Acute effects of delayed reperfusion on myocardial infarct shape and left ventricular volume: a potential mechanism of additional benefits from thrombolytic therapy

Why Is Infarct Expansion Such an Elusive Therapeutic Target?

Myocardial infarct expansion has been associated with an increased risk of infarct rupture and progression to heart failure, motivating therapies such as infarct restraint and polymer injection that aim to limit infarct expansion. However, an exhaustive review of quantitative studies of infarct remodeling reveals that only half found chronic in-plane expansion, and many reported in-plane compaction. Using a finite element model, we demonstrate that the balance between scar stiffening due to collagen accumulation and increased wall stresses due to infarct thinning can produce either expansion or compaction in the pressurized heart-potentially explaining variability in the literature-and that loaded dimensions are much more sensitive to changes in thickness than in stiffness. Our analysis challenges the concept that in-plane expansion is a central feature of post-infarction remodeling; rather, available data suggest that radial thinning is the dominant process during infarct healing and may be an attractive therapeutic target.

Physiological Implications of Myocardial Scar Structure

Once myocardium dies during a heart attack, it is replaced by scar tissue over the course of several weeks. The size, location, composition, structure, and mechanical properties of the healing scar are all critical determinants of the fate of patients who survive the initial infarction. While the central importance of scar structure in determining pump function and remodeling has long been recognized, it has proven remarkably difficult to design therapies that improve heart function or limit remodeling by modifying scar structure. Many exciting new therapies are under development, but predicting their long-term effects requires a detailed understanding of how infarct scar forms, how its properties impact left ventricular function and remodeling, and how changes in scar structure and properties feed back to affect not only heart mechanics but also electrical conduction, reflex hemodynamic compensations, and the ongoing process of scar formation itself. In this article, we outline the scar formation process following a myocardial infarction, discuss interpretation of standard measures of heart function in the setting of a healing infarct, then present implications of infarct scar geometry and structure for both mechanical and electrical function of the heart and summarize experiences to date with therapeutic interventions that aim to modify scar geometry and structure. One important conclusion that emerges from the studies reviewed here is that computational modeling is an essential tool for integrating the wealth of information required to understand this complex system and predict the impact of novel therapies on scar healing, heart function, and remodeling following myocardial infarction.

A remarkable medical story: benefits of angiotensin-converting enzyme inhibitors in cardiac patients

The development of angiotensin-converting enzyme inhibitors (ACE inhibitors) has been one of the most remarkable stories in the treatment of cardiovascular diseases. Angiotensin converting enzyme inhibitors have several acute and sustained hemodynamic effects that are beneficial in the presence of left ventricular (LV) dysfunction. They increase cardiac output and stroke volume and reduce systemic vascular resistance as well as pulmonary capillary wedge pressure. The hemodynamic benefits are associated with improvement in the signs and symptoms of congestive heart failure (CHF) as well as decreased mortality, regardless of the severity of CHF. In patients with asymptomatic LV dysfunction, therapy with ACE inhibitors prevented the development of CHF and reduced hospitalization and cardiovascular death. They also increase survival when administered early after an acute myocardial infarction (MI). Most recently, ACE inhibition was associated with improved clinical outcomes in a broad spectrum of high-risk patients with preserved LV function. The mechanism of ACE inhibitors benefits is multifactorial and includes prevention of progressive LV remodeling, prevention of sudden death and arrhythmogenicity and structural stability of the atherosclerotic process. Evidence suggests that ACE inhibitors are underutilized in patients with cardiovascular diseases. Efforts should be directed to prescribe ACE inhibitors to appropriate patients in target doses. It is reasonable to believe that ACE inhibitors have a class effect in the management of LV dysfunction with or without CHF and acute MI. Whether the same is true for ACE inhibitors in the prevention of ischemic events is not known yet.

Percutaneous coronary intervention for cardiogenic shock in the SHOCK Trial Registry

BACKGROUND

The SHOCK Registry prospectively enrolled patients with cardiogenic shock complicating acute myocardial infarction in 36 multinational centers.

METHODS

Cardiogenic shock was predominantly attributable to left ventricular pump failure in 884 patients. Of these, 276 underwent percutaneous coronary intervention (PCI) after shock onset and are the subject of this report.

RESULTS

The majority (78%) of patients undergoing angiography had multivessel disease. As the number of diseased arteries rose from 1 to 3, mortality rates rose from 34.2% to 51.2%. Patients who underwent PCI had lower in-hospital mortality rates than did patients treated medically (46.4% vs 78.0%, P < .001), even after adjustment for patient differences and survival bias (P = .037). Before PCI, the culprit artery was occluded (Thrombolysis In Myocardial Infarction grade 0 or 1 flow) in 76.3%. After PCI, the in-hospital mortality rate was 33.3% if reperfusion was complete (grade 3 flow), 50.0% with incomplete reperfusion (grade 2 flow), and 85.7% with absent reperfusion (grade 0 or 1 flow) (P < .001).

CONCLUSIONS

This prospective, multicenter registry of patients with acute myocardial infarction complicated by cardiogenic shock is consistent with a reduction in mortality rates as the result of percutaneous coronary revascularization. Coronary artery patency was an important predictor of outcome. Measures to promote early and rapid reperfusion appear critically important in improving the otherwise poor outcome associated with cardiogenic shock.

Implications of the timing of onset of cardiogenic shock after acute myocardial infarction: a report from the SHOCK Trial Registry. SHould we emergently revascularize Occluded Coronaries for cardiogenic shocK?

