Validity of myocardial oxygen consumption parameters

Impact of baseline coronary flow and its distribution on fractional flow reserve prediction

Model-based prediction of fractional flow reserve (FFR) in the context of stable coronary artery disease (CAD) diagnosis requires a number of modelling assumptions. One of these assumptions is the definition of a baseline coronary flow, ie, total coronary flow at rest prior to the administration of drugs needed to perform invasive measurements. Here we explore the impact of several methods available in the literature to estimate and distribute baseline coronary flow on FFR predictions obtained with a reduced-order model. We consider 63 patients with suspected stable CAD, for a total of 105 invasive FFR measurements. First, we improve a reduced-order model with respect to previous results and validate its performance versus results obtained with a 3D model. Next, we assess the impact of a wide range of methods to impose and distribute baseline coronary flow on FFR prediction, which proved to have a significant impact on diagnostic performance. However, none of the proposed methods resulted in a significant improvement of prediction error standard deviation. Finally, we show that intrinsic uncertainties related to stenosis geometry and the effect of hyperemic inducing drugs have to be addressed in order to improve FFR prediction accuracy.

Blood Biomarkers Predict Cardiac Workload Using Machine Learning

Introduction

Rate pressure product (the product of heart rate and systolic blood pressure) is a measure of cardiac workload. Resting rate pressure product (rRPP) varies from one individual to the next, but its biochemical/cellular phenotype remains unknown. This study determined the degree to which an individual's biochemical/cellular profile as characterized by a standard blood panel is predictive of rRPP, as well the importance of each blood biomarker in this prediction.

Methods

We included data from 55,730 participants in this study with complete rRPP measurements and concurrently collected blood panel information from the Health Management Centre at the Affiliated Hospital of Hangzhou Normal University. We used the XGBoost machine learning algorithm to train a tree-based model and then assessed its accuracy on an independent portion of the dataset and then compared its performance against a standard linear regression technique. We further determined the predictive importance of each feature in the blood panel.

Results

We found a fair positive correlation (Pearson r) of 0.377 (95% CI: 0.375-0.378) between observed rRPP and rRPP predicted from blood biomarkers. By comparison, the performance for standard linear regression was 0.352 (95% CI: 0.351-0.354). The top three predictors in this model were glucose concentration, total protein concentration, and neutrophil count. Discussion/

Conclusion

Blood biomarkers predict resting RPP when modeled in combination with one another; such models are valuable for studying the complex interrelations between resting cardiac workload and one's biochemical/cellular phenotype.

Autoregulation of Coronary Blood Supply in Response to Demand: JACC Review Topic of the Week

Although our coronary circulation evolved to meet demands during marked physical exertion for "fight or flight" survival, complex and multilayered control mechanisms reduce flow during other periods. Understanding homeostasis of resting flow provides essential insights into clinical pathophysiology. Several homeostatic mechanisms (myogenic, metabolic, endothelial, and neural) maintain sufficient baseline flow regardless of driving pressure (in aggregate, "autoregulation"). As a result, ventricular dysfunction does not arise until coronary perfusion pressure decreases to ∼40 mm Hg. Straightforward clinical parameters explain approximately one-half of observed absolute resting perfusion but with wide imprecision. Resting perfusion does not associate with clinical outcomes and remains unaffected by revascularization, recovery after myocardial infarction, and treating severe aortic stenosis, thereby supporting the notion that the heart was designed for peak performance.

Coronary Microcirculation in Aortic Stenosis: Pathophysiology, Invasive Assessment, and Future Directions

With the increasing prevalence of aortic stenosis (AS) due to a growing elderly population, a proper understanding of its physiology is paramount to guide therapy and define severity. A better understanding of the microvasculature in AS could improve clinical care by predicting left ventricular remodeling or anticipate the interplay between epicardial stenosis and myocardial dysfunction. In this review, we combine five decades of literature regarding microvascular, coronary, and aortic valve physiology with emerging insights from newly developed invasive tools for quantifying microcirculatory function. Furthermore, we describe the coupling between microcirculation and epicardial stenosis, which is currently under investigation in several randomized trials enrolling subjects with concomitant AS and coronary disease. To clarify the physiology explained previously, we present two instructive cases with invasive pressure measurements quantifying coexisting valve and coronary stenoses. Finally, we pose open clinical and research questions whose answers would further expand our knowledge of microvascular dysfunction in AS. These trials were registered with NCT03042104, NCT03094143, and NCT02436655.

Cardiac workload in patients with sleep-disordered breathing early after acute myocardial infarction

BACKGROUND

Sleep-disordered breathing (SDB) may promote an increase in cardiac workload early after acute myocardial infarction (AMI). We tested the hypothesis that in the early phase after AMI, SDB is associated with increased 24-h arterial BP, heart rate (HR), and, thus, cardiac workload.

METHODS

In this prospective study, 55 consecutive patients with AMI and subsequent percutaneous coronary intervention (78% men; mean age, 54 ± 10 y; mean BMI, 28.3 ± 3.6 kg/m²; mean left ventricular ejection fraction [LVEF], 47% ± 8%) underwent polysomnography and 24-h ambulatory BP and heart rate monitoring within 5 days after MI. Cardiac workload was calculated as systolic BP multiplied by HR. The presence of SDB was defined as ≥ 10 apneas and hypopneas per hour of sleep.

RESULTS

Fifty-five percent of the patients had SDB, of which 40% was predominantly central in nature. Patients with SDB had higher 24-h HR and systolic and diastolic BP compared with those without SDB (115 vs 108 mm Hg, P = .029; 71 vs 67 mm Hg, P = .034; 69 vs 64 beats/min, P = .050, respectively). Use of antihypertensive medication and β-receptor blockers was similar in both groups. In a multivariate linear regression analysis, SDB was significantly associated with an increased 24-h cardiac workload (β-coefficient, 0.364; 95% CI, 0.071-0.657; P = .016), independently of age, sex, BMI, LVEF, and antihypertensive medication.

CONCLUSION

Patients with AMI and SDB have significantly increased 24-h BP, HR, and cardiac workload. Treatment of SDB may be a valuable nonpharmacologic approach to lower cardiac workload in these patients.

Changes in cardiac physiology after severe burn injury

Cardiac stress, mediated by increased catecholamines, is the hallmark of severe burn injury typified by marked tachycardia, increased myocardial oxygen consumption, and increased cardiac output (CO). It remains one of the main determinants of survival in large burns. Currently, it is unknown for how long cardiac stress persists after a severe injury. Therefore, the aim of this study was to determine the extent and duration of cardiac stress after a severe burn. To determine persistence of cardiac alteration, the authors determined cardiac parameters of all surviving patients with burns ≥ 40% TBSA from 1998 to 2008. One hundred ninety-four patients were included in this study. Heart rate, mean arterial pressure, CO, stroke volume, cardiac index, and ejection fractions were measured at regular intervals from admission up to 2 years after injury. Rate pressure product was calculated as a correlate of myocardial oxygen consumption. All values were compared with normal nonburned children to validate the findings. Statistical analysis was performed using log transformed analysis of variance with Bonferroni correction and Student's t-test, where applicable. Heart rate, CO, cardiac index, and rate pressure product remained significantly increased in burned children for up to 2 years when compared with normal ranges (P < .05), indicating vastly increased cardiac stress. Ejection fraction was within normal limits for 2 years. Cardiac stress persists for at least 2 years postburn, and the authors suggest that attenuation of these detrimental responses may improve long-term morbidity.

Obstructive sleep apnea leads to transient uncoupling of coronary blood flow and myocardial work in humans

STUDY OBJECTIVES

Obstructive Sleep Apnea (OSA) is associated with a poor prognosis in patients with coronary artery disease. We hypothesized that abnormalities of coronary blood flow (CBF) associated with obstructive apneas may predispose patients to ischemia. We aimed to determine CBF during respiratory events in patients with OSA.

SETTING

University Hospital.

PATIENTS

Ten subjects undergoing elective percutaneous coronary intervention

DESIGN

We measured CBF and myocardial work (rate-pressure product [RPP]) in a non-culprit coronary artery in patients sleeping in the cardiac catheterization laboratory. Hemodynamic responses were matched to spontaneously occurring respiratory events.

MEASUREMENTS AND RESULTS

Events comprised a mixture of obstructive apneas, central apneas and hypopneas. RPP increased at the termination of each type of respiratory event. Following the rise in RPP, there was a delay, identified with breakpoint analysis, before CBF began to increase (P<0.001) that differed in duration with event type: 8 sec for obstructive apnea, 5 sec for central apnea, and 4 sec for hypopnea. The delay in CBF with obstructive apnea was associated with an increase in coronary vascular resistance of 16% +/- 4% (P < 0.05). Stepwise multilinear regression analysis showed the increase in CBF was predicted by the rise in RPP (R=0.52, P<0.001) and presence of arousal from sleep (R=0.30, P<0.05), but not the degree of O2 desaturation.

CONCLUSION

Following obstructive apneas there is a transient uncoupling of CBF from myocardial work and an increase in CVR. This disturbed flow-metabolic coupling may lead to nocturnal myocardial ischemia in patients with both OSA and coronary artery disease.

