Time-dependent increase in left ventricular contractility following acute volume loading in the dog

Model-Based Weaning Tests for VA-ECLS Therapy

Venoarterial extracorporeal life support (VA-ECLS) is used in ICUs (intensive care units) for the most extreme presentations of acute and severe cardiogenic shock, and one of the main issues the clinicians have to deal with is the weaning from VA-ECLS. In this study, a patient-specific model of the cardiovascular system connected to a VA-ECLS is built to improve the understanding of this complex system. Pig experiments are performed to validate the model, and the results are quite promising since the mean difference between experimental data and simulation is smaller than 5% for all the hemodynamic quantities. It is also a major objective of this paper to provide a proof-of-concept analysis that model-based approaches could improve the weaning strategy for VA-ECLS therapy.

The slow force response to stretch: Controversy and contradictions

When exposed to an abrupt stretch, cardiac muscle exhibits biphasic active force enhancement. The initial, instantaneous, force enhancement is well explained by the Frank-Starling mechanism. However, the cellular mechanisms associated with the second, slower phase remain contentious. This review explores hypotheses regarding this "slow force response" with the intention of clarifying some apparent contradictions in the literature. The review is partitioned into three sections. The first section considers pathways that modify the intracellular calcium handling to address the role of the sarcoplasmic reticulum in the mechanism underlying the slow force response. The second section focuses on extracellular calcium fluxes and explores the identity and contribution of the stretch-activated, non-specific, cation channels as well as signalling cascades associated with G-protein coupled receptors. The final section introduces promising candidates for the mechanosensor(s) responsible for detecting the stretch perturbation.

Effects of sustained length-dependent activation on in situ cross-bridge dynamics in rat hearts

The cellular basis of the length-dependent increases in contractile force in the beating heart has remained unclear. Our aim was to investigate whether length-dependent mediated increases in contractile force are correlated with myosin head proximity to actin filaments, and presumably the number of cross-bridges activated during a contraction. We therefore employed x-ray diffraction analyses of beat-to-beat contractions in spontaneously beating rat hearts under open-chest conditions simultaneous with recordings of left ventricle (LV) pressure-volume. Regional x-ray diffraction patterns were recorded from the anterior LV free wall under steady-state contractions and during acute volume loading (intravenous lactate Ringers infusion at 60 ml/h, <5 min duration) to determine the change in intensity ratio (I(1,0)/I(1,1)) and myosin interfilament spacing (d(1,0)). We found no significant change in end-diastolic (ED) intensity ratio, indicating that the proportion of myosin heads in proximity to actin was unchanged by fiber stretching. Intensity ratio decreased significantly more during the isovolumetric contraction phase during volume loading than under baseline contractions. A significant systolic increase in myosin head proximity to actin filaments correlated with the maximum rate of pressure increase. Hence, a reduction in interfilament spacing at end-diastole ( approximately 0.5 nm) during stretch increased the proportion of cross-bridges activated. Furthermore, our recordings suggest that d(1,0) expansion was inversely related to LV volume but was restricted during contraction and sarcomere shortening to values smaller than the maximum during isovolumetric relaxation. Since ventricular volume, and presumably sarcomere length, was found to be directly related to interfilament spacing, these findings support a role for interfilament spacing in modulating cross-bridge formation and force developed before shortening.

Preload-independent mechanisms contribute to increased stroke volume following large volume saline infusion in normal volunteers: a prospective interventional study

Introduction

Resuscitation with saline is a standard initial response to hypotension or shock of almost any cause. Saline resuscitation is thought to generate an increase in cardiac output through a preload-dependent (increased end-diastolic volume) augmentation of stroke volume. We sought to confirm this to be the mechanism by which high-volume saline administration (comparable to that used in resuscitation of shock) results in improved cardiac output in normal healthy volunteers.

Methods

Using a standardized protocol, 24 healthy male (group 1) and 12 healthy mixed sex (group 2) volunteers were infused with 3 l normal (0.9%) saline over 3 hours in a prospective interventional study. Individuals were studied at baseline and following volume infusion using volumetric echocardiography (group 1) or a combination of pulmonary artery catheterization and radionuclide cineangiography (group 2).