OBJECTIVES

We sought to examine the implications of the timing of onset of cardiogenic shock (CS) after acute myocardial infarction (MI).

BACKGROUND

Little information is available about the relationships between timing, clinical substrate, management and outcomes of shock.

METHODS

The multinational SHOCK Trial Registry enrolled MI patients with CS from 1993 to 1997. Cardiogenic shock was predominantly attributable to left ventricular (LV) failure in 815 Registry patients for whom temporal data were available. We examined factors related to the timing of shock onset and the relation of temporal onset to in-hospital outcomes.

RESULTS

Overall, shock developed a median of 6.2 h after MI symptom onset. Shock onset varied by culprit artery: left main, median 1.7 h; right, 3.5 h; circumflex, 3.9 h; left anterior descending (LAD), 11.0 h; saphenous vein graft, 10.9 h (p = 0.025). Early shock (< 24 h) occurred in 74.1% and was associated with chest pain at shock onset, ST-segment elevation in two or more leads, multiple infarct locations, inferior MI, left main disease and smoking. Late shock (> or = 24 h) was associated with recurrent ischemia, Q waves in two or more leads and LAD culprit vessel. Mortality was higher in patients with early versus late shock (62.6% vs. 53.6%, p = 0.022).

CONCLUSIONS

Shock onset after acute MI occurred within 24 h in 74% of the patients with predominant LV failure. Mortality was slightly higher in patients developing shock early rather than later. Many factors influence when shock develops, which has implications for its management.

Early angiotensin converting enzyme inhibitor therapy enhances the benefits of late coronary artery reperfusion on infarct expansion

BACKGROUND

Individually, both late reperfusion and early angiotensin converting enzyme (ACE) inhibitor treatment prevent infarct expansion after acute myocardial infarction.

OBJECTIVE

To examine the effect and mechanism of early post-myocardial infarction ACE inhibitor treatment, when used in combination with late coronary artery reperfusion, on infarct expansion.

METHODS

Sprague-Dawley rats underwent 8 h of coronary occlusion followed by permanent reperfusion. The treatment group received enalapril, started 1 h after coronary occlusion and continued for 13 days. A control group received placebo. Two weeks after acute myocardial infarction, hemodynamic, morphometric and histologic analyses were performed.

RESULTS

Hemodynamic parameters were similar in both groups (P = NS). Infarct size was similar in the ACE inhibitor and placebo treatment groups (44 +/- 4% compared with 39 +/- 4%, P = NS). Septal thickness was also similar in the two groups (2.8 +/- 0.3 mm compared with 2.7 +/- 0.3 mm, P = NS). The ACE inhibitor-treated group had thicker infarcts than those in the placebo-treated group (0.93 +/- 0.07 mm compared with 0.76 +/- 0.04 mm, P < 0.05) and these infarcts were less expanded (expansion index 1.17 +/- 0.12 compared with 1.57 +/- 0.12, P < 0.05). ACE inhibitor treatment was associated with hypertrophy of viable myocytes within the scar compared with placebo treatment (cell diameter 11.1 +/- 0.5 microns compared with 8.9 +/- 0.4 microns, P < 0.01).

CONCLUSIONS

Early post-myocardial infarction ACE inhibitor treatment enhances the benefits of late coronary reperfusion on infarct expansion. The benefits may be related to hypertrophy of still-viable myocytes within the infarcted zone.

Effects of Late Coronary Artery Reperfusion on Left Ventricular Remodeling Persist for 10 Weeks After Experimental Rat Myocardial Infarction and Are Associated with Improved Survival

Objective: To test the hypothesis that coronary artery reperfusion performed too late to reduce infarct size improves survival by altering left ventricular remodeling and preventing progressive left ventricular dilation. Background: Several clinical trials have suggested that late coronary artery reperfusion without infaret size reduction is associated with a survival benefit. Although the mechanism is not known, survival benefits could be related to decreased infarct expansion associated with late coronary artery reperfusion. Decreased infarct expansion results in decreased left ventricular volume, and the resulting decreased wall stress could prevent or attenuate progressive left ventricular dilation and improve survival. Methods: Rats (n = 84) were randomized to undergo sham operation, permanent left coronary artery ligation, or 2 hours of left coronary artery ligation followed by reperfusion. Ten weeks later, hemodynandic measurements were made before and after volume loading. The rats were killed, the hearts were removed, and passive pressure-volume curves were obtained. The hearts were fixed at a constant pressure and analyzed morphometrically. Results: When examined 10 weeks after experimental myocardial infarction late eperfusion's effects on left ventricular remodeling resulted in reduced left ventricular volume when compared to hearts with infarcts supplied by a permanently occluded coronary artery (1.9 +/- 0.1 ml/kg vs. 2.1 +/- 0.2 ml/kg; p < 0.01). Although there was a trend toward less thinning along (0.95 +/- 0.13 mm vs. 1.00 +/- 0.10 mm; p = NS) and less expansion (2.3 +/- 0.4 vs. 2.8 +/- 0.9; p = NS) in reperfused hearts compared to hearts with a permanently occluded coronary artery, changes in infarct shape 10 weeks after infarction were not significantly different. Reperfusion's beneficial effects on remodeling of noninfarcted myocardiurn were associated with improved survival. Mortality was higher in the permanently occluded rats than in the reperfused rats (35% vs. 12%; p < 0.05). Conclusion: Late coronary artery reperfusion has a beneficial effect on remodeling of noninfarcted myocardum that results in reduced left ventricular volume in rat hearts examined 10 weeks after infarction. These beneficial effects on left ventricular remodeling are associated with improved survival.