Coronary blood flow becomes uncoupled from myocardial work during obstructive sleep apnea in the presence of endothelial dysfunction

STUDY OBJECTIVES

Patients with obstructive sleep apnea (OSA) and coronary artery disease have a poor long-term prognosis. It is unknown whether the coronary blood flow (CBF) response to OSA is appropriate for myocardial metabolic requirements. Therefore, CBF was assessed during OSA, before and after the development of coronary artery endothelial dysfunction.

SETTING

University Hospital Animal Laboratory.

PATIENTS OR PARTICIPANTS

Newborn lambs.

INTERVENTIONS

Lambs were surgically instrumented for invasive hemodynamic monitoring and sleep-wake EEG recordings. A tracheostomy was inserted to control the upper airway and model OSA during sleep. Coronary artery endothelial dysfunction was created using infusions of lipopolysaccharide (LPS). The CBF response during OSA was assessed and compared to changes in myocardial work (rate-pressure product [RPP]), O2 saturation, and cortical arousal, before and after the LPS infusions.

MEASUREMENTS AND RESULTS

During OSA, CBF increased by 8.6% +/- 2.4% above baseline in the pre-LPS condition and 8.8% +/- 1.9% post-LPS, peaking following termination of the respiratory event. Pre-LPS, change in CBF post-apnea was independently correlated with change in RPP (R2 = 0.50), minimum SpO2 (R2 = 0.11) and the presence of cortical arousal (R2 = 0.04) (P < 0.01, forward stepwise regression analysis). Following LPS, the only predictor of CBF was degree of O2 desaturation (R2 = 0.14, P < 0.05).

CONCLUSION

Under baseline conditions, CBF correlates well with myocardial work following the termination of apnea in lambs. After the creation of coronary artery endothelial dysfunction with LPS, there is uncoupling of the normal CBF-myocardial work relationship.

In vivo hemodynamic responses to thoracic artificial lung attachment

A thoracic artificial lung (TAL) was attached to the pulmonary circulation in a porcine model. Proximal main pulmonary artery (PA) blood flow, in part or whole, was diverted to the TAL, and TAL outlet blood flow was split between the distal main PA and left atrium (LA). The right ventricle (RV) drove blood flow through the combined TAL/natural lung (NL) pulmonary system. Selective banding placed the TAL in parallel with the NLs, in series with the NLs, or in an intermediary hybrid configuration. Parallel TAL attachment lowered pulmonary system impedance, raised cardiac output (CO), and provided the greatest TAL blood flow rate, but reduced the NL blood flow rate which is important for pulmonary embolic clearance and metabolic blood processing. Hybrid or series TAL attachment raised pulmonary system impedance, lowered CO, increased RV oxygen consumption, and reduced RV oxygen supply. Redesign of the PA anastomoses, TAL inlet graft, and TAL entrance and exit would significantly improve hemodynamics and RV function with TAL attachment. Mean LA pressure increased throughout the experiment, which may indicate damage caused by graft attachment to the LA. Pulmonary resistance-flow rate curves may enable clinical prediction of tolerable TAL attachment configurations.

Coronary artery flow velocity is related to lumen area and regional left ventricular mass

BACKGROUND

Coronary flow velocity varies widely between individuals, even at rest. Because of this variation, indices with less apparent deviation, such as the ratio of hyperemic to resting velocity (coronary flow reserve), have been more commonly studied. We tested the hypothesis that the flow continuity principle could be used to model resting coronary flow, and we examined the resulting velocity relationship.

METHODS AND RESULTS

We studied coronary velocity in 59 patients using a Doppler wire to measure resting and hyperemic average peak velocities in the left anterior descending artery. Quantitative techniques were used to calculate lumen cross-sectional area and the lengths of all distal coronary branches. Branch lengths were used to estimate regional left ventricular mass. We then calculated the ratio of lumen area to regional mass (A/m). Regional perfusion was estimated from the double product of heart rate and systolic blood pressure. Resting velocity (V) varied inversely with A/m ratio [V=46.5/(A/m); r=0.68, P<0.001]. Disease in the left anterior descending artery was categorized as none or luminal irregularities only (n=22), mild (n=15), or moderate (n=22). The A/m ratio declined across these groups (8.7+/-4.0, 8.5+/-6.2, and 5. 6+/-3.0 mm(2)/100 g, respectively; P<0.04), and the resting average peak velocity increased (27+/-16, 33+/-11, and 37+/-20 cm/s, respectively; P=0.06).

CONCLUSIONS

Resting coronary artery flow velocity is inversely related to the ratio of lumen area to regional left ventricular mass. Higher resting velocities are found when insufficient lumen size exists for the distal myocardial bed, as occurs with diffuse mild or moderate coronary atherosclerosis.

Cardioprotective effects of the Na(+)/H(+) exchange inhibitor cariporide in patients with acute anterior myocardial infarction undergoing direct PTCA

BACKGROUND

Activation of Na(+)/H(+) exchange in myocardial ischemia and/or reperfusion leads to calcium overload and myocardial injury. Experimental studies have shown that Na(+)/H(+) exchange inhibitors can attenuate Ca(2+) influx into cardiomyocytes. We therefore performed a multicenter, randomized, placebo-controlled clinical trial to test the hypothesis that inhibition of Na(+)/H(+) exchange limits infarct size and improves myocardial function in patients with acute anterior myocardial infarction (MI) treated with direct PTCA.

METHODS AND RESULTS

One hundred patients were randomized to receive placebo (n=51) or a 40-mg intravenous bolus of the Na(+)/H(+) exchange inhibitor cariporide (HOE 642) (n=49) before reperfusion. Global and regional left ventricular functions were analyzed by use of paired contrast left ventriculograms performed before and 21 days after PTCA and myocardial enzymes (ie, creatine kinase ¿CK, CK-MB, and LDH) as markers for myocardial tissue injury were evaluated. At follow-up, the ejection fraction was higher (50% versus 40%; P<0.05) and the end-systolic volume was lower (69.0 versus 97.0 mL; P<0.05) in the cariporide group. Significant improvements in some indices of regional wall motion abnormalities were observed, such as the percentage of chords with hypokinesis < -2 SD (P=0.045) and the severity of hypokinesis in the border zone of the infarct region (P=0.052). In addition, CK, CK-MB, or LDH release was significantly reduced in the cariporide patients.

CONCLUSIONS

Our findings suggest that inhibition of Na(+)/H(+) exchange by cariporide may attenuate reperfusion injury and thereby improve the recovery from left ventricular dysfunction after MI.

Ultrastructural Evaluation of Postischemic Cell Death (Lethal Reperfusion Injury) in Porcine Hearts

This study investigated whether reperfusion results in an increase of ultrastructurally determined myocardial injury in pig hearts. The left anterior descending coronary artery (LAD) was distally occluded in 12 pigs for 35-45 minutes and then reperfused for 3 hours. At the end of ischemia, as well as after 3 hours of reperfusion, one transmural biopsy was removed from the center of the risk region and subdivided into four-specimens, representing the subendocardial (I), subendo-midmyocardial (II), subepi-midmyocardial (III), and subepicardial layers (IV). The degree of injury was assessed by electronmicroscopy and was scored as reversible (1), an almost equal mixture of reversible and irreversible (2), and totally irreversible (3) damage. In addition, infarct size was determined as the ratio of infarcted (tetrazolium stain) to ischemic (dye technique) myocardium. Infarct sizes ranged from 29.3% to 93% (mean 61.2%). The scores of injury of the four tissue layers before and after reperfusion did not differ significantly: layer I, 2.4 +/- 0.8/2.3 +/- 0.9; layer II, 2.2 +/- 0.9/2.0 +/- 0.9; layer III, 1.8 +/- 0.9/2.0 +/- 0.9; and layer IV, 1.6 +/- 0.9/1.3 +/- 0.6. The means of the four layers were almost identical at the end of ischemia (2.1 +/- 0.8) and after 3 hours of reperfusion (2.0 +/- 0.6). A linear regression analysis with 95% confidence limits of the score values before and after reperfusion indicated that maximally 25% of a mean final infarct size of about 50% may be due to lethal reperfusion injury. This study suggests that cell death in regional ischemia and reperfusion occurs predominantly during ischemia and not during reperfusion.

Growth hormone treatment in pediatric burns: a safe therapeutic approach

OBJECTIVE

To determine the safety and efficacy of recombinant human growth hormone (rhGH) in the treatment of children who are severely burned.

SUMMARY BACKGROUND DATA

During the last decade, we have used recombinant human growth hormone (rhGH; 0.2 mg/kg/day s.q.) to successfully treat 130 children with more than 40% total body surface area (TBSA) burns to enhance wound healing and decrease protein loss. A significant increase in the mortality of adult patients in the intensive care unit who were given rhGH has recently been reported in two large European trials which questions the therapeutic safety of rhGH.

