Infarct size reduction by propranolol before and after coronary ligation in dogs

Early donepezil monotherapy or combination with metoprolol significantly prevents subsequent chronic heart failure in rats with reperfused myocardial infarction

Despite the presence of clinical guidelines recommending that β-blocker treatment be initiated early after reperfused myocardial infarction (RMI), acute myocardial infarction remains a leading cause of chronic heart failure (CHF). In this study, we compared the effects of donepezil, metoprolol, and their combination on the progression of cardiac remodeling in rats with RMI. The animals were randomly assigned to untreated (UT), donepezil-treated (DT), metoprolol-treated (MT), and a combination of donepezil and metoprolol (DMT) groups. On day 8 after surgery, compared to the UT, the DT and DMT significantly improved myocardial salvage, owing to the suppression of macrophage infiltration and apoptosis. After the 10-week treatment, the DT and DMT exhibited decreased heart rate, reduced myocardial infarct size, attenuated cardiac dysfunction, and decreased plasma levels of brain natriuretic peptide and catecholamine, thereby preventing subsequent CHF. These results suggest that donepezil monotherapy or combined therapy with β-blocker may be an alternative pharmacotherapy post-RMI.

Beta-blocker effect on ST-segment: a prespecified analysis of the EARLY-BAMI randomised trial

Objective

The effect of early intravenous (IV) beta-blockers (BBs) administration in patients undergoing primary percutaneous coronary intervention (pPCI) on ST-segment deviation is unknown. We undertook a prespecified secondary analysis of the Early Beta-blocker Administration before primary PCI in patients with ST-elevation Myocardial Infarction (EARLY-BAMI) trial to investigate the effect of early IV BB on ST-segment deviation.

Methods

The EARLY-BAMI trial randomised patients with ST-elevation myocardial infarction (STEMI) to IV metoprolol (2×5 mg bolus) or matched placebo before pPCI. The prespecified outcome, evaluated by an independent core laboratory blinded to study treatment, was the residual ST-segment deviation 1 hour after pPCI (ie, the percentage of patients with >3 mm cumulative ST deviation at 1 hour after pPCI).

Results

An ECG for the evaluation of residual ST-segment deviation 1 hour after pPCI was available in 442 out of 683 randomised patients. The BB group had a lower heart rate after pPCI compared with placebo (71.2±13.2 vs 74.3±13.6, p=0.016); however, no differences were noted in the percentages of patients with >3 mm cumulative ST deviation at 1 hour after pPCI (58.6% vs 54.1%, p=0.38, in BB vs placebo, respectively) neither a significant difference was found for the percentages of patients in each of the four prespecified groups (normalised ST-segment; 1–3 mm; 4–6 mm;>6 mm residual ST-deviation).

Conclusions

In patients with STEMI, who were being transported for primary PCI, early IV BB administration did not significantly affect ST-segment deviation after pPCI compared with placebo. The neutral result of early IV BB administration on an early marker of pharmacological effect is consistent with the absence of subsequent improvement of clinical outcomes.

Metoprolol exerts a non-class effect against ischaemia-reperfusion injury by abrogating exacerbated inflammation

Aims

Clinical guidelines recommend early intravenous β-blockers during ongoing myocardial infarction; however, it is unknown whether all β-blockers exert a similar cardioprotective effect. We experimentally compared three clinically approved intravenous β-blockers.

Methods and results

Mice undergoing 45 min/24 h ischaemia–reperfusion (I/R) received vehicle, metoprolol, atenolol, or propranolol at min 35. The effect on neutrophil infiltration was tested in three models of exacerbated inflammation. Neutrophil migration was evaluated in vitro and in vivo by intravital microscopy. The effect of β-blockers on the conformation of the β1 adrenergic receptor was studied in silico. Of the tested β-blockers, only metoprolol ameliorated I/R injury [infarct size (IS) = 18.0% ± 0.03% for metoprolol vs. 35.9% ± 0.03% for vehicle; P < 0.01]. Atenolol and propranolol had no effect on IS. In the three exacerbated inflammation models, neutrophil infiltration was significantly attenuated only in the presence of metoprolol (60%, 50%, and 70% reductions vs. vehicle in myocardial I/R injury, thioglycolate-induced peritonitis, and lipopolysaccharide-induced acute lung injury, respectively). Migration studies confirmed the particular ability of metoprolol to disrupt neutrophil dynamics. In silico analysis indicated different intracellular β1 adrenergic receptor conformational changes when bound to metoprolol than to the other two β-blockers.

Conclusions

Metoprolol exerts a disruptive action on neutrophil dynamics during exacerbated inflammation, resulting in an infarct-limiting effect not observed with atenolol or propranolol. The differential effect of β-blockers may be related to distinct conformational changes in the β1 adrenergic receptor upon metoprolol binding. If these data are confirmed in a clinical trial, metoprolol should become the intravenous β-blocker of choice for patients with ongoing infarction.

Should We Be Using Upstream Beta-Blocker Therapy for Acute Myocardial Infarction?

Purpose of Review

Controversy exists whether beta-blockers should be given before primary percutaneous coronary intervention (PCI) or to defer their administration for up to 24 hours.

Recent Findings

Animal studies, most of them conducted in the 1970s and 1980s, showed evidence that early beta-blocker administration may reduce infarct size. Subsequent human studies had mixed results on infarct size and survival. More specifically, in the current primary PCI era, only four studies evaluated the impact of early intravenous beta-blocker administration after acute myocardial infarction, only two of them before PCI. All studies agree that in hemodynamically stable patients, early intravenous beta-blocker administration is safe and protected against malignant arrhythmias. Nevertheless, results on infarct size and mortality are equivocal.

Summary

Considering the heterogeneity of currently available data, further studies are still needed to assess the benefit of early injection of metoprolol in STEMI patients in a large double-blinded and randomized design versus placebo.

Dose-dependent Effects of Esmolol-epinephrine Combination Therapy in Myocardial Ischemia and Reperfusion Injury

BACKGROUND

Animal studies on cardiac arrest found that a combination of epinephrine with esmolol attenuates post-resuscitation myocardial dysfunction. Based on these findings, we hypothesized that esmololepinephrine combination therapy would be superior to a reported cardioprotective esmolol therapy alone in a mouse model of myocardial ischemia and reperfusion (IR) injury.

METHODS

C57BL/6J mice were subjected to 60 min of myocardial ischemia and 120 min of reperfusion. Mice received either saline, esmolol (0.4 mg/kg/h), epinephrine (0.05 mg/kg/h), or esmolol combined with epinephrine (esmolol: 0.4 mg/kg/h or 0.8 mg/kg/h and epinephrine: 0.05 mg/kg/h) during reperfusion. After reperfusion, infarct sizes in the area-at-risk and serum cardiac troponin-I levels were determined. Hemodynamic effects of drugs infused were determined by measurements of heart rate (HR) and mean arterial blood pressure (MAP) via a carotid artery catheter.

