Surgery is recommended for severe primary MR if persistent symptoms are present or if left ventricle dysfunction is present with an EF <60% or a left ventricle end-systolic diameter ≥40 mm. For secondary MR, therapy of heart failure with vasodilators and diuretics improves forward cardiac output. Coronary artery bypass grafts (CABG) or percutaneous coronary intervention (PCI) should be considered for severe MR due to ischemia. This review summarizes the pathophysiology, the characteristics, the management and the different interventions for high risk patients with chronic primary and secondary MR.

2020 ACC/AHA Guideline for the Management of Patients With Valvular Heart Disease: Executive Summary: A Report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines

AIM

This executive summary of the valvular heart disease guideline provides recommendations for clinicians to diagnose and manage valvular heart disease as well as supporting documentation to encourage their use.

METHODS

A comprehensive literature search was conducted from January 1, 2010, to March 1, 2020, encompassing studies, reviews, and other evidence conducted on human subjects that were published in English from PubMed, EMBASE, Cochrane, Agency for Healthcare Research and Quality Reports, and other selected database relevant to this guideline. Structure: Many recommendations from the earlier valvular heart disease guidelines have been updated with new evidence and provides newer options for diagnosis and treatment of valvular heart disease. This summary includes only the recommendations from the full guideline which focus on diagnostic work-up, the timing and choice of surgical and catheter interventions, and recommendations for medical therapy. The reader is referred to the full guideline for graphical flow charts, text, and tables with additional details about the rationale for and implementation of each recommendation, and the evidence tables detailing the data considered in developing these guidelines.

Dynamic Changes in the Molecular Signature of Adverse Left Ventricular Remodeling in Patients With Compensated and Decompensated Chronic Primary Mitral Regurgitation

BACKGROUND

There is no proven medical therapy that attenuates adverse left ventricular remodeling in patients with chronic primary mitral regurgitation (CPMR). Identification of molecular pathways important in the progression of left ventricular remodeling in patients with CPMR may lead to development of new therapeutic strategies.

METHODS AND RESULTS

We performed baseline echocardiographic, cardiac catheterization, and serum NT-pro-BNP analysis in patients with severe CPMR awaiting mitral valve surgery and stratified the study population into compensated or decompensated CPMR. We obtained left ventricular endomyocardial biopsies (n=12) for mRNA expression analysis, and compared baseline transcript levels of 109 genes important in volume-overload left ventricular remodeling with levels in normal hearts (n=5) and between patients with compensated (n=6) versus decompensated (n=6) CPMR. Patients were then randomized to treatment with and without carvedilol and followed until the time of surgery (mean follow-up 8.3 months) when repeat endomyocardial biopsies were obtained to correlate transcriptional dynamics with indices of adverse remodeling. CPMR was associated with increased NPPA expression levels (21.6-fold, P=0.004), decreased transcripts of genes important in cell survival, and enrichment of extracellular matrix genes. Decompensated CPMR was associated with downregulation of SERCA2 (0.77-fold, P=0.009) and mitochondrial gene expression levels and upregulation of genes related to inflammation, the extracellular matrix, and apoptosis, which were refractory to carvedilol therapy.

CONCLUSIONS

Transition to decompensated CPMR is associated with calcium dysregulation, increased expression of inflammatory, extracellular matrix and apoptotic genes, and downregulation of genes important in bioenergetics. These changes are not attenuated by carvedilol therapy and highlight the need for development of specific combinatorial therapies, targeting myocardial inflammation and apoptosis, together with urgent surgical or percutaneous valve interventions.

Endothelial Dysfunction in Patients with Severe Mitral Regurgitation

Mitral valve prolapse (MVP) is the most common cause of severe mitral regurgitation. It has been reported that MVP patients—candidates for mitral valve repair (MVRep)—showed an alteration in the antioxidant defense systems as well as in the L-arginine metabolic pathway. In this study, we investigate if oxidative stress and endothelial dysfunction are an MVP consequence or driving factors. Forty-five patients undergoing MVRep were evaluated before and 6 months post surgery and compared to 29 controls. Oxidized (GSSG) and reduced (GSH) forms of glutathione, and L-arginine metabolic pathway were analyzed using liquid chromatography-tandem mass spectrometry methods while osteoprotegerin (OPG) through the ELISA kit and circulating endothelial microparticles (EMP) by flow cytometry. Six-month post surgery, in MVP patients, the GSSG/GSH ratio decreased while symmetric and asymmetric dimethylarginines levels remained comparable to the baseline. Conversely, OPG levels significantly increased when compared to their baseline. Finally, pre-MVRep EMP levels were significantly higher in patients than in controls and did not change post surgery. Overall, these results highlight that MVRep completely restores the increased oxidative stress levels, as evidenced in MVP patients. Conversely, no amelioration of endothelial dysfunction was evidenced after surgery. Thus, therapies aimed to restore a proper endothelial function before and after surgical repair could benefit MVP patients.