METHODS

The records of 263 children who were burned were reviewed. Patients receiving either rhGH at 0.2 mg/kg/day subcutaneously as part of a randomized clinical trial (n = 48) or therapeutically (n = 82) were compared with randomized placebo-administered controls (n = 54), contiguous matched controls (n = 48), and matched patients admitted after August 1997, after which no patients were treated with rhGH (n = 31). Morbidity and mortality, which might be altered by rhGH therapy, were considered with specific attention to organ function or failure, infection, hemodynamics, and calcium, phosphorous, and albumin balance.

RESULTS

A 2% mortality was observed in both rhGH and saline placebo groups in the controlled studies, with no differences in septic complications, organ dysfunction, or heart rate pressure product identified. In addition, no difference in mortality could be shown for those given rhGH therapeutically versus their controls. No patient deaths were attributed to rhGH in autopsies reviewed by observers blinded to treatment. Hyperglycemic episodes and exogenous insulin requirements were higher among rhGH recipients, whereas exogenous albumin requirements and the development of hypocalcemia was reduced.

CONCLUSIONS

Data indicate that rhGH used in the treatment of children who were severely burned is safe and efficacious.

MK-801 and memantine inhibit a centrally induced increase in myocardial oxygen demand in rabbits

Electrical stimulation of the paraventricular nucleus of the hypothalamus in the anaesthetized rabbit induces an increase in indexes of myocardial oxygen demand. This increase in myocardial oxygen demand is due to the activation of sympathetic pathways which include glutamatergic relays. In this model, systemic injection of dizolcipine (MK-801) and memantine inhibited these responses. Because these drugs have only one pharmacological property in common i.e. blockade of the NMDA receptor channel complex, these results fit with our previous results concerning the possible involvement of NMDA receptors in the central control of sympathetic activation. Memantine appears to be an interesting prototype for centrally acting cardioprotective drugs devoid of serious side effects.

Evidence for a spinal origin of the effect of baclofen on the myocardial oxygen demand indexes

In a previous study in anaesthetized rabbits we observed that electrical stimulation of the hypothalamic paraventricular nucleus (PVN) elicited substantial rises in the maximum rate of change of left ventricular pressure (dP/dtmax) and in myocardial oxygen demand indexes (rate-pressure product and triple product), similar to the changes observed during stress or physical effort. Baclofen, a selective GABA(B) receptor agonist, injected intravenously prevented these responses. In the present study, we show that low doses of baclofen (0.1, 0.3 and 1 microgram/kg), injected intrathecally (i.t.) at the T9 level, reduced the myocardial oxygen demand during PVN stimulation. After 0.3 microgram/kg baclofen i.t., the peak value of the triple product during stimulation was 140 +/- 20 compared with 193 +/- 20 before treatment. An i.t. injection (500 micrograms/kg), of saclofen a selective GABA(B) receptor antagonist, did not modify the resting haemodynamics significantly but attenuated the inhibitory effects of baclofen (3 mg/kg i.v.). These results suggest that the main site of the effects of baclofen is located within the spinal cord and that GABA(B) receptors probably mediate these effects by modulating the central control of cardiac function. In conclusion, baclofen might be a useful tool to prevent the centrally evoked increases of myocardial oxygen demand.

Myocardial protection by Na(+)-H+ exchange inhibition in ischemic, reperfused porcine hearts

BACKGROUND

Studies in isolated myocytes and isolated heart preparations have suggested that Na(+)-H+ exchange is an important mechanism for myocardial ischemia-reperfusion injury. This study was undertaken to determine whether inhibition of Na(+)-H+ exchange limits infarct size and improves regional systolic shortening in regional ischemia and reperfusion in intact pigs.

METHODS AND RESULTS

The left anterior descending coronary artery was occluded in 18 anesthetized and thoracotomized pigs for 45 minutes and then reperfused for 24 hours. As main end points of this study, regional systolic shortening (sonomicrometry) and infarct size (percentage of infarcted to ischemic myocardium) were determined at the end of the experiments. Infarcted myocardium was assessed by histochemistry (tetrazolium stain) and by quantitative histology of one heart slice. The Na(+)-H+ exchange inhibitor Hoe 694 was administered intravenously at a dose of 3 mg/kg in 6 pigs each either 10 minutes before ischemia (group A) or 10 minutes before the onset of reperfusion (group B). Six pigs served as controls (group C). Treatment with Hoe 694 before ischemia decreased histochemical infarct size from 65 +/- 18% (control group) to 13 +/- 8% (P < .01) and histological infarct size from 49 +/- 20% (control group) to 14 +/- 4% (P < .01). Histochemical (55 +/- 19%) and histological (42 +/- 15%) infarct sizes of group B were insignificantly reduced by 15%. Myocardial protection in group A was associated with an attenuated contracture after 10 minutes of reperfusion and an improved regional systolic shortening after 24 hours of reperfusion (group A, 25 +/- 12%; control group, 6 +/- 5%; P = .01). These parameters remained unaffected in group B.

CONCLUSIONS

This study clearly demonstrates that Na(+)-H+ exchange is an important mechanism for cell death in myocardial ischemia and reperfusion in intact pigs; thus, inhibition of this exchange system may prove a promising new strategy in the clinical treatment of myocardial ischemia and reperfusion.

Functional effects of adrenochrome in isolated rabbit heart

The cardiotoxic effects of catecholamines have been explained in part by the generation of oxygen free radicals and aminochromes. The role of aminochromes remains however controversial. It has previously been demonstrated that adrenochrome, an oxidation product of adrenaline, shows cardiotoxic properties only at very high concentrations, and it has been suggested that the deleterious effects observed may be caused by a worsening in myocardial perfusion. The functional properties of adrenochrome were examined in isolated spontaneously-beating rabbit hearts with depleted catecholamine stores (reserpin 7.0 mg/kg 16-24 hr before preparation, Langendorff, constant pressure: 70 cm H2O, Tyrode solution, [Ca++]sol. 1.8 mmol/l, 37 degrees). Cumulative concentration-response curves show an adrenochrome-concentration-dependent increase of contractility (left ventricular pressure, EC50 = 3.6 x 10(-6) M; +dp/dtmax, EC50 = 1.6 x 10(-5) M), whereas myocardial relaxation was impaired (-dp/dtmax, EC50 = 2.6 x 10(-5) M; -dp/dtmax/+dp/dtmax = 0.68 at 10(-4) M). Heart-rate was only slightly enhanced (+10% at 10(-4) M), and the coronary flow was markedly influenced only by adrenochrome 10(-4) M (-17%). The relative coronary flow (= global coronary flow/pressure-rate product) was concentration-dependently reduced (EC50 = 10(-5) M; -49% at 10(-4) M). We conclude that in isolated rabbit hearts, adrenochrome has a positive inotropic action but impairs myocardial relaxation, and coronary constrictor activity prevents an increase of myocardial oxygen supply, thus worsening myocardial oxygen-demand/supply balance.

Inhibition of the centrally induced increases in myocardial oxygen demand in rabbits by chronic treatment with baclofen, a selective GABAB agonist

1. A previous study from our group demonstrated that neurones of the paraventricular nucleus of the hypothalamus (PVN) are selectively involved in the central control of the cardiac function. Moreover, in that study, it was shown that baclofen, a selective GABAB receptor agonist, is capable of modulating the increases in myocardial contractility and oxygen demand evoked by electrical or pharmacological stimulation of the PVN. Nevertheless, the acute administration of this compound was frequently accompanied by a cardiodepressant effect. 2. In the present study, the effects of a long term treatment (14 days) with baclofen (3 or 10 mg kg-1, i.p.) have been examined on the excitatory haemodynamic responses evoked by central pharmacological stimulation in anaesthetized rabbits. 3. The i.c.v. injection of L-glutamate (3 mg kg-1) induced marked increases in dP/dtmax (32%), mean arterial pressure (39%) and on two indices of myocardial oxygen consumption: the rate-pressure product (34%) and the triple product (78%). 4. Baclofen blunted the positive inotropic response and the increases in myocardial oxygen consumption induced by L-glutamate in a dose-related manner. The higher dose of baclofen (10 mg kg-1, i.p.), reduced by more than 50% these excitatory effects of L-glutamate without eliciting any significant negative effect on basal haemodynamics. The same doses of baclofen were not able to blunt the hypertensive response induced by central stimulation. 5. These results confirm and extend our previous findings suggesting that it is possible to discriminate the central control of vasomotor tone from that of cardiac function and also that baclofen can modulate the latter. It is concluded that when given chronically, baclofen modulates the increases in myocardial oxygen demand induced by activation of the central nervous system in doses which do not depress the resting cardiac function.

