Effect of magnesium on ischemic and reperfusion arrhythmias in a canine model with diminished collateral blood flow

Summary To assess the effects of elevated serum magnesium on ischemic and reperfusion arrhythmias, the left anterior descending coronary artery was cannulated and perfused by a shunt from a carotid artery in 20 open-chest anesthetized dogs. Ischemia was caused for 30 minutes by shunt occlusion and retrograde diversion of collateral blood flow. Dogs (10/group) were treated prior to occlusion with either saline or MgSO₄ (100 mg/kg IV). Plasma magnesium rose from 0.72±0.05 mM to 3.89±0.29 mM before occlusion (p<0.01) and fell to 3.28±0.21 mM just before reperfusion (p<0.01). Compared to saline, magnesium significantly slowed heart rate (113±4 beats/min vs. 124±3 beats/min, p<0.05), lowered arterial blood pressure (90±2 mmHg vs. 111±4 mmHg, p<0.05), and reduced myocardial blood flow to the ischemic zone before the occlusion (59±7 ml/min/100 g vs. 83±5 ml/min/100 g, p<0.01). The incidence of ventricular tachycardia during occlusion was 80% in the saline group and 70% in the magnesium group (p=1.0). The time required for a monophasic complex to develop in an electrogram over the ischemic zone was 4.5±0.24 minutes in the saline group and was not altered by magnesium (4.6±0.18 minutes). The incidence of reperfusion-induced ventricular fibrillation was 100% in both groups. The results suggest that acute infusion of magnesium offers little protection against ventricular tachyarrhythmias evoked by occlusion or reperfusion in a canine model of myocardial ischemia with diminished collateral blood flow.     

Effect of early coronary artery reperfusion on infarct development in a model of low collateral flow

To characterise collateral blood flow patterns after coronary artery occlusion in the rat and to determine whether tissue can be salvaged by reperfusion in this model, anaesthetised rats were subjected to 20, 30, 40, 60 min of coronary occlusion followed by 24 h of reperfusion or 24 h of permanent occlusion. Relative regional blood flow was measured by radioactive microspheres after 10 min of occlusion in rats undergoing 30 min of occlusion plus reperfusion and in those undergoing 24 h of permanent occlusion. The area at risk was determined by in vivo injection of fluorescent microspheres and necrosis delineated by in vitro tetrazolium staining. Tracings of heart slices were planimetered and the area of necrosis and transmural extent of the infarct measured. Blood flow in the area at risk during occlusion was similar in both the reperfused and permanent occlusion groups. In the 30 min group mean(SEM) subendocardial flow was reduced to 13(5)% of normal and subepicardial flow to 9(3)% and in the permanent occlusion group to 11(2)% and 8(3)% respectively. As delineated by fluorescent microspheres the area at risk was transmural in all rats; however, infarct size expressed as a percentage of the area at risk increased as the duration of occlusion increased. In rats reperfused up to 30 min after occlusion the area of necrosis as a percentage of the area at risk was significantly decreased compared with that in the permanent occlusion group (36.4(9.2)% in rats with 30 min occlusion plus 24 h reperfusion and 78.6(7.4)% in rats with permanent occlusion).(ABSTRACT TRUNCATED AT 250 WORDS)     

Reduction of myocardial reperfusion injury by intravenous adenosine administered during the early reperfusion period

