Effects of a calcium-entry blocker (diltiazem) on regional myocardial flow and function during exercise in conscious dogs

A pathophysiological compass to personalize antianginal drug treatment

Myocardial ischaemia results from coronary macrovascular or microvascular dysfunction compromising the supply of oxygen and nutrients to the myocardium. The underlying pathophysiological processes are manifold and encompass atherosclerosis of epicardial coronary arteries, vasospasm of large or small vessels and microvascular dysfunction - the clinical relevance of which is increasingly being appreciated. Myocardial ischaemia can have a broad spectrum of clinical manifestations, together denoted as chronic coronary syndromes. The most common antianginal medications relieve symptoms by eliciting coronary vasodilatation and modulating the determinants of myocardial oxygen consumption, that is, heart rate, myocardial wall stress and ventricular contractility. In addition, cardiac substrate metabolism can be altered to alleviate ischaemia by modulating the efficiency of myocardial oxygen use. Although a universal agreement exists on the prognostic importance of lifestyle interventions and event prevention with aspirin and statin therapy, the optimal antianginal treatment for patients with chronic coronary syndromes is less well defined. The 2019 guidelines of the ESC recommend a personalized approach, in which antianginal medications are tailored towards an individual patient's comorbidities and haemodynamic profile. Although no antianginal medication improves survival, their efficacy for reducing symptoms profoundly depends on the underlying mechanism of the angina. In this Review, we provide clinicians with a rationale for when to use which compound or combination of drugs on the basis of the pathophysiology of the angina and the mode of action of antianginal medications.

Myocardial stunning and hibernation revisited

Unlike acute myocardial infarction with reperfusion, in which infarct size is the end point reflecting irreversible injury, myocardial stunning and hibernation result from reversible myocardial ischaemia-reperfusion injury, and contractile dysfunction is the obvious end point. Stunned myocardium is characterized by a disproportionately long-lasting, yet fully reversible, contractile dysfunction that follows brief bouts of myocardial ischaemia. Reperfusion precipitates a burst of reactive oxygen species formation and alterations in excitation-contraction coupling, which interact and cause the contractile dysfunction. Hibernating myocardium is characterized by reduced regional contractile function and blood flow, which both recover after reperfusion or revascularization. Short-term myocardial hibernation is an adaptation of contractile function to the reduced blood flow such that energy and substrate metabolism recover during the ongoing ischaemia. Chronic myocardial hibernation is characterized by severe morphological alterations and altered expression of metabolic and pro-survival proteins. Myocardial stunning is observed clinically and must be recognized but is rarely haemodynamically compromising and does not require treatment. Myocardial hibernation is clinically identified with the use of imaging techniques, and the myocardium recovers after revascularization. Several trials in the past two decades have challenged the superiority of revascularization over medical therapy for symptomatic relief and prognosis in patients with chronic coronary syndromes. A better understanding of the pathophysiology of myocardial stunning and hibernation is important for a more precise indication of revascularization and its consequences. Therefore, this Review summarizes the current knowledge of the pathophysiology of these characteristic reperfusion phenomena and highlights their clinical implications.

Myocardial ischemia: lack of coronary blood flow, myocardial oxygen supply-demand imbalance, or what?

This opinionated article reviews current concepts of myocardial ischemia. Specifically, the historical background is briefly presented. Then, the prevailing paradigm of myocardial oxygen-supply-demand imbalance is criticized since demand is a virtual parameter that cannot be measured and data on measurements of myocardial blood flow and contractile function rather support matching between flow and function. Finally, a concept of myocardial ischemia that focusses on the reduction of coronary blood flow to below 8-10 µl/g per beat with consequences for myocardial electrical, metabolic, contractile and morphological features is advocated.

Guidelines for experimental models of myocardial ischemia and infarction

Myocardial infarction is a prevalent major cardiovascular event that arises from myocardial ischemia with or without reperfusion, and basic and translational research is needed to better understand its underlying mechanisms and consequences for cardiac structure and function. Ischemia underlies a broad range of clinical scenarios ranging from angina to hibernation to permanent occlusion, and while reperfusion is mandatory for salvage from ischemic injury, reperfusion also inflicts injury on its own. In this consensus statement, we present recommendations for animal models of myocardial ischemia and infarction. With increasing awareness of the need for rigor and reproducibility in designing and performing scientific research to ensure validation of results, the goal of this review is to provide best practice information regarding myocardial ischemia-reperfusion and infarction models.

Listen to this article’s corresponding podcast at ajpheart.podbean.com/e/guidelines-for-experimental-models-of-myocardial-ischemia-and-infarction/.

Acute and Chronic Exercise in Animal Models

Numerous animal cardiac exercise models using animal subjects have been established to uncover the cardiovascular physiological mechanism of exercise or to determine the effects of exercise on cardiovascular health and disease. In most cases, animal-based cardiovascular exercise modalities include treadmill running, swimming, and voluntary wheel running with a series of intensities, times, and durations. Those used animals include small rodents (e.g., mice and rats) and large animals (e.g., rabbits, dogs, goats, sheep, pigs, and horses). Depending on the research goal, each experimental protocol should also describe whether its respective exercise treatment can produce the anticipated acute or chronic cardiovascular adaptive response. In this chapter, we will briefly describe the most common kinds of animal models of acute and chronic cardiovascular exercises that are currently being conducted and are likely to be chosen in the near future. Strengths and weakness of animal-based cardiac exercise modalities are also discussed.

Preclinical pharmacology of diltiazem

The membrane site of action and intracellular effects of diltiazem on heart and blood vessels are briefly discussed and compared to those of other calcium entry blockers, mainly verapamil and nifedipine. Diltiazem seems to have another site of action in the membrane than verapamil and nifedipine. Even if its main action is exerted at the cell membrane level, diltiazem may, at high concentrations, appears to have intracellular effects. Similar to the haemodynamic effects of verapamil and nifedipine, those of diltiazem are determined not only by direct actions on heart and peripheral vessels, but also by sympathetic reflex activity which modulates the direct effects. Two aspects of the myocardial protective action of diltiazem are discussed, the ability of the drug to reduce the frequency of ventricular dysrhythmias associated with ischaemic damage, and the ability to protect the ischaemic myocardium during reperfusion.

Heart rate in the pathophysiology of coronary blood flow and myocardial ischaemia: benefit from selective bradycardic agents

Starting out from a brief description of the determinants of coronary blood flow (perfusion, pressure, extravascular compression, autoregulation, metabolic regulation, endothelium-mediated regulation and neurohumoral regulation) the present review highlights the overwhelming importance of metabolic regulation such that coronary blood flow is increased at increased heart rate under physiological circumstances and the overwhelming importance of extravascular compression such that coronary blood flow is decreased at increased heart rate through reduction of diastolic duration in the presence of severe coronary stenoses. The review goes on to characterize the role of heart rate in the redistribution of regional myocardial blood flow between a normal coronary vascular tree with preserved autoregulation and a poststenotic vasculature with exhausted coronary reserve. When flow is normalized by heart rate, there is a consistent close relationship of regional myocardial blood flow and contractile function for each single cardiac cycle no matter whether or not there is a coronary stenosis and what the actual blood flow is. beta-Blockade improves both flow and function along this relationship. When the heart rate reduction associated with beta-blockade is prevented by pacing, alpha-adrenergic coronary vasoconstriction is unmasked and both flow and function are deteriorated. Selective heart rate reduction, however, improves both flow and function without any residual negative effect such as unmasked alpha-adrenergic coronary vasoconstriction or negative inotropic action.

Analysis of Postsystolic Myocardial Thickening Work in Selective Myocardial Layers During Progressive Myocardial Ischemia

BACKGROUND

Myocardial function is transmurally heterogeneous. Postsystolic work may functionally reflect ischemic but viable myocardium. We calculated systolic and postsystolic regional myocardial work index (RMWi) in subendocardial and subepicardial layers of myocardium supplied by a slowly occluding coronary artery.

METHODS

Progressive stenosis of the left anterior descending coronary artery lasting 11 +/- 5 days (end point) was induced in 10 dogs, and pressure-strain loops were obtained from subendocardial and subepicardial layers of apical and middle anterior segments by intracardiac ultrasound.

RESULTS

At baseline, the RMWi was significantly higher (P < .05) in the subendocardial layer. At the end point, there was no significant change in the RMWi in ischemic myocardium; however, the postsystolic RMWi was higher (P < .05) in the subendocardial layer and accompanied a decrease in subendocardial myocardial blood flow, although viability was largely maintained.

CONCLUSION

A significant subendocardial postsystolic RMWi at rest suggests an impending ischemic injury in coronary artery disease when segmental function is still preserved.