Effects of different inotropes with antioxidant properties on acute regional myocardial ischemia in isolated rabbit hearts

1. The antiischemic properties of the flavonoids acetylvitexin-rhamnoside (AVR) and luteolin-7-glucoside-(LUT), combining phosphodiesterase (PDE)-inhibitory and antioxidant properties, were studied in comparison to amrinone (AMR) or superoxide dismutase (SOD). The effects of the new dihydropyridine-type calcium-agonist Bay T 5006 were studied in comparison to Bay K 8644. 2. In isolated Langendorff-rabbit hearts perfused at constant pressure, acute regional ischemia (MI) was induced by coronary artery occlusion (CAO) and quantitated from epicardial NADH-fluorescence photography. Drugs were applied either before or after CAO (pre-treatment or treatment) to permit distinguishing the influence of functional and direct cytoprotective actions in the poorly collateralized rabbit hearts. 3. SOD did not affect left ventricular pressure (LVP) or coronary flow (CF) and reduced MI only if applied before CAO. LVP and CF were enhanced by LUT or AMR but not by AVR. MI was reduced to a similar extent in hearts treated with either drug. Cardioprotection by LUT was not improved by starting drug application before CAO. 4. Bay K 8644 reduced LVP and particularly CF, whereas Bay T 5006 did not affect functional parameters. MI was enlarged by Bay K 8644 and remained unaffected by treatment or pretreatment with Bay T 5006. 5. AMR, LUT and AVR possess antiischemic properties related to an improvement of myocardial perfusion. Although oxygen free radicals contribute to ischemic tissue injury, as shown by the cardioprotective effectiveness of SOD, antioxidant properties of the flavonoids LUT and AVR do not seem to be relevant for the antiischemic effects. Our findings also give no evidence for antioxidant properties of dihydropyridines relevant for cardioprotection.

Functional and antiischemic effects of luteolin-7-glucoside in isolated rabbit hearts

1. The functional effects of the flavonoid luteolin-7-glucoside (LUT) were investigated in Langendorff-rabbit hearts perfused at constant pressure. Repetitive myocardial ischemia was induced by coronary artery ligature and quantified from NADH-fluorescence photography. 2. LUT significantly enhanced left ventricular pressure and the global and relative coronary flow (= global coronary flow/pressure-rate product). 3. LUT significantly diminished epicardial NADH-fluorescence area and intensity. 4. LUT is an inodilator possessing cardioprotective properties. These might be related to an improvement of myocardial perfusion and/or to free radical scavenging properties.

Cardiotoxicity of adrenochrome in isolated rabbit hearts assessed by epicardial NADH fluorescence

Noradrenaline in a micromolar concentration has recently been shown to contribute to ischemic tissue injury by direct cardiotoxic effects independent of functional alterations. Oxygen free radicals, generated during the auto-oxidation of catecholamines, are important mediators of catecholamine cardiotoxicity. However, the role of the oxidative products (aminochromes) is still unclear. We examined the effects of adrenochrome on functional parameters and on regional myocardial ischemia (MI) in isolated electrically-driven rabbit hearts with depleted catecholamine stores (reserpine 7.0 mg/kg i.p. 16-24 h before preparation, Langendorff, constant pressure: 70 cm H2O, Tyrode solution, Ca++ 1.8 mmol/l, 37 degrees C). Repetitive MI, separated by a reperfusion period of 50 min, was induced by coronary artery branch ligature, and MI was quantitated from epicardial NADH fluorescence photography. Adrenochrome-treatment (10(-6) M or 10(-4) M) was started after a reperfusion period of 20 min. The left ventricular pressure (LVP) was significantly enhanced by adrenochrome (p < 0.05), but it fell thereafter to below its initial value in hearts treated with adrenochrome 10(-4) M. The global coronary flow (CF) was not affected by adrenochrome 10(-6) M (P > 0.05), but it was significantly decreased by adrenochrome 10(-4) M (P < 0.05). The relative CF (= CF/LVP x heart-rate) was numerically decreased by adrenochrome 10(-6) M (p > 0.05) and more markedly by adrenochrome 10(-4) M (p < 0.05). Whereas epicardial NADH fluorescence was similar after repetitive coronary artery occlusions in controls and in hearts treated with adrenochrome 10(-6) M (p > 0.05), it was significantly enhanced by adrenochrome 10(-4) M (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative comparison of functional and anti-ischaemic effects of the phosphodiesterase-inhibitors, amrinone, milrinone and levosimendan in rabbit isolated hearts

1. The functional and anti-ischaemic effects of the phosphodiesterase (PDE)-inhibitors, amrinone, milrinone and levosimendan, a new agent combining PDE-inhibitory with calcium-sensitizing properties, were investigated in rabbit isolated hearts (Langendorff, constant pressure: 70 cmH2O, Tyrode solution, Ca2+ 1.8 mmol l-1, 37 degrees C). Anti-ischaemic effects were studied in electrically-driven hearts (200 beats min-1). Acute regional ischaemia was induced by ligature of a branch of the circumflex coronary artery and quantified from epicardial NADH-fluorescence photography. 2. Cumulative concentration-response curves in spontaneously beating hearts in the presence of isoprenaline (10(-10) M), showed a higher inotropic and coronary vasodilator potency for levosimendan (EC50: 7 x 10(-7) M) compared to milrinone (EC50: 7.7 x 10(-6) M) or amrinone (EC50: 2 x 10(-5) M). Although the maximal coronary dilator activity was similar for the three agents, the maximal inotropic and chronotropic effects were lower for levosimendan than for amrinone or milrinone (P < 0.05). 3. In regionally ischaemic hearts, milrinone (10(-5) M) or levosimendan (5 x 10(-6) M) similarly enhanced the left ventricular pressure (+15-20%) (P < 0.05) and the global coronary flow (+40-50%) (P < 0.05). The epicardial NADH-fluorescence area was significantly diminished by milrinone or levosimendan (-20-30%) (P < 0.05) and there was no significant difference between the anti-ischaemic effects of either agent (P > 0.05). 4. It is concluded that amrinone and milrinone possess similar functional profiles in rabbit isolated hearts and a higher inotropic and chronotropic efficacy than levosimendan. At functionally equieffective concentrations, milrinone and levosimendan show similar anti-ischaemic effects, related to an improvement of myocardial perfusion. The calcium-sensitizing properties seem not to be relevant for cardioprotection by levosimendan at the concentration used.

An approach to ventricular efficiency by use of carbon 11-labeled acetate and positron emission tomography

BACKGROUND

Positron emission tomography-derived 11C-labeled acetate kinetics have been shown to reflect myocardial oxidative metabolism. The objective of the study was to use this metabolic imaging technique in combination with an evaluation of left ventricular work as an index of ventricular mechanical efficiency.

METHODS AND RESULTS

The effects of ventricular ejection fraction and loading on this index were studied quantitatively in a canine experimental model. There was a curvilinear relationship between efficiency and the end-diastolic volume per unit mass (r = 0.84), which appeared to integrate the main determinants of left ventricular mechanical performance successfully and allowed the detection of a decreased ventricular efficiency in acute experimental heart failure.

CONCLUSIONS

This approach appears to have the potential to assess the energetic working point of the ventricle in clinical heart disease and follow the effects of therapy. The data demonstrate the feasibility of an estimate of ventricular efficiency that relies on noninvasive data-acquisition techniques.

Cardiac efficiency

Efficiency is defined as the ratio of the energy delivered by a system to the energy supplied to it. Depending on the particular question being addressed, there exist a plethora of definitions of efficiency in medical texts, thus hampering their comparison. If only the ventricular work seen by the arterial system is under investigation, pressure-volume work will serve as a useful numerator. If, on the other hand, external and internal work together, i.e. the total mechanical work, is of interest, the pressure-volume area might be employed. Total myocardial oxygen consumption (MVO2) will be a useful denominator in the case of aerobic energy production. The MVO2 for the unloaded contraction must be assessed if, as in other energy transfer systems, net efficiency is to be addressed. If even smaller steps in the chain of energy transfer are to be investigated MVO2 for the arrested heart must be assessed. With an appropriate therapy, hemodynamic determinants can be varied, to improve cardiac efficiency. Nonetheless, measurement of all variables necessary for the calculation of efficiency remains a challenge, in particular in the clinical setting. Separation of the direct effects of drugs on efficiency is even more difficult, since hemodynamic conditions can hardly be controlled throughout the observation period, and changes in efficiency might be secondary to changes in hemodynamics. Whether the heart by itself employs mechanisms to improve its efficiency is still a matter of discussion: there is evidence that when oxygen supply decreases, the heart can switch from one substrate to a less costly one, or possibly can improve efficiency through better use of oxygen. Moreover, the heart seems to "sense" an even more decreased oxygen supply and reduce function in response. Myocardial stunning could be regarded as a protective mechanism as well, with function remaining depressed and the oxygen supply being normal or close to normal. One may conclude from the decreased efficiency that the excess oxygen consumption is used up for repair processes. The improved efficiency found in hypertrophied hearts represents another adaptive process. The underlying mechanism is unclear: a shift towards isomyosin V3 or some undefined shift in metabolic pathway is discussed. It is also still a moot question towards which objective the efficiency of the heart is adjusted. It has been described that under physiologic conditions, the efficiency of both the left and the right ventricle ought to be maximized.(ABSTRACT TRUNCATED AT 400 WORDS)

Prevention by NMDA receptor antagonists of the centrally-evoked increases of cardiac inotropic responses in rabbits

