Differences in platelet aggregation in coronary sinus and aortic blood in patients with coronary artery disease: effect of propranolol

Effect of β-blockers on platelet aggregation: a systematic review and meta-analysis

AIMS

Platelets play an important role in cardiovascular disease, and β-blockers are often prescribed for cardiovascular disease prevention. β-Blockers may directly affect platelet aggregation, because β-adrenergic receptors are present on platelets. There is uncertainty about the existence and magnitude of an effect of β-blockers on platelet aggregation. The aim of this study was to perform a systematic review and meta-analysis of the effect of β-blockers on platelet aggregation.

METHODS

MEDLINE and EMBASE were searched until April 2014. Two reviewers independently performed data extraction and risk of bias assessment. Type of β-blocker, population, treatment duration and platelet aggregation were extracted. Standardized mean differences were calculated for each study and pooled in a random-effects meta-analysis.

RESULTS

We retrieved 31 studies (28 clinical trials and three observational studies). β-Blockers decreased platelet aggregation (standardized mean difference -0.54, 95% confidence interval -0.85 to -0.24, P < 0.0001). This corresponds to a reduction of 13% (95% confidence interval 8-17%). Nonselective lipophilic β-blockers decreased platelet aggregation more than selective nonlipophilic β-blockers.

CONCLUSIONS

Clinically used β-blockers significantly reduce platelet aggregation. Nonselective lipophilic β-blockers seem to reduce platelet aggregation more effectively than selective nonlipophilic β-blockers. These findings may help to explain why some β-blockers are more effective than others in preventing cardiovascular disease.

ADP-induced platelet aggregation and metoprolol treatment of myocardial infarction patients. A controlled study

The effect of metoprolol on platelet aggregation was investigated in a group of postmyocardial infarction (MI) patients. The study was double-blind and included 63 subjects; 30 patients were maintained on metoprolol and 33 received placebo treatment. Adenosine diphosphate (ADP)-induced platelet aggregation was carried out in each subject close to 4 weeks after the acute MI. It was demonstrated that metoprolol as compared to placebo did not influence ADP-induced platelet aggregation in the present clinical setting.

The Antihypertensive Efficacy and Safety of a Chronotherapeutic Formulation of Propranolol in Patients With Hypertension

This study evaluated the antihypertensive efficacy and tolerability of a chronotherapeutic formulation of propranolol designed for nighttime dosing (propranolol controlled release [CR]). A total of 434 patients with mild-to-moderate hypertension were randomized to placebo or to one of four doses of propranolol CR (80, 120, 160, or 640 mg/d). At baseline, the mean morning blood pressures were similar in each treatment group and averaged 152/101 mm Hg. After 8 weeks of treatment, morning diastolic blood pressure, the primary efficacy measurement, was significantly reduced from baseline in placebo (-6.98 mm Hg) and all propranolol groups (p<0.001). The decreases ranged from 10.1 mm Hg in the 80-mg/d group to 11.0 mm Hg in the 120-mg/d group and were significantly larger than placebo in the 120-, 160-, and 640-mg/d groups (p<0.05). Blood pressure measured in the evening (trough) demonstrated similar antihypertensive efficacy. Heart rate and rate-pressure product were reduced in a dose-related manner by propranolol CR. The formulation was well tolerated with only fatigue and dizziness being reported more frequently than in the placebo group. Propranolol CR is an effective antihypertensive formulation that may reduce blood pressure during the morning period of maximum cardiovascular risk.

Suppression of silent ischemia by metoprolol without alteration of morning increase of platelet aggregability in patients with stable coronary artery disease

To determine the effect of metoprolol on silent ischemia and platelet aggregability, 10 patients with coronary artery disease were studied with a randomized, double-blind, placebo-controlled, crossover trial. Patients were treated with metoprolol (200 mg b.i.d.) or placebo for 1 week and then received the alternate therapy. Two days before the end of each treatment period, platelet aggregability was studied for 24 hours, and a 48-hour ambulatory electrocardiogram was obtained. Compared with placebo, metoprolol significantly decreased the total number (from 26 to 4, p less than 0.1) and duration (from 735 to 84 minutes, p less than 0.01) of silent ischemic episodes. This decrease was accompanied by a decrease in the mean blood pressure (from 127/81 to 118/71 mm Hg, p less than 0.01) and the mean heart rate (from 70 to 54 beats/min, p less than 0.01). The incidence of silent ischemic episodes in the morning was significantly higher in untreated patients than in treated patients. The few episodes observed during metoprolol treatment occurred at the same time as the peak incidence observed during placebo treatment. During placebo treatment, platelet aggregability increased from 6:00 to 9:00 AM as reflected by a decrease in the threshold concentrations of ADP and epinephrine required to induce biphasic platelet aggregation (from 4.8 +/- 0.8 to 2.6 +/- 0.4 microM, p less than 0.02; and from 7.3 +/- 2.3 to 1.8 +/- 0.9 microM, respectively, p less than 0.02). Metoprolol did not alter the basal level nor blunt the morning increase of platelet aggregability.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha- and beta-blockade in angina pectoris