Adenosine influences the function of several cell types thought to be involved in the pathogenesis of myocardial reperfusion injury. We have previously demonstrated that intracoronary administration of adenosine enhances myocardial salvage 24 hours after reperfusion. To determine if these beneficial effects could be obtained during a prolonged period of reperfusion using an intravenous route of administration, 22 closed-chest dogs were subjected to 90 minutes of proximal left anterior descending coronary artery occlusion and 72 hours of reperfusion. Animals randomly received either intravenous adenosine (0.15 mg/kg/min) or an equal volume of Ringer's lactate during the first 150 minutes of reperfusion. The area at risk was defined in vivo with Monastral blue, and infarct size was measured histologically with Mallory's trichrome stain. Serial global and regional ventricular function were determined with contrast ventriculography and analyzed using a computerized radial shortening method. Biopsies were obtained from the central ischemic zone to assess endothelial ultrastructure and capillary obstruction. No significant effects in heart rate or blood pressure were noted during adenosine infusion. Transmural collateral blood flow during ischemia was similar in the groups. Infarct size expressed as a percentage of the anatomical area at risk was significantly less in the adenosine-treated group (35.3 +/- 4.3% in controls versus 17.1 +/- 4.3% in treated animals, p less than 0.01). A progressive decrease in transmural blood flow was noted in control animals during reperfusion, resulting in a significant reduction at 3 hours compared with the preocclusion value (0.69 +/- 0.11 ml/min/g [at baseline versus 0.45 +/- 0.10 ml/min/g at 3 hours, p less than 0.05]). In contrast, flow in adenosine animals at 3 hours was similar to baseline values (0.91 +/- 0.15 ml/min/g at baseline versus 0.98 +/- 0.14 ml/min/g at 3 hours, p = NS) and was significantly higher (p less than 0.05) than the control group. Radial shortening in the ischemic zone was significantly improved at 3 (-2.6 +/- 2.8% in controls versus 11.6 +/- 3.3% in treated animals, p less than 0.01) and 72 hours (5.5 +/- 2.0% in controls versus 17.3 +/- 3.5% in treated animals, p less than 0.01) after reperfusion in treated animals. Electron microscopy showed reduced neutrophil and erythrocyte plugging of capillaries with relative preservation of endothelial cell structure in the adenosine group.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of intracoronary adenosine infusion during coronary intervention on myocardial reperfusion injury in patients with acute myocardial infarction

Despite early recanalization of an occluded infarct artery, up to 33% of patients with acute myocardial infarction do not obtain complete myocardial reperfusion due to a process of reperfusion injury. This study assessed whether adjunctive therapy with adenosine might prevent or attenuate the phenomenon of myocardial reperfusion injury. Myocardial reperfusion was assessed in 79 consecutive patients receiving a 20-minute intracoronary infusion of adenosine during percutaneous coronary intervention (PCI) and in a historical cohort of 200 patients with acute myocardial infarction who were treated with PCI (controls). Myocardial reperfusion injury was defined as persistent (> or =50% of initial value) ST-segment elevation after successful recanalization. Its effect on infarct size was evaluated by calculating the Selvester QRS score before intervention and at follow-up. Myocardial reperfusion injury was present in 19% of patients receiving adenosine versus 35% of control patients (p = 0.004). Evaluation of infarct expansion over time showed almost no change in the QRS score in patients receiving adenosine (3.4 +/- 3.0 before PCI; 3.5 +/- 3.1 at follow-up). In contrast, infarct QRS score in the control group worsened from 3.1 +/- 2.7 before PCI to 4.5 +/- 3.2 at follow-up (p = 0.003 treatment with adenosine vs control). Multivariate analysis identified adjunctive therapy with adenosine as an independent protective determinant of myocardial reperfusion injury and of infarct expansion. The rate of major adverse cardiac events (death and myocardial infarction) at 1 month tended to be lower in patients receiving adenosine (4% vs 6.5%, p = 0.7) and was mainly observed in patients with evidence of myocardial reperfusion injury (cardiac event rate 2% in patients with ST-segment elevation of <50% vs 14% in patients with ST-segment elevation > or =50%, p = 0.003). Thus, impaired myocardial reperfusion is the most important determinant of clinical outcome in patients with acute myocardial infarction treated with PCI. Adjunctive therapy with intracoronary infusion of adenosine during PCI prevents the occurrence of severe myocardial reperfusion injury and is associated with less infarct expansion.     

静脉滴注尼可地尔对急性心肌梗塞犬冠状动脉侧支循环的作用

为探讨尼可地尔静脉滴注（静滴）对急性心肌梗塞（AMI）犬冠状动脉侧支循环和梗塞范围的影响，在12只麻醉开胸犬上结扎冠状动脉左前降支，远端插管建立侧支返流模型。实验组在缺血5min后静滴尼可地尔200μg／kg；对照组静滴等容积生理盐水。在不同时项上观察心脏血流动力学和侧支循环指标，缺血120min后以称重法测定心肌梗塞（MI）面积。结果：与缺血5min的基础值相比，实验组外周冠状动脉压显著下降，冠状动脉侧支流量显著增加，侧支系统阻力及外周冠状动脉阻力显著下降，而对照组无明显变化。实验组MI范围较对照组显著下降。表明：尼可地尔能改善AMI犬的冠状动脉侧支循环，并能限制MI范围。     

