Left ventricular response to experimentally induced chronic aortic regurgitation

Changes in diastolic function during development and correction of chronic LV volume overload produced by mitral regurgitation

BACKGROUND

Mitral regurgitation (MR) causes an augmentation in left ventricular (LV) diastolic function, increasing early diastolic filling rate and decreasing LV stiffness. Whether these changes in diastolic function persist, return to normal, or become abnormal after mitral valve replacement (MVR) is unknown.

METHODS AND RESULTS

Simultaneous LV echocardiography and catheterization studies were performed in six dogs in the baseline state (baseline), 3 months after creation of MR (chronic MR), and 3 months after MVR. Chronic MR caused LV dilation (end-diastolic dimension increased from 4.5 +/- 0.1 cm in baseline to 5.8 +/- 0.1 cm in chronic MR, p < 0.05) and eccentric LV hypertrophy (LV-to-body weight ratio increased from 3.6 +/- 0.2 g/kg in baseline to 4.9 +/- 0.4 g/kg in chronic MR, p < 0.05). Chronic MR caused an increase in LV early diastolic filling rate (peak rate of increase in minor-axis dimension increased from 11 +/- 1 cm/sec in baseline to 18 +/- 1 cm/sec in chronic MR, p < 0.05), did not change the time constant of myocardial relaxation (tau was 31 +/- 4 msec in baseline and 30 +/- 2 msec in chronic MR), and caused a decrease in the modulus of regional chamber stiffness from 7.7 +/- 1.2 in baseline to 2.4 +/- 0.03 in chronic MR, p < 0.05. MVR caused the resolution of LV dilation (end-diastolic dimension returned to normal [4.8 +/- 0.2 cm]), but three months after MVR, regression of LV hypertrophy was incomplete (LV-to-body weight ratio remained elevated [4.4 +/- 0.5 g/kg]). After MVR, LV early diastolic filling rate (8 +/- 1 cm/sec), the relaxation time constant (31 +/- 2 msec), chamber stiffness (7.1 +/- 1.8), myocardial stiffness (11.2 +/- 3.1), and LV end-diastolic pressure (8 +/- 1 mm Hg) returned to normal.

CONCLUSIONS

The enhanced diastolic function seen in chronic MR returned to normal after correction of the chronic volume overload by MVR.

Left ventricular passive diastolic properties in chronic mitral regurgitation

BACKGROUND

In chronic mitral regurgitation, the myocardium responds to the increased filling volume by geometric alteration and eccentric hypertrophy. This study was designed to evaluate the effects of a pure volume overload on left ventricular diastolic chamber and myocardial properties and to assess the relation of passive diastolic function to systolic ejection performance.

METHODS AND RESULTS

By use of simultaneous cineangiography and left ventricular micromanometry, left ventricular passive diastolic stiffness was evaluated in nine normal controls (group 1), 14 patients with chronic mitral regurgitation and a normal ejection fraction (greater than or equal to 57%, group 2), and 13 patients with mitral regurgitation and a reduced ejection fraction (less than 57%, group 3). Passive diastolic function was evaluated by using a three-constant elastic model. Left ventricular chamber properties were represented by the relation of pressure to volume; myocardial properties were evaluated by relating myocardial midwall stress to midwall strain. The constant of left ventricular chamber stiffness was decreased in group 2 compared with controls (p less than 0.05) but it was normal in group 3. The constant of myocardial stiffness was increased in group 3 compared with groups 1 and 2 (p less than 0.01). Among patients with mitral regurgitation, there was a significant inverse relation between ejection fraction and the constant of myocardial stiffness (r = -0.83).

CONCLUSIONS

The chronic adaptation to volume overload in chronic mitral regurgitation tends to decrease left ventricular chamber stiffness. Patients with mitral regurgitation and a depressed ejection fraction demonstrated diastolic myocardial dysfunction. Compromised diastolic function in patients with chronic mitral regurgitation and reduced systolic performance may contribute to the clinical manifestations of congestive heart failure.

Atrial natriuretic peptide secretion in rabbits with aortic regurgitation

We investigated whether or not the extent of secretion of atrial natriuretic peptide (ANP) was altered during the chronic course of aortic regurgitation (AR) in rabbits. AR was induced by aortic valve perforation in 20 rabbits. Left ventricular end-diastolic pressure (LVEDP) was used as an index of the severity of heart failure. LVEDP and plasma immunoreactive(IR)-ANP were measured before and at 15 min, 1 week, and 4 weeks after induction of AR. In each period a correlation between LVEDP and plasma IR-ANP was observed, and the coefficients and covariances did not vary throughout the experiment. We conclude, therefore, that a chronic change in ANP secretion does not develop during the first 4 weeks after induction of AR in rabbits.