RESULTS

Esmolol during reperfusion resulted in robust cardioprotection (esmolol vs. saline: 24.3±8% vs. 40.6±3% infarct size), which was abolished by epinephrine co-administration (38.1±15% infarct size). Increasing the esmolol dose, however, was able to restore esmolol-cardioprotection in the epinephrine-esmolol (18.6±8% infarct size) co-treatment group with improved hemodynamics compared to the esmolol group (epinephrine-esmolol vs. esmolol: MAP 80 vs. 75 mmHg, HR 452 vs. 402 beats/min).

CONCLUSION

These results confirm earlier studies on esmolol-cardioprotection from myocardial IR-injury and demonstrate that a dose optimized epinephrine-esmolol co-treatment maintains esmolol-cardioprotection with improved hemodynamics compared to esmolol treatment alone. These findings might have implications for current clinical practice in hemodynamically unstable patients suffering from myocardial ischemia.

Trigger, Signaling Mechanism and End Effector of Cardioprotective Effect of Remote Postconditioning of Heart

The hypothetical trigger of remote postconditioning (RPost) of the heart is the high-molecular weight hydrophobic peptide(s). Nitric oxide and adenosine serve as intermediaries between the peptide and intracellular structures. The role of the autonomic nervous system in RPost requires further study. In signaling mechanism RPost, kinases are involved: protein kinase C, PI3, Akt, JAK. The hypothetical end effector of RPost is aldehyde dehydrogenase-2, the transcription factors STAT, Nrf2, and also the BKCa channel.

Early intravenous beta-blockers in patients undergoing primary percutaneous coronary intervention for ST-segment elevation myocardial infarction: A patient-pooled meta-analysis of randomized clinical trials

BACKGROUND

Conflicting evidence is available on the efficacy and safety of early intravenous beta-blockers before primary percutaneous coronary intervention for ST-segment elevation myocardial infarction. We performed a patient-pooled meta-analysis of trials comparing early intravenous beta-blockers with placebo or routine care in ST-segment elevation myocardial infarction patients undergoing primary percutaneous coronary intervention.

AIM

The aim of this study was to evaluate the clinical and safety outcomes of intravenous beta-blockers in ST-segment elevation myocardial infarction patients undergoing primary percutaneous coronary intervention.

METHODS

Four randomized trials with a total of 1150 patients were included. The main outcome was one-year death or myocardial infarction. Secondary outcomes included biomarker-based infarct size, left ventricular ejection fraction during follow-up, ventricular tachycardia, and a composite safety outcome (cardiogenic shock, symptomatic bradycardia, or hypotension) during hospitalization.

RESULTS

One-year death or myocardial infarction was similar among beta-blocker (4.2%) and control patients (4.4%) (hazard ratio: 0.96 (95% confidence interval: 0.53-1.75, p=0.90, I²=0%). No difference was observed in biomarker-based infarct size. One-month left ventricular ejection fraction was similar, but left ventricular ejection fraction at six months was significantly higher in patients treated with early intravenous beta-blockade (52.8% versus 50.0% in the control group, p=0.03). No difference was observed in the composite safety outcome or ventricular tachycardia during hospitalization.

CONCLUSION

In ST-segment elevation myocardial infarction patients undergoing primary percutaneous coronary intervention, the administration of early intravenous beta-blockers was safe. However, there was no difference in the main outcome of one-year death or myocardial infarction with early intravenous beta-blockers. A larger clinical trial is warranted to confirm the definitive efficacy of early intravenous beta-blockers.

The Oxygen Paradox, the French Paradox, and age-related diseases

A paradox is a seemingly absurd or impossible concept, proposition, or theory that is often difficult to understand or explain, sometimes apparently self-contradictory, and yet ultimately correct or true. How is it possible, for example, that oxygen "a toxic environmental poison" could be also indispensable for life (Beckman and Ames Physiol Rev 78(2):547-81, 1998; Stadtman and Berlett Chem Res Toxicol 10(5):485-94, 1997)?: the so-called Oxygen Paradox (Davies and Ursini 1995; Davies Biochem Soc Symp 61:1-31, 1995). How can French people apparently disregard the rule that high dietary intakes of cholesterol and saturated fats (e.g., cheese and paté) will result in an early death from cardiovascular diseases (Renaud and de Lorgeril Lancet 339(8808):1523-6, 1992; Catalgol et al. Front Pharmacol 3:141, 2012; Eisenberg et al. Nat Med 22(12):1428-1438, 2016)?: the so-called, French Paradox. Doubtless, the truth is not a duality and epistemological bias probably generates apparently self-contradictory conclusions. Perhaps nowhere in biology are there so many apparently contradictory views, and even experimental results, affecting human physiology and pathology as in the fields of free radicals and oxidative stress, antioxidants, foods and drinks, and dietary recommendations; this is particularly true when issues such as disease-susceptibility or avoidance, "healthspan," "lifespan," and ageing are involved. Consider, for example, the apparently paradoxical observation that treatment with low doses of a substance that is toxic at high concentrations may actually induce transient adaptations that protect against a subsequent exposure to the same (or similar) toxin. This particular paradox is now mechanistically explained as "Adaptive Homeostasis" (Davies Mol Asp Med 49:1-7, 2016; Pomatto et al. 2017a; Lomeli et al. Clin Sci (Lond) 131(21):2573-2599, 2017; Pomatto and Davies 2017); the non-damaging process by which an apparent toxicant can activate biological signal transduction pathways to increase expression of protective genes, by mechanisms that are completely different from those by which the same agent induces toxicity at high concentrations. In this review, we explore the influences and effects of paradoxes such as the Oxygen Paradox and the French Paradox on the etiology, progression, and outcomes of many of the major human age-related diseases, as well as the basic biological phenomenon of ageing itself.

Cardioprotection: Where to from here?

The size of the myocardial infarction remains an important therapeutic target, because heart attack size correlates with mortality and heart failure. In this era, myocardial infarct size is reduced primarily by timely reperfusion of the infarct related coronary artery. Whereas numerous pre-clinical studies have shown that certain pharmacologic agents and therapeutic maneuvers reduce myocardial infarction size greater than reperfusion alone, very few of these therapies have translated to successful clinical trials or standard clinical use. In this review we discuss both the recent successes as well as recent disappointments, and describe some of the newer potential therapies from the preclinical literature that have not yet been tested in clinical trials.

