Preload-induced curvilinearity of left ventricular end-systolic pressure-volume relations. Effects on derived indexes in closed-chest dogs

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.

Reducing lung liquid volume in fetal lambs decreases ventricular constraint

BACKGROUND

This study evaluated whether an increased left ventricular (LV) pump function accompanying reduction of lung liquid volume in fetal lambs was related to increased LV preload, augmented LV contractility, or both.

METHODS

Eleven anesthetized preterm fetal lambs (gestation 128 ± 2 days) were instrumented with (1) an LV micromanometer-conductance catheter to obtain LV end-diastolic volume (EDV) and end-diastolic pressure (EDP), the maximal rate of rise of LV pressure (dP/dt_max), LV output, LV stroke work, and LV end-systolic elastance (E_es), a relatively load-independent measure of contractility; (2) an endotracheal tube to measure mean tracheal pressure and to reduce lung liquid volume. LV transmural pressure was calculated as LV EDP minus tracheal pressure.

RESULTS

Reducing lung liquid volume by 16 ± 4 ml kg^-1 (1) augmented LV output (by 16%, P = 0.001) and stroke work (29%, P < 0.001), (2) increased LV EDV (12%, P < 0.001), (3) increased LV transmural pressure (2.2 mmHg, P < 0.001), (4) did not change LV dP/dt_max normalized for EDV (P > 0.7), and (5) decreased LV E_es (20%, P < 0.01).

CONCLUSION

These findings suggest a rise in LV pump function evident after reduction of lung liquid volume in fetal lambs was related to increased LV preload secondary to lessening of external LV constraint, without any associated rise in LV contractility.

IMPACT

This study has shown that reducing the volume of liquid filling the fetal lungs lessens the degree of external constraint on the heart. This lesser constraint permits a rise in left ventricular dimensions and thus greater cardiac filling that leads to increased left ventricular pumping performance. This study has defined a mechanism whereby a reduction in lung liquid volume results in enhanced pumping performance of the fetal heart. These findings suggest that a reduction in lung liquid volume which occurs during the birth transition contributes to increases in left ventricular dimensions and pumping performance known to occur with birth.

A Comparison of Phenomenologic Growth Laws for Myocardial Hypertrophy

The heart grows in response to changes in hemodynamic loading during normal development and in response to valve disease, hypertension, and other pathologies. In general, a left ventricle subjected to increased afterload (pressure overloading) exhibits concentric growth characterized by thickening of individual myocytes and the heart wall, while one experiencing increased preload (volume overloading) exhibits eccentric growth characterized by lengthening of myocytes and dilation of the cavity. Predictive models of cardiac growth could be important tools in evaluating treatments, guiding clinical decision making, and designing novel therapies for a range of diseases. Thus, in the past 20 years there has been considerable effort to simulate growth within the left ventricle. While a number of published equations or systems of equations (often termed "growth laws") can capture some aspects of experimentally observed growth patterns, no direct comparisons of the various published models have been performed. Here we examine eight of these laws and compare them in a simple test-bed in which we imposed stretches measured during in vivo pressure and volume overload. Laws were compared based on their ability to predict experimentally measured patterns of growth in the myocardial fiber and radial directions as well as the ratio of fiber-to-radial growth. Three of the eight laws were able to reproduce most key aspects of growth following both pressure and volume overload. Although these three growth laws utilized different approaches to predict hypertrophy, they all employed multiple inputs that were weakly correlated during in vivo overload and therefore provided independent information about mechanics.

Nonlinear isochrones in murine left ventricular pressure-volume loops: how well does the time-varying elastance concept hold?

tThe linear time-varying elastance theory is frequently used to describe the change in ventricular stiffness during the cardiac cycle. The concept assumes that all isochrones (i.e., curves that connect pressure-volume data occurring at the same time) are linear and have a common volume intercept. Of specific interest is the steepest isochrone, the end-systolic pressure-volume relationship (ESPVR), of which the slope serves as an index for cardiac contractile function. Pressure-volume measurements, achieved with a combined pressure-conductance catheter in the left ventricle of 13 open-chest anesthetized mice, showed a marked curvilinearity of the isochrones. We therefore analyzed the shape of the isochrones by using six regression algorithms (two linear, two quadratic, and two logarithmic, each with a fixed or time-varying intercept) and discussed the consequences for the elastance concept. Our main observations were 1) the volume intercept varies considerably with time; 2) isochrones are equally well described by using quadratic or logarithmic regression; 3) linear regression with a fixed intercept shows poor correlation ( R2 < 0.75) during isovolumic relaxation and early filling; and 4) logarithmic regression is superior in estimating the fixed volume intercept of the ESPVR. In conclusion, the linear time-varying elastance fails to provide a sufficiently robust model to account for changes in pressure and volume during the cardiac cycle in the mouse ventricle. A new framework accounting for the nonlinear shape of the isochrones needs to be developed.

