Effects of ischemia and coronary reperfusion on myocardial digoxin uptake

The effects of coronary reperfusion on the uptake of digoxin by ischemic myocardium were studied in 17 open chest dogs undergoing anterior wall infarction produced by snaring confluent branches of the left coronary arterial system. Epicardial electrograms delineated ischemic, border and nonischemic zones. The hearts were reperfused by snare release after 1, 2 and 6 hours of occlusion. After 15 minutes of reperfusion, 1.0 mg of tritiated digoxin (³H-digoxin) was given intravenously, and 2 hours later the hearts were excised and endocardial and epfcardial samples from each zone were analyzed for ³H-digoxin concentration. In another group of eight dogs regional myocardial blood flow was assessed utilizing 15 μ of radio-labeled microspheres administered during occlusion and reperfusion. In five dogs with 1 hour of coronary occlusion and subsequent reperfusion, ³H-digoxin uptake was comparable in endocardial and epicardial layers of all three zones. In six dogs undergoing reperfusion after 2 hours of occlusion, mean ³H-digoxin concentration was significantly (P < 0.001) reduced from the mean nonischemic concentration, by 54 percent in endocardial and 35 percent in epicardial layers of the ischemic zone. Border zone endocardial and epicardial ³H-digoxin uptake was reduced by 21 percent and 16 percent, respectively (P < 0.05). In six dogs undergoing reperfusion after 6 hours of occlusion, ³H-digoxin uptake in the ischemic zone was significantly (P < 0.001) reduced by 85 percent in endocardial and 60 percent in epicardial layers from the concentration in the nonischemic zone. Border zone uptake was decreased by 54 percent in endocardial and 36 percent in epicardial regions (P < 0.01). These alterations of in vivo digoxin binding could not be explained by impaired reflow of blood to ischemic myocardium. We conclude that coronary reperfusion after 2 to 6 hours of occlusion is associated with a marked reduction in myocardial digoxin uptake, which is more pronounced in subendocardial than in subepicardial regions of ischemic tissue.     

Regional differences in postischemic recovery in the stunned canine myocardium

To determine if differences exist in the degree of ischemic damage and in postischemic recovery when different coronary arteries are occluded and reperfused, 40 barbital-anesthetized dogs were subjected to brief 15-minute periods of coronary artery occlusion followed by 3 hours of reperfusion ("stunned" myocardium) of the left anterior descending (LAD) or the left circumflex (LCX) coronary arteries. Myocardial segment shortening (%SS) in the subendocardium of nonischemic and ischemic reperfused areas was measured by sonomicrometry, and regional myocardial blood flow was measured by radioactive microspheres. Transmural tissue biopsies were taken at the end of reperfusion for the measurement of adenine nucleotides and total tissue water content. Arterial and local coronary venous blood samples were collected during preocclusion, during occlusion, and at 30 and 180 minutes of reperfusion for determination of blood oxygen content and oxygen consumption in the ischemic area. During occlusion, subendocardial blood flow (LAD flow = 0.11 +/- 0.02; LCX flow = 0.15 +/- 0.04 ml/min/gm), myocardial oxygen consumption (LAD = 2.4 +/- 0.7; LCX = 2.7 +/- 0.7 ml/min/100 gm), and areas of the left ventricle at risk (LAD = 27.4 +/- 2.3%; LCX = 32.4 +/- 2.4) were similar in both groups, thus indicating equivalent degrees of ischemia. There were no differences between groups in hemodynamics throughout the experiment or in the loss of myocardial high-energy phosphates or increase in total tissue water in the ischemic reperfused area at 3 hours of reperfusion. There was a significantly greater loss (p less than 0.05) of systolic wall function during LAD versus LCX occlusion and a greater recovery of segment function from 5 minutes throughout 1 hour of reperfusion after LCX occlusion (p less than 0.05), with no difference in %SS at 2 and 3 hours following reperfusion. Thus, although similar changes occurred in blood flow, metabolite parameters, tissue edema, wall function, and overall hemodynamics when either the LAD or LCX perfusion territories were occluded and reperfused, the loss of systolic wall function and recovery of segment shortening were more variable after regional stunning of the LCX perfusion bed. These data suggest that evaluation of pharmacologic or surgical interventions to improve postischemic functional recovery may be more reliably performed when the LAD coronary artery is the vessel occluded.     

