Relaxation-systolic pressure relation. A load-independent assessment of left ventricular contractility

Load dependent diastolic dysfunction in heart failure

Congestive heart failure may result from cardiovascular overload, from systolic or from diastolic dysfunction. Diastolic left ventricular dysfunction may result from structural resistance to filling such as induced by pericardial constraint, right ventricular compression, increased chamber stiffness (hypertrophy) and increased myocardial stiffness (fibrosis). A distinct and functional etiology of diastolic dysfunction is slow and incomplete myocardial relaxation. Relaxation may be slowed by pathological processes such as hypertrophy, ischemia and by asynchronous left ventricular function. The present contribution analyses the occurrence of slow and incomplete myocardial relaxation in response to changes in systolic pressure and in response to changes in venous return. The regulation of myocardial relaxation by load is critically dependent on the transition from myocardial contraction to relaxation, which occurs in dogs when 82% of peak isovolumetric pressure has developed or at a relative load of 0.82. This corresponds to early ejection in normal hearts, but is situated even before aortic valve opening in severely diseased hearts. When load is developed beyond this transition, relaxation becomes slow and even incomplete. This is load dependent diastolic dysfunction. Load dependent diastolic dysfunction occurs in normal hearts facing heavy afterload and in severely diseased hearts even with normal hemodynamic parameters. This dysfunction should contribute to elevating filling pressures in most patients with severe congestive heart failure. This dysfunction can be reverted by decreasing systolic pressures or by decreasing venous return. Load dependent diastolic dysfunction gives us an additional reason to aggressively treat CHF patients with diuretics and vasodilators.

Role of myofilaments and calcium handling in left ventricular relaxation

Myocardial relaxation is governed by the interplay of two macromolecular systems: (1) myofilaments and (2) calcium extruding pumps/exchangers. In myocardium from failing hearts, both systems act more slowly than normal, and cause relaxation to decelerate, which may impede early rapid filling and can often limit cardiac pumping ability--especially during exercise. Gene-based therapy to augment sluggish SERCA pumps is a possibility being currently investigated in research laboratories. In normal myocardium, the rate of dissociation of myosin crossbridges sets the rate of relaxation. In this case, relaxation is characterized by two features: (1) load-dependence and (2) displacement-dependence. Load-dependence derives from cooperative mechanisms acting among ensembles of crossbridges and myofilament regulatory proteins (troponin, tropomyosin); it allows contraction to be prolonged when more crossbridges are attached and mutually support each other. The rate of relaxation can still be rapid, however, as this cooperative system begins to collapse. Displacement-dependence is more important later in contraction, because tenuous crossbridge attachments cannot easily re-form after being disrupted when myofilaments slide along each other. Myofilaments control normal relaxation because the calcium extruding systems reduce calcium to near diastolic levels relatively early; however, when the relative timing of crossbridge dissociation versus calcium sequestration is altered, and calcium uptake is slowed (relative to crossbridges), then removal of calcium can become rate limiting instead. In this case, load- and displacement-dependence are less marked. Both the timing of calcium removal and the sensitivity of the myofilaments to calcium affect relaxation timing.

Assessment of diastolic dysfunction. Invasive modalities

In this article, the author sought to review the two primary components of diastolic function that are most directly and accurately determined using invasive methodologies. For chamber relaxation this is optimally achieved using a micromanometer catheter, whereas for chamber compliance (or its inverse stiffness) this is best achieved by combining this catheter with a measure of instantaneous volume from a conductance catheter, using data from multiple cycles. Even with the ideal data set, the analysis of both properties involves physiologic and often mathematical assumptions, and the extent to which the data do not match these assumptions, the derived indexes may be misleading. Care in the data collection, and awareness of the various factors and pitfalls involved with their analysis can undoubtedly improve the interpretations. As advances in noninvasive methods continue to evolve, reliance on invasive methodologies will continue to fade into the background. At present, however, they remain the gold standard for the two primary diastolic properties described, and have clearly played a central role in the evolution of our understanding of cardiac diastolic disease and its treatment.