Preload-Adjusted 2 Wave-Intensity Peaks Reflect Simultaneous Assessment of Left Ventricular Contractility and Relaxation

BACKGROUND

The magnitudes of the first (WI1) and the second wave-intensity peak (WI2) during the ejection period can be used as indices of left ventricular (LV) contractility and relaxation, respectively. However, use of WI to characterize LV dp/dt and the end-diastolic volume (V ed) relationship may be more problematic, as WI may be affected by changes in preload.

METHODS AND RESULTS

The LV pressure-volume data sets, consisting of 23 recordings obtained by the conductance method from 12 heart disease patients, were studied. End-systolic elastance (E es) and volume-axis-intercept (V0) were calculated with varying preload. Time constant of LV relaxation (tau), V ed, and WI were calculated from steady-state averaged data. The E es showed a weak correlation with WI1 (r = 0.46, p < 0.05) but a better correlation with preload-adjusted WI1 [WI1/V ed; r=0.86, WI1/V(ed)2; r = 0.92, WI1/(V ed - V0)2; r = 0.89, all p < 0.01]. Similarly, tau did not correlate with WI2 but did correlate with preload-adjusted WI2 [WI2/V ed; r = -0.73, WI2/V(ed) 2; r = -0.63, WI2/(V ed - V0)2; r = -0.78, all p < 0.01].

CONCLUSIONS

These data demonstrate the importance of preload-adjustment when using the WI index for simultaneous assessment of LV contractility and relaxation.

Effects of sodium nitroprusside in aortic stenosis associated with severe heart failure: pressure-volume loop analysis using a numerical model

In the recently published clinical study [Use of Nitroprusside in Left Ventricular Dysfunction and Obstructive Aortic Valve Disease (UNLOAD)], sodium nitroprusside (SNP) improved cardiac function in patients with severe aortic stenosis (AS) and left ventricular (LV) systolic dysfunction. We explored the possible mechanisms of these findings using a series of numerical simulations. A closed-loop lumped parameters model that consists of 24 differential equations relating pressure and flow throughout the circulation was used to analyze the effects of varying hemodynamic conditions in AS. Hemodynamic data from UNLOAD study subjects were used to construct the initial simulation. Systemic vascular resistance (SVR), heart rate, and aortic valve area were directly entered into the model while end-systolic and end-diastolic pressure-volume (P-V) relationships were adjusted using previously published data to match modeled and observed end-systolic and end-diastolic pressures and volumes. Initial simulation of SNP treatment by a reduction of SVR was not adequate. To obtain realistic model hemodynamics that reliably reproduce SNP treatment effects, we performed a series of simulations while simultaneously changing end-systolic elastance ( Ees), end-systolic volume at zero pressure (V0), and diastolic P-V shift. Our data indicate that either an Ees increase or V0 decrease is necessary to obtain realistic model hemodynamics. In five patients, we corroborated our findings by using the model to duplicate individual P-V loops obtained before and during SNP treatment. In conclusion, using a numerical model, we identified ventricular function parameters that are responsible for improved hemodynamics during SNP infusion in AS with LV dysfunction.

High power transcranial beam steering for ultrasonic brain therapy

A sparse phased array is specially designed for non-invasive ultrasound transskull brain therapy. The array is made of 200 single elements corresponding to a new generation of high power transducers developed in collaboration with Imasonic (Besançon, France). Each element has a surface of 0.5 cm2 and works at 0.9 MHz central frequency with a maximum 20 W cm(-2) intensity on the transducer surface. In order to optimize the steering capabilities of the array, several transducer distributions on a spherical surface are simulated: hexagonal, annular and quasi-random distributions. Using a quasi-random distribution significantly reduces the grating lobes. Furthermore, the simulations show the capability of the quasi-random array to electronically move the focal spot in the vicinity of the geometrical focus (up to +/- 15 mm). Based on the simulation study, the array is constructed and tested. The skull aberrations are corrected by using a time reversal mirror with amplitude correction achieved thanks to an implantable hydrophone, and a sharp focus is obtained through a human skull. Several lesions are induced in fresh liver and brain samples through human skulls, demonstrating the accuracy and the steering capabilities of the system.