Effects of hypoxemia on the extent of myocardial necrosis after experimental coronary occlusion

Arterial oxygen tension is variable in patients with acute myocardial infarction, and the effect of hypoxemia on the extent of myocardial necrosis after coronary occlusion has not been defined. In 11 anesthetized open chest dogs the left anterior descending coronary artery or one of its major branches was occluded for 20 minutes, and 10 to 15 epicardial electrocardiographic leads were recorded in the distribution and vicinity of the site of occlusion. Average S-T segment elevation and the number of sites showing S-T segment elevation greater than 2 mv, 15 minutes after occlusion were used as indexes of the severity and extent of ischemic injury. After occlusion with an inspired oxygen concentration of 20 percent these indexes were, respectively, 2.0 ± 0.5 mv (mean ± standard error) and 3.6 ± 0.8 sites; the respective values increased to 3.3 ± 0.5 mv (P < 0.01) and 6.7 ± 0.7 sites (P < 0.01) after occlusion with an inspired oxygen concentration of 10 percent, and arterial partial pressure of oxygen decreased from 92 ± 5 to 45 ± 3 mm Hg. In 23 dogs the occlusion was maintained for 24 hours and the S-T segment elevation 15 minutes after occlusion was compared with myocardial creatlne phosphokinase (CPK) activity and histologic appearance 24 hours later. In control dogs (inspired oxygen concentration of 20 percent) sites with no S-T segment elevation 15 minutes after occlusion showed normal myocardial CPK activity 24 hours later, whereas in sites with S-T segment elevation exceeding 2 mv there was an inverse relation between S-T segment elevation in each site and its myocardial CPK activity 24 hours later. Histologic examination revealed early myocardial necrosis in 98 percent (82 of 84) of sites with S-T segment elevation greater than 2 mv. In experimental dogs (inhaling a 10 percent Oxygen concentration for the first 8 of the 24 hours of occlusion) many sites that showed no S-T segment elevation before hypoxemia was induced exhibited S-T segment elevations 30 minutes later and showed abnormally low CPK activity and histologic evidence of early necrosis. We conclude that after experimental coronary occlusion, hypoxemia is deleterious because it substantially increases myocardial damage.     

Echocardiographic features of congenital mitral stenosis

The effects of 1 hour of occlusion of the left anterior descending coronary artery and subsequent reperfusion were studied for 3 hours in anesthetized closed chest pigs using an intracoronary balloon occluding technique. In 12 pigs subjected to reperfusion, S-T segment elevation decreased to control levels within 30 minutes and was significantly less than in 10 control pigs without reperfusion (P < 0.05). R wave amplitude was unaffected by the reperfusion. The more rapid fall of S-T segments in the reperfused group was accompanied by major hemorrhage in all reperfused hearts; hemorrhage occurred in only one control heart. The extent of hemorrhage was quantified and was significantly greater in the reperfused than in the control group (P < 0.01). Hemorrhage after reperfusion was unaffected by mannitol-induced increase in serum osmolallty but was greatly decreased by limitation of the occlusion period to 15 minutes. Myocardial wall motion was quantified angiographljcally. Shortening fraction of the area of the left ventricle most severely affected by occlusion decreased to similar levels in both groups. After reperfusion it remained similar in both groups at three hours. Adjacent left ventricular areas were likewise unaffected by reperfusion.This study thus documents the occurrence of postreperfusion myocardial hemorrhage in an animal with a coronary circulation similar to man's. Hemorrhage is directly related to the duration of occlusion but appears to be unaffected by mannitol given before reperfusion. Caution is advised both during bypass surgery, in which occlusion and reperfusion occur, and In efforts to restore coronary blood flow during acute myocardial infarction.     

Coronary hemodynamics during reperfusion following acute coronary ligation in dogs.

The coronary hemodynamic effects of re-establishing blood flow to ischemic myocardium and the regional distribution of myocardial flow during reperfusion were studied in anesthetized open-chest dogs. A large portion of the left ventricular wall was rendered ischemic by occlusion of the left anterior descending coronary artery for 2 hours. During reperfusion of the LAD, coronary resistance in the reperfused vasculature increased progressively for the first 3 hours, while resistance in the intact LC vasculature was unchanged. Minimal resistances in the reperfused vascular bed, calculated from mean aortic pressure and peak coronary reactive hyperemic blood flow following a 90 sec. LAD occlusion, were elevated significantly during reperfusion. The increased minimal resistance values, which reflect the passive physical component of resistance, indicate structural changes in the reperfused vascular bed which were evident shortly after the initiation of reperfusion and persisted throughout the experimental period. Coronary resistances (RH) in the reperfused (LAD) and intact (LC) vasculatures during the reactive hyperemia following 10 sec. coronary occlusions were evaluated. During reperfusion, RH in the reperfused vasculature increased progressively while RH in the intact bed was unchanged. The marked increase in RH in the LAD indicates that the reactive hyperemic flow response to a consistent period of coronary occlusion progressively diminished, and reflects a gradual reduction in the vasodilatory potential of the reperfused coronary circulation. The regional distribution of myocardial blood flow following 5 minutes, 2 hours, and 4 hours of reperfusion was measured with multiple injections of radioactive microspheres. These measurements demonstrated a progressive reduction of blood flow to the reperfused myocardium with no significant change in flow to the control myocardium. In contrast to the uniform transmural distribution of flow in the normal myocardium, the reperfused region showed a distinctly nonuniform distribution of flow after 2 hours and 4 hours of reperfusion, with more severe reduction of flow to the endocardial layer. These studies would suggest that rechannelling blood flow distal to an acute coronary occlusion in human subjects might not in itself be capable of reversing the myocardial injury. It is hoped that additional therapeutic measures might be applied to salvage the injured myocardium.     

