Biochemical and morphological correlates of acute experimental myocardial ischemia in the dog. IV. Energy mechanisms during very early ischemia

In the eye of the storm: mitochondrial damage during heart and brain ischaemia

We review research investigating mitochondrial damage during heart and brain ischaemia, focusing on the mechanisms and consequences of ischaemia-induced and/or reperfusion-induced: (a) inhibition of mitochondrial respiratory complex I; (b) release of cytochrome c from mitochondria; (c) changes to mitochondrial phospholipids; and (d) nitric oxide inhibition of mitochondria. Heart ischaemia causes inhibition of cytochrome oxidase and complex I, release of cytochrome c, and induction of permeability transition and hydrolysis and oxidation of mitochondrial phospholipids, but some of the mechanisms are unclear. Brain ischaemia causes inhibition of complexes I and IV, but other effects are less clear.

Energy metabolism in the reversible and irreversible phases of severe myocardial ischemia

In summary, myocardial ischemia is associated with the progressive depletion of HEP and the adenine nucleotide pool. Anaerobic glycolysis is essential for energy production in the severely ischemic myocyte and accounts for 80% of the HEP utilized by severely or totally ischemic myocardium. However, the rate of anaerobic glycolysis is too slow to prevent the progressive depletion of ATP. Anaerobic glycolysis stops entirely prior to the complete utilization of glycogen. Without remaining HEP stores or HEP production from anaerobic glycolysis, HEP utilization no longer can occur. This point occurs in vivo after about 40 minutes of severe ischemia and coincides with the onset of cell death. Modest depletion of ATP due to brief periods of transient ischemia may not cause cell death, but is associated with partial depletion of the adenine nucleotide pool. The slow repletion of this pool may be responsible for prolonged depression of contractile function.

A novel feature-tracking echocardiographic method for the quantitation of regional myocardial function: validation in an animal model of ischemia-reperfusion

OBJECTIVES

The aim of this study was to validate a novel, angle-independent, feature-tracking method for the echocardiographic quantitation of regional function.

BACKGROUND

A new echocardiographic method, Velocity Vector Imaging (VVI) (syngo Velocity Vector Imaging technology, Siemens Medical Solutions, Ultrasound Division, Mountain View, California), has been introduced, based on feature tracking-incorporating speckle and endocardial border tracking, that allows the quantitation of endocardial strain, strain rate (SR), and velocity.

METHODS

Seven dogs were studied during baseline, and various interventions causing alterations in regional function: dobutamine, 5-min coronary occlusion with reperfusion up to 1 h, followed by dobutamine and esmolol infusions. Echocardiographic images were acquired from short- and long-axis views of the left ventricle. Segment-length sonomicrometry crystals were used as the reference method.

RESULTS

Changes in systolic strain in ischemic segments were tracked well with VVI during the different states of regional function. There was a good correlation between circumferential and longitudinal systolic strain by VVI and sonomicrometry (r = 0.88 and r = 0.83, respectively, p < 0.001). Strain measurements in the nonischemic basal segments also demonstrated a significant correlation between the 2 methods (r = 0.65, p < 0.001). Similarly, a significant relation was observed for circumferential and longitudinal SR between the 2 methods (r = 0.94, p < 0.001 and r = 0.90, p < 0.001, respectively). The endocardial velocity relation to changes in strain by sonomicrometry was weaker owing to significant cardiac translation.

CONCLUSIONS

Velocity Vector Imaging, a new feature-tracking method, can accurately assess regional myocardial function at the endocardial level and is a promising clinical tool for the simultaneous quantification of regional and global myocardial function.

Gadolinium delayed enhancement cardiovascular magnetic resonance correlates with clinical measures of myocardial infarction

OBJECTIVES

The current study tested the hypothesis that gadolinium delayed enhancement assessment of infarct size correlates with clinical indices of myocardial infarction (MI) in humans. Acute infarct mass by cardiac magnetic resonance (CMR) was compared with peak troponin I, acute and chronic left ventricular (LV) systolic function, and chronic infarct mass in patients imaged after recent acute MI.

BACKGROUND

Cardiac magnetic resonance accurately determines myocardial viability in patients with chronic ischemic heart disease but is not well validated for recent MI.

METHODS

Patients with first acute MI (n = 33) or chronic MI (n = 10) underwent cine CMR followed by gadolinium delayed enhancement imaging. A follow-up CMR scan was performed on 20 of the 33 acute MI patients and all of the chronic MI patients.

RESULTS

In patients with acute percutaneous coronary intervention, acute MI mass correlated with peak troponin I (r = 0.83, p < 0.001, n = 23). In the 20 acute infarct patients with follow-up CMR scans, the acute infarct size correlated well with the follow-up LV ejection fraction (r = 0.86, p < 0.001). The transmural extent of delayed enhancement imaged acutely correlated inversely with wall thickening measured acutely (p < 0.001) and at follow-up (p < 0.001). Although chronic infarct size was reproducible (11 +/- 4% vs. 12 +/- 7%, p = NS), acute infarct size decreased from 16 +/- 12% to 11 +/- 9% (p < 0.003).

CONCLUSION

In humans imaged shortly after acute MI, gadolinium delayed enhancement acute CMR infarct size correlates with acute and chronic indices of infarct size but will appear to diminish in size on follow-up.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Cardioprotective effects of YM934, an ATP-sensitive potassium channel opener, on stunned myocardium in anesthetized dogs

The effects of YM934 [2-(3,4-dihydro-2,2-dimethyl-6-nitro-2H-1,4-benzoxazin-4-yl) pyridine N-oxide], an adenosine triphosphate (ATP)-sensitive potassium channel opener, on stunned myocardium were examined. Forty eight anesthetized dogs were subjected to 15 min of left anterior descending (LAD) coronary artery occlusion followed by 3 hours of reperfusion. To elucidate the possible contribution of the cardioprotective property of YM934 to stunned myocardium, a nonhypotensive dose of YM934 was directly injected into the LAD coronary artery before the ischemic insults. Intracoronary artery infusion (i.c.) of YM934 (0.1 microg/kg/min) produced a marked improvement in post-ischemic regional contractile dysfunction. The effects were not associated with improvement of hemodynamics, including regional myocardial blood flow during ischemia, heart rate and mean arterial blood pressure. The anatomic areas at risk expressed as a percentage of the left ventricle and regional myocardial blood flow were not significantly different between groups. The cardioprotective effect of YM934 was completely blocked by pretreatment with an ATP-sensitive potassium channel blocker, glibenclamide (1.0 mg/kg i.v. bolus). These results suggest that YM934 exerts cardioprotective effect on stunned myocardium through opening myocardial ATP-sensitive potassium channels.

