Depression of systolic and diastolic myocardial reserve during atrial pacing tachycardia in patients with dilated cardiomyopathy

Diminished contractile response to increased heart rate in intact human left ventricular hypertrophy. Systolic versus diastolic determinants

BACKGROUND

Experimental studies indicate that in addition to diastolic dysfunction, hypertrophied myocardium can display depressed contractile responses, particularly at rapid heart rates, compounding reserve limitations. This study tests whether such abnormalities exist in intact human subjects at physiological paced rates and, if so, whether they are linked to simultaneous rate-dependent deterioration in diastolic function.

METHODS AND RESULTS

Ten subjects with left ventricular hypertrophy (LVH) and 8 normal control subjects were studied. Most LVH patients presented with dyspnea and/or pulmonary edema and had concentric hypertrophy. Since rapid pacing simultaneously alters cardiac filling volumes and pressures, pressure-volume relation analysis was used to better define changes in contractile response. Patients were instrumented with a conductance catheter and micromanometer for pressure-volume data recording and a balloon occluder at the right atrial-inferior vena caval junction to vary filling and thus generate function relations. Data were obtained at baseline and at three atrial pacing rates (100, 120, 150 min-1). In addition, single-beat force-interval data were used to indirectly examine calcium cycling kinetics. LVH subjects demonstrated baseline diastolic abnormalities, including prolonged relaxation, elevated end-diastolic pressure, and reduced chamber compliance. However, systolic function was similar to that in control subjects. With rapid pacing, normal subjects displayed a positive contractile response, whereas this was markedly diminished in LVH subjects. With abrupt termination of pacing and return to slower sinus rhythm, LVH subjects displayed greater initial potentiation followed by a more rapid decline than control subjects, suggesting abnormalities of calcium handling. Despite contractile abnormalities, diastolic function did not further deteriorate with rapid pacing and thus did not appear to be tightly linked to the systolic changes.

CONCLUSIONS

Pacing stress in intact human LVH can result in systolic impairment superimposed on preexisting but not worsened diastolic dysfunction. Abnormal calcium handling probably contributes prominently to this response.

Effect of heart failure on the mechanism of exercise-induced augmentation of mitral valve flow

The exercise response of left ventricular (LV) filling dynamics may be altered by congestive heart failure (CHF). Accordingly, we studied 18 conscious dogs, instrumented to measure micromanometer LV and left atrial (LA) pressures and determine LV volume from three dimensions. CHF was produced by 4-5 weeks of right ventricular rapid pacing. Before CHF, exercise (5.5-8.5 mph for 8-15 minutes) increased the maximum rate of LV filling (dV/dtmax) (197 +/- 37 versus 297 +/- 56 ml/sec [mean +/- SD], p < 0.05) in response to an increase in the maximum early diastolic LA to LV pressure gradient (5.8 +/- 2.0 versus 9.8 +/- 1.9 mm Hg, p < 0.05) produced by a fall in minimum LV pressure (1.0 +/- 2.9 versus -3.9 +/- 3.1 mm Hg, p < 0.01), whereas mean LA pressure was unchanged (6.4 +/- 3.1 versus 6.4 +/- 4.2 mm Hg, p = NS). The time constant of LV relaxation was shortened (28.1 +/- 3.2 versus 21.0 +/- 4.2 msec, p < 0.05). After CHF, dV/dtmax (141 +/- 51 versus 200 +/- 59 ml/sec, p < 0.05) and the maximum LA to LV pressure gradient (6.0 +/- 1.1 versus 11.1 +/- 2.7 mm Hg, p < 0.05) continued to increase with exercise (3-5.0 mph for 4-8 minutes). However, the time constant of LV relaxation was prolonged (35.6 +/- 4.8 versus 38.9 +/- 5.5 msec, p < 0.05), and minimum LV pressure (15.1 +/- 5.6 versus 17.6 +/- 5.9 mm Hg, p < 0.05) and mean LA pressure increased (22.6 +/- 7.2 versus 29.1 +/- 7.3 mm Hg, p < 0.05). These altered effects of exercise on LV diastolic filling dynamics persisted when heart rate and wall stress during exercise before and after CHF were matched by varying the level of exercise. We conclude that, during normal exercise, mitral valve flow is augmented by a fall of early diastolic LV pressure without a rise in LA pressure. After CHF, early diastolic LV pressure does not fall but increases during exercise. Thus, the increase in the early diastolic LA to LV pressure gradient and the rate of mitral valve flow results from an increase in LA pressure during exercise after CHF. This study suggests that the failure of the enhancement of LV relaxation and an increase in early diastolic LV pressure with exercise after CHF may contribute to exercise intolerance in CHF.