Calcium antagonists

Calcium antagonists were introduced for the treatment of hypertension in the 1980s. Their use was subsequently expanded to additional disorders, such as angina pectoris, paroxysmal supraventricular tachycardias, hypertrophic cardiomyopathy, Raynaud phenomenon, pulmonary hypertension, diffuse esophageal spasms, and migraine. Calcium antagonists as a group are heterogeneous and include 3 main classes--phenylalkylamines, benzothiazepines, and dihydropyridines--that differ in their molecular structure, sites and modes of action, and effects on various other cardiovascular functions. Calcium antagonists lower blood pressure mainly through vasodilation and reduction of peripheral resistance. They maintain blood flow to vital organs, and are safe in patients with renal impairment. Unlike diuretics and beta-blockers, calcium antagonists do not impair glucose metabolism or lipid profile and may even attenuate the development of arteriosclerotic lesions. In long-term follow-up, patients treated with calcium antagonists had development of less overt diabetes mellitus than those who were treated with diuretics and beta-blockers. Moreover, calcium antagonists are able to reduce left ventricular mass and are effective in improving anginal pain. Recent prospective randomized studies attested to the beneficial effects of calcium antagonists in hypertensive patients. In comparison with placebo, calcium antagonist-based therapy reduced major cardiovascular events and cardiovascular death significantly in elderly hypertensive patients and in diabetic patients. In several comparative studies in hypertensive patients, treatment with calcium antagonists was equally effective as treatment with diuretics, beta-blockers, or angiotensin-converting enzyme inhibitors. From these studies, it seems that a calcium antagonist-based regimen is superior to other regimens in preventing stroke, equivalent in preventing ischemic heart disease, and inferior in preventing congestive heart failure. Calcium antagonists are also safe and effective as first-line or add-on therapy in diabetic hypertensive patients. Heart rate-lowering calcium antagonists (verapamil, diltiazem) may have an edge over the dihydropyridines in post-myocardial infarction patients and in diabetic nephropathy. Thus, calcium antagonists may be safely used in the management of hypertension and angina pectoris.

Fréquence cardiaque et ischémie myocardique expérimentale

Every increase in heart rate represents a poor prognostic factor in cardiology, and multiple arguments have now led to the belief that reducing heart rate is a major therapeutic challenge. A comparison of the pharmacological effects of If current inhibitors such as zatebradine, and more recently ivabradine (Procoralan) and beta-blockers, have demonstrated experimentally that reductions in heart rate and myocardial contractile force contribute equally to the reduction in myocardial oxygen consumption in the normal heart. Conversely, at a similar level of reduction in heart rate, the lack of a concomitant negative inotropic effect with ivabradine affords longer diastolic perfusion times than beta-blockers. In other words, a negative inotropic effect is deleterious when an increase in coronary blood flow is required. Hence, if the anti-ischaemic effects afforded by an If current inhibitor and a beta-blocker are roughly comparable, the former are clearly of higher benefit than beta-blockers in the treatment of myocardial dysfunction accompanying cardiac ischaemia-reperfusion, especially myocardial stunning.

Time-dependent cardioprotection with calcium antagonism and experimental studies with clevidipine in ischemic-reperfused pig hearts: part II

The intracellular calcium level is increased during ischemia and early reperfusion. The aim of this study was to study the role of the calcium influx in the development of myocardial ischemic and reperfusion injury during the early and late phases of ischemia and during early reperfusion. An ultrashort-acting calcium antagonist, clevidipine, was used as a tool for this investigation. Pentobarbital-anesthetized pigs were subjected to 45 minutes of LAD occlusion followed by 240 minutes of reperfusion. In the first set of experiments, clevidipine (0.3 nmol/kg per minute) was infused over 5 minutes into the ischemic myocardium via a catheter in the LAD, starting at 5, 35, or 44 minutes following the onset of ischemia (n = 6 in each group). The area at risk and the infarct size were determined after 240 minutes of reperfusion by staining with Evans blue dye and triphenyl tetrazolium chloride (TTC), respectively. In a second set of experiments, two groups of animals (n = 6 in each) were subjected to the same periods of ischemia and reperfusion; one group received no infusion during ischemia, whereas the other group received vehicle infusion during a 5-minute period between 5 and 10 minutes of ischemia. In the first set of experiments, there were no significant differences between the groups with regard to hemodynamic variables. The area at risk expressed as a percentage of the left ventricle was of similar magnitude in all three clevidipine-treated groups (about 18%). The infarct size, expressed as a percentage of the area at risk, was significantly smaller in pigs given clevidipine after 5 minutes (58 +/- 17%; p < 0.01) and after 44 minutes (42 +/- 6%; p < 0.01) of ischemia than in pigs receiving clevidipine after 35 minutes of ischemia (85 +/- 4%). The difference in infarct size between pigs given clevidipine after 5 or 44 minutes of ischemia was not significant. In the second set of experiments, there was a similar area at risk and no significant difference in infarct size between the noninfusion group and the 5-minute vehicle infusion group, indicating that the LAD infusion per se did not affect infarct size. The present results demonstrate that blockade of calcium influx by the short-acting dihydropyridine calcium antagonist clevidipine during the early phase of ischemia and at the time of reperfusion, but not during a late phase of ischemia, limits infarct size induced by ischemia and reperfusion. This indicates that the pathophysiological importance of calcium influx varies according to the different phases of myocardial ischemia and reperfusion.

Vasopeptidase inhibition in a canine model of exercise-induced left ventricular dysfunction

1. The present study compared the acute efficacies of vasopeptidase inhibition with omapatrilat, nitroglycerin and angiotensin-converting enzyme (ACE) inhibition in exercise-induced myocardial dysfunction. Omapatrilat, a vasopeptidase inhibitor, inhibits both neutral endopeptidase and ACE. Whereas vasopeptidase inhibitors have demonstrated clinical efficacy in hypertension and heart failure, their effects in myocardial ischaemia remain unclear. 2. Omapatrilat (0.3 mg/kg) was compared with vehicle (saline), an ACE inhibitor (fosinoprilat; 0.44 mg/kg) and nitroglycerin (8.0 microg/kg per min), in an established canine model of exercise-induced myocardial dysfunction induced by progressive closure of an ameroid constrictor placed about the proximal circumflex coronary artery. Maximal treadmill exercise tests, terminated when heart rate failed to increase with increasing workload or failure to continue exercise, were performed in chronically instrumented dogs. 3. During exercise, omapatrilat and nitroglycerin similarly increased ischaemic wall thickening (P< or = 0.0001, ANOVA, 12 d.f.), whereas fosinoprilat and vehicle were without effect. Ischaemic zone ST changes were decreased with nitroglycerin (P = 0.0006, ANOVA, 12 d.f.) and tended to decrease with omapatrilat (P = 0.07, ANOVA, 12 d.f.). Peak exercise capacity was increased with nitroglycerin (9.7 +/- 1.1 vs 11.2 +/- 1.0 kcal, control vs 4 h, respectively; n = 6) and omapatrilat (9.7 +/- 0.8 vs 11.4 +/- 0.6 kcal, control vs 4 h, respectively; n = 6) and was unchanged with ACE inhibition (9.0 +/- 1.2 vs 9.5 +/- 1.1 kcal, control vs 4 h, respectively; n = 7). Omapatrilat differentially increased double product during exercise (P = 0.001, ANOVA, 12 d.f.) compared with other treatments. 4. During exercise-induced myocardial dysfunction, acute ACE inhibition did not attenuate ischaemic changes and failed to improve exercise capacity. Increased exercise capacity with omapatrilat was accompanied by a differential increase in double product, consistent with increased oxygen supply and demand. Improvements in ischaemic function were comparable between omapatrilat and nitroglycerin, suggesting that omapatrilat may represent a novel therapy in demand-induced ischaemia.

Ischemic shortening of action potential duration as a result of KATP channel opening attenuates myocardial stunning by reducing calcium influx

Action potential duration (APD) shortening due to opening of sarcolemmal ATP-dependent potassium (KATP) channels has been postulated to protect the myocardium against postischemic damage by reducing Ca2+ influx. This hypothesis was assessed, assuming that increased postischemic stunning due to KATP channel inhibition with glibenclamide could be reverted by the addition of the Ca2+ channel blocker diltiazem. Percent wall thickening fraction (%WTh, conscious sheep) and APD (open-chest sheep) were obtained from the following groups: control: 12 min ischemia by anterior descending coronary artery occlusion followed by 2 h reperfusion; glibenclamide: same as control, with glibenclamide (0.4 mg/kg) infused 30 min before ischemia; diltiazem: same as control, with diltiazem (100 microg/kg) administered prior to ischemia; glibenclamide+diltiazem: both drugs infused as in glibenclamide and diltiazem groups. APD was reduced in control ischemia. Conversely, KATP-channel blockade by glibenclamide lengthened APD and increased postischemic stunning (p < 0.01 vs. control); glibenclamide+diltiazem did not shorten APD but enhanced functional recovery (p < 0.01 vs. glibenclamide). Ca2+ channel blockade improvement of increased stunning provoked by KATP channel inhibition supports the hypothesis that APD shortening due to opening of KATP channels protects against postischemic stunning by limiting Ca2+ influx.