Myocardial stunning: an overview

Over the past two decades, we have challenged the belief that transient ischemia is benign with little functional sequelae following resolution of ischemia. The phenomenon of prolonged postischemic contractile dysfunction, or of myocardial stunning, has been developed and is under investigation using multiple experimental and clinical models. Classifications of myocardial stunning have been suggested and include single and multiple reversible ischemic episodes, partially reversible episodes, and global ischemia. More challenging is the understanding of the mechanisms of myocardial stunning, including free radical protection, excitation-contraction uncoupling, altered calcium flux, microvascular dysfunction, and impaired energy production and use. Finally, advances have been made in the clinical arena, including development of new more sensitive technologies to detect dysfunction, and development of potentially important therapies, including free radical scavengers, adenosine-regulating agents, and calcium channel blockers. In this brief overview, we focus on myocardial stunning, including a historical perspective of coronary occlusion, and definition, classification, and clinical implications of myocardial stunning.

Visualization of altered myocardial lipids by 1H NMR chemical-shift imaging following ischemic insult

Acute myocardial infarction is associated with an accumulation of lipids. Spectroscopic and chemical-shift imaging strategies which can depict the spatial distribution of these chemical species are evolving. The present study was undertaken to test whether the Dixon method could detect spatially lipids known to accumulate in myocardium after an ischemic insult. Seven dogs underwent a 24-h coronary artery occlusion (LAD = 4, Cx = 3). Post mortem, hearts were removed and imaged ex vivo. Myocardial samples were also evaluated by high-resolution 1H NMR spectroscopy. Lipid images revealed regions of increased signal intensity, in the regions corresponding to the myocardial infarction, particularly in the periphery of the infarction. An increase in mobile lipids was observed by 1H NMR spectroscopy of myocardial samples with moderately reduced blood flow and corresponding to regions with increased signal intensity on the lipid image. This study shows that chemical-shift imaging may be useful for detecting alterations in myocardial lipid levels following an ischemic insult.

Myocardial stunning in dogs: preconditioning effect and influence of coronary collateral flow

In open-chest dogs the left anterior descending (LAD) coronary artery diagonal branch was encircled with a pneumatic occluder. Pairs of ultrasonic crystals were inserted into LAD myocardium and remote normal muscle. The coronary artery was occluded for 5 minutes, followed by 10 minutes of reperfusion. This occlusion-reperfusion cycle was repeated 12 times, and after a final 90-minute reperfusion period the hearts were removed and stained with triphenyltetrazolium chloride. No heart had evidence of necrosis. Baseline shortening normalized for end-diastolic length averaged 10.4 +/- 1.0% in the LAD area and 7.4 +/- 0.8% in the remote normal myocardium. When analyzed as a percentage of baseline, segment shortening in the normal myocardium was not significantly altered by LAD occlusion and reperfusion. In contrast, during occlusions the LAD myocardium paradoxically lengthened. With the initial reperfusion, shortening was significantly depressed to 28.6 +/- 8.6% of baseline. Although with subsequent reperfusions the return of function progressively decreased, the rate of deterioration was markedly attenuated after the first occlusion. By the end of the protocol many LAD segments lengthened paradoxically even after reperfusion, but in five hearts in which active contraction was preserved there was no significant change in regional function after the third cycle, suggesting a protective or preconditioning effect of earlier ischemia. There was a moderately good correlation between collateral blood flow and the degree of dysfunction following the initial 10-minute reperfusion (r = -0.73). This correlation deteriorated during subsequent reperfusion periods, implying that collateral blood flow can be a predictor of the extent of myocardial stunning only after the initial one or two reperfusion cycles. Thereafter other as yet unidentified factors make baseline collateral flow unimportant.

Hydrogen peroxide generation by mitochondria isolated from regionally ischemic and nonischemic dog myocardium

We occluded the left anterior descending coronary artery of anesthetized, open-chest dogs, for 1 or 2 h. Some hearts were reperfused for 1 h after 1 h of ischemia. We isolated mitochondria from the central ischemic zone (CIZ) and a surrounding nonischemic zone (NIZ) of the left ventricle, and assayed H2O2 production using a horseradish peroxidase-dual wavelength spectrophotometric technique. Mitochondria, studied in the absence of exogenous respiratory chain inhibitors, generated H2O2 during State 4 respiration with succinate as the substrate. NIZ mitochondria in all groups produced ca. 1.5 nmols H2O2/min/mg protein (no significant differences between groups). The State 4 O2 consumption rates of NIZ mitochondria from hearts subjected to 1 h ischemia plus reperfusion, or 2 h of ischemia (ca. 30 nmols/min/mg) were significantly higher than that of NIZ mitochondria of hearts subjected to only 1 h of ischemia (23 nmols/min/mg). Thus, the ratio between H2O2 produced and State 4 O2 consumption fell from 6.5% to 5%. Mitochondria from all CIZ samples had State 4 O2 consumption rates that were not different from corresponding NIZ values. However CIZ mitochondria of hearts subjected to 1 h ischemia without reperfusion produced less H2O2 (1.1 +/- 0.1 nmols/min/mg), and had a slightly reduced H2O2/O2 ratio (4.4 +/- 0.7%), compared with their NIZ samples (1.5 +/- 0.1 nmols/min/mg; 5.3%). Reperfusion after 1 h of ischemia abolished these regional differences. The CIZ mitochondria from hearts subjected to 2 h ischemia produced only 0.75 +/- 0.22 nmols H2O2/min/mg (2.5% of State 4 O2 consumption). These values were 50% of corresponding NIZ values, and were significantly less than for any other group or tissue region. If similar phenomena occur in conscious animals subjected to incomplete regional ischemia, especially of relatively brief duration or if accompanied by reduced intracellular defenses against oxidants such as H2O2, they suggest that mitochondria persist as H2O2 sources and so may contribute to the oxidant load and myocardial dysfunction.

