Myocardial force-frequency defect in mitral regurgitation heart failure is reversed by forskolin

Effects of Intravenous Pimobendan on Cardiovascular Parameters in Healthy Sedated Cats

This study aimed to investigate the effects of intravenous pimobendan on cardiovascular function and to determine the appropriate dose for clinical usage in cats. Six purpose-bred cats received one of the following treatments: intravenous pimobendan at a single dose of 0.075 mg/kg (low dose [LD] group), 0.15 mg/kg (middle dose [MD] group), 0.3 mg/kg (high dose [HD] group), or saline at 0.1 mL/kg (placebo group). Echocardiography and blood pressure measurements were performed before and 5, 15, 30, 45, and 60 minute after drug administration for each treatment. In the MD and HD groups, the fractional shortening, peak systolic velocity, cardiac output, and heart rate increased significantly. There were no significant differences in blood pressure among the groups. Intravenous pimobendan at 0.15-0.3 mg/kg increased the fractional shortening, peak systolic velocity, cardiac output in healthy cats.

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.

Influence of alterations in heart rate on left ventricular echocardiographic measurements in healthy cats

Objectives The purpose of this study was to evaluate the effect of sudden alterations in heart rate (HR) on left ventricular (LV) wall thickness and dimensions determined by echocardiography in healthy cats. Methods Six experimental cats were used. All cats were anaesthetised and HR was controlled with right atrial pacing. The interventricular septum and left ventricular free wall thickness at end diastole (IVSd and LVFWd, respectively), left ventricular end-diastolic and end-systolic diameter (LVIDd and LVIDs, respectively) and shortening fraction (FS) of each cat were assessed using echocardiography at pacing rates of 120, 130, 140, 150, 160, 170 and 180 ppm. Results There were significant relationships between HR and IVSd, LVFWd, LVIDd, LVIDs and FS. As the HR increased, LV wall thickness increased and chamber dimensions got smaller in a linear fashion. The maximum and minimum differences in wall thickness between 120 ppm and 180 ppm were 2.0 mm and 0.7 mm in single measurements, respectively. Conclusions and relevance LV wall thickness and dimensions were significantly influenced by alterations in HR.

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.

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.

Volume overload

Structural cardiac volume overload comprises a group of heterogeneous diseases, each creating a nearly unique set of loading conditions on the left ventricle and/or right ventricle. In turn, the heart responds to each with unique patterns of remodeling, leading to both adaptive and maladaptive consequences. An understanding of these different patterns of hypertrophy and/or remodeling should be useful in developing strategies for the timing and correction of cardiac volume overload.

Role of oxidative stress in disease progression in Stage B, a pre-cursor of heart failure

Oxidative stress represents a persistent imbalance between the production and the compensation of reactive oxygen species. Though predominantly found in advanced heart failure, the most frequent "at-risk" condition has been associated with underlying oxidative stress. It is therefore conceivable that timely detection and early intervention to reduce oxidative stress processes provide an opportunity to prevent disease progression to overt heart failure. This article reviews the current understanding of the current evidence of oxidative stress involvement in the pathophysiology of human heart failure and its potential therapeutic interventions in patients with Stage A and B heart failure.

Oxidative stress and myocardial remodeling in chronic mitral regurgitation

Mechanisms of left ventricular (LV) dysfunction in isolated mitral regurgitation (MR) are not well understood. Vasodilator therapy in other forms of LV dysfunction reduces LV wall stress and improves LV function; however, studies in isolated MR show no beneficial effect on LV remodeling using vasodilator drugs or renin-angiotensin system blockade. Therefore, the search for new therapies that improve LV remodeling and function in isolated MR is clinically significant. Recent work in the authors' laboratory has demonstrated increased oxidants from a number of sources including the enzyme xanthine oxidase (XO) in the LV of patients with isolated MR. In addition to being a major source of reactive oxygen species, XO is linked to bioenergetic dysfunction because its substrates derive from adenosine triphosphate catabolism. Correspondingly, there was also evidence of aggregates of small mitochondria in cardiomyocytes, which is generally considered a response to bioenergetic deficit in cells. Future studies are required to determine whether XO and persistent oxidative stress are causative in maladaptive LV remodeling and offer potential therapeutic targets in ameliorating LV damage in patients with isolated MR.

Increased oxidative stress and cardiomyocyte myofibrillar degeneration in patients with chronic isolated mitral regurgitation and ejection fraction >60%

OBJECTIVES

This study assessed myocardial damage in patients with chronic isolated mitral regurgitation (MR) and left ventricular ejection fraction (LVEF) >60%.

BACKGROUND

Typically, MR patients have decreased LVEF after mitral valve (MV) repair despite normal pre-operative LVEF.

METHODS

Twenty-seven patients with isolated MR had left ventricular (LV) biopsies taken at time of MV repair. Magnetic resonance imaging with tissue tagging was performed in 40 normal subjects and in MR patients before and 6 months after MV repair.

RESULTS

LVEF (66 +/- 5% to 54 +/- 9%, p < 0.0001) and LV end-diastolic volume index (108 +/- 28 ml/m(2) to 78 +/- 24 ml/m(2), p < 0.0001) decreased, whereas left ventricular end-systolic (LVES) volume index was 60% above normal pre- and post-MV repair (p < 0.05). The LV circumferential and longitudinal strain rates decreased below normal following MV repair (6.38 +/- 1.38 vs. 5.11 +/- 1.28, p = 0.0009, and 7.51 +/- 2.58 vs. 5.31 +/- 1.61, percentage of R to R interval, p < 0.0001), as LVES stress/LVES volume index ratio was depressed at baseline and following MV repair versus normal subjects (0.25 +/- 0.10 and 0.28 +/- 0.05 vs. 0.33 +/- 0.12, p < 0.01). LV biopsies demonstrated cardiomyocyte myofibrillar degeneration versus normal subjects (p = 0.035). Immunostaining and immunoblotting demonstrated increased xanthine oxidase in MR versus normal subjects (p < 0.05). Lipofuscin deposition was increased in cardiomyocytes of MR versus normal subjects (0.62 +/- 0.20 vs. 0.33 +/- 0.11, percentage of area: p < 0.01).