Early predictors of left ventricular remodeling after acute myocardial infarction

Ventricular remodeling after acute myocardial infarction is characterized by alteration in left ventricular (LV) size, shape, and wall thickness and involves both the infarcted and the noninfarcted regions of the ventricle. These structural changes are the result of several distinct pathologic processes that contribute to progressive LV dilation: rearrangement of wall structure, myocyte hypertrophy, and increasing muscle mass without an increase in wall thickness (eccentric hypertrophy). The pathogenesis of LV remodeling is multifactorial. Multiple factors may in fact contribute at different stages from the time of coronary occlusion until the development of ventricular dilation: These include the magnitude of the loss of contractile elements, the abrupt alteration in systolic and diastolic loading conditions, the activation of circulating neurohormones and local autocrine trophic factors, and the patency of the infarct-related artery. Although remodeling occurring early after infarction may be an appropriate compensatory response to preserve ventricular function, recent observations have suggested that this long process has a deleterious effect on LV function and prognosis. Thus attempts to inhibit these structural changes have been the focus of recent experimental and clinical studies. This review focuses on interactive factors that influence postinfarction LV remodeling, emphasizing the role of some new emerging determinants such as the extent of surviving myocardium within the infarcted and noninfarcted zones.

Myocardial salvage by reperfusion 12 hours after coronary ligation in dogs

It is not clear why late reperfusion therapy in patients with acute myocardial infarction is effective. An investigation was carried out as to whether or not reperfusion conducted 12 h after coronary occlusion causes myocardial salvage in dogs. Coronary arteries were occluded in 11 mongrel dogs and a portion of the occlusion (late reperfusion area; LR area) reperfused 12 h later; the other part was left occluded (permanent occlusion area; PO area). The dogs were maintained for 4 weeks after reperfusion. Regional myocardial blood flow (Qm) was measured by the non-radioactive colored microsphere method. In both areas, the transmurality of necrosis was measured by triphenyl tetrazolium chloride staining, and the amount of viable myocardium and the extent of fibrosis was determined by Azan-Mallory staining. Qm decreased markedly after coronary occlusion to similar levels in both areas until 12 h. Qm transiently increased in the LR area only following reperfusion after 12 h. The transmurality of necrosis in the PO area was 83.8+/-10.5%, but that in the LR area was 58.7+/-21.3%, a significant decrease (p<0.01). In the outer layer, the amount of viable myocardium was significantly greater, and the extent of myocardial fibrosis was significantly less in the LR area. Evaluation in the same heart of dogs confirmed the myocardial salvage effects of late reperfusion (12 h after coronary occlusion).

TIMI grade 3 flow and reocclusion after intravenous thrombolytic therapy: a pooled analysis

Early and sustained flow of grade 3 according to Thrombolysis in Myocardial infarction (TIMI) criteria and reocclusion rates are the key measures that define the physiologic efficacy of thrombolytic agents in the treatment of acute myocardial infarction. We performed a systematic overview of angiographic studies after intravenous thrombolysis with accelerated and standard-dose tissue-plasminogen activator (TPA), anisoylated plasminogen streptokinase activator complex (APSAC), and streptokinase. There were 5475 angiographic observations from 15 studies for TIMI flow analysis and 3147 angiographic observations from 27 studies for reocclusion. At 60 and 90 minutes, the rates of TIMI grade 3 flow were 57.1% and 63.2%, respectively, with accelerated TPA, 39.5% and 50.2% with standard-dose TPA, 40.2% and 50.1% with APSAC, and 31.5% at 90 minutes with streptokinase. Overall reocclusion with standard-dose TPA was 11.8% versus 6.0% for accelerated TPA, 4.2% for streptokinase, and 3.0% for APSAC. Although the incidence of TIMI grade 3 flow increased over time with all thrombolytic regimens, decreased patency was observed at 180 minutes with accelerated TPA. Still, accelerated TPA is the most effective agent to establish early (90-minute) TIMI grade 3 flow.

Reperfusion injury: a review of the pathophysiology, clinical manifestations and therapeutic options

Lack of blood supply or ischaemia underlies many of the most important cardiovascular and cerebrovascular diseases faced by clinicians in their daily practice. Many of these ischaemic episodes can be reversed at an early stage by surgical or pharmacological means with the ultimate aim of preventing infarction and cell necrosis in the ischaemic tissues. However, reperfusion of ischaemic areas, in particular the readmission of oxygen, may contribute to further tissue damage (reperfusion injury). For example, the use of thrombolytic therapy in acute myocardial infarction and other revascularisation procedures, such as percutaneous transluminal angioplasty and coronary artery bypass surgery, may be associated with reperfusion of ischaemic myocardium. Such ischaemia and reperfusion may result in injury to one of more of the biochemical, cellular and microvascular components of the heart. Our understanding of the significance of reperfusion injury is however restricted by the profuse literature in animal models and limited literature in the clinical situation. This article reviews the pathophysiology, clinical manifestations of reperfusion injury to the heart and discusses the possible therapeutic approaches to avoiding any adverse effects.