Myocardial protection by perfluorochemical infusion during transient ischemia produced by balloon coronary occlusion

To assess the efficacy of the perfluorochemical Fluosol-DA 20% for myocardial protection during repeated periods of balloon occlusion of the left circumflex coronary artery, 25 anesthetized dogs were randomized to receive either oxygenated perfluorochemical, (Fluosol-DA [F]; n = 10) or oxygenated Ringer's lactate (R; n = 6), at the rate of 30 ml/min, during inflation of the balloon. A control group (C; n = 9) received no infusion. A total of eight inflations were performed, each lasting 90 seconds, followed by an equivalent deflation time. Hemodynamics, ECGs, regional myocardial function, and biochemical parameters were studied. Significant differences were noted in ST segment elevation at 90 seconds of inflation in the F (1.5 mm +/- 0.6), C (3.7 mm +/- 0.75), and R (2.9 mm +/- 0.75) groups (F vs C or R p less than 0.05). This was associated with significant improvement in radial shortening in the jeopardized zone at 45 seconds into occlusion in the F group compared to the C and R groups (F = 21.1% +/- 5.1 vs C = 3.5% +/- 4.5 or R = 1.1% +/- 3.2; p less than 0.05). Results of electron microscopy showed reversible changes of ischemia within the mitochondria, and these were most marked in the C and R groups compared to the F group. Endothelial swelling was mild and was present only focally in the R and C groups. Thus perfluorochemicals may enhance the safety and efficacy of balloon angioplasty.

Substrate effects in the post-ischemic myocardium

A study was undertaken to examine the effects of glucose versus pyruvate as the sole substrate following severe myocardial ischemia. Glycolysis usually contributes only a small amount to total ATP production and may be rate limiting in providing tricarboxylic acid (TCA) cycle substrates. Consequently, pyruvate may be a more effective substrate by bypassing glycolysis to feed directly to the TCA cycle and oxidative phosphorylation. Isolated rat hearts were studied in a retrograde (Langendorff) perfusion apparatus while in an NMR spectrometer. Rate pressure product (RPP), myocardial oxygen consumption (MVO2), and the unidirectional Pi----ATP rate were measured in control and postischemic hearts with or without the inotrope dobutamine. The undirectional Pi----ATP rate was higher in the glucose than the pyruvate hearts and the difference increased further postischemia. This increase over that of the pyruvate hearts has been attributed to a glycolytic component of ATP metabolism. Oxygen consumption was higher in pyruvate hearts at equivalent levels of performance. It thus appears that the glycolysis rate is significant and may be elevated following severe myocardial ischemia. Perfusion with pyruvate requires increased rates of oxidative phosphorylation to make up for the loss of glycolytically produced ATP. Optimal postischemic substrate delivery may require several compounds, one of which should be glucose.

Sequential thallium-201 myocardial perfusion studies after successful percutaneous transluminal coronary artery angioplasty: delayed resolution of exercise-induced scintigraphic abnormalities

To characterize the sequential changes of myocardial perfusion scintigraphy in patients with coronary artery disease (CAD) after complete revascularization, 43 patients underwent exercise thallium-201 (201Tl) myocardial perfusion scintigraphy before and at 9 +/- 5 days, 3.3 +/- 0.6, and 6.8 +/- 1.2 months after percutaneous transluminal coronary angioplasty (PTCA). Only patients with single-vessel CAD, without previous myocardial infarction, and without evidence of restenosis at 6 to 9 months after PTCA were included. Perfusion scans were analyzed blindly with the use of a new quantitative method to define regional myocardial perfusion in the topographic distribution of each coronary artery, which was shown to be reproducible (r = .94 or higher and SEE of 7% or less, between repeated measures by one and two operators). At 4 to 18 days after PTCA, the mean treadmill walking time increased by 123 +/- 42 sec, mean exercise-induced ST segment depression decreased by 0.6 +/- 0.3 mm, group maximal heart rate increased by 20 +/- 9 beats/min, and group systolic blood pressure at peak exercise increased by 24 +/- 10 mm Hg, compared with pre-PTCA values (p less than .001). However, no group differences were noted in these variables between the three post-PTCA stages. Myocardial perfusion in the distribution of the affected (dilated) coronary artery, on the other hand, improved progressively. In the 45 degree left anterior oblique view for instance, myocardial perfusion increased at 9 days after PTCA (from 68 +/- 24% before PTCA to 91 +/- 9%, p less than .001) and at 3.3 months after PTCA (101 +/- 8%, p less than .05 vs 9 days after PTCA), but no further significant changes were seen at 6.8 months after PTCA (102 +/- 8%). Similar changes were noted in the other two views. No relationship between minor complications during PTCA and delayed improvement on the 201Tl was observed. Myocardial ischemia was diagnosed in 12 of the 43 scans recorded a few days after PTCA, but in none recorded at later stages. We conclude that 201Tl scans after PTCA often show delayed improvement and therefore, an abnormal myocardial perfusion scan soon after PTCA does not necessarily reflect residual coronary stenosis or recurrence.

Protection by verapamil of mitochondrial glutathione equilibrium and phospholipid changes during reperfusion of ischemic canine myocardium

