Relevance of the volume-axis intercept, V0, compared with the slope of end-systolic pressure-volume relationship in response to large variations in inotropy and afterload in rats

Science of left ventricular unloading

The concept of left ventricular unloading has its foundation in heart physiology. In fact, the left ventricular mechanics and energetics represent the cornerstone of this approach. The novel sophisticated therapies for acute heart failure, particularly mechanical circulatory supports, strongly impact on the mechanical functioning and energy consuption of the heart, ultimately affecting left ventricle loading. Notably, extracorporeal circulatory life support which is implemented for life-threatening conditions, may even overload the left heart, requiring additional unloading strategies. As a consequence, the understanding of ventricular overload, and the associated potential unloading strategies, founds its utility in several aspects of day-by-day clinical practice. Emerging clinical and pre-clinical research on left ventricular unloading and its benefits in heart failure and recovery has been conducted, providing meaningful insights for therapeutical interventions. Here, we review the current knowledge on left ventricular unloading, from physiology and molecular biology to its application in heart failure and recovery.

The left ventricle increases contractility in response to baroreceptor unloading, which is sympathetically mediated in the anesthetized rat

Contemporary discussion of the baroreflex includes the efferent vascular-sympathetic and cardiovagal arms. Since sympathetic postganglionic neurons also innervate the left ventricle (LV), it is often assumed that the LV produces a sympathetically mediated increase in contractility during baroreceptor unloading, but this has not been characterized using a load-independent index of contractility. We aimed to determine 1) whether LV contractility increases in response to baroreceptor unloading and 2) whether such increases are mediated via the sympathetic or parasympathetic arm of the autonomic nervous system. Ten male Wistar rats were anesthetized (urethane) and instrumented with arterial and LV pressure-volume catheters to measure mean arterial pressure (MAP) and load-independent LV contractility [maximal rate of increase in pressure adjusted to end-diastolic volume (PAdP/dtmax)], respectively. Rats were placed in a servo-controlled lower-body negative pressure (LBNP) chamber to reduce MAP by 10% for 60 s to mechanically unload baroreceptors under control conditions. LBNP was repeated in each animal following infusions of cardiac autonomic blockers using esmolol (sympathetic), atropine (parasympathetic), and esmolol + atropine. Under control conditions, PAdP/dtmax increased during baroreceptor unloading (26 ± 6 vs. 31 ± 9 mmHg·s-1·μL-1, P = 0.031). During esmolol, there was no increase in LV contractility during baroreceptor unloading (11 ± 2 vs. 12 ± 2, P = 0.125); however, during atropine, there was an increase in LV contractility during baroreceptor unloading (26 ± 6 vs. 31 ± 9, P = 0.019). During combined esmolol and atropine, there was a small increase in contractility versus control (13 ± 3 vs. 15 ± 4, P = 0.046). Our results demonstrate that, in anesthetized rats, LV contractility increases in response to baroreceptor unloading, which is largely sympathetically mediated.NEW & NOTEWORTHY This study empirically demonstrates a sympathetically mediated increase in LV contractility in response to baroreceptor unloading using a load-independent index of cardiac contractility in the anesthetized rat.

Mathematical analysis of left ventricular elastance with respect to afterload change during ejection phase

Since the left ventricle (LV) has pressure (Plv) and volume (Vlv), we can define LV elastance from the ratio between Plv and Vlv, termed as "instantaneous elastance." On the other hand, end-systolic elastance (Emax) is known to be a good index of LV contractility, which is measured by the slope of several end-systolic Plv-Vlv points obtained by using different loads. The word Emax originates from the assumption that LV elastance increases during the ejection phase and attains its maximum at the end-systole. From this concept, we can define another elastance determined by the slope of isochronous Plv-Vlv points, that is Plv-Vlv points at a certain time after the ejection onset time by using different loads. We refer to this elastance as "load-dependent elastance." To reveal the relation between these two elastances, we used a hemodynamic model that included a detailed ventricular myocyte contraction model. From the simulation results, we found that the isochronous Plv-Vlv points lay in one line and that the line slope corresponding to the load-dependent elastance slightly decreased during the ejection phase, which is quite different from the instantaneous elastance. Subsequently, we analyzed the mechanism determining these elastances from the model equations. We found that instantaneous elastance is directly related to contraction force generated by the ventricular myocyte, but the load-dependent elastance is determined by two factors: one is the transient characteristics of the cardiac cell, i.e., the velocity-dependent force drops characteristics in instantaneous shortening. The other is the force-velocity relation of the cardiac cell. We also found that the linear isochronous pressure-volume relation is based on the approximately linear relation between the time derivative of the cellular contraction force and the cellular shortening velocity that results from the combined characteristics of LV and aortic compliances.