CONCLUSIONS

Decreased LV strain rates and LVES wall stress/LVES volume index following MV repair indicate contractile dysfunction, despite pre-surgical LVEF >60%. Increased oxidative stress could cause myofibrillar degeneration and lipofuscin accumulation resulting in LV contractile dysfunction in MR.

Effect of losartan on exercise tolerance and echocardiographic parameters in patients with mitral regurgitation

OBJECTIVES

The aim of this study was to assess the effects of losartan treatment on exercise tolerance and echocardiographic parameters in patients with mitral regurgitation (MR) secondary to mitral valve prolapse or rheumatic heart disease.

METHODS

Twenty-seven patients (14 males, 13 females, mean age 51+/-11, range 21-76) with moderate MR due to mitral valve prolapse or rheumatic heart disease were examined by means of Doppler echocardiography. The subjects were submitted to treadmill exercise tests using the modified Bruce protocol at baseline, after six hours and after the six-week treatment period to be evaluated based on their exercise tolerance. Mitral Regurgitant Volume (MRV), effective regurgitant orifice diameter, left atrial volume, left ventricle (LV) end-diastolic volume index, LV end-systolic volume index, LV ejection fraction (LVEF), left ventricle mass index were calculated at baseline and after six weeks of treatment with single dose of losartan (50 mg/day).

RESULTS

Total treadmill exercise time increased from 477.7+/-147.9 to 535.7+/-149.0 seconds after six hours (p<0.01) and to 559.6+/-142.8 seconds after six weeks of treatment. Also, metabolic equivalent values increased following six hours of first dose and six weeks of losartan treatment (from 10.9+/-2.9 to 11.8+/-3.1, p=0.006 and 12.4+/-3.1, p=0.002; respectively). However, peak exercise systolic blood pressure (BP) was reduced after six hours and six weeks of treatment, and resting diastolic BP did not change after six hours but reduced at the end of the treatment period. MR volume decreased significantly from 29.3+/-14.1 ml to 25.1+/-14.8 ml, (p=0.025) without significant change in regurgitant orifice diameter (0.72+/-0.37 cm vs. 0.66+/-0.37 cm, p=NS), left atrium diameter and area while LVEF increased from 51.70+/-13.37 to 54.11-11.75 (p=0.015) with losartan.

CONCLUSION

We conclude that the angiotensin II receptor antagonist losartan improves exercise tolerance and echocardiographic parameters in patients with moderate MR.

Negative stress echo: Further prognostic stratification with assessment of pressure–volume relation

BACKGROUND

A maximal negative stress echo identifies a low risk for subsequent hard events subset. However, the potentially prognostically relevant information on global contractile reserve on the left ventricle is missed by standard regional wall motion assessment, and can be obtained by end-systolic pressure-volume relationship (PVR) evaluation.

AIM

To assess the relative prognostic value of PVR in patients with negative stress echo.

METHODS

We enrolled 99 consecutive patients (age=61+/-14 years; 81 males, LVEF 47+/-14%, WMSI=1.42+/-0.50) with negative exercise stress echo for standard wall motion criteria. To build the PVR, the force was determined at rest and peak stress as the ratio of the systolic pressure/end-systolic volume index. All patients were followed-up on medical therapy.

RESULTS

Median follow-up was 21 months (interquartile range 12-26). Twenty-nine events have been observed: 6 deaths, 10 heart failure related hospitalization and 13 worsening NYHA class of >or=1 grade. Using Cox's proportional hazard model the best independent predictor of total events was SP/ESV index change (rest-stress) <1.5 mm Hg/ml/m(2) as determined by ROC analysis cut-off (RR=29, p=0.001, sensitivity=80%, specificity=93%). The overall survival and event-free survival was 34% in patients with change (rest-stress) SP/ESV index<1.5 mm Hg/ml/m(2) and 97% in whose with >1.5 mm Hg/ml/m(2).

CONCLUSIONS

In patients with negative stress echo, a preserved global contractility response can be easily identified through stress-induced variation in SP/ESV index, with powerful further risk stratification.

Cardiac reflections and natural vibrations: force-frequency relation recording system in the stress echo lab

Background

The inherent ability of ventricular myocardium to increase its force of contraction in response to an increase in contraction frequency is known as the cardiac force-frequency relation (FFR). This relation can be easily obtained in the stress echo lab, where the force is computed as the systolic pressure/end-systolic volume index ratio, and measured for increasing heart rates during stress. Ideally, the noninvasive, imaging independent, objective assessment of FFR would greatly enhance its practical appeal.

Objectives

1 – To evaluate the feasibility of the cardiac force measurement by a precordial cutaneous sensor. 2 – To build the curve of force variation as a function of the heart rate. 3 – To compare the standard stress echo results vs. this sensor operator-independent built FFR.

Methods

The transcutaneous force sensor was positioned in the precordial region in 88 consecutive patients referred for exercise, dipyridamole, or pacing stress. The force was measured as the myocardial vibrations amplitude in the isovolumic contraction period. FFR was computed as the curve of force variation as a function of heart rate. Standard echocardiographic FFR measurements were performed.

Results

A consistent FFR was obtained in all patients. Both the sensor built and the echo built FFR identifiy pts with normal or abnormal contractile reserve. The best cut-off value of the sensor built FFR was 15.5 g * 10^-3 (Sensitivity = 0.85, Specificity = 0.77). Sensor built FFR slope and shape mirror pressure/volume relation during stress. This approach is extendable to daily physiological exercise and could be potentially attractive in home monitoring systems.

Altered myocardial shear strains are associated with chronic ischemic mitral regurgitation

BACKGROUND

Ischemic mitral regurgitation (IMR) limits life expectancy and can lead to postinfarction global left ventricular (LV) dilatation and remodeling, the pathogenesis of which is not completely known. We tested the hypothesis that IMR perturbs adjacent myocardial LV systolic strains.

METHODS

Thirteen sheep had three columns of miniature beads inserted across the lateral LV wall, with additional epicardial markers silhouetting the ventricle. One week later posterolateral infarction was created. Seven weeks thereafter, the animals were divided into two groups according to severity of IMR (< or = 1+, n = 7, IMR[-] vs > or = 2+, n = 6, IMR[+]). Four dimensional marker coordinates and quantitative histology were used to calculate ventricular volumes, transmural myocardial systolic strains, and systolic fiber shortening.

