Is microvascular protection by cariporide and ischemic preconditioning causally linked to myocardial salvage?

A historical literature review of coronary microvascular obstruction and intra-myocardial hemorrhage as functional/structural phenomena

The analysis of experimental data demonstrates that platelets and neutrophils are involved in the no-reflow phenomenon, also known as microvascular obstruction (MVO). However, studies performed in the isolated perfused hearts subjected to ischemia/reperfusion (I/R) do not suggest the involvement of microembolization and microthrombi in this phenomenon. The intracoronary administration of alteplase has been found to have no effect on the occurrence of MVO in patients with acute myocardial infarction. Consequently, the major events preceding the appearance of MVO in coronary arteries are independent of microthrombi, platelets, and neutrophils. Endothelial cells appear to be the target where ischemia can disrupt the endothelium-dependent vasodilation of coronary arteries. However, reperfusion triggers more pronounced damage, possibly mediated by pyroptosis. MVO and intra-myocardial hemorrhage contribute to the adverse post-infarction myocardial remodeling. Therefore, pharmacological agents used to treat MVO should prevent endothelial injury and induce relaxation of smooth muscles. Ischemic conditioning protocols have been shown to prevent MVO, with L-type Ca ²⁺ channel blockers appearing the most effective in treating MVO.

No-reflow phenomenon in the heart and brain

The no-reflow phenomenon refers to the observation that when an organ is made ischemic by occlusion of a large artery supplying it, restoration of patency in that artery does not restore perfusion to the microvasculature supplying the parenchyma of that organ. This has been observed after prolonged arterial occlusions in the heart (30–90 min), brain, skin, and kidney. In experimental models, zones of no reflow in the heart are characterized by ultrastructural microvascular damage, including focal endothelial swelling obstructing the lumen of small vessels. Blood elements such as neutrophil plugs, platelets, and stacking of erythrocytes have also been implicated. No reflow is associated with poor healing of the myocardial infarction. In patients, no reflow is associated with a poor clinical outcome independent of infarct size, suggesting that therapy for no reflow may be an important approach to improving outcome for ST elevation myocardial infarction. No reflow occurs after reperfusion of experimental cerebral ischemia and may be observed after only 5-min episodes of ischemia. Aggregation of blood elements may play a greater role than in cardiac no reflow. No reflow in the brain may involve cortical spreading depression with disturbed local vascular control and high, vasculotonic levels of extracellular K+ concentration, postischemic swelling in endothelial cells and abutting end feet of pericytes, pericyte contraction and death, interstitial edema with collapse of cerebral capillaries, and inflammatory reaction. New guidelines suggesting that reperfusion for stroke may be considered as late as 24 h after the onset of symptoms suggest that clinicians may be seeing more no reflow in the future.

Impact of electrical defibrillation on infarct size and no-reflow in pigs subjected to myocardial ischemia-reperfusion without and with ischemic conditioning

Ventricular fibrillation (VF) occurs frequently during myocardial ischemia-reperfusion (I/R) and must then be terminated by electrical defibrillation. We have investigated the impact of VF/defibrillation on infarct size (IS) or area of no reflow (NR) without and with ischemic conditioning interventions. Anesthetized pigs were subjected to 60/180 min of coronary occlusion/reperfusion. VF, as identified from the ECG, was terminated by intrathoracic defibrillation. The area at risk (AAR), IS, and NR were determined by staining techniques (patent blue, triphenyltetrazolium chloride, and thioflavin-S). Four experimental protocols were analyzed: I/R (n = 49), I/R with ischemic preconditioning (IPC; n = 22), I/R with ischemic postconditioning (POCO; n = 22), or I/R with remote IPC (RIPC; n = 34). The incidence of VF was not different between I/R (44%), IPC (45%), POCO (50%), and RIPC (33%). IS was reduced by IPC (23 ± 12% of AAR), POCO (31 ± 16%), and RIPC (22 ± 13%, all P < 0.05 vs. I/R: 41 ± 12%). NR was not different between protocols (I/R: 17 ± 15% of AAR, IPC: 15 ± 18%, POCO: 25 ± 16%, and RIPC: 18 ± 17%). In pigs with defibrillation, IS was 50% larger than in pigs without defibrillation but independent of the number of defibrillations. Analysis of covariance confirmed the established determinants of IS, i.e., AAR, residual blood flow during ischemia (RMBFi), and a conditioning protocol, and revealed VF/defibrillation as a novel covariate. VF/defibrillation in turn was associated with larger AAR and lower RMBFi. Lack of dose-response relation between IS and the number of defibrillations excluded direct electrical injury as the cause of increased IS. Obviously, AAR size and RMBFi account for both IS and the incidence of VF. IS and NR are mechanistically distinct phenomena.NEW & NOTEWORTHY Ventricular fibrillation/defibrillation is associated with increased infarct size. Electrical injury is unlikely the cause of such association, since there is no dose-response relation between infarct size and number of defibrillations. Ventricular fibrillation, in turn, is associated with a larger area at risk and lower residual blood flow.