Force-frequency-relation in human atrial and ventricular myocardium

In human heart failure, an increase in frequency of stimulation is followed by a reduced force of contraction in vivo and in vitro. The present study aimed to investigate whether a different origin of the myocardial sample or pretreatment with the cardioprotective agent 2,3-butanedione-monoxime (BDM) influences the force-frequency-relationship in electrically driven muscle strips taken from failing and nonfailing human myocardium. With as well as without pretreatment with BDM, the altered force-frequency-relationship in failing compared to nonfailing human ventricular myocardium can be observed. The effectiveness and the potency to increase force of contraction following an increase in frequency of stimulation was significantly higher in atrial than in ventricular myocardium in nonfailing and failing tissue. The different observations in atrial and ventricular myocardium provide evidence for functionally relevant differences in the electromechanical coupling between the human atrial and ventricular myocardium.

Central hemodynamic response to exercise in patients with chronic heart failure

Although the pathophysiology of exercise intolerance in patients with chronic heart failure (CHF) is not fully understood, it appears that the cardiac output response plays an important role in limiting exercise in this disorder. Although previous studies have demonstrated that peak VO2 is not related to left ventricular (LV) ejection fraction, studies have consistently identified peak exercise cardiac output as an important predictor of peak VO2. It is likely that a reduced cardiac output to work rate relationship in CHF causes hypoperfusion of both working skeletal muscle and visceral organs, which leads to early anaerobic metabolism and fatigue. Several factors may influence the cardiac output response in patients with severe systolic LV dysfunction, including heart rate, diastolic LV function, and the mitral regurgitation fraction. Although stroke volume increases through use of the Frank-Starling mechanism in many patients with severe systolic LV dysfunction, some patients with this disorder may not increase stroke volume during exercise due to diastolic LV dysfunction or pericardial constraint. The finding that this latter group has more severe exercise intolerance suggests that diastolic dysfunction may further decrease peak VO2 in this disorder. Variations in the mitral regurgitation fraction also have been found to have an important effect on exercise stroke volume in some patients with CHF. Therefore, the finding that LV ejection fraction at rest or during exercise is not related to peak VO2 in patients with systolic LV dysfunction does not necessarily indicate that central hemodynamics do not play a role in exercise intolerance. Rather, it is likely that variability in the LV ejection fraction with exercise, which does not take variable increases in LV end-diastolic volume or mitral regurgitation into account, plays only a modest role in determining the stroke volume and cardiac output response to exercise in patients with severe systolic dysfunction.

Altered myocardial force-frequency relation in human heart failure

BACKGROUND

In congestive heart failure (idiopathic dilated cardiomyopathy), exercise is accompanied by a smaller-than-normal decrease in end-diastolic left ventricular volume, depressed peak rates of left ventricular pressure rise and fall, and depressed heart-rate-dependent potentiation of contractility (bowditch treppe). We studied contractile function of isolated left ventricular myocardium from New York Heart Association class IV-failing and nonfailing hearts at physiological temperature and heart rates in order to identify and quantitate abnormalities in myocardial function that underlie abnormal ventricular function.

METHODS AND RESULTS

The isometric tension-generating ability of isolated left ventricular strips from nonfailing and failing human hearts was investigated at 37 degrees C and contraction frequencies ranging from 12 to 240 per minute (min-1). Strips were dissected using a new method of protection against cutting injury with 2,3-butanedione monoxime (BDM) as a cardioplegic agent. In nonfailing myocardium the twitch tension-frequency relation is bell-shaped developing 25 +/- 2 mN/mm2 at a contraction frequency of 72 min-1 and peaking at 44 +/- 3.7 mN/mm2 at a contraction frequency of 174 +/- 4 min-1. In failing myocardium the peak of the curve occurs at lower frequencies between 6 and 120 min-1 averaging 81 +/- 22 min-1, and it develops 48% (p less than 0.001) and 80% (p less than 0.001) less tension than in nonfailing myocardium at 72 and 174 min-1, respectively. Between 60 and 150 min-1 tension increases by 107% in nonfailing myocardium, but it does not change significantly in failing myocardium. Peak rates of rise and fall of isometric twitch tension vary in parallel with twitch tension as stimulation frequency rises in nonfailing myocardium but not in failing myocardium.