RESULTS

Seven weeks after infarction, end-diastolic (ED) volume increased similarly in both groups, end-systolic (ES) E13 (circumferential-radial) shear increased in both groups, but more so in IMR(+) than IMR(-) (+0.12 vs 0.04, p < 0.005), and E12 (circumferential-longitudinal) shear increased in IMR(-) but not IMR(+) (+0.04 vs -0.01, p < 0.005). There were no significant differences in ED or ES remodeling strains or systolic fiber shortening between IMR(-) and IMR(+).

CONCLUSIONS

An equivalent increase in LV end-diastolic (ED) volume in both groups, coupled with unchanged ED and end-systolic remodeling strains as well as systolic circumferential, longitudinal, and radial strains, argue against a global LV or regional myocardial geometric basis for the cardiomyopathy associated with IMR. Further, similar systolic fiber shortening in both groups militates against an intracellular (cardiomyocyte) mechanism. The differences in subepicardial E12 and E13 shears, however, suggest a causal role of altered interfiber (cytoskeleton and extracellular-matrix) interactions.

Force- and relaxation-frequency relations in patients with diastolic heart failure

BACKGROUND

Chronotropic effects on myocardial contractility (the positive force-frequency relation) and relaxation (the positive relaxation-frequency relation) are impaired in patients with congestive heart failure and depressed left ventricular (LV) ejection fraction (systolic heart failure [SHF]). However, the force- and relaxation-frequency relation and LV-arterial coupling in patients with diastolic heart failure (DHF) has not been fully investigated.

METHODS AND RESULTS

To examine inotropic and lusitropic responsiveness to atrial pacing, LV pressure-volume relations were measured using a conductance catheter and microtip manometer in patients with DHF (n = 18) and SHF (n = 11). In patients with SHF, an increase in heart rate by 40 beat/min did not affect LV end-systolic elastance (Ees), which reflects LV contractility, or the time constant of LV relaxation. By contrast, in patients with DHF, an increase in heart rate by 40 beat/min significantly enhanced Ees (2.1 vs 2.9 mm Hg/mL, P < .05) but not the time constant. Furthermore, LV-arterial coupling, quantified as Ees/arterial elastance, was impaired during pacing in patients with DHF (1.1 vs 0.8, P < .05) as well as SHF.

CONCLUSIONS

In patients with DHF, the force-frequency relation was preserved, but the relaxation-frequency relation was impaired. Furthermore, LV-arterial coupling was impaired as heart rate increased, which may be related to the impaired LV function. These results suggest that the impaired relaxation-frequency relation and exacerbated LV-arterial coupling during tachycardia may be an important therapeutic target in patients with DHF.

Controversies in ventricular remodelling

Ventricular remodelling describes structural changes in the left ventricle in response to chronic alterations in loading conditions, with three major patterns: concentric remodelling, when a pressure load leads to growth in cardiomyocyte thickness; eccentric hypertrophy, when a volume load produces myocyte lengthening; and myocardial infarction, an amalgam of patterns in which stretched and dilated infarcted tissue increases left-ventricular volume with a combined volume and pressure load on non-infarcted areas. Whether left-ventricular hypertrophy is adaptive or maladaptive is controversial, as suggested by patterns of signalling pathways, transgenic models, and clinical findings in aortic stenosis. The transition from apparently compensated hypertrophy to the failing heart indicates a changing balance between metalloproteinases and their inhibitors, effects of reactive oxygen species, and death-promoting and profibrotic neurohumoral responses. These processes are evasive therapeutic targets. Here, we discuss potential novel therapies for these disorders, including: sildenafil, an unexpected option for anti-transition therapy; surgery for increased sphericity caused by chronic volume overload of mitral regurgitation; an antifibrotic peptide to inhibit the fibrogenic effects of transforming growth factor beta; mechanical intervention in advanced heart failure; and stem-cell therapy.

Myocardial contractility in the echo lab: molecular, cellular and pathophysiological basis

In the standard accepted concept, contractility is the intrinsic ability of heart muscle to generate force and to shorten, independently of changes in the preload or afterload with fixed heart rates. At molecular level the crux of the contractile process lies in the changing concentrations of Ca²⁺ ions in the myocardial cytosol. Ca²⁺ ions enter through the calcium channel that opens in response to the wave of depolarization that travels along the sarcolemma. These Ca²⁺ ions "trigger" the release of more calcium from the sarcoplasmic reticulum (SR) and thereby initiate a contraction-relaxation cycle.

In the past, several attempts were made to transfer the pure physiological concept of contractility, expressed in the isolated myocardial fiber by the maximal velocity of contraction of unloaded muscle fiber (Vmax), to the in vivo beating heart. Suga and Sagawa achieved this aim by measuring pressure/volume loops in the intact heart: during a positive inotropic intervention, the pressure volume loop reflects a smaller end-systolic volume and a higher end-systolic pressure, so that the slope of the pressure volume relationship moves upward and to the left. The pressure volume relationship is the most reliable index for assessing myocardial contractility in the intact circulation and is almost insensitive to changes in preload and after load. This is widely used in animal studies and occasionally clinically. The limit of the pressure volume relationship is that it fails to take into account the frequency-dependent regulation of contractility: the frequency-dependent control of transmembrane Ca²⁺ entry via voltage-gated Ca2⁺ channels provides cardiac cells with a highly sophisticated short-term system for the regulation of intracellular Ca²⁺ homeostasis. An increased stimulation rate increases the force of contraction: the explanation is repetitive Ca²⁺ entry with each depolarization and, hence, an accumulation of cytosolic calcium. As the heart fails, there is a change in the gene expression from the normal adult pattern to that of fetal life with an inversion of the normal positive slope of the force-frequency relation: systolic calcium release and diastolic calcium reuptake process is lowered at the basal state and, instead of accelerating for increasing heart rates, slows down. Since the force-frequency relation uncovers initial alteration of contractility, as an intermediate step between normal and abnormal contractility at rest, a practical index to measure it is mandatory.