Pretreatment of the ischemic myocardium with verapamil protects against mitochondrial respiratory depression observed during ischemic arrest as well as during reperfusion. Since ischemic mitochondrial function appears not to be altered further by reperfusion, the purpose of this study is to identify a biochemical event affecting mitochondria that is specifically associated with reperfusion injury. It has been proposed that increased cellular Ca2+ influx and oxygen toxicity may result from reintroduction of coronary flow. Increased cytosolic Ca2+ is transmitted to the mitochondria with subsequent activation of Ca2+-dependent events, including phospholipase A2. Net production of lysophospholipids (and loss of total diacylphospholipids from the mitochondria) will proceed when reacylation mechanisms are inhibited. Since acyl-CoA:lysophospholipid acyltransferase is a sulfhydryl-sensitive enzyme and since increased activity of glutathione peroxidase shifts the levels of the mitochondrial sulfhydryl buffer, glutathione, towards oxidation, levels of glutathione and its oxidation state were measured during reperfusion in the absence or presence of verapamil pretreatment. Ischemia lowers total glutathione and reduces the redox ratio (reduced glutathione: oxidized glutathione) by 85%. Reperfusion partially returns the redox ratio to control by causing oxidized glutathione to disappear from the matrix. Verapamil maintains both the concentration and the redox potential of glutathione at control levels. Concomitant with alterations in reduced glutathione:oxidized glutathione is a decrease in ischemic mitochondrial phospholipid content. During reperfusion, phosphatidylethanolamine and its major constituent fatty acids (C 18:0 and C 20:4) are specifically lost from the mitochondrial membrane. Accompanying the significant loss of arachidonic acid during reperfusion is the decreased content of 11-OH, 12-OH, and 15-OH arachidonate. These lipid peroxidation products are not increased in ischemia. It is proposed that oxidation of matrix glutathione to glutathione disulfide during ischemia results in formation of glutathione-protein mixed disulfides and inhibition of sulfhydryl-sensitive proteins, including acyl-CoA lysophosphatide acyltransferase. Thus, metabolic events occurring within the ischemic period set the stage for prolonged dysfunction during reperfusion.

Disruption of myofibrillar energy use: dual mechanisms that may contribute to postischemic dysfunction in stunned myocardium

The abnormalities in regional function produced by myocardial ischemia persist after the ischemic episode resolves. Since a close functional coupling exists between myofibrillar creatine kinase and myosin ATPase, a disruption of this coupling could adversely influence myocardial function and might provide a mechanism for the myocardial dysfunction observed. The purpose of the present study was to determine if an alteration in the activity of creatine kinase associated with the myofibril occurs in the postischemic period. Anesthetized open-chest dogs (n = 6) underwent coronary occlusion for 15 minutes, followed by reperfusion for 15 minutes. In reperfused myocardium, adenine nucleotide content was decreased (72 +/- 10% of nonischemic myocardium, p less than 0.05), documenting the presence of previous ischemia. The creatine phosphate content of reperfused myocardium returned to normal, indicating resumption of myocardial energy production. The creatine kinase activity of purified myofibrils isolated from reperfused myocardium was decreased by 17 +/- 7% compared to that of nonischemic myofibrils (p less than 0.03). In addition, the free adenosine diphosphate concentration in reperfused myocardium was calculated to be 96 microM and was less than the Km of adenosine diphosphate determined for myofibrillar creatine kinase (105 microM). The results suggest two putative mechanisms for disruption of energy use in postischemic myocardium: decreased creatine kinase activity associated with the myofibril, and limitation of substrate necessary for maximal creatine kinase activity.

Time course of metabolic findings in coronary occlusion and reperfusion and their role for assessing myocardial salvage

The techniques currently used to assess myocardial infarction are limited in their ability to determine the amount of viable myocardium after a temporary ischemic event. Blood flow and segmental function may not necessarily demonstrate salvage, whereas metabolic parameters will determine cell survival. In an open chest dog model, short occlusion times of 20 min and subsequent reperfusion using C-11 palmitate as an index of fatty acid metabolism showed depression of fatty acid oxidation, which recovered after 3 hours of reperfusion, indicating the partial reversibility of the ischemic condition. In more extensive studies, using positron emission tomography (PET) and, as an indicator of glucose metabolism, fluoro-F-18-deoxyglucose (FDG); N-13 ammonia in addition to C-11 palmitate for the determination of blood flow; and ultrasonic crystals to measure shortening in the reperfused and control territories, the duration of occlusion was 3 h. Metabolic studies were repeated 24 h, 1 week, and 4 weeks after the ischemic injury. Reperfused viable myocardium exhibited residual glucose metabolism with FDG, whereas fatty acid oxidation remained impaired for a longer period. Gradual metabolic recovery during a 4-week period was associated with the prolonged recovery of regional function, whereas a lack of residual metabolic activity indicated that little change in function was likely to occur. Increased FDG uptake and impaired C-11 palmitate turnover are characteristic of reversibly injured tissue. Therefore, PET studies may offer a unique potential for the evaluation of therapeutic measures such as thrombolysis and early revascularization.

Effect of serial brief ischemic episodes on extracellular K+, pH, and activation in the pig

This study was performed to determine the reproducibility of the ionic and electrical changes associated with serial ischemic episodes. We used ion-selective and bipolar plunge electrodes to determine the changes in left ventricular extracellular potassium ([K+]e), extracellular pH (pHe), and local activation during sequential 10 min occlusions of the left anterior descending coronary artery separated by 50 min of reperfusion in open-chest anesthetized pigs. We found that uniformly during the initial occlusion, and in approximately 50% of animals during the second occlusion, [K+]e rose more rapidly but to a lower level than in subsequent occlusions. By the third occlusion the changes in [K+]e were reproducible. Extracellular acidosis was greatest in the first occlusion and decreased progressively with each subsequent occlusion. Local activation was characterized by a decrease in spontaneous improvement and increase in block with each successive occlusion. The occurrence of ventricular fibrillation could not be directly attributed to the magnitude of the change in [K+]e or pHe. Moreover, the occurrence of ventricular fibrillation in one occlusion did not necessarily predict its occurrence thereafter. Our results indicate that serial episodes of ischemia are associated with different but predictable changes in ionic and electrical events that may be clinically relevant and that must be appreciated before the results from similar protocols with serial ischemic episodes can be interpreted meaningfully.

Effects of repeated brief coronary occlusion on regional left ventricular function and dimension in dogs

The cumulative effects of repeated, brief episodes of regional ischemia on myocardial function and dimension were examined in 14 open-chest dogs. The left anterior descending coronary artery was occluded for 5 minutes, followed by 10 minutes of reflow, repeated 16 times, and then 1 hour recovery. Systolic function decreased progressively in segments made repetitively ischemic and remained depressed even after 1 hour of recovery. Average systolic shortening decreased 20% from baseline after recovery from the first occlusion, 82% after the 8th, 91% after the 16th, and 104% after the 1 hour recovery (p less than 0.015, analysis of variance). End-diastolic segment length progressively increased in regions made repetitively ischemic, lengthening 4% after the first occlusion, 10% after the third occlusion, 19% after the sixteenth occlusion, and 16% after 1 hour of recovery (p less than 0.02). Nonischemic end-diastolic segment length also showed a smaller but parallel increase, while non-ischemic systolic function showed compensatory improvement. After the dogs were killed, myocardial staining with triphenyl tetrazolium chloride revealed no necrosis. Electron microscopy, performed in 5 dogs, showed scattered mitochondrial swelling in both postischemic and nonischemic regions, but no evidence of irreversible injury. The ratio of myocardial blood flow in the region made repetitively ischemic to nonischemic flow, as measured with microspheres, was 1.00 +/- 0.02 before the occlusions and 0.90 +/- 0.03 just before death (difference not significant). Thus, in the dog progressively abnormal regional systolic function and regional and global diastolic dilatation can be produced by repetitive, brief, coronary occlusions, which are not associated with histochemical or ultrastructural evidence of myocardial necrosis.