The Role of Arterial Hypertension in Mitral Valve Regurgitation

PURPOSE OF REVIEW

To review medical literature for evidence of association between hypertension and mitral regurgitation (MR) and summarize potential favorable effects of antihypertensive drugs on MR natural history and treatment.

RECENT FINDINGS

Hypertension and MR are common diseases affecting a large proportion of the general population. Contemporary evidence suggests that hypertension may worsen the progression and prognosis of MR through augmented mechanical stress and increased regurgitation volume. Renin-angiotensin axis inhibitors, beta-blockers, and vasodilators have been tested in order to prevent or decrease primary or secondary MR. Although antihypertensive agents may improve hemodynamic parameters and left ventricular remodeling in primary MR, there is no strong evidence of benefit on clinical outcomes. On the other hand, a beneficial effect of these drugs on secondary MR is better established. Moreover, there are no studies evaluating a possible benefit of lower blood pressure targets in MR. Randomized controlled trials are warranted to elucidate the precise role of antihypertensive therapy on treatment of MR.

Left ventricular remodelling in chronic primary mitral regurgitation: implications for medical therapy

Surgical repair or replacement of the mitral valve is currently the only recommended therapy for severe primary mitral regurgitation. The chronic elevation of wall stress caused by the resulting volume overload leads to structural remodelling of the muscular, vascular and extracellular matrix components of the myocardium. These changes are initially compensatory but in the long term have detrimental effects, which ultimately result in heart failure. Understanding the changes that occur in the myocardium due to volume overload at the molecular and cellular level may lead to medical interventions, which potentially could delay or prevent the adverse left ventricular remodelling associated with primary mitral regurgitation. The pathophysiological changes involved in left ventricular remodelling in response to chronic primary mitral regurgitation and the evidence for potential medical therapy, in particular beta-adrenergic blockers, are the focus of this review.

Impact of Beta-Blockade on Cardiac Events in Patients with Chronic Severe Nonischemic Mitral Regurgitation

OBJECTIVES

The aim of this study was to examine the impact of beta-blockade on cardiac events among patients with initially asymptomatic chronic severe nonischemic mitral valve regurgitation (MR).

METHODS

Data from 52 consecutive patients in our prospective natural history study of isolated chronic severe nonischemic MR were assessed post hoc over 19 years to examine the relation of chronic beta-blockade use to subsequent cardiac events (death or indications for mitral valve surgery, MVS). At entry, all patients were free of surgical indications; 9 received beta-blockers. Cardiac event rate differences were analyzed by Kaplan-Meier log rank comparison.

RESULTS

During follow-up, cardiac events included sudden death (1), heart failure (8), atrial fibrillation (6), left ventricular dimensions at systole ≥4.5 cm (11), left ventricular ejection fraction <60% (6), right ventricular ejection fraction <35% (2), and a combination of cardiac events (7). The cardiac event risk was 4-fold higher among patients receiving beta-blockers (average annual risk = 60.6%) versus those not receiving beta-blockers (average annual risk = 15.2%; p = 0.001). These effects remained statistically significant (p = 0.005) when analysis was adjusted for other baseline covariates.

CONCLUSIONS

Beta-blockade appears to confer an increased risk of sudden cardiac death or indications for MVS among patients with chronic severe nonischemic MR. Randomized trials are needed to confirm these findings.

Targeting the sympatho-adrenergic link in chronic rheumatic mitral regurgitation: assessing the role of oral beta-blockers

INTRODUCTION

Chronic mitral regurgitation (MR) is characterized by adverse ventricular remodeling and progressive LV dysfunction leading to heart failure (HF). Beta-blockers (BB) improve LV remodeling and prognosis in patients with HF. As chronic severe MR results in neuroendocrine activation similar to HF, it is likely that BB may also exert favorable effects in these patients. No study has assessed the role of oral BB therapy in chronic rheumatic MR.