Measuring end-systolic elastance for increasing heart rates is impractical: increasing heart rates with atrial pacing has to be adjunct to the left ventricular conductance catheter, to the left ventricular pressure catheter, to the vena cava balloon, and to afterload changes. Furthermore, a noninvasive index is needed. Noninvasive measurement of the pressure/volume ratio for increasing heart rates during stress in the echo lab could be the practical answer to this new clinical demand in the current years of a dramatic increase in the number of heart failure patients.

Functional Consequences of Sarcomeric Protein Abnormalities in Failing Myocardium

Sarcomeric protein abnormalities have been recognized for many years in heart failure due to dilated cardiomyopathy (DCM). In contrast, virtually nothing is known about myofilament abnormalities in heart failure occurring in association with diastolic dysfunction. With the exception of sarcomeric protein mutations that cause DCM, the most important mechanism of myofilament dysfunction in DCM is probably altered post-translational modification, in particular the phosphorylation state of troponins I and T and possibly myosin light chain. Other modifications, including oxidation and glycation, may also play a role. Myosin heavy chain isoform switching occurs in human heart failure, but its functional significance is uncertain. Myofilament abnormalities contribute significantly to myocardial dysfunction in DCM, although their relative importance compared with abnormal calcium handling is debated. One consistent functional abnormality in DCM is increased myofilament calcium sensitivity of tension generation, which contributes to slowed or incomplete relaxation. More recently, decreases in the optimal frequency of myofilament work and power generation have been recognized. These changes may contribute to depression of the force-frequency relation in DCM. Altered mechanoenergetics constitute one of the most important manifestations of myofilament dysfunction in heart failure. DCM and hemodynamic overload are associated with more economical and efficient energy utilization by the contractile machinery, which may be protective of the myocardium. This change is strongly associated with depressed myofibrillar ATPase activity. We speculate that the effectiveness of mechanical therapies such as resynchronization may at least in part be related to improved mechanical function without loss of this mechanoenergetic advantage.

End-systolic pressure/volume relationship during dobutamine stress echo: a prognostically useful non-invasive index of left ventricular contractility

AIMS

Left ventricular end-systolic pressure-volume relationship (PVR) provides a robust, relatively load-insensitive evaluation of contractility and can be assessed non-invasively during exercise echo. Dobutamine might provide an exercise-independent alternative approach to assess inotropic reserve. The feasibility of a non-invasive estimation of PVR during dobutamine stress in the echo lab and its relationship with subsequent clinical events was assessed.

METHODS AND RESULTS

We enrolled 137 consecutive patients referred for dobutamine stress echo. To build the PVR, the force was determined at different heart rate increments during stepwise dobutamine infusion as the ratio of the systolic pressure/end-systolic volume index. The PVR at increasing heart rate was flat-biphasic in 65 and up-sloping in 72 patients: 42 patients underwent surgery and 95 patients were treated medically (median follow-up, 18 months; interquartile range, 12-24). Events occurred in 18 patients (death in eight, acute heart failure in 10); a flat-biphasic PVR was independent predictor of events (RR=10.16, P<0.01).

CONCLUSION

PVR is feasible during dobutamine stress. This index of global contractility is reasonably simple, does not affect the imaging time, and only minimally prolongs the off-line analysis time. It allows unmasking quite different, and heterogeneous, contractility reserve patterns underlying a given ejection fraction at rest. The best survival is observed in patients with up-sloping PVR, whereas flat-biphasic pattern is a strong predictor of cardiac events.

Noninvasive assessment of left ventricular contractility by pacemaker stress echocardiography

BACKGROUND

Estimating contractility of the left ventricle with noninvasive techniques is an important yet elusive goal. Positive inotropic interventions are mirrored by smaller end-systolic volumes and higher end-systolic pressures. An increased heart rate progressively increases the force of ventricular contraction (Bowditch treppe or staircase phenomenon).

AIM

To assess the feasibility of a noninvasive estimation of force-frequency relation (FFR) during pacing stress in the echo lab in patients with permanent pacemaker (PM).

METHODS

Transthoracic stress pacing echocardiography was performed in 26 patients with a permanent pacemaker (age 69+/-11 years; 21 men, 5 women). Seven patients had normal function at baseline and during stress ("normals"); eight had angiographically assessed coronary artery disease (three with and five without induced ischemia with stress echo); eleven patients had dilated cardiomyopathy (DC). To build the FFR, the force was determined at different steps as the ratio of the systolic pressure (SP, cuff sphygmomanometer)/end-systolic volume index (ESV, biplane Simpson rule/body surface area). Heart rate was determined from ECG.

RESULTS

The absolute value of the FFR slope was highest in controls and lowest in DC patients. A flat-downsloping FFR was found in 12/19 patients but not for normals (p<0.01).

CONCLUSIONS

Noninvasive pacemaker stress echocardiography (PASE) is a simple and efficient option to assess left ventricular (LV) contractility in patients with permanent pacemaker.

Regional differences in the force-frequency relation of human left ventricular myocardium in mitral regurgitation: implications for ventricular shape

Sphericalization of the left ventricular (LV) chamber shape in patients with mitral regurgitation (MR) contributes to increased LV wall stress and energy consumption. On the basis of previous observations, we hypothesized the existence of regional differences in the force-frequency relation (FFR) within the LV that may contribute to its shape. Accordingly, in the present study, we assessed regional variation in the FFR in patients undergoing surgery for chronic, nonischemic MR with class II–III heart failure symptoms and related our findings to the in vivo LV shape. FFRs (steady-state isometric twitches, 0.2–3.4 Hz, 37°C) were evaluated in MR myocardium from the LV subepicardial free wall (MR-FW) and papillary muscle (MR-PM) and from the subepicardial free wall in coronary artery bypass graft patients with normal LV contraction patterns [nonfailing (NF)]. Ascending slope, optimal stimulation frequency, and maximal twitch tension of the FFR were depressed in MR-FW and MR-PM compared with NF ( P < 0.05). FFR depression was greater in MR-PM than in MR-FW. Between 107 and 134 beats/min, twitch tension became weaker in MR-PM, whereas it increased in MR-FW. Elevation of intracellular cAMP with forskolin eliminated FFR depression in MR-FW but not in MR-PM. MR-PM also had a 35% lower myosin heavy chain content and slowed twitch kinetics. In MR patients, the echocardiographic end-diastolic LV shape (end-diastolic eccentricity index = long axis/short axis) correlated with the ratio of ascending FFR slopes such that the end-diastolic eccentricity index increased 10% per 15% increase in slope ratio ( r = 0.88, P = 0.01). These regional differences in the frequency dependence of contractility between the free wall and papillary myocardium may contribute to changes in LV shape in MR as well as during exercise.

