Changes in ischemic blood flow distribution and dynamic severity of a coronary stenosis induced by beta blockade in the canine heart

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 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.

Ivabradine in Patients with Stable Coronary Artery Disease: A Rationale for Use in Addition to and Beyond Percutaneous Coronary Intervention

Heart rate is an established prognostic marker for longevity and is an important contributor in the pathophysiology of various cardiovascular diseases, including ischemic heart disease and heart failure. Most ischemic episodes are triggered by an increase in heart rate, which causes an imbalance between myocardial oxygen delivery and consumption. In addition, increased heart rate is a modifiable risk factor for chronic heart failure. Ivabradine, an inhibitor of I_f ion channels, is an approved second-line anti-ischemic drug for the treatment of angina. Ivabradine has been shown to decrease the risk of hospitalization in patients with chronic heart failure who were previously treated with β-blockers, renin-angiotensin system blockers or mineralocorticoid receptor antagonists. This review describes the rationale for the pathophysiological and clinical use of ivabradine as an anti-ischemic agent in patients with stable coronary disease and highlights its benefits and drawbacks compared with other first- and second-line anti-anginal drugs. The review also highlights the role of ivabradine as a treatment for patients with high-risk coronary artery disease in whom first-line anti-anginal drugs are insufficient or inadequate and percutaneous coronary intervention is contraindicated or revascularization is incomplete or unsuitable.

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.

Increased exercise ejection fraction and reversed remodeling after long-term treatment with metoprolol in congestive heart failure: a randomized, stratified, double-blind, placebo-controlled trial in mild to moderate heart failure due to ischemic or idiop

BACKGROUND

the effects of long-term administration of beta-blockers on left ventricular (LV) function during exercise in patients with ischemic heart disease (IHD) and idiopathic dilated cardiomyopathy (DCM) are controversial.

PATIENTS AND METHODS

patients with stable congestive heart failure (CHF) (New York heart association [NYHA] class II and III) and ejection fraction (EF) < or =0.40 were randomized to metoprolol, 50 mg t.i.d. or placebo for 6 months. Patients were divided into two groups: ischemic heart disease (IHD) and idiopathic dilated cardiomyopathy (DCM). The mean EF was 0.29 in both groups and 92% were taking angiotensin-converting enzyme (ACE) inhibitors. In the IHD group, 84% had suffered a myocardial infarction (MI) and 64% had undergone revascularization at least 6 months before the study. LV volumes were measured by equilibrium radionuclide angiography. Mitral regurgitation was assessed by Doppler echocardiography. All values are changes for metoprolol subtracted by changes for placebo.

RESULTS

metoprolol improved LV function markedly both at rest and during sub-maximal exercise in both groups. The mean increase in EF was 0.069 at rest (P<0.001) and 0.078 during submaximal exercise (P<0.001). LV end-diastolic volume decreased by 22 ml at rest (P=0.006) and by 15 ml during exercise (P=0.006). LV end-systolic volume decreased by 23 ml both at rest (P=0.001) and during exercise (P=0.004). Exercise time increased by 39 s (P=0.08). In the metoprolol group, mitral regurgitation decreased (P=0.0026) and only one patient developed atrial fibrillation vs. eight in the placebo group (P=0.01).

CONCLUSION

metoprolol improves EF both at rest and during submaximal exercise and prevents LV dilatation in mild to moderate CHF due to IHD or DCM.

Contributions of heart rate and contractility to myocardial oxygen balance during exercise

