Contractility of the hypertrophied human left ventricle in chronic pressure and volume overload

Role of microtubules versus myosin heavy chain isoforms in contractile dysfunction of hypertrophied murine cardiocytes

In large mammals there is a correlation between microtubule network densification and contractile dysfunction in severe pressure-overload hypertrophy. In small mammals there is a similar correlation for the shift to beta-myosin heavy chain (MHC), a MHC isoform having a slower ATPase Vmax. In this study, murine left ventricular (LV) pressure overload invoked both mechanisms: microtubule network densification and beta-MHC expression. Cardiac beta-MHC was also augmented without altering tubulin levels by two load-independent means, chemical thyroidectomy and transgenesis. In hypertrophy, contractile function of the LV and its cardiocytes decreased proportionally; microtubule depolymerization restored normal cellular contraction. In hypothyroid mice having a complete shift from alpha-MHC to beta-MHC, contractile function of the LV and its cardiocytes also decreased, but microtubule depolymerization had no effect on cellular contraction. In transgenic mice having a cardiac beta-MHC increase similar to that in hypertrophy, contractile function of the LV and its cardiocytes was normal, and microtubule depolymerization had no effect. Thus, although both mechanisms may cause contractile dysfunction, for the extent of MHC isoform switching seen even in severe murine LV pressure-overload hypertrophy, microtubule network densification appears to have the more important role.

Left ventricular mechanics and geometry in patients with congenital complete atrioventricular block

BACKGROUND

Radiographic evidence of cardiomegaly is common in patients with congenital complete atrioventricular block (CCAVB). It has been speculated that left ventricular (LV) remodeling and increased stroke volume counteract the bradycardia, but the effects of slow heart rate and atrioventricular asynchrony on LV dimensions, geometry, wall stress, and function have not been examined in detail.

METHODS AND RESULTS

Thirty patients with CCAVB without associated congenital heart disease (mean age, 8.5+/-5.3 years; range, 0.2 to 20 years) were included in a cross-sectional two-institution study. Thirty-five echocardiograms were performed using standard techniques. ECG and 24-hour ECG recordings were reviewed. Seven patients did not receive a pacemaker, whereas 23 patients underwent pacemaker implantation after the echocardiogram. Compared with normal control subjects, LV volume (Z score=1.5+/-1.3) and LV mass (Z=1.2+/-1.5) were significantly increased, whereas LV mass-to-volume ratio (1.1+/-0.3) and geometry (short-axis diameter/length ratio=0.65+/-0.09) were normal. LV end-systolic stress (ESS) (a measure of afterload) was normal (Z score=0.2+/-2.3), whereas shortening fraction (Z=3+/-2.9) and velocity of circumferential fiber shortening (VCF) (Z=3+/-3.1) were increased. The relationship between VCF and ESS (a preload-insensitive and afterload-adjusted index of contractility) was increased (Z=2.2+/-2) with only small increase in preload (Z=1.02+/-1.1). Regression analyses showed no significant change over age in LV mass, volume, geometry, loading conditions, or systolic function. Patients who ultimately met criteria for pacemaker implantation did not differ from those who did not in terms of heart rate or LV function but did have increased LV volume (Z score=1.8+/-1.4 versus 0.4+/-0.9, P=.03) and LV mass (Z score=1.7+/-1.2 versus 0.2+/-1.7, P=.001) compared to the unpaced group.

CONCLUSIONS

In most patients with CCAVB, the LV was enlarged with normal geometry and enhanced systolic function during the first two decades of life. The degree of LV dilation and enhanced function did not significantly change with age. In patients who ultimately underwent pacemaker implantation LV function did not differ from those who remained unpaced, but evidence of a slightly increased load manifested as increased end-diastolic volume and mass.

Right and left ventricular function assessed by regional wall motion analysis in patients with tetralogy of Fallot

We studied pre- and postoperative regional right and left ventricular wall motion and global ejection fraction in 18 patients with tetralogy of Fallot who had successful repair, and compared these values to those of patients with a history of Kawasaki disease as controls. Right ventricular ejection fraction was significantly lower in the preoperative group (52 +/- 4%) than that in the control group (57 +/- 4%), and that in the postoperative state (49 +/- 4%) was significantly lower than those in the control and preoperative groups. Left ventricular ejection fraction was significantly lower in the preoperative group (56 +/- 7%) than that in the control group (61 +/- 5%), while that in the postoperative state (64 +/- 6%) was significantly higher than those in the control and preoperative groups. Regional ventricular wall motion analysis revealed that shortening fractions in the tricuspid valve region were reduced in the preoperative patients and were persistent even after successful repair; those in the right ventricular outflow tract region were decreased after the correction. Regional left ventricular wall motion showed that shortening fractions in the anterolateral region were improved after the correction. We conclude that right ventricular dysfunction was present in the preoperative patients with tetralogy of Fallot and was persistent even after total correction.