Mechanics of Relaxation of the Human Heart

Rapid and complete relaxation is a prerequisite for cardiac output adaptation to changes in loading conditions, inotropic stimulation, and heart rate. In the healthy human heart, the rate and extent of relaxation depend mainly on actomyosin cross bridge dissociation and on left ventricular end-systolic volume, rather than on the afterload level.

Effects of nicardipine and urapidil on length-dependent regulation of myocardial function in coronary artery surgery patients

OBJECTIVE

To assess effects of a decrease in left ventricular (LV) afterload (pharmacologically induced by nicardipine and urapidil) on myocardial contraction and relaxation, with emphasis on the effects on load dependence of myocardial function.

DESIGN

Prospective, blinded study.

SETTING

University hospital.

PARTICIPANTS

Coronary artery surgery patients.

INTERVENTIONS

Alterations of systolic load were effected by leg elevation in control conditions and after administration of either nicardipine or urapidil before and after cardiopulmonary bypass.

MEASUREMENTS AND MAIN RESULTS

High-fidelity LV pressure tracings were obtained at end-expiration while hearts were paced at a fixed rate of 90 beats/min. Hemodynamic effects of leg elevation were compared before and after nicardipine, 7 microg/kg (n = 15), and before and after urapidil, 0.4 mg/kg (n = 15). The effects of leg elevation on parameters of contraction and relaxation were coupled. Both nicardipine and urapidil similarly decreased systolic pressures and peripheral resistance. Nicardipine decreased rate of pressure development (dP/dtmax) and slowed LV pressure fall, whereas load dependence of LV relaxation was not altered. Urapidil did not alter dP/dtmax, rate of LV pressure fall, or load dependence of relaxation. Similar results were observed after cardiopulmonary bypass.

CONCLUSIONS

The results of the present study indicate that a pharmacologically induced moderate reduction in LV afterload with nicardipine or urapidil did not alter the length-dependent regulation of myocardial function.

Comparison of ventricular pressure relaxation assessments in human heart failure: quantitative influence on load and drug sensitivity analysis

OBJECTIVES

We contrasted various methods for assessing ventricular pressure decay time constants to test whether sensitivity to slight data instability or disparities between model-assumed and real decay are systematically altered by cardiac failure. We hypothesized that such discrepancies could result in apparent increased relaxation sensitivity to load and drug stimulation.

BACKGROUND

Deviation of relaxation behavior from model-assumed waveforms may be worsened by failure, enhancing instability and apparent load and drug sensitivity of commonly used indexes.

METHODS

Pressure-volume relations were measured in patients with normal (n = 14), hypertrophic (hypertrophic cardiomyopathy [HCM], n = 15) and dilated-myopathic (dilated cardiomyopathy [DCM], n = 37) hearts before and during preload reduction or inotropic stimulation. Relaxation parameters (monoexponential [ME] model assuming zero-T(in) or non-zero-T(D), T(F) asymptote:, hybrid logistic-T(L), linear-T(LR), and pressure halftime-T(1/2)) were contrasted regarding sensitivity to slight data range manipulation and loading or drug changes.

RESULTS

In DCM, T(D) and T(F) prolonged 15% to 25% (p < 0.0001) by deletion of only 1-2 data points, whereas this had minimal effect on controls or HCM. This stemmed from systematic deviation of relaxation from an ME decay in DCM. T(1/2) and T(in) were highly sensitive to pure pressure offsets, whereas T(L) was most stable to both manipulations in all hearts. As a result, T(D) and T(F) appeared to be much more sensitive to systolic load in DCM than T(1/2) or T(L) and disproportionately sensitive to increased cyclic adenosine monophosphate (cAMP).

CONCLUSIONS

Relaxation consistently deviates from an ME decay in DCM resulting in instability and amplified relaxation systolic load or drug dependence of ME-based indexes in failing versus control (or HCM) hearts. The hybrid-logistic method improves quantitative analyses by providing more consistent data fits with all three heart types.

