Dynamic myocardial contractile parameters from left ventricular pressure-volume measurements

A new dynamic model of left ventricular (LV) pressure-volume relationships in beating heart was developed by mathematically linking chamber pressure-volume dynamics with cardiac muscle force-length dynamics. The dynamic LV model accounted for >80% of the measured variation in pressure caused by small-amplitude volume perturbation in an otherwise isovolumically beating, isolated rat heart. The dynamic LV model produced good fits to pressure responses to volume perturbations, but there existed some systematic features in the residual errors of the fits. The issue was whether these residual errors would be damaging to an application where the dynamic LV model was used with LV pressure and volume measurements to estimate myocardial contractile parameters. Good agreement among myocardial parameters responsible for response magnitude was found between those derived by geometric transformations of parameters of the dynamic LV model estimated in beating heart and those found by direct measurement in constantly activated, isolated muscle fibers. Good agreement was also found among myocardial kinetic parameters estimated in each of the two preparations. Thus the small systematic residual errors from fitting the LV model to the dynamic pressure-volume measurements do not interfere with use of the dynamic LV model to estimate contractile parameters of myocardium. Dynamic contractile behavior of cardiac muscle can now be obtained from a beating heart by judicious application of the dynamic LV model to information-rich pressure and volume signals. This provides for the first time a bridge between the dynamics of cardiac muscle function and the dynamics of heart function and allows a beating heart to be used in studies where the relevance of myofilament contractile behavior to cardiovascular system function may be investigated.

Nonlinear myofilament regulatory processes affect frequency-dependent muscle fiber stiffness

To investigate the role of nonlinear myofilament regulatory processes in sarcomeric mechanodynamics, a model of myofilament kinetic processes, including thin filament on-off kinetics and crossbridge cycling kinetics with interactions within and between kinetic processes, was built to predict sarcomeric stiffness dynamics. Linear decomposition of this highly nonlinear model resulted in the identification of distinct contributions by kinetics of recruitment and by kinetics of distortion to the complex stiffness of the sarcomere. Further, it was established that nonlinear kinetic processes, such as those associated with cooperative neighbor interactions or length-dependent crossbridge attachment, contributed unique features to the stiffness spectrum through their effect on recruitment. Myofilament model-derived sarcomeric stiffness reproduces experimentally measured sarcomeric stiffness with remarkable fidelity. Consequently, characteristic features of the experimentally determined stiffness spectrum become interpretable in terms of the underlying contractile mechanisms that are responsible for specific dynamic behaviors.

Myofilament kinetics in isometric twitch dynamics

To better understand the relationship between kinetic processes of contraction and the dynamic features of an isometric twitch, studies were conducted using a mathematical model that included: (1) kinetics of cross bridge (XB) cycling; (2) kinetics of thin filament regulatory processes; (3) serial and feedback interactions between these two kinetic processes; and (4) time course of calcium activation. Isometric twitch wave forms were predicted, morphometric features of the predicted twitch wave form were evaluated, and sensitivities of wave form morphometric features to model kinetic parameters were assessed. Initially, the impulse response of the XB cycle alone was analyzed with the findings that dynamic constants of the twitch transient were much faster than turnover number of steady-state XB cycling, and, although speed and duration of the twitch wave form were sensitive to XB cycle kinetic constants. parameters of wave shape were not. When thin filament regulatory unit (RU) kinetics were added to XB cycle kinetics, the system impulse response was slowed with only little effect on wave shape. When cooperative neighbor interactions between RU and XB were added, twitch wave shape (as well as amplitude, speed and duration) proved to be sensitive to variation in cooperativity. Importantly, persistence and shape of the falling phase could be strongly modified. When kinetic coefficients of XB attachment were made to depend on sarcomere length, changes in wave shape occurred that did not occur when only sliding filament mechanisms were operative. Indeed, the force-length relationship proved to be highly sensitive to length-dependent XB attachment in combination with cooperative interactions. These model findings are the basis of hypotheses for the role of specific kinetic events of contraction in generating twitch wave form features.

Overall cardiac functional effect of positive inotropic drugs with differing effects on relaxation

Recent interest in so-called calcium-sensitizing positive inotropic drugs has highlighted the potential problem of a positive effect on force development being offset, at least partially, by the negative effect that many of these drugs have on relaxation. The purpose of this study was to examine the interplay of contraction and relaxation in determining the overall cardiac effect of different positive inotropic drugs. Using a buffer-perfused isolated rabbit heart preparation, we studied four drugs (calcium, dobutamine, EMD 57033, and CGP 48506) that were given at doses sufficient to increase similarly left ventricular pressure-generating capability by approximately 20%. We show that, even though they produce equivalent changes in pressure-generating capability, these four agents produce dissimilar changes in relaxation capability, with dobutamine speeding relaxation, EMD 57033 slowing relaxation, and calcium and CGP 48506 having little effect of relaxation. Similar relative effects were observed for drug-induced changes in the timing of pressure-generation events. These effects combine to produce different drug-induced changes in overall cardiac pump function judged by the relation between cardiac output and heart rate. Dobutamine shifted the maximal cardiac output to a higher heart rate. In contrast, both calcium sensitizers shifted the maximum in cardiac output to a lower heart rate, whereas calcium had no effect. Thus even though positive inotropic drugs may have similar effects on left ventricular pressure generation, the overall benefit of such drugs on ventricular pump function will depend on how the drug also affects ventricular relaxation and ejection capabilities.

Angiotensin IV has mixed effects on left ventricle systolic function and speeds relaxation

OBJECTIVE

A novel angiotensin receptor has been described and named AT4. Ligands for this receptor include the angiotensin II (Ang II) metabolite Ang II (3-8), known as angiotensin IV (Ang IV). There is 10-fold more AT4 receptor than AT1 receptor in rabbit myocardium. The AT4 receptor has a high affinity for Ang IV (Ki in rabbit myocardium < 2 x 10(-9)) and similar ligands, but very low affinity for Ang II (Ki in rabbit myocardium > 10(-6)). Although several functions have been attributed to the novel Ang IV peptide/AT4 receptor system, the effect of this system on left ventricular (LV) function has not been studied. We hypothesized (1) that Ang IV would affect LV function and (2) that any effects would be opposite to those of Ang II.

METHODS

Using the buffer-perfused (30 degrees C) isolated rabbit heart, we studied the effect of the AT4 agonist Nle1-Ang IV on LV systolic function, quantified using both Frank-Starling and end-systolic pressure-volume relationships, and relaxation. We also studied the effect of the AT1/AT2 agonist, Sar1-Ang II on LV function. Finally, because the profile of effect of Nle1-Ang IV was similar to the reported effect of nitric oxide (NO), we also studied the effect of Nle1-Ang IV in the presence of the NO synthase inhibitor NG-monomethyl-L-arginine.

RESULTS

Nle1-Ang IV reduced LV pressure-generating capability at any volume but increased the sensitivity of pressure development to volume change. Nle1-Ang IV reduced LV ejection capability. Sar1-Ang II had the opposite effect-increasing both pressure generation and ejection capability. Finally, both Sar1-Ang II and Nle1-Ang IV speeded LV relaxation. Inhibition of NO synthase did not alter the effect of Nle1-Ang IV on LV systolic function or relaxation.

CONCLUSIONS

AT4 receptor agonism has mixed effects on LV systolic function, depressing pressure-generation and ejection capabilities, but enhancing the sensitivity of pressure development to volume change. It also speeds relaxation. The effect of Ang IV on systolic function is generally opposite to the effect of Ang II, whereas the Ang IV influence on relaxation is similar to the effect of Ang II.

Contribution of vasomotion to vascular resistance: a comparison of arteries from virgin and pregnant rats

Intrinsic oscillatory activity, or vasomotion, within the microcirculation has many potential functions, including modulation of vascular resistance. Alterations in oscillatory activity during pregnancy may contribute to the marked reduction in vascular resistance. The purpose of this study was 1) to mathematically model the oscillatory changes in vessel diameter and determine the effect on vascular resistance and 2) to characterize the vasomotion in resistance arteries of pregnant and nonpregnant (virgin) rats. Mesenteric arteries were isolated from Sprague-Dawley rats and studied in a pressurized arteriograph. Mathematical modeling demonstrated that the resistance in a vessel with vasomotion was greater than that in a static vessel with the same mean radius. During constriction with the alpha1-adrenergic agonist phenylephrine, the amplitude of oscillation was less in the arteries from pregnant rats. We conclude that vasomotor activity may provide a mechanism to regulate vascular resistance and blood flow independent of static changes in arterial diameter. During pregnancy the decrease in vasomotor activity in resistance arteries may contribute to the reduction in peripheral vascular resistance.