The Coronary Circulation as a Target of Cardioprotection

The atherosclerotic coronary vasculature is not only the culprit but also a victim of myocardial ischemia/reperfusion injury. Manifestations of such injury are increased vascular permeability and edema, endothelial dysfunction and impaired vasomotion, microembolization of atherothrombotic debris, stasis with intravascular cell aggregates, and finally, in its most severe form, capillary destruction with hemorrhage. In animal experiments, local and remote ischemic pre- and postconditioning not only reduce infarct size but also these manifestations of coronary vascular injury, as do drugs which recruit signal transduction steps of conditioning. Clinically, no-reflow is frequently seen after interventional reperfusion, and it carries an adverse prognosis. The translation of cardioprotective interventions to clinical practice has been difficult to date. Only 4 drugs (brain natriuretic peptide, exenatide, metoprolol, and esmolol) stand unchallenged to date in reducing infarct size in patients with reperfused acute myocardial infarction; unfortunately, for these drugs, no information on their impact on the ischemic/reperfused coronary circulation is available.

Microvascular Alterations After Temporary Coronary Artery Occlusion: The No-Reflow Phenomenon

In experimental models of temporary coronary artery occlusion, tissue perfusion at the microvascular level remains incomplete even after patency of the infarct-related epicardial coronary artery is established, and distinct perfusion defects develop within the risk zone. This no-reflow phenomenon can be regarded as a basic cardiac response to ischemia-reperfusion. Perfusion defects observed in the clinical realm after reperfusion therapy for myocardial infarction may substantially be related to this mechanism in addition to microembolization and activation of platelets, as suggested in several recent studies. A major determinant of the amount of no-reflow seems to be infarct size itself. Reperfusion-related expansion of noreflow zones occurs within the first hours after the reopening of the coronary artery with a parallel reduction of regional myocardial flow, resulting in a potential therapeutic window. With various cardioprotective interventions, a close correlation between the size of the anatomic no-reflow and necrosis is a reproducible feature, which suggests a causal link between both entities of ischemic cardiac damage. Although vasodilating interventions failed to uncouple no-reflow zones from necrosis, the steps in the causal chain between microvascular and myocardial damage remain to be identified. On a long-term basis, tissue perfusion after ischemia-reperfusion remains markedly compromised for at least 4 weeks. Recent morphometric cardiac analyses suggested that the level of tissue perfusion after 4 weeks is a significant predictor of various indices of infarct healing, such as scar thickness, and infarct expansion index. As a consequence, improving tissue perfusion might concomitantly improve the healing process, which may provide the pathoanatomic basis for prognostic implications of no-reflow.

No-Reflow Phoenomenon by Intracoronary Thrombus in Acute Myocardial Infarction

Recently, percutaneous coronary intervention has been the treatment of choice in most acute myocardial infarction cases. Although the results of percutaneous coronary interventions have ben good, the no-reflow phenomenon and distal embolization of intracoronary thrombus are still major problems even after successful interventions. In this article, we will briefly review the deleterious effects of no-reflow and distal embolization of intracoronary thrombus during percutaneous coronary interventions. The current trials focused on the prevention and treatment of the no-reflow phenomenon and intracoronary thrombus.

Effect of Elevated Reperfusion Pressure on “No Reflow” Area and Infarct Size in a Porcine Model of Ischemia–Reperfusion

Background:

The “no reflow” phenomenon (microvascular obstruction despite restoration of epicardial blood flow) develops postreperfusion in acute myocardial infarction and is associated with poor prognosis. We hypothesized that increased reperfusion pressure may attenuate the no reflow phenomenon, as it could provide adequate flow to overcome the high resistance of the microvasculature within the no reflow zone. Thus, we investigated the effect of modestly elevated blood pressure during reperfusion on the extent of no reflow area and infarct size in a porcine model of ischemia–reperfusion.

Methods:

Eighteen farm pigs underwent acute myocardial infarction by occlusion of the anterior descending coronary artery for 1 hour, followed by 2 hours of reperfusion. Just prior to reperfusion, animals were randomized into 2 groups: in group 1 (control group, n = 9), no intervention was performed. In group 2 (n = 9), aortic pressure was increased by ∼20% (compared to ischemia) by partial clamping of the ascending aorta during reperfusion. Following 2 hours of reperfusion, animals were euthanized to measure area at risk, infarct size, and area of no reflow.