The respective contributions of heart rate (HR) reduction and left ventricular (LV) negative inotropy to the effects of antianginal drugs are debated. Accordingly, eight instrumented dogs were investigated during exercise at spontaneous and paced HR (250 beats/min) after administration of either saline, atenolol, or ivabradine (selective pacemaker current channel blocker). During exercise, atenolol and ivabradine (both 1 mg/kg iv) similarly reduced HR (-30% from 222 +/- 5 beats/min), and LV mean ejection wall stress was not altered. LV dP/dt(max) was reduced by atenolol but not ivabradine. Diastolic time (DT) was increased by atenolol versus saline (195 +/- 6 vs. 123 +/- 4 ms, respectively) and to a greater extent by ivabradine (233 +/- 11 ms). Myocardial oxygen consumption (MVo(2)) was lower under ivabradine and atenolol versus saline (6.7 +/- 0.6 and 4.7 +/- 0.4 vs. 8.1 +/- 0.6 ml/min, respectively, P < 0.05). Under pacing, DT and MVo(2) were similar between ivabradine and saline but significantly reduced with atenolol. Thus HR reduction and negative inotropy equally contribute to the reduction in MVo(2) during exercise in the normal heart. The negative inotropy limits the increase in DT afforded by HR reduction.

Esmolol and cardiopulmonary bypass during reperfusion reduce myocardial infarct size in dogs

BACKGROUND

Infarct size can be reduced by beta-blockade in acute myocardial ischemia. However it is unknown whether myocardial salvage is still effective when beta-blockade is limited to reperfusion.

METHODS

After initiation of cardiopulmonary bypass, 20 dogs were submitted to 2 hours of regional left ventricular ischemia, followed by 2 hours of reperfusion. In 11 dogs beta-blockade was started with the onset of reperfusion (esmolol group). The remaining dogs received no treatment (control, n = 9). Infarct size was determined by tetrazolium chloride staining. Myocardial water content (MWC) and ultrastructural damage (electronmicroscopy) were determined from transmural biopsies.

RESULTS

Infarct size was significantly smaller in the esmolol group compared with control (49% versus 68%, p < 0.05). After 2 hours ischemia there was no difference in MWC between groups, whereas after 2 hours reperfusion MWC of ischemic myocardium was significantly lower in the esmolol group than in the control (p < 0.05). Ultrastructural changes were typical for ischemia-reperfusion injury in both groups.

CONCLUSIONS

Beta-blockade may be cardioprotective during reperfusion through various mechanisms and may enhance myocardial salvage, even when treatment is initiated as late as with the onset of reperfusion.

Effects of critical coronary stenosis on global systolic left ventricular function quantified by pressure-volume relations during dobutamine stress in the canine heart

OBJECTIVES

In this study we quantified the effects of a critical coronary stenosis on global systolic function using pressure-volume relations at baseline and during incremental dobutamine stress.

BACKGROUND

The effects of coronary stenosis have previously been analyzed mainly in terms of regional (dys)function. Global hemodynamics are generally considered normal until coronary flow is substantially reduced. However, pressure-volume analysis might reveal mechanisms not fully exposed by potentially load-dependent single-beat parameters. Moreover, no systematic analysis by pressure-volume relations of the effects of dobutamine over a wide dose range has previously been presented.

METHODS

In 14 dogs left ventricular volume and pressure were measured by conductance and micromanometer catheters, and left circumflex coronary flow by Doppler probes. Measurements in control and with left circumflex stenosis were performed at baseline and at five levels of dobutamine (2.5 to 20 microg/kg/min). The end-systolic pressure-volume relation (ESPVR) dP/dtMAX vs. end-diastolic volume (dP/dtMAX - V(ED)) and the relation between stroke work and end-diastolic volume (preload recruitable stroke work [PRSW]) were derived from data obtained during gradual caval occlusion.

RESULTS

In control, dobutamine gradually increased heart rate up to 20 microg/kg/min, the inotropic effect blunted at 15 microg/kg/min. With stenosis, the chronotropic effect was similar, however, contractile state was optimal at approximately 10 microg/kg/min and tended to go down at higher levels. At baseline, the positions of ESPVR and PRSW, but not of dP/dtMAX - V(ED), showed a significant decrease in function with stenosis. No differences between control and stenosis were present at 2.5 microg/kg/min; the differences were largest at 15 microg/kg/min.

CONCLUSIONS

Pressure-volume relations and incremental dobutamine may be used to quantify the effects of critical coronary stenosis. The positions of these relations are more consistent and more useful indices than the slopes. The positions of the ESPVR and PRSW show a reduced systolic function at baseline, normalization at 2.5 microg/kg/min and a consistent significant difference between control and stenosis at dobutamine levels of 5 microg/kg/min and higher.