Comparison of Pyrroloquinoline Quinone and/or Metoprolol on Myocardial Infarct Size and Mitochondrial Damage in a Rat Model of Ischemia/Reperfusion Injury

The cardioprotective effectiveness of low-dose pyrroloquinoline quinone (PQQ, 3 mg/kg) was compared with metoprolol, a β1-selective adrenoceptor antagonist. Rats underwent 30 minutes of left anterior descending coronary artery occlusion and 2 hours of reperfusion. Metoprolol and/or PQQ were given at the onset of reperfusion to mimic clinical treatment. Metoprolol and/or PQQ reduced infarct size and protected against ischemia-induced left ventricular dysfunction after 2 hours of reper-fusion. Combined therapy augmented left ventricular developed pressure at the end of reperfusion. Metoprolol or PQQ alone enhanced mitochondrial respiratory ratios in ischemic and nonischemic myocardium. Although the PQQ/metoprolol combination therapy increased respiratory ratio values, the effects were small when compared with PQQ alone. Only PQQ decreased lipid peroxidation. Metoprolol and/or PQQ given at the onset of reperfusion reduce infarct size and improve cardiac function. Combination therapy further reduces infarct size. PQQ is superior to metoprolol in protecting mitochondria from ischemia/reperfusion oxidative damage

Early Intravenous Beta-Blockers in Patients With ST-Segment Elevation Myocardial Infarction Before Primary Percutaneous Coronary Intervention

BACKGROUND

The impact of intravenous (IV) beta-blockers before primary percutaneous coronary intervention (PPCI) on infarct size and clinical outcomes is not well established.

OBJECTIVES

This study sought to conduct the first double-blind, placebo-controlled international multicenter study testing the effect of early IV beta-blockers before PPCI in a general ST-segment elevation myocardial infarction (STEMI) population.

METHODS

STEMI patients presenting <12 h from symptom onset in Killip class I to II without atrioventricular block were randomized 1:1 to IV metoprolol (2 × 5-mg bolus) or matched placebo before PPCI. Primary endpoint was myocardial infarct size as assessed by cardiac magnetic resonance imaging (CMR) at 30 days. Secondary endpoints were enzymatic infarct size and incidence of ventricular arrhythmias. Safety endpoints included symptomatic bradycardia, symptomatic hypotension, and cardiogenic shock.

RESULTS

A total of 683 patients (mean age 62 ± 12 years; 75% male) were randomized to metoprolol (n = 336) or placebo (n = 346). CMR was performed in 342 patients (54.8%). Infarct size (percent of left ventricle [LV]) by CMR did not differ between the metoprolol (15.3 ± 11.0%) and placebo groups (14.9 ± 11.5%; p = 0.616). Peak and area under the creatine kinase curve did not differ between both groups. LV ejection fraction by CMR was 51.0 ± 10.9% in the metoprolol group and 51.6 ± 10.8% in the placebo group (p = 0.68). The incidence of malignant arrhythmias was 3.6% in the metoprolol group versus 6.9% in placebo (p = 0.050). The incidence of adverse events was not different between groups.

CONCLUSIONS

In a nonrestricted STEMI population, early intravenous metoprolol before PPCI was not associated with a reduction in infarct size. Metoprolol reduced the incidence of malignant arrhythmias in the acute phase and was not associated with an increase in adverse events. (Early-Beta blocker Administration before reperfusion primary PCI in patients with ST-elevation Myocardial Infarction [EARLY-BAMI]; EudraCT no: 2010-023394-19).

Impact of Heart Rate on Myocardial Salvage in Timely Reperfused Patients with ST-Segment Elevation Myocardial Infarction: New Insights from Cardiovascular Magnetic Resonance

BACKGROUND

Previous studies evaluating the progression of the necrotic wave in relation to heart rate were carried out only in animal models of ST-elevated myocardial infarction (STEMI). Aim of the study was to investigate changes of myocardial salvage in relation to different heart rates at hospital admission in timely reperfused patients with STEMI by using cardiovascular magnetic resonance (CMR).

METHODS

One hundred-eighty-seven patients with STEMI successfully and timely treated with primary coronary angioplasty underwent CMR five days after hospital admission. According to the heart rate at presentation, patients were subcategorized into 5 quintiles: <55 bpm (group I, n = 44), 55-64 bpm (group II, n = 35), 65-74 bpm (group III, n = 35), 75-84 bpm (group IV, n = 37), ≥85 bpm (group V, n = 36). Area at risk, infarct size, microvascular obstruction (MVO) and myocardium salvaged index (MSI) were assessed by CMR using standard sequences.

RESULTS

Lower heart rates at presentation were associated with a bigger amount of myocardial salvage after reperfusion. MSI progressively decreased as the heart rates increased (0.54 group I, 0.46 group II, 0.38 group III, 0.34 group IV, 0.32 group V, p<0.001). Stepwise multivariable analysis showed heart rate, peak troponin and the presence of MVO were independent predictor of myocardial salvage. No changes related to heart rate were observed in relation to area at risk and infarct size.

CONCLUSIONS

High heart rates registered before performing coronary angioplasty in timely reperfused patients with STEMI are associated with a reduction in salvaged myocardium. In particular, salvaged myocardium significantly reduced when heart rate at presentation is ≥85 bpm.

Rationale and design of a double-blind, multicenter, randomized, placebo-controlled clinical trial of early administration of intravenous β-blockers in patients with ST-elevation myocardial infarction before primary percutaneous coronary intervention: EARLY β-blocker administration before primary PCI in patients with ST-elevation myocardial infarction trial

BACKGROUND

β-Blockers have a class 1a recommendation in the treatment of patients with ST-elevation myocardial infarctions (STEMIs), as they are associated with a reduced mortality, recurrent myocardial infarction, life-threatening arrhythmias, and with prevention of unfavorable left ventricular remodeling. Whether early administration before primary percutaneous coronary intervention (PCI) of intravenous β-blockers reduces the infarct size in the current era is unknown.

HYPOTHESIS

We postulate that the early administration of β-blockers will reduce the myocardial infarcted area as assessed by magnetic resonance imaging (MRI) at 30 days.

DESIGN

In a multinational, multicenter, double-blind, placebo-controlled, randomized trial, patients with symptoms and signs of STEMI and transferred to a hospital for primary PCI will be randomized in a 1:1 fashion to intravenous metoprolol (5 mg twice daily) administration or placebo. Before admission, study treatment will be started as soon as possible after the diagnosis of STEMI. After admission, primary PCI will be performed as per standard of care. After primary PCI, medical treatment will occur as per current guidelines in all patients, including the use of oral β-blockers. The primary end point is the myocardial infarct size as assessed by MRI at 30 days. Based on a superiority design and assuming an 18% relative infarct size reduction (from 28% to 23.5%), 408 patients are required to be enrolled, accounting for 20% drop-out (α = .05 and power = 80%).

SUMMARY

The EARLY-BAMI trial is a multinational, multicenter, double-blind, placebo-controlled, randomized clinical trial that will investigate the impact of intravenous metoprolol administration before primary PCI for STEMI on myocardial infarct size as measured with MRI at 30 days.