Benefits of late coronary artery reperfusion on infarct expansion progressively diminish over time: relation to viable islets of myocytes within the scar

To define the time limit and mechanism of the effects of late coronary artery reperfusion on infarct expansion, rats were randomized into one of four groups: permanent left coronary artery occlusion; and 2, 8, and 16 hours of left coronary artery occlusion followed by reperfusion. Two weeks after coronary occlusion, morphometric and histologic analyses were performed. Benefits of late reperfusion on infarct expansion progressively diminished after increasingly long periods of coronary occlusion and were minimal but present after 16 hours of coronary occlusion. The extent of the benefits of late reperfusion on infarct expansion were related to preservation and hypertrophy of small islets of still viable myocytes located mainly in the subepicardium of the scar.

Significance of perfusion of the infarct related coronary artery for susceptibility to ventricular tachyarrhythmias in patients with previous myocardial infarction

OBJECTIVE

To study the significance of perfusion of the infarct related coronary artery for susceptibility to ventricular tachyarrhythmias in patients with a remote myocardial infarction.

SETTING

Tertiary referral cardiac centre.

METHODS

Angiographic filling of the infarct related artery was assessed in a consecutive series of 85 patients with different susceptibilities to ventricular tachyarrhythmias after previous (> 3 months) Q wave myocardial infarction: 30 patients had a history of cardiac arrest (n = 16) or sustained ventricular tachycardia (n = 14), and sustained ventricular tachyarrhythmia was inducible in these by programmed electrical stimulation (arrhythmia group); 47 patients had no clinical arrhythmic events and no inducible ventricular tachyarrhythmias during programmed ventricular stimulation (control group). Eight patients without a history of any arrhythmic events were inducible into ventricular tachycardia.

RESULTS

The patients in the arrhythmia group were older (63 (SD 8) years) than the control patients (59 (6) years, P < 0.05), and had larger left ventricular volumes in cineangiography (P < 0.01), but ejection fraction, severity of left ventricular wall motion abnormalities, previous thrombolytic therapy, and time from previous infarction did not differ between the groups. Patients with susceptibility to ventricular tachyarrhythmias more often had a totally occluded infarct related artery on angiography (77%) than patients without arrhythmia susceptibility (21%) (P < 0.001), and complete collateral filling of the infarct artery in cases without complete anterograde filling was less common in the arrhythmia group than in the control group (P < 0.001). Patients without a history of malignant arrhythmia but with inducible ventricular tachyarrhythmia also had no or poor perfusion of the infarct artery more often than the patients without inducible arrhythmia (P < 0.001). Logistic multiple regression showed that no or poor anterograde or collateral filling of the infarct related artery was the most powerful predictor of susceptibility to ventricular tachyarrhythmias (P < 0.001). Left ventricular size and function were not independently related to arrhythmic susceptibility.

CONCLUSIONS

No or poor angiographic filling of the infarct related artery is closely associated with susceptibility to ventricular tachyarrhythmias late after acute myocardial infarction, suggesting that perfusion of the infarct artery will modify favourably the electrophysiological substrate of the infarct scar independently of the myocardial salvage achieved by early reperfusion.

Deleterious effects of a systemic lytic state on reperfused myocardium. Minimization of reperfusion injury and enhanced recovery of myocardial function by direct angioplasty

BACKGROUND

The beneficial effects of flow restoration and the deleterious impact of reperfusion injury on ischemic myocardium are well known. However, most experimental studies have induced reperfusion by mechanical release of nonthrombotic occlusions, only occasionally in the presence of a systemic lytic state. Conditions differ markedly in patients undergoing pharmacological or mechanical recanalization of thrombotically occluded coronary arteries. Accordingly, this study was designed to determine whether the method of coronary occlusion and mode of recanalization influence the response of the heart to reperfusion.

METHODS AND RESULTS

The acute effects of reperfusion on right ventricular (RV) function and histology were studied in open-chest dogs subjected to right coronary artery (RCA) balloon occlusion and deflation alone (group 1), pharmacological lysis of thrombotic occlusions (group 2), balloon occlusion with reperfusion induced by balloon deflation in the presence of a systemic lytic state (group 3), and recanalization of thrombotically occluded vessels by direct angioplasty (group 4). In all groups, 1 hour of RCA occlusion led to RV free wall (FW) dyskinesis. In group 1, reperfusion promptly improved RVFW function, with normal RVFW thickness and only minimal edema by microscopy. In contrast, in group 2, clot lysis led to acute RVFW swelling and impaired recovery of RVFW contraction associated with striking interstitial edema, contraction band necrosis, and hemorrhage by microscopy. In group 3, balloon deflation in the presence of a lytic state led to a similar but less severe pattern of abrupt RVFW swelling and impaired recovery of RVFW function but lesser histological alterations than in group 2. However, mechanical recanalization of thrombotically occluded vessels (group 4) led to prompt recovery of RVFW function without significant RVFW swelling or histological abnormalities.

CONCLUSIONS

Our observations indicate that the responses of ischemic myocardium to reperfusion are influenced by factors beyond those effects attributable to ischemia and reperfusion per se. Pharmacological lysis of coronary thrombi results in alterations characteristic of reperfusion injury and associated with impaired functional recovery. Such changes are also evident, although to a lesser extent, when reperfusion of northrombotic occlusions is induced by mechanical recanalization in the presence of a systemic lytic state but not in its absence. However, such effects were not seen with direct mechanical recanalization of thrombotically occluded vessels. In aggregate, these findings indicate that induction of a systemic lytic state, together with products released by lysis of intracoronary thrombi, generates an inurious milieu that exerts adverse effects on reperfused myocardium.