The statistics of synergism

Biological scientists often want to determine whether two agents or events, for example, extracellular stimuli and/or intracellular signaling pathways, act synergistically when eliciting a biological response. When setting out to study whether two experimental treatments act synergistically, most biologists design the correct experiment--they administer four treatment combinations consisting of (1) the first treatment alone, (2) the second treatment alone, (3) both treatments together, and (4) neither treatment (i.e. the control). Many biologists are less clear about the correct statistical approach to determining whether the data collected in such an experimental design support a conclusion regarding synergism, or lack thereof. The non-additivity of two experimental treatments that is central to the definition of synergism leads to an algebraic formulation corresponding to the statistical null hypothesis appropriate for testing whether or not there is synergism. The resulting complex contrast among the four treatment group means is identical to the interaction effect tested in a two-way analysis of variance (ANOVA). This should not be surprising, because synergism, by definition, occurs when two treatments interact, rather than act independently, to influence a biological response. Hence, in the most readily implemented approach, the correct statistical analysis of a question of synergism is based on testing the interaction effect in a two-way ANOVA. This review presents the rationale for this correct approach to analysing data when the question is of synergism, and applies this approach to a recent published example. In addition, a common incorrect approach to analysing data with regards to synergism is presented. Finally, several associated statistical issues with regard to correctly implementing a two-way ANOVA are discussed.

Myocardial contractile depression from high-frequency vibration is not due to increased cross-bridge breakage

Experiments were conducted in 10 isolated rabbit hearts at 25 degrees C to test the hypothesis that vibration-induced depression of myocardial contractile function was the result of increased cross-bridge breakage. Small-amplitude sinusoidal changes in left ventricular volume were administered at frequencies of 25, 50, and 76.9 Hz. The resulting pressure response consisted of a depressive response [delta Pd(t), a sustained decrease in pressure that was not at the perturbation frequency] and an infrequency response [delta Pf(t), that part at the perturbation frequency]. delta Pd(t) represented the effects of contractile depression. A cross-bridge model was applied to delta Pf(t) to estimate cross-bridge cycling parameters. Responses were obtained during Ca2+ activation and during Sr2+ activation when the time course of pressure development was slowed by a factor of 3. delta Pd(t) was strongly affected by whether the responses were activated by Ca2+ or by Sr2+. In the Sr(2+)-activated state, delta Pd(t) declined while pressure was rising and relaxation rate decreased. During Ca2+ and Sr2+ activation, velocity of myofilament sliding was insignificant as a predictor of delta Pd(t) or, when it was significant, participated by reducing delta Pd(t) rather than contributing to its magnitude. Furthermore, there was no difference in cross-bridge cycling rate constants when the Ca(2+)-activated state was compared with the Sr(2+)-activated state. An increase in cross-bridge detachment rate constant with volume-induced change in cross-bridge distortion could not be detected. Finally, processes responsible for delta Pd(t) occurred at slower frequencies than those of cross-bridge detachment. Collectively, these results argue against a cross-bridge detachment basis for vibration-induced myocardial depression.

Relaxation effect of CGP-48506, EMD-57033, and dobutamine in ejecting and isovolumically beating rabbit hearts

Because it is not known whether ejection influences the negative effect of the Ca(2+)-sensitizing drugs on ventricular relaxation, we extended our previous analysis of stress-dependent relaxation in isovolumic beats to encompass ejecting beats and evaluated the relationships between both the time of onset of relaxation and the rate of relaxation and wall stress in a broader analysis framework. Furthermore, because the sites of action of the Ca(2+)-sensitizing drugs CGP-48506 and EMD-57033 may be different, and thus CGP-48506 may have fewer adverse effects on resting muscle length or force, we compared these two drugs to test the hypothesis that CGP-48506 would have less effect than EMD-57033 on relaxation in the isolated buffer-perfused rabbit heart. This analysis of stress-dependent relaxation in both ejecting and isovolumic beats readily differentiates between the negative lusitropic effect of 2 x 10(-6) M EMD-57033, the negligible lusitropic effect of 6 x 10(-6) M CGP-48506, and the positive lusitropic effect of 1.25 x 10(-6) M dobutamine. Furthermore, comparison of the effect of the two Ca(2+)-sensitizing drugs in ejecting versus isovolumic contractions shows that CGP-48506 affects relaxation differently in ejecting contractions than it does in isovolumic contractions, whereas EMD-57033 affects relaxation similarly in both ejecting and isovolumic contractions.

Left ventricular pressure response to small-amplitude, sinusoidal volume changes in isolated rabbit heart

The objective was to determine the dynamics of contractile processes from pressure responses to small-amplitude, sinusoidal volume changes in the left ventricle of the beating heart. Hearts were isolated from 14 anesthetized rabbits and paced at 1 beats/s. Volume was perturbed sinusoidally at four frequencies (f) (25, 50, 76.9, and 100 Hz) and five amplitudes (0.50, 0.75, 1.00, 1.25, and 1.50% of baseline volume). A prominent component of the pressure response occurred at the f of perturbation [infrequency response, delta Pf(t)]. A model, based on cross-bridge mechanisms and containing both pre- and postpower stroke states, was constructed to interpret delta Pf(t). Model predictions were that delta Pf(t) consisted of two parts: a part with an amplitude rising and falling in proportion to the pressure around that which delta Pf(t) occurred [Pr(t)], and a part with an amplitude rising and falling in proportion to the derivative of Pr(t) with time. Statistical analysis revealed that both parts were significant. Additional model predictions concerning response amplitude and phase were also confirmed statistically. The model was further validated by fitting simultaneously to all delta Pf(t) over the full range of f and delta V in a given heart. Residual errors from fitting were small (R2 = 0.978) and were not systematically distributed. Elaborations of the model to include noncontractile series elastance and distortion-dependent cross-bridge detachment did not improve the ability to represent the data. We concluded that the model could be used to identify cross-bridge rate constants in the whole heart from responses to 25- to 100-Hz sinusoidal volume perturbations.

The AT4 receptor agonist [Nle1]-angiotensin IV reduces mechanically induced immediate-early gene expression in the isolated rabbit heart

Angiotensin II (ANG II), acting principally at the AT1 receptor, modulates mechanically-induced cardiac growth. The ANG II metabolite Angiotensin IV (ANG IV) has been shown to inhibit ANG II-induced mRNA and protein synthesis in chick cardiomyocytes. This effect did not involve the AT1 receptor, but was likely an action at the AT4 receptor. To determine if ANG IV also modulates a mechanically-induced cardiac growth response, we studied the effects of two AT4 receptor ligands, [Nle1]-ANG IV and [divalinal]-ANG IV, on mechanically-induced immediate-early gene expression (c-fos, egr-1, and c-jun) in the buffer perfused (30 degrees C), ejecting, isolated rabbit heart. Mechanical load alone (high systolic pressure and high end-diastolic volume) induced approximately 23-, 49- and 5-fold increases in c-fos, egr-1 and c-jun mRNA (in comparison to control hearts). Perfusion with [Nle1]-ANG IV (10[-10] mol/l) reduced the mechanically-induced expression of c-/fos and egr-1 by 42% and 48%, respectively (P < 0.05). Mechanically-induced c-jun expression was not significantly reduced. Perfusion with [divalinal]-ANG IV (10[-8] mol/l) had no effect on mechanically-induced immediate-early gene expression. We conclude that AT4 receptor agonism influences mechanical immediate-early gene expression, and propose the hypothesis that AT1 and AT4 receptors initiate opposing effects on mechanically-induced immediate-early gene expression in the isolated rabbit left ventricle.