Results:

Partial clamping of the ascending aorta resulted in modest elevation of blood pressure during reperfusion. The area at risk did not differ between the 2 groups. The no reflow area was significantly increased in group 2 compared to control animals (50% ± 13% vs 37% ± 9% of the area at risk; P = .04). The infarcted area was significantly increased in group 2 compared to control animals (75% ± 17% vs 52% ± 23% of the area at risk; P = .03). Significant positive correlations were observed between systolic aortic pressure and no reflow area, between systolic aortic pressure and infarcted area and between infarcted area and no reflow area during reperfusion.

Conclusions:

Modestly elevated blood pressure during reperfusion is associated with an increase in no reflow area and in infarct size in a clinically relevant porcine model of ischemia–reperfusion.

Influence of cardiac decentralization on cardioprotection

The role of cardiac nerves on development of myocardial tissue injury after acute coronary occlusion remains controversial. We investigated whether acute cardiac decentralization (surgical) modulates coronary flow reserve and myocardial protection in preconditioned dogs subject to ischemia-reperfusion. Experiments were conducted on four groups of anesthetised, open-chest dogs (n = 32): 1- controls (CTR, intact cardiac nerves), 2- ischemic preconditioning (PC; 4 cycles of 5-min IR), 3- cardiac decentralization (CD) and 4- CD+PC; all dogs underwent 60-min coronary occlusion and 180-min reperfusion. Coronary blood flow and reactive hyperemic responses were assessed using a blood volume flow probe. Infarct size (tetrazolium staining) was related to anatomic area at risk and coronary collateral blood flow (microspheres) in the anatomic area at risk. Post-ischemic reactive hyperemia and repayment-to-debt ratio responses were significantly reduced for all experimental groups; however, arterial perfusion pressure was not affected. Infarct size was reduced in CD dogs (18.6±4.3; p = 0.001, data are mean±1SD) compared to 25.2±5.5% in CTR dogs and was less in PC dogs as expected (13.5±3.2 vs. 25.2±5.5%; p = 0.001); after acute CD, PC protection was conserved (11.6±3.4 vs. 18.6±4.3%; p = 0.02). In conclusion, our findings provide strong evidence that myocardial protection against ischemic injury can be preserved independent of extrinsic cardiac nerve inputs.

Concurrent Microvascular and Infarct Remodeling After Successful Reperfusion of ST-Elevation Acute Myocardial Infarction

Background—

Connection between the course of microvascular and infarct remodeling processes over time after reperfused ST-elevation acute myocardial infarction has not been fully elucidated. The aim of this study is to investigate the association of temporal changes in hemodynamics of microcirculation in the infarcted territory and infarct size (IS) after primary percutaneous coronary intervention in patients with ST-elevation acute myocardial infarction.

Methods and Results—

Thirty-five patients admitted with ST-elevation acute myocardial infarction undergoing primary percutaneous coronary intervention were enrolled in the study. Coronary flow reserve (CFR), index of microvascular resistance (IMR), and IS were assessed 2 days after primary percutaneous coronary intervention and at the 5-month follow-up. The predictors of the 5-month IS were the baseline values of IS (β=0.6, P <0.001), IMR (β=0.280, P =0.013), and CFR (β=−0.276, P =0.017). There were significant correlations between relative change in IS and relative change in measures of microvascular function (IS and CFR [ r =−0.51, P =0.002]); IS and IMR ([ r =0.55, P =0.001]). In multivariate model, relative changes in IMR (β=0.552, P =0.001) and CFR (β=−0.511, P =0.002) were the only predictors of relative change in IS. In patients with an improvement in IMR >33%, the mean IS decreased from 32.3±16.9% to 19.3±14% ( P =0.001) in the follow-up. Similarly, in patients with an improvement in CFR >41%, the mean IS significantly decreased from 29.9±20% to 15.8±12.4% ( P =0.003). But in patients with an improvement in IMR and CFR, which were below than the mean values, IS did not significantly decrease during the follow-up.

Conclusions—

Improvement in microvascular function in the infarcted territory is associated with reduction in IS after reperfused ST-elevation acute myocardial infarction. This link suggests that further investigations are warranted to determine whether therapeutic protection of microvascular integrity results in augmentation of infarct healing.