Results

Saline infusion resulted in minor effects on heart rate and arterial pressures. Stroke volume index increased significantly (by approximately 15–25%; P < 0.0001). Biventricular end-diastolic volumes were only inconsistently increased, whereas end-systolic volumes decreased almost uniformly. Decreased end-systolic volume contributed as much as 40–90% to the stroke volume index response. Indices of ventricular contractility including ejection fraction, ventricular stroke work and peak systolic pressure/end-systolic volume index ratio all increased significantly (minimum P < 0.01).

Conclusion

The increase in stroke volume associated with high-volume saline infusion into normal individuals is not only mediated by an increase in end-diastolic volume, as standard teaching suggests, but also involves a consistent and substantial decrease in end-systolic volumes and increases in basic indices of cardiac contractility. This phenomenon may be consistent with either an increase in biventricular contractility or a decrease in afterload.

Heart rate modulates the slow enhancement of contraction due to sudden left ventricular dilation

In isovolumic blood-perfused dog hearts, left ventricular developed pressure (DP) was recorded while a sudden ventricular dilation was promoted at three heart rate (HR) levels: low (L: 52 +/- 1.7 beats/min), intermediate (M: 82 +/- 2.2 beats/min), and high (H: 117 +/- 3.5 beats/min). DP increased instantaneously with chamber expansion (Delta(1)DP), and another continuous increase occurred for several minutes (Delta(2)DP). HR elevation did not alter Delta(1)DP (32.8 +/- 1.6, 33.6 +/- 1.5, and 34.3 +/- 1.2 mmHg for L, M, and H, respectively), even though it intensified Delta(2)DP (17.3 +/- 0.9, 20.7 +/- 1.0, and 26.8 +/- 1.2 mmHg for L, M, and H, respectively), meaning that the treppe phenomenon enhances the length dependence of the contraction component related to changes in intracellular Ca(2+) concentration. Frequency increments reduced the half time of the slow response (82 +/- 3.6, 67 +/- 2.6, and 53 +/- 2.0 s for L, M, and H, respectively), while the number of beats included in half time increased (72 +/- 2.9, 95 +/- 2.9, and 111 +/- 3.2 beats for L, M, and H, respectively). HR modulation of the slow response suggests that L-type Ca(2+) channel currents and/or the Na(+)/Ca(2+) exchanger plays a relevant role in the stretch-triggered Ca(2+) gain when HR increases in the canine heart.

Load as an acute determinant of end-diastolic pressure-volume relation

Afterload-induced changes in myocardial relaxation are a mechanism for diastolic dysfunction when afterload is elevated beyond certain limits. The present study investigated the effects of acute afterload and preload changes on the position of the end-diastolic (ED) pressure-volume (P-V) relation. Beat-to-beat afterload elevations were induced in seven open-chest rabbits by gradually occluding the ascending aorta to increase peak left ventricular pressure (LVP) from baseline to isovolumetric level. Afterload elevations were performed at three ED LVP: 2.0 +/- 0.2 (low), 5.7 +/- 0.2 (mid), and 9.6 +/- 0.6 (high) mmHg. Preload was altered with caval occlusions and/or intravenous dextran. Afterload elevations induced an upward shift of the diastolic P-V relation, which became more important as afterload and/or preload increased. For instance, maximal afterload elevations shifted this relation upward 2.2 +/- 0. 5, 5.1 +/- 0.8, and 12.1 +/- 1.7 mmHg at low, mid, and high preload, respectively. These effects were partially due to changes in relaxation rate and time available to relax. In conclusion, load is an acute determinant of the ED P-V relation, which, therefore, does not provide a load-independent assessment of diastolic function.

Load dependent diastolic dysfunction in heart failure

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

Right ventricular function in respiratory distress syndrome and subsequent partial liquid ventilation. Homeometric autoregulation in the right ventricle of the newborn animal