CONCLUSIONS

The quantitative agreement between these results from isolated myocardium and those from catheterization laboratory measurements on intact humans suggest that alterations of myocardial origin, independent of systemic factors, may contribute to the above mentioned abnormalities in left ventricular function seen in dilated cardiomyopathy.

Alterations in left ventricular mechanics, energetics, and contractile reserve in experimental heart failure

The contributions of changes in primary systolic and diastolic properties, limitations of contractile reserve, and alterations in energy efficiency to the left ventricular dysfunction seen with chronic pacing tachycardia were investigated. Seven dogs (heart failure group) were ventricularly paced at 250 beats per minute for 26.3 +/- 2.9 days and compared with a separate control group (n = 8). STudies were performed with isolated, metabolically supported hearts coupled to a computer-controlled loading system. Pressure-volume relations and myocardial oxygen consumption (MVO2) were measured to assess chamber systolic and diastolic properties and efficiency (relation between MVO2 and pressure-volume area [PVA]). Systolic function was reduced in failure hearts versus controls as assessed by the slope of the end-systolic pressure-volume relation (1.29 +/- 0.94 versus 2.71 +/- 0.98 mm Hg/ml, p less than 0.01) and lowered end-systolic stiffness at a matched stress (956.1 +/- 123.5 versus 1,401.7 +/- 431.7 g/cm2, p less than 0.05). Diastolic chamber and myocardial stiffness were unaltered in failure hearts, but the unstressed diastolic-arrested volume was significantly larger (33.3 +/- 3.9 versus 21.9 +/- 7.6 ml, p less than 0.01). Inotropic response to increased heart rate and exogenous beta-adrenergic stimulation (dobutamine HCl) was significantly impaired in failure compared with control hearts. Most interestingly, failure hearts had a lowered slope of the MVO2-PVA relation (2.1 +/- 1.1 versus 2.9 +/- 1.4 ml O2.mm Hg-1.ml-1.100 g left ventricle-1, p less than 0.001), indicating increased efficiency of chemomechanical energy conversion. The y intercept of the MVO2-PVA relation, which reflects oxygen costs of basal metabolism and excitation-contraction coupling, was unchanged in the two groups despite decreased contractility of the heart failure hearts. These results demonstrate reduced chamber and myocardial contractility, dilatation without alteration of passive myocardial properties, impaired contractile reserve, and novel alterations in cardiac efficiency in this model of heart failure.

Mechanism of augmented rate of left ventricular filling during exercise

At rest, most of left ventricular (LV) filling occurs early in diastole. This LV filling occurs in response to the pressure gradient produced as LV pressure falls below left atrial (LA) pressure. Because mitral valve flow occurs in response to an LA to LV pressure gradient, augmented diastolic mitral valve flow during exercise may be due to an increased mitral valve pressure gradient resulting from a rise in LA pressure and/or a fall in LV early diastolic pressure. Accordingly, we studied 13 conscious dogs, instrumented to measure micromanometer LV and LA pressures, and determined LV volume from three ultrasonic dimensions during exercise. The animals ran on a treadmill for 8-15 minutes at 5-8 miles/hr. With reflexes intact, during exercise, the heart rate increased from 116 +/- 20 to 189 +/- 24 beats per minute (mean +/- SD, p less than 0.01), the maximum rate of change of LV volume (dV/dtmax) increased from 185 +/- 44 to 282 +/- 76 ml/sec (p less than 0.01), the ejection fraction and cardiac output increased, and the duration of diastole decreased from 296 +/- 83 to 162 +/- 71 msec (p less than 0.01). Mitral valve opening pressure, mean LA pressure (10.9 +/- 4.4 versus 10.2 +/- 3.9 mm Hg, p = NS), and LV end-diastolic pressure (12.8 +/- 4.8 versus 13.1 +/- 3.3 mm Hg, p = NS) were all relatively unchanged. The time constant of the fall of isovolumic LV pressure decreased from 28 +/- 3.3 to 21 +/- 4.4 msec (p less than 0.05). The early diastolic portion of the LV pressure-volume loop was shifted downward during exercise, with the minimum LV pressure decreasing from 3.3 +/- 2.8 to -2.8 +/- 3.4 mm Hg (p less than 0.05) and the maximum mitral valve pressure gradient increasing from 5.5 +/- 1.7 to 11.8 +/- 3.5 mm Hg (p less than 0.01). A similar downward shift of the early diastolic portion of the LV pressure-volume loop was produced by infusion of dobutamine (6 micrograms/kg/min i.v.) at rest, as well as by exercise when the heart rate was held constant by right ventricular pacing at 190-210 beats per minute. The downward shift during exercise was prevented by beta-blockade (metoprolol, 0.5 mg/kg i.v.). We conclude that during exercise, sympathetic stimulation and tachycardia produce a downward shift of the early diastolic portion of the LV pressure-volume loop.(ABSTRACT TRUNCATED AT 400 WORDS)