Calcium antagonist protects the myocardium from reperfusion injury by interfering with mechanisms directly related to reperfusion: an experimental study with the ultrashort-acting calcium antagonist clevidipine

To test the hypothesis that calcium antagonists protect the myocardium from reperfusion-induced damage by local myocardial mechanisms just at the time of reperfusion, the myocardioprotective effects of the dihydropyridine clevidipine were investigated, taking advantage of its ultrashort-acting effect. Pigs were subjected to 45 min of myocardial ischemia by occlusion of the left anterior descending coronary artery followed by 4 h of reperfusion. Either clevidipine (0.3 nmol/kg/min, n = 6) or the corresponding amount of vehicle (n = 6) was administered to the ischemic myocardium by retrograde coronary venous infusion over a 30-min period starting 10 min before reperfusion. Hemodynamic variables (heart rate, left ventricular systolic and end-diastolic pressure, max dP/dt, and mean arterial blood pressure) as well as coronary blood flow were measured throughout the experiment. At the end of reperfusion, the area at risk (percentage of left ventricle) was determined by infusion of Evans blue into the left atrium, and the infarct size, by triphenyl tetrazolium chloride (TTC) staining. The plasma level of endothelin-like immunoreactivity (ET-LI) was analyzed in blood from the aorta and the anterior coronary vein before ischemia and at different times during reperfusion. The area at risk was similar in the vehicle and the clevidipine groups. The infarct size, expressed as a percentage of the area at risk, was 80 +/- 9.2 in the vehicle group, whereas it was significantly reduced to 51 +/- 9.2% in the clevidipine group (p < 0.01). Clevidipine did not influence any of the hemodynamic variables measured throughout the study. A nonsignificant trend toward decreased total ET-LI overflow during 4-h reperfusion was observed in the clevidipine-treated pigs compared with vehicle-treated ones (5.3 +/- 1.4 vs. 7.1 +/- 3.4 pmol). These results demonstrate that, in this model of ischemia/reperfusion-induced myocardial infarction, clevidipine reduced the damage to the myocardium when given in association with reperfusion. The local administration of the compound together with its short blood half-life shows that clevidipine reduces reperfusion-induced damage by local mechanisms within the ischemic tissue rather than by peripheral mechanisms.

High level of cholesterol increases coronary vasomotor tone during exercise

BACKGROUND

Coronary vasomotor tone plays an important role in the regulation of myocardial perfusion and influences ischemic threshold significantly. Endothelial dysfunction occurs in the presence of coronary risk factors and is closely linked to the development of atherosclerosis affecting myocardial perfusion and decreasing ischemic threshold.

OBJECTIVE

To study the effect of hypercholesterolemia on coronary vasomotor tone in normal and stenotic coronary arteries at rest and during exercise.

PATIENTS AND METHODS

In total 48 patients were included in the present analysis. Patients were divided into two groups according to the actual levels of serum cholesterol: 18 patients had normal (mean 181 +/- 28 mg%; group 1) and 30 had elevated (mean 263 +/- 46 mg%; group 2) levels of serum cholesterol according to the 4S criteria with a cutoff level of 213 mg% (5.5 mmol/l). Coronary vasomotor tone at rest and during supine bicycle exercise was calculated by dividing mean aortic pressure by radius of coronary vessel obtained using biplanar quantitative coronary angiography. A normal as well as a stenotic vessel segment in each patient were studied.

RESULTS

Normal vessel segments in patients with normal levels of cholesterol (group 1) exhibited no exercise-induced change in coronary vascular tone (+3%, NS), whereas a significant increase in tone (+24%, P < 0.01 versus rest) occurred in those with high levels of cholesterol (group 2). In contrast, stenotic segments in members of both groups exhibited an increase in vascular tone irrespective of the actual level of serum cholesterol.

CONCLUSIONS

Hypercholesterolemia causes a pathologic increase in coronary vasomotor tone of angiographically normal vessel segments during exercise. A similar pathologic response occurs in stenotic arteries, but this is independent of the actual level of serum cholesterol. These findings suggest that hypercholesterolemia influences vasomotor tone of the nonstenosed coronary arteries in patients with coronary artery disease probably through the occurrence of endothelial dysfunction.

Beneficial effects of the T- and L-type calcium channel antagonist, mibefradil, against exercise-induced myocardial stunning in dogs

Abnormalities in calcium homeostasis such as calcium overload have been shown to participate in the pathogenesis of myocardial stunning. The goal of this study was to investigate the effects of mibefradil, a mixed T- and L-type calcium channels antagonist on exercise-induced ischemia (i.e., high-flow ischemia). Nine dogs were permanently instrumented to measure left ventricular wall thickening (Wth) and coronary blood flow (Doppler). Infusion of saline or mibefradil (30 and 40 microg/kg/min, i.v., for 20 min) was started 10 min before exercise (10 min, 14 km/h; slope, 13%) and stopped at its end. Circumflex coronary artery stenosis (pneumatic occluders) was set up 5 min before exercise to suppress exercise-induced increase in mean coronary blood flow without simultaneously affecting Wth at rest. Mibefradil (30 microg/kg/min) was also administered at the beginning of the recovery period in a subset of four dogs. During exercise with saline, Wth was dramatically reduced (-77 +/- 7%; p < 0.05) and recovered only after 24 h. Mibefradil at both doses significantly limited tachycardia during exercise (211 +/- 7 and 210 +/- 5 beats/min vs. 240 +/- 8 beats/min for mibefradil, 30 microg/kg/min, mibefradil, 40 microg/kg/min, and saline, respectively) but exerted no negative inotropic effects. Mibefradil at both doses significantly reduced the intensity of myocardial stunning and the time to recovery in Wth (3 h). Administration of mibefradil at the beginning of the recovery period did not protect against myocardial stunning. Administration of a mixed T- and L-type calcium channel antagonists before ischemia confers cardioprotection against exercise-induced myocardial stunning. This may potentially be related to the limitation of exercise-induced tachycardia and/or the prevention of calcium overload.

Verapamil reduces dipyridamole-induced myocardial ischemia in patients with coronary artery disease

The aim of this study was to evaluate the effects of verapamil administration on dipyridamole-induced transient wall-motion abnormalities as detected by two-dimensional echocardiographic monitoring in patients with coronary artery disease. Twenty-eight patients (16 men and 12 women; mean age, 60+/-7 years) with angiographic evidence of significant coronary artery disease, positive dipyridamole echocardiography test results at basal condition on two consecutive days, were prospectively studied. Patients were randomized to verapamil (360 mg/day) or placebo treatments, given in three divided doses daily for 7 days; at the end of this time, each patient crossed over to the alternate regimen. Dipyridamole echocardiographic testing was repeated at the end of each treatment period. Our data demonstrate that verapamil significantly reduces the dipyridamole-induced wall-motion score index, a quantitative marker of acute myocardial ischemia (1.7+/-0.4 vs. 1.3+/-0.2; p<0.001). Hemodynamic data show that the drug reduces heart rate and rate-pressure product at basal condition (heart rate from 75+/-8 to 67+/-9 beats/min; p<0.001; rate-pressure product from 99+/-13 to 86+/-13 U x 10(-2); p<0.001) and at peak dipyridamole infusion (heart rate from 96+/-8 to 89+/-6 beats/min; p<0.001; rate pressure product from 127+/-21 to 118+/-13 U x 10(-2); p<0.05) with respect to placebo treatment. We conclude that verapamil is able to reduce dipyridamole-induced ischemia, as detected by two-dimensional echocardiographic monitoring, in patients with coronary artery disease by reducing, at least partially, myocardial oxygen consumption. Moreover, its beneficial action could be related to the effects of the drug on coronary collateral circulation and on sympathetic modulation.

Cardioprotective effects of diltiazem infusion in the perioperative period

OBJECTIVE

To evaluate the perioperative effects of intravenous diltiazem infusion on left ventricular functions, hemodynamics and as an anti-ischemic and antiarrhythmic agent in patients undergoing coronary artery bypass grafting (CABG).

METHODS

A double blind, randomised study was performed on 71 patients undergoing elective CABG. Infusion of diltiazem (0.1 mg/kg per h, n = 34) or nitroglycerin (1 microgram/kg per min, n = 37) was given for 24 h starting from onset of cardiopulmonary bypass. Holter monitoring, electrocardiogram and serum cardiac enzymes levels were used to diagnose myocardial ischemia. Myocardial function was assessed by perioperative transesophageal echocardiography.