Effects of physiologic load of pregnancy on left ventricular contractility and remodeling

Left ventricular (LV) adaptation to the hemodynamic load of pregnancy has been studied with load-sensitive ejection-phase indexes, but the results of these studies are conflicting. The aim of this study was to examine the effects of the hemodynamic load of pregnancy on the contractile state of the left ventricle by using load-adjusted indexes of contractility. Thirty-four healthy women were prospectively studied by serial echo and Doppler examinations at six periods during pregnancy and after delivery. LV volume increased 10.5%, paralleling the change in stroke volume. End-systolic stress, an index of myocardial afterload, decreased 28.8% because of a decrease in end-systolic pressure and an increase in LV thickness/diameter ratio. Despite the increase in preload and the decrease in afterload, ejection phase indexes did not change during or after pregnancy. Although remaining within the normal range, the afterload-adjusted velocity of circumferential fiber shortening, an index of contractility that is relatively insensitive to preload, transiently decreased by 1.75 SDs during gestation, returning to non-pregnant values 2 to 4 weeks postpartum. Thus the increase in hemodynamic load that characterizes normal pregnancy is associated with preservation of global pump function. The transient decrease in contractile state may represent an adaptation phase of the contractile elements of the myocardium to the rapid changes in loading conditions observed during the first trimester of pregnancy.

Metabolic regulation of in vivo myocardial contractile function: multiparameter analysis

To gain insight into the mechanisms of myocardial regulation as it relates to the interaction of mechanical and metabolic function and perfusion, intact animal models were instrumented for routine physiological measurements of mechanical function and for measurements of metabolism (31P NMR, NADH fluorescence (redox state)) and perfusion (2H NMR and Laser doppler techniques). These techniques were applied to canine and cat models of volume and/or pressure loading, hypoxia, ischemia and cardiomyopathic states. Data generated using these techniques indicate that myocardial bioenergetic function is quite stable under most loading conditions as long as the heart is not ischemic. In addition, these data indicate that there is no universal regulator and that different biochemical regulators appear to mediate stable function under different physiological and pathophysiological conditions: for example; during hypoxia, NADH redox state appears to play a regulatory role; and in pressure loading, ADP, phosphorylation potential and free energy of ATP hydrolysis as well as NADH redox state appear to be regulatory.

Creatinine kinase kinetics studied by phosphorus-31 nuclear magnetic resonance in a canine model of chronic hypertension-induced cardiac hypertrophy

To determine whether cardiac hypertrophy secondary to chronic renovascular hypertension is associated with altered in vivo myocardial metabolism, phosphorus-31 nuclear magnetic resonance saturation transfer techniques were used to study creatine kinase (CK) kinetics in six chronically hypertensive dogs with moderate cardiac hypertrophy and eight control dogs. The forward rate constant of CK and the flux of phosphocreatine to adenosine triphosphate were determined in both groups of dogs before and during norepinephrine administration (1 microgram/kg per min), used to increase heart rate x systolic blood pressure (rate-pressure product), cardiac output and oxygen consumption. Baseline and norepinephrine-induced changes in rate-pressure product, cardiac output and oxygen consumption were similar in both groups of dogs, as were baseline forward rate constant and flux of phosphocreatine to adenosine triphosphate. However, the norepinephrine-induced changes in forward rate constant and flux were significantly less in hypertensive than in control dogs (p less than 0.05) even though changes in hemodynamic and functional variables were similar in both groups. These data demonstrate that moderate myocardial hypertrophy is associated with altered CK kinetics, which do not appear to affect the heart's ability for global mechanical recruitment at this stage in the hypertensive process. It is possible that the changes in myocardial enzyme kinetics may contribute to diastolic dysfunction previously reported in this model and may be a precursor for ultimate development of heart failure if hypertension is maintained for prolonged periods.(ABSTRACT TRUNCATED AT 250 WORDS)

Size dependence of the end-systolic stress/volume ratio in humans: implications for the evaluation of myocardial contractile performance in pressure and volume overload