A cross-species validation of single-beat metrics of cardiac contractility

The assessment of left ventricular (LV) contractility in animal models is useful in various experimental paradigms, yet obtaining such measures is inherently challenging and surgically invasive. In a cross-species study using small and large animals, we comprehensively tested the agreement and validity of multiple single-beat surrogate metrics of LV contractility against the field-standard metrics derived from inferior vena cava occlusion (IVCO). Fifty-six rats, 27 minipigs and 11 conscious dogs underwent LV and arterial catheterization and were assessed for a range of single-beat metrics of LV contractility. All single-beat metrics were tested for the various underlying assumptions required to be considered a valid metric of cardiac contractility, including load-independency, sensitivity to inotropic stimulation, and ability to diagnose contractile dysfunction in cardiac disease. Of all examined single-beat metrics, only LV maximal pressure normalized to end-diastolic volume (EDV), end-systolic pressure normalized to EDV, and the maximal rate of rise of the LV pressure normalized to EDV showed a moderate-to-excellent agreement with their IVCO-derived reference measure and met all the underlying assumptions required to be considered as a valid cardiac contractile metric in both rodents and large-animal models. Our findings demonstrate that single-beat metrics can be used as a valid, reliable method to quantify cardiac contractile function in basic/preclinical experiments utilizing small- and large-animal models KEY POINTS: Validating and comparing indices of cardiac contractility that avoid caval occlusion would offer considerable advantages for the field of cardiovascular physiology. We comprehensively test the underlying assumptions of multiple single-beat indices of cardiac contractility in rodents and translate these findings to pigs and conscious dogs. We show that when performing caval occlusion is unfeasible, single-beat metrics can be utilized to accurately quantify cardiac inotropic function in basic and preclinical research employing various small and large animal species. We report that maximal left-ventricular (LV)-pressure normalized to end-diastolic volume (EDV), LV end-systolic pressure normalized to EDV and the maximal rate of rise of the LV pressure waveform normalized to EDV are the best three single-beat metrics to measure cardiac inotropic function in both small- and large-animal models.

Various Approaches to Define the Volume Intercept of the Ventricular End-Systolic Pressure-Volume Relationship: Implications for Statistical Analysis

Ventricular pump function is often characterized by the (non)linear end-systolic pressure-volume relationship (ESPVR). For each working point on that curve the tangent along with the intercept (Vo) reflect contractile state. Vo on the abscissa is an extrapolated point without physiological meaning, and may be negative. To obtain positive values for the intercept, investigators often choose a non-zero pressure level. Although this preference is mathematically sound, we demonstrate that statistical evaluations may yield different results, depending on the pressure level selected. Published data on 17 cardiac patients representing three diagnostic groups were analyzed, showing dicrotic notch pressure based values -14<Vo<119 mL/m²• At 20, 40, 60, 80 and 100 mmHg levels all intercepts were positive, but statistical outcomes varied when comparing the 3 groups, depending on the pressure level selected, e.g. ranging from P=0.03 to 0.28. Also, we found that the correlation between intercept value Vo and end-systolic volume varies with the choice made for the pressure level. However, as there is no need to explore alternatives for the intercept, we conclude that the traditional route for the estimation of Vo (i.e. extrapolated to zero pressure) is to be preferred and this practice warrants adequate comparison of outcomes generated by all studies. Clinical Relevance- Personal preference for an alternative intercept definition for the ESPVR does not serve any relevant goal but rather implies that the statistical outcomes are altered thus hampering interpretation of data when comparing patient groups or assessing the effect of clinical intervention.