Aspirin enhances the benefits of late reperfusion on infarct shape. A possible mechanism of the beneficial effects of aspirin on survival after acute myocardial infarction

BACKGROUND

The time window of the benefits of late reperfusion on infarct shape is limited. In rats, these benefits diminish in a wave front over time, with minimal benefits when reperfusion follows 16 hours of coronary occlusion. The mechanism of the benefits of aspirin on survival after acute myocardial infarction is unknown. The purpose of this study was to test the ability of aspirin to enhance the benefits of late coronary artery reperfusion on infarct shape and to examine the mechanism of the benefits of aspirin on infarct shape.

METHODS AND RESULTS

Rats were entered into two different protocols, the morphometric and the histological protocols. In the morphometric protocol, rats were randomized into two groups: the aspirin group, in which rats underwent left coronary artery occlusion followed by treatment with aspirin (12 mg/kg i.v.), and the control group, in which rats underwent left coronary artery occlusion followed by treatment with placebo. Rats in both groups were reperfused 8 hours after coronary occlusion. Rats in the aspirin group received aspirin in the drinking water (12 +/- 2 mg/kg daily). Morphometric analysis was performed 2 weeks after acute myocardial infarction. In the histological protocol, rats underwent the same randomization, coronary occlusion, and reperfusion protocols. Hearts were removed 24 hours after coronary occlusion, and microvessels were assessed for patency. Infarct size expressed as a percent of circumference was similar in the aspirin and placebo treatment groups (28 +/- 2% versus 33 +/- 3%, P = NS). Septal thickness was also similar in both groups (1.8 +/- 0.1 versus 2.1 +/- 0.1 mm, P = NS for aspirin versus placebo). The aspirin-treated group had thicker infarcts compared with the placebo-treated group (0.8 +/- 0.1 versus 0.5 +/- 0.1 mm, P < .05) and less expanded infarcts (expansion index, 1.2 +/- 0.1 versus 2.0 +/- 0.2, P < .05). Aspirin was associated with increased patency of the microvessels in the infarcted area compared with the placebo group (96% versus 64% of microvessels patent, P < .001).

CONCLUSIONS

Aspirin enhances the benefit of late coronary artery reperfusion on infarct shape after 8 hours of coronary occlusion. The benefits of aspirin on infarct shape after late reperfusion are related to increased patency of the microvessels in the infarcted area.

Quantitative two-dimensional echocardiographic assessment of regional wall motion during transient ischemia and reperfusion in the rat

Nonlethal myocardial ischemia produces profound and long-lasting effects on regional ventricular function and metabolism (myocardial stunning) and protects against myocardial infarction from subsequent prolonged ischemia (ischemic preconditioning). Two-dimensional echocardiography (2DE) is an essential tool for quantitative analysis of regional and global left ventricular (LV) function during myocardial ischemia and reperfusion and the study of these phenomena. However, the inability to perform 2DE in the open-chest rat heart has seriously limited the use of this model. To investigate the effect of transient coronary occlusion on segmental wall motion and LV geometry, we employed a 20 MHz intravascular ultrasound catheter placed on the epicardial surface of the rat heart (n = 15) to yield 2DE images suitable for quantitative analysis. Three 2-minute left coronary occlusions were made, separated by 5 minutes of reperfusion, with imaging during occlusion and at 5 and 60 minutes of reperfusion. Ischemic and nonischemic wall thicknesses, LV cross-sectional area, estimated LV volume, and the fractional changes of these parameters were measured. In eight animals these values were also compared with necropsy measurements of wall thickness, LV cross-sectional area, and volume. LV and right ventricular structures were well visualized in short-axis cross-sectional images in all animals, and images suitable for quantitative analysis were obtained in 92% of the periods. Coronary occlusion caused immediate, marked LV cavitary expansion, which rapidly returned to normal by 5 minutes of reperfusion. Active systolic thickening of the anterior wall at baseline (47% +/- 3%) became passive thinning during occlusion (-6% +/- 2%) and recovered partially, to 30% +/- 3% at 5 minutes of reperfusion and 42% +/- 4% at 60 minutes (p < 0.0005 at 5 minutes of reperfusion vs baseline; p not significant at 60 minutes). Recovery of thickening after 5 minutes of reperfusion was not different after the first versus third occlusion (23% +/- 4% vs 30% +/- 3%; p = 0.19). Measurements made by 2DE correlated well with those made by necropsy, although wall thickness was slightly thicker by 2DE. We conclude that epicardial echocardiography with an intravascular ultrasound catheter provides quantifiable 2DE images in this model and yields accurate information on segmental wall thickening and ventricular geometry not available by other techniques. Left coronary occlusion in the rat is associated with marked global and segmental LV expansion, which rapidly reverses with reperfusion. Postischemic regional wall motion abnormalities are present after coronary occlusion as brief as 2 minutes and can be measured accurately. The effect of multiple brief occlusions is not cumulative.(ABSTRACT TRUNCATED AT 400 WORDS)

Left ventricular remodeling after acute myocardial infarction

The loss of myocytes as a consequence of myocardial infarction results in a prompt reduction in regional wall motion and often leads to more protracted and progressive changes in ventricular architecture. The recognition that the process of ventricular enlargement following myocardial infarction is modifiable provided the initial rationale for the use of angiotensin-converting enzyme (ACE) inhibitors as therapy to prevent deterioration in ventricular size and function following infarction. Experimental and clinical studies have documented the effectiveness of this therapy in preventing this late enlargement following infarction. Increasing clinical evidence indicates that this new use of ACE inhibitor therapy in survivors of acute myocardial infarction will lead to an improvement in clinical outcome.