The end-systolic stress/volume ratio is currently recognized as a relatively load-independent index of myocardial contractile performance, but its dependence on ventricular size may limit its value for interpatient comparisons. In this study, the relation between the end-systolic stress/volume ratio and left ventricular end-diastolic volume was angiographically analyzed in 104 patients with normal coronary angiograms. Eighteen patients had a normal ventricle, 24 had aortic stenosis, 18 had aortic regurgitation, 9 had mitral regurgitation and 35 had cardiomyopathy. An inverse relation between the end-systolic stress/volume ratio and left ventricular end-diastolic volume was demonstrated in the normal group (r = 0.72, p less than 0.001); subjects with a larger left ventricle had a reduced index but, presumably, the same degree of contractility as that of subjects with a smaller ventricle. Attempts to normalize values by using end-diastolic volume or body surface area were unsuccessful. A similar inverse relation was demonstrated in the aortic stenosis group (r = 0.48, p less than 0.05), probably because hypertrophy helps to keep wall stress normal or low despite progressive ventricular enlargement in these patients. The end-systolic stress/volume ratio was also inversely related to left ventricular chamber size in patients with volume overload due to aortic regurgitation (r = 0.80, p less than 0.001) and in those with cardiomyopathy (r = 0.84, p less than 0.001). However, at a given left ventricular end-diastolic volume, the end-systolic stress/volume ratio was higher in patients with aortic regurgitation than in those with cardiomyopathy, suggesting better contractile performance for a comparable degree of ventricular dilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute volume loading studied in cat myocardium with 31P nuclear magnetic resonance

To study the effects of acute volume loading on myocardial metabolic and mechanical function, seven cats were volume loaded via anastomosis of the abdominal aorta to the vena cava (AV shunt). Metabolic effects were evaluated with 31P nuclear magnetic resonance (NMR). Mechanical function was evaluated with heart rate X systolic blood pressure product (HR X SBP). Shunts were opened for 1-2 h during which time phosphocreatine (PCr), adenosine triphosphate (ATP), inorganic phosphate (Pi), and HR X SBP were monitored. High-energy phosphate energetics as determined by Pi/PCr and PCr/ATP ratios were correlated with HR X SBP. Opening of the AV shunts was associated with an increase (four cats) or a decrease (three cats) in HR X SBP. Pi/PCr ratios increased and PCr/ATP ratios decreased in cats with an increase in HR X SBP. In cats with a decrease in HR X SBP, Pi/PCr and PCr/ATP generally did not change significantly. In summary, acute volume loading could be associated with an increase or decrease in myocardial external work as evaluated by HR X SBP, accompanied by metabolic changes suggestive of appropriate induction of state 3 metabolism (active metabolic state: ADP + Pi----ATP) in those cats with increased mechanical work, and minimal change in bioenergetics in cats with no or minimal increase in mechanical work. These induced metabolic responses to myocardial mechanical loading can be evaluated with 31P NMR techniques and may provide insight into in vivo metabolic control mechanisms.

Central and peripheral components of cardiac failure

Chronic heart failure results from two processes, i.e., myocardial and congestive failure. Myocardial failure is clinically silent, most often progresses slowly, and is documented by a depressed left ventricular ejection fraction. Multiple etiologic factors include systolic and diastolic overloads, myocardial necrosis and/or ischemia, and, perhaps, microvascular spasm. Myocardial failure ultimately leads to exaggerated neurohumoral compensatory mechanisms and derangements of the peripheral circulation, which are the hallmarks of congestive heart failure. At that stage of the syndrome, patients have symptoms, initially, with exercise and, later, at rest. Objective assessment of severity is afforded by determination of maximal oxygen uptake during maximal exercise testing. When congestive heart failure supervenes, the prognosis is poor. Current medical therapy is aimed at improving the derangements of the peripheral circulation, which relieves the symptoms but leaves the primary myocardial process unaffected. The goal of future therapy is to intervene at an earlier stage of the syndrome to halt or even partially reverse the myocardial failure.

Relation of midwall circumferential systolic stress to equatorial midwall fibre shortening in chronic aortic regurgitation. Value as a predictor of postoperative outcome

Nineteen patients with chronic aortic regurgitation and a large increase in heart size were studied before aortic valve replacement. By relating midwall circumferential systolic stress to midwall circumferential fibre shortening (Cs/Cd) before operation the patients could be divided into two well defined groups. Twelve patients (group 1) had a pronounced decrease in heart size as measured by the cardiothoracic ratio and an excellent clinical outcome six months after operation. Seven patients (group 2) had no significant decrease in heart size and a less good clinical outcome. The ratio of midwall circumferential systolic stress to end systolic volume index was significantly higher in group 1 than in group 2. Group 2 had more severe left ventricular hypertrophy determined by the ratio of the wall thickness to the minor internal radius of the left ventricle (h:r ratio), total left ventricular mass, and left ventricular mass to end diastolic volume ratio. There were no significant differences in any other haemodynamic or angiographic indices between the two groups. Thus the relation of midwall circumferential systolic stress to fibre shortening is useful in determining the prognosis in individual patients with chronic aortic regurgitation undergoing aortic valve replacement.