Non-linearity of end-systolic pressure-volume relation in afterload increases is caused by an overlay of shortening deactivation and the Frank-Starling mechanism

The linearity and load insensitivity of the end-systolic pressure-volume-relationship (ESPVR), a parameter that describes the ventricular contractile state, are controversial. We hypothesize that linearity is influenced by a variable overlay of the intrinsic mechanism of autoregulation to afterload (shortening deactivation) and preload (Frank-Starling mechanism). To study the effect of different short-term loading alterations on the shape of the ESPVR, experiments on twenty-four healthy pigs were executed. Preload reductions, afterload increases and preload reductions while the afterload level was increased were performed. The ESPVR was described either by a linear or a bilinear regression through the end-systolic pressure volume (ES-PV) points. Increases in afterload caused a biphasic course of the ES-PV points, which led to a better fit of the bilinear ESPVRs (r2 0.929 linear ESPVR vs. r2 0.96 and 0.943 bilinear ESPVR). ES-PV points of a preload reduction on a normal and augmented afterload level could be well described by a linear regression (r2 0.974 linear ESPVR vs. r2 0.976 and 0.975 bilinear ESPVR). The intercept of the second ESPVR (V0) but not the slope demonstrated a significant linear correlation with the reached afterload level (effective arterial elastance Ea). Thus, the early response to load could be described by the fixed slope of the ESPVR and variable V0, which was determined by the actual afterload. The ESPVR is only apparently nonlinear, as its course over several heartbeats was affected by an overlay of SDA and FSM. These findings could be easily transferred to cardiovascular simulation models to improve their accuracy.

Understanding cardiac systolic performance beyond left ventricular ejection fraction

Left ventricular ejection fraction is the critical parameter used for heart failure classification, decision making and assessing prognosis. It is defined as a volumetric ratio and is essentially a composite of arterial and ventricular elastances, but not intrinsic contractility. The clinician should be aware of its numerous limitations when measuring and reporting it. And make a step toward more insightful understanding of hemodynamics.

Longitudinal measures of deformation are associated with a composite measure of contractility derived from pressure-volume loop analysis in children

Aims

The relationship between echocardiographic measures of left ventricular (LV) systolic function and reference-standard measures have not been assessed in children. The objective of this study was to assess the validity of echocardiographic indices of LV systolic function via direct comparison to a novel composite measure of contractility derived from pressure-volume loop (PVL) analysis.

Methods and results

Children with normal loading conditions undergoing routine left heart catheterization were prospectively enrolled. PVLs were obtained via conductance catheters. A composite invasive composite contractility index (ICCI) was developed using data reduction strategies to combine four measures of contractility derived from PVL analysis. Echocardiograms were performed immediately after PVL analysis under the same anesthetic conditions. Conventional and speckle-tracking echocardiographic measures of systolic function were measured. Of 24 patients, 18 patients were heart transplant recipients, 6 patients had a small patent ductus arteriosus or small coronary fistula. Mean age was 9.1 ± 5.6 years. Upon multivariable regression, longitudinal strain was associated with ICCI (β = -0.54, P = 0.02) while controlling for indices of preload, afterload, heart rate, and LV mass under baseline conditions. Ejection fraction and shortening fraction were associated with LV mass and load indices, but not contractility.

Conclusion

Speckle-tracking derived longitudinal strain is associated ICCI in children with normal loading conditions. Longitudinal measures of deformation appear to accurately assess LV contractility in children.