Determinants of left ventricular function two weeks and one year after an acute myocardial infarction

This study examines possible covariates of left ventricular function two weeks and sixteen months after an acute infarction. It was performed in a group of 312 patients randomized double blindly to recombinant tissue plasminogen activator (rt-PA) (n = 156) or placebo treatment and followed thereafter for at least one year. Two weeks after the infarction, enzymatic infarct size, infarct-related vessel, and number of diseased coronary arteries were significant determinants of the infarct-related regional wall motion (centerline method) (R2 = 0.25 to 0.60, P = 0.0001). Enzymatic infarct size, regional wall motion of both infarct-related and remote areas, reinfarction, and treatment allocation were significant independent correlates of ejection fraction (R2 = 0.76), end-diastolic volume (R2 = 0.20), and end-systolic volume (R2 = 0.69, P < 0.0001). Infarct-related coronary artery and predischarge end-systolic volume were significant independent covariates of ejection fraction at rest (R2 = 0.47) after sixteen months. Age, enzymatic infarct size, and predischarge end-diastolic volume were independent determinants of the maximal (R2 = 0.49, P < 0.0001) and peak exercise ejection fraction (R2 = 0.49, P < 0.0001).

Randomized comparison of rescue angioplasty with conservative management of patients with early failure of thrombolysis for acute anterior myocardial infarction

BACKGROUND

When used in the setting of acute myocardial infarction, intravenous thrombolytic agents fail to achieve early infarct artery patency in 15% to 50% of patients. We tested the hypothesis that immediate balloon angioplasty applied to patients with failed early reperfusion would improve left ventricular function and clinical outcome at 30 days compared with conservative management alone.

METHODS AND RESULTS

One hundred fifty-one patients with first anterior wall infarction treated with any accepted intravenous thrombolytic regimen and angiographically demonstrated to have an occluded infarct vessel within 8 hours of chest pain onset were randomized to aspirin, heparin, and coronary vasodilators (conservative therapy) or to this therapy and balloon angioplasty supplemented by further thrombolytic therapy as needed. Left ventricular function was assessed using multiple-gated equilibrium radionuclide technique to determine ejection fraction, and adverse clinical outcome was assessed evaluating death, ventricular tachycardia, and class III or IV heart failure at 30 days. Seventy-three patients were randomized to conservative therapy and 78 to angioplasty. The two groups were well balanced for patient age (59 +/- 11 years), sex (82% were male), and time to randomization (4.5 +/- 1.9 hours). Angioplasty was technically successful in 72 of 78 randomized patients (92%). Two patients randomized to conservative therapy crossed over to angioplasty within 72 hours. Resting 30-day ejection fraction was 40 +/- 11% in the angioplasty group and 39 +/- 12% in the conservative group (P = .49), but ejection fraction with exercise was 43 +/- 15% and 38 +/- 13% for the angioplasty and conservatively treated groups, respectively (P = .04). Adverse clinical outcomes included death in 5% and 10% (P = .18), severe heart failure in 1% and 7% (P = .11), and either death or severe heart failure in 6% and 17% (P = .05) of the angioplasty and conservatively managed groups, respectively.

CONCLUSIONS

When applied to patients with first anterior infarction, rescue angioplasty appears to be useful in the prevention of death or severe heart failure, with improvement in exercise, but not resting, ejection fraction. This strategy deserves further study and highlights the potential advantage of early mechanical restoration of infarct vessel patency when thrombolytic therapy has failed.

Effects of reperfusion on ischemic right ventricular dysfunction. Disparate mechanisms of benefit related to duration of ischemia

BACKGROUND

Right ventricular free wall (RVFW) ischemia impairs global RV performance and may result in acute hemodynamic compromise. However, RV function and hemodynamic performance typically improve spontaneously over time. This study was designed to determine whether reperfusion facilitates recovery of function in the ischemic right ventricle.

METHODS AND RESULTS

Closed chest dogs underwent right coronary balloon occlusion for 1 hour (n = 9), 4 hours (n = 6), or 8 hours (n = 7). In all animals, occlusion depressed RVFW function and global RV performance. After 1 hour of ischemia, reperfusion led to immediate improvement in RVFW function and consequently global RV performance, with complete recovery over 4 weeks and scar in < 1% of total RVFW area. Reperfusion after 4- and 8-hour occlusions resulted in acute improvement in global RV performance but to a lesser extent and by different mechanisms, since RVFW contraction remained severely impaired. This disproportionate recovery of global RV function was attributable to diminished RVFW dyskinesis associated with reperfusion-induced increments in RVFW diastolic thickness (characterized histopathologically in 6 additional animals subjected to 4-hour occlusions but killed 1 hour after reperfusion by interstitial edema, contraction band necrosis, and hemorrhage). Although later reperfusion was associated with a slower pace and lesser extent of recovery, RVFW contraction improved markedly over time. At 4 weeks, there was trivial RVFW scar in 4-hour animals (2% of total RVFW area), and, although fibrosis was significantly greater in 8-hour animals (7% of RVFW area), infarction was minimal relative to the extent of jeopardized myocardium.