Contraction frequency dependence of twitch and diastolic tension in human dilated cardiomyopathy (tension-frequency relation in cardiomyopathy)

We studied isometric twitch tension and diastolic tension at 37 degrees C as a function of stimulation frequency (12-240 min-1) in very thin (.07-.5 mm2), parallel fibered strips of left-ventricular myocardium. Non-failing control tissue (C) was obtained from epicardial biopsies taken during myocardial revascularization surgery on patients with normal ventricular function. End-stage failing tissue was obtained from endocardial and epicardial biopsies from explanted hearts with idiopathic dilated cardiomyopathy (DCM). The methods and apparatus for biopsy and dissection of myocardium are described. Maximal peak twitch tension at optimal stimulation frequency of 163 +/- 5 min-1 was 41.8 +/- 10 mN/mm2 in non-failing myocardium and it was reduced by 70% (p less than .02) to 12.9 +/- 1.6 mN/mm2 at an optimal frequency of 72 +/- 17 min-1 in DCM. The peaks of the tension-frequency curves occurred at frequencies between 12 and 60 min-1 in most DCM strips (5/9), while in C most of the peaks (8/9) fell between 156 and 180 min-1. The peaks from four DCM hearts fell in an intermediate range of frequencies (96-144 min-1) which also included one non-failing peak at 132 min-1. Diastolic tension declined in both groups as stimulation frequency increased above 12 min-1 and it began increasing when stimulation frequency rose above optimal frequency by 19 +/- 5% and 110 +/- 50% in C and DCM, respectively. Total duration of the isometric twitch diminished with tachycardia remaining shorter than stimulation intervals up to 140 +/- 16 min-1 (3.1 +/- 1 times optimal frequency) in DCM and up to 161 +/- 14 min-1 (not different than optimal frequency) in C. Decline in peak twitch tension above optimal stimulation frequency was 4 to 6 times larger than the accompanying rise in diastolic tension in both groups. The premature decline in tension at lower than normal degrees of tachycardia in DCM does not arise from incomplete relaxation of the twitch response. The 70% deficit in tension generating ability of DCM may be a major contributor to heart failure. Moderate shift in the peak of the tension-frequency curves to lower frequencies (130 min-1) in C does not appear to predispose end-stage failure, but it may make the ventricle more susceptible to dilation.

Effects of beta-adrenergic stimulation with dobutamine on isovolumic relaxation in the normal and failing human left ventricle

BACKGROUND

We tested the hypothesis that beta-adrenergic receptor-stimulated acceleration of left ventricular (LV) isovolumic relaxation (i.e., positive lusitropic response) is attenuated in patients with severe congestive heart failure (CHF) compared with patients without LV dysfunction or CHF.

METHODS AND RESULTS

The beta-adrenergic agonist dobutamine was infused by the intracoronary route in 14 subjects (normal group, six; CHF patients, eight) and by the intravenous route in a second group of 14 subjects (normal group, four; CHF patients, 10). The positive inotropic response to intracoronary or intravenous dobutamine was substantially and significantly reduced in the patients with CHF. LV isovolumic relaxation rate was determined by the methods of Weiss (TL), Mirsky (T1/2), and by a nonlinear regression technique (TNL). LV isovolumic relaxation assessed by all three methods was significantly prolonged in CHF patients compared with normal subjects. Intracoronary and intravenous infusions of dobutamine caused significant acceleration of LV isovolumic relaxation in both normal subjects and patients with CHF. The magnitude of the dobutamine-stimulated acceleration of isovolumic relaxation in patients with CHF was comparable with that in normal subjects.