Mechanical pathophysiology of some heart diseases: a theoretical model study

Sarcomere dynamics are related to the global left ventricular (LV) function in some representative pathological states, by using a theoretical model which combines sarcomere function, LV fibrous structure and geometry with the haemodynamic loading conditions. The analysis shows that pressure (concentric) hypertrophy due to hypertension or aortic stenosis is associated with an increase of the normal endocardial-to-epicardial gradient(s) of oxygen demand, which may be one of the causes for the development of endocardial fibrosis. The analysis also indicates that sarcomere shortening is relatively normal in compensated volume (eccentric) hypertrophy. Mitral stenosis demonstrates a case of decreased LV function, secondary to a chronic decrease in LV end diastolic volume, with sarcomeres that operate at their lowest length range. Conversely, the sarcomere function is depressed in cardiomyopathy; the heart's pumping function is maintained by appropriate adjustment mechanisms. However, the sarcomeres show minimal shortening and function at their highest length range with low (or zero) functional reserve. The study thus provides a quantitative tool that relates global LV function to local sarcomere dynamics in various pathological states.

Impact of Fontan operation on left ventricular size and contractility in tricuspid atresia

Left ventricular dimensions and contractility were determined by echocardiography in 33 patients with tricuspid atresia in 1985 and again in 1988. Eight patients remained palliated throughout the 3-year period; neither the left ventricular end-diastolic diameter (153 +/- 15% of normal vs. 157 +/- 19%, p = NS) nor a load-independent index of contractility (rate-corrected velocity of shortening [VCFc]/end-systolic meridional stress [ESSM]) changed. Eleven patients underwent a Fontan operation during the study and were reevaluated at least 6 months after surgery; left ventricular dimension decreased (130 +/- 15% vs. 114 +/- 19%, p less than 0.001), and the contractility index VCFc/ESSM improved (p less than 0.05). Fourteen patients had undergone a Fontan operation 0.9-9.5 years (mean, 4.2 years) before initial examination in 1985. Over the 3-year period, left ventricular dimensions did not change (121 +/- 17% vs. 118 +/- 11%, p = NS), but the contractility index showed significant improvement (p less than 0.01). Eight additional patients were studied just before and after a Fontan operation to examine the early effects of surgery. Left ventricular dimensions decreased from 130 +/- 14% to 100 +/- 13% by 10 days p less than 0.001) with no further change at 2 months. An inappropriate degree of ventricular hypertrophy was observed in only the early postoperative period. Successful Fontan repair results in rapid reduction of left ventricular size, followed by regression of hypertrophy to a normal mass-to-volume ratio. Operating at more favorable dimensions and loading conditions results in an early increase in left ventricular contractility, which further improves in the medium term follow-up.

Left ventricular changes resulting from chronic aortic regurgitation in dogs

Chronic aortic regurgitation (AR) was induced by aortic valve perforation using catheterization techniques in 7 closed-chest puppies. Approximately 2 years after the creation of AR in these puppies, their growth was similar to that in littermate controls. A gradually progressive degree of left ventricular (LV) dilation and hypertrophy occurred as LV end-diastolic volume (average 116%) and mass (average 114%) increased in animals with AR compared with that in the littermate controls. The technique described to induce AR did not interfere with the normal growth and development of the animals, and permitted study of the functional characteristics of the resulting LV dilation and hypertrophy without added effects of thoracotomy and pericardiotomy.

The contractile state as the major determinant in the evolution of left ventricular dysfunction in the spontaneously hypertensive rat

Female spontaneously hypertensive and normotensive rats were studied at 6, 12, 18, and 24 months of age to determine which characteristics of myocardial performance herald the onset of left ventricular dysfunction. Peak ejection fraction index was derived from measurements of peak stroke volume (in vivo volume loading) and passive pressure-volume relations. The myocardial stiffness constant (km, slope of the incremental modulus-stress relation, EINC = km sigma), chamber stiffness constant (kc, slope of the chamber stiffness-pressure relation, dP/dV = kcP), and left ventricular cavitary volume-to-wall volume ratio at 10 mm Hg) were calculated from the pressure-volume data and the contractile state was assessed from the ejection fraction index-afterload relations. In the normotensive rats, the myocardial stiffness constant was not affected by age, whereas, in the spontaneously hypertensive rats, the myocardial stiffness constant remained within normal limits until 18 months, at which time a significant increase in this index of myocardial stiffness occurred. Baseline and maximal cardiac indices and ejection fraction index of spontaneously hypertensive rats were normal from 6 to 18 months, but were markedly reduced at 24 months. This reduction in cardiac performance was associated with a decrease in the left ventricular chamber stiffness constant, i.e., kc. This decreased chamber stiffness, which occurred at a time when myocardial stiffness was increased, was due to a greater increase in cavity size than in myocardial stiffness. The left ventricular cavity-to-wall volume ratio of normotensive rats was not affected by age, whereas, in the spontaneously hypertensive rats, this ratio markedly declined by 18 months. The ejection fraction index-afterload relations i.e., a measure of the contractile state, of the 6- and 12-month-old spontaneously hypertensive rats were similar to those of the normotensive rats of all ages. However, a depression in the contractile state of the spontaneously hypertensive rats occurred at 18 months and was further depressed at 24 months. This abnormality of the contractile state was evident before the deterioration of cardiac performance, as reflected in a decrease in baseline and maximal cardiac indices, and dilation of the left ventricle occurred. The contractile state (ejection fraction index-afterload relation) is thus the most sensitive indicator of left ventricular dysfunction in spontaneously hypertensive rats.