CONCLUSIONS

The responses of ischemic RV myocardium to reperfusion are complex, with disparate effects according to the duration of preceding ischemia. Early reperfusion results in prompt improvement in and subsequent complete recovery of RVFW contraction and global RV performance, with trivial or no RVFW scar. Late reperfusion leads to little acute recovery of RVFW function, but global performance improves owing to diminished RVFW dyskinesis associated with reperfusion-induced increments in RVFW diastolic thickness. Nevertheless, RVFW function improves over time, with minimal evidence of infarction. Therefore, reperfusion facilitates recovery of RV function and minimizes the extent of infarction even after prolonged ischemia.

Reperfusion reduces left ventricular dilatation by preventing infarct expansion in the acute and chronic phases of myocardial infarction

Reperfusion reduces left ventricular dilatation in patients with acute myocardial infarction, but it is unclear to what extent this is a primary effect or only a consequence of the limiting effect of reperfusion on infarct size. To address this issue, 56 consecutive patients were examined by means of two-dimensional echocardiography on day 1, on day 3, before discharge, and at 6 months after an acute myocardial infarction. From this population two groups of 12 patients each, perfectly matched for site of myocardial infarction, extent of ventricular asynergy at two-dimensional echocardiography (akinesis + dyskinesis), and clinical characteristics were identified according to the creatine kinase (CK) time to peak, which was regarded as a marker of spontaneous or induced reperfusion: (1) CK time to peak of 12 hours or less (reperfused patients, n = 12), and (2) CK time to peak of more than 12 hours (nonreperfused patients, n = 12). In these two groups of patients end-diastolic and end-systolic left ventricular volumes and endocardial lengths of asynergic and normal ventricular segments, imaged in a cross-sectional view at the level of the papillary muscles, were then computed. At the first examination end-diastolic volume, end-systolic volume, and endocardial segment lengths of normal and asynergic segments were similar in the two groups of patients. Patients with late CK time to peak, however, showed a progressive increase in left ventricular systolic volumes and in asynergic endocardial segment lengths between the first and third (predischarge) examinations (p < 0.05 for both), with no change in systolic length of the normal myocardium. The left ventricular end-systolic volume and the asynergic endocardial segment length of patients with early CK time to peak, however, did not increase during hospitalization. The increment in end-systolic volume and in systolic infarct segment length from the first to the third examinations was higher in nonreperfused patients (p = 0.018 and p = 0.04, respectively). Changes similar to those detected in systole were found for diastolic volume and diastolic infarcted and noninfarcted segment length in both groups, but they did not reach statistical significance. After 6 months, an increases in volume and endocardial length were found in both groups of patients. Relative to the first examination, however, the increase in systolic volume and in asynergic systolic endocardial lengths remained greater for nonreperfused patients (p = 0.077 and p = 0.01, respectively).(ABSTRACT TRUNCATED AT 400 WORDS)

Transvalvular left ventricular assistance in cardiogenic shock secondary to acute myocardial infarction. Evidence for recovery from near fatal myocardial stunning

OBJECTIVES

The purpose of this study was to test the hypothesis that transvalvular left ventricular assistance would support the circulation in patients with cardiogenic shock secondary to acute myocardial infarction and allow recovery of function in patients with a reversibly damaged (stunned) left ventricle.

BACKGROUND

Cardiogenic shock occurs in 7.5% of patients presenting with acute myocardial infarction, resulting in survival of only 20%. Despite the use of aggressive interventional therapy in patients with shock secondary to anterior myocardial infarction, survival remains as low as 33%.

METHODS

We studied 11 patients with acute myocardial infarction and cardiogenic shock, as defined by a cardiac index < 2 liters/min per m2, pulmonary capillary wedge pressure > 18 mm Hg and systolic blood pressure < 90 mm Hg during positive inotropic therapy. Patients were 57 +/- 13 years old (mean +/- SD) and had a mean left ventricular ejection fraction of 25 +/- 11%, mean arterial pressure of 69 +/- 13 mm Hg and mean cardiac index of 1.6 +/- 0.4 liters/min per m2 on admission to the study.

RESULTS

During the 1st 24 h of left ventricular assistance, pulmonary capillary wedge pressure decreased from 26 +/- 4 to 16 +/- 4 mm Hg (p = 0.01), cardiac index increased from 1.6 +/- 0.4 to 2.4 +/- 0.4 liters/min per m2, and the dopamine hydrochloride dose decreased from 51 +/- 92 to 18 +/- 12 micrograms/kg body weight per min. In survivors, cardiac index improved to 3.2 +/- 0.5 liters/min per m2 (p = 0.01), and left ventricular ejection fraction improved to 34 +/- 5% (p < 0.05). The overall survival in the study group was 4 (36%) of 11 patients (95% confidence interval [CI] 8% to 65%), and 4 (66%) of 6 patients (95% CI 29% to 100%) with a Q wave anterior myocardial infarction survived.

CONCLUSIONS

Transvalvular left ventricular support during cardiogenic shock complicating acute myocardial infarction is feasible and results in significant hemodynamic and functional improvement.