Left ventricular function in chronic aortic regurgitation with reference to end-systolic pressure, volume and stress relations

Left ventricular muscle and pump performance were evaluated in 12 normal subjects and 21 patients with aortic regurgitation (10 with minimal symptoms and 11 with congestive heart failure). A computer-based quantitative analysis of biplane left ventriculograms was used. Both patient groups had significant aortic regurgitation documented by ventriculography. Contractile function measured by peak systolic stress/end-diastolic volume and end-systolic pressure/volume curves was poorer than that in normal subjects in patients with heart failure but not in asymptomatic patients. When normalized for muscle mass, stroke work was not depressed in either asymptomatic patients (mean +/- standard error of the mean 0.008 +/- 0.001 joules/g) or patients with heart failure (0.009 +/- 0.004) by comparison with the value in normal subjects (0.010 +/- 0.001). Angiographically determined cardiac index (CI) increased with increasing volume overload even though forward cardiac index measured by the Fick method remained essentially unchanged: normal subjects (total CI 3.7 +/- 0.4 liters/min per m2, Fick CI 2.4 +/- 0.1); asymptomatic patients (total CI 7.6 +/- 0.7, Fick CI 2.3 +/- 0.2); patients with heart failure (total CI 9.1 +/- 0.82, Fick CI 2.1 +/- 0.18). Left ventricular peak stress increased significantly in patients with heart failure (511 +/- 55 dynes/cm2 x 10(-3)) compared with values in normal subjects (360 +/- 33) and asymptomatic patients (428 +/- 50). The combination of decreased muscle function and increased demands on pump function causes a significant increase in end-diastolic pressure only in patients with heart failure (23 +/- 2 mm Hg), which results in pulmonary congestive symptoms.

Systolic and diastolic properties of the human left ventricle during valve replacement for chronic mitral regurgitation

M mode and two dimensional echocardiography were combined with pressure-flow data to analyze systolic mechanics and diastolic compliance in nine patients during valve replacement for chronic mitral regurgitation. Both M mode (six patients) and two dimensional (four patients) echographic analyses revealed large decreases in early postoperative shortening fraction (-24 +/- 17 [standard deviation] percent M mode study, p < 0.01: -30 +/- 12 percent two dimensional study, p < 0.02), which were significantly different from small changes observed in control subjects (M mode study, +7 +/- 10 percent, 25 subjects and two dimensional study, -7 +/- 14 percent, 8 subjects). Additional data suggest that ventricular compliance is increased in chronic mitral regurgitation and that elimination of the low impedance left atrial pathway by valve replacement is associated with a significant increase in wall stress (five patients, p < 0.02) that appears to be responsible for the decreased ejection fraction postoperatively. Analysis of hemodynamic variables other than ejection fraction and rate of circumferential shortening revealed no difference between five postoperative patients with chronic mitral regurgitation and five with coronary artery disease. These results in human subjects confirm predictions from studies in animal models and suggest that unique properties of chronic mitral regurgitation and demand special attention when patients with this condition are being evaluated for surgery.

Current status of radionuclide imaging in valvular heart disease

In valvular heart disease, there is a different radionuclide angiographic pattern in each of three left-sided valve abnormalities: pressure overload (aortic stenosis), volume overload (aortic or mitral regurgitation) and inflow obstruction (mitral stenosis). In pressure overload, the left ventricle is usually normal in size or minimally dilated. The ejection fraction may be normal, increased or decreased. In volume overload, there is left ventricular dilatation with a normal or reduced ejection fraction at rest. Scans may be performed during exercise to unmask abnormalities of ventricular function not evident at rest. In inflow obstruction, left ventricular function is usually normal but may be depressed. Right ventricular function may be abnormal secondary to pulmonary hypertension. Radionuclide angiography in valvular heart disease evaluates the impact of the valve abnormality on cardiac chamber size and function, which is useful in managing the patient, in determining the prognosis and in evaluating the success of valve surgery. Thallium-2-1 imaging evaluates regional myocardial blood flow and cell integrity and can be used to assess associated coronary artery disease.