Impact of pre-procedural beta blockade on inpatient mortality in patients undergoing primary percutaneous coronary intervention for ST elevation myocardial infarction

Early use of β blockers (BBs) in acute myocardial infarction remains controversial, with some studies demonstrating benefit and others harm. The aim of this study was to assess the association between pre-percutaneous coronary intervention (PCI) BB use and in-hospital outcomes in patients who underwent primary PCI for ST-segment elevation myocardial infarction between 2007 and 2009 at institutions participating in the Blue Cross Blue Shield of Michigan Cardiovascular Consortium (BMC-2). Inverse propensity score weighting was used to account for the nonrandomized use of pre-PCI BBs. The cohort comprised 7,667 patients, with 4,769 (62%) receiving pre-PCI BBs. These patients were older, with higher rates of diabetes mellitus, hypertension, and previous myocardial infarction, PCI, or coronary artery bypass grafting. In adjusted models, pre-PCI BB use was associated with lower rates of intraprocedural ventricular tachycardia or ventricular fibrillation (odds ratio [OR] 0.58, p <0.01) and lower in-hospital mortality (OR 0.65, p = 0.022), with increases in rates of emergent coronary artery bypass grafting (OR 1.56, p <0.01) and repeat PCI (OR 1.93, p <0.01). There were no significant increases in rates of cardiogenic shock and congestive heart failure. In conclusion, pre-PCI BB use in this population was associated with decreased arrhythmia and mortality, without increasing rates of cardiogenic shock and heart failure but with higher rates of repeat PCI and emergent coronary artery bypass grafting, suggesting that there may yet remain a role for early BB use in pre-PCI patients with ST-segment elevation myocardial infarctions.

Intracoronary followed by intravenous administration of the short-acting β-blocker landiolol prevents myocardial injury in the face of elective percutaneous coronary intervention

BACKGROUND

Myocardial injury during elective percutaneous coronary intervention (PCI) is associated with higher subsequent cardiac events and mortality. β-Blockers have been used to reduce myocardial injury during ischemia and reperfusion. We investigated whether intracoronary followed by intravenous administration of the short-acting β-blocker landiolol prevents myocardial injury in the face of elective PCI.

METHODS AND RESULTS

Patients undergoing elective PCI (n=70) were randomly assigned to the landiolol (n=35) or control (n=35) group. Landiolol or saline was administered into target vessels through a balloon catheter for 1min before and after first balloon inflation followed by continuous intravenous administration for 6h after PCI. The incidence of myocardial injury defined by cardiac troponin-I (cTnI) >/=0.05 ng/ml was 79% of the patients in the control group compared to 56% in the landiolol group (p=0.04). The cTnI level at 24h after PCI tended to be lower in the landiolol group (0.57 ± 1.14 versus 1.27 ± 2.48 ng/ml; p=0.07), while the CK-MB level was not significantly different between the landiolol and control groups. The incidence of peri-procedural myocardial infarction defined by cTnI >/=0.12 ng/ml was significantly (p=0.02) lower in the landiolol group (41%) compared to the control group (70%). There was no incidence of coronary spasm, hypotension, bradycardia or heart failure during and after PCI in the two groups.

CONCLUSIONS

Brief intracoronary followed by continuous intravenous administration of landiolol is safe and effective for myocardial protection in the face of elective PCI.

The cellular and molecular origin of reactive oxygen species generation during myocardial ischemia and reperfusion

Myocardial ischemia-reperfusion injury is an important cause of impaired heart function in the early postoperative period subsequent to cardiac surgery. Reactive oxygen species (ROS) generation increases during both ischemia and reperfusion and it plays a central role in the pathophysiology of intraoperative myocardial injury. Unfortunately, the cellular source of these ROS during ischemia and reperfusion is often poorly defined. Similarly, individual ROS members tend to be grouped together as free radicals with a uniform reactivity towards biomolecules and with deleterious effects collectively ascribed under the vague umbrella of oxidative stress. This review aims to clarify the identity, origin, and progression of ROS during myocardial ischemia and reperfusion. Additionally, this review aims to describe the biochemical reactions and cellular processes that are initiated by specific ROS that work in concert to ultimately yield the clinical manifestations of myocardial ischemia-reperfusion. Lastly, this review provides an overview of several key cardioprotective strategies that target myocardial ischemia-reperfusion injury from the perspective of ROS generation. This overview is illustrated with example clinical studies that have attempted to translate these strategies to reduce the severity of ischemia-reperfusion injury during coronary artery bypass grafting surgery.

Lethal myocardial reperfusion injury: a necessary evil?

Despite being the most effective means of limiting infarct size, coronary reperfusion comes at a price and induces additional damage to the myocardium. Lethal reperfusion injury (death of myocytes that were viable at the time of reperfusion) is an increasingly acknowledged phenomenon. There are many interconnected mechanisms involved in this type of cell death. Calcium overload (generating myocyte hypercontracture), rapid recovery of physiological pH, neutrophil infiltration of the ischemic area, opening of the mitochondrial permeability-transition-pore (PTP), and apoptotic cell death are among the more important mechanisms involved in reperfusion injury. The activation of a group of proteins called reperfusion injury salvage kinases (RISK) pathway confers protection against reperfusion injury, mainly by inhibiting the opening of the mitochondrial PTP. Many interventions have been tested in human trials triggered by encouraging animal studies. In the present review we will explain in detail the main mechanism involved in reperfusion injury, as well as the various approaches (pre-clinical and human trials) performed targeting these mechanisms. Currently, no intervention has been consistently shown to reduce reperfusion injury in large randomized multicenter trials, but the research in this field is intense and the future is highly promising.

Cardiac protection during acute myocardial infarction: Where do we stand in 2004?

Despite better outcomes with early coronary artery reperfusion for the treatment of acute ST-elevation myocardial infarction (MI), morbidity and mortality from acute myocardial infarction (AMI) remain significant, the incidence of congestive heart failure continues to increase, and there is a need to provide better cardioprotection (therapy that reduces the amount of necrosis that may be coupled with better clinical outcome) in the setting of AMI. Since the introduction of the concept of cardiac protection over a quarter of a century ago, various interventions have been investigated to reduce myocardial infarct size. Intravenous beta-blockers administered in the early hours of infarction were clearly shown to be of benefit. Intravenous adenosine appeared promising for anterior wall AMIs, as did cariporide in some studies. Glucose-insulin-potassium infusion was beneficial in certain subgroups of patients, particularly diabetics. A variety of other medications were studied with negative or marginal results. The best strategy to limit infarct size is early reperfusion with percutaneous coronary stenting or thrombolytic therapy. Stenting is superior and should be adopted whenever there is a qualified laboratory available. Available resources should focus on decreasing time from onset of symptoms to start of reperfusion and maintaining vessel patency. Future studies powered to better assess clinical outcome are needed for adjunctive therapy with adenosine, K(ATP) channel openers, Na(+)/H(+) exchange inhibitors, and hypothermia.