Esmolol and percutaneous cardiopulmonary bypass enhance myocardial salvage during ischemia in a dog model

Despite recent advances in techniques of reperfusion for acute myocardial ischemia, myocardial salvage remains suboptimal. Beta-blockers have been shown to limit infarct size during acute ischemia, but their negative inotropic properties have limited their use. Cardiopulmonary bypass is an attractive technique for cardiac resuscitation because it can stabilize a hemodynamically compromised patient and potentially reduce myocardial oxygen consumption. In an attempt to maximize myocardial salvage in the setting of acute ischemia, the combination of esmolol, an ultrashort-acting beta-blocker, with percutaneous cardiopulmonary bypass was evaluated. Four groups of instrumented dogs underwent 2 hours of myocardial ischemia induced by occlusion of the proximal left anterior descending coronary artery, followed by 1 hour of reperfusion. Throughout the period of ischemia and reperfusion, esmolol plus percutaneous cardiopulmonary bypass was compared with esmolol alone, percutaneous cardiopulmonary bypass alone, and control conditions. After the reperfusion period, the extent of infarction of the left ventricle at risk was determined. Four animals had intractable arrhythmias: one in the esmolol plus bypass group, one in the esmolol group, and two in the control group. The extent of infarction of the left ventricle at risk was significantly reduced in the esmolol plus bypass group (30%) compared with bypass alone (52%), with esmolol alone (54%), and with the control groups (59%; p < 0.05). We conclude that in this experimental model the combination of esmolol with bypass improves myocardial salvage after ischemia and reperfusion.

Neuronal control of coronary blood flow

Controversies on acetylcholine-induced increases or decreases in coronary blood flow arise from obvious species differences, the role of endothelium in mediating vascular smooth muscle responses, and the marked negative chronotropic and inotropic effects of acetylcholine. In man, there appears to be a predominant dilation of intact epicardial coronary arteries and a constriction of artherosclerotic segments. However, at present there is no evidence for a vagal initiation of myocardial ischemia. Coronary vascular beta-adrenergic receptors mediate dilation, but appear to be functionally insignificant during sympathetic activation. The beta-adrenergic mechanism contributing to myocardial ischemia are indirect, mediated by a tachycardia-related redistribution of blood flow away from the ischemic myocardium. alpha-Adrenergic receptors mediating epicardial coronary artery constriction in experimental studies appear not to be responsible for the initiation of ischemia in patients with angina at rest. However, alpha-adrenergic constriction of coronary resistance vessels resulting in the precipitation of post-stenotic myocardial ischemia was demonstrated in experimental studies and recently confirmed in patients with effort angina. Non-adrenergic, non-cholinergic neurotransmitters exist; however, their role in regulating coronary blood flow remains entirely unclear.

Sudden cardiac death and the potential role of beta-adrenoceptor-blocking drugs

Sudden cardiac death is a major health problem in the industrially developed countries. The risk of sudden cardiac death may be reduced by early detection of coronary heart disease, elimination of the risk factors, treatment of the ischaemia in patients known to have coronary heart disease and suppression of ventricular arrhythmias. Of all the therapeutic measures currently available to reduce the risk of sudden cardiac death, beta-adrenoceptor-blocking drugs (beta blockers) appear to be the most effective. In this paper their actions are reviewed and evidence for their efficacy is presented.

