Myocardial contractile function in aortic stenosis as determined from the rate of stress development during isovolumic systole

CARDIAC CONTRACTILITY MEASURES OF LEFT VENTRICULAR SYSTOLIC FUNCTIONAL ASSESSMENT OF NORMAL AND DISEASED HEARTS

Left ventricular (LV) contraction is the basis of LV systolic function, impairment of which underlies heart failure pathophysiology. Its accurate quantification in the form of LV contractility indices is imperative for diagnostic and follow-up assessment of LV systolic function in heart failure.

Herein, we analyze LV contractile performance by focusing on LV contractility indices at different physiological organizational levels: from sarcomere dynamics to LV myocardial properties (such as elastic modulus and elastance), and from LV wall contractile stress development to the generation of intra-LV blood flow velocities and pressure distributions. Further, we present the development analyses of these indices and their medical applications. Using improved development of invasive and noninvasive techniques for measuring ventricular pressure, geometry, and volume, we show how these indices have become more amenable for clinical usage to obtain better patient assessment.

The purpose of this paper is to present a comprehensive coverage of LV contraction physiology, indices to qualify LV contraction, formulation, and medical applications of some major intrinsic LV contractility indices, so as to provide the basis of functional assessment of normal versus diseased hearts.

Validation of a novel noninvasive cardiac index of left ventricular contractility in patients

Although there are several excellent indexes of myocardial contractility, they require accurate measurement of pressure via left ventricular (LV) catheterization. Here we validate a novel noninvasive contractility index that is dependent only on lumen and wall volume of the LV chamber in patients with normal and compromised LV ejection fraction (LVEF). By analysis of the myocardial chamber as a thick-walled sphere, LV contractility index can be expressed as maximum rate of change of pressure-normalized stress (dσ*/d tmax, where σ* = σ/P and σ and P are circumferential stress and pressure, respectively). To validate this parameter, dσ*/d tmax was determined from contrast cine-ventriculography-assessed LV cavity and myocardial volumes and compared with LVEF, dP/d tmax, maximum active elastance ( Ea,max), and single-beat end-systolic elastance [ Ees(SB)] in 30 patients undergoing clinically indicated LV catheterization. Patients with different tertiles of LVEF exhibit statistically significant differences in dσ*/d tmax. There was a significant correlation between dσ*/d tmax and dP/d tmax (dσ*/d tmax = 0.0075dP/d tmax − 4.70, r = 0.88, P < 0.01), Ea,max (dσ*/d tmax = 1.20 Ea,max + 1.40, r = 0.89, P < 0.01), and Ees(SB) [dσ*/d tmax = 1.60 Ees(SB) + 1.20, r = 0.88, P < 0.01]. In 30 additional individuals, we determined sensitivity of the parameter to changes in preload (intravenous saline infusion, n = 10 subjects), afterload (sublingual glyceryl trinitrate, n = 10 subjects), and increased contractility (intravenous dobutamine, n = 10 patients). We confirmed that the index is not dependent on load but is sensitive to changes in contractility. In conclusion, dσ*/d tmax is equivalent to dP/d tmax, Ea,max, and Ees(SB) as an index of myocardial contractility and appears to be load independent. In contrast to other measures of contractility, dσ*/d tmax can be assessed with noninvasive cardiac imaging and, thereby, should have more routine clinical applicability.

[Usefulness of the Doppler index delta P/delta t in the evaluation of left ventricular systolic dysfunction]

INTRODUCTION AND OBJECTIVES

It has been shown that the delta P/delta t index, derived from the continuous Doppler mitral regurgitation signal correlates strongly with dP/dt. This study evaluates the feasibility, reproducibility and correlation of the index with ejection fraction and other conventional echocardiographic parameters.

MATERIAL AND METHODS

One hundred and ten patients with mitral regurgitation demonstrated by colour Doppler were studied. delta P/delta t were calculated by the ratio between the interval of pression between two points of the Doppler signal (-1 and -3 m/s; 32 mmHg, applying the modified Bernouilli equation) and the interval of time (s) which separates both. Ejection fraction was measured in 70 patients by non-echocardiographic methods (isotopic ventriculography, n = 52, and angiography, n = 18).

RESULTS

The index was feasible in 91 cases, the variability of intra and interobserver was 5% and 7% respectively. The correlation between delta P/delta t and ejection fraction was significant although weak (r = 0.59; p < 0.001; n = 70). It was better in the group of dilated idiopathic myocardiopathy (r = 0.72; p < 0.001; n = 18) than in the group of myocardial infarction (r = 0.54; p < 0.01; n = 25). No significant correlation was founded in the cases with mitral rheumatic valvulopathy. Regarding to the echocardiographic parameters, the best correlation was obtained with end systolic diameter (r = -0.64; p < 0.001; n = 49). Finally, a value of delta P/delta t < 1,000 mmHg/s predicted the existence of left ventricular systolic dysfunction with high accuracy (84%), sensitivity (80%) and specificity (92%).