1. The purpose of this study was to investigate further the role of the excitatory amino acid (EAA) system of neurotransmission, particularly of the NMDA receptor, in the central regulation of cardiac function. 2. Electrical stimulation of the paraventricular nucleus of the hypothalamus (PVN) in pentobarbitone anaesthetized rabbits induced a cardiovascular response mainly characterized by a positive inotropic effect, hypertension and a marked increase in the myocardial oxygen demand index. 3. The intracerebroventricular (i.c.v.) or intravenous (i.v.) injection of different EAA antagonists acting on different sites of the NMDA receptor/channel complex dose-dependently blunted the excitatory cardiovascular effects of PVN stimulation. 4. 5,7 Dichlorokynurenic acid was used as a specific glycine site antagonist and 2-amino-5-phosphonovaleric acid was used to block the agonist recognition site; ketamine was used as a channel blocker site antagonist and ifenprodil as a blocker of the polyamine binding site. 5. 5,7 Dichlorokynurenic acid (125 and 250 micrograms kg-1, i.c.v.) virtually abolished the cardiovascular responses, inducing only haemodynamic depression at the highest dose used. 2-Amino-5-phosphonovaleric acid (0.1 to 1.0 mg kg-1, i.c.v.) elicited a reduction of the peak values observed during PVN stimulation which was accompanied by a decrease of the basal cardiovascular parameters. Ketamine (2.5 and 10 mg kg-1) and ifenprodil (1 mg kg-1), injected intravenously, blocked the haemodynamic response induced by PVN stimulation without marked reduction of the basal haemodynamics. 6. It is concluded that glutamate neurotransmission is not only involved in vasomotor tone control but also in the central control of cardiac function and can therefore modulate the myocardial oxygen demand.

Functional and antiischaemic effects of the phosphodiesterase inhibitor levosimendan in isolated rabbit hearts

Several phosphodiesterase inhibitors have been reported to possess antiischaemic properties by improving myocardial oxygen demand/supply balance. Levosimendan is a new phosphodiesterase inhibitor with calcium-sensitizing properties. We investigated the functional and antiischaemic properties of levosimendan in isolated electrically-driven rabbit hearts (Langendorff, constant perfusion pressure: 70 cm H2O, Tyrode solution, Ca++ 1.8 mmol/l, 37 degrees, 200 beats/min.). Acute regional myocardial ischaemia was induced by ligature of a circumflex artery branch and quantified from epicardial NADH-fluorescence photography.

RESULTS

The left ventricular pressure was similarly enhanced by levosimendan 10(-7) M or 5 x 10(-6) M (+15%) (P < 0.05). The global coronary flow was increased more markedly by levosimendan 5 x 10(-6) M (+35-50%) compared to 10(-7) M (+25%) (P < 0.05). The relative coronary flow (= global coronary flow/pressure-rate-product) was significantly enhanced only by levosimendan 5 x 10(-6) M (+20-35%) (P < 0.05), but not by levosimendan 10(-7) M (+10-15%) (P > 0.05). Epicardial NADH-fluorescence area and intensity were significantly diminished by levosimendan (-20%) (P < 0.05), and there was no significant difference between myocardial ischaemia reduction by levosimendan 10(-7) M and 5 x 10(-6) M (P > 0.05).

CONCLUSION

Levosimendan is an inotrope with coronary dilator activity, showing antiischaemic effects in isolated rabbit hearts. These may be caused by (1) phosphodiesterase-inhibition and improvement of myocardial perfusion and/or (2) by oxygen-sparing effects, related to calcium sensitization of myofilaments, particularly at lower concentrations associated with a lower coronary dilator activity.

Effect of epinephrine treatment during late ischemia and early reperfusion on regional myocardial function and infarct size in partially infarcted reperfused porcine hearts

This study investigated whether epinephrine treatment during late ischemia and early reperfusion improves systolic shortening after 45 minutes of reperfusion at the cost of increased infarct size. A model consisting of both stunned and dead myocytes was used. The left anterior descending coronary arteries of 10 control and 10 treated pigs were occluded distally for 40 minutes and then reperfused for 3 days. Regional systolic shortening was determined by sonomicrometry, and infarct size was assessed as the percentage of infarcted (tetrazolium stain) to ischemic (dye technique) myocardium. Intravenous administration of epinephrine was started 10 minutes before the onset of reperfusion (5 micrograms/min) and continued until 45 minutes of reperfusion (mean 18 micrograms/min). Immediately before and during 45 minutes of reperfusion, left ventricular peak pressure, dp/dtmax, and heart rate were significantly increased in the treated animals. After 45 minutes of reperfusion, epinephrine treatment improved systolic shortening of the reperfused myocardium (treated group 9% +/- 8%; control group -1% +/- 6%; p < 0.01). Transient beta-adrenergic stimulation of the reperfused myocardium did not increase infarct size (treated group 57.2% +/- 19%; control group 55.4% +/- 17%). In conclusion, epinephrine treatment during late ischemia and early reperfusion improved systolic shortening after 45 minutes of reperfusion without affecting infarct size.

Beneficial effect of amrinone on the size of acute regional ischemia in isolated rabbit hearts

The effect of inotropes on myocardial ischemia is difficult to predict because they may influence the determinants of myocardial O2 demand and O2 supply differently. Several PDE-inhibitors have been reported to possess antiischemic properties related to their hemodynamic and O2-sparing effects. To assess whether PDE-inhibitors also possess direct cardioprotective properties, the effects of amrinone (2.5 x 10(-5) mol/L) in comparison to isoproterenol (5 x 10(-9) mol/L) and ouabain (1.5 x 10(-7) mol/L) were studied in isolated rabbit hearts perfused according to Langendorff at a constant pressure (70 cmH2O) and electrically driven at a constant pacing rate. Regional ischemia was induced by coronary artery ligation and quantified by epicardial NADH fluorescence. All substances significantly increased the actively developed left ventricular pressure to a similar extent (+20%) (P < 0.05). Coronary flow was significantly decreased by ouabain (-15%) and significantly increased by isoproterenol (+25%) and particularly by amrinone (+50%) (P < 0.05). Neither ouabain nor isoproterenol significantly changed the intensity or the distribution pattern of NADH fluorescence, whereas the size of the ischemic zone was significantly reduced by amrinone (-25%) (P < 0.05). The PDE-inhibitor amrinone was shown to possess a direct cardioprotective effect by improving myocardial perfusion and O2 supply in isolated rabbit hearts.

Baclofen prevents the increase of myocardial oxygen demand indexes evoked by the hypothalamic stimulation in rabbits

The electrical stimulation of the paraventricular nucleus (PVN) of the hypothalamus in anaesthetized rabbits elicited important cardiovascular responses which were mainly characterized by increases in arterial pressure, dP/dtmax, and of the indexes of myocardial oxygen consumption, rate-pressure product (from 34 +/- 2 to 40 +/- 2 mmHg.bpm.10(-3) and triple product (from 102 +/- 12 to 162 +/- 19 mmHg2.s-1.bpm.10(-6). The hemodynamic alterations induced by PVN stimulation were similar to those observed during physical effort and stressful situations. Intracerebroventricular (0.1, 0.3 and 1 microgram.kg-1) or intravenous administration (1, 3 and 10 mg.kg-1) of baclofen, a selective GABAB receptor agonist, induced a dose-related decrease in the peak values of dP/dtmax and of the indexes of myocardial oxygen consumption (rate-pressure and triple products) during the electrical PVN stimulation. After 1 microgram.kg-1 baclofen (i.c.v.), the peak value of the triple product during PVN stimulation was 101 +/- 21 as compared to 149 +/- 15 before treatment. At the 10 mg.kg-1 dose (i.v.), the triple product during stimulation only reached 90 +/- 20 vs. 150 +/- 20 before treatment. These results suggested that a type B GABAergic transmission system is involved in the modulation of central control of the cardiac function. Drugs modulating this system could therefore be designed to blunt the myocardial oxygen demand increases.

Postischemic cell death in reperfused porcine hearts is not attenuated by the spin trap agent PBN during early reperfusion

Ischemic, reperfused porcine hearts were used to investigate whether the spin trap agent PBN (N-tert-butyl-alpha-phenylnitrone) attenuates postischemic cell death by scavenging of free radicals. The left anterior descending coronary artery (LAD) was ligated distally in 16 pigs for 45 min and then reperfused for 3 h. PBN (coronary concentration approximately 1 mM) was infused into the LAD of eight pigs during the first 45 min of reperfusion. Electron spin resonance spectroscopy (ESR) was performed to identify free radical adducts in the reperfused coronary venous blood. Regional systolic shortening (SS%) was determined by sonomicrometry. Infarct size was evaluated as the percentage of infarcted (tetrazolium stain) to ischemic (dye technique) myocardium. The transmural ultrastructural degree of myocardial injury as well as myocardial ATP levels were assessed at the end of the experiment. Intracoronary treatment with PBN during early reperfusion did not attenuate myocardial damage. Infarct sizes (control group 59 +/- 19%, treated group 55 +/- 14%), transmural ultrastructural alterations, myocardial ATP concentrations (control group 1.8 +/- 0.3 mumol/mg frozen weight, treated group 1.7 +/- 0.4 mumol/mg) and regional systolic shortening at the end of the experiments (control group -1 +/- 5%, treated group -2 +/- 6% did not differ significantly. Furthermore, under various experimental conditions of spin trapping, free radical adducts could not be identified in coronary venous blood during early reperfusion. The results suggest that the spin trap agent PBN (1 mM) does not affect postischemic cell death in porcine hearts.