Infant respiratory distress syndrome (IRDS) and subsequent partial liquid ventilation (PLV) cause increased pulmonary vascular resistance, thus raising afterload. In nine newborn lambs the effects of IRDS and subsequent PLV on right (RV) and left ventricular (LV) contractility and systolic pump function were assessed using indices derived from RV and LV pressure-volume relations, obtained by micromanometric and conductance catheters during transient inferior vena cava occlusion. Pulmonary function deteriorated during IRDS with a significant decrease in the ratio of arterial oxygen pressure to fraction of inspired oxygen (Pa(O(2))/FI(O(2))) whereas pulmonary artery pressure (Ppa) showed a significant increase and pulmonary vascular resistance showed a substantial though not significant increase. Cardiac output (Q), stroke volume (SV), and end-diastolic volume (EDV) did not change. RV contractility showed a significant increase during IRDS: the slope of the end-systolic pressure-volume relation (RV-E (ES)) increased whereas its volume intercept at 5 kPa (RV-V(5)) decreased. The preload-corrected time derivative of ventricular pressure (RV-dP/dt(max)), however, did not change significantly. LV pump function and contractility were unchanged. During PLV pulmonary function showed a recovery but Ppa and pulmonary vascular resistance remained high; indices for RV contractility showed a sustained significant increase compared with baseline conditions whereas indices for LV pump function and contractility remained unchanged. These results show that the right ventricle of the newborn heart, in the face of increased pulmonary vascular resistance, is able to maintain cardiac output through homeometric autoregulation.

Right ventricular pressure and dilation during pressure overload determine dysfunction after pressure overload

Volume expansion and inotropic stimulation are used clinically to augment cardiac output during acute right ventricular (RV) pressure overload. We previously showed that a brief period of RV pressure overload causes RV free wall dysfunction that persists after normal loading conditions have been restored. However, the impact of volume expansion and inotropic stimulation on the severity of RV dysfunction after acute pressure overload is unknown. We hypothesized that the severity of RV dysfunction after RV pressure overload would be related to the level of RV free wall systolic stress during RV pressure overload, rather than to the specific interventions used to augment RV function. Chloralose-anesthetized, open-chest pigs were subjected to 1 h of RV pressure overload caused by pulmonary artery constriction, followed by 1 h of recovery after release of pulmonary artery constriction. A wide range of RV free wall systolic stress during RV pressure overload was achieved by either closing or opening the pericardium (to simulate volume expansion) and by administering or not administering dobutamine. The severity of RV free wall dysfunction 1 h after RV pressure overload was strongly and directly correlated with the values of two hemodynamic variables during RV pressure overload: RV free wall area at peak RV systolic pressure (determined by sonomicrometry) and peak RV systolic pressure, two of the major determinants of peak RV free wall systolic stress. Opening or closing the pericardium, and using or not using dobutamine during RV pressure overload, had no independent effects on the severity of RV dysfunction. The findings suggest that the goal of therapeutic intervention during RV pressure overload should be to achieve the required augmentation of cardiac output with the smallest possible increase in RV free wall systolic stress.

Altered LV inotropic reserve and mechanoenergetics early in the development of heart failure

To test the hypothesis that alterations in left ventricular (LV) mechanoenergetics and the LV inotropic response to afterload manifest early in the evolution of heart failure, we examined six anesthetized dogs instrumented with LV micromanometers, piezoelectric crystals, and coronary sinus catheters before and after 24 h of rapid ventricular pacing (RVP). After autonomic blockade, the end-systolic pressure-volume relation (ESPVR), myocardial O(2) consumption (MVO(2)), and LV pressure-volume area (PVA) were defined at several different afterloads produced by graded infusions of phenylephrine. Short-term RVP resulted in reduced preload with proportionate reductions in stroke work and the maximum first derivative of LV pressure but with no significant reduction in baseline LV contractile state. In response to increased afterload, the baseline ESPVR shifted to the left with maintained end-systolic elastance (E(es)). In contrast, after short-term RVP, in response to comparable increases in afterload, the ESPVR displayed reduced E(es) (P < 0.05) and significantly less leftward shift compared with control (P < 0.05). Compared with the control MVO(2)-PVA relation, short-term RVP significantly increased the MVO(2) intercept (P < 0.05) with no change in slope. These results indicate that short-term RVP produces attenuation of afterload-induced enhancement of LV performance and increases energy consumption for nonmechanical processes with maintenance of contractile efficiency, suggesting that early in the development of tachycardia heart failure, there is blunting of length-dependent activation and increased O(2) requirements for excitation-contraction coupling, basal metabolism, or both. Rather than being adaptive mechanisms, these abnormalities may be primary defects involved in the progression of the heart failure phenotype.