Functional effects of EMD-57033 in isovolumically beating isolated rabbit hearts

The results of isolated myocyte and cardiac muscle experiments indicate that inotropic agents that increase responsiveness of myofilaments to Ca2+ (so-called Ca2+ sensitizers) may prolong myocardial contraction and increase diastolic tone, but the importance of these effects in the whole heart is unclear. Therefore, we studied the effects of the Ca2+ sensitizer EMD-57033 (EMD) on left ventricular (LV) contractile events and passive properties in isovolumically beating isolated rabbit hearts that were buffer perfused at 30 degrees C. Several LV pressure and timing variables were evaluated, including the passive pressure-volume relationship, the Frank-Starling relationship, and the wall stress dependence of the duration of relaxation during perfusion with 0, 2, and 4 microM EMD. EMD (2 microM) increased average peak developed pressure of the Frank-Starling relationship by approximately 18%. In contrast, the peak developed pressure of the Frank-Starling relationship decreased toward control with 4 microM EMD, and therefore all the results presented pertain to 2 microM EMD. The maximum developed pressure at baseline volume was increased by approximately 19% by 2 microM EMD, and this was accompanied by an increase in contraction duration of approximately 13%, due exclusively to slowed relaxation. The relative contributions of maximal wall stress (sigma max) versus an independent negative lusitropic effect of EMD were determined at three LV volumes. At baseline volume, just less than one-half of the effect to slow relaxation was ascribable to an increase in sigma max, whereas the remainder was due to an independent EMD effect. LV passive properties were unchanged by perfusion with 2 microM EMD. We conclude that EMD is a potent inotrope in our isolated rabbit heart preparation, which has no effect on diastolic tone and causes a modest prolongation of contraction duration due to slowed relaxation. At baseline volume, approximately 50% of the slowed relaxation was ascribable to positive inotropy leading to increased sigma max, whereas the remaining approximately 50% was ascribable to a direct negative lusitropic effect of EMD. We discuss our results in terms of the current hypotheses regarding the mechanism of action of the Ca2+ sensitizers.

Immediate-early gene responses to different cardiac loads in the ejecting rabbit left ventricle

Clinical and experimental observations in humans and animals have shown that different cardiac adaptations occur in response to different types of hemodynamic overload. However, very little is known about how different hemodynamic loads lead to these different cardiac adaptations. Accordingly, we studied the acute response of ejecting isolated rabbit hearts to independently varied systolic and diastolic mechanical loads at constant coronary perfusion pressure. We studied the combined effects of low end-diastolic volume (EDV) and low systolic ejection pressure (Pej), compared to low EDV and high Pej, high EDU and low Pej, and high EDV and high Pej, on the expression of c-fos, c-jun, and egr-1. Further, although we did not seek to clarify the role of these immediate-early genes in cardiac hypertrophy, we hypothesized that they should not all respond in the same manner to these different mechanical loads. In these ejecting hearts we found that the expression of these immediate-early genes did not all respond alike to the different mechanical loads: both c-fos and egr-1 were strongly induced at both 30 and 60 min. However, at 30 min only c-fos depended on the level of EDV (P = 0.01). Neither c-fos nor egr-1 was influenced by EDV at 60 min. The expression of c-jun was largely insensitive to all loading conditions. We conclude that EDV, independent of Pej, influences the pattern and time course of expression of some immediately-early genes and that these different immediate-early genes do not respond in parallel to changes in cardiac loading.

Previous-beat contraction history is not influenced by mechanosensitive ion channel blockade

OBJECTIVES

Prior studies have shown that the performance of the left ventricle on any one beat is influenced by the mechanical events of the previous beat, a phenomenon called "previous-beat contraction history". This previous-beat contraction history, which appears to be an interplay between the mechanical events of one contraction and the activation state of the next contraction, could depend, at least in part, on mechanosensitive ion channels. The purpose of this study, therefore, was to test the hypothesis that mechanosensitive ion channels contribute to previous-beat contraction history: If previous-beat contraction history depends on mechanosensitive ion channels, the magnitude of its effect should be decreased by blocking mechanosensitive ion channels.

METHODS

We performed experiments in buffer-perfused isolated rabbit hearts in which left ventricular pressure and volume were controlled with a servo-motor system. We evaluated the pulse interval-dependent expression of previous-beat contraction history under control conditions (no drug) and in the presence of 100 and 500 microM streptomycin, a blocker of mechanosensitive ion channels.

RESULTS

Under control conditions, previous-beat contraction history nor its dependence on pulse interval was influenced significantly by either concentration of streptomycin.

CONCLUSION

Mechanosensitive ion channels do not play a role in the expression of previous-beat contraction history in the left ventricle of the isolated rabbit heart.

Characterization of chronotropic and dysrhythmogenic effects of atropine in dogs with bradycardia

OBJECTIVE

To characterize the magnitude, character, and time course of chronotropic and dysrhythmogenic responses of dogs with vagally mediated bradycardia to atropine sulfate.

DESIGN

Latin square design.

ANIMALS

Seven clinically normal adult mixed-breed dogs.

PROCEDURE

Vagally mediated bradycardia was induced with morphine and fentanyl citrate. Atropine (0.02 mg/kg of body weight) was administered i.v., s.c., or i.m.. Electrocardiograms were recorded continuously for 5 minutes before and for 35 minutes after atropine administration or until a sustained parasympatholytic response was observed. Data were digitized, analyzed independently for changes in atrial and ventricular rate, and compared between different routes of administration.

RESULTS

All dogs developed second-degree atrioventricular (AV) block after i.v. administration of atropine, and 71% of dogs developed AV block after s.c. or i.m. administration. The AV block arose and resolved more rapidly with i.v. administration than with s.c. or i.m. administration. The AV block was principally attributable to an increase in the atrial rate prior to increases in the ventricular rate. Atropine, regardless of route of administration, potentiated baseline ventricular bradycardia in 62% of the experiments (mean heart rate decrease of 16 beats/min; decreased to < 20 beats/min in 2 dogs for < or = 10 seconds). Duration of the bradycardic potentiation was longer with s.c. administration (9.1 minutes, s.c., vs 1.4 minutes, i.v., and 4.6 minutes, i.m.). Parasympatholytic rate was higher for i.v. than s.c. or i.m. administration (128 beats/min vs 92 beats/min and 101 beats/min). Two dogs given atropine s.c. failed to resolve the AV block and attain sinus rhythm.

CONCLUSIONS

Administration of 0.02 mg of atropine/kg by i.v., i.m., and s.c. routes for vagally mediated bradycardia in dogs consistently induces AV block and occasional brief potentiation of ventricular bradycardia.

CLINICAL RELEVANCE

Parasympathomimetic effects occur and resolve most rapidly and consistently, and the stable parasympatholytic effect is of greatest magnitude after i.v. administration. Thus, vagally mediated bradycardia in clinically normal dogs appears to be best abolished by i.v. administration of atropine.

Mechanical determinants of left ventricular relaxation in isovolumically beating hearts

Both pressure and volume have been proposed to determine the speed of left ventricular (LV) relaxation, but their relative importance is not known. Accordingly, we used isolated, buffer-perfused, isovolumically beating ferret hearts to study the effects of maximal developed pressure (Pdmax) and LV volume (V) on the speed of LV relaxation. Experiments were performed at 30 degrees C, and the hearts were paced at a baseline interbeat interval (BI) of 800 ms. Pdmax was varied independently of V by use of seven BI (75 to 133% of baseline BI), which resulted in test beats that developed a range of Pdmax due to varying degrees of restitution. Pdmax was also varied by setting V at five levels (80 to 120% of baseline V) during the test beats. Speed of relaxation was quantified as the time period of pressure decay from 75 to 25% Pdmax (T75-25). Data were analyzed by multiple linear regression. Increases in both Pdmax and V independently prolonged T75-25, and T75-25 was 1.45 times more sensitive to Pdmax than to V. However, when Pdmax and V were combined to estimate maximal wall stress (sigma max), the effects of Pdmax and V, as well as relative circumferential muscle length (estimated by V1/3), were not important determinants, and T75-25 depended on sigma max alone. Thus we conclude that 1) Pdmax and V are both determinants of the speed of LV relaxation and that Pdmax is approximately 1.5 times more important than V, and 2) the effects of Pdmax and V on relaxation act via the common mechanism of sigma max.

Beta-adrenergic blockade reduces myocardial injury during experimental cardiopulmonary resuscitation

OBJECTIVES

We attempted to determine the effects of beta-adrenergic blockade during cardiopulmonary resuscitation (CPR) on defibrillation rates and postresuscitation left ventricular function.

BACKGROUND

The results of previous studies suggest that propranolol administration can both reduce myocardial oxygen requirements and increase coronary perfusion pressure during CPR.