Reperfusion injury induced by augumented oxygen uptake in the initial reperfusion period. Possible efficacy of extreme hemodilution

This study was designed to investigate the relationship between myocardial oxygen consumption and oxygen-induced myocardial injury. Dog hearts were exposed to 40 min normothermic ischemia and then reperfused for 10 min with three oxygenated perfusates containing different hemoglobin concentrations and with moderate hemodilution. The experimental groups consist of a moderate hemodilution group (Group M) receiving 8 g/dl of hemoglobin, an extreme hemodilution group (Group E) given 4 g/dl of hemoglobin, and a hemoglobin-free autologous plasma group (Group F). Hearts in the non-ischemic control group (Group C) were also perfused with moderate hemodilution throughout. In Group C, Group M and Group E, the O2 tension of perfusates was maintained at about 100 mmHg and in Group F, at over 300 mmHg. Oxygen extraction at 5 min after reperfusion in Group C was 14%, in Group M and Group E less than 10%, but in Group F 70%. Oxygen consumption in Group F was more than twice that in Group E and surpassed that in Group C. Group M also showed a significant increase in oxygen consumption compared with Group E at 5 min after reperfusion. Among the ischemic groups, Group E showed improvement of left ventricular function almost comparable to Group C accompanied by a rapid decrease in myocardial lactate, improved preservation of myocardial adenine nucleotides and prevention of myocardial lipid peroxidation. In contrast, Group F showed persistently higher values of lipid peroxides and lactate, the poorest recovery of adenine nucleotides, and impairment of left ventricular function.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased regional end-diastolic wall thickness early after reperfusion: a sign of irreversibly damaged myocardium

Two-dimensional echocardiographic measurements of regional left ventricular end-diastolic wall thickness and systolic wall thickening were studied during coronary artery occlusion and early after reperfusion and compared with measurements of regional myocardial infarct size. In 25 closed chest anesthetized dogs with left anterior descending coronary artery occlusion followed by reperfusion, the occlusion period was 3 minutes in group I (n = 4), 20 minutes in group II (n = 4), 60 minutes in group III (n = 5) and 180 minutes in group IV (n = 12). Infarct size in groups III and IV was quantitated using the triphenyltetrazolium chloride technique. After coronary occlusion, wall thickening was replaced by thinning in the center of the ischemic region at the midpapillary echographic short-axis section, and no improvement in function occurred up to 60 minutes after reperfusion, except in group I. Ischemic zone end-diastolic wall thickness did not change significantly from control to the end of the coronary occlusion period, except Group IV. At 60 minutes after reperfusion, end-diastolic wall thickness increased only slightly in groups I and II (by 7.2 and 0.24%, respectively), but a marked increase was observed in groups III and IV (by 41 and 50%, respectively). The percent change in ischemic zone end-diastolic wall thickness from before reperfusion to 60 minutes after reperfusion correlated well with the amount of myocardial necrosis in corresponding segments (r = 0.936, standard error of estimate = 11.4%); an increase in segmental end-diastolic wall thickness of more than 25% was generally associated with 20% or more segmental necrosis. It is concluded that significantly increased regional end-diastolic wall thickness early after reperfusion is associated with irreversibly damaged myocardium, and this might be used as an index of myocardial salvage.

Relationship between regional myocardial blood flow and mitochondrial function

The purpose of this study was to clarify the relationship between myocardial mitochondrial dysfunction and the degree plus duration of restricted coronary blood flow. 135 anesthetized and open-chest dogs were divided into 3 groups according to coronary occlusion time: 10, 20, and 60 min. Regional myocardial blood flow (MBF) was determined in both ischemic and nonischemic areas before and during coronary occlusion using the hydrogen gas clearance method. Myocardial mitochondria were prepared from each area in which MBF was determined after 10, 20, or 60 min of coronary ligation, and their respiratory control index (RCI), ADP/O, and rate of oxygen consumption in state III O2 (St. III O2) were measured. The MBF measured in 135 dogs before coronary ligation was 103 +/- 25 ml/min/100 g (mean +/- SD) for the area to be rendered ischemic and 101 +/- 24 ml/min/100 g for the control area. The MBF in the ischemic area did not cease completely following coronary ligation, and the distribution of MBF showed variations which seemed attributable to individual differences. In the 10-min group, no index of mitochondrial function of the ischemic area differed from that of the nonischemic area at any level of MBF. When MBF was less than 20 ml/min/100 g, RCI of mitochondria from the ischemic area was significantly lower than that from the nonischemic area, in the 20- and 60-min groups. When MBF was less than 20 ml/min/100 g, St. III O2 of mitochondria from the ischemic area significantly decreased compared with that from the nonischemic area, in the 20-min group. In the 60-min group, MBF less than 30 ml/min/100 g, St. III O2 of mitochondria from the ischemic area was likewise significantly decreased. Moreover, with MBF below 20 ml/min/100 g, both RCI and St. III O2 of mitochondria from the ischemic area were significantly lower in the 60-min group than in the 20-min group. These results indicate that ischemia-induced mitochondrial dysfunction depends on the degree of decrease in the blood flow of the area involved as well as on the duration of ischemia, and the blood flow that is critical for survival, based on mitochondrial function, is approximately 20 ml/min/100 g, i.e., a reduction to 20% of normal value.