Postconditioning fails to improve no reflow or alter infarct size in an open-chest rabbit model of myocardial ischemia-reperfusion

Postconditioning (PoC) with brief intermittent ischemia after myocardial reperfusion has been shown to lessen some elements of postischemic injury including arrhythmias and, in some studies, the size of myocardial infarction. We hypothesized that PoC could improve reflow to the risk zone after reperfusion. Anesthetized, open-chest rabbits were subjected to 30 min of coronary artery occlusion followed by 3 h of reperfusion. In protocol 1, rabbits were randomly assigned to the control group ( n = 10, no further intervention after reperfusion) or to the PoC group, which consisted of four cycles of 30-s reocclusions with 30 s of reperfusion in between starting at 30 s after the initial reperfusion (4 × 30/30, n = 10). In protocol 2, rabbits were assigned to the control group ( n = 7) or the PoC group, which received PoC consisting of four cycles of 60-s intervals of ischemia and reperfusion starting at 30 s after the initial reperfusion (4 × 60/60, n = 7). No reflow was determined by injecting thioflavine S (a fluorescent marker of capillary perfusion), risk zone by blue dye, and infarct size by triphenyltetrazolium chloride. In protocol 1, there were no statistical differences in hemodynamics, ischemic risk zone, or infarct size (35 ± 6% of the risk zone in the PoC group vs. 29 ± 4% in the control group, P = 0.38) between the groups. Similarly, in protocol 2, PoC failed to reduce infarct size compared with the control group (45 ± 4% of the risk zone in the PoC group vs. 42 ± 6% in the control group, P = 0.75). There was a strong correlation in both protocols between the size of the necrotic zone and the portion of the necrotic zone that contained an area of no reflow. However, PoC did not affect this relationship. PoC did not reduce infarct size in this model, nor did it reduce the extent of the anatomic zone of no reflow, suggesting that this intervention may not impact postreperfusion microvascular damage due to ischemia.

Adenosine receptor-mediated coronary vascular protection in post-ischemic mouse heart

This study evaluated the ability of A1 and A3 adenosine receptor (AR) agonism, and A1, A2A, A2B and A3AR antagonism (revealing "intrinsic" responses), to modify post-ischemic coronary dysfunction in mouse heart. Vascular function was assessed before and after 20 min global ischemia and 30-45 min reperfusion in Langendorff perfused C57/Bl6 mouse hearts. Ischemic insult impaired coronary sensitivity to the endothelial-dependent dilators ADP (pEC50=6.8+/-0.1 vs. 7.6+/-0.1, non-ischemic) and acetylcholine (pEC50=6.1+/-0.1 vs. 7.3+/-0.1 in non-ischemic), and for the mixed endothelial-dependent/independent dilator 2-chloroadenosine (pEC50=7.5+/-0.1 vs. 8.4+/-0.1, non-ischemic). Endothelium-independent dilation in response to nitroprusside was unaltered (pEC50=7.0+/-0.1 vs. 7.1+/-0.1 in non-ischemic). Pre-treatment with a selective A1AR agonist (50 nM CHA) failed to modify coronary dysfunction, whereas A1AR antagonism (200 nM DPCPX) worsened the effects of I/R (2-chloroadenosine pEC50=6.9+/-0.1). Conversely, A3AR agonism (100 nM Cl-IB-MECA) did reduce effects of I/R (pEC50s=8.0+/-0.1 and 7.3+/-0.1 for 2-chloroadenosine and ADP, respectively), whereas antagonism (100 nM MRS1220) was without effect. While A2AAR agonism could not be assessed (due to pronounced vasodilatation), A2AAR antagonism (100 nM SCH58261) was found to exert no effect, and antagonism of A2BARs (50 nM MRS1754) was also ineffective. The protective actions of A3AR agonism were also manifest as improved reactive hyperemic responses. Interestingly, post-ischemic coronary dysfunction was also limited by: Na+-H+ exchange (NHE) inhibition with 10 or 50 microM BIIB-513 (2-chloroadenosine pEC50s=7.8+/-0.1, either dose), an effect not additive with A3AR agonism; Ca2+ antagonism with 0.3 microM verapamil (2-chloroadenosine pEC50=7.9+/-0.1); and Ca2+ desensitization with 5 mM BDM (2-chloroadenosine pEC50=7.8+/-0.1). In contrast, endothelin antagonism (200 nM PD142893) and anti-oxidant therapy (300 microM MPG+150 U/ml SOD+600 U/ml catalase) were ineffective. Our data collectively confirm that ischemia selectively impairs endothelial function and reactive hyperemia independently of blood cells. Vascular injury is intrinsically limited by endogenous (but not exogenous) activation of A1ARs, whereas exogenous A3AR activation further limits dysfunction (improving post-ischemic vasoregulation). Finally, findings suggest this form of post-ischemic coronary injury is unrelated to endothelin or oxidant stress, but may involve modulation of Ca2+ overload and/or related ionic perturbations.

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.