Cardiac-specific norepinephrine mass transport and its relationship to left ventricular size and systolic performance

Objectives of this study were to develop a technique for quantifying cardiac-specific norepinephrine (NE) mass transport and determine whether cardiac NE kinetic modeling parameters were related to physiological variables of left ventricular (LV) size and systolic performance in nine patients with chronic mitral regurgitation. Biplane contrast cineventriculograms were used to determine LV size and ejection fraction (EF), micromanometer LV pressures and radionuclide LV volumes from a range of loading conditions to calculate LV end-systolic elastance, and [(3)H]NE infusions with LV and coronary sinus sampling for [(3)H]NE and endogenous NE during and after termination of infusions to model NE mass transport. Total NE release rate into cardiac interstitial fluid (M(IF)(R)) averaged 859 +/- 214 and NE released de novo into cardiac interstitial fluid (M(IF)(u,r,en)) averaged 546 +/- 174 pmol/min. Both M(IF)(R) and M(IF)(u,r,en)correlated directly with LV end-systolic volume (r = 0.84, P = 0.005; r = 0.86, P = 0.003); inversely with LV EFs (r = -0.75, P = 0.02; r = -0.81, P = 0.008); and inversely with LV end-systolic elastance values, optimally fit by a nonlinear function (r = 0.89, P = 0.04; r = 0.96, P = 0.01). We conclude that total and newly released NE into interstitial fluid of the heart, determined by regional mass transport kinetic model, are specific measures of regional cardiac-specific sympathetic nervous system activity and are strongly related to measures of LV size and systolic performance. These data support the concept that this new model of organ-specific NE kinetics has physiological relevance.

Is it ever too late to operate on the patient with valvular heart disease?

All valvular heart disease imparts a hemodynamic burden on the left and/or right ventricle. This burden can only be removed effectively by correcting the responsible valvular lesion. Although a percutaneous approach is usually used to correct mitral stenosis, other valve lesions require surgical intervention. Over the past 40 years there has been a persistent improvement in our understanding of the pathophysiology of valvular heart disease and in the surgical techniques for correcting it. These factors have acted in concert to alter our view of the proper timing and applicability of surgery. On one hand it is no longer necessary or even advisable to delay surgery until advanced symptoms are present, and thus surgery is timed earlier today than it was even a decade ago. On the other hand, many but not all patients with far advanced disease, once considered inoperable, are now often helped substantially by valve surgery. However, selection of which of these very ill patients will or will not benefit from valve surgery remains a challenge for all of us. It is this group of patients that is addressed in the review.

Changes in systemic sympathetic nervous system activity after mitral valve surgery and their relationship to changes in left ventricular size and systolic performance in patients with mitral regurgitation

BACKGROUND

We have shown that the systemic sympathetic nervous system (SNS) is activated in patients with chronic mitral regurgitation (MR). However, the fate of systemic SNS activity after surgical correction of MR is currently unknown.

METHODS

We examined 14 patients with MR who had normal sinus rhythm with an investigational, preoperative cardiac catheterization, including arterial norepinephrine (NE) sampling and [(3)H]-NE infusions and arterial blood sampling to determine NE kinetic parameters using a 2-compartment modeling analysis. The arterial NE and NE kinetic parameters were determined in all patients after mitral valve surgery (MVS) at a mean of 12 months. A 2-dimensional echocardiographic examination was also performed before and after MVS.

RESULTS

The average extravascular NE release rates (NE(2)) before and after MVS were 1.89 +/- 0.66 and 2.26 +/- 0.82 microg/min/m(2) (P =.24), respectively. The average left ventricular (LV) end-diastolic dimension, fractional shortening, and ejection fraction decreased, whereas the mean LV end-systolic dimension did not change between the pre- and post-MVS echocardiographic studies. However, these group averages were comprised of patients with MR in whom the NE(2) and echocardiographic values both increased and decreased. This lack of homogeneity was a reflection of our new observation that the pre- to post-MVS changes in NE(2) were directly proportional to the changes in LV end-systolic dimension (r = 0.91, P <.001) and inversely related to the changes in LV fractional shortening (r = -0.82, P <.001) and ejection fraction (r = -0.78, P <.001).

CONCLUSIONS

The response in systemic SNS activity in patients with MR after MVS is not homogeneous, and these changes are concordant with the post-MVS changes in LV size and systolic performance. These data further support our earlier observations and extend them to suggest that systemic SNS activation in patients with chronic MR is related to LV remodeling and impaired systolic performance.

Force-frequency relationship in the echocardiography laboratory: a noninvasive assessment of Bowditch treppe?

BACKGROUND

Estimation of contractility of the left ventricle is an important, and as yet elusive, goal with noninvasive techniques.

OBJECTIVE

We sought to assess the feasibility of a totally noninvasive estimation of force-frequency relation (FFR) during exercise stress in the echocardiography laboratory.

METHODS

We enrolled 13 healthy control patients (12 men, age 38 +/- 15 years) as group I, and 50 patients (38 men, age 64 +/- 11 years) referred for exercise echocardiography as group II. To build the FFR, the force was determined at each step as the ratio of the systolic pressure (cuff sphygmomanometer)/end-systolic volume index (biplane Simpson's rule/body surface area). The slope of the relationship was calculated with the linear best fit of the FFR.