Combined adenosine and lidocaine administration limits myocardial reperfusion injury

The endogenous compound adenosine may play a role in limiting myocardial ischemia-reperfusion injury through its ability to cause vasodilation, modulate cardiac adrenergic responses, inhibit neutrophil function, or modulate energy supply and demand of the myocardium. The local anesthetic lidocaine has been shown to be protective against myocardial ischemia-reperfusion injury, although its mechanism of action remains unresolved. We hypothesized that administration of exogenous adenosine during reperfusion would limit the size of the infarct that results from a period of ischemia and reperfusion only when the animals are treated with lidocaine. Male, mongrel dogs (13.0-20.0 kg) were anesthetized (30 mg/kg i.v. sodium pentobarbital), and a left thoracotomy was performed. The left circumflex coronary artery (LCx) was isolated and instrumented with an electromagnetic flow probe, a 25-gauge nonobstructing intracoronary catheter, and a critical stenosis. The dogs were allocated randomly to one of four groups: 1) control, n = 13, (saline), 2) adenosine, n = 13, (0.15 mg/kg/ml/min i.c. for the first hour of reperfusion), 3) lidocaine, n = 9, (2.0 mg/kg i.v. given immediately before coronary artery occlusion and just before reperfusion), or 4) adenosine plus lidocaine, n = 11. The LCx was occluded for 90 minutes and reperfused for 6 hours. Regional myocardial blood flow (RMBF) was determined (n = 6 per group) at 80 minutes of occlusion and at 45 minutes of reperfusion with radiolabeled microspheres. RMBF determinations revealed an increase in blood flow to the inner two thirds of the myocardium at 45 minutes of reperfusion only in the presence of the combined treatment. Adenosine treatment alone or lidocaine treatment alone did not affect RMBF. Quantification of infarct size (triphenyltetrazolium method) expressed as a percent of the area at risk revealed a significant limitation of infarct size only in the group treated with both adenosine and lidocaine: control, 47.8 +/- 6.6%; adenosine, 45.0 +/- 3.2%; lidocaine, 46.9 +/- 6.0%; and adenosine and lidocaine, 20.8 +/- 5.6%. Statistical analyses were performed with two-way analysis of variance to account for the two individual drug treatments. The findings show that intracoronary administration of exogenous adenosine, at the dose used, is only effective at limiting myocardial infarct size when administered to lidocaine-treated animals.     

Extracoronary collateral myocardial blood flow during cardioplegic arrest

Extracoronary blood flow to the myocardium was studied in 54 patients during cold cardioplegic arrest. Coronary venous return was measured with the aorta and the pulmonary artery cross-clamped, both venae cavae occlusively snared, and the heart completely drained. Cold St. Thomas' cardioplegic solution was infused into either the aortic root or the coronary ostia. Myocardial septal temperature was continuously monitored. The amount of blood in the right atrial effluent was determined by means of the hematocrit and was considered to be the extracoronary collateral myocardial blood flow (QE), originating from the systemic circulation. QE ranged from 0 to 1470 ml-100 min-1 (x = 241.1 ml-100min-1). The myocardial spontaneous rewarming rate was not significantly correlated to QE. QE was lowest in pure mitral valve stenosis (x = 39.9 ml-100 min-1) and higher in aortic valve disease (x = 165.5 ml-100 min-1). Very high QE values (greater than 800 ml-100 min-1) were only observed in patients with severe three vessel coronary artery disease. Patients with angina at rest appear to have lower QE values than patients with equally severe coronary artery disease suffering from angina under excise only. It is concluded that extracoronary collateral blood flow may unpredictably influence the efficacy of clinical cardioplegia and may to some extent compensate for severe coronary artery disease.     