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)

Tedisamil (KC 8857) is a new specific bradycardic drug: does it also influence myocardial contractility? Analysis by the conductance (volume) technique in coronary artery disease

To determine whether inotropism influences the bradycardic action of tedisamil, hemodynamic assessment was performed in 13 patients with ischemic coronary artery disease including analysis of end-systolic pressure-volume relationships after an infusion of tedisamil, 0.3 mg/kg, at rest, and during paced tachycardia stress. Slope Emax fell by 14% at rest (13 patients) and by 10% during tachycardia (6/13 patients), whereas loops of end-systolic pressure-volume relationships moved rightward; all parameter changes indicated a lack of significant inotropism loss with tedisamil (p > 0.05). Although the mean heart rate decreased from 77.5 to 64.7 beats/min and QTc duration increased by 14% (p < 0.05), filling pressure and dp/dtmin remained unchanged and vascular resistance increased by 30%. Parameters of left ventricular pump function (ejection fraction, stroke volume, left ventricular efficiency) decreased slightly (between 3% and 13%), whereas left ventricular volumes increased (end-diastolic volume by 6%, end-systolic volume by 23%). The respective parameter changes during tachycardia were comparable in tendency, and angina could no longer be induced during postdrug pacing stress. We concluded that the bradycardic effects of tedisamil are selectively generated without impairing either ventricular pump function or contractility in a clinically relevant fashion, whereas the postdrug anginal threshold appears elevated. Thus tedisamil can be used safely in ischemic coronary artery disease.

Local beta-adrenergic blockade does not reduce infarct size after coronary occlusion and reperfusion: a study of coronary venous retroinfusion of metoprolol

Previous studies have demonstrated pronounced ischemic zone myocardial concentrations of metoprolol following coronary venous retroinfusion in pigs with coronary artery ligation. The effect of coronary venous retroinfusion of metroprolol on myocardial infarct size was studied in 16 pentobarbital-anesthetized open-chest pigs undergoing 60-minute occlusion of the left anterior descending coronary artery followed by 3 hours of reperfusion. Pigs in the experimental group (n = 8) were given 0.4 mg/kg (1.0 mg/ml) of metroprolol via the anterior interventricular vein over a period of 5 minutes, beginning immediately after coronary occlusion followed by 0.2 mg/kg/hr intravenously. Control pigs (n = 8) received the same volume of saline as the treated group. The risk area and the necrotic area were assessed by monastral blue dye and triphenyl tetrazolium chloride staining, respectively. Metoprolol did not influence hemodynamics. Plasma concentrations of metoprolol were within therapeutic levels. The administration of the beta-blocker resulted in a trend toward reduced norepinephrine concentrations, both in the aorta and coronary vein after coronary occlusion, but it did not prevent norepinephrine overflow following reperfusion. Infarct size expressed as a percentage of the risk area was 77 +/- 11% in the control group and 75 +/- 12% (mean +/- SD; NS) in the treated group. Thus, metoprolol retroinfusion did not reduce infarct size and did not prevent catecholamine overflow after reperfusion. It is concluded that the beneficial effects of metroprolol in acute infarction are probably unrelated to local beta-adrenergic blockade, at least in the pig, an animal with a paucity of coronary collateral blood flow.

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)

Long-term cardiovascular effects of metoprolol therapy: a review article

Long-term studies in primary and secondary prevention using metoprolol have showed improvement in prognosis. This review summarizes the clinical results of such studies, and puts these results into the perspective of findings in mechanistic and explanatory studies. Many possible factors may have contributed to explain the beneficial effects on prognosis. The antihypertensive action, cardiac anti-ischemic action, antifibrillatory action, antiatherogenic and antithrombotic actions of metoprolol are discussed. The additive effects of these actions may explain the clinically observed beneficial effects. In the evaluation of a specific intervention, it is important to realize the multiple actions that may contribute, and not only discuss single factors.