Comparison between apexcardiographic and angiographic indexes of left ventricular performance in patients with aortic incompetence

Left ventricular (LV) apexcardiogram (ACG) and its first derivative (dA/dt) was obtained in 104 normal subjects and 34 patients with chronic aortic incompetence (AI). In the patients with AI the ACG was recorded simultaneously with LV pressure (tipmanometer). The systolic upstroke time (SUT), the time to peak dA/dt (t-dA/dt) and the a wave percentage amplitude (a/H) of the ACG was measured. In normal subjects SUT averaged 99 +/- 17 (SD) msec. In 17 patients with AI and normal ejection fraction (EF) (group 1) SUT was within normal limits; in 17 patients with AI and decreased EF (group 2) it was prolonged (142 +/- 19 msec) (P less than 0.001). The SUT was closely correlated with EF (r = 0.85) and less with contractile indexes derived from pressure curves. The indexes t-dA/dt and a/H were not significantly different in groups 1 and 2; they were weakly correlated only with the time to peak rate of LV pressure rise (r = +0.56) and the LV end-diastolic pressure (r = +0.59), respectively. These results demonstrate the superiority of SUT over the other apexcardiographic parameters. The measure provides another means of noninvasive assessment of the LV performance in patients with AI.

Reduced left ventricular myocardial blood flow per unit mass in aortic stenosis

Myocardial blood flow (MBF) per unit mass was measured in 10 patients (pts) with severe aortic stenosis (AS) and no significant aortic insufficiency, normal ejection fractions, and normal coronary arteriograms, using xenon-133 and a multiple crystal scintillation camera. MBF per unit mass was reduced in AS (53 +/- 13 mg/100g.min) in comparison to a group of seven normal control patients (69 +/- 12 ml/100g.min) (P less than 0.05). When normalized for heart rate. MBF remained depressed in aortic stenosis (0.65 +/- 0.11 ml/100 g.beat). MBF/beat was strongly related to peak left ventricular wall stress in both groups (r = 0.97). Individual values of MBF/beat were normalized for peak stress using an analysis of covarience; the adjusted mean values were 0.62 +/- 0.03 ml/100g.beat for the AS patients and 0.84 +/- 0.03 ml/100 g.beat for the control patients. There was no overlap between groups in adjusted MBF per beat. Values of MBF per beat and peak stress for a group of ten cardiomyopathy patients with depressed contractility were observed to fall close to the regression line for AS patients. The results suggest that variability in resting MBF in these AS patients is due primarily to differences in LV stress and that reduction in MBF per beat in this group may be due to reduced contractility.

Left ventricular performance in patients with left ventricular hypertrophy caused by systemic arterial hypertension

To assess the adaptation of the left ventricle to a chronic pressure overload we used echocardiography to study 18 patients with left ventricular hypertrophy caused by systemic arterial hypertension. Increased values for either posterior wall or interventricular septal thickness or both confirmed the presence of left ventricular hypertrophy in all patients and an increase in the average wall thickness to radius ratio was consistent with the development of concentric hypertrophy. No patient had clinical evidence of ischaemic heart disease. Ejection phase indices of left ventricular performance (mean Vcf, fractional per cent of shortening, normalised posterior wall velocity, and ejection fraction) were within the normal range in the basal state in 16 of the 18 patients. The hypothesis is advanced that patients with concentric left ventricular hypertrophy resulting from systemic arterial hypertension usually have normal left ventricular performance in the basal state because values for wall stress remain within the normal range. We conclude that the hypertrophic response to a chronic increase in systemic arterial pressure does not per se result in depression of the basal inotropic state of the left ventricle.

The evaluation of left ventricular function by echocardiography

Echocardiographic assessment of left ventricular size and function correlates well with that made by cineangiography. The technic is particularly suited to the early detection and serial monitoring of left ventricular hypertrophy and dysfunction in valvular heart disease, hypertensive heart disease and the cardiomyopathies. It also has advantages over angiography in experimental situations in which frequently repeated or continuous assessment of left ventricular size and function may be required. The limited usefulness of a single ultrasound beam in segmental heart disease, such as ischemic heart disease, may be overcome in the future with improvements in two-dimensional echocardiographic technics.

Effects of hypertrophy on contractile function in man

In patients with chronic pressure and volume overload matched with respect to elevated left ventricular muscle mass contractile function assessed by isovolumic as well as by ejection phase indexes is depressed to a similar extent. Pressure overload hypertrophy in aortic stenosis is associated with a reduced inotropic state of the individual contractile units. Despite this diminution of contractility hypertrophy may be effective in maintaining a normal ejection fraction as long as afterload does not become excessive. Aortic valve replacement leads to a significant reduction of the preoperatively elevated ventricular muscle mass and to a significant improvement of contractile function.