Pressure-volume-based single-beat estimations cannot predict left ventricular contractility in vivo

The end-systolic pressure-volume relationship is regarded as a useful index for assessing the contractile state of the heart. However, the need for preload alterations has been a serious limitation to its clinical applications, and there have been numerous attempts to develop a method for calculating contractility based on one single pressure-volume loop. We have evaluated four of these methods. Pressure-volume data were obtained by combined pressure and conductance catheters in 37 pigs. All four methods were applied to 88 steady-state pressure-volume files, including eight files sampled during dopamine infusions. Estimates of single-beat contractility (elastance) were compared with preload-varied multiple-beat elastance [E(es(MB))]. All methods had a low average bias (-0.3 to 0.5 mmHg/ml) but limits of agreement (+/-2 SD) were unacceptably high (+/-2.6 to +/-3.8 mmHg/ml). In the dopamine group, E(es(MB)) showed an increase of 1.7 +/- 0.8 mmHg/ml (mean +/- SD) compared with baseline (P < 0.001). None of the single-beat methods predicted this increase in contractility. It is therefore doubtful whether any of the methods allow for single-beat assessment of contractility.

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.

Hypovolemic shock and cardiac contractility: assessment by end-systolic pressure-volume relations

The end-systolic pressure-volume relation (ESPVR) is accepted as a load-independent measure of cardiac contractility. Potential curvilinearity of the ESPVR, dependency on coronary perfusion pressure (CPP) and sensitivity to the type of loading intervention might limit its use in hemorrhagic shock. This study compared ESPVRs obtained by caval and aortic occlusion under physiological loading conditions at baseline with those obtained during hemorrhagic shock (mean arterial pressure 45 mmHg). The left ventricular (LV) pressure (tip manometer) and volume (conductance catheter) were measured in ten anesthetized pigs. ESPVRs were fitted to linear and quadratic models. Within end-systolic pressure (Pes) ranges obtained under baseline conditions, ESPVR displayed only minimal curvilinearity (second-order coefficient a < 0.007) and could be accurately described by a linear model. However, nonlinearity of ESPVRs obtained over wider load ranges is suggested by negative volume axis intercepts of the linear model. Steeper ESPVR with aortic than with caval occlusion (2.28 +/- 0.22 vs 3.41 +/- 0.51 mmHg/ml, ns) could not be proven owing to the large interindividual variance of ESPVR slopes with both loading interventions. During shock the Pes range obtained by caval occlusion decreased to very low levels (from 49 +/- 2 to 34 +/- 1 mmHg), ESPVR did not adequately fit either of the two models (mean R < 0.66), and critical reduction of CPP induced negative ESPVR slope in four of ten experiments. In contrast, aortic occlusion at shock resulted in linear ESPVR (R = 0.927 +/- 0.029), Pes ranges (92 +/- 3 to 58 +/- 4 mmHg) comparable to the ones obtained by caval occlusion at control (113 +/- 5 to 73 +/- 6 mmHg), and steeper ESPVR than at control (3.41 +/- 0.51 to 7.38 +/- 1.0 mmHg/ml, P < 0.05). Interpretation of the increased ESPVR slope obtained with aortic occlusion as due to increased contractility in shock is, however, complicated by different Pes ranges. It is concluded that within Pes ranges obtained with caval or aortic occlusion in situ the ESPVR can be adequately fitted to a linear model. For assessment of the inotropic response to shock the ESPVR is of limited value because (1) caval occlusion is not suitable to generate ESPVR during shock, and (2) Pes ranges obtained with identical loading interventions differ greatly between baseline and shock and, therefore, apparent ESPVR changes are influenced by the potential nonlinearity of the ESPVR. Combining caval occlusion at baseline with aortic occlusion at shock would result in comparable Pes ranges. Interpretation of results is, however, complicated by diverging effects of the different loading interventions on the shape and slope of the ESPVR.

The relationship between systolic pressure and stroke volume describes myocardial contractility

OBJECTIVE

To develop a method of measuring end-systolic elastance from information obtained outside the ventricle and thereby simplify its transduction.

DESIGN

Prospective, within-animal comparative analysis.

SETTING

University-based laboratory study.

PARTICIPANTS

Six mixed-breed dogs.

INTERVENTIONS

Instrumentation included minor axis sonomicrometry, ascending aortic flow probe, aortic and ventricular pressure transducers, and constricting cuffs on the vena cavae and aorta.

MEASUREMENTS AND MAIN RESULTS

Elastance was determined from the equation PES = EES (VED - VES), where VED - VES is stroke volume and PES is end-systolic arterial pressure. EES was derived from both preload and afterload manipulation. Cardiac performance indices were calculated automatically by computer under conditions of varying load and inotropy. This extraventricular method of elastance calculation was compared by linear regression and analysis of variance to preload recruitable stroke work, traditional EES determination (using ventricular dimension instead of volume), and LVdP/dt at 50 mmHg. EES measured from the aortic sites correlated well with the other contractility indicators (p < 0.003 in all cases) and demonstrated more sensitivity and stability under loading manipulation than traditional EES. A strong relationship between the change in stroke volume and end-systolic ventricular diameter during acute aortic constriction (r = 0.924, p < 0.0001) was observed, and the mean r value for the individual outflow elastance measurements was 0.97 +/- 0.02.