The reperfusion era. Strategies for establishing or maintaining coronary patency

Only 3 decades ago, controversy existed over the origin of the thrombus that occluded coronary arteries during myocardial infarction (MI). Then, the acute clinical angiographic studies of DeWood butterssed new pathological and experimental evidence that the thrombus was the proximate cause of MI and unstable angina. The remaining years of the 20th century saw an explosion of knowledge delineating a spectrum of related disorders now collectively called acute coronary syndromes (ACS). The clinician managing patients is confronted with an array of evidence as more than 75,000 patients worldwide have been randomized to clinical trials in ACS. This article reviews key pathophysiological concepts, presents an initial strategy for triage of patients, and summarizes evidence-based medicine guiding therapy for acute coronary lesions.

Failure of intravenous metoprolol to limit acute myocardial infarct size in a nonreperfused porcine model

The usefulness of intravenous beta-adrenergic receptor blockade in limiting infarct size when neither reperfusion nor collateral flow occurs is unknown. The effect of intravenous metoprolol on limiting myocardial infarct size was therefore examined in a nonreperfused porcine model. Closed-chest techniques were used to occlude the left anterior descending coronary artery, after which animals were randomized at 20 minutes to receive intravenous metoprolol, 0.75 mg/kg, or placebo. Infarct size examined at 5 hours with Evans blue and triphenyltetrazolium staining techniques was expressed as a percentage of total ventricular myocardium at ischemic risk. This percentage was not significantly different between the groups (84% +/- 5% with metoprolol vs 90% +/- 4% with placebo; p = 0.4). Myocardial infarct size was not significantly decreased at 5 hours by early administration of intravenous metoprolol when the infarct artery remained occluded and collateral flow was minimal.

Infarct size in rabbits: a modified method illustrated by the effects of propranolol and trimetazidine

Following a 45-min period of coronary occlusion the myocardial infarct that developed after 24 h of blood reperfusion in the rabbit heart was studied in three groups of animals: controls (n = 7), and those pretreated with 3 mg.kg-1 of the piperazine derivative, trimetazidine (n = 7) or propranolol at 0.3 mg.kg-1 (n = 6). Twenty-four hours after coronary artery ligation for 45 min infarct size was measured in myocardial slices using trinitrophenyl-tetrazolium staining, and the "area at risk" was determined by injection of zinc/cadmium particles and delineated by imaging under fluorescent light the areas of tissue that did not fluoresce. The range of "area at risk" was similar in all of the groups. There was a significant reduction in the size of infarct that developed in the trimetazidine treated group when compared with the controls. In the propranolol treated group there was a slight reduction in infarct size when compared to control, but this was less than in the trimetazidine treated group and did not reach statistical significance. It is concluded that pretreatment with trimetazidine in the blood perfused rabbit heart is effective in reducing myocardial infarct size.

Cardioprotective effect of pindolol in ischemic-reperfused isolated rat hearts

The effects of pindolol and timolol on ischemia reperfusion damage were studied in isolated working rat hearts. Ischemia (15 min) decreased the mechanical function and the energy state, and increased the tissue levels of free fatty acids (FFA). During reperfusion (20 min), the mechanical function did not recover, but the energy state recovered incompletely, whereas FFA increased further. Pindolol (50 microM) accelerated recovery of the mechanical function and the energy state that had been decreased by ischemia during reperfusion, and inhibited the accumulation of FFA during ischemia and reperfusion, especially when it was applied during the whole period of reperfusion. Timolol (50 microM), however, did not accelerate recovery of the mechanical function and the energy state during reperfusion, although it attenuated FFA accumulation during reperfusion. The pindolol-induced recovery of the mechanical function during reperfusion was reduced by timolol. The results suggest that the intrinsic sympathomimetic activity of pindolol may play an important role, at least in part, in producing the cardioprotective effect, especially during reperfusion.

Determinants of infarct size in non-human primates

To achieve a better understanding of the major factors that determine infarct size in non-human primates, a mathematical model was constructed using stepwise regression analysis. The model was developed on the basis of infarct size measurements, including the anatomical area at risk, regional myocardial blood flow measurements and hemodynamic determinants obtained in 23 control baboons undergoing up to 2 h of coronary artery thrombosis followed by thrombolysis. In this model, the size of the perfusion bed of the occluded coronary artery and the duration of coronary artery occlusion were found to be the only important predictors of infarct size (expressed as a percentage of left ventricular mass). R2 (square or the multiple correlation coefficient) was 70% in this model. Collateral blood flow and rate-pressure product were not identified as important predictors of infarct size. In a second group of eight baboons, atenolol (0.1 mg.kg-1) was administered intravenously 15 min after the onset of coronary artery thrombosis. Predicted infarct size (based on the mathematical model obtained in the control group) was larger than the observed infarct size in seven out of eight cases. In four instances observed infarct size was smaller than the 95% lower limit of the predicted value. It is concluded that the determinants of infarct size in non-human primates differ from those in canine models with respect to collateral flow and estimates of myocardial oxygen consumption (rate pressure product). The developed mathematical model of infarct size prediction allows the detection of cardioprotective drug effects with an acceptable efficacy.

The effects of propranolol on regional cardiac metabolism during ischemia and reperfusion assessed by magnetic resonance spectroscopy

Sixteen anesthetized New Zealand white rabbits were subjected to thoracotomy, and a reversible snare occluder was attached around a large branch of the left circumflex coronary artery. A 1.3 cm. diameter nuclear magnetic resonance (NMR) surface coil was placed adjacent to the myocardium perfused by this vessel. The animals were divided into two groups of eight animals each, treatment and control. The rabbits were studied using a 2.0 T magnetic resonance (MR) spectrometer, and baseline spectra were acquired. The treatment animals then received intravenous propranolol (1.5 mg/kg) and the control animals received an equal volume of saline. Spectra were then acquired during a 20-minute occlusion period and during subsequent reperfusion. Animals in both groups showed expected decreases in phosphocreatine and adenosine triphosphate and an increase in inorganic phosphate during occlusion; these changes reverted toward baseline values with reperfusion. There were no significant differences between the two groups. The myocardium became acidotic during occlusion in both groups, but significantly more so in the control animals: during the first 10 minutes of occlusion pH was 7.30 +/- 0.41 in the treatment group versus 6.55 +/- 0.24 for controls (p = 0.0005). During the second 10 minutes of occlusion pH was 7.05 +/- 0.65 in the treatment group versus 6.24 +/- 0.25 in controls (p = 0.0053). We conclude that attenuation of intracellular acidosis by propranolol during myocardial ischemia was evident by MR spectroscopy in this animal model.