腺苷对急性心肌梗死病人PCI术后心肌再灌注损伤的临床观察

目的观察腺苷对急性心肌梗死(AM I)患者行PC I后再灌注损伤心肌的保护作用。方法AM I患者分为腺苷组和生理盐水组,行PC I时一组冠脉内推注腺苷,一组推注生理盐水,并测定血浆SOD、MDA、CK-MB峰值。结果腺苷组MDA较生理盐水组低(5.01±0.80 vs 6.97±0.86,P<0.05),腺苷组SOD较生理盐水组高(80.70±3.23 vs 61.63±3.49,P<0.05),腺苷组CK-MB峰值较生理盐水组明显降低(123.6±84.3 vs186.1±92.2,P<0.05)。结论AM I患者行急诊PC I术时冠脉内应用腺苷能减轻心肌再灌注损伤。     

Comparison of maximal myocardial blood flow during adenosine infusion with that of intravenous dipyridamole in normal men.

OBJECTIVE. This study compared quantitatively the efficacy of intravenous adenosine and dipyridamole for pharmacologic induction of myocardial hyperemia. BACKGROUND. Pharmacologic vasodilation is used increasingly for induction of myocardial hyperemia in conjunction with radionuclide imaging of myocardial blood flow. Although both intravenous dipyridamole and adenosine have been used, the magnitude of hyperemia induced by these agents and the hyperemia to baseline blood flow ratios have not been quantified and compared. METHODS. Twenty normal volunteers were studied with dynamic positron emission tomography (PET) and intravenous nitrogen-13 ammonia. Myocardial blood flow was quantified with a two-compartment tracer kinetic model. RESULTS. Myocardial blood flow at rest averaged 1.1 +/- 0.2 ml/min per g and increased significantly to 4.4 +/- 0.9 ml/min per g during adenosine and 4.3 +/- 1.3 ml/min per g after dipyridamole administration. Hyperemia to baseline flow ratios averaged 4.3 +/- 1.6 for adenosine and 4.0 +/- 1.3 for dipyridamole. The average flow ratios and the maximal flows achieved were similar for both agents, but there was considerable variation in the individual response to these agents, as indicated by the range of hyperemia to baseline flow ratios (from 2.0 to 8.4 for adenosine and from 1.5 to 5.8 for dipyridamole). In addition, the hyperemic responses to dipyridamole and to adenosine differed by greater than 1 ml/min per g in nine subjects. CONCLUSIONS. Despite these inter- and intraindividual differences, we conclude that both agents are equally effective in producing myocardial hyperemia.     

腺苷对猪急性心肌梗死再灌注后无再流的影响

目的评价腺苷防治猪急性心肌梗死(AMI)再灌注后无再流的作用。方法中华小型猪24只随机分成对照组、腺苷组(100μg·kg-1·min-1持续静点)和假手术组,每组8只。前2组行冠状动脉结扎3h,松解1h建立AMI再灌注模型。AMI前、后和再灌注后均行血流动力学测定和心肌声学造影检查,最终行病理学分析。结果(1)与AMI前相比,对照组AMI后3h主动脉收缩和舒张压、左室收缩压、心排量和左室内压最大收缩和舒张变化速率(±dp/dtmax)均显著下降(P<0.05～0.01),肺毛细血管楔压和左室舒张末压均显著升高(P<0.01);再灌注后1h仅左室舒张末压显著恢复(P<0.05)然而±dp/dtmax继续显著下降(P<0.05);而腺苷组AMI后3h各项指标变化与对照组相同;但再灌注后1h左室收缩压、左室舒张末压、±dp/dtmax和心排量均显著恢复(P均<0.05),且比对照组更显著(P均<0.05)。(2)对照组心肌声学造影和病理染色所测的冠状动脉结扎区心肌范围高度一致,再灌注后无再流面积分别为67.5%和69.3%,心肌坏死面积(NA)占结扎区心肌面积(LA)的98.5%;而腺苷组LA均与对照组相当,但两方法所测无再流面积仅分别为21.5%和20.8%,NA仅为75.2%,均显著小于对照组(P<0.05～0.01)。(3)对照组再灌注即刻和再灌注后1h冠状动脉血流量仅占AMI前的45.8%和50.6%(P均<0.01),而腺苷组冠     