Aortic valve replacement in childhood: evaluation of left ventricular function by electrocardiography, echocardiography and graded exercise testing

Preoperative and postoperative left ventricular (LV) performance was evaluated noninvasively in 15 children who survived aortic valve replacement (AVR). The noninvasive evaluation included electrocardiography, M-mode echocardiography, and graded exercise testing. Clinically, there was dramatic improvement postoperatively: All but 2 patients were asymptomatic. No conduction defects or arrhythmias were detected preoperatively; however, in the late postoperative period there was a variety of intraventricular conduction abnormalities, myocardial infarctions, and ventricular or supraventricular arrhythmias. On M-mode echocardiography, children with aortic stenosis continued to have increased LV mass postoperatively. Shortening fraction and left-sided systolic time intervals returned to normal. The children with aortic regurgitation also had persistently abnormal LV mass on echocardiography postoperatively. Exercise data indicated no improvement in working capacity after AVR. Also, 9 children (63%) continued to have ST-segment depression with maximal exercise. These data indicate that AVR does not result in a return to normal of myocardial performance in children with severe aortic valve disease.

Exercise testing in asymptomatic or minimally symptomatic aortic regurgitation: relationship of left ventricular ejection fraction to left ventricular filling pressure during exercise

Exercise radionuclide angiography is being used to evaluate left ventricular function in patients with aortic regurgitation. Ejection fraction is the most common variable analyzed. To better understand the rest and exercise ejection fraction in this setting, 20 patients with asymptomatic or minimally symptomatic severe aortic regurgitation were studied. All underwent simultaneous supine exercise radionuclide angiography and pulmonary gas exchange measurement and underwent rest and exercise measurement of pulmonary artery wedge pressure (PAWP) during cardiac catheterization. Eight patients had a peak exercise PAWP less than 15 mm Hg (group 1) and 12 had a peak exercise PAWP greater than or equal to 15 mm Hg (group 2). Group 1 patients were younger and more were in New York Heart Association class I. Group 1 patients also had a higher mean rest ejection fraction (0.64 +/- 0.08 vs 0.49 +/- 0.13, p less than 0.01, higher exercise ejection fraction (0.63 +/- 0.10 vs 0.40 +/- 0.18, p less than 0.01), lower end-systolic volume (38 +/- 13 vs 79 +/- 36 ml/m2, p less than 0.01) and higher peak oxygen uptake (24.9 +/- 5.1 vs 16.6 +/- 4.9 ml/kg/min, p less than 0.01) than group 2 patients. However, the two groups had similar cardiothoracic ratios, changes in ejection fractions with exercise, and rest and exercise regurgitant indexes. Using multiple regression analysis, the best correlate of the exercise PAWP was peak oxygen uptake (r = -0.78, p less than 0.01). No other measurement added significantly to the regression. When peak oxygen uptake was excluded, rest and exercise ejection fraction also correlated significantly (r = -0.62 and r = -0.60, respectively, p less than 0.01). Patients with asymptomatic or minimally symptomatic severe aortic regurgitation have a wide spectrum of cardiac performance in terms of the PAWP during exercise. The absolute rest and exercise ejection fraction and the level of exercise achieved are noninvasive variables that correlate with exercise PAWP in aortic regurgitation, but the change in ejection fraction with exercise by itself is not.

Cardiac response to exercise in patients with chronic aortic regurgitation

Rest and exercise measurements of left ventricular (LV) ejection fraction (EF) and volumes were obtained by radionuclide angiocardiography (RNV) in 30 patients with severe aortic regurgitation (AR). The ratio of peak systolic pressure to end-systolic volume was used as an index of contractility. Volumetric cardiac output (CO) averaged 11.7 +/- 3.8 L/min at rest and 18.4 +/- 5.6 L/min during exercise. Much individual variation occurred in LVEF and end-diastolic volume (EDV) responses to exercise, and there was no consistent change in these measurements. Resting hemodynamic parameters and clinical history correlated poorly with changes observed during exercise. An increase in heart rate was one mechanism used by all 30 patients to increase CO during exercise. An inverse relationship was defined between the change in myocardial contractility and the change in EDV during exercise. Patients with the greatest increase in contractility during exercise showed the greatest decrease in EDV. Less use of an exercise increase in contractility was associated with an exercise increase in EDV to meet the demand for greater CO. Therefore, exercise measurements of LV function provide unique information regarding the degree of impairment of the LV myocardium in these patients with chronic AR.