Myocardial salvage after regional beta-adrenergic blockade

The aim of the study was to determine whether regional beta-adrenergic blockade via the coronary sinus limited myocardial damage after coronary artery occlusion in the canine model. Accordingly, open-chest anesthetized dogs were randomly allocated to one of three groups: a control group and groups treated with propranolol (in doses of 0.02, 0.2, and 2.0 mg/kg) given either intravenously or via the coronary sinus. The hypoperfused zone (i.e., risk area) and the extent of myocardial damage were assessed by autoradiography and triphenyltetrazolium chloride staining, respectively. Myocardial damage expressed as a percent of the hypoperfused zone was 84 +/- 5% in the control group (n = 9) and 78 +/- 7% (0.02 mg/kg, n = 7, NS), 63 +/- 6% (0.2 mg/kg, n = 7, p less than 0.05), and 62 +/- 7% (2.0 mg/kg, n = 9, p less than 0.02) in the groups receiving intravenous propranolol and 73 +/- 6% (0.02 mg/kg, n = 7, NS), 58 +/- 7% (0.2 mg/kg, n = 7, p less than 0.01), and 44 +/- 9% (2.0 mg/kg, n = 9, p less than 0.001) in groups receiving propranolol via the cardiac veins. There was a significant enhancement of myocardial salvage with increasing doses of propranolol delivered via the cardiac veins (linear regression trend, p less than 0.05). In contrast, myocardial damage expressed as a percent of the hypoperfused zone remained comparable with propranolol doses of 0.2 and 2.0 mg/kg administered intravenously (linear regression trend, NS).

IN CONCLUSION

(1) regional beta-adrenergic blockade via the cardiac veins afforded significant myocardial salvage and (2) the regional administration of propranolol resulted in significant reduction of myocardial damage in a dose-dependent fashion.

Relation of peak creatine kinase levels during acute myocardial infarction to presence or absence of previous manifestations of myocardial ischemia (angina pectoris or healed myocardial infarction)

The hypothesis that acute myocardial infarction (MI) is more extensive in patients without previous angina or healed MI was evaluated in 177 patients with documented recent acute MI. Ninety-nine patients (56%) had no previous angina or healed MI (negative history group), and the remaining 78 patients (44%) had a previous history of angina or healed MI (positive history group). The mean peak creatine kinase (CK) level in the negative history group was 784 compared with 419 IU in the positive history group (p less than 0.0001). The mean peak CK-MB level in the negative history group was 128 compared with 76 IU in the positive history group (p less than 0.001). The mean peak CK-MB level was higher in the negative history group after controlling for age, streptokinase administration, previous coronary artery bypass grafting or treatment with beta-blocking agents. Despite the high frequency of healed MI in the positive history group (73%), the rates of in-hospital complications were similar for the 2 groups. Patients with acute MI without previous angina or healed MI have substantially higher peak CK and CK-MB levels; this implies a larger MI than in patients with previous angina or healed MI.

Comparison of the effects of acute and chronic beta-blockade on infarct size in the dog after circumflex occlusion

In order to compare the effects of acute and chronic beta-blockade on infact size, the left circumflex coronary artery was occluded for 6 hours in 33 anesthetized dogs. The dogs (18 to 22 kg) were divided into three groups; group 1 (N = 10) served as controls, group 2 received intravenous nadolol (average dose 1.25 mg/kg) just prior to coronary occlusion, and group 3 received oral nadolol (80 mg) twice daily for 16 days prior to coronary occlusion. To ensure equivalent degrees of beta-blockade at the time of occlusion, group 2 and 3 dogs were given incremental doses of intravenous nadolol to abolish the chronotropic response to isoproterenol (2 mu/kg IV). Left ventricular pressure, its first derivative (dP/dt), and heart rate were monitored. The anatomic risk region was determined antemortem by Evan's blue staining while the infarct zone was delineated postmortem by tetrazolium staining. Compared to Group 1, heart rate was 22% lower in group 2 and 15% lower in group 3 dogs 6 hours after occlusion (p less than 0.05). There were no differences among groups in peak left ventricular systolic pressure or mean arterial pressure. Infarct size as a function of the area at risk was 68 +/- 3% in group 1, 52 +/- 7% in group 2, and 44 +/- 8% in group 3. A significant difference was found only between groups 3 and 1. The data suggest that chronic beta-blockade provides greater protection against ischemic-induced necrosis than does acute beta-blockade. The greater protective effect of chronic beta-blockade may be due to chronic adaptive changes in either blood flow or metabolism.

Effects of verapamil and timolol on cellular morphometric changes in cat hearts with regional ischaemia

In twenty-one anaesthetized open chest cats the left anterior descending coronary artery (LAD) was occluded for three hours. Seven cats were pretreated with a bolus injection of Verapamil, followed by a continuous infusion of Verapamil during the ischaemic period. Seven cats were pretreated with a bolus injection of Timolol to a heart rate reduction of 20 beats/min or more and seven cats were given saline. In the latter two groups the cats received a continuous infusion of saline during the period of coronary occlusion. Biopsies were taken from the mid-myocardium of the normal, border and ischaemic zones, as defined by fluorescein staining, and verified by blood flow measurements with radiolabelled microspheres. Standard point counting techniques were used for calculations of fractional volumes of mitochondria, cytoplasm and myofibrils as well as of mitochondrial surface density and surface to volume ratio. We observed a cytoplasmic oedema in the border and ischaemic zones, that was not altered by medical treatment. In the border zone of the control cats there is greater mitochondrial swelling than in the ischaemic zone. This particular swelling is not seen in the treatment groups. However, in the normal and border zones of the verapamil group the mitochondria are smaller when compared with the respective zones in the two other groups, but increases relatively more in size in the border and ischaemic zones. Furthermore, we measured the water content, sarcomere length and per cent heavily damaged cells. These variables were not altered by Verapamil or Timolol in any zone when compared with the respective zones in the control group.

Comparison of different pharmacological interventions on enzymatic parameters during acute myocardial infarction

The concept that acute myocardial infarction is a dynamic event and that different interventions can modify the extent of the necrosis, has led to renewed interest in early pharmacological and surgical treatments designed to reduce the ischemic injury. To evaluate the effects of different pharmacological interventions aimed to reduce the extent of necrosis, we studied 166 patients (138 male and 28 female, mean age of 59.4 +/- 11.3 years) admitted within 6 h after chest pain and treated with a single therapy during the first 72 h. Enzymatic infarct size (IS) was calculated by serial creatine kinase isoenzyme MB determinations using a compartmental model. Six groups of patients were evaluated: 33 patients were treated only with antiplatelet drugs, 38 with anticoagulants, 34 with intravenous thrombolytic therapy, 20 with calcium channel blockers, 25 with nitrates, and 16 with beta-blockers. Estimated IS (gEq/m2) and elimination constant (Kd, U/L/h) did not differ in the six groups, but patients treated with streptokinase had higher release constant (Ka, U/L/h) and shorter time to peak CK-MB value. Early treatment (less than or equal to 2 h after chest pain) had a favourable effect on the enzymatic IS only in patients treated with calcium channel blockers (p less than 0.005).