RESULTS

Noninvasive systolic pressure/end-systolic volume ratio was obtained in all patients. The slope of the linear best fit of the force-frequency curve was lower in group II compared with group I (group II = 10.1 +/- 9.3 x 10(-2) vs group I = 14.9 +/- 9.9 x 10(-2) group I, P =.04). By regional wall-motion analysis, 2 subgroups were identified in group II: group IIA (n = 8) had a positive echocardiogram; and group IIB (n = 42) had a negative echocardiogram. The slope of the force-frequency curve was lower in patients with ischemia compared with those without (group IIA = 3.5 +/- 4.2 x 10(-2) vs group IIB = 11.4 +/- 9.5 x 10(-2); P =.012). Heart rate-systolic pressure/end-systolic volume index relation was biphasic, with an initial positive slope and a subsequent negative slope in 1 patient of group I, 4 patients of group IIA, and 15 patients of group IIB (P <.05 vs group I).

CONCLUSION

A noninvasive estimation of FFR can be easily determined during exercise echocardiography. This index of global contractility is theoretically appealing for identification of limited contractile reserve and latent global left ventricular dysfunction.

Disparate force-frequency effects of pimobendan and dobutamine in conscious dogs with tachycardia-induced cardiomyopathy

BACKGROUND

This study was designed to examine how a calcium sensitizer, pimobendan, affected a force-frequency response (FFR) as compared to the beta-adrenergic agonist dobutamine.

METHODS AND RESULTS

Left ventricular (LV) contractility and relaxation were evaluated by the slope (Ees) of the LV end-systolic pressure-volume relation and the time constant (Tau) of LV pressure decay. Using 6 conscious dogs with tachycardia-induced heart failure, the FFR was examined before and after administration of dobutamine (6 microg/kg/min) or pimobendan (0.5 mg/kg). Despite the similar inotropic and lusitropic action at the baseline heart rate, pimobendan and dobutamine showed different FFR and relaxation-frequency responses. Before administration of these drugs, there was no significant increase in LV contractility and relaxation by increasing heart rate. However, dobutamine amplified FFR (Ees: +3.1 +/- 1.4, P <.05) as compared with Ees for a comparable increase in heart rate before administration of the drug. On the other hand, pimobendan showed relatively mild amplification of FFR compared with dobutamine (Ees: +1.9 +/- 1.1, P <.05). The relaxation-frequency response tended to increase with dobutamine but not with pimobendan.

CONCLUSIONS

Mild amplification of FFR observed in pimobendan suggests that this agent could be used more safely than beta-adrenergic agent when heart rate is increased, as seen with exercise.

SERCA2A overexpression decreases the incidence of aftercontractions in adult rabbit ventricular myocytes

K. Davia, E. Bernobich, H. K. Ranu, F. del Monte, C. M. N. Terracciano, K. T. MacLeod, D. L. Adamson, B. Chaudhri, R. J. Hajjar and S. E. Harding. SERCA2a Overexpression Decreases the Incidence of Aftercontractions in Adult Rabbit Ventricular Myocytes. Journal of Molecular and Cellular Cardiology (2001) 33, 1005-1015. Slow relaxation and poor contractile response to increasing stimulation frequency in failing human heart have been strongly linked to a decrease in the activity of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a). Restoration of SERCA2a levels using gene transfer has beneficial effects on contractile function but, like beta -adrenoceptor stimulation, could potentially produce excess SR Ca(2+), arrhythmias and cell death. We have examined the effects of SERCA2a overexpression in adult rabbit cardiac myocytes, and compared changes in relaxation with those following beta -adrenoceptor stimulation. Myocytes were infected with an adenovirus carrying both SERCA2a and green fluorescent protein (GFP) for positive identification of infected cells. Myocyte survival was significantly enhanced in the infected cultures. There was a reduction in both time-to-peak contraction and time-to-50% relaxation (R50) 48 h after infection. Time-to-90% relaxation (R90) was particularly improved (non-infected 516+/-41 ms, AD.SERCA2a-GFP 230+/-23 ms, n=7 preparations, P<0.001). There was also a decreased incidence of aftercontractions in Ad.SERCA2a-GFP infected myocytes (21+/-5%v 41+/-4% in controls, P<0.01). This contrasts with beta -adrenoceptor stimulation, which reduced R50 but prolonged R90 by 158+/-76 ms (P<0.02, n=16). At higher stimulation frequencies (2-3 Hz) contraction amplitude and SR calcium content were increased and diastolic contracture was reduced following SERCA2a overexpression. Overall, increasing levels of SERCA2a resulted in an improvement in systolic and diastolic function and a reduction in cell death and arrhythmic aftercontractions. SERCA2a overexpression therefore lacks the detrimental effects associated with some other inotropic interventions.

Progress in mitral and aortic regurgitation

Over the past 15 years there has been rapid and dramatic change in the therapy for valvular heart disease. When mitral and aortic regurgitation are severe, they inevitably cause left ventricular damage, eventually resulting in death. However, when surgical correction of these lesions is timed appropriately, longevity can approach that of a normal population after surgery. As surgical techniques have improved, surgery is now indicated earlier in the course of these diseases. It is clear that some patients with mitral and aortic regurgitation require surgery even though they are entirely asymptomatic. However, it must be emphasized that mitral and aortic regurgitation are quite different from one another. These different lesions result in different loading conditions, different pathophysiologies, and have different means for surgical correction. All of these issues impact on the proper timing of surgery and are discussed.

Normal myocardial function in severe right ventricular volume overload hypertrophy

Severe left ventricular volume overloading causes myocardial and cellular contractile dysfunction. Whether this is also true for severe right ventricular volume overloading was unknown. We therefore created severe tricuspid regurgitation percutaneously in seven dogs and then observed them for 3.5-4.0 yr. All five surviving operated dogs had severe tricuspid regurgitation and right heart failure, including massive ascites, but they did not have left heart failure. Right ventricular cardiocytes were isolated from these and from normal dogs, and sarcomere mechanics were assessed via laser diffraction. Right ventricular cardiocytes from the tricuspid regurgitation dogs were 20% longer than control cells, but neither the extent (0.171 +/- 0.005 microm) nor the velocity (2.92 +/- 0.12 microm/s) of sarcomere shortening differed from controls (0.179 +/- 0.005 microm and 3.09 +/- 0.11 microm/s, respectively). Thus, despite massive tricuspid regurgitation causing overt right heart failure, intrinsic right ventricular contractile function was normal. This finding for the severely volume-overloaded right ventricle stands in distinct contrast to our finding for the left ventricle severely volume overloaded by mitral regurgitation, wherein intrinsic contractile function is depressed.