AIMS

A total of 100 patients of chronic rheumatic MR (mean age 30±13.48 years, NYHA 2.2±0.5) were randomized to BB (Metoprolol, 37±13.5 mg, n=48) vs no BB (n=52) in addition to standard therapy.

RESULTS

Baseline BNP and echocardiographic parameters were comparable in the two groups. At 3 months, BB therapy resulted in significantly lower NYHA class (1.97 vs 2.35), BNP (141 vs 207 pg/mL), LV end-systolic (35.89 vs 51.30) and LV end-diastolic volumes (101 vs 128 mL/m(2) ), LV end-systolic stress (81.1 vs 93.3 dyn/cm(2) ), LV mass (122 vs 154 gm/m(2) ), and LV work (737.02 vs 952.82 mm Hg L/min, all P significant). Therapy with BB resulted in a -15.6%, -10.4%,-12.1%, and -7.3% reduction in LV end-systolic and end-diastolic dimensions and LVESVi and LVEDVi, respectively. Following BB therapy, BNP levels, end-systolic stress, indexed LV mass, and LV work also reduced significantly by 27.3%, 15.6%, 8.7%, and 28%, respectively. The control group had no significant change. The MR grade reduced from severe to moderate in 11% of those on BB (controls: no change). At 6 months, the BB group had further improvement in all echocardiographic parameters ranging from +9.1 to -18.2%.

CONCLUSION

In this first study of BB in rheumatic MR, targeting the sympatho-adrenergic axis exerted favorable effects on NYHA class, LV volumes, LV end-systolic stress, and LV work. Further studies are required to elucidate the role of BB in rheumatic MR.

Drug Therapy for Heart Valve Diseases

Valvular heart diseases (VHDs) are progressive. When not caused by acute comorbidities they are generally characterized by long asymptomatic phases during which hemodynamic severity may progress leading to morbidity and mortality. Treatment depends on VHD type and severity but when severe and symptomatic, usually involves mechanical intervention. Asymptomatic patients, and those who lack objective descriptors associated with high risk, are closely observed clinically with optimization of associated cardiovascular risk factors until surgical indications develop. Though often prescribed based on theory, no rigorous evidence supports pharmacological therapy in most chronic situations though drugs may be appropriate in acute valvular diseases, or as a bridge to surgery in severely decompensated patients. Herein, we examine evidence supporting drug use for chronic VHDs.

Comparative effectiveness of Mitraclip plus medical therapy versus medical therapy alone in high-risk surgical patients: a comprehensive review

In recent years, Mitraclip has become available as a treatment option for mitral regurgitation in high-risk surgical patients. Focusing on the incremental effectiveness of Mitraclip versus the current standard of care, this article provides a comparative review of the evidence on Mitraclip and standard medical therapy (MT) in high-risk mitral regurgitation patients. Evidence was retrieved from seven major databases. Results suggest that Mitraclip presents a high safety profile and a good middle-term effectiveness performance. Evidence on long-term effectiveness is limited both for Mitraclip and MT. Few studies allow a comparison with MT and comparative results on different endpoints are mixed. Therefore, the available evidence does not conclusively inform whether or under which circumstances Mitraclip should be preferred over MT in the treatment of high-risk patients. Head-to-head real-world studies would be needed, as they would provide great and timely insights to support policy decisions when medical devices are at stake.

The multiple mechanistic faces of a pure volume overload: implications for therapy

: Mitral regurgitation and other conditions marked by a pure isolated volume overload (VO) of the heart result in a progressive form of eccentric left ventricular remodeling and dysfunction. As opposed to the more extensively studied pressure overload, there are no approved medical therapies because an understanding of the underlying pathological mechanisms at work in VO is lacking. Over the past 20 years, our laboratory has identified multiple key biological functions involved in the pathological remodeling in VO. Specifically, we have noted perturbed matrix homeostasis, detrimental adrenergic signaling, increased intracellular reactive oxygen species and an intense inflammatory response that implicates mast cells and their product chymase, which seems to cause extensive remodeling both inside and outside the cardiomyocyte. How these multiple pathways intersect over the course of VO and their response to various single and combined interventions are now the subject of intense investigation.