Reduction in infarct size and enhanced recovery of systolic function after coronary thrombolysis with tissue-type plasminogen activator combined with beta-adrenergic blockade with metoprolol

The effect of beta-adrenergic blockade on the salvage and functional recovery of reperfused myocardium was investigated in anesthetized dogs. Immediately after thrombotic occlusion of the left anterior descending coronary artery, the cardioselective beta-blocking agent metoprolol was given intravenously at a dose of 0.5 mg/kg infused over 10 min. One hour after the onset of occlusion, recanalization was initiated by intravenous infusion of recombinant human tissue-type plasminogen activator (rt-PA, 10 micrograms/kg/min for 30 min). Anatomic infarct size expressed as percent of the left ventricular mass (I/LV), global ejection fraction, and mean systolic shortening of the segmental radii (SS) of the infarcted area were measured either after 24 hr or 1 week in six groups of six dogs each: group I (rt-PA + metoprolol, evaluated at 24 hr), group II (rt-PA + metoprolol, evaluated at 1 week, group III (rt-PA alone, evaluated at 24 hr), group IV (rt-PA alone, evaluated at 1 week), group V (persistent occlusion, evaluated at 24 hr), and group VI (persistent occlusion, evaluated at 1 week). The smallest infarcts were found in reperfused dogs given metoprolol, but the differences from dogs receiving rt-PA alone were not statistically significant (I/LV, expressed as mean +/- SEM: 5.5 +/- 0.9% in group I, 6.7 +/- 1.9% in group II, 15.4 +/- 5.0% in group III, 11.4 +/- 3.5% in group IV, 23.6 +/- 2.5% in group V, and 26.9 +/- 2.3% in group VI).(ABSTRACT TRUNCATED AT 250 WORDS)

Non-Q wave myocardial infarction: recent changes in occurrence and prognosis--a community-wide perspective

A community-wide study of patients hospitalized with validated acute myocardial infarction (MI) in all 16 hospitals in the Worcester metropolitan area, during the calendar years 1975, 1978, and 1981, is examining time trends in the incidence rates, in-hospital case fatality rates, and long-term survival rates of 2451 patients hospitalized with acute MI further classified into those with Q wave and those with non-Q wave MI. The age-adjusted hospital attack rates (per 100,000) of initial events of Q wave MI increased from 153 in 1975 to 197 in 1981 (p less than 0.05), while the attack rates of non-Q wave MI increased from 46 in 1975 to 89 in 1981 (p less than 0.001). Patients with non-Q wave MI were at significantly decreased risk of dying during hospitalization (12.0%) as compared to those with Q wave MI (25.2%), overall as well as for each of the three time periods studied (p less than 0.05). For patients discharged alive from the hospital, however, there were no significant differences in long-term survival rates according to MI type over an 8-year follow-up period. These results, obtained from a community-wide sample of patients with validated acute MI, indicate a significant change in the occurrence rates and/or recognition of non-Q wave MI, a significantly lower in-hospital case fatality rate for patients with non-Q wave MI, and comparable long-term prognosis for patients with Q wave and non-Q wave acute MI.

Infarct size--can it be measured or modified in humans?

Despite more than 15 years of intensive experimental and clinical research in the general area of limiting infarct size, no treatment has been shown to be so efficacious and relatively free of side effects that its routine use can be recommended. In addition, there is no ideal means of measuring infarct size as yet. However, considerable progress has been made in understanding mechanisms responsible for irreversible cellular injury and in identifying factors and anatomic alterations responsible for or contributing to the development of transmural (Q wave) and non-transmural (non-Q wave) myocardial infarcts. Interventions are available that are capable of causing rapid coronary thrombolysis, and techniques are becoming available tht have increasing power to size myocardial infarcts and estimate both segmental and ventricular function. Experimental studies have also suggested a potential benefit from a combination of reperfusion therapy with selected pharmacologic intervention in reducing infarct size and preserving ventricular function. It seems likely that this general area will remain an intensive area of clinical research in the immediate future.

Comparative effects of nicardipine, a new calcium antagonist, on size of myocardial infarction after coronary artery occlusion in dogs

To examine whether nicardipine, a dihydropyridine derivative, limits size of myocardial infarction, and to compare the protective effects of nicardipine administered before and early and late after coronary artery occlusion, 99mTc-labeled albumin microspheres were injected into the left atrium during 5 min temporary coronary artery occlusion to determine the extent of the hypoperfused zone (the area at risk). The coronary arteries were then reperfused for 45 min before 6 hr permanent coronary artery occlusion. Fifteen minutes before permanent occlusion, dogs were randomly assigned to a control group (n = 11), a pretreatment group (n = 9), which received at this point 10 micrograms/kg of nicardipine as a loading dose followed by a continuous infusion of 8 micrograms/kg/hr for 6 hr, an early treatment group (n = 9), in which nicardipine treatment was initiated 15 min after occlusion, or a late treatment group (n = 8), in which nicardipine administration was delayed for 3 hr. Six hours after coronary artery occlusion, the hearts were excised and the left ventricle of each was cut into 3 mm thick slices and stained with triphenyltetrazolium chloride. The extent of myocardial necrosis was measured by planimetry of the unstained areas. Thereafter, the same slices were autoradiographed and the extent of the hypoperfused zone was measured by planimetry of the "cold spot." The extent of the hypoperfused zone was identical among the four groups. In the control group, the ratio of the extent of myocardial necrosis to the extent of the hypoperfused zone was 95.8 +/- 3.8% (mean +/- SEM). However, it was significantly smaller in the pretreatment group (59.9 +/- 13.3%, p less than .05) and the early treatment group (49.0 +/- 10.6%, p less than .01) than in the control group. In the late treatment group, this value was not different from that in the control group (86.5 +/- 7.1%). There was a close inverse correlation between reduction of infarct size and the extent of the hypoperfused zone in the pretreatment and early treatment groups. Thus, nicardipine administered before or early after coronary artery occlusion limited infarct size by 37% to 49%, whereas when administration was delayed for 3 hr infarct size was not reduced. Furthermore, nicardipine had more striking effects on the ischemic myocardium of dogs with small hypoperfused zones than on that of dogs with large hypoperfused zones.