三磷酸腺苷后处理对心肌缺血/再灌注损伤的影响

目的:观察三磷酸腺苷(ATP)和腺苷A2a受体激动剂CGS-21680后处理对心肌缺血/再灌注损伤(MIRI)的影响,并探讨其作用的机制。方法: 健康新西兰大白兔60只,随机分成5组（n=12）:即对照组、缺血/再灌注(I/R)组、缺血后处理(IPO)组、ATP组及CGS-21680组。建立兔心肌I/R模型,于再灌注末颈动脉取血,应用放射免疫测定法(RIA)测定血清中白细胞介素-1β(IL-1β)、肿瘤坏死因子-α(TNF-α)的含量;用TUNEL法检测心肌细胞的凋亡;实时荧光定量RT-PCR检测各组心肌组织中IL-1β mRNA和TNF-α mRNA的表达。 结果: ①与I/R组比较, IPO组、ATP组和CGS21680组血清IL-1β、TNF-α的含量明显降低（P<0.01）;而3组组间相比较无显著差异。②IPO组、ATP组和CGS-21680组的细胞凋亡指数(AI)较I/R组明显降低,心肌组织损伤也显著减轻。同时,IPO组、ATP组和CGS-21680组IL-1β、TNF-α mRNA的表达明显低于I/R组;但3组间比较无显著差异。细胞凋亡和IL-1β、TNF-α mRNA的表达之间呈正相关（分别为r=0.91和r=0.93,P<0.05）。结论: ATP后处理可减轻MIRI,其作用机制可能与抑制炎症反应,减少细胞凋亡有关。     

腺苷A_(2a)受体激动剂后处理对兔缺血再灌注心肌的保护作用

目的观察三磷酸腺苷(ATP)、腺苷A_(2a)受体激动剂CGS-21680及缺血后处理对兔缺血再灌注心肌的影响,并探讨后处理与腺苷受体亚型的关系及其保护机制。方法健康新西兰大耳白兔48只,随机分成4组:缺血再灌注组(IR组)、缺血后处理组(IPO组)、ATP后处理组(ATP组)和CGS-21680后处理组(CGS-21680组),每组12只。建立兔急性心肌缺血再灌注模型,实验终点测定血清肌酸激酶同工酶(CK-MB)、丙二醛(MDA)水平及超氧化物歧化酶(SOD)活力;采用Evans蓝和四氮唑蓝(NBT)双重染色测定心肌梗死面积。光镜下观察心肌组织病理形态变化。结果 IR组心肌损伤较重,有心肌断裂、坏死,间质肿胀、出血及中性粒细胞浸润,IPO组、ATP组及CGS-21680组心肌组织损伤明显轻于IR组。与IR组比较,IPO组、ATP组和CGS-21680组血清MDA生成量减少[(3.68±0.60)U/ml、(3.75±0.82)U/ml和(4.05±0.86)U/ml比(5.05±0.65)U/ml,均为P0.05],CK-MB活性降低[(231.83±16.22)U/L、(225.83±9.22)U/L和(238.33±22.26)U/L比(343.42±21.55)U/L,均为P0.01],心肌梗死面积降低(13.86%±2.77%、14.22%±2.24%和14.57%±1.66%比30.49%±1.57%,均为P0.01)以及SOD活力升高[(238.08±38.22)nmol/ml、(261.75±40.27)nmol/ml和(294.78±23.38)nmol/ml比(149.98±42.42)nmol/ml,均为P0.05];而CGS-21680组、IPO组和ATP组组间比较,差异无统计学意义。结论 ATP和CGS-21680后处理可减轻心肌缺血再灌注损伤,保护强度与机械性缺血后处理相似,且后处理对再灌注心肌的保护与腺苷A_(2a)受体的激活有关,其机制可能与清除氧自由基,抑制脂质过氧化反应从而提高机体的抗氧化能力有关。     

Salvage of ischaemic myocardium by reperfusion: importance of collateral blood flow and myocardial oxygen demand during occlusion