CONCLUSIONS

In this study, measurement of EES from the ventricular outflow tract during progressive aortic constriction produced results more consistent and descriptive than EES by traditional techniques and has the potential for obtaining elastance measurements from possibly less invasive techniques.

Chronic changes of end-systolic pressure-volume relationship after regional myocardial infarction

The chronic changes of the end-systolic pressure-volume relationship (ESPVR) after regional myocardial infarction were evaluated in a sheep model. Pressure-volume area (PVA) obtained from the pressure-volume diagram and left ventricular oxygen consumption (LVO2) were studied. The regional myocardial infarction was created by ligating distal branches of the left coronary artery. ESPVR was obtained using a conductance catheter during transient inferior vena cava occlusion. Measurements were performed at baseline (n = 13), 1 hour (n = 8), 3 months (n = 9), and 6 months (n = 4) after infarction. Ees, the slope of the ESPVR did not change at 1 hour after infarction and remained the same at 3-month and 6-month measurements (baseline 2.26 +/- 1.24 mmHg/mL, 1 hour 2.71 +/- 1.06, 3 months 3.46 +/- 1.51, 6 months 2.45 +/- 0.64, NS). Because of the ventricular dilatation, which was demonstrated as an increase in changes of end-systolic volume (Ves) correlating with the time course after infarction (y = -3.21 + 0.12x, r = 0.454, p < 0.05), V0, the volume intercept of the ESPVR increased at 1 hour after the infarction, and showed a tendency to increase at 3 months and 6 months after the infarction (baseline -18.0 +/- 22.5 mL; 1 hour -0.9 +/- 11.6; 3 months 5.4 +/- 10.9, 6 months 9.2 +/- 23.1, baseline vs 3 months p < 0.05, baseline vs 6 months p < 0.05). PVA and LVO2 were unchanged over time after infarction (PVA: baseline 2097 +/- 1526 mmHg/mL per 100 g-1; 1 hour 1771 +/- 699; 3 months 2483 +/- 1086; 6 months 1,608 +/- 1,010, NS), (LVO2: baseline 40.6 +/- 13.1 x 10(-3) mL/100 g-1 per beat-1; 1 hour 42.9 +/- 9.7; 3 months 35.0 +/- 8.6; 6 months 31.2 +/- 18.1, NS). Chronic regional infarction in the sheep model did not affect Ees over 6 months, but significantly increased V0 after the increase in the acute phase. PVA and LVO2 were not affected by this regional infarction either acutely or over 6 months.

Hypertonic saline dextran does not increase cardiac contractile function during small volume resuscitation from hemorrhagic shock in anesthetized pigs

Small volumes of hypertonic saline dextran (10% of shed blood volume [SBV] restore cardiac output (CO) and increase arterial pressure in hemorrhagic shock. Besides rapid expansion of plasma volume, a positive inotropic effect has been proposed as an additional mechanism for the immediate onset of the cardiovascular response. This study compares the effects of 7.2% saline/10% dextran 60 (HSDex, n = 8) and normal saline (NS; n = 6) on central hemodynamics and cardiac contractility assessed by end-systolic elastance (Ees; conductance technique) and segmental preload recruitable stroke work (sPRSW; sonomicrometry). In anesthetized open chest pigs (28 +/- 1 kg, mean +/- SEM) shock was induced by blood withdrawal (40% of blood volume) to maintain mean arterial pressure (MAP) at 45 mm Hg for 75 min. Resuscitation was started by bolus infusion (2 min) of either HSDex (10% of SBV) or the identical sodium load of NS (80% of SBV); 30 min later both groups received 6% dextran (10% of SBV). Hemorrhagic shock reduced CO (-45%) and left ventricular end-diastolic volume (Ved; -70%) while Ees increased (NS:2.2 +/- 0.4 to 7.5 +/- 1.8 mm Hg/mL, P < 0.05; HSDex: 1.9 +/- 0.2 to 9.1 +/- 2.6 mm Hg/mL, P = 0.085). Within 5 min after infusion of either solution CO returned to baseline values and MAP (NS +55%, HSDex +64%) and Ved (+100%) increased. Neither HSDex nor NS increased Ees above shock levels (NS, 8.7 +/- 4.9 mm Hg/mL; HSDex, 7.3 +/- 2.6 mm Hg/mL) and no group differences occurred in other measurements of contractility (dP/dt40,sPRSW). Plasma osmolality increased to 328 +/- 3 mOsmol/kg with HSDex.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between afterload and contractility in the newborn heart: evidence of homeometric autoregulation in the intact circulation

OBJECTIVES

We undertook the present study to determine whether afterload and contractility interact in the hearts of newborn lambs. We specifically investigated whether stepwise increases in afterload increase contractility.