Left ventricular volume analysis as a basic tool to describe cardiac function

The heart is often regarded as a compression pump. Therefore, determination of pressure and volume is essential for cardiac function analysis. Traditionally, ventricular performance was described in terms of the Starling curve, i.e., output related to input. This view is based on two variables (namely, stroke volume and end-diastolic volume), often studied in the isolated (i.e., denervated) heart, and has dominated the interpretation of cardiac mechanics over the last century. The ratio of the prevailing coordinates within that paradigm is termed ejection fraction (EF), which is the popular metric routinely used in the clinic. Here we present an insightful alternative approach while describing volume regulation by relating end-systolic volume (ESV) to end-diastolic volume. This route obviates the undesired use of metrics derived from differences or ratios, as employed in previous models. We illustrate basic principles concerning ventricular volume regulation by data obtained from intact animal experiments and collected in healthy humans. Special attention is given to sex-specific differences. The method can be applied to the dynamics of a single heart and to an ensemble of individuals. Group analysis allows for stratification regarding sex, age, medication, and additional clinically relevant covariates. A straightforward procedure derives the relationship between EF and ESV and describes myocardial oxygen consumption in terms of ESV. This representation enhances insight and reduces the impact of the metric EF, in favor of the end-systolic elastance concept advanced 4 decades ago.

Cardiophysiology Illustrated by Comparing Ventricular Volumes in Healthy Adult Males and Females

Recent advances in cardiac imaging techniques have substantially contributed to a growing interest in the analysis of global cardiac chamber dimensions and regional myocardial deformation. During the cardiac cycle, ventricular luminal volume varies due to the contraction process, which also confers a shape change including substantial alteration of long axis length, as well as rotation of the base compared to the apex. Local deformation can be assessed by strain (rate) analysis. Reviewing the present literature, it must be concluded that there is no single metric available to comprehensively characterize ventricular function. Every candidate advanced thus far has been found to incompletely reflect ventricular performance. This observation is not surprising in view of the complexity of the cardiac pump system. Additionally, sex-specific modifiers may play a role. More than three decades ago, it was shown that on average the ventricular volume is smaller in healthy women compared to matched males. Therefore, the present contribution concerns the interpretation of data derived from the healthy heart in both men and women. Starting from the classical Starling concept, we apply a simple mathematical transformation which permits an insightful representation of ventricular mechanics. Relating end-systolic volume (ESV) to end-diastolic volume creates the ventricular volume regulation graph which features the pertinent working point of an individual heart. This fundamental approach illustrates why certain proposed performance indexes cannot individually reveal the essence of ventricular systolic function. We demonstrate that particular metrics are highly interconnected and just tell us the same story in a different disguise. It is imperative to understand which associations exist and if they expectedly are (nearly) linear or frankly nonlinear. Notably, ejection fraction (EF) is primarily determined by ESV, while in turn EF is not much different from ventriculo-arterial coupling (VAC). Insight into cardiac function is promoted by identification of the paramount/essential components involved. The smaller ESV (p < 0.0001) implies that EF is higher in women and may also have consequences for VAC.

A novel single-beat approach to assess right ventricular systolic function

Clinical assessment of right ventricular (RV) contractility in diseases such as pulmonary arterial hypertension (PAH) has been hindered by the lack of a robust methodology. Here, a novel, clinically viable, single-beat method was developed to assess end-systolic elastance (E_es), a measure of right ventricular (RV) contractility. We hypothesized that this novel approach reduces uncertainty and interobserver variability in the estimation of the maximum isovolumic pressure (P_iso), the key step in single-beat methods. The new method was designed to include a larger portion of the RV pressure data and minimize subjective adjustments by the operator. Data were obtained from right heart catheterization of PAH patients in a multicenter prospective study ( data set 1) and a single-center retrospective study ( data set 2). To obtain P_iso, three independent observers used an established single-beat method (based on the first derivative of the pressure waveform) and the novel method (based on the second derivative). Interobserver variability analysis included paired t-test, one-way ANOVA, interclass correlation (ICC) analysis, and a modified Bland-Altman analysis. The P_iso values obtained from the two methods were linearly correlated for both data set 1 ( R² = 0.74) and data set 2 ( R² = 0.91). Compared with the established method, the novel method resulted in smaller interobserver variability ( P < 0.001), nonsignificant differences between observers, and a narrower confidence interval. By reducing uncertainty and interobserved variability, this novel approach may pave the way for more effective clinical management of PAH. NEW & NOTEWORTHY A novel methodology to assess right ventricular contractility from clinical data is demonstrated. This approach significantly reduces interobserver variability in the analysis of ventricular pressure data, as demonstrated in a relatively large population of subjects with pulmonary hypertension. This study may enable more accurate clinical monitoring of systolic function in subjects with pulmonary hypertension.