Contractile and biochemical effects of coronary reperfusion after extended periods of coronary occlusion.

The effects of coronary reperfusion on recovery of regional myocardial contractility and high energy pegmental changes in myocardial contractility were measured by means of a strain gauge-tipped, two-pronged catheter probe that measures myocardial fiber shortening. The curves of contraction are sensitive to the effects of ischemia. Coronary occlusion resulted in a rapid replacement of fiber shortening by passive fiber lengthening. If coronary occlusion was released and blood flow restored within 45 minutes, myocardial contractility returned promptly; adenosine triphosphate and creatine phosphate values were restored to normal. With coronary occlusion of 1 hour or longer, contractility failed to return in the immediate postperfusion period, but delayed return was recorded after 2 weeks of reperfusion. The extent of such recovery varied with the duration of preceding occlusion. Thus, reperfusion after 1 hour of occlusion was followed by return of fiber shortening over the entire reperfused region. With 2 hours of occlusion, recovery occurred over 75 percent of the reperfused myocardium. With 3 hours of occlusion followed by reperfusion, recovery of contractility was only partial, comprising approximately 60 percent of the reperfused region. High energy phosphate content of the reperfused myocardium showed a similar pattern of recovery. With occlusion of longer duration, reperfusion failed to restore contractility to any significant extent. These findings indicate that reperfusion after coronary occlusion of 1 to 3 hours may restore contractility over a period of 2 weeks, but the extent of such recovery diminishes with the increase in the duration of occlusion.     

Effects of amlodipine on myocardial ischemia-reperfusion injury in dogs

The effects of the dihydropyridine calcium channel blocker amlodipine on subendocardial segment shortening (%SS), regional myocardial blood flow, myocardial high-energy phosphate levels and tissue water content were compared with those of a saline-treated group of barbital-anesthetized dogs subjected to a 45-minute coronary artery occlusion followed by 60 minutes of reperfusion. Saline or amlodipine (200 micrograms/kg administered intravenously) was given 15 minutes before coronary occlusion. There were no significant differences between groups in ischemic bed size or hemodynamics although dP/dt was higher after amlodipine administration. Subepicardial collateral blood flow was higher in the amlodipine group during coronary occlusion. After occlusion, %SS in the ischemic region was markedly decreased in both series and passive systolic lengthening resulted. Despite similar decreases in %SS during occlusion, the amlodipine-treated dogs showed a marked improvement in myocardial segment function of the ischemic reperfused region throughout 60 minutes of reperfusion compared with saline-treated dogs. In addition, amlodipine prevented the rebound increase in phosphocreatine and attenuated the loss of adenine nucleotides and increase in tissue water in the ischemic reperfused area at 60 minutes of reperfusion. These results suggest that amlodipine has a favorable effect on the functional and metabolic recovery of the ischemic reperfused myocardium and may have potential as a therapeutic agent for the treatment of coronary artery disease. The mechanism of action of amlodipine in this model is unknown but may be partially related to a drug-induced increase in coronary collateral blood flow or a decrease in afterload.     

F(ab')2 fragments of anti-Mo1 (904) monoclonal antibodies do not prevent myocardial stunning

To determine if inhibition of leukocyte adhesion and aggregation could improve postischemic ventricular dysfunction ("stunning"), a monoclonal antibody (904) that binds to the adhesion-promoting Mo1 glycoprotein on the cell surface of leukocytes was administered intravenously (0.5 mg/kg) to open-chest dogs before a 15-minute coronary occlusion. Ultrasonic crystals placed in ischemic and control myocardium were used to measure systolic wall thickening during a 15-minute occlusion of the left anterior descending artery and for 3 hours after reperfusion. Myocardial blood flow was measured with tracer-labeled microspheres before occlusion, after 10 minutes of occlusion, 3 minutes of reperfusion, and at 1 and 3 hours after reperfusion. Six animals receiving anti-Mo1 antibody had antibody excess demonstrated with immunofluorescence techniques at 5 minutes and 3 hours of reperfusion; this finding indicated saturation of binding sites. Five animals served as controls and received an antibody (murine immunoglobulin G) that does not influence neutrophils. The two groups did not differ hemodynamically during ischemia and reperfusion. Risk areas and myocardial blood flow were also not significantly different between the two groups. The main parameter used to define regional myocardial stunning, percentage systolic wall thickening in the ischemic/reperfused area, did not differ significantly between the two groups. Specimens from nonischemic myocardium were compared with ischemic specimens for myeloperoxidase content. There were no significant differences within or between groups. These data indicate that the anti-Mo1 monoclonal antibody (904) is not effective in improving the profound myocardial dysfunction that persists for 3 hours of reperfusion after 15 minutes of ischemia.     