BACKGROUND

Several studies in the isolated and intact adult dog heart have shown that afterload and contractility are not independent determinants of cardiac performance; rather, they interact. Afterload and contractility are unlikely to interact in the newborn heart because the factors that may mediate the interaction in the adult are missing in the newborn.

METHODS

We measured contractility at different steady state levels of afterload in seven newborn lambs under complete anesthesia. Contractility was measured by three different indexes: end-systolic pressure-volume relations (slope and volume position); preload-corrected first derivative of left ventricular pressure (dP/dtmax); and preload-corrected stroke work. Left ventricular pressure and volume were measured with a micromanometer and conductance catheter, respectively. Preload and afterload were manipulated by inflating or deflating a balloon catheter in the inferior vena cava and descending thoracic aorta, respectively. Data are expressed as mean value +/- 1 SD.

RESULTS

Stepwise increases in afterload increased contractility, independent of which of the three indexes was used. The slope of the end-systolic pressure-volume relation increased from a mean baseline value of 4.44 +/- 2.43 to 6.69 +/- 2.89 kPa/ml at the highest level of afterload. Concomitantly, volume at 14 kPa of the end-systolic pressure-volume relation decreased from 3.34 +/- 1.52 ml at baseline to 1.12 +/- 0.83 ml at the highest afterload. The other two indexes showed qualitatively similar changes. Beats selected from unloading interventions on the basis of the same end-diastolic volume for each level of afterload showed no difference in stroke volume.

CONCLUSIONS

This study in newborn lambs demonstrates that stepwise increases in afterload increase contractility considerably and that this enables the heart to maintain stroke volume at different levels of afterload. This forms direct evidence for the existence of homeometric autoregulation in the intact newborn heart.

Timing and velocity of ejection as major determinants of end-systolic pressure in isolated rabbit hearts

BACKGROUND

Systolic shortening is known to produce muscle deactivation. The present study was designed to analyze whether the velocity and the timing of ejection play a role on end-systolic pressure-volume relations (ESPVR).

METHODS AND RESULTS

In isolated rabbit hearts, left ventricular pressure and volume were recorded and digitized, and left ventricular volume was controlled by a servosystem (4-millisecond cycles) to alter the timing of ejection. A significant deficit in end-systolic pressure was observed when ejection was late in systole with respect to earlier ejection. This was associated with a significantly reduced end-systolic elastance. End-systolic pressure of beats with slow ejection was intermediate between that of the beats with early ejection and that of beats with late ejection with a significantly increased end-systolic volume compared with beats with early rapid ejection. The same results were obtained with hypertrophied hearts (abdominal aortic stenosis). Pressure-volume loop areas were significantly increased in beats with slow ejections and with rapid delayed ejections versus early rapid ejections. No change in the positive peak of dP/dt was observed when the timing and the velocity of ejection were modified.

CONCLUSIONS

ESPVR is modified by the ejection profile, with a decreased end-systolic pressure and an increased pressure-volume loop area related to the velocity and the amount of shortening during the end-systolic phase. These indices of ventricular function thus must be used with caution when the timing of ejection is altered, and the end-diastolic volume-peak dP/dt relation may be a better index of ventricular function.

Left ventricular end-systolic elastance is incorrectly estimated by the use of stepwise afterload variations in conscious, unsedated, autonomically intact dogs

BACKGROUND

End-systolic elastance (Ees), the slope parameter of the end-systolic pressure (ESP)-volume (ESV) relation (ESPVR), is usually estimated in patients by producing stepwise, steady-state pharmacological afterload variations and collecting one ESP-ESV point from each step. The ESPVR is then constructed by fitting a linear equation to these points. In sedated, autonomically blocked dogs, it has been shown that when one point from control, one point from a state of increased afterload, and one point from a state of decreased afterload are used, the resulting Ees incorrectly estimates true Ees, defined as the slope of the ESPVR obtained by transient vena caval occlusion. We investigated if this was also the case in unsedated, autonomically intact dogs when the points used belonged to steady states of progressively decreasing or progressively increasing afterload pressure.