The effects of beta-adrenergic stimulation on the length-dependent regulation of myocardial function in coronary surgery patients

Increasing cardiac load by leg elevation identifies patients with load-dependent impairment of left ventricular (LV) function. This impairment is related to a deficient length-dependent regulation of LV function. We investigated the effects of dobutamine on length-dependent regulation of LV function in coronary surgery patients (n = 25). High-fidelity LV pressure tracings were obtained at end-expiration, while hearts were paced at a fixed rate of 90 bpm. Effects of leg elevation on contraction and relaxation were compared before and during dobutamine 5 microg x kg(-1) x min(-1). Effects on contraction were evaluated by analysis of changes in dP/dtmax. Effects on relaxation were assessed by analysis of R (slope of the relation between the time constant of isovolumic relaxation and end-systolic pressure). Correlations were calculated with linear regression analysis using Pearson's coefficient r. The effects of leg elevation on variables of contraction and relaxation were coupled. We found a close relationship between changes in dP/dtmax and individual values of R (r = 0.84; P < 0.001). Dobutamine improved myocardial function and accelerated LV pressure decrease. Under dobutamine, the increase in dP/dtmax with leg elevation was larger (P < 0.001) and load dependence of LV relaxation was reduced (P = 0.001). Dobutamine improved the effects of leg elevation on LV function, reflecting improved length-dependent regulation of LV function.

IMPLICATIONS

This study demonstrated that beta-adrenoreceptor stimulation with dobutamine improved length-dependent regulation of myocardial function assessed during leg elevation in cardiac surgical patients.

Pharmacologic evaluation of isometric contraction-relaxation coupling indexes in rabbit ventricular muscle

Investigations of the coupling between contraction and relaxation (contraction-relaxation [CRC] process) in isometric conditions are essential in determining whether pharmacologic interventions or cardiac diseases specifically modify isometric relaxation (intrinsic lusitropic effect) or change it in proportion with the accompanying changes in contractility (or inotropy). For this purpose, the CRC process is quantified by various indexes, derived from differentiation and/or curve fitting the whole or relaxation phase of the isometric twitch, one of the most used being tau, the time constant of the final iso(volu)metric phase of relaxation. Nevertheless, the possible redundancy and validity of such indexes have not been thoroughly investigated. Accordingly, we performed a pharmacologic evaluation of such indexes in isolated rabbit ventricular muscles isometrically contracting in vitro, using modifiers of either intracellular Ca(2)+ handling (nifedipine, ryanodine, 2,5-di-tert-butyl-benzohydroquinone, all negative inotropic compounds, and BAY K 8644, a positive inotropic drug), or myofibrillar Ca(2)+ sensitivity (CGP 48506, a Ca(2)+ sensitizer, and butanedione monoxime, a Ca(2)+ desensitizer, respectively positive and negative inotropic compounds). The isometric twitch in control conditions and in the presence of increasing concentration of each compound was analyzed to determine the classically used CRC and/or lusitropic indexes, derived either from single parameters such as the maximal rate or contraction and relaxation (+dT(max) and -dT(max), respectively), or from curve fitting of the whole, or part, of the twitch. As the rate of isometric relaxation is dependent on myofilament properties, we expected that compounds modifying myofibrillar Ca(2)+ sensitivity in an opposite direction (CGP 48506 vs butanedione monoxime) would be the only drugs exerting an intrinsic lusitropic and opposite effect on a validated CRC index. Results showed that (1) none of the tested compounds affected the slope of the linear relationship between peak twitch tension and dT(max), a previously assumed CRC index, sensitive only to myofibrillar Ca(2)+ sensitivity modifiers; (2) the lusitropic parameter B, derived from mathematical curve fitting of the whole isometric twitch, and the ratio +dT(max)/dT(max), exhibited similar drug- and dose-dependency, but no opposite sensitivity to CGP 48506 and BDM for either index; and (3) negative inotropic compounds dose-dependently slowed relaxation (and conversely for positive inotropes), whether the latter was quantified by the rate constant beta, derived from double exponential curve fitting of the whole relaxation phase, or by the time constants tau(L) and tau(E), derived from the curve fitting (logistic and monoexponential, respectively) of the final phase of relaxation. Nevertheless, the pharmacologicly induced changes in beta were statistically significant at lower concentrations and exhibited less individual variability, compared with the time constants. We demonstrate that intrinsic lusitropic changes can be quantified by the value of the slope of the relationship relating beta to peak isometric tension: the slope value was unchanged by Ca(2)+ handling modifiers, decreased by CGP 48506, and reversed by BDM (indicating number, negative, and positive intrinsic lusitropic effects respectively). Based on these data, we propose that the linear relationship between beta and peak isometric tension could be used a new method to assess whether pharmacologic interventions or cardiac diseases exert intrinsic effects on isometric relaxation.