Dissociation between contractile function and oxidative metabolism in postischemic myocardium. Attenuation by ruthenium red administered during reperfusion.

The oxidative metabolic rate may be disproportionately high compared with contractile function in postischemic reperfused myocardium. To study the potential involvement of intracellular calcium transport in high energy expenditure after reperfusion, we determined in isolated rat hearts the effect of ruthenium red, an inhibitor of mitochondrial calcium transport, on recovery of contractile function and oxidative metabolic rate. Hearts subjected to 60 minutes of no-flow ischemia exhibited, at 15 minutes after the onset of reperfusion, poor recovery of left ventricular pressure development to only 7% of the corresponding value measured in control hearts (p less than 0.01). However, myocardial oxygen consumption was recovered to 84% of control (p = NS). The ratio of isovolumic contractile performance (expressed as the product of heart rate and left ventricular pressure development) to myocardial oxygen consumption was severely depressed to 6% of control (p less than 0.01). Supplementation of the perfusate with 6 microM ruthenium red during the initial 40 minutes of reperfusion resulted in a reduction of myocardial oxygen consumption to 65% of the value measured after 15 minutes of reperfusion in hearts reperfused without ruthenium red (p less than 0.01), despite a threefold increase of left ventricular pressure development (p less than 0.05). Oxidation of both palmitate and glucose was reduced to a comparable extent by ruthenium red. The ratio of contractile performance to myocardial oxygen consumption increased progressively during infusion of ruthenium red and did not differ further from control hearts after 30 minutes of reperfusion. Cumulative myocardial release of creatine kinase was reduced by 47% (p less than 0.05) in hearts reperfused with ruthenium red-containing medium. The results provide circumstantial evidence for the hypothesis suggesting that enhanced energy expenditure by intracellular calcium transport may be involved in the mechanisms underlying the dissociation between left ventricular performance and myocardial oxidative metabolic rate early after postischemic reperfusion.     

Myocardial salvage by intracoronary thrombolysis in evolving acute myocardial infarction: evaluation using intracoronary injection of thallium-201

Immediate objective assessment of viabillty of reperfused myocardium following intracoronary (IC) thrombolysis by evaluation of ventricular function may be limited due to delay in restoration of function. Thus we assessed myocardial uptake of thallium-201 (TI-201) following IC injection postreperfusion as an index of myocardial salvage in 12 experimental dogs and in five patients with evolving acute myocardial infarction (AMI). In seven dogs with mean of 313 minutes of experimental coronary occlusion, immediate postreperfusion IC TI-201 images revealed absence of myocardial uptake in prevlously occluded zones. These TI-201 defects correlated with presence of necrosis as demonstrated by histochemical staining with triphenyl-tetrazolium chloride (TTC). In contrast, in five dogs with mean of 37 minutes of coronary occlusion, reperfused myocardium showed normal TI-201 uptake following its IC injection; this normal TI-201 uptake pattern correlated with absence of necrosis by TTC technique in all five dogs. In five patients with evolving AMI, control TI-201 images obtained following IV injection prior to IC thrombolysis showed myocardial perfusion defects corresponding to distribution of the occluded vessel. Following reperfusion, 30 to 50 mCi of TI-201 was injected into the reopened coronary artery. In two patients with mean symptom onset of reperfusion time of $\text{2}\text{1}\text{2}\text{hours}$, immediate postreperfusion IC TI-201 images demonstrated normal or improved TI-201 uptake in reperfused myocardium. By radionuclide ventriculography, segmental wall motion remained abnormal in the reperfused regions 6 hours postreperfusion and showed improvement by the time of 10-day study. In the remaining three patients with symptom onset to reperfusion time of 5 hours, immediate postreperfusion IC TI-201 images did not show improvement, correlating with persistent wall motion abnormalities 10 days postreperfusion. In all five patients, repeat 10-day IV TI-201 images were unchanged from the immediate postreperfusion IC TI-201 images. We conclude that (1) prereperfusion TI-201 imaging with repeat TI-201 injection into the reopened coronary artery appears to delineate the extent of myocardial salvage in both experimental and clinical studies and (2) this method of IC TI-201 imaging allows immediate assessment of myocardial viabillty which may facilltate decisions regarding the need for additional myocardial revascularization modalities.     

Temporal relation of epicardial electrographic, contractile and biochemical changes after acute coronary occlusion and reperfusion.