Angina pectoris encompasses a clinically diverse group of syndromes in which the common factor is myocardial ischaemia resulting from an imbalance between oxygen requirement and delivery. Anginal pain is most frequently precipitated by exercise, but may occur without any apparent cause at rest. Fixed obstructions in the coronary vessels, often attended by thickening of the overlying coronary artery media, are the most frequent cause of ischaemic cardiac pain. The resulting myocardial fibrosis impairs efficient filling and emptying of the ventricles, further aggravating the functional embarrassment resulting from the reduced coronary blood flow. In stable coronary heart disease the increased energy demands during exercise are associated with the rapid development of a haemodynamic profile characteristic of acute left ventricular failure. This results in further substantial increases in pressure work, wall stress and oxygen consumption of the left ventricle. The reflex sympathoadrenal consequences of these primary haemodynamic changes lead to further mechanical and electrical embarrassment of the ischaemic heart. Increased stimulation of beta 1- and beta 2-receptors in the heart increases heart rate and contractility and thereby myocardial oxygen demand. Increased stimulation of alpha-receptors in the peripheral veins and arteries indirectly increases left ventricular oxygen demand still further by increasing preload and afterload, respectively. The reduced blood flow to the endocardium enhances its sensitivity to increased sympathoadrenal stimulation and facilitates initiation of arrhythmias. Blockade of all adrenergic activity, particularly in the myocardium, coronary arteries and peripheral blood vessels should, therefore, help alleviate the myocardial ischaemia. There is a rational argument for the use of alpha-blockade in coronary heart disease, particularly in conjunction with beta-blockade. Attenuation of the risk of coronary spasm and ventricular arrhythmias and reduction of pressure work and left ventricular afterload are amongst the potential attributes of alpha-blockade. Alone, however, their utility is severely limited by the risks of hypotension and reduction in coronary perfusion pressure and reflex oxygen-wasting tachycardia. Alone, alpha-adrenoceptor antagonists have no place in the treatment of angina pectoris. beta-Adrenoceptor blocking drugs competitively inhibit catecholamines at both cardiac and peripheral vascular beta-adrenergic receptors. Their main advantage is that they reduce many of the important determinants of myocardial oxygen consumption, particularly by reducing the heart rate during exercise.(ABSTRACT TRUNCATED AT 400 WORDS)

The peripheral platelet count and ADP-induced platelet aggregation in response to metoprolol and propranolol as studied in young healthy male volunteers

In previous studies, we have demonstrated that the oral administration of one single dose of metoprolol as well as of propranolol causes a significant increase in the venous platelet concentration. In the present study, we investigated the effect of metoprolol, administered twice daily over a period of 1 week, on the venous platelet count in a group of healthy volunteers. The results showed that the drug significantly raised the venous platelet concentration over the whole study period. Further, the effect of 1 week's metoprolol and propranolol medication on ADP-induced platelet aggregation was investigated in another group of healthy subjects; the results were compared to those obtained during 1 week of placebo medication. It was demonstrated that neither of the two beta-blocking drugs affected ADP-induced platelet aggregability; this was true on the first as well as on the last day of study.

Technical considerations for platelet aggregation and related problems

Platelet aggregation generally is ordered by the physician to evaluate platelet function in hemorrhagic or thrombotic disorders. Malfunction of the platelet may be the result of an intrinsic congenital defect or an acquired problem induced by drugs or certain circulating plasma factors. It is necessary to obtain information from the patient with respect to family history, drug ingestion, physical or mental stress. In addition, other laboratory studies should be obtained to rule out general coagulation disorders affecting the plasma factors. A bleeding time will be helpful in establishing the severity of any platelet dysfunction. Technical considerations with regard to the preparation of the samples are of primary importance in determining platelet aggregation. Aggregating studies require the use of a variety of binding agents. (Studies on shape change, adhesion of platelets, release of platelet granule substance, and or lysis with extrusion of cytoplasmic constituents may be helpful in certain cases.) Instrumentation for platelet aggregation presently is available in many hospitals. The technical factors to be considered for routine aggregation studies include the type and strength of anticoagulant, centrifugation technique used in preparing the platelet-rich and platelet-poor plasma, platelet concentration, time of storage of the sample after venipuncture and after centrifugation, temperature, and the mixing of the sample. In general, critical concentrations of each reagent should be employed to improve the discrimination capability of the assay. Small differences in response may be obliterated by using excessive concentrations of a given reagent. Comparison in response to the test platelets with control platelets is best done at the same time by performing the aggregation in a dual instrument so that handling procedures will be identical and artifactual differences eliminated.