DITPA prevents the blunted contraction-frequency relationship in myocytes from infarcted hearts

Loss of the positive force-frequency relationship is a characteristic finding in failing hearts. The mechanisms of this change are not well understood. Myocardial infarction (MI) was induced in rabbits to produce left ventricular (LV) dysfunction. Beginning 1 day after MI, a subgroup of rabbits received diiodothyropropionic acid (DITPA) (3.75 mg x kg(-1) x day(-1) sc) for 3 wk. We measured contractions, Ca(2+) transients, action potentials, and sarcoplasmic reticulum (SR) Ca(2+) content at different stimulation rates in single LV myocytes. The shortening-frequency relationship was markedly flattened in MI myocytes compared with control myocytes. In addition, Ca(2+) transients, action potentials, and contractions were prolonged. Myocytes from DITPA-treated MI rabbits had preserved inotropic responses to increased stimulation rate and normal duration of action potentials and Ca(2+) transients. SR Ca(2+) content increased significantly when stimulation rate was increased from 0.5 to 2.0 Hz in control myocytes but did not change significantly in MI myocytes. Myocytes from DITPA-treated MI rabbits had a greater frequency-dependent increase in SR Ca(2+) content compared with the untreated MI rabbits. Thus single myocytes from infarcted rabbit hearts have frequency-dependent abnormalities of contractility, Ca(2+) cycling, and action potential repolarization. The flattened contraction-frequency relationship can be partially explained by an attenuation of the normal enhancement of SR Ca(2+) content that occurs when stimulation rate is increased. Chronic DITPA administration after MI largely prevents the development of these abnormalities.

Bradycardia and the role of beta-blockade in the amelioration of left ventricular dysfunction

BACKGROUND

It is clear that beta-blockers are effective for treatment of congestive heart failure, but their mechanism of action remains controversial. Hypothesized mechanisms include normalization of beta-receptor function and myocardial protection from the effects of catecholamines, possibly by the institution of bradycardia. We hypothesized that beta-blockade-induced bradycardia was an important mechanism by which these agents were effective for correction of LV dysfunction.

METHODS AND RESULTS

In 2 groups of dogs with mitral regurgitation and LV dysfunction, beta-blockers were instituted. In 1 group that received beta-blockers and pacing (group beta+P), a pacemaker prevented the natural bradycardia that beta-blockers cause. In both groups, substantial LV dysfunction developed. Before beta-blockade, the end-systolic stiffness constant decreased from 3. 5+/-0.1 to 2.7+/-0.2 (P<0.01) at 3 months in group beta+P. A similar reduction occurred in the group that eventually received only beta-blockers (group betaB). In group betaB, end-systolic stiffness improved after 3 months of beta-blockade from 2.9+/-0.2 to 3.5+/-0.4 and was not different from baseline. However, in group beta+P, end-systolic stiffness failed to improve (2.7+/-0.2) after 3 months of mitral regurgitation, and was 2.9+/-0.2 at the end of the studies. The contractile function of cardiocytes isolated from the ventricles at the end of the studies confirmed these in vivo estimates of contractility.

CONCLUSIONS

We conclude that institution of bradycardia is a major mechanism by which beta-blockers are effective for restoration of contractile function in a model of LV dysfunction.

Human heart failure: cAMP stimulation of SR Ca(2+)-ATPase activity and phosphorylation level of phospholamban

Failing human myocardium has been associated with decreased sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity. There remains controversy as to whether the regulation of SR Ca(2+)-ATPase activity is altered in heart failure or whether decreased SR Ca(2+)-ATPase activity is due to changes in SR Ca(2+)-ATPase or phospholamban expression. We therefore investigated whether alterations in cAMP-dependent phosphorylation of phospholamban may be responsible for the reduced SR Ca(2+)-ATPase activity in human heart failure. Protein levels of phospholamban and SR Ca(2+)-ATPase, detected by Western blot, were unchanged in failing compared with nonfailing human myocardium. There was decreased responsiveness to the direct activation of the SR Ca(2+)-ATPase activity by either cAMP (0.01-100 micromol/l) or protein kinase A (1-30 microgram) in failing myocardium. Using the backphosphorylation technique, we observed a decrease of the cAMP-dependent phosphorylation level of phospholamban by 20 +/- 2%. It is concluded that the impaired SR function in human end-stage heart failure may be due, in part, to a reduced cAMP-dependent phosphorylation of phospholamban.

Electrical and hemodynamic correlates of the maximal rate of pressure increase in the human left ventricle

BACKGROUND

The rate of left ventricular (LV) pressure increase (LV + dP/dt) may be related to QRS duration, as well as to a number of hemodynamic parameters.

METHODS AND RESULTS

We studied the relation between basal LV + dP/dt and QRS duration in 43 patients with normal LV function and 81 patients with heart failure undergoing diagnostic catheterization. We also examined the relationship between LV + dP/dt and heart rate, as well as measures of both LV preload and afterload. In patients with normal LV function, there was a strong relationship between basal LV + dP/dt and resting heart rate, whereas the relationship with QRS duration was of borderline significance. In patients with heart failure, the relationship with heart rate was lost; however, LV systolic pressure, QRS duration, and LV end-diastolic pressure all made significant contributions to a model predicting LV + dP/dt.

CONCLUSIONS

These data show a strong relationship between resting heart rate and LV + dP/dt in the healthy human LV. In patients with heart failure, the relationship with heart rate is not maintained; however, there is a systematic relationship between LV + dP/dt and both the time-course of the electrical activation and measures of LV loading conditions.

Effect of milrinone and atrial pacing on stunned myocardium

OBJECTIVE

Most mammalian cardiac muscles show a positive force-frequency relation, which is turned into a negative relation in failing hearts. Stunned myocardium shows similar defects as failing myocardium, it has a functional reserve recruitable by positive inotropic interventions, and possibly shows a disturbed response to increased heart rate. The present experiments compare in vivo the response of stunned and intact myocardium to atrial pacing before and during inotropic stimulation by milrinone.