METHODS AND RESULTS

In 10 conscious dogs instrumented with left ventricular (LV) endocardial sonomicrometers to measure LV volume, a LV pressure transducer, and an inferior vena caval (IVC) occluder, two protocols were carried out on separate days. In each protocol, an ESPVR was generated by IVC occlusion in the control state and in two steady-state levels of afterload change produced by stepwise infusion of nitroprusside (protocol 1, afterload decrease) and angiotensin II (protocol 2, afterload increase). In each protocol, steady-state ESP-ESV data points were averaged from the control state and from each level of afterload variation. Linear equations were fitted to the three steady-state points from each protocol, and the estimated Ees values obtained (EesEST) were compared with the Ees values of the control ESPVRs obtained by IVC occlusion (EesTRUE). In protocol 1, EesEST underestimated EesTRUE by about 16% (EesEST, 6.49 +/- 1.55 mm Hg/mL; EesTRUE, 7.48 +/- 1.29 mm Hg/mL; P < .02). In protocol 2, EesEST overestimated EesTRUE by about 37% (EesEST, 9.99 +/- 3.97 mm Hg/mL; EesTRUE, 6.43 +/- 3.88 mm Hg/mL; P < .007).

CONCLUSIONS

In conscious, autonomically intact dogs, the use of stepwise, steady-state afterload variations to obtain ESP-ESV data points to construct the ESPVR incorrectly estimates Ees. In the case of afterload reduction, EesTRUE is underestimated an average of 16.3%, and in the case of afterload increase, EesTRUE is overestimated an average of 37.1%. These errors should be taken into account when interpreting clinical studies using this methodology.

Characteristics of left-ventricular isovolumic pressure waves in isolated dog hearts

The peak pressure which a chamber would develop in isovolumic contraction at end-diastolic distention (peak source pressure) is an expression of contractile vigor and a determinant of systolic performance. One can predict source pressure of an ejecting beat by fitting its isovolumic phases with a model isovolumic-wave function. Characteristics of the left-ventricular isovolumic pressure wave (amplitude, duration, shape) were studied in isolated, perfused, artificially loaded dog hearts, where strictly isovolumic conditions could be obtained over a wide range of cavity volumes at constant heart rate and approximately constant contractile state. The characterization involved two steps: (1) beginning and ending points were identified by a transition-locating algorithm, and (2) Fourier analysis was performed on points in between. The amplitude of the isovolumic pressure wave increased with cavity volume as expected, the duration of contraction increased with cavity volume, and the shape of the wave (normalized Fourier coefficients) depended slightly on the cavity volume. Duration of contraction declined slightly with increasing heart rate, but the shape of the isovolumic pressure wave was independent of heart rate. The mean shape was similar to that found in dog hearts subjected to one-beat aortic-root clamping in vivo-the wave being less sharply peaked than a cosine wave and tilted to the left because relaxation was slower than contraction. When ejecting beat duration declined linearly with increasing ejection fraction. This relation could be used to predict the duration of the isovolumic beat corresponding to the duration of an ejecting beat. Source pressure could then be predicted by fitting a model isovolumic wave of predicted duration to the isovolumic contraction phase of the ejecting beat. In 270 comparisons, the ratio of predicted peak source pressure to observed peak source pressure was 1.04 +/- 0.10 (SD). This method provides a reasonably accurate prediction of an important determinant of systolic performance.

Comparative efficacy of three indexes of left ventricular performance derived from pressure-volume loops in heart failure induced by tachypacing

OBJECTIVES

The purpose of this study was to serially evaluate the response and variability of the end-systolic pressure-volume relation, the left ventricular end-diastolic volume-peak positive first derivative of left ventricular pressure (dP/dt) relation and the left ventricular end-diastolic volume-stroke work relation in the development of progressive left ventricular dysfunction.

BACKGROUND

Evaluation of systolic performance of the failing left ventricle may be enhanced by using relatively load-insensitive measures of left ventricular performance. The end-systolic pressure-volume, left ventricular end-diastolic volume-peak positive dP/dt and left ventricular end-diastolic volume-stroke work relations adequately define left ventricular performance under multiple loading conditions, but efficacy has not been fully assessed in the failing heart, particularly in the intact circulation.

METHODS

Fourteen dogs underwent instrumentation and rapid pacing to heart failure. Variably loaded pressure-volume beats were produced by inferior vena cava occlusion. The dogs were evaluated at baseline and at three progressively more severe levels of left ventricular dysfunction.