Early reperfusion after coronary artery occlusion is used to treat acute myocardial infarction, but the factors that determine whether salvage of ischaemic myocardium actually occurs remain poorly defined. Differences in collateral blood flow to the region at risk, and haemodynamic variables during occlusion, may contribute to uncertainty as to the time beyond which reperfusion no longer reduces infarct size. To clarify this issue, open chest anaesthetised dogs underwent 1, 2, 3, 4, or 6 hours of left anterior descending coronary artery occlusion followed by reperfusion or permanent occlusion (n = 8 per group). Microspheres were injected before occlusion and 15 minutes after occlusion for regional myocardial blood flow determination, and heart rate and arterial blood pressure were measured before occlusion and 10 minutes and 30 minutes after occlusion. At 96 hours after occlusion haemodynamic variables were again measured; the animals were then killed, and occluded bed size was determined by in vitro dye perfusion. The area of necrosis was quantified from histological sections and expressed as a percentage of occluded bed size (AN/OB). If duration of occlusion is considered alone, reperfusion beyond two hours did not salvage ischaemic myocardium in this model. If the results for occlusion equal to and greater than two hours are combined, the mean area of necrosis (27(2)%) was significantly greater than that produced by one hour of occlusion followed by reperfusion (10(4)%). For the animals undergoing occlusion for two or more hours or permanent occlusion, collateral blood flow significantly influenced the area of necrosis. When epicardial flow during occlusion was high (greater than 0.30 ml X min-1 X g-1 tissue) 13 out of 14 dogs undergoing occlusion for two or more hours or permanent occlusion developed small (AN/OB less than 27%) infarcts (mean AN/OB 17(2)%). In contrast, when epicardial collateral flow was low (less than 0.30 ml X min-1 X g-1) 14 out of 23 animals had large (AN/OB greater than 27%) infarcts (mean AN/OB 34(3)%). For the 23 dogs in which epicardial flow was low, heart rate during occlusion significantly influenced infarct size: the 14 dogs that developed large infarcts (AN/OB greater than 27%) had a higher mean heart rate (152(6) beats X min-1) than the nine that developed small infarcts (AN/OB less than 27%) (130(5) beats X min-1; p less than 0.025). Thus reperfusion at one hour after occlusion salvaged ischaemic myocardium.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evaluation of collateral blood flow by myocardial contrast enhanced echocardiography

Contrast enhanced cross sectional echocardiography is a new method for the real-time evaluation of regional myocardial perfusion. Two patients with a history of anteroseptal myocardial infarction and echocardiographically detected septal dyskinesia were examined by this new method. The first patient had two severe stenoses of the left anterior descending coronary artery and normal echocontrast opacification of the interventricular septum caused by collaterals from the right coronary artery. The second patient had good patency of left anterior descending coronary artery and no septal opacification. Thus contrast enhanced cross sectional echocardiography can be used to assess the importance of collateral blood flow in the myocardium.     

Effect of echinochrom on experimental myocardial reperfusion injury]

Echinochrom, a new antioxidant of the polyhydroxynaphthaquinone class, was tested for its cardioprotective activity in a model of occlusive reperfusion myocardial infarction (90-min occlusion and 4-hour reperfusion) in the acute experiments with open-chest dogs. The bolus intravenous injection of echinochrom in a dose of 1 mg/kg 5 min before reperfusion caused a significant (over 40%) reduction in the size of a necrotic focus. A supplementary administration of echinochrom 5 min after the onset of ischemia failed to contribute to a significant enhancement of its protective effect, suggesting that there is no substantial effect of the agent on ischemic lesion. The efficacy of echinochrom given after prolonged ischemia, low effective doses, and no adverse effects create prerequisites for using the drug in the clinical setting.     

The effects of intravenous infusions of selective adenosine A1-receptor and A2-receptor agonists on myocardial reperfusion injury.

To determine the efficacy of very low doses of adenosine on myocardial reperfusion injury and whether its effect is receptor mediated, 78 rabbits underwent 30 minutes of left circumflex artery occlusion and 48 hours of reperfusion. Animals were randomly assigned to receive one of three doses of adenosine, cyclopentyladenosine (a selective A1-receptor agonist), or CGS 21680C (a selective A2-receptor agonist). The drugs were infused for 65 minutes beginning 5 minutes before reperfusion. A significant reduction in histologically determined infarct size was noted with all three doses of adenosine, intermediate and low doses of the A1-receptor agonist (cyclopentyladenosine), and high and intermediate doses of the A2-receptor agonist (CGS 21680C). Furthermore, all three adenosine receptor agonists afforded similar degrees of protection. Results of this study demonstrate that intravenous infusions of very low doses of adenosine significantly enhance myocardial salvage and this protection is receptor mediated. Furthermore, the administration of the A1-receptor agonist would be clinically appealing, since it would avoid the potential side effects associated with activation of A2 receptors.