CONCLUSIONS

High feasibility when mitral regurgitation exists, adequate reproducibility and heightened precision in diagnosing left ventricular systolic dysfunction, are characteristics which make delta P/delta t useful in the echocardiographic routine practice.

Acute changes in myosin heavy chain synthesis rate in pressure versus volume overload

The left ventricular hypertrophy that develops with the volume overload of mitral regurgitation is relatively less than that which develops with the pressure overload of aortic stenosis even when both lesions are severe. The hypertrophy that develops must be the sum of changes in the rate of myocardial protein synthesis and degradation. In the present canine study, we explored early changes in the synthesis rate of myosin heavy chain in response to severe acute pressure overload versus that of the severe acute volume overload of mitral regurgitation. We tested the hypothesis that in acute overload, the rate of protein synthesis would increase less in the volume-overload model than in the pressure-overload model, a potential partial mechanism for the discrepancy in the eventual total amount of hypertrophy that develops in these two lesions. Acute pressure overload was produced by inflating a balloon in the descending aorta, and acute volume overload was produced by using our closed-chest mitral chordal rupture technique. In both models, the hemodynamic lesion that was created was severe. In eight dogs with pressure overload, the average gradient across the balloon was 119.8 +/- 6.1 mm Hg. In six dogs with volume overload, the average regurgitant fraction was 0.67 +/- 0.06. Six other dogs served as controls. The average rate of myosin heavy chain synthesis in control dogs was 2.7 +/- 0.2% per day, virtually identical to the rate we found in the severe volume-overload model. In contrast, the rate was increased in the pressure-overload model by 30% to 3.5 +/- 0.3% per day (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Noninvasive estimation of the instantaneous first derivative of left ventricular pressure using continuous-wave Doppler echocardiography

BACKGROUND

The complete continuous-wave Doppler mitral regurgitant velocity curve should allow reconstruction of the ventriculoatrial (VA) pressure gradient from mitral valve closure to opening, including left ventricular (LV) isovolumic contraction, ejection, and isovolumic relaxation. Assuming that the left atrial pressure fluctuation is relatively minor in comparison with the corresponding LV pressure changes during systole, the first derivative of the Doppler-derived VA pressure gradient curve (Doppler dP/dt) might be used to estimate the LV dP/dt curve, previously measurable only at catheterization (catheter dP/dt).

METHODS AND RESULTS

This hypothesis was examined in an in vivo mitral regurgitant model during 30 hemodynamic stages in eight dogs. Contractility and relaxation were altered by inotropic stimulation and hypothermia. The Doppler mitral regurgitant velocity spectrum was recorded along with simultaneously acquired micromanometer LV and left atrial pressures. The regurgitant velocity profiles were digitized and converted to VA pressure gradient curves using the simplified Bernoulli equation. The instantaneous dP/dt of the VA pressure gradient curve was then derived. The instantaneous Doppler-derived VA pressure gradients, instantaneous Doppler dP/dt, dP/dtmax, and -dP/dtmax were compared with corresponding catheter measurements. This method of estimating dP/dtmax from the instantaneous dP/dt curve was also compared with a previously proposed Doppler method of estimating dP/dtmax using the Doppler-derived mean rate of LV pressure rise over the time period between velocities of 1 and 3 m/sec on the ascending slope of the Doppler velocity spectrum. Both instantaneous Doppler-derived VA pressure gradients (r = 0.95, p less than 0.0001) and Doppler dP/dt (r = 0.92, p less than 0.0001) correlated well with corresponding measurements by catheter during systolic contraction and isovolumic relaxation (pooled data). The Doppler dP/dtmax (1,266 +/- 701 mm Hg/sec) also correlated well (r = 0.94) with the catheter dP/dtmax (1,200 +/- 573 mm Hg/sec). There was no difference between the two methods for measurement of dP/dtmax (p = NS). Although Doppler -dP/dtmax was slightly lower than the catheter measurement (961 +/- 511 versus 1,057 +/- 540 mm Hg/sec, p less than 0.01), the correlation between measurements by Doppler and catheter was excellent (r = 0.93, p less than 0.0001). The alternative method of mean isovolumic pressure rise (896 +/- 465 mm Hg/sec) underestimated the catheter dP/dtmax (1,200 +/- 573 mm Hg/sec) significantly (on average, 25%; p less than 0.001).

CONCLUSIONS

The present study demonstrated an accurate and reliable noninvasive Doppler method for estimating instantaneous LV dP/dt, dP/dtmax, and -dP/dtmax.