Basal metabolism adds a significant offset to unloaded myocardial oxygen consumption per minute

Myocardial oxygen consumption (MVO2) includes components for 1) mechanical energy generation, 2) activation, and 3) basal metabolism. Whereas the first two components are expected to increase in proportion with heart rate, a significant basal level of metabolism would consume oxygen even if the heart rate were zero. Contrary to this expectation, however, a previous study reported that, during unloaded beats, MVO2 per beat (which includes basal metabolism) was independent of heart rate. Accordingly, unloaded MVO2 per minute would extrapolate to zero at zero heart rate; this result is unexpected considering basal metabolism. To resolve this inconsistency, we varied heart rate over a wide range after inducing atrioventricular block in eight isolated cross-circulated canine hearts that contracted isovolumically. We examined whether a term representing rate-independent basal metabolism was needed to describe MVO2 per minute. Mechanical energy generated by the left ventricle was evaluated from the pressure-volume area, which was altered by changing isovolumic ventricular volume over at least five levels at each heart rate. Contractility, evaluated by the slope of the end-systolic pressure-volume relation, did not vary significantly with heart rate in this study. In contrast to the previous report, unloaded MVO2 per beat (i.e., MVO2 extrapolated to a pressure-volume area of zero) was not constant but fell monotonically with increases in heart rate in every heart. We considered that this trend was caused by a significant rate-independent basal level of MVO2 per minute. Multiple linear regression analysis confirmed that this rate-independent basal term differed significantly from zero in seven of the eight hearts studied.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of Trolox, a water-soluble vitamin E analogue, in regionally ischemic, reperfused porcine hearts

Myocardial protection by the water-soluble vitamin E analogue, Trolox, was investigated in 18 regionally ischemic, reperfused porcine hearts. The left anterior descending coronary artery was distally ligated for 45 min and was reperfused for three days. Five grams of Trolox (n = 9) were infused intravenously before coronary occlusion. Treatment was continued with an intravenous dose of 5 grams Trolox/24 hours until the end of the experiment. Infarct size was determined as the ratio of infarcted (tetrazolium stain) to ischemic myocardium (dye technique). Regional systolic shortening was assessed by sonomicrometry. Generation of free radicals by stimulated neutrophils was evaluated by luminol-enhanced chemiluminescence. Plasma concentrations of Trolox were measured by high-performance liquid chromatography. Aside from heart rate before ischemia, global hemodynamic values including calculated left ventricular oxygen consumption did not differ significantly between the two groups. Plasma concentrations of Trolox measured 1.8 +/- 0.3 mmol/l (before ischemia), 0.96 +/- 0.13 mmol/l (before reperfusion), 0.77 +/- 0.1 mmol/l (40 min of reperfusion), and 0.08 mmol/l (end of the experiment). Generation of free radicals by stimulated neutrophils was reduced by about 30% in the treatment group before ischemia and immediately before reperfusion, but was not reduced at the end of the experiment. Risk regions (control group 19.4 +/- 6 g, treatment group 19.3 +/- 7 g) and infarct sizes (control group 69.3 +/- 8%, treatment group 69.3 +/- 12%) were almost identical. Regional systolic shortening of a control segment and of the risk region were similar in both groups before ischemia, before reperfusion, and after 45 min of reperfusion. After 3 days of reperfusion, regional systolic shortening of the reperfused myocardium of the treated group had recovered to a significantly greater extent (P = 0.027). This parameter amounted to 9 +/- 6% in the treated group and to 3 +/- 3% in the control group. Improved functional recovery was not accompanied by higher tissue concentrations of adenosine triphosphate. It is concluded that the chosen treatment with Trolox does not reduce infarct size but accelerates functional recovery. This finding suggests that the mechanisms resulting in myocardial necrosis during ischemia/reperfusion and in post-ischemic myocardial dysfunction may differ.

Relation between maximum time-varying elastance pressure-volume areas and myocardial oxygen consumption in dogs

To establish whether pressure-volume areas (PVAs) calculated using the maximum time-varying elastance (Emax) have a relation with myocardial oxygen consumption (MVO2) that improves on other indexes of myocardial oxygen demand, we studied nine dogs of either sex weighing 19-39 kg, which were instrumented with a micromanometer left ventricular (LV) catheter and a Wilton-Webster coronary sinus flow catheter and had red blood cells tagged with technetium-99m for radionuclide angiography. Hemodynamics, coronary sinus flow determinations, and radionuclide angiograms were obtained under control conditions and during three to five steady-state loading conditions (mean +/- SD, 5.6 +/- 0.7). Isochronal pressure-volume data points from each pressure-volume loop were subjected to linear regression analysis to calculate Emax. The Emax relations, diastolic curves, and systolic portions of each pressure-volume loop were used to obtain calibrated PVAs. The Emax PVA (mm Hg.ml.beat-1.100 g-1) and MVO2 (ml O2.beat-1.100 g-1) values correlated in each animal (r = 0.77 to 0.99). Their slopes averaged (3.48 +/- 1.68) x 10(-5) ml O2.mm Hg-1.ml-1, and their y-axis intercepts averaged 0.07 +/- 0.04 ml O2.beat-1.100 g-1. When the MVO2 relations were compared with Emax PVA, LV systolic pressure-rate product, LV stroke work, and a modification of the LV pressure-work index, the Emax PVA, LV systolic pressure-rate product, and LV pressure-work index had similar relations with MVO2, whereas LV stroke work was a weaker index of MVO2 (p less than 0.05 versus Emax PVA). This occurred because the Emax PVA:MVO2 slopes and y-axis intercepts differed in each dog, which was due to differences in basal LV contractility. The Emax PVA:MVO2 slopes correlated with Emax (r = 0.73, p less than 0.05), and the y-axis intercepts were also weakly related to Emax (r = 0.48, p = 0.19). We conclude that the Emax PVAs calculated using data acquisition techniques that are clinically applicable have relations with MVO2 that in general do not improve on other indexes of myocardial oxygen demand in this animal preparation.

Myocardial energetics in dilated cardiomyopathy

To assess hemodynamic and energetic effects of different drug interventions on idiopathic dilated cardiomyopathy (IDCM), we determined hemodynamic variables of myocardial oxygen consumption (MVO2) in 37 patients with IDCM. Hemodynamics were measured during routine left and right heart catheterization. MVO2 was analyzed from myocardial blood flow (measured by the argon method) and aortocoronary sinus blood oxygen difference. The hemodynamic variable which correlated best with MVO2 was shown to be the systolic stress time integral (STI). Four different representative compounds were tested with respect to their acute effects on myocardial energetics (MVO2/STI) in patients with IDCM who were in compensated heart failure (NYHA class II-III). The drug interventions were performed at rest. Intravenous injection of the vasodilator nitroprusside yielded a 35% reduction in STI and a 30% reduction in MVO2; in other words, the ratio MVO2/STI was not altered. Injection of the calcium sensitizer and phosphodiesterase inhibitor pimobendan also did not alter this ratio, as both STI (36%) and MVO2 (33%) were lowered. The profound reduction in STI (60%) seen with the phosphodiesterase inhibitor enoximone was accompanied by a much smaller decrease in MVO2 (19%); therefore, the ratio of MVO2/STI increased significantly. An increase of this ratio was also seen with the partial beta-1 receptor agonist xamoterol. However, in this case STI did not change, whereas MVO2 increased by 26%. In summary, vasodilation has energy-saving effects, whereas positive inotropism is an energy-consuming process. We conclude that the overall effect on myocardial energetics of a drug which possesses both positive inotropic and vasodilating properties depends on the balance of the two properties.

Regional myocardial oxygen consumption determined noninvasively in humans with [1-11C]acetate and dynamic positron tomography

Experimental studies of animals have previously demonstrated the validity of [1-11C]acetate as a tracer of oxidative metabolism for use with positron emission tomography. The present study was undertaken to define in normal human volunteers the relation between myocardial clearance kinetics of [1-11C]acetate, and the rate-pressure product as an index of myocardial oxygen consumption. Twenty-two studies were performed of 12 volunteers. The rate-pressure product was increased with continuous supine bicycle exercise in six studies. Of the 16 resting studies, seven were performed in the fasted state and nine following an oral glucose load, to define possible effects of substrate availability on the tracer-tissue kinetics. Myocardial tissue time-activity curves were biexponential. Clearance of activity was homogeneous throughout the myocardium. The rate constants k1, obtained from biexponential fitting, and kmono, obtained by monoexponential fitting of the initial linear portion of the time-activity curves, correlated well with the rate-pressure product. Although the correlation coefficient was higher for k1 than for kmono (0.95 vs. 0.91), analysis on a sectorial basis showed less regional variability in kmono. This suggests that kmono, which is more practical than k1 because it requires shorter acquisition times, may be more clinically and experimentally useful for detection of myocardial segments with abnormal oxygen consumption. Overall, changes in myocardial substrate supply were without significant effect on the relation between the rate constants (k1 and kmono) and the rate-pressure product, although a small decrease in kmono/rate-pressure product was observed following oral glucose by paired analysis in four subjects. It is concluded that [1-11C]acetate can be used for the noninvasive measurement of myocardial oxygen consumption in humans with positron emission tomography, and, thus, has clinical and experimental potential as a tool for the understanding and diagnosis of myocardial disease.