Functional mitral regurgitation and left ventricular systolic dysfunction in the recent era of cardiovascular clinical practice, an observational cohort study

Functional mitral regurgitation (MR) is frequently associated with left ventricular systolic dysfunction (LVSD). Ventricular volume overload that occurs in patients with MR may lead to a progression of myocardial dysfunction. However, the prevalence and clinical outcomes of functional MR in Japanese patients with LVSD remain unclear. The aim of the present study is to clarify the prevalence and prognosis of functional MR in Japanese LVSD patients in the contemporary era. We followed patients with LVSD (LV ejection fraction (LVEF) ⩽ 40%) who were listed within a single, hospital-based cohort in the Shinken Database from 2004 to 2011, which was composed of all new patients (n = 17,517) who visited the Cardiovascular Institute. A total of 506 patients were included: 86 FMR (moderate-to-severe functional MR) patients and 420 non-FMR (none or mild functional MR) patients. FMR patients were older, had lower rates of hypertension and ischemic heart disease but had higher rates of chronic kidney disease, dilated cardiomyopathy and New York Heart Association III/IV classification. FMR patients had higher brain natriuretic peptide levels and lower LVEF. The Kaplan-Meier curves revealed that the incidence of all-cause death, cardiovascular death and heart failure (HF) admission was significantly higher in FMR patients. The presence of FMR was independently associated with a significantly higher risk of composite end point, including all-cause death and/or HF admission (hazard ratio 1.551, 95% confidence interval 1.045-2.303, P = 0.029). FMR was common in Japanese patients with LVSD and was associated with adverse long-term outcomes. Future study is warranted to establish the optimal therapeutic strategy for FMR and LVSD.

Increased sarcolipin expression and adrenergic drive in humans with preserved left ventricular ejection fraction and chronic isolated mitral regurgitation

BACKGROUND

There is currently no therapy proven to attenuate left ventricular (LV) dilatation and dysfunction in volume overload induced by isolated mitral regurgitation (MR). To better understand molecular signatures underlying isolated MR, we performed LV gene expression analyses and overlaid regulated genes into ingenuity pathway analysis in patients with isolated MR.

METHODS AND RESULTS

Gene arrays from LV tissue of 35 patients, taken at the time of surgical repair for isolated MR, were compared with 13 normal controls. Cine-MRI was performed in 31 patients before surgery to measure LV function and volume from serial short-axis summation. LV end-diastolic volume was 2-fold (P=0.005) higher in MR patients than in normal controls, and LV ejection fraction was 64±7% (50%-79%) in MR patients. Ingenuity pathway analysis identified significant activation of pathways involved in β-adrenergic, cAMP, and G-protein-coupled signaling, whereas there was downregulation of pathways associated with complement activation and acute phase response. SERCA2a and phospholamban protein were unchanged in MR versus control left ventricles. However, mRNA and protein levels of the sarcoplasmic reticulum Ca2+ ATPase (SERCA) regulatory protein sarcolipin, which is predominantly expressed in normal atria, were increased 12- and 6-fold, respectively. Immunofluorescence analysis confirmed the absence of sarcolipin in normal left ventricles and its marked upregulation in MR left ventricles.

CONCLUSIONS

These results demonstrate alterations in multiple pathways associated with β-adrenergic signaling and sarcolipin in the left ventricles of patients with isolated MR and LV ejection fraction>50%, suggesting a beneficial role for β-adrenergic blockade in isolated MR.

Effects of early, late, and long-term nonselective β-blockade on left ventricular remodeling, function, and survival in chronic organic mitral regurgitation

BACKGROUND

Mitral regurgitation (MR) produces sympathetic nervous system activation which is detrimental in other causes of heart failure. However, whether β-blockade is beneficial in MR has not been determined.

METHODS AND RESULTS

Eighty-seven rats with significant organic MR were randomized to the β-blockade group (n=43) or the control group (n=44). Carvedilol was started in week 2 post MR induction and given for 23 to 35 weeks in the β-blockade group. Echocardiography was performed at baseline and at weeks 2, 6, 12, 24, 30, and 36 after MR induction. After 23 weeks of β-blockade, heart rates were significantly reduced by carvedilol (308 ± 25 versus 351 ± 31 beats per minute; P<0.001). Left ventricular end-diastolic (2.2 ± 0.7 versus 1.59 ± 0.6 mL; P<0.001), end-systolic volumes (0.72 ± 0.42 versus 0.40 ± 0.19 mL; P<0.001), and mass index (2.40 ± 0.55 versus 2.06 ± 0.62 g/kg; P<0.001) were significantly higher, and left ventricular fraction shortening (33 ± 7% versus 38 ± 7%; P<0.001) and ejection fraction (69 ± 11% versus 75 ± 7%; P<0.001) were significantly lower in the β-blockade group than in the control group. Systolic blood pressure was lower in the β-blockade group than in the control group (114 ± 10 versus 93 ± 12 mm Hg; P<0.005). Survival probability was significantly lower in the early β-blockade group than in the control group (88% versus 96%; P=0.03).