A microcomputer-based device to simulate biomechanical environments for cultured cells

We describe software and hardware for a microcomputer-based cyclic strain device which applies programmed cycles of elongation and relaxation to cultured cells. This system has the potential to simulate many of the complex mechanically active environments found in living systems. As a sample application, we use it to simulate the cyclic stresses to which vascular smooth muscle cells in the arterial system are exposed.

Left ventricular myocardial structure in aortic valve disease before, intermediate, and late after aortic valve replacement

Left ventricular biplane cineangiography, micromanometry, and endomyocardial biopsies were performed in 27 patients with aortic stenosis (AS) and in 17 patients with aortic insufficiency (AI). Twenty-three patients with AS and 15 with AI were restudied at an intermediate time (18 months after successful valve replacement), and nine patients with AS and six with AI were restudied late (70 and 62 months after surgery). Biopsy samples were evaluated for muscle fiber diameter, percent interstitial fibrosis, and volume fraction of myofibrils. In control biopsy samples obtained from five donor hearts at transplantation, these morphometric variables averaged 21.2 microns, 7.0%, and 57.2%, respectively. After surgery, mass determined by cineangiography decreased from 186 to 115 and 94 g/m2 in patients with AS and from 201 to 131 and 93 g/m2 in patients with AI. At the three studies, muscle fiber diameter was 30.9, 28.0, and 28.7 microns in patients with AS and was 31.4, 27.6, and 26.4 microns in patients with AI. Percent interstitial fibrosis was 18.2, 25.8, and 13.7% in patients with AS and was 20.4, 23.7, and 19.2% in patients with AI. Left ventricular fibrous content decreased from 34.2 to 29.8 and to 12.7 g/m2 in patients with AS and from 42.1 to 28.9 and to 18.9 g/m2 in patients with AI. Volume fraction of myofibrils was 57.7, 56.8, and 49.0% in patients with AS and was 56.8, 56.6 and 48.8% in patients with AI. Thus, the decrease of muscle mass determined by cineangiography at the intermediate time after valve replacement is mediated by regression of myocardial cellular hypertrophy in patients with AS and AI and in addition by a decrease of fibrous content in patients with AI. Late after surgery, left ventricular fibrous content also decreases in patients with AS. This late decrease associated with minor changes of end-diastolic volume may be important for improvement of increased diastolic myocardial stiffness. Even 6-7 years after valve replacement, incomplete regression of structural abnormalities of left ventricular hypertrophy still exists compared with the normal myocardium. The residually increased relative interstitial fibrosis and the small late postoperative decrease of volume fraction of myofibrils, associated with a prosthesis-related slight left ventricular pressure increase, are at the origin of a persistent systolic overload at the myofibrillar level.

Degree of reversibility of left ventricular systolic dysfunction after aortic valve replacement for isolated aortic valve stenosis

To determine whether a low preoperative left ventricular (LV) ejection fraction (EF) returns to normal late after aortic valve replacement for aortic stenosis, 42 patients with critical aortic stenosis (valve area 0.7 cm2 or less), LV systolic dysfunction (EF 0.45 or less), angiographically normal coronary arteries, and no other significant valvular disease were studied at 10 to 84 months (mean 41 +/- 21) postoperatively. All patients survived aortic valve replacement and were discharged clinically improved. There were 4 late deaths; these patients were older (79 +/- 6 vs 64 +/- 13 years, p = 0.007) and had lower preoperative mean valve gradients (51 +/- 6 vs 68 +/- 23 mm Hg, p = 0.003) than late survivors. Of 23 survivors who returned for follow-up radionuclide angiography and Doppler echocardiography, 21 were asymptomatic. EF returned to normal (0.50 or more) in 14 patients (group 1) and remained low in 9 patients (group 2). Doppler peak prosthetic valve gradient was 24 +/- 8 mm Hg in group 1 and 25 +/- 10 mm Hg in group 2 (difference not significant). Six of the 9 patients in group 2 underwent early postoperative radionuclide imaging, and LVEF was normal in 4 (0.65 +/- 0.14 early vs 0.41 +/- 0.06 late, p = 0.02). Of 77 preoperative and intraoperative variables analyzed, only paroxysmal nocturnal dyspnea (0 of 14 vs 4 of 9, p = 0.01) distinguished group 1 from group 2. Thus, LVEF does not always normalize after aortic valve replacement for AS, implying impaired myocardial contractility.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of aortic valve disease on systolic stiffness of the human left ventricular myocardium