Mechanisms of regional ischemia and antianginal drug action during exercise

Several mechanisms involved in the production of regional exercise-induced ischemia are described. Each offers the potential for modification using different types of antianginal drugs operating to alter regional O2 demands, improve regional perfusion, or both, leading to reduced ischemia and increased contractile function in the ischemic zone. Evidence is presented for matching of regional subendocardial myocardial blood flow and flow per beat with regional myocardial contraction at various levels of ischemia at rest, during steady-state exercise, and after antianginal drugs, signifying a particularly important role for heart rate control. In addition to reducing myocardial O2 demand per minute, beta-blockers and bradycardic drugs cause improvement of absolute subendocardial blood flow and particularly flow per beat by producing vasoconstriction in the epicardial region of the ischemic zone, with improvement of transmural blood flow distribution. Vasodilator drugs can act at several locations to increase regional blood flow and also to decrease O2 demands. A recruitable vasodilator reserve has been shown to exist during exercise-induced ischemia either in native resistance vessels, collateral channels, or both, which appears to be due at least in part to reduction of increased alpha-adrenergic constrictor tone to the coronary vessels during exercise, even in the presence of severe ischemia. The potential for additive effects using combinations of bradycardic and vasodilating agents are described within a framework relating regional subendocardial blood flow to regional systolic contraction. The experimental findings described suggest some potential new directions for antianginal therapy and, along with recent clinical observations, support the use of combinations of antianginal agents that act by different mechanisms.

Treatment of pain in acute myocardial infarction

The treatment of pain in acute myocardial infarction varies with local practice. Narcotic analgesics are still the usual treatment in many hospitals. Knowledge of optimal doses, duration of pain relief, and time between drug administration and pain relief is inadequate. Many studies indicate that the relief of pain is often incomplete after treatment with narcotic analgesics. There is often a need for alternative treatments. Large randomised studies consistently show that beta blockade, initially given intravenously and then orally, relieves pain and reduces the need for analgesics. Studies also indicate that early administration of streptokinase and glyceryl trinitrate relieves pain. There is evidence that drugs that limit ischaemic damage also relieve pain.

An evaluation of the perioperative efficacy of selective beta 1-blockade in coronary surgery. Studies with a late preoperative dose of metoprolol

The plasma concentration of metoprolol was measured hourly following an oral dose on two consecutive days--the day before, and the day of, coronary surgery. No significant difference was found between the two sampling days, though there was a tendency to lower concentrations during and after extracorporeal circulation. After a median dose of 50 mg the peak concentration (reached on average after 1.5 h) was 545 +/- 70 nmol/l on the first day and 388 +/- 57 on the day of surgery. The respective elimination rates from plasma, expressed as half-life, were 3.4 +/- 0.21 and 3.5 +/- 0.19 hours (NS). On the day of surgery the heart rate rose during the second half of the observation period, peaking above 130 beats/min. Heart rate was inversely correlated to metoprolol concentration in plasma with coefficient -0.68 before induction of anesthesia and -0.77 two hours after termination of extracorporeal circulation. The perioperative efficacy of beta-blockade following a late preoperative oral dose of the agent thus appeared to be reduced and inadequate.

Amelioration by beta-adrenergic blockade of regional myocardial dysfunction induced by coronary artery occlusion after, but not before collateral development in conscious dogs

Effects of beta-adrenergic blockade on regional myocardial dysfunction induced by coronary artery occlusion were studied in chronically instrumented, conscious dogs before (n = 8) and after (n = 7) collateral development. Intravenous atenolol or propranolol produced no beneficial effects on systolic shortening in the area rendered ischemic during 2 minutes of circumflex occlusion, before collateral development. After the collateral development by repetitive 2-minute coronary occlusions, regional asynergy recovered to the preocclusive level during 2 minutes of occlusion. Both atenolol and propranolol significantly improved the peak reductions of regional shortening by 19 +/- 9% and 18 +/- 9%, respectively (p less than 0.05 versus without beta-blockade). These beneficial actions of beta-blockade were again noted during atrial tachypacing at matched heart rates of 160 +/- 11 beats/min. Thus the beneficial effects of beta-blockade on regional asynergy during coronary occlusion depend on the level of functional state of the collaterals and cannot be totally accounted for by the reduction in heart rate or by the cardioselectivity.