RESULTS

The two groups did not differ with respect to preoperative and operative data. Diltiazem had no influence on hemodynamic parameters except for significant reduction in post operative heart rate and pulse pressure rate. Transient ischemic events (dilitiazem 10.2% versus nitroglycerin 33.3%, P = 0.15) and transient coronary spasm (diltiazem-6.8% versus nitroglycerin 25.9%, P = 0.15) were reduced in the diltiazem group as compared with the nitroglycerin group. The postoperative incidence of atrial fibrillation (diltiazem 3% versus nitroglycerin 22%, P = 0.03), supra ventricular tachycardia (diltiazem-3% versus nitroglycerin-22%, P = 0.03) and average ventricular premature contraction per h (diltiazem-40.2 +/- 10.2 versus nitroglycerin 53.8 +/- 12.3, P < 0.01) were significantly lower in the diltiazem group. Transesophageal echocardiography showed no significant difference in left ventricular functions and better preservation of left ventricular diastolic functions in post cardiopulmonary bypass period in diltiazem group. In addition mean deceleration time for the E wave on a 12 h post cardiopulmonary bypass period was significantly lower in the diltiazem group as compared with nitroglycerin (diltiazem 131 +/- 6 versus nitroglycerin 171 +/- 6, P < 0.01).

CONCLUSION

The present study demonstrates that diltiazem infusion provides superior anti-ischemic protection and control of supraventricular arrhythmias as compared to nitroglycerin and does not produce any negative inotropic effect, as demonstrated by transesophageal echocardiography.

Cardiac effects of calcium antagonists in systemic hypertension

The calcium antagonists are a class of heterogeneous drugs, with a wide spectrum of direct and indirect cardiac effects that vary a great deal from one drug to another and depend upon formulation and duration of action. Calcium antagonists act by decreasing total peripheral resistance to lower arterial pressure. As a consequence, reflex tachycardia, increased cardiac output, and increased plasma catecholamine and plasma renin activity are commonly seen, particularly with the initial dose and with short-acting dihydropyridines. The abrupt vasodilation can paradoxically elicit angina and even acute myocardial infarction. These hemodynamic and neuroendocrine changes are less pronounced with the long-acting formulations. Most calcium antagonists diminish automaticity of the sinus node, slow conduction in the atrioventricular node, and have little, if any, effect on the automaticity of the myocytes. The dihydropyridines generally have less effect on automaticity and cardiac conduction than nondihydropyridines. The negative inotropic effect is most profound with nondihydropyridines and is greatly reduced or absent with newer dihydropyridines, such as isradipine, felodipine, amlodipine, and nisoldipine. Long-acting calcium antagonists generally improve myocardial oxygenation by unloading the heart, increasing coronary blood flow, and reducing myocardial oxygen consumption. Thus, calcium antagonists have a variety of beneficial effects in patients with hypertensive heart disease: they reduce left ventricular hypertrophy and its sequelae, such as ventricular dysrhythmias, impaired filling and contractility, and myocardial ischemia. Ongoing studies should provide a more conclusive answer regarding the efficacy and safety of calcium antagonists.

A method for repeated high-fidelity micromanometer measurement of intracardiac pressures

Dial-tipped, high-fidelity micromanometers were inserted through polyurethane catheters to acutely measure blood pressures within the chambers of the heart and the great vessels of baboons, rhesus monkeys, and goats. Repeated measurements of atrial, ventricular, aortic, and pulmonary artery pressure were possible with this method, with calibration of micromanometers accomplished immediately prior to and after pressure recordings to assure data accuracy. All attempts to pass micromanometers into the atria in all species were successful. Passage of micromanometers from the left ventricle across the aortic valve and into the aorta was successful in 97% of the attempts in baboons, 100% for rhesus monkeys, and 75% for goats; while insertions into the pulmonary artery from the right ventricle were successful in 64% of the baboons, 40% of the rhesus monkeys, and 75% of the goats. Advantages of this technique are that a permanent conduit for cardiac vascular access is available and that high-fidelity pressure signals may be acquired.

Mechanism of the antiischemic effect of mibefradil, a selective T calcium channel blocker in dogs: comparison with amlodipine

Calcium channel blockers are active in variant angina principally by preventing coronary vasospasm. However, a direct antiischemic effect may also occur. In open-chest dogs, an attack of variant angina was mimicked by a 2-min critical coronary stenosis, and the following reversible myocardial ischemia was assessed by measuring the decrease of segmental shortening. We compared the antiischemic mechanism of mibefradil, a T and L calcium channel blocker, with that of amlodipine, a pure L channel blocker. Both drugs showed a similar relationship between the decrease of the rate-pressure product and the antiischemic effect, but only mibefradil reduced heart rate. Amlodipine and mibefradil at the highest doses tested (20 and 70 micrograms/kg/min, respectively) restored 68 +/- 8 and 76 +/- 5% of segmental shortening in the ischemic area, respectively, as compared with preischemic values. Matching blood pressure (by intraaortic balloon) or heart rate (by atrial pacing) to predrug values showed that the antiischemic effect was mainly afterload-dependent for amlodipine and heart rate-dependent for mibefradil. We conclude that in variant angina, in addition to their antivasospastic effects, calcium channel blockers may be antiischemic by a direct myocardial effect associated with a decrease of the rate pressure product. Blockade of the T channel does not seem to participate in the direct antiischemic effect of mibefradil but could explain the decrease of heart rate.

Contemporary approaches in medical management of patients with stable coronary artery disease

CAD continues to be the principal cause of mortality in the United States, and the largest group of patients with CAD are those with stable angina. Among this group of patients, the most common manifestation of CAD is presence of transient episodes of myocardial ischemia. The presence of transient ischemia and not the severity of angina has been found to be associated with poor clinical outcome in patients with stable CAD. As part of a global treatment strategy for patients with stable CAD, changes in lifestyle and modification of coronary risk factors should be emphasized as an integral part of treatment. Conventional antianginal therapy is quite effective in controlling anginal attacks. Currently, several drugs and therapeutic strategies are available for the treatment of patients with angina (see Table 5). Nitrates are highly effective antianginal drugs with complex beneficial actions in patients with CAD, but their usefulness is limited by development of tolerance during long-term use. When clinically indicated, the use of nitrates should be supplemented with another longer-acting antianginal drug, such as a beta-blocker or a calcium channel blocker. Based on the available data, beta-blockers, when tolerated, seem to be the most effective antianginal drugs for most patients with stable CAD. Beta-blockers are also the most effective anti-ischemic drugs that reduce the magnitude of myocardial ischemia detected during routine daily activities. Calcium channel blockers are also effective vasodilators and good antianginal drugs. The clinician should become familiar with the different actions that this heterogeneous group of drugs has on the heart and vessels. This knowledge allows the clinician to choose the appropriate combination of different antianginal drugs for patients on an individualized basis. It is also critical to develop the treatment strategy by carefully taking into account other associated medical conditions that are frequently encountered in patients with CAD.

Stunned myocardium and the attenuation of stunning by calcium antagonists

Myocardial "stunning" is characterized by a reversible postischemic contractile dysfunction despite full restoration of blood flow. The underlying mechanisms are not clearly understood. Inadequate energy supply and impaired sympathetic neurotransmission may have been excluded. Potential mechanisms, which are not mutually exclusive, may include damage to membranes and enzymes by free radicals, an increase in free cytosolic calcium during ischemia and reperfusion, and a lower calcium sensitivity of myofibrils. The equally pronounced increases in regional contractility in normal and stunned myocardium during postextrasystolic potentiation and the infusion of calcium or the calcium-sensitizing agent AR-L-57, however, suggest an unchanged calcium sensitivity in reperfused myocardium. Pretreatment with calcium antagonists before ischemia attenuates myocardial stunning. This effect is probably related to a lessened myocardial calcium overload during early ischemia. The potential benefit of treatment with calcium antagonists after reperfusion is established remains controversial.

Adenylate cyclase activation promotes the recruitment of coronary vasodilator reserve and improves subendocardial contractility during coronary hypoperfusion

This study was designed to examine the effects of an adenylate cyclase activator, NKH477, on epicardial and endocardial contraction and coronary blood flow (CoF) in the presence or absence of ischemia and to compare it to those of adenosine. We measured coronary pressures (CoP), coronary blood flow, epicardial and endocardial wall thickening (i.e., %EPWT and %ENWT, respectively, by sonomicrometry) in 18 anesthetized dogs. The left circumflex coronary artery was perfused with arterial blood using a pressure controlled servo pump. Propranolol (0.5 mg/kg) and atropine (0.25 mg) were used to minimize the neurogenic effects. CoP decreased from 100 mm Hg to 40 mm Hg with and without drugs. At CoP of 100 mm Hg, intracoronary infusion of NKH477 (10(-8) M/kg/min) produced a two-fold increase in CoF, but there were no changes in either the %EPWT or the %ENWT. During coronary hypofusion at coronary pressures equal to 40 mm Hg, NKH477 increased CoF from 16 +/- 2 to 28 +/- 4 mL/min (p < 0.05) and improved %ENWT significantly from 6 +/- 7 to 23 +/- 7% (p < 0.05). However %EPWT was not improved by NKH477. On the other hand, the intracoronary infusion of adenosine (10 micrograms/kg/min) increased CoF from 16 +/- 5 to 21 +/- 6 mL/min (p < 0.05) at CoP of 40 mm Hg. However, this dose of adenosine failed to improve %ENWT (16 +/- 10% vs. 14 +/- 10%, n.s.). Thus, the improvement of subendocardial function by NKH477 might be related to the improvement of subendocardial perfusion which could be induced by the potentiation of endogenously released adenosine as well as the direct vasodilator effect. This contrasts with the effects of exogenously administered adenosine, which failed to improve subendocardial contractility.