Cellular mechanisms for the slow phase of the Frank-Starling response

Following a step increase in sarcomere length, isometric cardiac muscle tension increases instantaneously by the Frank-Starling mechanism. In isolated papillary muscle and myocytes, there is an additional significant rise in developed tension over the following 15 min due to an unknown mechanism. This slow change in tension could not be explained by mechanical heterogeneity of the muscle preparations or by an increase in myofilament sensitivity to Ca2+. The slow change in tension was not dependent on sarcoplasmic reticulum Ca2+ loading assessed with rapid cooling contractures, and was not significantly altered by sarcoplasmic reticulum Ca2+ depletion (ryanodine) or inhibition of sarcoplasmic reticulum Ca2+ reuptake (cyclopiazonic acid). We used the Luo-Rudy ionic model of the ventricular myocyte together with a model of the length-dependent myofilament activation by Ca2+ to examine the effects of step changes in the parameters of sarcolemmal ion fluxes as possible mechanisms for the slow change in stress. The slow increase in tension was simulated by step changes in the Na+-K+ pump or Na+ leak currents, suggesting that the slow change in stress may be caused by length induced changes in Na+ fluxes. The model also predicted a slow increase in the magnitude of the initial repolarization during phase 1 of the action potential. The combination of experimental and computational models used in this investigation represents a valuable technique in elucidating the cellular mechanisms of fundamental processes in cardiac excitation-contraction coupling.

Myocardial tactile stiffness: a variable of regional myocardial function

OBJECTIVES

We developed a new sensor system for in situ measurement of myocardial tactile stiffness-stiffness in a direction perpendicular to the wall-and validated its use for providing a reasonable estimation of regional myocardial function.

BACKGROUND

Numerous attempts have been made to directly assess regional myocardial function. The complexity and highly invasive nature of the measuring devices have hampered their in situ application.

METHODS

In open chest mongrel dogs, myocardial tactile stiffness, ventricular pressure and ventricular volume were monitored. Under the preload reduction, these variables were measured to determine the relation between the end-systolic pressure-volume relation (ESPVR) and the end-systolic tactile stiffness-volume relation (ESSVR). The changes in myocardial tactile stiffness were monitored in the regional ischemic myocardial model and infarcted model to evaluate their usefulness as indexes of regional myocardial function.

RESULTS

Myocardial tactile stiffness changed cyclically and followed a time course similar to left ventricular pressure. When preload was altered, the ESSVR was as linear as the ESPVR. The slope of the ESSVR and that of the ESPVR showed a strong correlation over a wide range of contractility. These results suggest that myocardial tactile stiffness can be a good index of regional wall stress or fiber stress. End-systolic myocardial tactile stiffness of ischemic and infarcted regions decreased significantly, with a concomitant increase in end-diastolic stiffness compared with that of intact myocardium.

CONCLUSIONS

Using our tactile sensor system, regional myocardial tactile stiffness of a beating heart was measured with reasonable temporal resolution. We consider myocardial tactile stiffness to be a useful index of regional myocardial function.

Mechanisms of length history-dependent tension in an ionic model of the cardiac myocyte

The ionic model of the ventricular myocyte developed by Luo and Rudy (Circ. Res. 74: 1071-1096, 1994) was used to investigate potential mechanisms of the slow changes in stress (SCS) that follow step changes in muscle length. A step change in myofilament sensitivity alone caused an immediate increase in active tension, but no SCS. The effects of additional step changes in the parameters of sarcolemmal ion fluxes were examined for each ion flux in the model. Changes in the coefficients of Ca2+ or K+ channels did not produce SCS. SCS was produced by step changes in parameters of the Na(+)-K+ pump or the Na+ leak current. This simulated mechanism was mediated through a slow increase in intracellular Na+ concentration and a resulting increase in systolic Ca2+ entry through the Na+/Ca2+ exchanger. The model reproduced the effects of several experimental interventions such as sarcoplasmic reticulum Ca2+ depletion, "diastolic" length changes, and changes in extracellular Ca2+. Thus SCS in cardiac muscle may be caused by length-induced changes in sarcolemmal Na+ fluxes.