The new concept of systolic myocardial stiffness was applied to the study of ejection mechanics in aortic valve disease. Frame-by-frame analysis of stress (sigma) and volume (V) was performed for two differently loaded beats in 26 patients who underwent simultaneous cineangiography and micromanometry: nine normal subjects, eight with isolated aortic regurgitation (AR), and nine with aortic stenosis (AS). Maximum myocardial stiffness (maxEav) was defined as the slope of the end-systolic (es) stress-strain relationship. End-systole was defined as the frame where stiffness was maximal, and strain was defined as epsilon = loge (Dm/Dom), where Dm is left ventricular midwall diameter and Dom is the theoretical Dm at zero stress. Expressed in terms of cavity volume, epsilon = gamma X loge (V/Vo), where gamma is the geometric factor relating Dm to V during systole. Vo was obtained by extrapolating to sigma es = 0 the function, sigma es = maxEav X gamma X loge (Ves/Vo), which was fit to the end-systolic data. Vo always had a value greater than zero. MaxEav was preserved in the AR group (1575 +/- 565) and increased in the AS group (1877 +/- 544; p = .02) compared with normal (1320 +/- 268), suggesting maintenance of contractile force per unit of myocardium in these two lesions. However, theoretical "unloaded" shortening fraction (SFo) was depressed in the AS group (0.30 +/- 0.06; p = .01) compared with normal (0.37 +/- 0.04), preserved in the AR group (0.34 +/- 0.07; p = .24), and inversely related to maxEav (r = -.66, p = .01), suggesting a disparity between shortening potential and force potential.(ABSTRACT TRUNCATED AT 250 WORDS)

Contractile function, myosin ATPase activity and isozymes in the hypertrophied pig left ventricle after a chronic progressive pressure overload

Experimental right ventricular pressure-overload hypertrophy in small mammals is associated with early muscle dysfunction, even before the onset of overt pump failure. Experimental results are quite heterogeneous regarding muscle function of the pressure hypertrophied left ventricle. Muscle dysfunction of the right or left ventricle, when found, may be causally related to alterations of myosin ATPase activity and isozyme type. However, the effect of a gradual pressure overload, analogous to that which occurs in human aortic stenosis, on myocardial contractile function and myosin ATPase activity has not been studied in a large animal whose normal myosin isozyme pattern resembles that of man. We therefore studied pump performance, myocardial contractile function, and myosin ATPase activity and isozyme pattern in pigs with severe, gradually applied left ventricular pressure overload. Thirteen weeks after supravalvular aortic banding, 10 pigs grew more than 7-fold in body weight and were found to have an aortic stenosis area of 0.5 +/- 0.1 cm2 with a gradient of 93 +/- 12 mm Hg. Compared with nine control animals, the banded animals had a 67% increase in left ventricular mass relative to body weight without overt pump failure as measured by cardiac index and pulmonary artery wedge pressure. Left ventricular ejection performance, measured as shortening fraction, was maintained except in three animals with extreme hypertrophy, in which depressed ejection performance may have been due to an afterload mismatch, myocardial dysfunction, or both. Myocardial contractile function, determined from the end-systolic stress-diameter relationship, was normal except in two pigs in which ejection performance was depressed and left ventricular mass was more than doubled. Only the slow V3 isozyme of myosin ATPase was found in both normal and hypertrophied pig myocardium, and the ATPase activity was normal in pigs with all degrees of hypertrophy. Thus, in a large animal model of severe, gradual left ventricular pressure overload, in which myosin isozyme pattern remains apparently unaltered, moderate hypertrophy can be associated with normal myosin ATPase activity and contractile function that is normal by current methods of evaluation.

Cardiac hypertrophy: useful adaptation or pathologic process?

An extensive body of evidence supports the concept that cardiac hypertrophy and normal cardiac growth develop in response to increased hemodynamic loading and abnormal systolic and diastolic stresses at the myocardial fiber level. The pattern of hypertrophy reflects the nature of the inciting stress. Experimental studies indicate that if the stress is moderate, gradually applied, and the animal young and healthy, physiologic hypertrophy of muscle with normal contractility develops. In this circumstance, cardiac hypertrophy may be regarded as a useful adaptation to increased hemodynamic loading. When the inciting stress is severe, abruptly applied, or the animal old or debilitated, pathologic hypertrophy develops: in this circumstance, the cardiac muscle produced is abnormal and exhibits depressed contractility. Of particular clinical relevance is the intermediate situation which seems to develop in many patients with chronic left ventricular pressure-overload and perhaps also in left ventricular volume-overload. In this situation, chronic left ventricular pressure or volume overload is initially matched by adequate hypertrophy in the appropriate pattern. Eventually, in some patients, hypertrophy fails to keep pace with the hemodynamic overload so that a systolic stress imbalance occurs at the myocardial fiber level and left ventricular pump failure ensues. If this situation persists uncorrected, it is possible that the increasingly high wall stresses will convert physiologic to pathologic hypertrophy. The task of the clinician is to identify this intermediate stage and to correct the abnormal hemodynamic loading before the transition to pathologic hypertrophy becomes complete.