Energetically optimal left ventricular pressure for the failing human heart

Sex differences in right ventricular adaptation to pressure overload in a rat model

With severe right ventricular (RV) pressure overload, women demonstrate better clinical outcomes compared with men. The mechanoenergetic mechanisms underlying this protective effect, and their dependence on female endogenous sex hormones, remain unknown. To investigate these mechanisms and their impact on RV systolic and diastolic functional adaptation, we created comparable pressure overload via pulmonary artery banding (PAB) in intact male and female Wistar rats and ovariectomized (OVX) female rats. At 8 wk after surgery, right heart catheterization demonstrated increased RV energy input [indexed pressure-volume area (iPVA)] in all PAB groups, with the greatest increase in intact females. PAB also increased RV energy output [indexed stroke or external work (iEW)] in all groups, again with the greatest increase in intact females. In contrast, PAB only increased RV contractility-indexed end-systolic elastance (iE_es)] in females. Despite these sex-dependent differences, no statistically significant effects were observed in the ratio of RV energy output to input (mechanical efficiency) or in mechanoenergetic cost to pump blood with pressure overload. These metrics were similarly unaffected by loss of endogenous sex hormones in females. Also, despite sex-dependent differences in collagen content and organization with pressure overload, decreases in RV compliance and relaxation time constant (tau Weiss) were not determined to be sex dependent. Overall, despite sex-dependent differences in RV contractile and fibrotic responses, RV mechanoenergetics for this degree and duration of pressure overload are comparable between sexes and suggest a homeostatic target.NEW & NOTEWORTHY Sex differences in right ventricular mechanical efficiency and energetic adaptation to increased right ventricular afterload were measured. Despite sex-dependent differences in contractile and fibrotic responses, right ventricular mechanoenergetic adaptation was comparable between the sexes, suggesting a homeostatic target.

Four-quadrant visualization of systemic circulatory equilibrium: right ventricular failure after left ventricular assist device implantation

Right ventricular failure (RVF) is a serious adverse event after left ventricular assist device (LVAD) implantation but difficult to be characterized. This study aimed to visualize the dynamic circulatory equilibrium of acute RVF after LVAD implantation using a new four-quadrant diagram constructed by 1) cardiac function with central venous pressure (CVP) and cardiac index (CI) axes, 2) arterial vascular resistance with CI and mean blood pressure (mBP) axes, 3) pressure-diuretic function with mBP and net urinary sodium output (net U-Na) axes, and 4) venous compliance with net U-Na and CVP axes. Twenty LVAD patients were stratified into two groups, group S (≤10 days) and group L (>10 days), according to duration of postoperative inotropic support. The preoperative equilibrium loops were small in both groups. In the early postoperative phase, the loop in group S became dramatically enlarged to the left and upward, indicating increased CVP and CI by LVAD support. In group L, however, augmentation of CI was smaller despite similarly increased CVP, and net U-Na was decreased despite increased mBP. In the late postoperative phase, the equilibrium loop in group L recovered as similar to that seen in group S. Thus, acute RVF, as shown in group L, was characterized by the shape of the loop constructed by marked increased CVP, a relatively small increase in CI, and concomitant impairment of pressure natriuresis. In conclusion, the novel four-quadrant presentation of systemic circulatory equilibrium provides clear visualization of RVF after LVAD implantation, thus serving as a useful guide for prompt and optimal management.NEW & NOTEWORTHY Systemic circulatory dynamics are regulated by various negative feedback systems, including cardiac, arterial, venous, and renal functions, as well as autonomic nervous systems. The present novel four-quadrant presentation of their functions allows clear visualization of dynamic organ-to-organ interactions that can lead to a new circulatory equilibrium after therapeutic intervention. This new system physiological framework can serve as a useful guide for prompt and optimal management of circulatory malfunction.

Cardiac work is related to creatine kinase energy supply in human heart failure: a cardiovascular magnetic resonance spectroscopy study

BACKGROUND

It has been hypothesized that the supply of chemical energy may be insufficient to fuel normal mechanical pump function in heart failure (HF). The creatine kinase (CK) reaction serves as the heart's primary energy reserve, and the supply of adenosine triphosphate (ATP flux) it provides is reduced in human HF. However, the relationship between the CK energy supply and the mechanical energy expended has never been quantified in the human heart. This study tests whether reduced CK energy supply is associated with reduced mechanical work in HF patients.

METHODS

Cardiac mechanical work and CK flux in W/kg, and mechanical efficiency were measured noninvasively at rest using cardiac pressure-volume loops, magnetic resonance imaging and phosphorus spectroscopy in 14 healthy subjects and 27 patients with mild-to-moderate HF.