The increasing use of changes in the S-T segment of local epicardial electrograms to quantitate myocardial infarct size has led to the need for a better understanding of this method. Accordingly, we studied the local electrographic, tension and biochemical changes that occurred after coronary occlusion and subsequent reperfusion in 44 dogs using epicardial electrograms from 10 to 12 sites, Walton-Brodie strain gauge arches and myocardial ratios of potassium ion to sodium ion (K+/Na+). After coronary occlusion for 1 hour, total S-T segment elevation increased from 10.2 +/- 2.4 to 78.3 +/- 13.7 mv (P less than 0.001) and tension development decreased to 63.6 +/- 7.0% of control value (P less than 0.001); occlusion for 3 hours resulted in a total S-T segment elevation increase from 5.8 +/- 3.4 to 56.7 +/- 8.7 mv (P less than 0.001) and a tension decrease to 61.4 +/- 5.3% (P less than 0.001) of control value. After reperfusion two types of response were observed. In nine experiments new local pathologic Q waves appeared in an average of 5.3 of 8.2 ischemic electrode sites within 5 to 10 minutes of reperfusion concomitant with a marked further decrease in total tension from 67.3 +/- 5.5% to 42.4 +/- 6.0% of control value (P less than 0.001). Simultaneously, total S-T elevation decreased from 66.1 +/- 8.2 to 25.3 +/- 3.4 mv (P less than 0.001). In seven experiments no Q waves appeared after reperfusion and there was no significant change in tension. Total S-T elevation again decreased from 58.3 +/- 12.7 to 27.1 +/- 5.7 mv (P less than 0.025). When normal saline solution was perfused distal to the coronary arterial occlusion total S-T elevation decreased from 68.0 +/- 3.6 to 36.3 +/- 5.2 mv (P less than 0.001). After 3 hours of coronary occlusion, myocardial K+ decreased and Na+ increased in the ischemic zone, resulting in a significant decrease in the K+/Na+ ratio (P less than 0.005). Reperfusion for 2 hours resulted in a further depletion of K+ and an increase in Na+ with a resultant complete reversal of the K+/Na+ ratio (P less than 0.001). In summary, after reperfusion the S-T segment abnormalities rapidly decreased in all experiments despite the appearance of new Q waves in more than half of these studies concomitant with either a decrease or no change in contractile ability and continuing myocardial K+ loss and Na+ accumulation. S-T segment mapping therefore appears to be of limited value in assessing the effect of reperfusion on infarct size. The decrease in S-T segments that occurred with perfusion of either blood or saline solution suggests a "washout" phenomenon.     

Ischemic preconditioning reduces infarct size in swine myocardium

We evaluated the hypothesis that stunning swine myocardium with brief ischemia reduces oxygen demand in the stunned region and increases tolerance of myocardium to longer periods of ischemia. Wall function was quantified with ultrasonic crystals aligned to measure wall thickening, and stunning was achieved with two cycles of left anterior descending coronary artery (LAD) occlusion (10 minutes) and reperfusion (30 minutes), after which the LAD was occluded for 60 minutes and reperfused for 90 minutes. Infarct size (as a percent of risk region) was then determined by incubating myocardium with para-nitro blue tetrazolium. Regional oxygen demand was measured as myocardial oxygen consumption before the 60-minute LAD occlusion in the stunned region; tracer microspheres were used to determine blood flow, and blood from the anterior interventricular vein and left atrium was used to calculate oxygen saturations. After the second reperfusion period, wall thickening in the stunned region was reduced to 1.4 +/- 2.4% compared with 36.7 +/- 2.5% (mean +/- SEM) before ischemia (p less than 0.001). Regional myocardial oxygen consumption after stunning (3.1 +/- 0.7 ml O2/min/100 g) was no different from regional myocardial oxygen consumption before stunning (3.7 +/- 0.6 ml O2/min/100 g). In the nine pigs "preconditioned" by stunning, infarct size was 10.4 +/- 6.3% of the risk region compared with 48.0 +/- 12.7% in the six control pigs subjected to 60 minutes of ischemia without prior stunning (p less than 0.005). The risk regions were similar (14.4 +/- 1.5% vs. 14.6 +/- 1.9% of the left ventricle, preconditioned vs. control pigs, respectively). We conclude that stunning swine myocardium with two cycles of a 10-minute LAD occlusion followed by reperfusion increases ischemic tolerance but that changes in regional demand in stunned myocardium do not predict the marked reduction in infarct size that follows a subsequent 60-minute period of ischemia.     

Regional redistribution of myocardial blood flow after coronary occlusion and reperfusion in the conscious dog

Early and late changes in regional myocardial blood flow distribution within the left circumflex coronary arterial bed after occlusion and after occlusion and reperfusion were compared with the extent of myocardial tissue necrosis. Radiolabeled microspheres, 15 μm, were used to study regional myocardial blood flow in conscious dogs at 5 minutes, 2 and 6 hours and 1 month after coronary occlusion. Blood flow was measured in conscious dogs whose hearts were reperfused for 72 hours after 2, 6 and 24 hours of occlusion. Blood flow was measured in four distinct transmural myocardial zones dellneated by dye injections and gross infarct features of the occluded left circumflex coronary bed. After occlusion, myocardial flow was redistributed from deep layers to outer layers, and within 6 hours after occlusion collateral flow was increased to the outer zones in excess of redlstributed flow. After reperfusion, blood flow greatly increased to regions containing predominantly normal tissue, and flow was redlstrlbuted away from the necrotic zones. The indigenous collateral circulation was a major determinant of infarct size in the occluded and reperfused myocardium. The concept of a migrating and narrowing marginal zone is discussed.     