RESULTS

There was a progressive increase in left ventricular volumes at end-diastole ([mean value +/- SE] 60 +/- 28 to 73 +/- 29 ml, p < 0.001) and end-systole (39 +/- 19 to 61 +/- 27 ml, p < 0.001) during the 3 weeks of rapid pacing and a progressive decline in peak positive dP/dt (1,631 +/- 410 to 993 +/- 222 mm Hg/s, p < 0.001) and ejection fraction (37 +/- 8% to 16 +/- 11%, p < 0.001). There was a corresponding decline in the slope of each of the three relations: for end-systolic pressure-volume, 6.3 +/- 2.2 to 2.8 +/- 0.7 (p < 0.05); for left ventricular end-diastolic volume-stroke work, 61.9 +/- 9.1 to 26.5 +/- 2.4 (p < 0.05); and for left ventricular end-diastolic volume-peak positive dP/dt, 47.1 +/- 13.6 to 20.31 +/- 6.8 (p < 0.05). There was also a corresponding increase in position volumes: for end-systolic pressure-volume, 33.6 +/- 3.9 to 61.2 +/- 6.6 ml (p < 0.05); for left ventricular end-diastolic volume-stroke work, 46.2 +/- 3.6 to 89.3 +/- 7.6 ml (p < 0.05); and for left ventricular end-diastolic volume-peak positive dP/dt, 29.1 +/- 19.1 to 68.6 +/- 25.9 ml (p < 0.05). The relative degree of change in each of the three relations was similar as more severe heart failure developed. The coefficients of variation for position were significantly less than the variation for slopes. The response of volume intercepts was heterogeneous. For left ventricular end-diastolic volume-stroke work, the intercept increased as ventricular performance decreased. The intercept of the end-systolic pressure-volume relation was significantly more variable than the left ventricular end-diastolic volume-stroke work relation and did not change with progressive heart failure. The intercept for left ventricular end-diastolic volume-peak positive dP/dt was highly variable and showed no consistent changes as left ventricular function declined.

CONCLUSIONS

All three relations consistently describe changes in left ventricular performance brought about by tachypacing. Evolution of left ventricular dysfunction causes a decline in slope and a rightward shift of these relations. The position of the relation is the most sensitive and least variable indicator of left ventricular systolic performance.

Cardiac sympathetic denervation does not change the load dependence of the left ventricular end-systolic pressure/volume relationship in dogs

It has been shown that in the intact canine heart the left-ventricular end-systolic pressure/volume relation (ESPVR) depends on loading conditions: an increase in arterial vascular resistance causes a leftwards shift and a steeper slope of the ESPVR, suggesting an increased inotropic state. Our purpose was to investigate the possible contribution of the sympathetic nervous system to this load sensitivity of the ESPVR, using intact, but denervated, hearts with normal coronary perfusion and afterload. We used two types of loading intervention: venous volume infusion and gradual occlusion of the descending aorta. ESPVRs were obtained in six anaesthetized open-chest dogs, both before and after bilateral ablation of the stellate ganglia. To exclude the influence of heart rate changes, bilateral vagotomy was performed and the heart was paced. The absence of (unpaced) heart rate changes in response to pressure alterations was used to confirm total denervation. Left ventricular pressure was measured with a micromanometer and volume with a conductance catheter. ESPVRs were essentially linear and characterized by their slope (Ees) and volume intercept at 12 kPa (V12). We found that Ees (P < 0.0001) and V12 (P < 0.05) were both significantly different during pressure and volume interventions (0.67 +/- 0.29 and 0.41 +/- 0.18 kPa/ml for Ees and 16.2 +/- 8.2 and 18.2 +/- 8.4 ml for V12 respectively). Denervation did not significantly affect the parameters of the ESPVR obtained by either volume infusion or aortic occlusion. Two-way analysis of variance revealed no significant interactive effect between denervation and intervention, indicating that the sympathetic nervous system does not influence the load dependency of the ESPVR.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of single isovolumic left-ventricular pressure waves of dog hearts in situ

By fitting isovolumic phases of an ejecting beat with a model-wave function, one can predict source pressure of the ejecting beat (Sunagawa et al. Trans Biomed Eng 1980; 27:299-305), this being a major determinant of systolic performance. Prior applications of this principle have involved two assumptions: (1) that the isovolumic pressure wave is shaped like an inverted cosine wave, and (2) that duration of an isovolumic beat is the same as that of an ejecting beat. The first assumption might cause overestimation of source pressure, since an isovolumic pressure wave begins declining before the midpoint of the wave. The second assumption might cause underestimation of source pressure, since an ejecting beat is always shorter than an adjacent isovolumic beat at the ejecting beat's end-diastolic volume. Although the two errors tend to cancel, it would be more rational and accurate to use a realistic model wave shape and a realistic isovolumic beat duration. To acquire the information necessary for this, pressure and volume time courses were measured during ejecting beats and adjacent isovolumic beats in dogs under the following steady-state conditions: basal, atrial pacing at various rates, infusion of dobutamine, infusion of verapamil, coronary ligation(s), and ventricular pacing at various sites. These conditions affected the amplitude and duration of isovolumic pressure waves substantially but did not affect the shape of the waves significantly. The duration of each isovolumic beat exceeded that of the previous ejecting beat to a degree which corresponded approximately to the ejecting beat's normalized pressure reserve (source pressure minus peak ejection pressure)/(source pressure). A more accurate source-pressure prediction should be possible by use of a realistic isovolumic pressure-wave shape and by taking account of the effect of pressure reserve on contraction duration.