METHODS

Left ventricular pressure and segment length were measured before and after 5 min of CPR in 22 dogs either given epinephrine (0.015 mg/kg body weight at the onset and after 4 min) or pretreated with propranolol (2 mg/kg) and given epinephrine during CPR.

RESULTS

Despite identical epinephrine doses, coronary perfusion pressure during CPR was higher in the epinephrine plus propranolol group (p < 0.05), and defibrillation was successful in 9 of 11 dogs given both epinephrine and propranolol versus 6 of 11 dogs given epinephrine alone (p = NS). Peak and developed left ventricular pressures, left ventricular end-diastolic pressure and the peak rate of left ventricular pressure development (+dP/dt) did not differ between study groups when measured either 5 or 15 min after successful defibrillation. However, when survivors in the epinephrine group were given propranolol after CPR to eliminate compensatory sympathetic stimulation, left ventricular developed pressure and peak +dP/dt were lower (p < 0.05) despite trends toward higher left ventricular end-diastolic pressures and normalized end-diastolic segment lengths compared with dogs given propranolol before CPR.

CONCLUSIONS

These findings suggest that beta-adrenergic blockade reduces myocardial injury during CPR without decreasing the likelihood of successful defibrillation or compromising spontaneous postresuscitation left ventricular function.

Previous beat contraction history alters mechanical restitution in the isolated left ventricle

OBJECTIVE

The aim was to test directly the hypothesis that the magnitude of previous beat contraction history will be greatest for short pulse intervals, will become smaller as pulse interval is lengthened, and will vanish when pulse interval is long enough to allow complete restitution.

METHODS

Experiments were performed in isolated rabbit and ferret left ventricles in which pressure and volume were controlled with a servo-motor system. Two restitution curves were generated, each constructed from isovolumetric beats that were preceded by a beat with differing amounts of ejection: one curve was constructed from isovolumetric beats preceded by non-ejecting beats, while the other was constructed from isovolumetric beats preceded by a beats that had a high level of ejection (ejection fraction approximately 50%).

RESULTS

When the isovolumetric beats used to construct a restitution curve were preceded by ejecting beats, the restitution curve was shifted upward (that is, higher activation at a given interval between beats) when compared to a restitution curve constructed from isovolumetric beats that were preceded by non-ejecting beats.

CONCLUSIONS

Mechanical restitution is affected by the mechanical events of preceding contractions, implying that previous beat contraction history and restitution share a common mechanism and that restitution sets the stage for previous beat contraction history. Hence restitution is not simply a useful tool for assessing calcium cycling or interval dependent behaviour and should be viewed more broadly as a fundamental process in the beat to beat regulation of cardiac contraction.

Effects of free wall ischemia and bundle branch block on systolic ventricular interaction in dog hearts

Systolic direct ventricular interaction is thought to occur via the ventricular septum and the coordinated contraction of common fibers shared by both ventricles. The purpose of the present study was to evaluate the effects of transient free wall ischemia and bundle branch block, which disrupt the coordinated contraction of shared common fibers, on left-to-right systolic ventricular interaction. We produced transient right and left ventricular free wall ischemia by 2-min coronary artery occlusions and bundle branch block by ventricular pacing in nine in situ dog hearts. To eliminate any confounding effect of series interaction, we used an abrupt hemodynamic perturbation (aortic constriction), and we measured systolic interaction gain (IG) as delta right ventricular peak systolic pressure/delta left ventricular peak systolic pressure (IG(peak)) and instantaneous delta right ventricular pressure/delta left ventricular pressure at matched data sampling times (IG(inst)), along with changes in right ventricular stroke volume and stroke work before and on the beat immediately after the aortic constriction. To achieve equivalence of the interventricular septal pressure transmission contribution to ventricular interaction, the delta left ventricular peak systolic pressure produced by the aortic constriction was matched under all experimental conditions [average increase: 64 +/- 19 (SD) mmHg]. Control IG(peak) was 0.12 +/- 0.05, and control IG(inst) was 0.11 +/- 0.05. These values did not change with either free wall ischemia or ventricular pacing, with or without an intact pericardium. The changes in right ventricular stroke volume and stroke work produced by the aortic constriction were not different from zero, during either ischemia or ventricular pacing, with or without an intact pericardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenylephrine plus propranolol improves the balance between myocardial oxygen supply and demand during experimental cardiopulmonary resuscitation

Epinephrine increases coronary blood flow but may not improve the balance between myocardial oxygen supply and demand during cardiopulmonary resuscitation (CPR). The objective of this study was to determine whether this balance can be improved by administering a relatively pure alpha-adrenergic vasoconstrictor, alone or in combination with a beta-adrenergic blocker. We measured coronary perfusion pressures during CPR and myocardial adenosine 5'-triphosphate (ATP) and lactate concentrations in biopsies obtained immediately after 10 minutes of CPR in six control dogs and in three groups of six dogs each given large doses of epinephrine, phenylephrine, or phenylephrine plus propranolol during CPR. Coronary perfusion pressure during CPR was higher in the three treated groups than in the control group, although differences were limited to the early portion of CPR in dogs given epinephrine or phenylephrine alone. Postresuscitation myocardial ATP concentration was significantly higher (29.5 +/- 3.0 vs 22.6 +/- 1.8 nmol/mg of protein, p < 0.05) and myocardial lactate concentration tended to be lower (52.8 +/- 13.6 vs 78.5 +/- 15.2 nmol/mg of protein) than in the control group in dogs given both phenylephrine and propranolol. In contrast, myocardial ATP concentration tended to be lower than in the control group in epinephrine-treated dogs, and myocardial lactate concentrations were higher than in the control group in dogs treated with either epinephrine (p < 0.05) or phenylephrine alone (p = 0.052). We conclude that the balance between myocardial oxygen supply and demand during CPR can be improved by administering a combination of phenylephrine and propranolol, but not by administering large doses of epinephrine or phenylephrine alone.

Use of 2,3-butanedione monoxime to estimate nonmechanical VO2 in rabbit hearts

The purpose of the present study was to evaluate the feasibility of partitioning myocardial O2 consumption (VO2) into mechanical and nonmechanical components in the whole heart preparation using a negative inotrope, 2,3-butanedione monoxime (BDM), which has been reported to have a selective effect on the contractile proteins in a low concentration range (< 6 mM). In six isolated bovine red blood cell-perfused rabbit hearts, VO2 and force-time integral (FTI) were measured during infusion of varying concentrations of BDM at a constant left ventricular volume chosen such that control left ventricular peak isovolumic pressure was approximately 100 mmHg. The VO2-FTI relation with BDM concentrations < or = 5 mM was highly linear (median r = 0.98). Its VO2-axis intercept at zero FTI had a positive value (mean 23% of control, 0.014 ml O2.beat-1 x 100 g-1). To confirm the selective effect of BDM on the contractile proteins, the intracellular free Ca2+ transient was measured with the fluorescent indicator indo 1 in three isolated buffer-perfused rabbit hearts. The amplitude of the Ca2+ transient was not altered by BDM at concentrations < or = 10 mM, although left ventricular developed pressure was markedly depressed. This finding indicates that BDM < or = 10 mM does not affect excitation-contraction coupling. We conclude that the VO2-axis intercept value of the VO2-FTI relation during BDM infusion in a low concentration range represents VO2 for nonmechanical energy utilization. The BDM method to partition VO2 into mechanical and nonmechanical components is thus feasible in the whole rabbit heart.