Normothermic ischemic cardiac arrest of the isolated working rat heart. Effects of time and reperfusion on myocardial ultrastructure, mitochondrial oxidative function, and mechanical recovery

The ischemic state of the myocardium of the isolated working rat heart after induction of normothermic ischemic cardiac arrest was assessed by the interrelationship among changes in myocardial ultrastructure, mitochondrial oxidative phosphorylation, and tissue high energy phosphate contents. At all time intervals (10-40 minutes) studied, the ultrastructural changes were more severe in the subendocardium than in the subepicardium. After 25-40 minutes of normothermic ischemic cardiac arrest, the mitochondrial oxygen uptake (state 3) became increasingly depressed, particularly in mitochondria isolated from the subendocardium. Mitochondrial oxidative function, as measured in vitro, did not correlate well with mitochondrial ultrastructural damage. In addition, the effects of coronary reperfusion on the ability of the ischemic heart to recover in terms of ultrastructure, mechanical, and metabolic function were evaluated. Hearts subjected to 10-40 minutes of normothermic ischemic cardiac arrest showed almost complete ultrastructural recovery of the subepicardium upon reperfusion; regression of ultrastructural changes occurred to a lesser extent in the subendocardium. Reperfusion for 30 minutes did not alleviate the depression in mitochondrial oxidative function, while tissue ATP levels did not return to control, preischemic levels. After 20 minutes of normothermic ischemic cardiac arrest, the mechanical performance of the working heart during reperfusion was significantly depressed, compared with pre-ischemic control values. Normal ultrastructure of the subendocardium always accompanied mechanical recovery, while improvement of mitochondrial oxidative function was not essential.

Pathobiology of acute myocardial ischemia: metabolic, functional and ultrastructural studies

Acute myocardial ischemia induced by coronary occlusion in dogs is most severe in the subendocardial region, whereas more collateral blood flow is often present in the subepicardial region. Initially, all ischemic myocytes are reversibly injured, but beginning at 15 to 20 minutes after the onset, and continuing for 3 to 6 hours, there is a wave front of cell death from the subendocardial region to the less ischemic subepicardial region, such that by 6 hours, the final transmural extent of the infarct is established. Thus, ischemic myocardium cannot be salvaged by reperfusion after greater than or equal to 6 hours of coronary occlusion in open-chest anesthetized dogs. In the severely ischemic subendocardial region, most of the creatine phosphate is lost within the first 3 minutes of ischemia in vivo, and adenosine triphosphate (ATP) is depleted to 35% of control by 15 minutes (when cellular injury is still reversible), and to less than 10% of control at 40 minutes (when injury is irreversible). Tissue ATP content and other indexes of subcellular damage have also been compared after different periods of ischemia using a model of total myocardial ischemia in vitro. As long as the ATP of the tissue was not depleted below 5 mumols/g dry weight, incubated slices of injured myocardium resynthesized high-energy phosphates and excluded inulin. However, lower tissue ATP was associated with depressed high-energy phosphate resynthesis and failure of cell volume regulation. Overt membrane damage, as measured by an increased inulin-diffusible space, was detected only after the tissue ATP decreased to less than 2.0 mumols/g of dry weight. Thus, marked ATP depletion is associated with the onset of structural and functional indexes of irreversible injury. However, whether irreversibility is caused by the marked ATP depletion or by other concomitant metabolic consequences of ischemia is not known. Myocardial ischemic cellular injury is reversible despite depletion of 70% of the control ATP. Nevertheless, when myocyte injury is reversible, there is slow repletion of adenine nucleotides. This slow metabolic recovery may explain the delayed recovery of contractile function observed after reperfusion of ischemic myocardium.

A new model for testing therapeutic interventions during myocardial ischemia

An open-chest dog heart model is presented which shows highly reproducible hemodynamic, electrophysiologic, and metabolic changes during and after short-term coronary occlusions. We repeatedly performed 3-min coronary ligations of the proximal ramus interventricularis anterior followed by reperfusion periods of 45 min. Thus, hemodynamic, metabolic, and electrophysiologic parameters of myocardial damage were fully reversible. The experimental model presented seems well suited to evaluate interventions which aim at influencing the extent and damage of myocardial ischemia without needing a large number of separate experiments.

Evaluation of high-energy phosphate metabolism during cardioplegic arrest and reperfusion: a phosphorus-31 nuclear magnetic resonance study

Hypothermic potassium cardioplegia is now commonly used to protect the myocardium during surgically induced ischemia. Because the potassium-related membrane depolarization has been shown to increase calcium influx, we undertook this study to define the effects of varying the calcium content in hyperkalemic perfusates and the effects of using magnesium instead of or in addition to potassium as the arresting agent on the ability of hearts to recover normal function after ischemic arrest. We subjected isolated perfused working rat hearts to 60 minutes of cardioplegic arrest followed by 30 minutes of reperfusion, and measured high-energy phosphate levels every 2 1/2 minutes by phosphorus-31 nuclear magnetic resonance spectroscopy. These data were correlated with postischemic recovery of function. Our results show that potassium cardioplegia may be harmful when the calcium concentration is greater than 1 mM. The kalemic injury is significantly reduced when the calcium content is lowered to 0.25 mM and the greatest extent of preservation is provided by a calcium-poor perfusate (0.25 mM) containing 13 mM magnesium. The beneficial effects of magnesium are not enhanced by subsequent addition of potassium. Close correlations were found between all observed metabolic changes during arrest and the degree of recovery of contractile performance after reperfusion. We conclude that the ability of the myocardium to maintain or resynthesize high-energy phosphate after cardioplegic arrest may be an important determinant of postischemic mechanical performance. These results show that phosphorus-31 nuclear magnetic resonance spectroscopy is a valuable method for evaluating interventions to reduce the severity of ischemic damage.

Myocardial protection during surgical coronary reperfusion

Reperfusion injury in the surgical setting is defined as those metabolic, functional and structural consequences of restoring coronary flow (that is, aortic unclamping and revascularization) that can be avoided or reversed by modification of the conditions of reperfusion by the operating surgeon. The potential for reperfusion damage exists during cardiac surgery because temporary myocardial ischemia (that is, aortic clamping) is needed to produce a quiet, bloodless surgical field. Cold cardioplegic techniques have decreased the risks of ischemic myocardial damage during aortic clamping, but reperfusion damage can still occur when there is poor cardioplegic distribution (that is, coronary artery disease) or in hearts that have suffered ischemic damage before extracorporeal circulation is started (such as extending myocardial infarction, cardiogenic shock and the like). The surgical setting affords the ideal opportunity for reperfusate modification because the components and conditions of the reperfusate are in the surgeon's control. This study reviews present understanding of the nature of reperfusion damage in the surgical setting and summarizes studies over the past 6 years which suggest that much of reperfusion damage can be avoided or reversed by adjusting the temperature, pressure and composition of reperfusate blood.