CONCLUSIONS

These data demonstrate that beta-adrenergic receptor stimulation causes significant acceleration of LV isovolumic relaxation in both normal subjects and patients with severe CHF. Coronary to our hypothesis, the lusitropic response to beta-adrenergic stimulation is well preserved in patients with severe CHF despite substantial attenuation of the beta-adrenergic positive inotropic response. These findings have potentially important implications regarding the physiology and pharmacology of adrenergically mediated LV relaxation in humans.

Usefulness of physiologic dual-chamber pacing in drug-resistant idiopathic dilated cardiomyopathy

The beneficial effects of physiologic dual-chamber (DDD) pacing in the treatment of end-stage idiopathic dilated cardiomyopathy were evaluated in 16 patients in whom conventional drug therapy had failed. Candidates for cardiac transplantation as well as patients not accepted for transplantation participated. During DDD pacing at an atrioventricular delay of 100 ms, left ventricular ejection fraction increased from 16.0 +/- 8.4 to 25.6 +/- 8.6% (p less than 0.001) accompanied by a striking improvement in clinical symptoms, such as severe dyspnea at rest and pulmonary edema. The New York Heart Association class decreased from 3.6 +/- 0.4 to 2.1 +/- 0.5 (p less than 0.001). The decrease in cardiothoracic ratio from 0.60 +/- 0.06 to 0.56 +/- 0.05 (p less than 0.001) coincided with a decrease in left atrial and right ventricular echocardiographic dimensions, indicating a decrease in preload. Systolic blood pressure increased from 108 +/- 29 to 126 +/- 21 mm Hg (p less than 0.01) and diastolic blood pressure from 67 +/- 15 to 80 +/- 11 mm Hg (p less than 0.01). Normalization of heart rate was achieved. No major complications developed as a consequence of DDD pacing. All patients could be discharged from the hospital within 3 weeks after pacemaker implantation and return to a relatively normal life. Within 1 year after onset of DDD pacing only 4 of the patients died (from either sudden death or stroke). DDD pacing could represent an alternative approach to the management of chronic heart failure due to dilated cardiomyopathy, especially for heart transplant candidates and patients who are not accepted for cardiac transplantation, but no longer respond to drug therapy.

Is tau a preload-independent measure of isovolumetric relaxation?

Several studies have been performed in patients with a variety of myocardial diseases that have identified a prolongation of tau. However, it is not clear whether prolongation of tau represents abnormal myocardial physiology or the effect of excessive load associated with a particular disease process. Accordingly, we evaluate the effect on tau of an isolated decrease in preload induced by inferior vena cava occlusion before the appearance of reflex changes in six patients designated as normal by catheterization criteria. A computer-based digitization routine identified cardiac contractions in all patients early after inferior vena cava occlusion where left ventricular end-diastolic pressure decreased (18.3 +/- 6.3 to 9.3 +/- 5.8, p less than 0.05) while left ventricular systolic pressure (113.3 +/- 13.8 to 111.8 +/- 14.0, p = NS) and heart rate (66.0 +/- 10.0 to 65.9 +/- 10.3, p = NS) did not change. After this alteration in preload, no change in tau from baseline, as calculated by the logarithmic (TL), derivative (TD), or method of Mirsky (T1/2), was noted: TL, 47.4 +/- 6.5 to 44.6 +/- 7.6; TD, 39.3 +/- 8.1 to 39.8 +/- 8.4; T1/2, 33.0 +/- 4.0 to 31.8 +/- 4.6; all p = NS. The baseline pressure extrapolated from isovolumetric relaxation did not change in these preload beats compared with baseline (+4.26 +/- 6.20 to -0.80 +/- 4.87, p = NS). Subsequent beats were identified where left ventricular systolic pressure showed a numeric decrease compared with baseline (113.3 +/- 13.8 to 100.8 +/- 14.3, p = NS) despite no change in heart rate (66.0 +/- 10.0 to 66.8 +/- 10.5, p = NS).(ABSTRACT TRUNCATED AT 250 WORDS)

The challenge of cardiomyopathy

The combined clinical and pathophysiologic characteristics and diagnostic features as well as current concepts of pathogenesis, therapy and prevention of the principal forms of cardiomyopathy are reviewed. These include hypertrophic cardiomyopathy, dilated cardiomyopathy, restrictive cardiomyopathy and specific cardiac muscle disease. Emphasis is placed on recent developments and unresolved questions requiring application of newer techniques of molecular biology and genetics and adult myocyte culturing.