Effect of ventricular stretch on contractile strength, calcium transient, and cAMP in intact canine hearts

Isovolumic contractions were imposed by intraventricular balloon in 39 isolated, blood-perfused canine hearts to investigate the effects of myocardial stretch on contractile force. After stabilization at 37 degrees C, left ventricular volume was increased so that end-diastolic pressure increased from 0 to 5 mmHg. After the immediate increase in developed pressure [DP; from 37 +/- 14 to 82 +/- 22 mmHg (means +/- SD)], there was a slow secondary rise in DP (97 +/- 27 mmHg) that peaked at 3 min. However, DP subsequently decreased over the next 7 min back to the initial value (84 +/- 25 mmHg). Light emission from microinjected aequorin (n = 10 hearts) showed that changes in intracellular calcium [3 min: 124 +/- 15% (P < 0.01); 10 min: 99 +/- 18% of baseline] paralleled DP changes. Increases in myocardial adenosine 3',5'-cyclic monophosphate (cAMP) content (n = 12) accompanied the secondary rise in DP. In contrast, the gradual elevation of DP after the stretch was not exerted during continuous beta-adrenergic stimulation by isoproterenol. Thus, in contrast to isolated muscle, stretch only transiently increases intracellular calcium and contractile strength in intact hearts. The findings of changes in cAMP and abolition of the phenomena by beta-stimulation suggest that a primary stretch-mediated influence on cAMP metabolism may underlie these phenomena.

Time to dP/dtmax, a preload-independent index of contractility: open-chest dog study

A mathematical model of left ventricular pressure (LVP) during isovolumic contraction in the time domain shows the following predictions: 1) td, the time from onset of contraction to dP/dtmax and (dP/dt)/P, reflect only the time-dependent aspects of contraction, and are independent of preload; 2) dP/dtmax depends on both preload and the time-dependent aspects of contraction. To test preload independence we reduced filling volume (FV) by the method of ventricular volume clamps with a remote-controlled mitral valve in 7 anesthetized open-chest dogs. A decrease in FV of 80 +/- 15% produced a 29 +/- 12% (p < 0.001) decrease in LVP, 34 +/- 13% (p < 0.001) decrease in dP/dtmax, 13 +/- 4% (p < 0.001) decrease in t-dP/dtneg, and no change in td (-3 +/- 5%, NS). The heart rate (HR) dependence on td was assessed in other 5 anesthetized open-chest dogs. HR was changed with atrial pacing (50-240 bpm). td was linearly and inversely related to HR in each dog, and at each HR: dobutamine lowered and propranolol elevated this relation when compared to control (p < .001, both). Since dP/dtmax occurs usually before the opening of the aortic valve, td is, thus, also afterload-independent. Conclusion. This study supports the theoretical predictions that td is independent of preload and that it can serve, at any given HR, as a reliable index of contractility, provided that dP/dtmax occurs before the opening of the aortic valve.

Effect of hydration on cavity obliteration during dobutamine stress echocardiography

The purpose of this study was to determine whether it is possible to prevent or delay the onset of midventricular cavity obliteration during a dobutamine stress test with standard hydration. Left ventricular (LV) intracavitary obstruction has been reported as the mechanism for hypotension seen in approximately 20% of patients undergoing dobutamine stress echocardiography. In addition, it has been proposed that administration of a normal saline bolus prior to dobutamine infusion may avert the dynamic ventricular obstruction. We performed a standard graded dobutamine stress echocardiogram before and after fluid loading with 10 ml/kg of normal saline in 10 mongrel dogs. Measurements were made of left atrial pressure, aortic pressure, and the area of the LV cavity at the papillary muscle level throughout each infusion. Although hydration produced an increase in baseline left atrial pressure (5.7 +/- 3.2 to 8.1 +/- 2.7 mmHg, p < 0.01) and systolic blood pressure (128 +/- 18 to 139 +/- 22 mmHg, p = 0.03), there was no significant change in pre-dobutamine heart rate or systolic area. With dobutamine infusion, there was a similar change in heart rate, systolic blood pressure, diastolic area, and systolic area (SA) at each dose of dobutamine regardless of hydration status. In addition, the dose at which cavity obliteration occurred was not altered by hydration (p = NS). Although all dogs developed cavity obliteration (SA < 1.0 cm2) with dobutamine infusion, none experienced hypotension. In this canine model, cavity obliteration does not lead to systemic hypotension and cannot be prevented or delayed by volume loading.