Effect of increasing degrees of ischemic injury on myocardial oxidative metabolism early after reperfusion in isolated rat hearts

The present investigation studied the effect of increasing severities of ischemic injury on recovery of oxidative metabolism after reperfusion in isolated rat hearts perfused retrogradely with erythrocyte-containing medium. Hearts subjected to 60 minutes of low-flow ischemia (5% of control perfusion) exhibited delayed but sustained recovery of left ventricular pressure development during reperfusion and preservation of ultrastructure delineated with electron microscopy. Immediately after reperfusion, myocardial oxygen consumption returned to control values, well before left ventricular pressure development recovered. Early after reperfusion release of 14CO2 from [1-14C]palmitate was reduced (-53%, p less than 0.01). Conversely, release of 14CO2 from [U-14C]glucose was increased (+131%, p less than 0.05). After 60 minutes of reperfusion 14CO2 release had completely returned to normal for both labeled substrates. Pulse-labeling experiments indicated that during transient depression of [1-14C]palmitate oxidation more tracer was incorporated into myocardial lipid esters, primarily triglycerides. In contrast to hearts subjected to low-flow ischemia, hearts subjected to 60 minutes of no-flow ischemia exhibited poor recovery of contractile function during the reperfusion period. Electron microscopic examination of reperfused hearts showed advanced myocyte damage consistent with irreversible injury. Interestingly, myocardial oxygen consumption in this group also recovered to control values. The substrate pattern during the early reperfusion period was similar to that of hearts subjected to low-flow ischemia. After 120 minutes of no-flow ischemia, recovery of oxidative metabolism was virtually absent. The results indicate a pronounced dissociation between recovery of oxidative metabolism and of contractile function in reperfused myocardium. The oxidative metabolic rate was disproportionately high compared with contractile function, not only in reversibly "stunned" hearts, but also in severely damaged hearts exhibiting signs of irreversible injury.     

Reversibly injured, postischemic canine myocardium retains normal contractile reserve

Transient coronary occlusion (15 minutes) does not result in irreversible myocardial injury but is associated with a depression of contractile function sustained for several hours to days ("stunned myocardium"). The defect in the contractile process responsible for this phenomenon has been suggested to be causally related to a reduced energetic state, altered excitation or excitation-contraction coupling, or damaged contractile filaments. The purpose of this study was to attempt to exclude one or more of these hypotheses by evaluating the contractile reserve of reperfused myocardium. Regional subendocardial segment function was measured (sonomicrometry) in a control region and in an area (treatment region) perfused by a carotid artery to anterior descending coronary artery bypass in 13 chloralose-anesthetized dogs. Dose-response curves were constructed from changes in segment shortening (%SS) in response to intracoronary calcium infusion before ischemia and following 5 or 15 minutes of occlusion and reperfusion (30 minutes). Calcium infusion before ischemia resulted in dose-dependent increases in %SS in the treatment area to a maximum value of 36.6% from a preinfusion value of 25.5% (p less than 0.01), in the absence of changes in control region shortening (23.7%). After 15 minutes of occlusion and reperfusion, treatment area %SS had fallen to a depressed but stable level (46% of preischemic values; p less than 0.01). Subsequent calcium infusion at the same doses as in the preischemic trial produced increases in treatment segment function with return of shortening to control levels at an intermediate dose. At the highest dose, %SS was 35.4%, which was not different from the maximal value found in the preischemic trial. Alterations in heart rate and left ventricular systolic and diastolic pressures during calcium infusion were minor and similar before and after ischemia. Calcium-induced increases in regional segment shortening above control levels (113% of control) in reperfused myocardium were sustained with continuous infusion (30 minutes) without deleterious effects on subsequent function. These results demonstrate that stunned myocardium in this model retains a normal contractile reserve in response to calcium, suggesting that the mechanism responsible for postischemic contractile dysfunction involves calcium.     