Bisoprolol improves perfusion of ischaemic myocardium in anaesthetized pigs

1. The ability of the cardioselective beta-adrenoceptor antagonist bisoprolol ((+/-)-1-[4-(2-isopropoxyethoxymethyl)-phenoxy]-3-isopropyl-amino -2-propanol hemifumarate, EMD 33512) to suppress isoprenaline-induced increases in heart rate and maximal rate of rise in left ventricular pressure (LVdP/dtmax) was studied in 6 anaesthetized pigs given 4 cumulative doses (16, 64, 256 and 1024 micrograms kg-1). Bisoprolol was about 2 times more effective in suppressing isoprenaline-induced increases in LVdP/dtmax than those in heart rate. 2. In 8 animals which had a partial stenosis of the left anterior descending coronary artery (LADCA), the effects of 3 consecutive doses (50, 200 and 750 micrograms kg-1) of bisoprolol were studied on systemic haemodynamics, regional myocardial perfusion and function. The effects of the drug were compared with those obtained in a group of 9 animals with LADCA stenosis which did not receive any treatment. 3. The lowest dose of bisoprolol (50 micrograms kg-1) increased perfusion of the ischaemic myocardium (which had been reduced from 123 +/- 20 ml min-1 100 g-1 to 42 +/- 11 ml min-1 100 g-1) by 21 +/- 10 ml min-1 100 g-1 (P less than 0.05). In particular the subendocardial layers, which were most severely affected by the stenosis (a decrease from 128 +/- 19 ml min-1 100 g-1 to 20 +/- 6 ml min-1 100 g-1) benefited from the administration of the drug (an increase of 30 +/- 10 ml min-1 100 g-1). Perfusion of the subepicardium was not significantly affected. With the higher dose only a minor additional improvement in perfusion of the ischaemic myocardium was observed. 4. The negative chronotropic response is the most likely factor leading to the improvement in perfusion. 5. Myocardial wall thickening, which decreased from 41 +/- 2% to 9 +/- 4% (P less than 0.05) due to the hypoperfusion, did not improve after administration of the drug. This lack of improvement may possibly be due to the duration of ischaemia before and the magnitude of the flow deficit after bisoprolol administration. 6. Between 15 and 60 min of ischaemia, 5 of the 9 untreated animals had an episode of ventricular fibrillation compared with only 1 of the 8 animals treated with bisoprolol, in spite of an initially larger flow reduction in the treated animals. The more homogeneous flow distribution after bisoprolol might account for the lower incidence of arrhythmias in this group. 7. It was demonstrated that bisoprolol improves perfusion of ischaemic myocardium in anaesthetized pigs even at doses (50.pgkg-1) that only moderately antagonize isoprenaline-induced cardiostimulatory effects.

Beta-adrenoceptor stimulation and blockade during myocardial ischemia in dogs: effect on cardiac O2 supply and consumption

The effect of beta-adrenoceptor blockade and activation on ischemic regional and microregional myocardial O2 supply/consumption parameters was assessed in 28 open chest, anesthetized dogs. Ten minutes after LAD occlusion, dogs were given i.v. saline, 2 mg/kg propranolol, 0.2 mg/kg pindolol, or 1 microgram/kg per min isoproterenol. Coronary blood flow was determined using radioactive microspheres before and 2 h after LAD occlusion while O2 supply/consumption parameters were determined using microspectrophotometry. Ischemia resulted in a 66% reduction in subendocardial flow in controls in the ischemic zone and no experimental treatment significantly altered this flow. Pindolol resulted in a significant improvement in the ischemic regional subendocardial/subepicardial flow ratio (from 0.69 in the control ischemic region to 0.88 during pindolol treatment). O2 extractions were significantly increased and O2 consumptions were significantly depressed in the ischemic regions of all groups. O2 extractions were increased to a lesser degree in the ischemic region with the use of pindolol and propranolol. Propranolol and pindolol both significantly decreased the proportion of veins with low (0-20%) O2 saturations in the ischemic region indicating an improved microregional distribution of blood flow and/or O2 consumption within the ischemic region.