Perioperative Myocardial Protection with Continuous Infusion of Diltiazem in Coronary Bypass Surgery

In a randomized study 120 patients undergoing elective coronary artery bypass grafting were investigated to evaluate the perioperative antiischemic and antiarrhythmic efficacy of diltiazem. The patients received a continuous, perioperative infusion of either diltiazem 0.1 mg/kg/h, N = 60) or nitroglycerin (control group lpg/kg/min, N = 60) over a period of 24 hours. Perioperative monitoring included hemodynamic measurements and 3-channel Holter monitoring up to 24 hours postoperatively; repeated assessment of 12–lead electrocardiogram; and analysis of ischenlia-specific laboratory parameters (CK-MB and troponin-T). Myocardial function was assessed preoperatively at 1 and 4 hours after cardiopulmonary bypass by transesophageal echocardiography (TEE, short axis view, monoplane 5 MHz faced array transducer). The 2 groups did not differ with respect to preoperative and operative data. Except for a significant reduction in perioperative heart rate by an average of 9 beats/min, diltiazem had no influence on hemodynamic parameters. The antiischemic efficacy of diltiazem led to a reduction of the number (17 ± 9 vs. 25 ± 5, p < 0.05) and duration (69 ± 47 vs. 104 ± 87 min, p < 0.05) of transient ischemic events and a lower incidence of perioperative myocardial infarction (3.3 vs. 6.7%) as compared to the nitroglycerin group. Peak values of CK-MB and troponin-T were significantly lower in the diltiazem group. Patients treated with diltiazem had a lower incidence of perioperative atrial fibrillation (5 vs. 18%, p < 0.05) and lower numbers of ventricular premature beats/hour (10 ± 8 vs. 19 ± 22, p < 0.05). The postoperative increase in myocardial function was more pronounced in the diltiazem group. The perioperative infusion of diltiazem does not adversely affect perioperative hemodynamics and myocardial contractility but provides potent antiischemic and antiarrhythmic protection of patients undergoing coronary artery bypass grafting. Future investigations must focus on the role of diltiazem in the improvement of long-term prognosis after coronary bypass surgery.

The role of heart rate in myocardial ischemia and infarction: implications of myocardial perfusion-contraction matching

The pathophysiology of myocardial ischemia traditionally has been attributed to disturbances of oxygen demand, as observed in classic effort-induced angina pectoris, or to a primary disruption of coronary blood supply, as in unstable angina or acute myocardial infarction. Laboratory research eliciting various types of perfusion-contraction matching has challenged such a historical distinction between supply and demand-induced determinants of myocardial ischemia. A growing number of clinical studies analyzing the role of heart rate in the course of coronary heart disease suggest the possibility that a common perfusion-contraction scheme may underlie these diverse clinical manifestations. During experimental myocardial ischemia, produced by a low coronary blood flow, regional perfusion-contraction matching exists in which the energy demands and regional contraction are reduced to match the diminished myocardial substrate supply. Heart rate is a major factor influencing transmural blood flow distribution and regional function, because when coronary vasodilation is maximal there is an inverse relation between the level of heart rate and subendocardial perfusion. Thus, in experimental regional ischemia, increasing heart rate reduces subendocardial flow and contraction, whereas slowing of heart rate causes improvement of contraction associated with increased subendocardial blood flow, accompanied by a decrease in outer wall blood flow. Also, "interventricular steal" of blood from the left ventricle by the right ventricle during ischemia can be reversed by slowing the heart rate in the presence of regional ischemia. Improvement of contraction by heart rate slowing is more than would be expected on the basis of the increase in subendocardial perfusion alone, reflecting a combination of decreased oxygen demand and increased oxygen supply, and separate curves relating blood flow per minute to contractile function are observed at different heart rates. However, when perfusion is normalized for heart rate by expressing subendocardial blood flow in units per beat, a single relation is observed at different heart rates. This observation supports the concept of a close coupling between subendocardial blood flow per beat and regional performance, or perfusion-contraction matching, during various levels of ischemia. Based on these principles, it can be predicted that exercise-induced regional ischemia in the presence of coronary stenosis will be attenuated by several mechanisms when heart rate is slowed using either a beta-blocking agent, or a specific bradycardic drug.(ABSTRACT TRUNCATED AT 400 WORDS)

Ischemia in collateral-dependent myocardium: effects of nifedipine and diltiazem in man

It has recently been shown that ischemia in collateral-dependent myocardium may develop at a very variable threshold in anginal patients; accordingly, the aim of this study was to assess whether nifedipine and diltiazem can increase blood flow to collateralized myocardium in man. Nine patients with complete coronary occlusion filled by collaterals, with no other coronary stenosis, normal left ventricular function, and reproducibly positive exercise tests were studied. They underwent exercise tests off therapy and after acute randomized administration of nifedipine (10 mg sublingually), diltiazem (120 mg orally), and nitroglycerin (0.5 mg sublingually), the latter a drug known to increase blood flow to collateralized myocardium. Following nifedipine, time to 1 mm ST segment depression increased significantly (from 430 +/- 176 to 576 +/- 205 seconds, p < 0.01), while heart rate and rate-pressure product remained unchanged (115 +/- 16 vs 121 +/- 17 beats/min and 199 +/- 29 vs 204 +/- 44 beats/min.mm Hg.10(2), respectively, p = NS for both). Similarly, diltiazem significantly increased time to ischemic threshold from baseline to 638 +/- 125 seconds (p < 0.01), but did not change heart rate and rate-pressure product at 1 mm ST segment depression. Submaximal rate-pressure products were significantly lowered by both nifedipine and diltiazem. Nitroglycerin not only significantly improved time to ischemic threshold (from baseline to 666 +/- 76 seconds, p < 0.01), but also increased heart rate (from baseline to 137 +/- 16 beats/min, p < 0.01) and rate-pressure product (from baseline to 242 +/- 48 beats/min.mm Hg.10(2), p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Atenolol or nicardipine alone is as efficacious in stable angina as their combination: a double blind randomised trial

BACKGROUND

Beta blockers and calcium antagonists are widely used in the management of angina pectoris in the belief that the combination is more efficacious than either drug alone.

METHODS

This double blind randomised crossover placebo controlled study compares the effects of nicardipine, atenolol and their combination in 30 patients with chronic stable angina. Each treatment period lasted 6 weeks with dose titration after 3 weeks. Symptom limited treadmill exercise testing and radionuclide ventriculography at rest was carried out at the end of each treatment period.

RESULTS

Total exercise duration and time to 1-mm ST-segment depression was significantly prolonged by nicardipine and atenolol when compared to placebo, the combination offered no additional benefit. Time to onset of angina was significantly prolonged by nicardipine and the combination but not by atenolol. Indices of left ventricular function were not significantly affected by any treatment other than an increase in left ventricular end diastolic volume on atenolol and the combination.

CONCLUSIONS

Nicardipine and atenolol are equally effective in prolonging exercise duration and time to onset of ischemia in patients with chronic stable angina while the combination appeared to offer no additional benefit. Nicardipine prolonged the time to onset of angina significantly; again there was no further improvement with the combination. Neither drug appears to have an important effect on the parameters of diastolic function studied in patients with chronic stable angina.