Contraction-relaxation coupling: determination of the onset of diastole

Left ventricular relaxation is dependent on afterload conditions during systole. An abrupt increase in afterload while the ventricle is actively contracting prolongs the duration of systole. An increase in afterload during ventricular relaxation shortens the duration of systole. Therefore, we hypothesized that the point during systole when an abrupt increase in afterload had no effect on the duration of systole represented the onset of ventricular relaxation. To determine when this point occurs, we performed aortic occlusions progressively throughout the duration of systole in six dogs. We determined the change in systolic time (t(sys)) after an intervention normalized to t(sys) of a control beat (t(sys,i)/t(sys, c)) as a function of systolic occlusion time as a percentage of total systolic time (t(occ)/t(sys,c)), where t(sys) is the duration from time of left ventricular end-diastolic pressure to the time of minimum first derivative of left ventricular pressure. Our results show the onset of left ventricular relaxation during normal ejection occurs at 34 +/- 3% of systolic time and approximately 16% after the onset of ejection. Thus the beginning of relaxation occurs soon after the beginning of ejection, suggesting that relaxation is modulated by variable loading conditions during ejection, significantly before what has been conventionally been assumed to be the beginning of ventricular relaxation.

Effects of reconstructive surgery for left ventricular anterior aneurysm on ventriculoarterial coupling

OBJECTIVE

To investigate left ventricular elastance (Emax) and effective arterial elastance (Ea) in postinfarction left ventricular aneurysm and evaluate their role in left ventricular function improvement after endoventricular circular patch plasty (EVCPP). Ventriculoarterial coupling has never been studied in these patients.

PATIENTS

22 consecutive patients (49 to 73 years) with left ventricular anterior aneurysm.

METHODS

Haemodynamic studies were done before and two weeks after EVCPP. Ventriculography was performed during atrial pacing (100 beats/min). Pressure/volume loops were analysed and derived parameters measured. Emax was estimated by applying the "single beat" method. Ea was calculated as end systolic pressure/stroke volume.

RESULTS

Left ventricular volumes and Ea decreased after surgery: end diastolic volume index from mean (SD) 155 (53) to 106 (29); end systolic volume index from 112 (51) to 62 (30) ml/m2 (both p < 0.0001); Ea from 1.65 (0.70) to 1.39 (0.41) mm Hg/ml (p = 0.04). Ejection fraction and Emax increased, without significant changes in stroke volume and work. The decrease in Ea was directly correlated with its preoperative value. The time interval between left ventricular pressure upstroke and peak systolic pressure decreased, from 237 (39) to 191 (41) ms (p < 0.0001), paralleling morphological changes in pressure tracings.

CONCLUSIONS

After EVCPP, ventriculoarterial coupling improves because of the fall in Ea caused by end systolic pressure reduction. The improvement is related to aortic pressure waveform changes and improved relaxation.

The application of individualised fetal growth curves

Individually adjusted or 'customised' growth charts aim to optimise the assessment of fetal growth by taking individual variation into account, and by projecting an optimal curve which delineates the potential weight gain in each pregnancy. This results in an increased detection rate of true growth restriction and a reduction in false positive diagnoses for IUGR. An adjustable standard can apply across geographical boundaries, as individual variation exceeds that between different maternity populations.