Effects of propranolol on myocardial damage resulting from coronary artery occlusion followed by reperfusion

To evaluate the effects of propranolol on myocardial metabolism after coronary reperfusion, serial measurements of myocardial creatine kinase (CK) and calcium (Ca) contents and CK and lactic acid (LA) concentrations in coronary sinus blood were carried out in 33 open-chest dogs. The left anterior descending coronary artery was occluded for 60 minutes and was then reopened. Twelve of the dogs were given propranolol before occlusion. Reperfusion for 30 minutes in dogs with and without propranolol pretreatment resulted in reduced myocardial CK in the ischemic region and rapidly elevated plasma CK and LA. However, when compared with the control group, the propranolol-treated group showed smaller changes in myocardial CK and plasma LA. Myocardial Ca in the ischemic region was significantly higher than that in the nonischemic region in the control group, but not in the propranolol-treated group. It was concluded that propranolol was protective against myocardial damage resulting from coronary occlusion followed by reperfusion.

Effect of early treatment with propranolol on left ventricular function four weeks after myocardial infarction

Left ventricular function and exercise capacity were assessed in 79 patients randomised to receive intravenous and oral propranolol (n = 44) or conventional therapy (n = 35) within four hours of onset of their first myocardial infarction. Cineangiocardiography and exercise testing were performed four weeks after infarction to allow for maximum recovery of myocardial function. Left ventriculography showed no improvement in ejection fraction or preservation of regional contractile function in patients treated with propranolol compared with controls. A trend towards smaller end diastolic volumes was seen in the propranolol group (mean (SD) 151(42) ml) compared with controls (167(42) ml). Exercise duration and frequency of angina were not significantly different in the two groups. It is concluded that limitation of infarct size by propranolol does not lead to a significant improvement in ventricular systolic function, although left ventricular dilatation may be reduced. These findings are consistent with the known effect of early intravenous beta blockade which limits infarct size by preservation of subepicardial myocardium.

Effects of a combination of metoprolol and dazmegrel on myocardial infarct size in rats

The effects of acute pretreatment with metoprolol, dazmegrel and a combination of these two drugs has been examined on myocardial infarct size in rats. Ischaemic damage was assessed 4 h after coronary artery occlusion in anaesthetized rats and after 48 h of ischaemia in conscious rats. Infarct size was measured histochemically (by using periodic-acid-Schiff diastase reaction for glycogen) and by standard histological examination (haematoxylin and eosin stain). There was some evidence of protection of the myocardium by metoprolol following 4 h of ischaemia (determined histologically) but this was not apparent 48 h after occlusion. When given alone, dazmegrel had no significant effects on infarct size assessed by either method. A clear reduction in the extent of glycogen depletion and histological damage was observed with the combination of metoprolol and dazmegrel 48 h after the onset of ischaemia. This protection was seen to occur in the horizontal plane of the heart, preventing the extension of the infarct towards the posterior wall of the left ventricle and showing some salvage of the epicardial surfaces.

Reduction of sympathetic inotropic response after ischemia in dogs. Contributor to stunned myocardium

Eight open chest dogs underwent 25 min of coronary occlusion to determine whether brief myocardial ischemia disrupts the normal myocardial inotropic response to sympathetic nervous stimulation. If so, this could represent a mechanism contributing to postischemic myocardial dysfunction. Myocardial segment shortening was measured using ultrasonic dimension crystals before and after coronary artery occlusion and reperfusion. Left ansa subclavia stimulation and systemic norepinephrine (NE) infusion were used to test the myocardial inotropic response to neural stimulation and direct exposure to the sympathetic mediator, respectively. Before coronary artery occlusion, base-line preischemic segment shortening (12.5 +/- 1.6%) (SEM) increased during both sympathetic stimulation (20.2 +/- 1.4%) and NE infusion (19.7 +/- 1.1%). The control segment responded similarly. After ischemia and reperfusion there was no significant change in heart rate, aortic or left ventricular pressures, nor changes in control segment shortening. In contrast, shortening in the postischemic segment was markedly reduced compared to baseline (4.1 +/- 2.4%), and no longer responded to sympathetic stimulation (2.4 +/- 2.8%), while responsiveness to systemic NE was maintained (12.9 +/- 2.0%), P less than 0.001, which suggested injury to the sympathetic-neural axis during the period of ischemia. This reduced response to neural stimulation was persistent for up to 2 h after reperfusion. Left atrial or intracoronary infusion of bretylium tosylate, which releases norepinephrine from nerve terminals, resulted in an immediate inotropic response in the postischemic segment, which indicated that total depletion of NE from nerve terminals during the ischemic period had not occurred. Disruption of sympathetic neural responsiveness is likely a component of the mechanism of postischemic myocardial dysfunction whenever there is appreciable sympathetic drive to the heart.

Effects of beta-adrenergic blockade on the natural progression of myocardial infarct size and compensatory hypertrophy

Using contrast-enhanced computed tomography, the effects of beta-adrenergic blockade were assessed on experimentally produced myocardial infarcts in dogs evaluated serially over the course of approximately 1 month. Infarct size, initial perfusion defect (jeopardized segment) and noninfarcted muscle mass were studied in two groups of conditioned mongrel dogs. Group 1 (n = 11) served as the control group and Group 2 (n = 10) was pretreated with propranolol (2 mg/kg). Each animal in the propranolol-treated group was given identical amounts of the agent twice daily for 7 days after coronary occlusion. Both groups developed increases in the noninfarcted muscle mass of the left ventricle (compensatory hypertrophy). The mean increase averaged 19.8% over 30 days when the two groups were included together. Infarct size was smaller in the propranolol-treated group, and averaged 28% less (p less than 0.05) than that of the control group 30 days after initial myocardial infarction. Thus, pharmacologic interventions were shown by computed tomography to alter the size of an acute experimental myocardial infarct, particularly when examined over the time course of infarct healing. Moreover, compensatory hypertrophy occurred in both the control and propranolol-treated groups.

Nifedipine limits infarct size for 24 hours in closed chest coronary embolized dogs

We studied the ability of nifedipine, a calcium antagonist, to limit infarct size in the closed chest, coronary embolized dog. Immediately after embolization 141Ce labelled microspheres were administered into the left ventricle. Myocardium not receiving microspheres was considered to be the region at risk. The nifedipine group (10 dogs) received a bolus (16 micrograms/kg i.v. over 8 minutes as a loading dose) followed by continuous infusion (1,000 micrograms/24 hours) 10 min after embolization. The control group (9 dogs) received an equal volume of saline. Twenty-four hours after embolization the dogs were sacrificed, the heart sectioned into 4-mm slices and the slices were stained with tetrazolium to reveal the infarct. The region at risk was determined by autoradiography of the microspheres in the heart slices. Infarct and risk zone volume were determined by planimetric methods. The nifedipine group had a significantly smaller infarct volume to risk zone volume ratio than the control group (38.7 +/- 4.7% vs. 79.5 +/- 4.3%, p less than 0.001). We conclude that nifedipine produces a sustained limitation of infarct size following permanent occlusion of a dog's coronary artery.