Newer concepts in the pathophysiology of ischemic heart disease

Beta-adrenergic blockade during severe ischemia

OBJECTIVE

To examine the effect of acebutolol, a beta-adrenergic-receptor blocker, on severe regional myocardial ischemia, specifically the effects on regional myocardial function and metabolism.

DESIGN

Randomized study.

SETTING

Animal laboratory of the Department of Anesthesiology and Critical Care, University of Stellenbosch Medical School.

PARTICIPANTS

Anesthetized open-chest pig model (n = 18).

INTERVENTIONS

Regional left ventricular function and metabolism were evaluated. Severe stenosis was applied to the left anterior descending coronary artery. After establishing regional myocardial ischemia, acebutolol was administered intravenously, and results were compared with controls who did not receive acebutolol. Animals were prospectively randomized to 1 of the groups.

MEASUREMENTS AND MAIN RESULTS

Regional myocardial function and metabolism were assessed by end-systolic pressure relationship, regional systolic shortening, postsystolic shortening, regional myocardial oxygen consumption, and lactate dynamics. Coronary blood flow was determined with a Doppler flow probe. Results indicated that acebutolol increased regional myocardial blood flow, and this resulted in less severe regional myocardial ischemia, improved function, and an increase in regional myocardial oxygen consumption.

CONCLUSION

The beta-Adrenergic-receptor antagonist was successful in reducing regional myocardial ischemia in this model. This reduction was achieved by an increase in coronary blood flow, which resulted in an improvement in regional mechanical function and an increase in oxygen consumption.

Risk stratification after acute myocardial infarction in the reperfusion era

Historically, risk stratification for survivors of acute myocardial infarction (AMI) has centered on 3 principles: assessment of left ventricular function, detection of residual myocardial ischemia, and estimation of the risk for sudden cardiac death. Although these factors still have important prognostic implications for these patients, our ability to predict adverse cardiac events has significantly improved over the last several years. Recent studies have identified powerful predictors of adverse cardiac events available from the patient history, physical examination, initial electrocardiogram, and blood testing early in the evaluation of patients with AMI. Numerous studies performed in patients receiving early reperfusion therapy with either thrombolysis or primary angioplasty have emphasized the importance of a patent infarct related artery for long-term survival. The predictive value of a variety of noninvasive and invasive tests to predict myocardial electrical instability have been under active investigation in patients receiving early reperfusion therapy. The current understanding of the clinically important predictors of clinical outcomes in survivors of AMI is reviewed in this article.

Does alpha 1-adrenergic blockade influence regional blood flow regulation in hearts with coronary artery occlusion?

The effects of selective alpha 1-adrenergic blockade with doxazosin on regional myocardial tissue blood flow was studied in anaesthetized cats with acute coronary artery occlusion. Reflex tachycardia was prevented by selective beta 1-adrenergic blockade with atenolol and coronary perfusion pressure was kept constant by partial stenosis of the descending aorta. Administration of atenolol reduced cardiac mechanical work-load by its negative inotropic and chronotropic effects, and reduced myocardial tissue blood flow in normally perfused myocardium. This reduction was most pronounced in the endocardial half-layer of the myocardium adjacent to the ischaemic region. Administration of doxazosin in this situation clearly reduced peak systolic and coronary perfusion pressure. But when coronary perfusion pressure was raised to pre-administration values, measurements of regional blood flow revealed no changes either in ischaemic or non-ischaemic myocardium. Also, there was no sign of redistribution of blood flow between endocardial and epicardial tissue in any area. This study, therefore, indicates that alpha 1-adrenoceptors play a minor role in the regulation of coronary blood flow in normal myocardium as well as ischaemic myocardium.

Profile of an ideal antianginal agent

Many antianginal agents are available for the treatment of coronary artery disease. These agents act by influencing the determinants of myocardial oxygen supply and demand. The 3 main classes of agents are the nitrates, beta-adrenergic blocking agents and the calcium entry blockers. Agents from all 3 classes have shown efficacy in treating both symptomatic and asymptomatic myocardial ischaemia. However, some patients cannot be treated with these agents because of side effects or contraindications. An ideal antianginal drug should effectively treat both angina and silent ischaemia. Additionally, it should be free of side effects, allow for maintenance of physical performance and be metabolically neutral. New agents are being developed which strive for this goal.

Effects of an increase in haemoglobin O2 affinity produced by BW12C on myocardial function in the erythrocyte-perfused rabbit heart in vitro and myocardial infarct size in the dog

The effects of BW12C on myocardial function in the erythrocyte-perfused rabbit heart and on myocardial infarct size in the anaesthetized dog have been evaluated. Perfusion of rabbit hearts with erythrocytes pretreated with BW12C (10(-3) M-4 X 10(-3) M) produced concentration-dependent decreases in left ventricular pressure (LVP), LVP dP/dt and coronary perfusion pressure. A concomitant decrease in PO2 and an increase in lactate production by the myocardium was also observed. Perfusion of rabbit hearts with Krebs Henseleit buffer containing BW12C (10(-5)-10(-4) M) caused no change in measured variables. Although BW12C (10(-3) M) caused a small decrease in LVP, coronary perfusion pressure and heart rate, these changes were not significant. In anaesthetized dogs, an infusion of BW12C (total dose 50 mg kg-1, i.v.) caused small, but significant, changes in haemodynamic status. The oxygen saturation curve was shifted to the left and relative % oxygenation (P20) was shifted to the left throughout the course of the experiment. (P20, control 16.3 +/- 0.4 mmHg; after BW12C 7.9 +/- 1.4 mmHg). Pretreatment with BW12C (total dose 50 mg kg-1) caused no change in area at risk but significantly increased the myocardial infarct size by 410%. These studies with BW12C demonstrate that alteration in haemoglobin-oxygen affinity can induce adaptive physiological changes in tissue function and metabolism and can assume a critical role when oxygen supply may be impaired due to a flow-limiting stenosis.