Obesity and cardiac function

We studied 10 obese volunteers, mean age 36.5 +/- 10.3 years, who weighed 123.56 +/- 28.7 g and were 69.96 +/- 22.5 kg overweight. The subjects did not have diabetes, arterial hypertension or signs of cardiac and respiratory failure or disease and all underwent right- and left-heart catheterization. cardiac output and stroke volume were high, according to increased oxygen consumption and to the degree of obesity. Ventricular end-diastolic and atrial pressures ranged from normal to high and correlated with body weight, signs of volume overloading and reduced left ventricular (LV) compliance. The mean pulmonary artery pressure was elevated and correlated well with weight, pulmonary resistance being normal; mean aortic pressure did not correlate with weight, and systemic arterial resistance tended to have a negative correlation. The LV function curve showed impaired ventricular function, particularly for the heaviest subjects, in whom Vmax and the ratio of the stroke work index to LV end-diastolic pressure were reduced. These indexes correlated well with each other and both correlated negatively with the degree of obesity. In contrast, maximal dP/dt was normal and did not correlate with excess weight. These observations show that depressed LV function is already present in relatively young obese people, even if they are free from signs of cardiopathy and other associate diseases. The degree of impairment of heart function seems to parallel the degree of obesity.

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.

Regression of left ventricular dilation and hypertrophy after removal of volume overload. Morphological and ultrastructural study

Gross and ultrastructural changes occurring in the left ventricle in response to chronic volume overload were studied in dogs. One group of dogs was examined 28-43 days after an aortocaval fistula had been created; congestive heart failure had developed at the time of the examination. Two other groups of dogs were investigated 78 ± 17 ( SD ) days and 178 ± 4 days after similar fistulas had been closed. A group of control dogs was also studied. Aortocaval fistulas produced significant left ventricular dilation and hypertrophy. Ultrastructural changes included enlargement and distortion of intercalated disks, increase in number but decrease in size and relative volume of mitochondria, and loss of lateral alignment of sarcomeres. Left ventricles, 78 days after the fistulas had been closed, were not different ultrastructurally or grossly from those in dogs with patent fistulas. After 178 days of closure, ventricular mass and volume had decreased; mass was still significantly greater than that in normal dogs, but cavity volume was not. Ultrastructural abnormalities in dogs after 178 days of closure were much less marked than those in dogs with patent fistulas, and ultrastructure was hardly distinguishable from normal. The findings in these experiments indicate that gross and ultrastructural abnormalities produced in the left ventricle by chronic volume overload are largely, if not completely, reversible.

Digitalis-induced increase in aortic regurgitation and the contrasting effect of glucagon in the sedated dog

The hemodynamic and phasic ascending aortic flow changes induced by acetylstrophanthidin and glucagon were studied in closed-chest sedated dogs with aortic regurgitation. While the positive inotropic effect of both agents was reflected in an increase in peak rate of rise of left ventricular pressure, acetylstrophanthidin increased aortic regurgitation, while glucagon decreased it. With the former, left ventricular end-diastolic pressure rose from 20+/-6 to 27+/-6 mm Hg (P < 0.005), but fell from 18+/-4 to 11+/-3 mm Hg (P < 0.001) with glucagon. Acetylstrophanthidin increased systemic vascular resistance, aortic diastolic pressure, and diastolic regurgitant flow rate, and, heart rate and the duration of regurgitation per beat and per minute being unchanged, regurgitant flow per beat increased 32+/-15% (P < 0.001). Glucagon decreased regurgitant flow per beat 27+/-14% (P < 0.001) because of abbreviation of diastole associated with tachycardia, and because of reduction in regurgitant flow rate. Despite tachycardia, the duration of regurgitation per minute was unchanged, and the small fall in regurgitant blood flow per minute was not significant, but this pertained in the face of 47% increase in effective cardiac output (P < 0.001). In contrast, acetylstrophanthidin increased regurgitant flow per minute 28+/-14% (P < 0.001) without change in effective cardiac output. The increase in cardiac contractility, tachycardia, and systemic vasodilatation induced by glucagon preferentially enhanced forward blood flow, which led to reduction in left ventricular volume overload, while it increased cardiac output. Contrarily, acetylstrophanthidin increased aortic regurgitation and, despite its inotropic effect, increased left ventricular volume overload without an increase in cardiac output.