Regional redistribution of myocardial perfusion by UL-FS 49, a selective bradycardic agent

The effects of UL-FS 49, a specific bradycardic agent, on systemic hemodynamics, regional myocardial function (sonomicrometry, percentage of segment shortening), and regional coronary blood flow (radioactive microspheres) were studied in open-chest, anesthetized dogs with severe left circumflex coronary artery (LCX) stenosis. UL-FS 49 was administered as two sequential bolus injections of 0.25 mg/kg. Heart rate decreased from 149 +/- 13 beats/min to 102 +/- 6 and 77 +/- 4 beats/min after 0.25 and 0.5 mg/kg cumulative doses of UL-FS 49, respectively. The reduction in heart rate was not associated with any significant change in left ventricular pressure or mean arterial pressure, left ventricular dp/dt, or coronary vascular resistance. Similarly no hemodynamic changes occurred with atrial pacing to the initial heart rate. Application of an LCX stenosis of sufficient severity to produce a 50% reduction in mean LCX blood flow (44 +/- 4 to 22 +/- 2 ml/min) resulted in a significant reduction in the percentage of segment shortening in the ischemic zone (9.8 +/- 1.6% to 6.5 +/- 1.1%). The percentage of segment shortening in the ischemic zone progressively improved to 8.4 +/- 1.2% and 9.4 +/- 0.5% after 0.25 and 0.5 mg/kg UL-FS 49, respectively. Subepicardial perfusion in the ischemic zone was decreased and subendocardial perfusion was increased after administration of UL-FS 49. Consequently the ischemic zone endocardial/epicardial ratio increased from 0.43 +/- 0.08 to 1.12 +/- 0.22 and 1.48 +/- 0.32 with low and high doses of UL-FS 49.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of nitroglycerin and diltiazem on well-developed coronary collateral circulation in conscious dogs

The purpose of the present study was to compare the effects of nitroglycerin and diltiazem on coronary collateral circulation. Studies were conducted in 8 conscious dogs instrumented for the measurement of left circumflex coronary artery (LCCA) flow, subendocardial segment lengths in areas perfused by the LCCA, and left anterior descending coronary artery (LAD). Brief, repeated LCCA occlusions sufficiently developed collateral vessels for the resting metabolic requirement in the LCCA region. One week following the cessation of repeated LCCA occlusions, two-minute coronary occlusions with and without drug pretreatment were performed on separate days. The ischemic responses to coronary occlusions were not altered by diltiazem (50 micrograms/kg, IV), but nitroglycerin (5 micrograms/kg, IV) attenuated myocardial ischemia definitely. The authors conclude that nitroglycerin produces greater effects than diltiazem in attenuating myocardial ischemia in the collateral dependent zone when effects of each drug on systemic and coronary circulation were minimized by pretreatment with small doses.

Effect of exercise intensity and duration on regional function during and after exercise-induced ischemia

BACKGROUND

Transient reversible myocardial dysfunction has been documented after episodes of exercise-induced ischemia. This study was undertaken to determine whether the duration or intensity of exercise affects the severity of postischemic dysfunction in this setting.

METHODS AND RESULTS

Ten dogs were instrumented with ultrasonic microcrystals for measurement of wall thickening, with circumflex coronary artery flow probes, and with hydraulic occluders. Dogs performed low-intensity exercise, which was sufficient to increase coronary perfusion 50% above control, and high-intensity exercise, which was sufficient to double coronary blood flow. To investigate the effects of exercise intensity on postischemic dysfunction, we had dogs perform high-intensity exercise for 5 minutes in the presence of a stenosis. On the alternate day, dogs performed low-intensity exercise for 10 minutes in the presence of a stenosis. These two protocols provide equivalent coronary flow debts. Mean transmural blood flow during high-intensity exercise without stenosis (2.61 +/- 0.54 ml/min/g) was significantly higher than that during low-intensity exercise (1.74 +/- 0.61 ml/min/g, p less than 0.002). During high-intensity exercise with coronary artery stenosis, subendocardial blood flow was significantly lower than that during low-intensity exercise with stenosis (0.64 +/- 0.40 versus 1.08 +/- 0.28 ml/min/g, p less than 0.02). This difference in subendocardial perfusion was associated with greater degrees of regional dysfunction during exercise (circumflex wall thickening was 44 +/- 23% of control for high-intensity exercise versus 60 +/- 18% of control for low-intensity exercise, p less than 0.01). In addition, from 10 to 30 minutes after exercise, wall thickening in myocardium perfused by the circumflex coronary artery remained significantly lower after high-intensity exercise than that after low-intensity exercise. To assess the effects of exercise duration on the severity of postischemic dysfunction, we had dogs perform low-intensity exercise in the presence of a coronary stenosis for 10 minutes and low-intensity exercise for only 5 minutes on alternate days. Systolic wall thickening was significantly lower after low-intensity exercise for 10 minutes than after low-intensity exercise for 5 minutes.

CONCLUSIONS

High-intensity exercise results in greater degrees of subendocardial hypoperfusion and greater degrees of regional dysfunction both during and after exercise-induced ischemia than does low-intensity exercise. Second, exercise duration also exerts an effect on the severity of postischemic dysfunction, although the magnitude of this effect is less important than the effect of exercise intensity.

Inhibition of dipyridamole-induced ischemia by antianginal therapy in humans. Correlation with exercise electrocardiography

BACKGROUND

Dipyridamole echocardiography test (DET: two-dimensional echocardiographic monitoring with dipyridamole infusion up to 0.84 mg/kg in 10 minutes) is a useful tool for the noninvasive diagnosis of coronary artery disease. Aims of the present study were to assess the effects of antianginal drugs on dipyridamole-induced ischemia and to evaluate whether drug-induced changes in DET response may predict variations in exercise tolerance.

METHODS AND RESULTS

Fifty-seven patients with angiographically assessed significant coronary artery disease (greater than 70% lumen reduction in at least one major coronary vessel) performed a DET and an exercise electrocardiography test (EET) in random order both off treatment and on antianginal drugs (beta-blockers, calcium antagonists and nitrates, alone or in various combinations). The criterion for DET positivity was a transient dyssynergy of contraction absent or of a lesser degree in the baseline examination. In DET, two parameters were evaluated: the dipyridamole time (i.e., the time from onset of dipyridamole infusion to obvious dyssynergy) and the wall motion score index. DET sensitivity was 91% off therapy and fell to 65% under therapy (p less than 0.01). In the 37 patients who had a positive DET both off and on therapy, the dipyridamole time increased from 6 +/- 3 (off therapy) to 8 +/- 3 minutes (on therapy) (p less than 0.01). The wall motion score index at peak dipyridamole went from 1.38 +/- 0.14 to 1.31 +/- 0.14 (p less than 0.01). EET and DET yielded concordant (positive versus negative) results in 41 of 57 (71%) patients off and in 35 of 57 (61%) on therapy (p = NS). In the subgroup of 38 patients with both positive DET and EET without treatment, the therapy-induced variations in exercise time were significantly correlated with the variations in dipyridamole time (r = 0.5; p less than 0.01), not with variations in wall motion score index (r = 0.3; p = NS).

CONCLUSIONS

1) Antianginal therapy can protect from dipyridamole-induced ischemia and 2) the therapy-induced changes in DET response parallel variations in exercise tolerance and might be useful for the objective, exercise-independent assessment of the therapy efficacy.

Transmural steal with isoproterenol and exercise in poststenotic myocardium

Transmural coronary steal describes the phenomenon that can occur when coronary narrowing is severe enough to eliminate or nearly eliminate vasodilator reserve in the subendocardial layers. Because blood flow in a maximally vasodilated vascular bed is linearly dependent on perfusion pressure, additional reductions in perfusion pressure will decrease subendocardial blood flow. The subepicardial layers, operating on a different autoregulatory pressure-flow curve, may have vasodilator reserve available and display normal or even elevated blood flow when the subendocardium has reduced perfusion. Therefore, it appears as if subendocardial blood flow has been "stolen" by the subepicardial layers. Blood flow is not actually stolen but redistributed distal to a flow-limiting stenosis and the redistribution tends to favor the subepicardium because it can autoregulate to a lower pressure than the subendocardium. Physiologic interventions such as exercise can alter myocardial oxygen requirements substantially. Vasodilator reserve will be utilized in those parts of the myocardium that have it available, in order to meet the augmented myocardial flow requirements associated with exercise. In poststenotic myocardium, however, decreased vascular resistance in subepicardial layers may reduce poststenotic perfusion pressure which will lead, in turn, to a decrease in blood flow to the subendocardial layers if they are maximally vasodilated. Because transmural systolic function (measured as wall thickening, for example) is largely dominated by changes in subendocardial perfusion, transmural steal during exercise may aggravate the level of dysfunction that occurs by augmenting the subendocardial flow deficit.