Effects of Cardiopulmonary Exercise Rehabilitation on Left Ventricular Mechanical Efficiency and Ventricular-Arterial Coupling in Patients With Systolic Heart Failure

BACKGROUND

Success of cardiac rehabilitation (CR) is generally assessed by the objective improvement in peak volume of inhaled oxygen (VO2) measured by cardiopulmonary exercise test (CPX). However, cardiac mechanical efficiency and ventricular-arterial coupling (VAC) are the other important dimensions of the heart failure pathophysiology, which are not included in CPX-derived data. The effect of cardiac rehabilitation on left ventricular (LV) efficiency or VAC in unselected heart failure patients has not been studied thus far.

METHODS AND RESULTS

Thirty patients with an ejection fraction of ≤45% were recruited for 20 sessions of exercise-based CR. Noninvasive LV pressure-volume loops were constructed and VAC was calculated with the help of applanation tonometry and echocardiography before and after CR. VAC showed an improved mechanical efficiency profile and increased significantly from 0.56±0.18 to 0.67±0.21 (P=0.02). LV mechanical efficiency improved from 43.9±9.1% to 48.8±9.1% (P=0.01). The change in peak VO2 was not in a significant correlation with the change in VAC (r=-0.18; P=0.31), mechanical efficiency (r=-0.16, P=0.39), or the change in ejection fraction (r=-0.07; P=0.68).

CONCLUSIONS

CR is associated with an improvement in VAC and LV mechanical efficiency in heart failure patients. Further studies are needed to determine the incremental value of VAC and mechanical efficiency over CPX-derived data in predicting clinical outcomes.

Effects of exercise on postexercise ventricular-arterial coupling and pulsatile efficiency in patients with systolic dysfunction

BACKGROUND

A suboptimal ventricular-arterial (VA) interaction may have a prolonged depressing effect on the failing heart after functional reserves forced to their limits under stress conditions such as exercise. The continuation of excessive load in the postexercise period may be more important than the load during exercise, because the sum of postexercise periods generally exceeds exercise time itself. We sought that exercise-induced changes in postexercise VA coupling and pulsatile efficiency in patients with heart failure (HF).

METHODS

Thirty consecutive HF with reduced ejection fraction (EF) and thirty age-, sex- and peak VO2 -matched subjects with preserved EF were enrolled. Pre- and postexercise echocardiographic and tonometric measurements were taken to calculate left ventricular and arterial elastances, arterial compliance and wave reflections, and steady and pulsatile power.

RESULTS

VA coupling significantly deteriorated in HF group (from 1·50 ± 0·47 to 2·00 ± 0·75 mmHg/mL, P < 0·01), but control group maintained basal favourable coupling status after exercise (from 1·04 ± 0·29 to 1·03 ± 0·24 mmHg/mL, P = 0·77). Pulsatile percentage of total power significantly increased with exercise in HF group, whereas it showed a significant decrease in control group. The change in pulsatile power fraction was correlated with the change in augmentation pressure (r = 0·41, ß = 3·00, P < 0·01) and inversely correlated with the change in total arterial compliance (r = -0·29, ß = -8·52, P = 0·02).

CONCLUSION

Our data indicate that exercise-induced VA decoupling and pulsatile inefficiency extend into postexercise phase in patients with systolic dysfunction. The exact duration of these derangements requires further studies.

Biomechanics of Cardiac Function

The heart pumps blood to maintain circulation and ensure the delivery of oxygenated blood to all the organs of the body. Mechanics play a critical role in governing and regulating heart function under both normal and pathological conditions. Biological processes and mechanical stress are coupled together in regulating myocyte function and extracellular matrix structure thus controlling heart function. Here, we offer a brief introduction to the biomechanics of left ventricular function and then summarize recent progress in the study of the effects of mechanical stress on ventricular wall remodeling and cardiac function as well as the effects of wall mechanical properties on cardiac function in normal and dysfunctional hearts. Various mechanical models to determine wall stress and cardiac function in normal and diseased hearts with both systolic and diastolic dysfunction are discussed. The results of these studies have enhanced our understanding of the biomechanical mechanism in the development and remodeling of normal and dysfunctional hearts. Biomechanics provide a tool to understand the mechanism of left ventricular remodeling in diastolic and systolic dysfunction and guidance in designing and developing new treatments.