CONCLUSIONS

Early and long-term nonselective β-blockade was associated with adverse left ventricular remodeling, systolic dysfunction, and a reduction in survival in the experimental rat model of organic MR.

A randomized controlled phase IIb trial of beta(1)-receptor blockade for chronic degenerative mitral regurgitation

OBJECTIVES

The purpose of the study was to evaluate the effect of long-term β(1)-aderergic receptor (AR) blockade on left ventricular (LV) remodeling and function in patients with chronic, isolated, degenerative mitral regurgitation (MR).

BACKGROUND

Isolated MR currently has no proven therapy that attenuates LV remodeling or preserves systolic function.

METHODS

Thirty-eight asymptomatic subjects with moderate to severe, isolated MR were randomized either to placebo or β(1)-AR blockade (Toprol-XL, AstraZeneca, London, United Kingdom) for 2 years. Magnetic resonance imaging with tissue tagging and 3-dimensional analysis was performed at baseline and at 6-month intervals for 2 years. Rate of progression analysis was performed for endpoint variables for primary outcomes: LV end-diastolic volume/body surface area, LV ejection fraction, LV end-diastolic (ED) mass/ED volume ratio, LV ED 3-dimensional radius/wall thickness; LV end-systolic volume/body surface area, LV longitudinal strain rate, and LV early diastolic filling rate.

RESULTS

Baseline LV magnetic resonance imaging or demographic variables did not differ between the 2 groups. Significant treatment effects were found on LV ejection fraction (p = 0.006) and LV early diastolic filling rate (p = 0.001), which decreased over time in untreated patients on an intention-to-treat analysis and remained significant after sensitivity analysis. There were no significant treatment effects found on LV ED or LV end-systolic volumes, LV ED mass/LV ED volume or LV ED 3-dimensional radius/wall thickness, or LV longitudinal strain rate. Over 2 years, 6 patients treated in the placebo group and 2 patients in the β(1)-AR blockade group required mitral valve surgery (p = 0.23).

CONCLUSIONS

β(1)-AR blockade improves LV function over a 2-year follow-up in isolated MR and provides the impetus for a large-scale clinical trial with clinical outcomes. (Molecular Mechanisms of Volume Overload-Aim 1 [SCCOR in Cardiac Dysfunction and Disease]; NCT01052428).

Mitral regurgitation: current trends in diagnosis and management

Mitral regurgitation is the most prevalent heart valve disorder in the United States. Individuals with mitral regurgitation may be asymptomatic or may present with dyspnea, orthopnea, fatigue, and/or heart rhythm disturbances. Long-standing mitral regurgitation causes chronic left ventricular volume overload, which leads to left ventricular dilation and contractile dysfunction. Without treatment, mitral regurgitation results in biventricular failure and death. Echocardiography is the preferred diagnostic test to assess the presence and severity of mitral regurgitation. Mitral valve surgery, the only effective treatment for patients with severe mitral regurgitation, is recommended early in the course of the disease to prevent the development of heart failure. Early recognition of mitral regurgitation and timely referral for mitral valve surgery significantly improve symptoms and long-term survival.

Changes in mitral annular geometry and dynamics with ß-blockade in patients with degenerative mitral valve disease

BACKGROUND

remodeling of the mitral annulus contributes to progression of mitral regurgitation (MR). In patients with moderate-to-severe MR, short-term treatment with β-blockers has been shown to increase left ventricular (LV) end-diastolic and end-systolic volume, and this could deleteriously increase mitral valve annular dimensions. The objective of this study was to quantify the effects of a short duration of β-blocker treatment on mitral annular dimensions and dynamics in patients with MR due to primary degenerative valve disease.