The cardiovascular and platelet effects of epoprostenol (prostacyclin, PGI2) are unaffected by beta-adrenoceptor blockade in man

1 Atenolol 0.2 mg/kg i.v., propranolol 0.2 mg/kg i.v. or placebo were given in a double-blind crossover study to six healthy male subjects, and the effects of a subsequent infusion of epoprostenol (prostacyclin, PGI2) 0-6 ng kg-1 min-1 monitored. 2 PGI2 caused a tachycardia, a fall in diastolic blood pressure, a rise in pulse pressure, reduction in pre-ejection period (PEP) and rise in left ventricular ejection time index (LVETI), headache and facial flushing at doses of PGI2 greater than 2 ng kg-1 min-1,, (P less than 0.05). 3 Beta-adrenoceptor blockade did not prevent the tachycardia in response to PGI2, and did not interact with any of the other dynamic effects of PGI2. 4 In vitro, PGI2 at 1 and 2 ng/ml inhibited platelet aggregation to ADP (P less than 0.01), although no significant effect on platelet aggregation was seen in the in vivo study. Atenolol and propranolol at a final concentration of 1 microgram/ml did not affect this in vitro study. Atenolol and propranolol at a final concentration of 1 microgram/ml did not affect in vitro effect of PGI2 on platelet aggregation. 5 Pretreatment with atropine 0.04 mg/kg i.v. in three subjects did not attenuate the tachycardia caused by PGI2 infusion, even though the baseline heart rate was increased. 6 Adverse effects to PGI2 infusion included sudden bradycardia, pallor and sweating, suggesting that the Bezold-Jarisch reflex seen in animals in response to PGI2 may also occur in humans. 7 Neither increased sympathetic drive nor vagal withdrawal are likely causes of the tachycardia following PGI2 infusion.

Role of blood platelets and prostaglandins in coronary artery disease

In the last decade, several studies evaluating blood platelet function in patients with coronary heart disease have been reported. Although several platelet function abnormalities such as enhanced platelet aggregation, decreased platelet survival and increase in platelet release reaction in the stable condition and during stress in patients with myocardial ischemia have been recognized, the mechanism of these abnormalities is just beginning to be understood. Discovery of certain platelet and endothelium-generated prostaglandins has provided some information as to the possible mechanism of platelet dysfunction. Abnormalities of prostaglandin production and platelet sensitivity to various prostaglandins may have an important bearing on the enhanced platelet aggregation in vivo, genesis of atherosclerosis and probably precipitation of acute ischemic events. Since the discovery of these prostaglandins, the precise mode of action of several commonly used platelet-active drugs has been clarified. Development of new drugs acting at selective steps in the prostaglandin pathways may provide some exciting novel therapeutic procedures in patients with coronary heart disease.

Ultrastructure of platelets before and during atrial stimulation. Differences in the coronary sinus and aortic blood in patients with angina pectoris

We studied 15 patients with strain-induced angina pectoris performing an ultrastructural analysis of platelets withdrawn simultaneously from the aorta and coronary sinus. In coronary sinus blood, all platelets exhibited shape changes with pseudopod formation but without degranulation. In the aortic blood we observed on average fewer shape changes of the platelets. During tachycardia stress, shape change formation did not increase in most patients. In three patients however, micro-aggregation of the platelets was observed only in the aortic blood. These patients had a paucity of risk factors. They also had fewer critical coronary artery stenoses compared with the other patients. We conclude that there may be minority among the patients with angina pectoris exhibiting hyper-aggregation of the platelets.

Platelet aggregates in peripheral and coronary-sinus blood in patients with spontaneous coronary-artery spasm

Five out of twenty patients with vasotonic angina had frequent spontaneous attacks with dramatic ST-segment alterations. These attacks were consistently associated with an increase in platelet aggregates in coronary-sinus blood but not arterial blood. This increase in platelet aggregates during coronary-artery spasm suggests a potential causal role for the platelet in this subset of patients with ischaemic heart-disease. However, the increase in platelet aggregates could be secondary to reduced coronary blood-flow.