2,3-Butanedione monoxime increases contractile efficiency in the rabbit ventricle

The efficiency of chemomechanical energy transduction (contractile efficiency) of the left ventricle (LV) has been calculated from the linear correlation of the systolic pressure-volume area (PVA) of the LV and its O2 consumption (VO2). Thus far, a wide range of acute interventions, including adrenergic agents, Ca(2+)-sensitizing drugs, and Ca2+ channel blockers have not altered contractile efficiency. In contrast, hyperthyroidism has been reported to decrease contractile efficiency, an effect attributed at the cross-bridge level to an increase in the V1/V3 myosin isoenzyme ratio. We hypothesized that an acute intervention which directly alters cross-bridge cycling would also change contractile efficiency. Accordingly, 2,3-butanedione monoxime (BDM), a negative inotropic agent that is thought to directly inhibit cross-bridge formation, was administered to seven excised, red blood cell-perfused, isovolumically beating rabbit LVs. At 3-4 mM perfusate concentration, BDM resulted in the following reversible mechanical and energetic effects compared with control. Contractility, assessed by the slope (Emax) of the end-systolic pressure-volume relation, decreased by 11% (196.6 +/- 25.9 vs. 222 +/- 28 mmHg/ml). Both the time to end systole (Tmax) and relaxation half time (T1/2) decreased. The slope of the VO2-PVA relation decreased by 20% (1.55 +/- 0.44 x 10(-5) vs. 1.95 +/- 0.52 x 10(-5) ml O2 x mmHg-1 x ml-1), equivalent to an increase in contractile efficiency from 36.5 +/- 10.4 to 46.4 +/- 14.4%, while the O2 costs of the mechanically unloaded LV decreased by 12% (0.0258 +/- 0.0060 vs. 0.0292 +/- 0.0064 ml O2 x beat-1 x 100 g-1). Finally, BDM also produced coronary vasodilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Absence of right ventricular isovolumic relaxation in open-chest anesthetized dogs

During the left ventricular (LV) pump cycle, peak negative first derivative of pressure vs. time (dP/dt) occurs very close to the end of LV ejection, and there is a well-defined isovolumic relaxation period. Despite similarities between the right ventricular (RV) and LV pump cycles, recent studies indicate uncertainty as to whether peak negative RV dP/dt occurs simultaneously with RV end ejection and whether there is an isovolumic relaxation period during the RV pump cycle. To study these questions, we recorded relative timing of peak negative RV dP/dt, RV end ejection, and right atrial-RV pressure crossover in the open-chest anesthetized dog. The data demonstrate that peak negative RV dP/dt occurs an average of 60 ms before end ejection and that there is no RV isovolumic relaxation period. These findings have implications for the possible use of peak negative RV dP/dt as a marker of RV end ejection and for how time constants of pressure decay obtained during RV relaxation can be interpreted.

Single perturbed beat vs. steady-state beats for assessing systolic function in the isolated heart

Single-beat and steady-state techniques for evaluating end-systolic pressure-volume relationship (ESPVR) and Frank-Starling mechanism (FSM) in the crystalloid-perfused isolated rabbit heart were compared. In the single-beat technique, a train of stable isovolumic beats was interrupted with a single perturbed beat that either ejected against various levels of imposed isobaric load (ESPVR protocol) or beat isovolumically against various levels of end-diastolic volume (V(ED); FSM protocol). In steady-state technique, sustained beating was established, isobarically, at each of various loads (ESPVR protocol) or, isovolumically, at each of various V(ED) values (FSM protocol). ESPVR from steady-state technique lay above and to the left of that from single-beat technique. Contractile state was not uniform within steady-state technique, whereas it was uniform within single-beat technique. In the FSM protocol, single-beat technique exhibited the following features relative to steady-state technique: 1) greater range of developed pressures, 2) steeper ascending limb and more sharply defined maxima, 3) higher maximal developed pressure (Pdmax), and 4) greater volume at Pdmax(Vmax). Again. a common contractile state existed within single-beat technique but not within steady-state technique. It was concluded that single-beat technique was preferable to steady-state technique for evaluating ESPVR and FSM because 1) single-beat technique required less time for obtaining data, 2) single-beat technique allowed identification of uncomplicated values of Pdmax and Vmax, and 3) single-beat technique provided a common contractile-state reference for all data, whereas steady-state technique did not.

Effects of amrinone and isoproterenol on mechanoenergetics of blood-perfused rabbit heart

To compare the effects of amrinone (AMR) and isoproterenol (Iso) on left ventricular contractility and energetics, we assessed Emax (ventricular contractility index) and the relation between oxygen consumption per beat (VO2) and systolic pressure-volume area (PVA, a measure of left ventricular total mechanical energy) in isolated cross-circulated (blood-perfused) rabbit hearts during infusion of AMR or Iso in either a constant-flow (CF) or constant-pressure (CP) perfusion mode. Both Emax and the VO2 intercept of the linear VO2-PVA relation increased significantly during AMRCP (increase in Emax 15% and increase in VO2 intercept 11%), ISOCF (49 and 43%), and ISOCP (55 and 54%) but not during AMRCF. However, neither drug changed the slope of the VO2-PVA relation (reciprocal of contractile efficiency) in either perfusion mode. Furthermore, with both drugs the relation between increases in Emax and the VO2 intercept fell on a single regression line (r = 0.92). We conclude that 1) although the mechanism of action and inotropic potency of the two drugs differ, their effects on cardiac energetic cost are essentially the same, i.e., both drugs increase the nonmechanical oxygen cost in proportion to the increase in contractility without changing contractile efficiency, and 2) a significant portion of the inotropic effect of AMR in the whole ventricle is likely due to increased coronary blood flow, i.e., Gregg's phenomenon.

The sinus node inhibitor UL-FS 49 lacks significant inotropic effect

UL-FS 49 is a sinus node inhibitor that has been reported to reduce heart rate and may be useful in improving myocardial oxygen supply vs. demand. However, previous studies performed in a variety of preparations have produced mixed results regarding the independent inotropic effect of UL-FS 49. To determine whether UL-FS 49 has an inotropic effect, we measured both steady-state hemodynamic responses and transient hemodynamic responses to random preload and afterload changes, both with and without UL-FS 49. We found that under steady-state conditions, the effect of UL-FS 49 is so small that it would be of doubtful physiologic significance: a 3% increase in stroke volume (p = 0.007) and 7% increase in peak positive dP/dt (p = 0.051), in the presence of no statistically significant differences in end-diastolic pressure, end-diastolic volume, peak systolic pressure, end-systolic pressure, or heart rate. The more powerful multiple linear regression modeling of hemodynamic transient sequences resulting from random preload and afterload changes showed that UL-FS 49 is without a statistically significant direct effect on left ventricular function. We conclude that UL-FS 49 has no physiologically important direct effect on left ventricular pump function.

Stability of myocardial O2 consumption-pressure-volume area relation in red cell-perfused rabbit heart

We evaluated the mechanical and energetic stability of the isolated rabbit heart perfused with a suspension of bovine red cells in Krebs-Henseleit buffer in terms of the pressure-volume area (PVA) concept. PVA, the area surrounded by the end-systolic and end-diastolic pressure-volume (P-V) relations and the systolic P-V trajectory of the P-V diagram, represents the total mechanical energy generated by each cardiac contraction. Myocardial O2 consumption (VO2) per beat has been reported to be highly linearly correlated with PVA. We used the slope and VO2-axis intercept of the VO2-PVA relation as energetic parameters and the maximum P-V ratio (Emax) as a contractility index of the left ventricle (LV) and compared them every 30 min for 120 min. Emax, the slope, and VO2 intercept of the VO2-PVA relation did not change significantly over 120 min compared with their control values [7.3 +/- 2.9 mmHg.ml-1.100 g LV, (1.67 +/- 0.40) x 10(-5) ml O2.mmHg-1.ml-1, and (3.26 +/- 1.01) x 10(-2) ml O2.beat-1.100 g LV-1, respectively]. However, the goodness of the linear fit of the VO2-PVA relation decreased after 90 min (r = 0.94 control, 0.62 at 90 min, and 0.64 at 120 min). Therefore, we conclude that the isolated bovine red cell-perfused rabbit heart preparation is stable for mechanical and energetic studies for at least 60 min.