METHODS

In anaesthetised (piritramide) open chest pigs, heart rate, left ventricular and aortic pressure, left descending (LAD) and circumflex (LCX) coronary artery and aortic blood flow, myocardial systolic shortening in the LAD and LCX area were monitored, and myocardial power was calculated. The LAD region was subjected to ischaemia and reperfused. Heart rate was raised by right atrial pacing after 90 min reperfusion before and during i.v. milrinone (105 microg/kg bolus + 8 microg/kg per min infusion). The ischaemic/reperfused area was sliced post mortem and stained by triphenyl tetrazolium chloride to exclude myocardial infarction. Data from ten experiments are presented.

RESULTS

After 90 min LAD reperfusion, LAD blood flow and power were 110 and 36% of preischaemic control, respectively, indicating myocardial stunning. The power of the intact area was not changed (102% of control). Pacing from 87 to 164 per min increased the power of the intact area (+96%), the power of the stunned myocardium decreased (-64%). Milrinone increased the power of the stunned region to 72% of the pre-stunning level and the power of the intact area by +51%. Pacing from 111 to 164 per min during milrinone increased the power of the intact myocardium to the same level as before milrinone, the power of the stunned region did not change.

CONCLUSIONS

Stunned myocardium responds pathologically to atrial pacing with a negative staircase in contrast to the positive staircase of intact myocardium. Inotropic stimulation by the phosphodiesterase inhibitor milrinone recruited the functional reserve of stunned myocardium. Milrinone did not restore a positive staircase in stunned myocardium, but power was maintained during atrial pacing. The pathological staircase of stunned myocardium may arise from an impaired availability of cyclic AMP, but the data do not exclude defects in calcium handling, a dysfunction of the sarcoplasmic reticulum, or an impaired Ca-sensitivity of the myofilaments.

Role of cAMP in modulating relaxation kinetics and the force-frequency relation in mitral regurgitation heart failure

The report is a discussion of previously published and newly analyzed results concerning the association between heart diseases and alterations in the force-frequency relation (FFR). The optimum stimulation frequency of the FFR is measured and compared in isolated left ventricular myocardium from non-failing hearts with atrial septal defect, coronary artery disease (without and with insulin dependent diabetes mellitus) and from failing hearts with mitral regurgitation, or idiopathic dilated cardiomyopathy. Specifically, we examine the role of altered control of the excitation-contraction coupling system in blunting the force-frequency relation. We use the percent slope of the FFR as a measure of changes in the frequency sensitivity of this control. Our finding of a linear, direct relation between optimum stimulation frequency and % slope across all disease types suggests both parameters are coupled to the same underlying mechanism. To investigate the possible role of altered control of the calcium pump in this mechanism, we analyzed the detailed relation between isometric twitch relaxation kinetics and stimulation frequency in mitral regurgitation myocardium (MR). In the presence of 0.5 microM forskolin the depressed slope and optimum frequency of the FFR and the prolonged half-time of twitch relaxation were all restored to values found in non-failing myocardium. We use the kinetics of isometric twitch relaxation as an index of changes in pumping rate that occur in response to changes in stimulation frequency or in intracellular cyclic adenosine monophosphate concentration. A mathematical model based on the Hill relations for calcium pump uptake rate and for isometric tension as a function of intracellular pCa is developed to simulate isometric twitch relaxation in MR and non-failing myocardium. The success of this model in simulating non-failing and failing twitch relaxation supports a proposed mechanism for the prolonged relaxation time and depressed FFR in MR involving depressed protein kinase-A activity (due to lowered cAMP or to a defect in the Ser16 site of phospholamban) as a mechanism of altered control of the calcium pump in MR heart disease.

Altered inotropic and lusitropic responses to heart rate in conscious dogs with tachycardia-induced heart failure

OBJECTIVES

The effects of increasing heart rate on left ventricular contraction and relaxation were examined in conscious dogs with tachycardia-induced heart failure under autonomically blocked conditions.

BACKGROUND

Previous studies using isolated myocardium have shown attenuated positive inotropic responses to stimulation frequency in heart failure. However, these responses have not been well examined in intact preparations in the presence of heart failure with autonomic system blockade, where the intrinsic ventricular responses to increasing heart rate could be revealed.

METHODS

Seven dogs were instrumented with a micromanometer and a conductance volume catheter. After autonomic blockade to eliminate neural reflexes, left ventricular contractile properties were quantified by the slope of the end-systolic pressure-volume relation (ventricular elastance), and left ventricular relaxation was assessed by the time constant of isovolumetric ventricular pressure decay.

RESULTS

Increasing the heart rate by 60 beats/min enhanced ventricular elastance by 71 +/- 18% (mean +/- SD) and decreased end-systolic volume by 6 +/- 5% in normal hearts. In failing hearts, ventricular elastance increased by only 21 +/- 20%, and end-systolic volume did not change appreciably. Although the reduction in left ventricular end-diastolic and minimal pressures by tachycardia was smaller in the failing heart, ventricular relaxation rate remained unaltered both in the normal heart and in the failing heart.

CONCLUSIONS

Under conscious but autonomically blocked conditions, effects of increasing heart rate on the failing left ventricle are characterized by a predominant attenuation of the inotropic response rather than of the lusitropic response.

The force-interval relationship in human myocardium

The force-interval relationship is an important modulator of contractility in mammalian myocardium. In a number of mammalian species, increasing the frequency of stimulation results in an increase in force of contraction. Over the last 10 years, the effects of atrial pacing have been closely examined in normal human subjects and in patients with dilated cardiomyopathy, and the effects of the stimulation frequency have been investigated in isolated preparations from nonfailing and failing human hearts. An abnormal force-interval relationship in vivo and in vitro has been a consistent finding in patients with dilated cardiomyopathy, whereby an increase in stimulation frequency fails to increase the contractile response. The force-interval relationship of cardiac muscle has been shown to reflect intracellular calcium cycling and sarcoplasmic reticulum function. Therefore, agents that affect excitation-contraction coupling, in particular intracellular calcium mobilization and sarcoplasmic reticulum function, modulate the response of contraction force to stimulation frequency.