RESULTS

In HF, the resting CK flux (126 ± 46 vs. 179 ± 50 W/kg, p < 0.002), the average (6.8 ± 3.1 vs. 10.1 ± 1.5 W/kg, p  <0.001) and the peak (32 ± 14 vs. 48 ± 8 W/kg, p < 0.001) cardiac mechanical work-rates, as well as the cardiac mechanical efficiency (53% ± 16 vs. 79% ± 3, p < 0.001), were all reduced by a third compared to healthy subjects. In addition, cardiac CK flux correlated with the resting peak and average mechanical power (p < 0.01), and with mechanical efficiency (p = 0.002).

CONCLUSION

These first noninvasive findings showing that cardiac mechanical work and efficiency in mild-to-moderate human HF decrease proportionately with CK ATP energy supply, are consistent with the energy deprivation hypothesis of HF. CK energy supply exceeds mechanical work at rest but lies within a range that may be limiting with moderate activity, and thus presents a promising target for HF treatment.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT00181259 .

The physiologic implications of isolated alpha(1) adrenergic stimulation

Phenylephrine and methoxamine are direct-acting, predominantly α(1) adrenergic receptor (AR) agonists. To better understand their physiologic effects, we screened 463 articles on the basis of PubMed searches of "methoxamine" and "phenylephrine" (limited to human, randomized studies published in English), as well as citations found therein. Relevant articles, as well as those discovered in the peer-review process, were incorporated into this review. Both methoxamine and phenylephrine increase cardiac afterload via several mechanisms, including increased vascular resistance, decreased vascular compliance, and disadvantageous alterations in the pressure waveforms produced by the pulsatile heart. Although pure α(1) agonists increase arterial blood pressure, neither animal nor human studies have ever shown pure α(1)-agonism to produce a favorable change in myocardial energetics because of the resultant increase in myocardial workload. Furthermore, the cost of increased blood pressure after pure α(1)-agonism is almost invariably decreased cardiac output, likely due to increases in venous resistance. The venous system contains α(1) ARs, and though stimulation of α(1) ARs decreases capacitance and may transiently increase venous return, this gain may be offset by changes in afterload, venous compliance, and venous resistance. Data on the effects of α(1) stimulation in the central nervous system show conflicting changes, while experimental animal data suggest that renal blood flow is reduced by α(1)-agonists, and both animal and human data suggest that gastrointestinal perfusion may be reduced by α(1) tone.

On coupling a lumped parameter heart model and a three-dimensional finite element aorta model

Aortic flow and pressure result from the interactions between the heart and arterial system. In this work, we considered these interactions by utilizing a lumped parameter heart model as an inflow boundary condition for three-dimensional finite element simulations of aortic blood flow and vessel wall dynamics. The ventricular pressure-volume behavior of the lumped parameter heart model is approximated using a time varying elastance function scaled from a normalized elastance function. When the aortic valve is open, the coupled multidomain method is used to strongly couple the lumped parameter heart model and three-dimensional arterial models and compute ventricular volume, ventricular pressure, aortic flow, and aortic pressure. The shape of the velocity profiles of the inlet boundary and the outlet boundaries that experience retrograde flow are constrained to achieve a robust algorithm. When the aortic valve is closed, the inflow boundary condition is switched to a zero velocity Dirichlet condition. With this method, we obtain physiologically realistic aortic flow and pressure waveforms. We demonstrate this method in a patient-specific model of a normal human thoracic aorta under rest and exercise conditions and an aortic coarctation model under pre- and post-interventions.

Cardiac magnetic resonance elastography. Initial results

OBJECTIVES

To develop cardiac magnetic resonance elastography (MRE) for noninvasively measuring left ventricular (LV) pressure-volume (P-V) work.

MATERIAL AND METHODS

The anterior chest wall of 8 healthy volunteers was vibrated by 24.3-Hz acoustic waves for stimulating oscillating shear deformation in myocardium and adjacent blood. The induced motion was recorded by an electrocardiogram-gated, vibration-synchronized and segmented gradient-recalled echo MRE sequence acquiring 360 phase-contrast wave images with a temporal resolution of 5.16 milliseconds in the short-axis view during controlled breathing. Relative changes in wave amplitudes served as a measure of LV pressure variation during the cardiac cycle. MRE pressure data were combined with LV volumes obtained from segmentation of 3D cine-steady-state free precession data sets.