Inotropic stimulation of reperfused myocardium with dopamine: effects on infarct size and myocardial function

Prolonged postischemic ventricular dysfunction ("stunned myocardium") may be responsible for heart failure after myocardial reperfusion. Although inotropic stimulation can enhance the contractility of stunned myocardium, it could potentially increase infarct size and thereby impair ultimate recovery of myocardial function. In 16 anesthetized dogs, the left anterior descending coronary artery was occluded for 2 hours, and then reperfused. At 45 minutes of reperfusion, the dogs were randomized to receive a 3 hour intravenous infusion of either saline solution or dopamine (10 micrograms/kg per min), and 1 hour after the infusion was discontinued the area of necrosis and an in vivo area at risk of necrosis were determined. All dogs had serial two-dimensional echocardiograms with computer-assisted analysis and in vivo biopsies for determination of adenosine triphosphate and creatine phosphate levels. Dopamine caused an increase in the contractility of the reperfused myocardium, with systolic wall thickening increasing from -4.1 +/- 2.6 (mean +/- SEM) to +24.0 +/- 3.7% (p less than 0.002) and short-axis cross-sectional ejection fraction increasing from 27.1 +/- 4.7 to 71.6 +/- 4.4% (p less than 0.002) after 15 minutes of infusion. Regional myocardial blood flow in the previously ischemic epicardium was increased from 1.18 +/- 0.11 ml/min per g with saline to 2.95 +/- 0.36 ml/min per g with dopamine (p less than 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)     

Transmural gradients of experimental myocardial ischemia: limited correlation of ultrastructure with epicardial S-T segment elevation.

A standardized preparation was developed to investigate the precise relationship between epicardial S-T segment elevation and myocardial ultrastructure. The distal left anterior descending coronary artery was occluded for 20 minutes and epicardial S-T segments were recorded at five-minute intervals. Immediate perfusion-fixation with glutaraldehyde preserved premortem anatomic relationships and allowed precise sampling of the myocardium immediately underlying sites of S-T segment recording. Ischemic and non-ischemic zones were defined both anatomically and by S-T segment mapping. Transmural samples at 0.2 cm. intervals were compared at sites in ischemic and non-ischemic zones. In non-ischemic zones, S-T segment elevations ranged from 0 to 2 mm., and myocardial ultrastructure was normal. In ischemic zones, S-T segment elevations ranged from 0 to 14 mm., and samples showed a gradient of ischemic injury with greatest change in subendocardial blocks. The extent of ultrastructural change at any point was not consistently proportional to the height of S-T segment elevation. S-T segment elevations greater than 2 mm. were always associated with a marked transmural gradient of change, but S-T segment elevations less than 2 mm. in ischemic zones could also be associated with severe subendocardial and mid-myocardial change. In a second group of dogs the coronary artery snare was released after 20 minutes of occlusion and recovery was allowed for 60 minutes before killing and subjecting to perfusion-fixation. These hearts exhibited no abnormality in myocardial ultrastructure when sampled in the same fashion as the first group.In this model a reproducible gradient of transmural, myocardial ultrastructural change was demonstrated under conditions of reversible injury. Prominent S-T segment elevation in ischemic zones always indicated the presence of underlying ultrastructural change, but marked changes were also present when S-T segment elevations were minimal, indicating that the S-T segment maps tended to underestimate the extent of early ischemic change. We were unable to establish a quantitative relationship between the extent of S-T segment elevation and the extent of transmural ultrastructural change.     

Early differentiation of infarcted and noninfarcted reperfused myocardium in dogs by quantitative analysis of regional myocardial echo amplitudes

This study tests the hypothesis that ischemic but viable reperfused myocardium can be differentiated from infarcted reperfused myocardium by regional analysis of myocardial echo amplitudes. In eight closed-chest, anesthetized dogs, the left anterior descending coronary artery was occluded for 3 hours, followed by 1 hour of reperfusion, and sacrifice. Infarct size was measured by the triphenyl tetrazolium chloride technique in a 1-cm-thick mid-left ventricular transverse slice, and matched with a corresponding end-diastolic two-dimensional echo short-axis cross-section. Outlining of epi- and endocardial surfaces, along with construction of a mid-myocardial outline, allowed measurements of regional myocardial echo intensities and grey-level histograms in subendo- and subepicardial regions. In 36 eventually infarcted subendocardial segments (greater than 20% wall necrosis), average pixel intensity (arbitrary units) was 73.7 +/- 33.1 (SD) in control, 75.8 +/- 33.0 at 3 hours of occlusion, and 107.8 +/- 40.9 at 5 minutes, 105.5 +/- 38.9 at 15 minutes, and 101.1 +/- 37.6 at 60 minutes postreperfusion P less than 0.05 vs. control or occlusion); intensity in normal segments (no or less than 20% wall necrosis) was 60.0 +/- 18.6 in control, 57.4 +/- 20.3 at 3 hours of occlusion, and 63.5 +/- 14.8, 68.0 +/- 27.9, and 64.2 +/- 22.3 at 5, 15, and 60 minutes postreperfusion, respectively (no significant change). The skew of the grey-level distribution in infarcted subendocardial segments did not change from control (0.49 +/- 0.72) to 3 hours of occlusion (0.41 +/- 0.52), but decreased (shift to higher echo amplitude) significantly at 5 minutes (-0.31 +/- 0.53), 15 minutes (-0.22 +/- 0.50), and 60 minutes (-0.28 +/- 0.45) after reperfusion (P less than 0.05 vs. control or occlusion); in normal subendocardial segments, there was no significant change throughout the study. In 31 partly infarcted subepicardial segments (greater than 50% wall necrosis), changes in postreperfusion echo amplitudes were less significant. Average pixel intensity was 71.3 +/- 28.6 in control, 71.8 +/- 29.2 after coronary occlusion, and 89.2 +/- 35.3, 83.7 +/- 37.5, and 85.6 +/- 34.9 at 5, 15, and 60 minutes after reperfusion, respectively. It is concluded that reperfusion of irreversibly injured myocardium is associated with consistent early increase in regional myocardial echo intensities and changes in the grey-level distribution. Such alterations might be used to detect the extent of tissue necrosis within minutes after reperfusion.     