Effect of altered contractility on the linearity of regional left ventricular end-systolic relations in intact hearts

The purpose of these studies was to determine the effect of altered regional contractility on the linearity of regional left ventricular end-systolic relations. Significant change in the shape of these relations would limit their application as load-independent indices of regional contractility. In a paced, open-chest pig heart preparation (n = 7), the left ventricular end-systolic pressure-segment length relation (ESPLR) and pressure-wall thickness relation (ESPTR) were obtained over a wide range of end-systolic pressures (134 +/- 9 to 70 +/- 6 mm Hg). Regional inotropic state was varied with intracoronary calcium and verapamil. The shapes of the ESPLR and ESPTR were characterized by using linear and quadratic models. Both provided a good fit, although the quadratic model showed a slight concavity to the segment length and thickness axes (second-order coefficient < 0). In the linear model, calcium increased the slope of the ESPLR by 111% (p < 0.01) and the slope of the ESPTR by 170% (p < 0.01). At a pressure of 100 mm Hg, end-systolic segment length (L100) shifted to the left (p < 0.05) and end-systolic wall thickness (T100) to the right (p < 0.025). Verapamil decreased the slope of the ESPLR by 45% (p < 0.01) and of the ESPTR by 33% (p not significant) and produced significant shifts in L100 (p < 0.001) and T100 (p < 0.025). The values of L100 and T100 determined by the quadratic fit were nearly identical to those for the linear fit, and both showed similar significant shifts with altered contractility. There was no significant change in the shape of the quadratic fit (as assessed by the second-order coefficient) with different contractile states. It is concluded that the curvilinearity of the ESPLR and ESPTR under physiologic conditions is slight and appears to be independent of the contractile state. Furthermore, a linear model of regional end-systolic relations can be used to assess regional left ventricular function in intact hearts.

Assessment of regional myocardial performance with end-systolic pressure length and thickness relationships

Although end-systolic pressure length and thickness relationships (ESPLR, ESPTR) are now widely used as substitutes for the end-systolic pressure volume relationships, there are some reservations about their use as an index of left ventricular (LV) performance. This study addressed three issues, namely: (1) which loading technique (decreasing preload by inferior vena cava (IVC) balloon occlusion or increasing systolic pressure by aortic constriction) is the most likely to yield usable data; (2) reproducibility of these relationships over a 30 min period; and (3) whether by using end-ejection (zero aortic flow) as a definition of end-systole, ESPLR and ESPTR can be used to characterize myocardial performance independent of load. Thirteen anesthetized beagles, weighing 16-25 kg, were used for this study, and were instrumented with sonomicrometers. We found that when ESPLR and ESPTR were constructed from data derived during aortic constriction, the slopes of these relationships were steeper and more curvilinear than when they were constructed from data recorded during IVC occlusion. In addition, the mean between ESPLR, ESPTR obtained 30 min apart was small, although there was a fair degree of variability between the first and second measurements. Using end-ejection to define end-systole, both ESPLR and ESPTR were relatively insensitive to loading conditions (LV end-diastolic pressure of 8-12 mmHg and 14-18 mmHg, aortic systolic pressure of 7-10 mmHg and 20-25 mmHg above baseline (in terms of the slope and shift (leftward or rightward) in these relationships, but were sensitive to inotropic interventions (dobutamine 2.5 micrograms/kg per min and 5 micrograms/kg per min). We conclude that, ESPLR and ESPTR, defined from measurements at end-ejection, can be used as adequate descriptors of regional myocardial performance if they were constructed from data over a similar pressure range during IVC balloon occlusion.

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)

Geometric and muscle physiological factors of the Frank-Starling mechanisms

The relation between left-ventricular stroke volume (SV) and end-diastolic volume (EDV) was determined based on angiocardiographic measurements in 10 open-chest minipigs under varying filling conditions (blood letting or infusions). The results were compared with a theoretical relation calculated under the assumption of varying EDV but constancy of myocardial properties. In contrast to the linear increase of SV as a function of EDV as found in the animal experiments, the calculated curve reveals a maximum near the normal operating point with a decrease in the range of higher EDV. It can be concluded that the well-known increase of SV with increasing ventricular filling, beyond the normal EDV, is almost completely due to muscle physiological factors (mainly increase in Ca2+ sensitivity of the contractile apparatus), whereas the decrease of SV in the range of low filling pressure is mainly due to the geometrical conditions.