Mechanisms of benefit in the ischemic myocardium due to heart rate reduction

The studies reviewed here examine the role of heart rate reduction in the beneficial effect observed following beta-adrenoceptor blockade during exercise-induced ischemia in conscious dogs. To further study the effects of heart rate reduction on regional blood flow in an ischemic bed without collateral circulation, anesthetized swine with controlled coronary perfusion were also studied. Measurements of regional myocardial blood flow (microspheres) and contractile function (sonomicrometers) during steady state exercise in dogs with chronic coronary artery stenosis indicated the existence of severe regional contractile dysfunction and subendocardial ischemia. The administration of beta-adrenoceptor blockade (1.0 mg/kg atenolol) improved regional contractile function when heart rate was reduced from 220 to 165 beats/min. Atrial pacing during exercise to prevent the bradycardia following beta-adrenoceptor blockade eliminated the improved regional function and blood flow. Thus, the beneficial effect of beta-blockade was only apparent when exercise heart rate was reduced. In anesthetized swine with constant inflow coronary perfusion, two levels of coronary hypoperfusion were examined at heart rates of 91 beats/min or 55 beats/min. Bradycardia was produced using the bradycardic agent UL-FS 49 (0.3 mg/kg). Regional contractile function and subendocardial blood flow were markedly improved at the lower heart rate for either level of reduced coronary perfusion, indicating a redistribution of blood flow towards the subendocardium. The improvement in contractile function was larger than predicted on the basis of the improvement in blood flow per min to the subendocardium. Independent relationships between regional contractile function and the subendocardial blood flow per min were observed for each heart rate. Thus, the studies in conscious exercising dogs indicated that heart rate reduction is an essential mechanism for the improvement of ischemic regional myocardial contractile function during exercise by beta-blockade. This is likely the result of the marked improvement in subendocardial blood flow per beat which accompanies the reduced heart rate; regional myocardial blood flow per beat appears to be a predictor of regional contractile function during ischemia both at rest and during exercise.

Prevention of alpha-adrenergic coronary constriction by calcium-antagonists

This manuscript reviews the experimental evidence for a functional antagonism of Ca-antagonists against alpha-adrenoceptor-mediated increases in coronary vasomotor tone. In studies on anesthetized dogs, intravenous nifedipine effectively prevented the alpha 1-adrenoceptor-mediated increase in epicardial coronary resistance, as well as the increase in end-diastolic resistance mediated by both alpha 1- and alpha 2-adrenoceptors during cardiac sympathetic nerve stimulation. Both intracoronary and intravenous administration of nifedipine also prevented the alpha 2-adrenoceptor-mediated increase in coronary resistance distal to severe stenoses, as well as the resulting ischemic dysfunction and net lactate production during cardiac sympathetic nerve stimulation. Felodipine was equally effective as nifedipine in preventing an alpha 2-adrenoceptor-mediated increase in coronary resistance and the resulting contractile dysfunction distal to severe coronary stenoses. alpha 1- and alpha 2-Adrenergic coronary constriction also contribute to the severity of myocardial ischemia in conscious dogs during treadmill exercise. Again, nifedipine improved regional myocardial blood flow and attenuated regional contractile dysfunction during exercise-induced ischemia in conscious dogs with a chronic coronary stenosis. This beneficial effect of nifedipine was attributed to a recruitment of coronary dilator reserve and not to a reduction in heart rate or afterload. In conclusion, there is solid experimental evidence for a functional antagonism of Ca-antagonists against alpha-adrenergic coronary constriction and its contribution to myocardial ischemia.

Possible survival benefit from concomitant beta-but not calcium-antagonist therapy during reperfusion for acute myocardial infarction

To test the hypothesis that long-term beta- or calcium-antagonist therapy begun before the time of myocardial infarction and coronary reperfusion might improve patient in-hospital survival compared with reperfusion alone, 424 consecutive patients successfully reperfused with coronary angioplasty within 12 hours of infarct symptom onset were carefully and retrospectively characterized. Forty-seven patients (11%) were taking beta antagonists and 74 patients (17%) were taking calcium antagonists at the time of infarction. Patients receiving beta antagonists had a more frequent history of hypertension (p less than or equal to 0.001) and prior infarction (p less than or equal to 0.01) than those not so treated and patients receiving calcium antagonists had a more frequent history of prior infarction, prior angina, hypertension and diabetes (all p less than or equal to 0.001) than their nontreated counterparts. Stepwise logistic regression analysis found significant independent correlations between in-hospital death and the following variables: recurrent ischemia (p less than or equal to 0.001); proximal left anterior descending coronary infarct (p less than or equal to 0.001); 3-vessel disease (p = 0.002); patient age (p = 0.004); and initial total occlusion of the infarct artery (p = 0.022). After adjustment for these factors, beta antagonist use (mortality = 0 vs 8% without treatment) was still significantly correlated with improved survival (p = 0.048), whereas calcium-antagonist therapy made no difference in survival. Heart rate and left ventricular end-diastolic pressure upon presentation were significantly lower in patients treated with beta antagonists. Thus, beta-antagonists therapy, but probably not calcium-antagonist therapy, taken before reperfusion for acute myocardial infarction, may improve early survival compared to reperfusion alone. Larger studies will be required to confirm or refute these observations.

Stable angina pectoris: 3. Medical treatment

The therapeutic goals for the patient with angina pectoris are to minimize the frequency and severity of angina and to improve functional capacity at a reasonable cost and with as few side effects as possible. An integrated approach necessitates attention to conditions that might be aggravating angina, such as anemia or hypertension. Alterations in life-style and personal habits, such as cessation of cigarette smoking, are often necessary and should be continually reinforced by the physician. Certain concomitant diseases, such as chronic obstructive pulmonary disease, may influence the selection of drug therapy. Nitrates, beta-adrenergic blockers, and calcium entry blockers are the major classes of drugs that can be used alone or in combination in a program that is designed for the individual patient.

Selective thromboxane A2 receptor blockade in experimental exercise-induced myocardial ischaemia in dogs

Thromboxane A2 receptor stimulation induces blood platelet aggregation and vasoconstriction, both potential causes of impaired perfusion of ischaemic myocardium. To study the potential role of thromboxane A2 receptor blockade in exercise-induced myocardial ischaemia and post-exercise myocardial dysfunction, nine conscious chronically instrumented dogs with single-vessel coronary artery stenosis (ameroid constrictor) were studied before, during and after steady-state treadmill runs which induced regional myocardial ischaemia. Three hours after a control run, the dogs were exercised again after the infusion of a selective thromboxane A2 receptor blocker: BM 13.177 (10 mg kg-1 i.v.). In the control run, systolic wall thickening (WTh, sonomicrometer) in the post-stenotic myocardium decreased from 22.1 +/- 9.1% at rest to 8.8 +/- 5.2% (mean +/- SD). Subendocardial blood flow (microspheres) in the ischaemic area decreased from 0.75 +/- 0.25 to 0.45 +/- 0.27 (ml min-1 g). The WTh in the ischaemic region remained depressed at 20 min after the run. BM 13.177 reduced peak left ventricular (+) dP/dt (micromanometer) and WTh in both control and post-stenotic myocardium at rest, during and after the run. WTh in the ischaemic area was reduced to approximately the same levels during running with BM 13.177 (not significantly different from control exercise) and remained depressed for at least 30 min after the run. Regional myocardial blood flow was not affected by BM 13.177. Thus, selective thromboxane A2 receptor blockade with BM 13.177 had a modest negative inotropic effect and did not improve regional function or blood flow in post-stenotic ischaemic subendocardium.

Complementary mechanisms of atenolol and diltiazem in the clinical improvement of patients with stable angina

The combination of atenolol with diltiazem has been shown to be useful in the treatment of patients with coronary artery disease. Eighteen patients with proven coronary artery disease, stable angina, and no previous myocardial infarction were studied before and after treatment with atenolol (100 mg/day) (9 patients) or diltiazem (180 mg/day) (9 patients). Ischemic threshold at stress test, pressure-rate product at ischemic threshold, direct oxygen consumption at ischemic threshold, and exercise ejection fraction were determined. There was a slight increase in the duration of exercise, maximal oxygen consumption, and ischemic threshold after treatment with each drug. Double product at ischemic threshold decreased from 20.9 to 19.8 (p = NS) with atenolol but increased from 20.1 to 21.9 (p = NS) with diltiazem. Conversely oxygen consumption at ischemic threshold increased with atenolol to nearly significant values from 17.2 to 23.6 (p = 0.067) but not with diltiazem (16.2 to 22.3; p = 0.16). Before treatment, exercise ejection fraction increased less than 10% or decreased from its resting values in all patients but 1 with atenolol and 1 with diltiazem, but exercise ejection fraction increased significantly after treatment with atenolol (60.6 to 67.5; p = 0.02) but not with diltiazem. This improvement was due to a significant reduction in end systolic volume (103.8 to 78.6; p = 0.019), despite a similar increase in heart rate and blood pressure in all patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary collateral reserve during exercise induced ischemia in swine

We determined coronary collateral vasodilator reserve during exercise-induced ischemia in 17 mini-swine. We induced coronary collateral development in the left circumflex bed by placing an ameroid occluder on that artery. Four weeks later we studied the animals at rest and during exercise (EX) eliciting heart rates (HR) of 240 and 265 beats/min. We measured myocardial blood flow with microspheres and myocardial function by wall thickness sonomicrometry gauges. At matched exercise HRs we treated the animals with nifedipine (10 micrograms/kg IV) (EXN 10), nifedipine (100 micrograms/kg IV), (EXN 100), and adenosine infusion (1.2 mg/min/kg) EXAD. EXN 10 did not significantly alter hemodynamics compared to EX but EXN 100 and EXAD both decreased blood pressure significantly (p less than 0.05). Ischemic endocardial/nonischemic endocardial flow ratios and collateral resistance served as indices of vasodilator reserve. In the ischemic zone exercise reduced vasodilator reserve to 24 +/- 3% in the endocardium and 64 +/- 7% in the epicardium. Neither EXN 10 nor EXAD improved exercise-induced ischemia measured either as flow or function. However EXN 100 improved function during exercise-induced ischemia without improving coronary collateral flow. We conclude there is no additional coronary flow reserve during exercise-induced ischemia in the collateral dependent bed of the pig a few days after occlusion that can be recruited. Large doses of nifedipine improve function by direct action on the myocardium or by reducing afterload. The lack of development and deep myocardial distribution of the coronary collateral vessels in the pig may be an important factor of why these nifedipine responses differ from those reported in species which have primarily large epicardial coronary collaterals.