Reperfusion of ischemic myocardium: ultrastructural and histochemical aspects

The effects of reperfusion on ischemic myocardium generally depend on the severity of the preceding ischemic injury. Reperfusion of myocardium, irreversibly injured by ischemia, produces further progression of myocardial necrosis that is accompanied by simultaneously occurring stimulation of interstitial cell proliferation resulting in scar formation. Reperfusion of reversibly injured myocardium leads to structural improvement and reorganization. Thus, it may be stated from the ultrastructural part of this study that reperfusion of ischemic myocardium induces 1) slow structural recuperation after reversible injury, and 2) accelerated cellular destruction and symptoms of scar formation after irreversible ischemic injury. We observed that the reduced tissue content of nicotinamide adenine dinucleotide (NAD), rather than reduced dehydrogenase activity, is the basis of histochemical reactions employing tetrazolium salts. Directly measured NAD tissue content in ischemic tissue correlated well with the degree of ultrastructural injury and with macroscopic differential staining. Occlusion of two small coronary arteries in the same heart followed by reperfusion of only one artery (identical occlusion times for both arteries) showed identical infarct sizes for reperfused and nonreperfused myocardium for occlusion times of 3 and 6 hours. When the effects of occlusion times of less than 3 hours are studied with tetrazolium salts, a difficult technical problem arises: during that time, tissue-NAD concentrations have not decreased enough to enable differential staining. Reperfusion leads to washout of NAD, thus producing differential staining; this may be a harmful effect of reperfusion. However, because early reperfusion leads to significant structural and functional recovery and to small infarcts, reperfusion injury is unlikely to occur. Both ultrastructural and histochemical evidence suggest that reperfusion is beneficial for reversibly injured tissue but accelerates the decay of irreversibly injured tissue.

Impaired function of salvaged myocardium: two-dimensional echocardiographic quantification of regional wall thickening in the open-chest dog

This study was designed to examine the functional properties of myocardium subjected to acute coronary occlusion but surviving the ischemic insult. Ten conscious mongrel dogs underwent mild-circumflex coronary occlusion and were treated for 6 hours with prostacyclin, 540 ng/kg/min, and ibuprofen, 110 micrograms/kg/min, or dipyridamole (7-9.7 micrograms/kg/min). At 7 days, each dog was anesthetized, the chest was opened, and cross-sectional two-dimensional echocardiograms were obtained through the middle of the occluded vascular bed. A computer-aided contouring system was used to assess percent systolic thickening in 16 equally spaced segments around the left ventricle. Metal markers sewn to the epicardium permitted precise regional correlation of histology, percent systolic thickening, and flow, as measured by radioactive microspheres. Necrosis was minimal, averaging only 2.2 +/- 0.8% (+/- SEM) of the left ventricular ring corresponding to the echocardiographic cross section. Percent systolic thickening was 28.6 +/- 4.7 in the nonischemic anterior wall, but was reduced to -4.5 +/- 3.1 in the occluded bed (p less than 0.01). In individual echo segments, percent systolic thickening correlated with local flow (r = 0.69, p less than 0.001), but was still depressed even when flow was normal. In six segments within the occluded bed that had normal histology and flow, percent systolic thickening was 52% less than that in the nonischemic region (p less than 0.02). Thus, coronary artery occlusion combined with drug treatment results in myocardium that, although histologically normal and supplied by normal myocardial blood flow, remains functionally abnormal 7 days after occlusion.

Prolonged depletion of ATP because of delayed repletion of the adenine nucleotide pool following reversible myocardial ischemic injury in dogs

Sixty-five percent of the ATP and 50% of the total adenine nucleotide (sigma Ad) pool is lost from the subendocardial myocardium after 15 min of severe ischemia induced by circumflex artery occlusion in open-chest dogs (12). In the present experiment, we assessed the effects of various periods of arterial reflow following 15 min of ischemic injury on resynthesis of ATP and sigma Ad. The circumflex artery was occluded for 15 min and reperfused for 20 or 60 min or 24 or 96 hr. The mean ATP after 15 min of ischemia was reduced 62% from 5.42 +/- 0.33 to 2.08 +/- 0.21 mumol/g; and the total nucleotide content was reduced by 50%. ATP content recovered slightly during the first 20 min of reperfusion but remained markedly depressed for at least 24 hr because of the initial depletion of adenine nucleotides and because minimal salvage of de novo repletion occurred in the injured muscle during this time period. By 4 days, ATP and total adenine nucleotides were still slightly depressed but had recovered to 88% and 91% of control. Electrolyte changes and an increased inulin-diffusible space, which are characteristic of irreversibly injured myocardium, reperfused for 20 or 60 min, were not observed. Also, tissue necrosis was absent in the hearts reperfused for 24 or 96 hr. These observations indicate that the marked depression of ATP and adenine nucleotides and the slow recovery of these metabolites occurred in myocardium that nevertheless was reversibly injured in terms of cellular viability.

Intermittent brief periods of ischemia have a cumulative effect and may cause myocardial necrosis

We investigated the effects of brief intermittent periods of ischemia on myocardial viability. Brief periodic coronary occlusions were produced up to 18 times by inflating and deflating the balloon of an intracoronary No. 2F catheter for periods of 15, 10 or 5 minutes, followed by 15-minute periods of reperfusion. Creatine kinase (CK) release, triphenyl tetrazolium chloride staining, and light and electron microscopy were used to detect the presence of myocardial necrosis. For the study of CK release, blood was taken from the great cardiac vein and the aorta before and at 5-minute intervals during each left anterior descending coronary occlusion, as well as during and 1, 5, 10 and 15 minutes after balloon deflation. In seven of 24 dogs with 15-minute occlusions, in five of 21 dogs with 10-minute occlusions, and in three of 32 dogs with 5-minute occlusions, small but distinct areas of subendocardial necrosis were present. In all dogs with morphologic proof of necrosis, there was periodic release of CK into the great cardiac vein, which peaked immediately after reperfusion, reflecting CK washout. Thus, brief periods of ischemia, which when single do not cause necrosis, have a cumulative effect and may cause myocardial necrosis. This mechanism of necrosis may be relevant clinically in patients with frequent anginal episodes. Since many dogs of this study did not have any myocardial necrosis, the findings also suggest that intermittent reperfusion has a beneficial effect and may prevent necrosis, even when total occlusion time exceeds 200 minutes.