RESULTS

Shear wave amplitudes decreased from diastole to systole, which reflects the dynamics of myocardial shear modulus variations during the cardiac cycle. Assuming spherical shear stress, a linear relationship between myocardial stiffness and LV pressure was derived. The MRE-measured pressure was plotted as a function of LV volumes. Characteristic P-V cycles displayed an isovolumetric increase in pressure during early systole, whereas less pronounced volume conservation was observed in early diastole. Mean cardiac P-V work in all volunteers was 0.85 +/- 0.11 J.

CONCLUSION

In vivo cardiac MRE is a noninvasive method for measuring pressure-related heart function determined by shear modulus variations in the LV wall. This is the first noninvasive mechanical test of cardiac work in the human heart and is potentially useful for assessing pathologies associated with increased myocardial stiffness such as diastolic dysfunction.

Operative contractility: A functional concept of the inotropic state

1. Initial unsuccessful attempts to evaluate ventricular function in terms of the 'heart as a pump' led to focusing on the 'heart as a muscle' and to the concept of myocardial contractility. However, no clinically ideal index exists to assess the contractile state. The aim of the present study was to develop a mathematical model to assess cardiac contractility. 2. A tri-axial system was conceived for preload (PL), afterload (AL) and contractility, where stroke volume (SV) was represented as the volume of the tetrahedron. Based on this model, 'operative' contractility ('OperCon') was calculated from the readily measured values of PL, AL and SV. The model was tested retrospectively under a variety of different experimental and clinical conditions, in 71 studies in humans and 29 studies in dogs. A prospective echocardiographic study was performed in 143 consecutive subjects to evaluate the ability of the model to assess contractility when SV and PL were measured volumetrically (mL) or dimensionally (cm). 3. With inotropic interventions, OperCon changes were comparable to those of ejection fraction (EF), velocity of shortening (Vcf) and dP/dt-max. Only with positive inotropic interventions did elastance (Ees) show significantly larger changes. With load manipulations, OperCon showed significantly smaller changes than EF and Ees and comparable changes to Vcf and dP/dt-max. Values of OperCon were similar when AL was represented by systolic blood pressure or wall stress and when volumetric or dimensional values were used. 4. Operative contractility is a reliable, simple and versatile method to assess cardiac contractility.

Spectral analysis of heart rate variability in dogs with mild mitral regurgitation

OBJECTIVE

To assess autonomic function in dogs with mild mitral regurgitation (MR) that did not have clinical signs of the condition.

ANIMALS

6 healthy adult Beagles.

PROCEDURE

Mild MR was experimentally induced. A 24-hour ambulatory ECG was recorded before and after induction of MR. Heart rate variability was analyzed in frequency domains by use of the ambulatory ECG. Low-frequency (LF) and high-frequency (HF) power were calculated by integrating over their frequency intervals, and the ratio of LF to HF was also calculated. Measurements of frequency domains were analyzed for 4 time periods (midnight to 6 AM, 6 AM to noon, noon to 6 PM, and 6 PM to midnight).

RESULTS

Dogs with experimentally induced MR were classified as International Small Animal Cardiac Health Council class Ia. The HF power of dogs with MR was significantly decreased between 6 AM and noon. The ratio of LF to HF in dogs with MR was significantly increased for the periods between midnight and 6 AM, 6 AM and noon, and noon and 6 PM.

CONCLUSIONS AND CLINICAL RELEVANCE

Compensatory response through autonomic modulation was observed in dogs with mild MR that did not have abnormalities, except for cardiac murmur, during clinical examination. This result suggests that treatment during the early stages of mild MR may be beneficial. Additional studies are necessary to determine whether such treatment will delay the onset of congestive heart failure and prolong survival in dogs affected with mild MR.

Direct compression of the failing heart reestablishes maximal mechanical efficiency

BACKGROUND

In failing hearts, homeostatic mechanisms contrive to maximize stroke work and maintain normal arterial blood pressure at the expense of energetic efficiency. In contrast dobutamine reestablishes maximal mechanical efficiency by promoting energetically optimal loading conditions. However, dobutamine also wastefully increases nonmechanical oxygen consumption. We investigated whether direct mechanical cardiac compression would reestablish maximal mechanical efficiency without the oxygen-wasting effect.

METHODS

The pressure-volume relationship and myocardial oxygen consumption were derived in sheep using left ventricular pressure and volume from manometer-tipped and conductance catheters, and coronary flow from Transonics flow probe.

RESULTS

Propranolol hydrochloride and atropine sulfate were administered to reduce ejection fraction to 21% when ventricular elastance fell to 1.35 mm Hg/mL and mechanical efficiency to 79% of maximal. Low-pressure direct mechanical compression of the failing heart restored mechanical efficiency to 94% of maximal and realigned optimal left ventricular end-systolic pressure with operating left ventricular end-systolic pressure without altering nonmechanical oxygen consumption.