Detection of oxygen-derived free radical generation in the canine postischemic heart during late phase of reperfusion

To define the relation between oxygen-derived free radical (oxy-radical) generation in the reperfused ischemic myocardium and the progression of myocardial damage, we measured oxy-radical generation in the ischemic myocardium and the propagating infarct size in a model of canine coronary occlusion (90 minutes) and reperfusion. We used electron paramagnetic resonance spin-trapping techniques (5,5-dimethyl-1-pyrroline N-oxide [DMPO]) to detect oxy-radicals in the rapidly frozen myocardial samples taken by needle biopsy. There was no detectable generation of DMPO adducts in the normal myocardium before or after reperfusion. In the reperfused ischemic myocardium, electron paramagnetic resonance signals of DMPO-OOH (superoxide anion) and DMPO-OH (hydroxyl radical) were detected, with peak concentrations at 1 hour after reperfusion for DMPO-OOH and at 3 hours after reperfusion for DMPO-OH, respectively. These DMPO adducts were also detected during the early phase (15 seconds) of reperfusion, but the concentrations of these signals were much less than those during the late phase of reperfusion. Treatment with human recombinant superoxide dismutase (2.5 mg/kg/hr) and catalase (2.5 mg/kg/hr) during the course of experiments abolished DMPO-OOH formation but had little effect on DMPO-OH formation. Infarct size (percent of risk area infarcted), quantified by a dual staining method with Evans blue dye and triphenyltetrazolium chloride, was 18.3 +/- 4.8% (mean +/- SEM) at 90 minutes of occlusion. After 5 hours of reperfusion, infarct size increased to 43.6 +/- 7.2%. These results indicate that a greater magnitude of oxy-radical generation was sustained in the ischemic myocardial tissue during the late phase (1-3 hours) of reperfusion, associated with the progression of myocardial infarction. The concurrent appearance of oxy-radicals and progressive infarction may support the view that a chain reaction of oxy-radicals contributes to the propagation of myocardial cell damage in the postischemic heart.     

Relation between myocardial glutathione content and extent of ischemia-reperfusion injury

The relation between the extent of myocardial injury sustained during reperfusion and total glutathione (GSH) content in the ischemic myocardium was examined in anesthetized open-chest pigs subjected to coronary occlusion for 45 minutes and reperfusion for 2 hours. In pigs infused with saline during reperfusion (n = 6) there was a decrease in myocardial GSH content from 380 +/- 48 micrograms/g in normally perfused myocardium to 182 +/- 36 micrograms/g in the ischemic reperfused myocardium (p less than 0.02). Myocardial infarct size (expressed as a percentage of the ischemic area) was 12.5 +/- 0.8%. There was a delay of recovery of contractile function before returning to 60% of preocclusion value. In pigs pretreated with buthionine sulfoximine (BSO) (n = 5), an inhibitor of cellular GSH synthesis, there was reduction in GSH content to 215 +/- 25 micrograms/gm in normally perfused myocardium and to 77 +/- 8 micrograms/gm in the ischemic reperfused myocardium. The extent of injury was greater as evidenced by an increase in infarct size to 30.4 +/- 4.0% (p less than 0.001), severe destructive changes in subepicardial ultrastructure, which were absent in saline-infused pigs, and persistence of dyskinesia throughout reperfusion. In pigs infused with glutathione intravenously (0.8 gm/kg) at a rate of 6.5 mg/kg/min (n = 6), 5 minutes before and continuously during reperfusion, there was an increase in GSH content to 582 +/- 67 micrograms/g in normally perfused myocardium and to 312 +/- 80 micrograms/g in ischemic reperfused myocardium. The increase in myocardial GSH was associated with a reduction in infarct size to 7.5 +/- 1.3% (p less than 0.05, compared with saline-infused pigs) and an early recovery of contractile function of the ischemic myocardium. GSH infusion into pigs pretreated with BSO (n = 4) failed to increase myocardial GSH content and failed to reduce the extent of myocardial injury. Thus, the extent of myocardial injury sustained during reperfusion is very dependent on the effectiveness of its antioxidant defenses. Markedly increased susceptibility to injury occurs when the GSH content in the ischemic myocardium becomes depleted.