The stunned myocardium: prolonged, postischemic ventricular dysfunction

Myocardial ischemia has, for many decades, been viewed as an all-or-none process that causes myocardial necrosis when prolonged and severe, but whose effects are transient when it is brief or mild. In view of the evidence that the ischemic process may "hit, run and stun," perhaps our thinking about the consequences of myocardial ischemia should be expanded. According to this formulation, an ischemic insult not of sufficient severity of duration to produce myocardial necrosis may acutely affect myocardial repolarization and cause angina (hit); but these changes wane rapidly (run), when the balance between myocardial oxygen supply and demand has been reestablished. However, the ischemia may interfere with normal myocardial function, biochemical processes and ultrastructure for prolonged periods (stun). The severity and duration of these postischemic changes depend on the length and intensity of the ischemia, as well as on the condition of the myocardium at the onset of the ischemic episode. Furthermore, it is likely that when the myocardium is repeatedly stunned, it may exhibit chronic postischemic left ventricular dysfunction, an ill-defined condition. If prolonged, chronic postischemic left ventricular dysfunction can progress to myocardial scarring and ischemic cardiomyopathy, it may be important to determine how often it can be ameliorated by permanent improvement of myocardial perfusion by surgical treatment.

Ventricular performance and biochemical alteration of regional ischemic myocardium after reperfusion in the pig

Reperfusion of acutely ischemic myocardium may cause profound alterations in left ventricular wall performance and metabolism. This study evaluates regional left ventricular wall thickness, analyzes metabolic and biochemical alterations, and examines tissue hemorrhage during 15, 30, and 120 minutes of myocardial ischemia, each followed by 120 minutes of reperfusion. Reperfusion after 15 minutes of ischemia showed nearly normal ventricular wall thickening and motion, intact metabolic and biochemical function, and no tissue hemorrhage. However, reperfusion after 30 and 120 minutes of ischemia was associated with ventricular wall thickening and failure to resume systolic and diastolic wall motion. Furthermore, adverse metabolic and biochemical alterations and reperfusion zone hemorrhaging increased proportionally with the duration of ischemia. These findings suggest critical myocardial damage occurring between 15 and 30 minutes of ischemia in an animal model without preexisting coronary collateral circulation. The observed metabolic and biochemical changes are consistent with irreversible cell membrane defects, allowing calcium ion accumulation and thus adversely affecting diastolic relaxation and systolic thickening.

Limitations of potassium cardioplegia during cardiac ischemic arrest: a phosphorus 31 nuclear magnetic resonance study

Cold K+ cardioplegia is commonly used to preserve the myocardium during surgical ischemia. Since the K+-induced membrane depolarization could cause a Ca2+-mediated breakdown of adenosine triphosphate, this study compared the influence of different electrolytes on high-energy phosphate metabolism during cardioplegic arrest phosphate metabolism during cardioplegic arrest and subsequent recovery of mechanical function. An isolated working heart was subjected to hypothermic ischemia for one hour. Metabolic studies were assessed on phosphorus 31 nuclear magnetic resonance (NMR). Results show that (1) K+ cardioplegia is harmful when the Ca2+ content is equal to 2 mEq/I; (2) deleterious effects of K+ are markedly reduced by lowering the Ca2+ content; (3) the most adequate preservation is provided by a Mg2+-rich-Ca2+-poor perfusate; (4) this protection is not enhanced by addition of K+. Finally, 31P NMR appears particularly appropriate for evaluating myocardial protection techniques since it allows noninvasive serial monitoring of high-energy phosphate content and subsequent correlation with functional recovery after ischemia.

Recovery from prolonged abnormalities of canine myocardium salvaged from ischemic necrosis by coronary reperfusion

The purpose of this study was to determine whether or not the biochemical, functional, and ultrastructural abnormalities produced by brief temporary coronary occlusions (unassociated with necrosis) ever resolve and, if so, when they do. Anesthetized open-chest dogs were subjected to 15 min of coronary artery occlusion followed by 72 hr, 7 days, or 14 days of reperfusion. Serial in vivo myocardial biopsies were performed for measurement of ATP and for ultrastructural analysis. Regional function was evaluated by sonomicrometry. Mean (+/- SEM) myocardial ATP concentration was 36.6 +/- 1.2 nmol/mg of cardiac protein in nonischemic subendocardium and 18.9 +/- 1.5 in ischemic subendocardium after 15 min of ischemia. ATP remainede performed for measurement of ATP and for ultrastructural analysis. Regional function was evaluated by sonomicrometry. Mean (+/- SEM) myocardial ATP concentration was 36.6 +/- 1.2 nmol/mg of cardiac protein in nonischemic subendocardium and 18.9 +/- 1.5 in ischemic subendocardium after 15 min of ischemia. ATP remainede performed for measurement of ATP and for ultrastructural analysis. Regional function was evaluated by sonomicrometry. Mean (+/- SEM) myocardial ATP concentration was 36.6 +/- 1.2 nmol/mg of cardiac protein in nonischemic subendocardium and 18.9 +/- 1.5 in ischemic subendocardium after 15 min of ischemia. ATP remained depressed in the reperfused previously ischemic subendocardium at both 90 min (68% of nonischemic value) and 72 hr (78% of nonischemic value) but returned to normal at 7 days. Regional systolic function and cardiac ultrastructural abnormalities required 7 days for full recovery. Histologic and histochemical analysis did not reveal necrosis at any time. Therefore, biochemical, functional, and ultrastructural abnormalities induced by brief periods of transient coronary occlusion not associated with necrosis do resolve completely but the recovery period is prolonged.