METHODS AND RESULTS

twenty-five patients with moderate-to-severe degenerative MR and normal LV systolic function were studied in a double-blind crossover experiment using a β1-selective adrenergic blocker and placebo administered for 14±3 days. Cardiac MRI images were acquired after each treatment period to quantify mitral annular dimensions. At end diastole, there was no change in annular area (1659±331 versus 1632±299 mm(2); P<0.19), annular perimeter (154.3±16.4 versus 152±13.9 mm; P<0.13), septal-lateral (SL) dimension (38.0±5 versus 39.0±4.5 mm; P<0.15), or annular height (9.8±3.8 versus 9.5±2.5 mm; P<0.53). β-blockade resulted in significant end-diastole decreases in commissure-commissure dimension (48.9±4.6 versus 47.2±4.0 mm; P<0.01) and eccentricity (1.3±0.2 versus 1.2±0.1; P<0.01). At end systole (ES), β-blockade conferred a small, but significant decrease in annular perimeter (161.0±19.3 versus 156.8±16.9 mm; P<0.04) and eccentricity (1.2±0.1 versus 1.1±0.1; P<0.02), and the SL dimension significantly increased (41.5±5.7 versus 43.0±5.3 mm; P<0.03). Commissure-commissure dimension, annular area, and annular height at ES were not significantly different.

CONCLUSIONS

despite significant increases in LV end-diastolic and end-systolic volume, short-term β-blocker treatment of patients with moderate-to-severe MR reduced or preserved all mitral annular dimensions except SL at ES.

Serial changes in left ventricular shape following early mitral valve repair

Mitral valve (MV) repair has become the preferred treatment for mitral regurgitation associated with degenerative MV disease. Although the functional benefits of early MV repair are known, the associated alterations in left ventricular (LV) shape have not been studied. The aim of this study was to evaluate serial changes in LV performance and 3-dimensional (3D) shape after MV repair using a new analytic technique. Fifty patients (mean age 59 +/- 12 years) with severe asymptomatic mitral regurgitation and ejection fractions >55% who underwent MV repair were enrolled. Transthoracic real-time 3D echocardiography was performed the day before and 6 and 12 months after MV repair. An age-matched control group of 50 normal subjects was studied for comparison. Endocardial surfaces were extracted to compute 3D shape indexes of sphericity and conicity, which were compared in patients between different time points and versus the normal group. Postoperatively, LV volumes were decreased compared to presurgical values. Ejection fractions were slightly decreased 6 months after surgery but had restored by 12 months. These changes were associated with modifications in end-diastolic LV shape: before surgery, compared to normal controls, sphericity was augmented and conicity was decreased; at 6 months, these shape changes were reversed, with no further improvements at 12 months. In conclusion, patients with asymptomatic mitral regurgitation and preserved LV function already exhibit changes in LV shape. Early MV repair leads to near normal morphology after surgery, indicating the benefits of this procedure. Real-time 3D echocardiography and novel 3D shape analysis allow detailed serial examination of the complex relation between LV performance and shape.

Effect of electrical stimulus parameters on the development and propagation of action potentials in short excitable fibres

Intracellular action potentials (IAPs) produced by short fibres in response to their electrical stimulation were analysed. IAPs were calculated on the basis of the Hodgkin-Huxley (1952) model by the method described by Joyner et al. (1978). Principal differences were found in processes of activation of short (semilength L less than 5 lambda) and long fibres under near-threshold stimulation. The shorter the fibre, the lower was the threshold value (Ithr). Dependence of the latency on the stimulus strength (Ist) was substantially non-linear and was affected by the fibre length. Both fibre length and stimulus strength influenced the IAP amplitude, the instantaneous propagation velocity (IPV) and the site of the first origin of the IAP (and, consequently, excitability of the short fibre membrane). With L less than or equal to 2 lambda and Ithr less than or equal to Ist less than or equal to 1.1Ithr, IPV could reach either very high values (so that all the fibre membrane fired practically simultaneously) or even negative values. The latter corresponded to the first origin of the propagated IAP, not at the site of stimulation but at the fibre termination or at a midpoint. The characters of all the above dependencies were unchanged irrespective of the manner of approaching threshold (variation of stimulus duration or its strength). Reasons for differences in processes of activation of short and long fibres are discussed in terms of electrical load and latency. Applications of the results to explain an increased jitter, velocity recovery function and velocity-diameter relationship are also discussed.