Combined treatment with vitamins E and C in experimental myocardial infarction in pigs

The effect of two different combined treatments with vitamin E acetate and vitamin C on infarct size and recovery of regional myocardial function was investigated in ischemic, reperfused porcine hearts. The left anterior descending coronary artery was distally ligated in 30 thoracotomized pigs for 45 minutes followed by 3 days of reperfusion. Infarct size was determined as the ratio of infarcted (tetrazolium stain) to ischemic (dye technique) myocardium. Regional myocardial function was assessed by sonomicrometry. Ten pigs received vitamin E acetate (12 gm intravenously three times for 1 week) before ischemic and vitamin C (4.4 gm intravenously) before reperfusion (therapy A). Another 10 pigs were treated with vitamin E acetate (12 gm intraarterially) and vitamin C (4.4 gm intravenously) during ischemia (therapy B). An additional 10 pigs served as a control group. Global hemodynamics did not differ significantly among the groups before and during ischemia. Mean plasma concentrations of vitamin E amounted to 107 micrograms/ml in group A, 16 micrograms/ml in group B, and 0.9 micrograms/ml in the control group at the onset of reperfusion. Therapy A reduced the size of the infarct from 73 +/- 12% to 47 +/- 16% of the region at risk (p less than 0.005) and improved regional systolic shortening from 0 +/- 7% to 11 +/- 6% at 3 days after reperfusion (p less than 0.01). Therapy B decreased the size of the infarct to 64 +/- 9% of the region at risk (p = 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Improved myocardial oxygen utilization following propranolol infusion in adolescents with postburn hypermetabolism

The purpose of this study was to determine if propranolol (0.5 mg/kg and 1 mg/kg), administered intravenously (IV) at the height of the postburn hypermetabolic response, would decrease myocardial oxygen requirements, without adversely affecting overall oxygen delivery or total body oxygen consumption. To test this hypothesis, six nonseptic patients age 17 +/- 3 years with burns over 82% +/- 11% total body surface area were given propranolol with continuous hemodynamic monitoring. Propranolol was administered to these patients 20 +/- 15 days postburn. Two clinically derived indices of myocardial oxygen consumption, pressure-work index (PWI) and rate-pressure product (RPP), were used to estimate the energy expenditure of the working heart. Both PWI and RPP were significantly decreased from baseline after 0.5 mg/kg propranolol, 31% for PWI (P less than .001) and 30% for RPP (P less than .01). Similarly, a decrease from baseline was seen after 1.0 mg/kg propranolol, 32% for PWI (P less than .001) and 35% for RPP (P less than .01). Cardiac index (L/min/m2) demonstrated no significant change [7.4 +/- 1.1 (prepropranolol), 6.5 +/- 1.3 (after 0.5 mg/kg propranolol), and 6.8 +/- 1.0 (after 1.0 mg/kg propranolol)] and exceeded the upper limits of normal (hyperdynamic state) throughout the study. Oxygen delivery index (962 +/- 209 mL/min/m2) and oxygen consumption indices [(254 +/- 78 mL/min/m2 by Fick method and 236 +/- 78 mL/min/m2 by inspired and expired gases)] were elevated at baseline and unaffected by propranolol. The decrease in PWI and RPP was achieved mainly by propranolol's effect to lower both heart rate and BP.(ABSTRACT TRUNCATED AT 250 WORDS)

[Histamine effects on the heart with special reference to cardiac side effects of H2 receptor antagonists]

The existence of cardiac h1- and h2-receptors is evidenced by pharmacologic studies. Despite of the relatively high content of cardiac histamine it is not clarified whether histamine actually plays a physiologic role - apart from pharmacologic effects - in the regulation of myocardial function and coronary blood flow. Under pathophysiologic conditions (during anaphylaxis, surgical procedures, accidents, stress etc.), however, when a local or systemic histamine release occurs both hemodynamic and arrhythmogenic effects are evident. Numerous studies in animal models conclusively demonstrated a role of cardiac histamine as a major mediator of serious arrhythmias. Consequently, a combination of h1- and h2-receptor antagonists (f.e. Dimetinden/Cimetidin) was recommended as a prophylactic treatment against severe anaphylaxis including life-threatening arrhythmias for cardiac patients at risk. There is pharmacologic evidence of both a positive inotropic and chronotropic effect in the human heart via h2-receptor and stimulation of adenylate cyclase. Histamine-induced coronary effects such as vasoconstriction via h1-receptor and coronary dilatation via h2-receptor are not yet sufficiently validated. Studies on the human heart in vitro using coronary strips from explanted hearts and in vivo investigations on the intact coronary system yielded conflicting results. H2-receptor blocking agents cimetidine, ranitidine and famotidine have qualitatively a different pharmocodynamic spectrum of side effects due to differences in chemical structure. Data on cardiac arrhythmias are mostly associated to cimetidine. Symptomatic bradycardia were reported for both ranitidine and cimetidine. A possible negative inotropic effect of famotidine, although presently not validated, requires further studies. Causative and adverse side effects of cimetidine on the cardiovascular system, however, are to be expected extremely seldom due to easily reversible competetive h2-receptor binding. For prophylaxis rapid intravenous injections of h2-blockers, particularly in elder patients with cardiac diseases, should be avoided. Compared to cimetidine, a tendency of explainable difference seems to become apparent for ranitidine and famotidine due to higher receptor affinity.

Myocardial energetics in patients with dilated cardiomyopathy. Influence of nitroprusside and enoximone

Cardiotonic agents influence myocardial energy consumption by vasodilation, which may reduce energy demand, and by inotropism, which may increase it. To distinguish between the two effects, myocardial oxygen consumption must be analyzed in relation to its hemodynamic determinants. The coupling of myocardial oxygen consumption with its determinants was investigated in 22 patients with idiopathic dilated cardiomyopathy (NYHA Class II and III). Predicted myocardial oxygen consumption by the pressure-work index, the Bretschneider index, and the pressure-volume area correlated moderately with measured myocardial oxygen consumption (r = 0.57, p less than 0.001; r = 0.52, p less than 0.005; and r = 0.63, p less than 0.001). Multiple regression analysis, including left ventricular peak systolic wall stress, systolic stress-time integral, pressure-volume work, maximum rate of left ventricular pressure rise, and mean velocity of circumferential fiber shortening indicated that systolic stress-time integral is the major determinant of myocardial oxygen consumption (r = 0.75, p less than 0.001) in these patients. Enoximone, a phosphodiesterase inhibitor, has an inotropic and a vasodilating effect. To investigate the inotropic portion of the energy cost of this phosphodiesterase inhibitor, the influence of enoximone on myocardial oxygen consumption and systolic stress-time integral was compared with the effects of nitroprusside, which is a vasodilator only. Nitroprusside (10 patients) and enoximone (12 patients) reduced left ventricular systolic stress-time integral from 109 +/- 22 to 71 +/- 21 (p less than 0.005) and from 104 +/- 23 to 42 +/- 10 (p less than 0.001) 10(3) dynes.sec/cm2, respectively. Myocardial oxygen consumption decreased from 159 +/- 44 to 112 +/- 23 (p less than 0.005) and from 134 +/- 28 to 109 +/- 21 (p less than 0.001) microliters/beat/100 g, respectively. In both groups, there was a significant correlation between the decrease in myocardial oxygen consumption and the decrease in systolic stress-time integral. The slopes of the respective linear regression lines were significantly different (1.27 for nitroprusside and 0.51 nl.cm2/100 g.dynes.sec for enoximone, p less than 0.05), indicating a smaller decrease of myocardial oxygen consumption for a given decrease of stress-time integral with enoximone. Applying the pressure-work index or the pressure-volume area instead of systolic stress-time integral yielded comparable results. Thus, vasodilation reduces myocardial oxygen consumption in proportion to the reduction of stress-time integral. With enoximone, the energy-saving effect of vasodilation is counteracted in part by the increased energy d

The effects of nitroglycerin on regional myocardial contractile dysfunction produced by treadmill exercise or isoprenaline stimulation in dogs

1. To compare different methods of cardiac stress testing that are clinically applied in the management of coronary heart disease, 2 groups of dogs each were chronically instrumented and subjected to treadmill exercise or isoprenaline infusion in the presence of coronary stenosis. 2. It was of interest to determine differences in haemodynamic and regional myocardial contractile parameters, the response to antianginal therapy (nitroglycerin 15 micrograms kg-1 15 min-1, i.v.), and, in particular, whether this response differed according to the mode of cardiac stimulation, i.e. treadmill exercise or isoprenaline infusion. 3. After stenosis of the circumflex branch of the left coronary artery which affected resting myocardial function only minimally, treadmill exercise or isoprenaline infusion induced transient regional contractile dysfunction. Heart rate, arterial blood pressure, left ventricular end-diastolic pressure and left ventricular dp/dtmax were registered and myocardial oxygen demand was calculated. Regional contractile performance was assessed by ultrasonic distance measurement in the underperfused and in a normally perfused area. 4. Treadmill exercise led to an increase in systolic arterial and left ventricular end-diastolic pressure. In contrast, isoprenaline-induced stimulation led to a decrease in diastolic arterial and left ventricular end-diastolic pressure. Regional contractile function in the critically underperfused area showed a deterioration during both modes of stress. Nitroglycerin completely abolished stress-induced contractile dysfunction only in the group where treadmill exercise was employed for stimulation. 5. The inability of nitroglycerin to prevent myocardial dysfunction in the isoprenaline group may be due to exhaustion of the arterial and/or venous vasodilator potency of nitroglycerin in the presence of adrenoceptor vasodilatation induced by isoprenaline. 6. These findings indicate that clinical antianginal drug testing and the evaluation of the course of disease in patients with coronary heart disease may be highly dependent on the test method chosen.