Left Ventricle-Arterial System Interaction in Heart Failure

Ejection fraction (EF) has been viewed as an important index in assessing the contractile state of the left ventricle (LV). However, it is frequently inadequate for the diagnosis and management of heart failure (HF), as a significant subset of HF patients have been found to have reduced EF (HFrEF) whereas others have preserved EF (HFpEF). It should be noted that the function of the LV is dependent on both preload and afterload, as well as its intrinsic contractile state. Furthermore, stroke volume (SV) is dependent on the properties of the arterial system (AS). Thus, the LV-arterial system interaction plays an important role in those patients with HF. This aspect is investigated through the analysis of the specific parameters involved in the coupling of the LV and AS. This includes contractility and the systolic/diastolic indices of the LV. Furthermore, AS afterload parameters such as vascular stiffness and arterial compliance, and their derived coupling coefficient, are also investigated. We conclude that those parameters, which relate to LV structural changes, are most appropriate in quantifying the LV-AS interaction.

Patient-specific assessment of cardiovascular function by combination of clinical data and computational model with applications to patients undergoing Fontan operation

The assessment of cardiovascular function is becoming increasingly important for the care of patients with single-ventricle defects. However, most measurement methods available in the clinical setting cannot provide a separate measure of cardiac function and loading conditions. In the present study, a numerical method has been proposed to compensate for the limitations of clinical measurements. The main idea was to estimate the parameters of a cardiovascular model by fitting model simulations to patient-specific clinical data via parameter optimization. Several strategies have been taken to establish a well-posed parameter optimization problem, including clinical data-matched model development, parameter selection based on an extensive sensitivity analysis, and proper choice of parameter optimization algorithm. The numerical experiments confirmed the ability of the proposed parameter optimization method to uniquely determine the model parameters given an arbitrary set of clinical data. The method was further tested in four patients undergoing the Fontan operation. Obtained results revealed a prevalence of ventricular abnormalities in the patient cohort and at the same time demonstrated the presence of marked inter-patient differences and preoperative to postoperative changes in cardiovascular function. Because the method allows a quick assessment and makes use of clinical data available in clinical practice, its clinical application is promising.

Stroke volume-to-wall stress ratio as a load-adjusted and stiffness-adjusted indicator of ventricular systolic performance in chronic loading

Load-adjusted measures of left ventricle (LV) systolic performance are limited by dependence on LV stiffness and afterload. To our knowledge, no stiffness-adjusted and afterload-adjusted indicator was tested in models of pressure (POH) and volume overload hypertrophy (VOH). We hypothesized that wall stress reflects changes in loading, incorporating chamber stiffness and afterload; therefore, stroke volume-to-wall stress ratio more accurately reflects systolic performance. We used rat models of POH (ascending aortic banding) and VOH (aorto-cava shunt). Animals underwent echocardiography and pressure-volume analysis at baseline and dobutamine challenge. We achieved extreme bidirectional alterations in LV systolic performance, end-systolic elastance (Ees), passive stiffness, and arterial elastance (Ea). In POH with LV dilatation and failure, some load-independent indicators of systolic performance remained elevated compared with controls, while some others failed to decrease with wide variability. In VOH, most, but not all indicators, including LV ejection fraction, were significantly reduced compared with controls, despite hyperdynamic circulation, lack of heart failure, and preserved contractile reserve. We related systolic performance to Ees adjusted for Ea and LV passive stiffness in multivariate models. Calculated residual Ees was not reduced in POH with heart failure and was reduced in VOH, while it positively correlated to dobutamine dose. Conversely, stroke volume-to-wall stress ratio was normal in compensated POH, markedly decreased in POH with heart failure, and, in contrast with LV ejection fraction, normal in VOH. Our results support stroke volume-to-wall stress ratio as a load-adjusted and stiffness-adjusted indicator of systolic function in models of POH and VOH.