Comparison of noninvasive measures of contractility in dilated cardiomyopathy

Left ventricular performance is usually quantified by ejection phase indices such as ejection fraction, cardiac output, and fractional shortening. The load-dependence of these measures may result in inaccurate estimation of intrinsic myocardial contractility in states of chronic pressure or volume overload. End-systolic and stress-shortening relations have been proposed as measures of contractile state insofar as they are theoretically independent of preload and incorporate afterload. This article examines the behavior of these relations in response to changes in loading conditions and contractile state and reviews their application utilizing noninvasive methodology, particularly in the setting of dilated cardiomyopathy.

Ventricular load optimization by unloading therapy in patients with heart failure

The effects of unloading a depressed heart were assessed in terms of optimal coupling between the ventricle and arterial system. To assess the effects of preload on ventricular load coupling, preload was reduced with a lower body negative pressure of -20 mm Hg. Nitroprusside was used to evaluate the effects of afterload on the coupling under the condition that preload reduction was comparable to that with lower body negative pressure. In 13 patients with heart failure (ejection fraction 32 +/- 3%, mean +/- SE), direct arterial pressure was simultaneously recorded with the left ventricular echocardiogram as the pressure was elevated by phenylephrine. Left ventricular contractile properties were defined by the slope (Ees) of the end-systolic pressure-volume relation. The effective arterial elastance (Ea) was expressed by the slope of the end-systolic pressure-stroke volume relation. Left ventricular external work, end-systolic potential energy and work efficiency, defined as external work per pressure volume area (external work + potential energy), were determined. Baseline ventricular load coupling in these patients was characterized by an increase in the ratio of arterial elastance to ventricular elastance (Ea/Ees) (1.96 +/- 0.31). This ratio decreased significantly, to 1.45 +/- 0.22, with nitroprusside, and increased to 2.37 +/- 0.34 with lower body negative pressure. Therefore, end-systolic potential energy was decreased by nitroprusside but was unaltered by lower body negative pressure while external work was comparably decreased by both manipulations, indicating that work efficiency was significantly augmented with nitroprusside but declined with lower body negative pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of beta-adrenergic blockade on myocardial function and energetics in congestive heart failure. Improvements in hemodynamic, contractile, and diastolic performance with bucindolol

The hemodynamic effects of beta-adrenergic blockade with bucindolol, a nonselective beta-antagonist with mild vasodilatory properties, were studied in patients with congestive heart failure. Fifteen patients (New York Heart Association class I-IV) underwent cardiac catheterization before and after 3 months of oral therapy with bucindolol. The left ventricular ejection fraction increased from 0.23 +/- 0.12 to 0.29 +/- 0.14 (p = 0.007), and end-systolic elastance, a relatively load-independent determinant of contractility, increased from 0.60 +/- 0.40 to 1.11 +/- 0.45 mm Hg/ml (p = 0.0049). Both left ventricular stroke work index (34 +/- 13 to 47 +/- 19 g-m/m2, p = 0.0059) and minute work (5.5 +/- 2.2 to 7.0 +/- 2.6 kg-m/min, p = 0.0096) increased despite reductions in left ventricular end-diastolic pressure (19 +/- 8 to 15 +/- 5 mm Hg, p = 0.021). There was an upward shift in the peak + dP/dtmax-end-diastolic volume relation (p = 0.0005). These data demonstrate improvements in myocardial contractility after beta-adrenergic blockade with bucindolol. At a matched paced heart rate of 98 +/- 15 min-1, the time constant of left ventricular isovolumic relaxation was significantly reduced by bucindolol therapy (92 +/- 17 versus 73 +/- 11 msec, p = 0.0013), and the relation of the time constant to end-systolic pressure was shifted downward (p = 0.014) with therapy. The slope of the logarithm left ventricular end-diastolic pressure-end-diastolic volume relation was unchanged (p = 0.51) after bucindolol. These data suggest that chronic beta-adrenergic blockade with bucindolol improves diastolic relaxation but does not alter myocardial chamber stiffness. Myocardial oxygen extraction, consumption, and efficiency were unchanged despite improvement in contractile function and mechanical work. Thus, in patients with congestive heart failure, chronic beta-adrenergic blockade with bucindolol significantly improves myocardial contractility and minute work, yet it does not do so at the expense of myocardial oxygen consumption. Additionally, bucindolol improves myocardial relaxation but does not affect chamber stiffness.