Ventricular remodeling and infarct expansion

Infarct expansion, defined as an alteration in the ventricular topography due to thinning and lengthening of the infarcted segment, develops within the first few hours of the acute symptoms, mostly in patients with a large, transmural, anterior myocardial infarction. Shape changes, peculiar to risk region location and due to disparity in regional ventricular architecture, could be posited as the first step in the process of infarct expansion, with various cellular mechanisms contributing to subsequent continued early and late ventricular dilation. Because the increase in left ventricular volume is expected to be linearly dependent on the extent of the infarction, limiting infarct size, by thrombolysis, would proportionally reduce enlargement of the cavity. The effect of thrombolysis on left ventricular volume, however, seems not to be completely accounted for by the lessening effect of reperfusion on infarct size, because data suggest a restraining effect of reperfusion on the process of ventricular dilation in addition to the lessening effect on infarct size. If this turns out to be true, then the achievement of a patent vessel even beyond the time period when that patency may be expected to salvage myocardium would be further justified. Theoretical predictions substantiate the potential effectiveness in restraining ventricular dilation of stiffening of the necrotic region alone, independently of myocardial salvage in infarcted patients. The process of progressive ventricular dilation involves not only a primary alteration in function of the infarcted region, but also a time-dependent secondary change in the noninfarcted tissue itself, finalized to restore stroke volume despite a persistently depressed ejection fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of the recovery of right ventricular performance following experimental chronic right coronary artery occlusion

BACKGROUND

Patients with acute ischemic right ventricular dysfunction often develop hemodynamic compromise, yet most manifest spontaneous early clinical improvement and later recovery of global right ventricular performance. This study was designed to delineate the determinants of right ventricular performance following chronic right coronary artery occlusion.

METHODS AND RESULTS

Thrombotic right coronary artery occlusion was induced in 16 closed-chest dogs. Acute occlusion depressed right ventricular free wall motion (motion score, 1.0 +/- 0.0 to 3.4 +/- 0.1*) and global performance (right ventricular fractional area change, 29.2 +/- 1.8% to 2.3 +/- 1.9%*). There was right ventricular dilatation and reversed septal curvature, with elevated and equalized diastolic filling pressures. At 5 days, despite persistent severe right ventricular free wall dysfunction (motion score, 3.4 +/- 0.1 to 2.7 +/- 0.1*), global right ventricular performance improved (fractional area change, 2.3 +/- 1.9% to 17.0 +/- 3.8%*), in part due to reduced right ventricular free wall dyskinesis associated with increased right ventricular free wall end-diastolic thickness. At 7 weeks, collateral flow had restored right ventricular free wall perfusion to baseline values. The increased right ventricular free wall thickness had resolved, right ventricular free wall motion was improved (2.7 +/- 0.1 to 1.6 +/- 0.1*), and global right ventricular performance had recovered further (17.0 +/- 3.8 to 20.9 +/- 0.9). Right ventricular dilatation was reduced, septal curvature normalized, and there was resolution of equalized filling pressures. Histopathological analysis demonstrated minimal right ventricular infarction.

CONCLUSIONS

Acute right coronary artery occlusion results in right ventricular free wall dyskinesis and depressed global right ventricular performance. Global right ventricular function improves early, in part due to reduced right ventricular free wall dyskinesis associated with increased right ventricular free wall diastolic thickness. Collateral restoration of perfusion facilitates late recovery of right ventricular free wall function (*P < .05).

Remodelling of the heart after myocardial infarction

In the first few hours after the onset of coronary occlusion the infarct zone stretches due to myocyte slippage. Subsequently the noninfarct zone develops volume overload hypertrophy with series addition of new sarcomeres and fibre elongation. Dilatation is detrimental as it increases ventricular wall stress and oxygen demand, and re-entry of electrical impulses may be influenced by stretching of the ischaemic scar resulting in ventricular fibrillation. Left ventricular remodelling and dilatation is a progressive process which begins early and continues in the months after infarction. The major determinants of the extent of remodelling are infarct size and patency of the infarct-related artery. Late reperfusion may reverse initial infarct dilatation and decrease left ventricular volumes by inducing calcium-activated contracture of the actomyosin complex. Expansion may also be inhibited by acceleration of healing, splinting of the infarct zone by salvage of subepicardial cells, and blood in the coronary arteries and veins supporting the infarct zone. End-systolic volume is the strongest predictor of long-term prognosis after infarction. A number of therapies including thrombolysis, angiotensin-converting enzyme (ACE) inhibition and nitrates have been shown to decrease left ventricular dilatation. The optimal time for commencement, dose, duration and the effects of combinations of therapy are yet to be determined.

Patency of the infarct-related coronary artery and ventricular geometry

The pathogenesis of acute myocardial infarction (AMI) involves a sudden thrombotic occlusion of a coronary artery. Spontaneous or pharmacologic thrombolysis may lead to myocardial salvage if patency is achieved within a narrow time window. However, patients in whom thrombolysis occurs late seem to demonstrate improved left ventricular (LV) function and prognosis, which may be independent of myocardial salvage. Preservation of normal LV geometry by reducing expansion of the infarcted segment is a likely mechanism for this benefit. Infarct expansion is most pronounced in patients with anterior wall AMI who have a persistently occluded infarct-related vessel. This process of expansion leads to early increases in LV volume and distortions of LV contour (abnormal LV geometry). Patients whose infarct segment is largest, patients who have manifested infarct expansion, and patients with a persistently occluded infarct-related artery are at highest risk for progressive LV dilation. Experimental data support the concept that reperfusion of occluded vessels that occurs too late for myocardial salvage will preserve LV geometry by limiting infarct expansion. Prospective clinical trials should address whether there is a late, "second time window" during which infarct expansion and distortions of LV geometry may be reduced by (1) therapy with thrombolytic agents applied late after infarction, (2) late mechanical reperfusion with percutaneous transluminal coronary angioplasty (PTCA) or related methods, and (3) load-reducing agents to decrease remodeling, such as angiotensin-converting enzyme inhibitors or nitroglycerin.