Combining transmural left ventricular mechanics and energetics to predict oxygen demand

This study relates to our earlier study which predicts the transmural distribution as well as the global left ventricular (LV) function and oxygen demand, based on the LV structure, geometry and sarcomere function. Here, we test the predicted global oxygen demand against experimental data in anesthetized, open chest dogs under changing working conditions. The experimental oxygen demand was calculated from the arterio-venous difference in oxygen content times the measured coronary flow. LV load was manipulated by a combination of a pressurized chamber connected to the femoral artery, phenylephrine infusion and an adjustable arteriovenous shunt. The heart was paced in two present heart rates. The study demonstrates that the global predictions, based on the local distributed oxygen demand model, are comparable to those obtained by other methods of global metabolic predictions. However, unlike other global methods, the distributed model gives spatial information and predicts an endo/epi ratio of oxygen demand ranging between 1.05 to 1.14, depending on the loading conditions, which is comparable to available experimental data. For the experimental conditions studied here (stroke volume, heart rate, aortic pressure), the theoretical analysis shows that only the end diastolic volume is significantly correlated to the endo/epi ratio of the transmural oxygen demand.

Comparison of the effects of right atrial, right ventricular apex and atrioventricular sequential pacing on myocardial oxygen consumption and cardiac efficiency: a laboratory investigation

As the impact of cardiac pacing on myocardial energetics has not yet been established, this laboratory investigation was undertaken to evaluate the effects of right atrial (AP), right ventricular apex (VP) and atrioventricular sequential pacing (AVP) on cardiac energetics in a closed-chest model. Ninety-two pacing interventions were performed in ten anesthetized mongrel dogs with normal loading conditions and contractile states. The energetic effects of pacing were assessed in terms of myocardial oxygen consumption (MVO2), its hemodynamic determinants and cardiac efficiency. Efficiency was calculated as the ratio of O2-equivalent of external cardiac work to MVO2, using standard definitions. In the first series of experiments 36 intra-individual comparisons were made between AP and VP at identical rates (95-210 beats/min). In the second series AVP was compared to VP in 10 intra-individual comparisons at identical rates (109-190 beats/min). MVO2 was lower (p less than 0.001) during AP (8.30 +/- 2.14 ml O2/min.100 g) compared to VP (10.16 +/- 3.15 ml O2/min.100 g) at the same rate (158 +/- 32 beats/min). Efficiency (p less than 0.001) was considerably higher during AP (21.6 +/- 5.7%) compared to VP (12.8 +/- 5.9%). During AVP, MVO2 (10.85 +/- 1.76 ml O2/min.100 g) was not significantly different from VP (10.57 +/- 1.34 ml O2/min.100 g) at the same rate (146 +/- 25 beats/min). Hemodynamics were superior with AVP compared to VP. Efficiency was significantly higher (p less than 0.01) with sequential (15.4 +/- 3.9%) as compared to ventricular pacing (12.0 +/- 3.2%). In conclusion, this study indicated that VP exerts disadvantageous effects on MVO2 and cardiac efficiency. AP has beneficial effects on cardiac energetics because it improves the relationship between mechanical performance of the heart and its energy requirements. AVP results in a higher efficiency than VP due to superior hemodynamics, despite MVO2 levels comparable to those of VP. The mechanism of energy waste with right ventricular apex pacing is probably related to an asynchronous contraction in the ventricular myocardium due to a nonphysiological spread of excitation.

Effects of CI-914 in congestive heart failure due to coronary artery disease or idiopathic cardiomyopathy

The hemodynamic effects of CI-914, a phosphodiesterase inhibitor, were studied in 12 patients with left ventricular (LV) dysfunction who were undergoing diagnostic cardiac catheterization. CI-914 was infused intravenously at a rate of 0.8 to 7.0 micrograms/kg/min for 30 to 60 minutes; hemodynamic values were measured every 10 minutes. No effect was seen in the patient receiving 0.8 microgram/kg/min. At infusion rates of 1.2 to 2.4 micrograms/kg/min, cardiac index increased by 14% (p less than 0.025). At infusion rates of 4.5 to 7.0 micrograms/kg/min, cardiac index increased by 21% (n = 8, difference not significant [NS]). Among 4 patients (group B) with an initial pulmonary artery wedge pressure greater than 20 mm Hg and cardiac index less than 2.5 liters/min/m2, cardiac index increased by 50% (p less than 0.001); it did not change among the 4 patients with an initial pulmonary artery wedge pressure of less than 20 mm Hg and cardiac index of more than 2.5 liters/min/m2 (group A). Although systemic vascular resistance decreased in all 8 patients by 26% (p less than 0.01), the reduction was greater in group B (33%, p less than 0.01) than in group A (16%, NS). Peak +dP/dt increased in all 8 patients by 13% (p less than 0.01). Mean stroke work index increased from 29 +/- 15 to 34 +/- 13 g-m/m2; the double product fell from 101 +/- 31 to 91 +/- 23 (NS). In all 12 patients, a linear correlation between peak venous blood concentration and peak effect on cardiac index, systemic vascular resistance and pulmonary artery wedge pressure was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Left ventricular mechanics related to the local distribution of oxygen demand throughout the wall

The complex interactions between left ventricular mechanics and the oxygen demand is studied by relating the left ventricular transmural oxygen demand to the myocardial structural and dynamic characteristics. The study utilizes a recent model of left ventricular contraction, which is based on a nested shell spheroidal geometry, a fan-like fibrous structure, the twisting motion of the left ventricle over its long axis, a transmural electrical activation propagation and the basic laws of sarcomere dynamics. The local "axial" stress (in the direction of the fibers) and the instantaneous sarcomere length are used to calculate the spatial distribution of the intramural oxygen demand per beat Vo2(y), where y is the distance from the endocardium. The normalized local sarcomere stress-length area SLAn(y) is related linearly to Vo2(y) by: Vo2(y) = K1 X SLAn(y) + K2, where K1 and K2 are constants. The calculations show a transmural metabolic gradient which is characterized by higher values of Vo2(y) in the endocardial layers than in the epicardial layers. Shorter endocardial sarcomeres and the twisting motion of the left ventricle around the long axis decrease the metabolic gradient across the wall, while a slow transmural electrical propagation wave as well as a wider angle of distribution of the fan-like fiber architecture increases the transmural metabolic gradient. Integration of the local oxygen demand across the left ventricular wall yields global values in agreement with those based on Suga's pressure-volume area approach. The model thus provides a qualitative and quantitative tool to assess the relation of the local and global oxygen demand to the complex left ventricular structure, fiber mechanics, and the dynamics of contraction.

Preservation of myocardium in transient ischemia by the thromboxane synthetase inhibitor UK 38.485

This study was performed to examine potential protective effects of UK 38.485, an inhibitor of thromboxane synthetase, in canine myocardium stressed by transient ischemia. On anesthetized open-chest mongrel-dogs (n = 9) repeated ischemia (3 min) was produced by proximal, intermittent occlusion of the left anterior descending artery. A total of 18 occlusions after 3 mg UK 38.485/kg body wt. and 12 occlusions after 5 mg UK 38.485/kg body wt. were compared to a total of 24 occlusions under control conditions. In each experiment, 2-3 control occlusions and 3-4 therapy occlusions were performed. The drug was applied i.v. in a dose of 3 or 5 mg/body wt. 30 min before the first therapy occlusion. In both groups, hemodynamics and energetics did not significantly change as compared to control. The efficiency of the drug in protecting ischemically stressed myocardium was examined by (a) quantification of oxygen debt and oxygen repayment in the occlusion and reperfusion periods and (b) the amounts of inorganic phosphate, lactate, and potassium released in the first minute of reperfusion. Compared to control occlusions, premedication with either 3 or 5 mg UK 38.485 led to a significantly reduced oxygen debt combined with a significant decrease of the release of inorganic phosphate, lactate, and potassium. The protective effect is suggested to be mainly due to enhanced flow to ischemic areas. Data obtained in this study suggest protective effects of the compound in the preservation of myocardium in transient ischemia and attest to the concept that thromboxane A2 may aggravate the metabolic and energetic situation of myocardium in circumstances with reduced oxygen supply.