CONCLUSIONS

We conclude that direct cardiac compression restores mechanical efficiency to normal maximum without wasting energy on additional nonmechanical activity.

Noninvasive single-beat determination of left ventricular end-systolic elastance in humans

OBJECTIVES

The goal of this study was to develop and validate a method to estimate left ventricular end-systolic elastance (E(es)) in humans from noninvasive single-beat parameters.

BACKGROUND

Left ventricular end-systolic elastance is a major determinant of cardiac systolic function and ventricular-arterial interaction. However, its use in heart failure assessment and management is limited by lack of a simple means to measure it noninvasively. This study presents a new noninvasive method and validates it against invasively measured E(es).

METHODS

Left ventricular end-systolic elastance was calculated by a modified single-beat method employing systolic (P(s)) and diastolic (P(d)) arm-cuff pressures, echo-Doppler stroke volume (SV), echo-derived ejection fraction (EF) and an estimated normalized ventricular elastance at arterial end-diastole (E(Nd)): E(es(sb)) = [P(d) - (E(Nd(est)) x P(s) x 0.9)[/(E(Nd(est)) x SV). The E(Nd) was estimated from a group-averaged value adjusted for individual contractile/loading effects; E(es(sb)) estimates were compared with invasively measured values in 43 patients with varying cardiovascular disorders, with additional data recorded after inotropic stimulation (n = 18, dobutamine 5 to 10 microg/kg per min). Investigators performing noninvasive analysis were blinded to the invasive results.

RESULTS

Combined baseline and dobutamine-stimulated E(es) ranged 0.4 to 8.4 mm Hg/ml and was well predicted by E(es(sb)) over the full range: E(es) = 0.86 x E(es(sb)) + 0.40 (r = 0.91, SEE = 0.64, p < 0.00001, n = 72). Absolute change in E(es(sb)) before and after dobutamine also correlated well with invasive measures: E(es(sb)): DeltaE(es) = 0.86 x DeltaE(es(sb)) + 0.67 (r = 0.88, p < 0.00001). Repeated measures of E(es(sb)) over two months in a separate group of patients (n = 7) yielded a coefficient of variation of 20.3 +/- 6%.

CONCLUSIONS

The E(es) can be reliably estimated from simple noninvasive measurements. This approach should broaden the clinical applicability of this useful parameter for assessing systolic function, therapeutic response and ventricular-arterial interaction.

A new framework for echocardiographic assessment of left ventricular mechanics: sensitivity to heart failure

A recent report describes an approach to ventricular mechanics that employs mean end-systolic fiber stress and an exact mathematical strain index based on wall thickness referenced to myocardial mass. We used echocardiography and mean arterial pressures to determine the strain index and wall stress in (1) normal hearts from patients and swine, (2) swine with pacing-induced congestive heart failure, and (3) patients with dilated cardiomyopathy. Pigs were also studied under afterload variation with phenylephrine. Paired values of stress and strain index from control hearts (both swine and human) were tightly clustered. Values from animals and patients with congestive heart failure deviated from this cluster. Excellent separation (sensitivity 83%, specificity 94%) was displayed between control and paced pigs, despite confounding effects of varying afterload. We conclude that these variables display little change over a large range of normal cardiac mass, but deviate from this range during heart failure.

Rational manipulation of oxygen delivery

BACKGROUND

Optimization of oxygen delivery remains the best method to prevent and the only way to treat common intensive care unit syndromes such as sepsis, multiple organ dysfunction, and acute lung injury. This paper reviews the elements of oxygen delivery, describes how clinical interventions work through those elements to alter oxygen delivery, reviews theoretical and empirical data relating to manipulation of each element, and distinguishes between therapeutic means and clinical endpoints in the care of the critically ill.

MATERIALS AND METHODS

Recent literature is reviewed. Relevant equations are detailed. Computer models and patient data illustrate key points.

RESULTS

Clinical interventions intended to improve oxygen delivery all work through at least one of seven variables (oxygen saturation, hemoglobin concentration, heart rate, mean arterial blood pressure, systemic vascular resistance, end-diastolic volume, and ejection fraction). Because interventions that increase oxygen delivery are always accompanied by physiologic costs, cavalier application of any therapy in the intensive care unit may actually decrease oxygen delivery, harming the critically ill patient. Various clinical indicators may be used as endpoints to guide therapy.

CONCLUSIONS

While a systematic consideration of the elements of oxygen delivery reveals weaknesses in experimental evidence guiding optimal treatment of shock, reasonable strategies as well as avoidable pitfalls emerge from the data. Furthermore, facility with each of the elements of oxygen delivery makes ICU management easier to teach and to apply.