Left ventricular function depends on previous beat ejection but not previous beat pressure load

Previous beat contraction history, in which the performance of the left ventricle on any one beat is influenced by the mechanical events of the previous beat, may be important in the beat-to-beat regulation of left ventricular performance in the intact cardiovascular system. Prior studies of this phenomenon have established that mechanical events of the previous beat influence the function of the current beat, but it is not known whether the important mechanical influence is exerted by previous beat ejection or previous beat pressure. In addition, the magnitude of the effect of previous beat contraction history on left ventricular performance is unknown. To make these determinations, we performed experiments in six isolated rabbit left ventricle preparations buffer perfused at 30 degrees C. Left ventricular pressure and volume were controlled precisely with a servo-controlled linear motor system. After steady-state ejecting conditions were established by clamping left ventricular ejection pressure at 60% of peak isovolumic pressure, single test beats, which were pressure clamped at 40%, 60%, 80%, and 100% of peak isovolumic pressure, were introduced and followed by an isovolumic reference beat. As the level of pressure clamp decreased from 100% to 40%, developed pressure on the isovolumic beat following the single test beats increased from 139 +/- 15 (mean +/- SD) to 151 +/- 13 mm Hg. Similarly, peak positive left ventricular dP/dt increased from 1,718 +/- 209 to 1,864 +/- 181 mm Hg.sec-1 (both p less than 0.01). Multiple regression analysis showed that this increase in left ventricular function was related to previous beat ejection but not to previous beat pressure load or relaxation.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiac cycle length modulates cardiovascular regulation that is dependent on previous beat contraction history

Previous studies of the beat-to-beat regulation of left ventricular performance in the intact cardiovascular system have shown that the performance of the left ventricle on any one beat is influenced by the mechanical events of the previous beat, so-called previous beat contraction history. The general hypothesis investigated in this study is that previous beat contraction history occurs because of the perturbation of one or more biochemical processes with time courses that are long relative to one cardiac cycle. If this is true, then previous beat contraction history should depend on the interval between beats and, therefore, should extend beyond the previous beat to two, or even three, previous beats as heart rate is increased. Hemodynamic responses to random preload and afterload changes were measured in 11 anesthetized open-chest dogs on right heart bypass at three paced heart rates. Multiple linear regression was used to analyze these hemodynamic sequences and identify variables from the previous one, two, or three beats that were important in the mechanical history dependence of left ventricular function. The results of this analysis showed that under baseline conditions, all 11 hearts showed one beat of history dependence, with only two of 11 hearts showing a dependence on the previous two or three beats. At the highest heart rate, all 11 hearts still showed one beat of history dependence, but 10 of 11 hearts showed two beats of history dependence, and four of 11 showed three beats of history dependence (p less than 0.05). A general framework for the mechanism underlying these findings, which relates previous beat contraction history to the interval-dependent mechanical restitution phenomenon, is proposed and discussed.

Effect of coronary hyperemia on Emax and oxygen consumption in blood-perfused rabbit hearts. Energetic consequences of Gregg's phenomenon

To assess the relation between increases in contractile function and oxygen consumption (VO2) during increased coronary flow (Gregg's phenomenon), we measured the end-systolic pressure-volume relation and the relation between VO2 and left ventricular systolic pressure-volume area (PVA, a measure of total mechanical energy output) in blood-perfused, isovolumically contracting rabbit hearts during control and intracoronary adenosine infusion. During adenosine infusion at a constant perfusion pressure (93 +/- 11 mm Hg), coronary flow increased by 99 +/- 76% (p less than 0.01), and the slope of the end-systolic pressure-volume relation, Emax (ventricular contractility index), increased by 18 +/- 15% (p less than 0.01). When compared at the same left ventricular volume, PVA increased by 20 +/- 14% (p less than 0.01) and VO2 by 19 +/- 15% (p less than 0.01) with adenosine. The VO2-PVA relation was linear under each condition (both median r = 0.98). With increased coronary flow, the VO2-intercept of the VO2-PVA relation (unloaded VO2) increased by 22 +/- 18% (p less than 0.01) without a change in the slope; that is, a parallel upward shift was observed, indicating that the contractile efficiency (energy conversion efficiency of the contractile machinery) remained constant. These increases in Emax and unloaded VO2 were not eliminated by beta-adrenergic blockade with propranolol. We conclude that increased coronary flow with adenosine at a constant perfusion pressure augments both Emax and the nonmechanical energetic cost for excitation-contraction coupling and basal metabolism via nonadrenergic mechanisms, without changing contractile efficiency.

Pentoxifylline (Trental) does not inhibit dipyridamole-induced coronary hyperemia: implications for dipyridamole-thallium-201 myocardial imaging

Dipyridamole-thallium-201 imaging is often performed in patients unable to exercise because of peripheral vascular disease. Many of these patients are taking pentoxifylline (Trental), a methylxanthine derivative which may improve intermittent claudication. Whether pentoxifylline inhibits dipyridamole-induced coronary hyperemia like other methylxanthines such as theophylline and should be stopped prior to dipyridamole-thallium-201 imaging is unknown. Therefore, we studied the hyperemic response to dipyridamole in seven open-chest anesthetized dogs after pretreatment with either pentoxifylline (0, 7.5, or 15 mg/kg i.v.) or theophylline (3 mg/kg i.v.). Baseline circumflex coronary blood flows did not differ significantly among treatment groups. Dipyridamole significantly increased coronary blood flow before and after 7.5 or 15 mm/kg i.v. pentoxifylline (p less than 0.002). Neither dose of pentoxifylline significantly decreased the dipyridamole-induced hyperemia, while peak coronary blood flow was significantly lower after theophylline (p less than 0.01). We conclude that pentoxyifylline does not inhibit dipyridamole-induced coronary hyperemia even at high doses.

Decreased contractile efficiency and increased nonmechanical energy cost in hyperthyroid rabbit heart. Relation between O2 consumption and systolic pressure-volume area or force-time integral

Both systolic pressure-volume area (PVA) and force-time integral (FTI) have been used as measures of oxygen consumption per beat (VO2) in the isolated left ventricle. The reciprocal of the slope of the VO2-PVA relation has been considered to reflect the chemomechanical energy transduction efficiency of the contractile machinery (contractile efficiency), whereas its VO2 intercept consists of energy cost of excitation-contraction coupling and basal metabolism. To examine whether the increase in myosin isoform V1/V3 ratio in hyperthyroid rabbits decreases contractile efficiency and to determine overall mechanisms of higher oxygen consumption in hyperthyroid hearts, the VO2-PVA and VO2-FTI relations as well as the end-systolic pressure-volume relation were assessed in cross-circulated, isovolumically beating hearts isolated from normal, hyperthyroid, and hypothyroid rabbits. Normalized initial slopes of the rising limb of the curvilinearly fitted end-systolic pressure-volume relation (E'max, ventricular contractility index) were similar for normal and hyperthyroid groups. However, the slopes and VO2 intercepts of the VO2-PVA and VO2-FTI relations were greater in hyperthyroid hearts than in normal hearts. Accordingly, in the hyperthyroid hearts, the contractile efficiency (27 +/- 6%) was lower and left ventricular VO2 for excitation-contraction coupling (0.028 +/- 0.004 ml O2/beat/100 g) was higher than in normal hearts (40 +/- 4% and 0.021 +/- 0.005 ml O2/beat/100 g, respectively). This decreased contractile efficiency in the hyperthyroid hearts was attributable to myosin isoform alteration rather than to increased beta-adrenoceptors because isoproterenol did not affect the slope of the VO2-PVA relation in all groups. In contrast, the slope of the VO2-FTI relation was significantly increased by isoproterenol in all groups. Neither the VO2-PVA nor the VO2-FTI relations in hypothyroid hearts were different from those in normal hearts except for significantly lower VO2 for basal metabolism. We conclude that in hyperthyroid rabbits, the left ventricle has decreased contractile efficiency and increased energy cost of excitation-contraction coupling and that the decreased contractile efficiency in hyperthyroid hearts is probably due to the increased V1/V3 ratio of the myosin isoform component. In addition, this study demonstrates that the VO2-PVA and VO2-FTI relations dissociate depending on the intervention, even in the same isovolumic contraction mode.

Missing data in two-way analysis of variance

We previously described a regression approach to analysis of variance computations that permitted analysis of unbalanced designs and experiments with missing data in two-way (or higher) analyses of variance Am. J. Physiol. 255 (Regulatory Integrative Comp. Physiol. 24): R353-R367, 1988. That approach can only be used correctly under a set of narrow, and relatively uninteresting, circumstances. In fact, in the example we worked, we extended that approach beyond its intended scope and incorrectly computed F statistics for testing hypotheses about the main effects of strain of rat and nephron site in a study of renal Na(+)-K(+)-adenosinetriphosphatase. This paper presents the correct approach, which can be generalized to most situations likely to be encountered when two-way, or higher, analyses of variance are used.