Calcium-channel blockers for combined angina pectoris and systemic hypertension

Calcium-channel blockers have been successfully used in the treatment of angina of effort and systemic hypertension. Many patients present with concomitant angina pectoris and hypertension. Controlled clinical trials demonstrate that the calcium-channel blockers are safe and effective as monotherapy in the treatment of these patients, and that their use compares favorably with that of propranolol. The effectiveness of these agents in hypertension appears to be primarily due to their ability to induce systemic vasodilation. Calcium-channel blockers have several therapeutic effects in angina pectoris. Beneficial actions on the major determinants of oxygen consumption, i.e. heart rate, blood pressure and contractility, are generally seen. The potent coronary vasodilating actions of these agents allow for increased coronary blood flow. Improvements in ventricular compliance, regression of left ventricular hypertrophy and cardioprotection appear to be additional effects of the calcium-channel blockers; their contribution to the drugs' overall therapeutic efficacy is presently being evaluated. Calcium-channel blockers are a welcome addition to drug regimens available for the management of patients with coexisting angina pectoris and hypertension.

The surgeon's role in the treatment of acute myocardial infarction

Most acute myocardial infarctions are associated with thrombotic occlusion superimposed on a significant coronary arterial atherosclerotic plaque. Restoration of coronary flow can limit ultimate infarct size if accomplished within 4 to 6 hours of the onset of symptoms. Thrombolytic therapy with intracoronary and intravenous streptokinase or by tissue-specific plasminogen activator is effective as a rapid means of reperfusing infarcting myocardium. Percutaneous coronary angioplasty and surgically constructed bypass grafts can convert a tenuous source of coronary flow to reliable, unrestricted myocardial perfusion. Although acute surgical revascularization can be performed during an evolving infarction, the time restraints imposed by ischemic cellular injury dictate that bypass grafting serve as an adjunct to thrombolytic therapy as a means of preventing rethrombosis and providing global as well as regional revascularization. The vast potential benefit for minimizing morbidity and mortality resulting from quantitative limitation of acute myocardial necrosis mandates scientific evaluation of the new armamentarium directed at medical-surgical reperfusion.

Evidence for isosorbide dinitrate (ISDN) promoting effect on prostacyclin release by the lung and prostacyclin implication in ISDN-induced inhibition of platelet aggregation in humans

We investigated thromboxane B2 (TxB2), 6-keto-PGF1 alpha (6KPGF1 alpha reflecting prostacyclin), PGE2 and PGF2 alpha plasma levels; TxB2, PGE2 and PGF2 alpha platelet production and platelet aggregation response in ascending aorta (reflecting trans-pulmonary difference) and in venous coronary sinus (reflecting transcardiac difference) simultaneously in patients with ischemic heart disease, before and after right-atrial administration of 3 mg ISDN bolus. Transcardiac differences were scarce before as well as after ISDN administration. In aortic blood, ISDN administration into the right atria resulted in a significant increase in prostacyclin and PGF2 alpha plasma levels (472% and 242%, respectively), a decrease of both PGE2 plasma level (-173%) and PGE2 platelet production (-485%) and a marked lowering of platelet aggregation response to ADP, concomittantly. In contrast, TxB2-related features were poorly affected by ISDN. In coronary sinus blood, the aortic increase in 6KPGF1 alpha and PGF2 alpha plasma levels was detected to a lower extent whereas the characteristics of platelet aggregation had returned to control levels. By contrast, PGE2 plasma level (-191%) and PGE2 platelet production (-133%) were lower than prior ISDN administration. The results we report here, strongly support the view that ISDN promotes release of prostacyclin and PGF2 alpha from the lung and inhibit PGE2 production. These prostanoids may be responsible for the concomittant platelet reactivity lowering, thus providing a basis for understanding how ISDN might relieve myocardial ischemia favoring prostanoid mediated vasodilation and inhibition of platelet reactivity.

Anti-arrhythmic effects of prazosin and propranolol during coronary artery occlusion and re-perfusion in dogs and pigs

Open-chest dogs and pigs anaesthetized with pentobarbitone were used to evaluate the anti-arrhythmic effect of prazosin and propranolol during a 30 min period of occlusion of the left anterior descending coronary artery followed by 15 min of re-perfusion. In dogs, both prazosin and propranolol reduced the incidence of ventricular premature depolarizations and ventricular tachycardia during the occlusion period. During the 45 min period of occlusion and re-perfusion, the incidence of ventricular fibrillation was significantly reduced in the prazosin-treated and propranolol-treated dogs. In pigs prazosin reduced the incidence of ventricular premature depolarizations during occlusion and propranolol reduced the incidence of both ventricular premature depolarizations and ventricular tachycardia during occlusion, but the incidence of ventricular fibrillation was not significantly reduced in the prazosin- and propranolol-treated pigs. Prazosin reduced arterial pressure and propranolol lowered heart rate in both dogs and pigs, but a comparison of mean arterial pressure and heart rate in animals surviving and those not surviving the 30 min of coronary artery occlusion and 15 min of re-perfusion showed no significant difference.