Postinfarction angina

In conclusion, the PIA patient is at high risk, with higher early as well as late mortality. The pathophysiology of PIA is complex and may vary from patient to patient. The concepts of ischemia at a distance and ischemia in the infarct zone have led to a better understanding of early PIA. Coronary spasm may play an important role in most PIA patients as in the general population of patients with angina pectoris. Medical therapy is efficacious in many, although it may on rare occasion aggravate myocardial ischemia. Urgent coronary arteriography is generally safe and should be performed as soon as possible for medically refractory PIA. CABG appears to be safe in experienced hands, but its timing must be individualized. The IABP should be reserved for more unstable patients for fear of vascular complications. Randomized controlled trials such as the BARI Trial will further compare PTCA with CABG.

Systemic and regional hemodynamic, antiarrhythmic and antiischemic effects of bevantolol in anesthetized pigs

In doses of 0.5 to 3.0 mg/kg bevantolol caused dose-dependent decreases in cardiac output (10% to 35%), primarily due to negative chronotropic actions, as heart rate decreased by 10% to 25%. Stroke volume decreased after the highest dose (15%), due to a negative inotropic action (maximum left ventricular dP/dt decreased by 40%) and a mild vasoconstriction in systemic vascular beds. Decreases in perfusion of the heart, kidneys, liver, spleen, stomach, muscles and adrenals were similar to those in cardiac output. However, blood flow to the brain and small intestine was not significantly affected. Bevantolol (0.5 or 1.5 mg/kg IV) gave full protection against ventricular fibrillation during the first period (10 minutes) of proximal left anterior descending (LAD) coronary artery occlusion in the highest dose. After the third and last reperfusion period, 70% of these animals survived, while only 8% of the untreated and 15% of the animals treated with the lower dose survived. After permanent ligation of the LAD coronary artery at midpoint, bevantolol prevented ventricular fibrillation during the first phase of early ventricular arrhythmias but was unable to prevent it during the second phase of early arrhythmias. Administration of bevantolol (1.5 mg/kg) to animals in which the LAD coronary artery blood flow was reduced to 35% of baseline did not improve transmural myocardial blood flow to the ischemic zone. However, the drug caused a redistribution in favor of the endocardial layers. The changes in flow were accompanied by a narrowing of the arterial-coronary venous differences in pH and pCO2.(ABSTRACT TRUNCATED AT 250 WORDS)

Course of chest pain and its relation to CK release and ST/QRS vector changes in patients with acute myocardial infarction randomized to treatment with intravenous timolol or placebo

Assessments of hourly pain scores (0 to 4) were made in 135 patients during the initial 24 hours after admission to the hospital. The duration of chest pain and the cumulative pain score obtained by adding the pain scores hour by hour were compared to ST and QRS vector changes and CK release. The cumulative pain score over a 24-hour period after admission correlated to the maximal QRS vector difference (r = 0.51) and the cumulative CK release (r = 0.58). The time until patients had complete relief of pain was closely related to the time during which QRS vector changes were seen to continue (r = 0.73). No corresponding correlation was found between pain duration and CK release time (r = 0.24). The ST decline time correlated to the duration of the first uninterrupted episode of chest pain in the placebo group only (r = 0.50). Pain duration showed no correlation to our indices of infarct size. There was a recurrence of pain in 41% of the patients, of whom 36% had a time-associated further increase of the ST vector magnitude. We conclude that chest pain is an important clinical symptom that signals ongoing necrosis. Furthermore, assessments of a "soft" parameter, such as the cumulative pain score, can add valuable information concerning the severity of myocardial damage.

Vasodilating beta-adrenoceptor blocking drugs do not redistribute myocardial flow during acute coronary ligation in dogs

The effects of two nonselective beta-adrenoceptor blocking drugs, additionally endowed with vasodilating properties with different mechanisms: labetalol which blocks alpha-adrenoceptors, and bucindolol which nonspecifically relaxes vascular smooth muscle, were investigated on regional myocardial blood flow (RMBF) distribution in ischemic and nonischemic areas and on ST-segment elevation in ischemic areas during intermittent coronary artery occlusion in dogs. Both labetalol and bucindolol reduced heart rate, arterial blood pressure and myocardial oxygen consumption. However, they did not affect coronary vascular resistance and did not induce any favorable coronary blood flow redistribution phenomenon either from the epicardium to the endocardium or from the nonischemic to the ischemic areas. These results indicate that the inability of labetalol and bucindolol to favorably redistribute RMBF was due to the fact that their vasodilating properties counteract one major determinant of RMBF redistribution. i.e. the beta-adrenoceptor blockade-induced increase in regional coronary vascular resistance. Finally, both labetalol and bucindolol dose dependently decreased ST-segment elevation in ischemic myocardial areas.