Systolic direct ventricular interaction affects left ventricular contraction and relaxation in the intact dog circulation

Changes in right ventricular systolic function directly influence left ventricular systolic function. Most of our knowledge of this systolic direct ventricular interaction comes from studies of isolated hearts, which suggest that changes in right ventricular size only affect left ventricular systolic function at low pressures and volumes. However, almost nothing is known about systolic direct ventricular interaction in a heart functioning in situ as a part of the intact circulation. We used sudden constriction of the pulmonary artery to assess the immediate effect of a change in right ventricular pressure and contraction pattern on left ventricular contraction and relaxation on the beat following the pulmonary artery constriction in anesthetized open-chest dogs. By focusing on this first beat, we were able to avoid the confounding effect of series ventricular interaction, which changes left ventricular filling and, thus, indirectly influences left ventricular function. At baseline left ventricular end-diastolic pressure of 9.6 +/- 2.1 mm Hg (mean +/- SD), sudden pulmonary artery constriction increased left ventricular peak systolic pressure by 3 +/- 2 mm Hg (2% change), left ventricular stroke volume by 2 +/- 2 ml (8% change), and monoexponential time constant of left ventricular pressure fall during relaxation by 9 +/- 6 msec (22% change). This increased left ventricular relaxation time constant was associated with altered regional segment length changes in the posterior and anterior left ventricular free walls during relaxation. We conclude that systolic direct ventricular interaction affects left ventricular systolic function and relaxation under normal conditions in the intact circulation.

Nonhomogeneous left ventricular regional shortening during acute right ventricular pressure overload

Acute right ventricular pressure overload shifts the interventricular septum leftward and decreases systolic shortening of the left ventricular (LV) septal-lateral diameter. These changes should alter regional shortening in the LV minor axis. To test this hypothesis, LV minor axis circumferential segment lengths of the septum and anterior, lateral, and posterior walls were measured during pulmonary artery or venae caval constriction in seven open-chest dogs with intact pericardia. Starting at an end-diastolic pressure of 10 mm Hg, venae caval constriction decreased LV end-systolic pressure by 19 +/- 6% and stroke volume by 40 +/- 15% and produced uniform decreases in systolic shortening and end-diastolic length around the minor axis. However, during pulmonary artery constriction resulting in similar decreases in end-systolic pressure (22 +/- 7%) and stroke volume (39 +/- 11%), decreases in systolic shortening were significantly larger in the anterior (-34 +/- 10%) and posterior (-33 +/- 21%) walls than in the septum (-10 +/- 9%) or lateral wall (-8 +/- 13%). The mechanisms of these large anterior and posterior shortening decreases differed: anterior end-diastolic length decreased more than posterior and lateral end-diastolic lengths, while posterior end-systolic length decreased less than anterior and lateral end-systolic lengths. Similar changes were seen at starting end-diastolic pressures of 5 and 15 mm Hg. Propranolol did not alter this nonuniform response, while pericardiectomy attenuated the regional variations. Thus, changes in LV geometry during acute right ventricular pressure overload are associated with nonuniform regional changes in systolic shortening in the LV minor axis that are enhanced by the pericardium.

Beat-to-beat regulation of left ventricular function in the intact cardiovascular system

A variety of regulatory mechanisms have evolved to control the heart's pump function because the cardiovascular system must continually adapt to the changing demands that body functions place on it. This regulation takes place through many physiological systems; however, fine adjustments in cardiac pumping probably require adaptations more quickly than external control mechanisms (such as the autonomic nervous system) can compensate. Thus cardiac pumping is also regulated by mechanisms intrinsic to the heart. To better understand these intrinsic control mechanisms, we studied the beat-to-beat response of left ventricular function to continually varying changes in loading conditions produced by transiently occluding the pulmonary artery, venae cavae, and aorta. We used multiple linear regression to identify and quantify the important beat-to-beat determinants of left ventricular systolic function, quantified as stroke work. We could not adequately explain or predict beat-to-beat changes in stroke work with traditional determinants of ventricular function, preload, afterload, and heart rate, because a large systematic error remains after taking these traditional determinants of function into account. To eliminate this systematic error, we had to include some function of previous beat stroke volume and end-systolic size and pressure. This additional information significantly improved both our ability to model the observed transient changes in left ventricular stroke work and to predict additional observations that were not used to develop our model. We conclude that previous beat contraction history is an important determinant of left ventricular function and implies an important regulatory mechanism whereby the left ventricle can fine tune its function from beat to beat in response to continually changing loading conditions.

Direct diastolic ventricular interaction gain measured with sudden hemodynamic transients

Changes in right ventricular volume affect left ventricular function via direct ventricular interaction mediated by the septum, common myocardial fibers in the free wall, and the pericardium, and also via series interaction mediated by changes in right ventricular output reaching the left ventricle through the pulmonary circulation. To study direct interaction, series interaction must be held constant or removed from the experimental preparation. Because there has been no way to directly measure direct ventricular interaction in the intact circulation, we developed a new method to experimentally separate these two components of ventricular interaction by combining abrupt occlusion of both venae cavae and quick withdrawal of 10-15 ml of blood from the right ventricle. This procedure decreased right ventricular end-diastolic pressure (RVEDP) on the next beat without changing pulmonary venous flow, left ventricular end-diastolic segment lengths, or left ventricular systolic function. The direct interaction gains, quantified as delta LVEDP/delta RVEDP, where LVEDP is left ventricular end-diastolic pressure, and delta refers to the change between the beats before and after reducing right ventricular volume, were (means +/- SD) 0.32 +/- 0.32 at steady-state LVEDP = 5 mmHg, 0.38 +/- 0.23 at LVEDP = 10 mmHg, and 0.28 +/- 0.32 at LVEDP = 15 mmHg. These gains were not significantly different (P greater than 0.50). Therefore, we calculated an overall average gain by pooling data from the three base-line LVEDP conditions. This value is 0.33 with 95% confidence interval 0.16-0.51. This 95% confidence interval indicates our data are consistent with many previous reports of diastolic direct interaction.

Multiple linear regression is a useful alternative to traditional analyses of variance

Physiologists often wish to compare the effects of several different treatments on a continuous variable of interest, which requires an analysis of variance. Analysis of variance, as presented in most statistics texts, generally requires that there be no missing data and often that each sample group be the same size. Unfortunately, this requirement is rarely satisfied, and investigators are confronted with the problem of how to analyze data that do not strictly fit the traditional analysis of variance paradigm. One can avoid these pitfalls by recasting the analysis of variance as a multiple linear regression problem. When there are no missing data, the results of a traditional analysis of variance and the corresponding multiple regression problem are identical; when the sample sizes are unequal or there are missing data, one can use a regression formulation to analyze data that cannot be easily handled in a traditional analysis of variance paradigm and thus overcome a practical computational limitation of traditional analysis of variance. In addition to overcoming practical limitations of traditional analysis of variance, the multiple linear regression approach is more efficient because in one run of a statistics routine, not only is the analysis of variance done but also one obtains estimates of the size of the treatment effects (as opposed to just an indication of whether such effects are present or not), and many of the pairwise multiple comparisons are done (they are equivalent to t tests for significance of the regression parameter estimates). Finally, interaction between the different treatment factors is easier to interpret than it is in traditional analysis of variance.

Similar normalized Emax and O2 consumption-pressure-volume area relation in rabbit and dog

The end-systolic pressure-volume relation (ESPVR), a measure of ventricular contractile state, and systolic pressure-volume area (PVA), a primary determinant of cardiac oxygen consumption per beat (VO2), have been derived from the pressure-volume diagram of the cross-circulated dog left ventricle. The slope of the PVA-VO2 relation represents the efficiency of chemomechanical energy transduction of the contractile machinery. To see whether these relationships were similar in other animals, we studied the isovolumic ESPVR and the VO2-PVA relation in nine excised, cross-circulated rabbit left ventricles. The base-line ESPVR was linear (r = 0.94-0.99) with the slope (Emax) and volume-axis intercept (V0) equal to 83.4 +/- 18.3 mmHg/ml and 0.43 +/- 0.17 ml, respectively. When normalized for left ventricular weight, Emax (4.1 +/- 1.1 mmHg.ml-1.100 g) and V0 (8.9 +/- 3.7 ml/100 g) were similar to values reported for the dog left ventricle. The correlation between PVA and VO2 was linear (r = 0.93-1.00), and the slope (1.90 X 10(-5) +/- 0.44 X 10(-5) ml O2.mmHg-1.ml-1) and VO2-axis intercept (0.040 +/- 0.009 ml O2.beat-1.100 g-1) were similar to values found in the dog left ventricle. Hence, despite the greatly different heart size, the base-line contractile state and chemomechanical energy conversion efficiency of the excised, cross-circulated rabbit left ventricle are similar to those of the dog left ventricle.