Role of calcium channel blockers in experimental exercise-induced ischemia

Calcium channel blockers, which induce vasodilation by relaxing vascular smooth muscle cells, have proven effective in the treatment of angina pectoris. To study mechanisms of calcium blockade in ischemic heart disease, conscious chronically instrumented dogs with a single coronary artery ameroid constrictor were studied during steady-state treadmill runs which induced regional myocardial ischemia. During exercise-induced ischemia, regional systolic wall thickening and subendocardial blood flow were both significantly reduced in the ischemic zone. Calcium channel blockade with verapamil, diltiazem, or nifedipine enhanced regional systolic wall thickening. Regional subendocardial blood flow in the ischemic region, measured during diltiazem and nifedipine experiments, improved during exercise. Reduced coronary artery resistance in the native vessels and/or recruitment of collaterals appears to largely explain the increased total myocardial blood supply in the jeopardized area and the increased function. However, after diltiazem, reduced exercise heart rates as well as reduced left ventricular end-diastolic pressure also contributed to the improvement in the oxygen-supply imbalance in the ischemic myocardium. These data provide a basis for understanding the efficacy of calcium channel blocker treatment in patients with coronary artery disease.

Dissociation between regional myocardial dysfunction and subendocardial ST segment elevation during and after exercise-induced ischemia in dogs

The onset and resolution of electrical and functional measures of regional myocardial ischemia were examined in nine conscious dogs during control exercise and exercise after beta-receptor blockade. The dogs had been instrumented with an ameroid constrictor and were studied when no regional dysfunction was evident at rest, although severe coronary stenosis or coronary occlusion with collateral circulation development was present. ST segment elevation was measured on subendocardial electrograms, and regional wall motion was studied by sonomicrometry. During control exercise, subendocardial myocardial blood flow in the ischemic zone, normalized to blood flow in the nonischemic zone, decreased. Subendocardial ST elevation increased slowly, was significantly different from control standing values by 2.5 minutes of exercise and returned quickly to control values within 5 minutes after exercise. Percent systolic wall thickening decreased rapidly, was significantly depressed by 1 minute of exercise and did not return to control values until 30 minutes after exercise. A second, identical exercise stress was performed on the same day after a single oral dose (1 mg/kg body weight) of atenolol. In the ischemic zone during exercise after atenolol compared with control exercise, normalized subendocardial myocardial blood flow was improved and significantly less ST elevation occurred, but the onset and resolution of ST elevation were not altered. Systolic wall dysfunction during exercise was significantly less after atenolol, and function returned toward preexercise values by 1 minute after exercise, even more rapidly than ST segment resolution.(ABSTRACT TRUNCATED AT 250 WORDS)

An easy and quick implantation procedure for the measurement of myocardial wall thickness using sonomicrometry

Ultrasonic techniques for the measurement of ventricular dimensions are widely used in acute and chronic experiments. Implantation of ultrasonic crystals is associated with reversible and irreversible myocardial damage which might limit the interpretation of the obtained results, in particular during acute experiments. We therefore developed a sonomicrometric device which can be easily and quickly implanted, and thus reduces the reversible myocardial damage induced by prolonged surgical implantation.

Mechanism of beneficial effect of beta-adrenergic blockade on exercise-induced myocardial ischemia in conscious dogs

We examined the importance of decreased heart rate in the beneficial effect of beta-adrenergic blockade on exercise-induced regional myocardial ischemia and contractile dysfunction in conscious dogs with single vessel coronary stenosis (ameroid constrictor). Studies were performed during control treadmill exercise, which produced regional myocardial ischemia (blood flow measured with microspheres) and wall dysfunction (measured using sonomicrometers). A second run was performed after the administration of atenolol (0.3-1.0 mg/kg i.v.), and the reduced heart rate caused by atenolol during early steady-state running was then prevented by atrial pacing during the latter portion of the run. Atenolol reduced the exercise heart rate from 217 +/- 25 beats per minute (SD, n = 9) to 166 +/- 15, and ischemic zone wall thickening during systole improved from 27 +/- 22% of the resting value in the control run to 50 +/- 25% of the resting value in the atenolol run (p less than 0.01). Atrial pacing then increased heart rate to 217 +/- 23 beats per minute, and regional wall thickening deteriorated to 15 +/- 25% of the resting value. Regional subendocardial blood flow in the ischemic zone during atrial pacing with atenolol was slightly less than that observed in the control run, in both ischemic and control zones, indicating no remaining beneficial effect of atenolol when heart rate reduction was eliminated. We conclude that the only significant mechanism for the improvement in exercise-induced ischemia and wall motion produced by atenolol is a reduction in the exercise heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Elimination of exercise-induced regional myocardial dysfunction by a bradycardiac agent in dogs with chronic coronary stenosis

We have previously demonstrated that the beneficial effect of cardioselective beta-blockade on exercise-induced ischemia is due entirely to negative chronotropism. Therefore we studied the effect of a new bradycardiac agent (UL-FS 49) in 10 dogs with chronic coronary artery stenosis produced by an ameroid constrictor. Regional myocardial function (sonomicrometers, wall thickness) and blood flow (microspheres) were measured during a control treadmill exercise bout and an identical run 3 hr later after the administration of UL-FS 49 (1.0 mg/kg iv). In the control run, heart rate increased from 114 +/- 20 to 230 +/- 19 beats/min and systolic wall thickening (%WT) in the poststenotic myocardium decreased from 23.3 +/- 5.2% at rest to 9.3 +/- 5.0%, a 60% reduction. Subendocardial blood flow in the ischemic area decreased from 1.04 +/- 0.30 to 0.55 +/- 0.40 ml/min/g, blood flow per beat decreased from 9.1 X 10(-3) to 2.5 X 10(-3) ml/g, and mean transmural flow failed to increase (1.06 +/- 0.30 vs 1.08 +/- 0.39 ml/min/g). During exercise with UL-FS 49, heart rate increased from 89 +/- 10 to only 139 +/- 10 beats/min. End-diastolic left ventricular pressure was increased compared with that during the control run (35.7 +/- 3.0 vs 28.9 +/- 5.5 mm Hg) but left ventricular peak systolic pressure and dP/dt were unchanged. %WT in the ischemic zone did not change significantly during exercise with UL-FS 49 (23.3 +/- 7.9% at rest, 21.5 +/- 8.4% during the run), and in the nonischemic zone it increased to the same extent as during the control run.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of exercise-induced myocardial ischemia in dogs with recruitment of coronary vasodilator reserve by nifedipine

There is now evidence that under resting conditions coronary vasodilator reserve exists even in the presence of myocardial ischemia. Therefore, we tested the hypothesis that a vasodilator reserve may exist during exercise so that during exercise-induced ischemia a reduction in coronary constrictor tone can be produced that attenuates the decreases in regional myocardial blood flow and function distal to a severe coronary stenosis without changing the determinants of myocardial oxygen demand. Nine dogs were instrumented with an ameroid constrictor on the left circumflex coronary artery and were studied 2 to 3 weeks later. During a control treadmill run, heart rate increased from 119 +/- 20 to 225 +/- 20 beats/min and peak left ventricular pressure increased from 144 +/- 17 to 163 +/- 28 mm Hg. Poststenotic subendocardial blood flow (measured by a microsphere technique) fell from 1.19 +/- 0.36 to 0.51 +/- 0.30 ml/min X g and systolic wall thickening (by sonomicrometry) decreased from 24.3 +/- 5.8% to 6.0 +/- 6.1%. During an identical run after nifedipine (10 micrograms/kg iv), systemic hemodynamics were not significantly altered. However, subendocardial blood flow was increased to 0.85 +/- 0.51 ml/min X g (p less than .05) and systolic wall thickening to 11.4 +/- 7.8% (p less than .01). We conclude that in this study the amelioration of exercise-induced myocardial ischemia was due to the recruitment by nifedipine of coronary vasodilator reserve.