Hemodynamics of human arterial stenoses

We assessed the hemodynamic and geometric changes in compliant, human arterial stenoses in response to manipulation of vascular tone, perfusion pressure and distal resistance. Coronary and popliteal arteries were harvested from human cadavers shortly after death. Following incubation for several hours to permit recovery of physiologic energy stores and ion gradients, the vessels were attached to a perfusion apparatus and perfusion pressure (PP), distal pressure (DP), and flow (F) were recorded as perfusion pressure and distal resistance (DR) were varied. The experiments were then repeated in the presence of a vasoconstrictor (100 mM KCl). Orthogonal arteriograms were performed at maximums of vasoconstriction and vasodilation. Stenotic resistance (SR) was calculated as (PP-DP)F. Minimum cross-sectional area was determined by computer assisted analysis of the arteriograms. Stenosed vessels with normal wall segments at the stenosis responded to vasoconstriction with a large stenotic resistance increase (111 +/- 15%; P less than 0.05) and a flow decrease averaging 39.3 +/- 6.2% (P less than 0.05). In addition, decreased perfusion pressure also increased stenotic resistance significantly (P less than 0.05). Stenotic resistance changes were of sufficient magnitude to be both statistically significant and clinically important. These results confirm the existence of dynamic arterial stenoses in humans and further support the assertion that dynamic stenotic severity changes elicited by manipulation of proximal and distal vascular tone and pressure are of sufficient magnitude to create acute ischemia. This information may apply to clinical situations in which compliant stenoses and acute ischemia coexist.

Regional myocardial tissue blood flow during sequential beta 1- and beta 2-adrenergic blockade in cat hearts with acute ischaemia

beta-Adrenergic blockade was imposed on cats with ischaemic regions of the left ventricle produced by coronary artery occlusion. Ten animals first received a beta 1-blocking agent (atenolol) followed by a beta 2-blocking agent (IPS 339). In ten more animals this sequence was reversed. Combined blockade, obtained after both agents were administered, showed clear reduction of tissue blood flow in all areas of the ventricle, except for the central ischaemic zone. The flow reduction could be ascribed to bradycardia and reduced coronary perfusion pressure. By analysing the sequential changes it was evident that blockade of beta 1-adrenergic receptors was responsible for the haemodynamic changes, and the coronary vascular resistance rose so as to match the quantity of blood flow to the functional state of the ventricle. Blockade of beta 2-receptors by IPS 339, however, showed no evidence of coronary vasoconstriction but rather maintained vascular resistance at an unchanged level despite a weak beta 1-adrenergic blocking effect.

Influence of propranolol on uptake of radioiodinated heptadecanoic acid and thallium-201 in the dog heart

In an experimental study, the influence of propranolol on myocardial uptake of radioiodinated heptadecanoic acid (131I-HDA) and thallium-201 (201Tl) in the dog heart was assessed. Uptake of 131I-HDA and 201Tl was evaluated in ten control dogs and in ten dogs 20 min after IV administration of propranolol (0.15 mg/kg). In both groups, four healthy dogs were studied and six dogs were studied after coronary artery occlusion. It was shown that both total uptake of 131I-HDA and 201Tl did not alter significantly, regardless of significant changes in hemodynamic parameters and total arterial plasma FFA levels. However, distribution of both 131I-HDA and 201Tl was markedly affected by propranolol, since the endocardial to epicardial ratio showed significantly higher values in the ischemic myocardial regions. The results of our study indicate that propranolol (1) preserves myocardial perfusion in the normal and acutely ischemic dog heart, and (2) gives a more favorable distribution in the ischemic myocardial region towards the subendocardial layers.