Accuracy of volume measurement of rabbit left ventricle by balloon method

Although the intraventricular balloon method has been commonly used to measure left ventricular volume of the isolated heart preparation of various animals, the accuracy of the method has not been examined in the ventricle of small animals such as the rabbit. To assess the accuracy of the intraventricular balloon method in measuring left ventricular volume, we measured the space between the balloon inflated in the left ventricle and the endocardial surface in eight Formalin-fixed rabbit left ventricles. The space was only 0.05-0.12 ml (1.5-3.6% of total left ventricular volume) at intraballoon pressures of 40-160 mmHg. However, it increased to 0.37 ml (14.6% of total ventricular volume) with a decrease in pressure to 5 mmHg. We conclude that the intraventricular balloon method accurately measures left ventricular volume in the rabbit, especially in the high pressure range.

Right heart pressure does not equal pericardial pressure in the potassium chloride-arrested canine heart in situ

Recently proposed concepts of pericardial surface pressure, as opposed to liquid pressure, have advanced our understanding of the relationship between pericardial and heart chamber pressures. However, the subsequent suggestion that right heart intracavitary pressure equals, or nearly equals, pericardial surface pressure is not strictly consistent with the physiology of pericardial constraint. If right heart pressure equals pericardial surface pressure, then transmural right heart pressure equals zero. Because of the difficulty in measuring pericardial pressure directly in the beating heart we designed an experiment in the recently arrested canine heart in situ to measure pericardial pressure indirectly and to test the hypothesis that right heart transmural pressure is zero under reasonably physiologic, static equilibrium conditions. According to a static equilibrium analysis of the pressures acting across the walls of the heart, at a given volume the change in right heart pressure caused by removing the pericardium is equal to the pericardial pressure when the pericardium is intact. We found that this drop in pressure caused by pericardiectomy did not equal right heart pressure and therefore that right heart transmural pressure does not equal zero.

Chronic pressure overload hypertrophy decreases direct ventricular interaction

We studied the relative roles of direct (via the interventricular septum) and series (via the pulmonary circulation) ventricular interaction in hearts with concentric left ventricular hypertrophy by using statistical models to analyze the transient responses in right and left ventricular pressures and dimensions to occlusions of the venae cava and pulmonary artery in five open-chest anesthetized dogs. The left ventricles of these dogs had moderate concentric hypertrophy (31% increase in mass) induced by 3 mo of renovascular hypertension [peak left ventricular pressure = 160 +/- 13 (SD) mmHg]. At end diastole we found that direct interaction was only about one-tenth as important as series interaction in determining left ventricular size with the pericardium around the heart. At end systole we found that direct interaction was about one-fifth as important as the end-systolic pressure-volume relationship in determining left ventricular size. Removing the pericardium decreased the importance of direct interaction. Direct interaction is less important in these hearts than in normal hearts, probably because the septum is thicker and, hence, less distensible. This change in the relative importance of direct ventricular interaction with hypertrophy complicates comparison of pressure-volume relationships between normal and hypertrophied hearts.

End-systolic and end-diastolic ventricular interaction

Right ventricular volume affects left ventricular volume via direct interaction across the interventricular septum and series interaction because the right and left hearts are connected in series through the lungs. Because it is difficult to sort out complex physiological mechanisms in the intact circulation, the relative importance of these two effects is unknown. We used statistical analyses of transient changes in left and right ventricular pressures and dimensions following pulmonary artery and venae caval constrictions to separate and quantitate the direct (immediate) from the series (delayed) interaction effects on left ventricular size at end systole and end diastole. With the pericardium closed, direct interaction was one-half as important as series interaction at end diastole and was one-third as important at end systole. With the pericardium removed, direct interaction was one-fifth as important as series interaction at end diastole and one-sixth as important at end systole. These results suggest that differences between transient and steady-state end-systolic pressure-volume relationships are largely explained by direct interaction and that direct end-systolic interaction is important for maintaining balanced right and left heart outputs.

Multiple regression for physiological data analysis: the problem of multicollinearity

Multiple linear regression, in which several predictor variables are related to a response variable, is a powerful statistical tool for gaining quantitative insight into complex in vivo physiological systems. For these insights to be correct, all predictor variables must be uncorrelated. However, in many physiological experiments the predictor variables cannot be precisely controlled and thus change in parallel (i.e., they are highly correlated). There is a redundancy of information about the response, a situation called multicollinearity, that leads to numerical problems in estimating the parameters in regression equations; the parameters are often of incorrect magnitude or sign or have large standard errors. Although multicollinearity can be avoided with good experimental design, not all interesting physiological questions can be studied without encountering multicollinearity. In these cases various ad hoc procedures have been proposed to mitigate multicollinearity. Although many of these procedures are controversial, they can be helpful in applying multiple linear regression to some physiological problems.

The accuracy of inferring left ventricular volume from dimension depends on the frequency of information needed to answer a given question

Left ventricular volume is an important variable in cardiovascular physiology. Because volume cannot be transduced directly, by necessity people measure one or more cardiac dimensions, then implicitly or explicitly estimate volume. Despite its common use, this practice has never been adequately justified. Previous validations of the use of left ventricular dimensions to infer volume have ignored the complexity of the relation between dimensions and volume, particularly over a broad range of hemodynamic states, and have not considered the frequency content of the volume signal needed to answer a given physiological question. We show that, strictly speaking, the assumptions underlying this practice are false. The resulting errors are of little practical consequence under many circumstances, whereas they can be quite important under other circumstances. We consider the frequency content of the volume signal to organize our examples and findings. In this general framework, we find that, for most assessments of volume change from dimension change at or below a frequency of about the heart rate (e.g., beat-to-beat end-diastolic volume), there is no limitation, provided all relevant dimensions are measured. However, for information at frequencies above the heart rate (i.e., rapid changes in volume or dimension), it is probably not possible to draw accurate conclusions about left ventricular volume from dimension data.

Mechanical determinants of transient changes in stroke volume

To better define beat-to-beat regulation of stroke volume (SV), the several-beat transient response of the left ventricle (LV) to sudden changes in hydraulic loading impedance was studied. Data were collected from eight canine isolated heart-lung preparations with controlled LV loading impedance. At a selected diastolic interval, a sudden increase in hydraulic loading resistance was induced. The resulting transient response in SV, end-diastolic pressure (EDP), and end-systolic pressure (ESP) was analyzed by comparing the relative predictive capability of six competing models, each incorporating different degrees of complexity in the relationship between SV and EDP and ESP. The basic model assumed linear LV pressure-volume relationships at both end diastole and end systole. Incorporation of nonlinear, end-state interaction or coronary perfusion pressure effects into the basic model did not improve predictive performance. Models incorporating SV and ESP of the preceding beat as well as ejecting beat ESP and EDP were consistently superior to all other models. The ranking of the relative influence of the determinants of SV was 1) ejecting beat ESP, 2) preceding beat ESP, 3) ejecting beat EDP, and 4) preceding beat SV.

Arterial baroreflex control of heart rate in the horse, pig, and calf

The heart rate (HR) response to stepwise changes in mean arterial pressure (MAP) produced by methoxamine and sodium nitroprusside was studied in conscious horses, pigs, and calves. The respective steady-state arterial baroreflex sensitivities (delta HR/delta MAP expressed as beats min-1 . mm of Hg-1) were -1.20, -1.21, and -0.39 for decreasing MAP and -0.08, -0.91, and -0.56 for increasing MAP. After parasympathetic nervous system (PNS) blockade with atropine, the reflex sensitivity to decreasing MAP in horses and the pigs decreased to -0.62 and -0.65, respectively (P less than 0.05), whereas the sensitivity in calves was not significantly different from the calves' base-line sensitivity (P greater than 0.05). In pigs, the sensitivity to increasing MAP decreased to -0.43 after PNS blockade (P less than 0.05). Horses and calves had a response to small increases in MAP similar to that observed in the absence of PNS blockade. There was no additional change in HR with larger increases in MAP in these 2 species. These responses indicate that varying degrees of mixed PNS and sympathetic nervous system activity changes mediate the baroreflex change in HR seen for increasing and decreasing MAP in pigs and for decreasing MAP in horses. Predominantly PNS activity is responsible for the response to increasing MAP in the horses and calves, whereas predominantly sympathetic nervous system activity is responsible for the response to decreasing MAP in calves. Comparisons were made between the observations in these 3 species and other mammalian species.