Keith A. Reimer

Preconditioning with ischemia: a delay of lethal cell injury in ischemic myocardium.

We have previously shown that a brief episode of ischemia slows the rate of ATP depletion during subsequent ischemic episodes. Additionally, intermittent reperfusion may be beneficial to the myocardium by washing out catabolites that have accumulated during ischemia. Thus, we proposed that multiple brief ischemic episodes might actually protect the heart from a subsequent sustained ischemic insult. To test this hypothesis, two sets of experiments were performed. In the first set, one group of dogs (n = 7) was preconditioned with four 5 min circumflex occlusions, each separated by 5 min of reperfusion, followed by a sustained 40 min occlusion. The control group (n = 5) received a single 40 min occlusion. In the second study, an identical preconditioning protocol was followed, and animals (n = 9) then received a sustained 3 hr occlusion. Control animals (n = 7) received a single 3 hr occlusion. Animals were allowed 4 days of reperfusion thereafter. Histologic infarct size then was measured and was related to the major baseline predictors of infarct size, including the anatomic area at risk and collateral blood flow. In the 40 min study, preconditioning with ischemia paradoxically limited infarct size to 25% of that seen in the control group (p less than .001). Collateral blood flows were not significantly different in the two groups. In the 3 hr study, there was no difference between infarct size in the preconditioned and control groups. The protective effect of preconditioning in the 40 min study may have been due to reduced ATP depletion and/or to reduced catabolite accumulation during the sustained occlusion. These results suggest that the multiple anginal episodes that often precede myocardial infarction in man may delay cell death after coronary occlusion, and thereby allow for greater salvage of myocardium through reperfusion therapy.

The wavefront phenomenon of ischemic cell death. 1. Myocardial infarct size vs duration of coronary occlusion in dogs.

Irreversible ischemic myocardial cell injury develops in an increasing number of cells as the duration of coronary occlusion is prolonged. The present study quantitates myocardial necrosis produced by 40 minutes, 3 hours, or 6 hours of temporary circumflex coronary occlusion (CO) followed by 2 to 4 days of reperfusion, or by 24 or 96 hours of permanent circumflex ligation In pentobarbital anesthetized open chest dogs. After 40 minutes of ischemia, myocyte necrosis was subendocardial but with increasing duration of coronary occlusion, irreversible injury progressed as a wavefront toward the subepicardium. Transmural necrosis was 38 ± 4% after 40 mi, 57 ± 7% after 3 hours, 71 ± 7% after 6 hours and 85 ± 2% after 24 hours of ischemic injury. These results document the presence of a subepicardial zone of ischemic but viable myocardium which is available for pharmacologic or surgical salvage for at least three and perhaps six hours following circumflex occlusion in the dog.

Ischemic preconditioning slows energy metabolism and delays ultrastructural damage during a sustained ischemic episode.

We have shown previously that preconditioning myocardium with four 5-minute episodes of ischemia and reperfusion dramatically limited the size of infarcts caused by a subsequent 40-minute episode of sustained ischemia. The current study was undertaken to assess whether the same preconditioning protocol slowed the loss of high energy phosphates, limited catabolite accumulation, and/or delayed ultrastructural damage during a sustained ischemic episode. Myocardial metabolites and ultrastructure in the severely ischemic subendocardial regions were compared between control and preconditioned canine hearts. Hearts (four to 10 per group) were excised after 0, 5, 10, 20, or 40 minutes of sustained ischemia. All groups had comparable collateral blood flow. Preconditioned hearts developed ultrastructural injury more slowly than controls; evidence of irreversible injury was observed after 20 minutes in controls but not until 40 minutes in preconditioned hearts. Furthermore, after 40 minutes of ischemia, irreversible injury was homogeneous in controls but only focal in preconditioned myocardium. Preconditioning reduced starting levels of ATP by 29%. Nevertheless, it also slowed the rate of ATP depletion during the episode of sustained ischemia, so that after 10 minutes of ischemia, preconditioned hearts had more ATP than controls. However, after 40 minutes, ATP contents were not significantly different between groups. Preservation of ATP resulted from reduced ATP utilization and was not due to increased ATP production. Accumulation of purine nucleosides and bases (products of adenine nucleotide degradation) was limited in preconditioned myocardium. Accumulation of glucose-1-phosphate, glucose-6-phosphate, and lactate also was reduced markedly by preconditioning, due to reduced rates of glycogen breakdown and and anaerobic glycolysis. We propose that preconditioning reduces myocardial energy demand during ischemia, which results in a reduced rate of high energy phosphate utilization and a reduced rate of anaerobic glycolysis. Either preservation of ATP or reduction of the cellular load of catabolites may be responsible for delaying ischemic cell death.

Evaluation of a QRS scoring system for estimating myocardial infarct size

This study correlated the location and size of posterolateral myocardial infarcts (MIs) measured anatomically with that estimated by quantitative criteria derived from the standard 12-lead ECG. Twenty patients were studied who had autopsy-proved, single, posterolateral MIs and no confounding factors of ventricular hypertrophy or bundle branch block in their ECG. Left ventricular anatomic MI size ranged from 1 to 46%. No patient had a greater than or equal to 0.04-second Q wave in any electrocardiographic lead and only 55% had a 0.03-second Q wave. A 29-point, simplified QRS scoring system consisting of 37 weighted criteria was applied to the ECG. Points were scored by the ECG in 85% of the patients (range 1 to 8 points). MI was indicated by a wide variety of QRS criteria; 19 of the 37 criteria from 8 different electrocardiographic leads were met. The correlation coefficient between MI size measured anatomically and that estimated by the QRS score was 0.72. Each point represented approximately 4% MI of the left ventricular wall.

Effect of reperfusion late in the phase of reversible ischemic injury. Changes in cell volume, electrolytes, metabolites, and ultrastructure.

The acute effects of reperfusion on myocardium reversibly damaged by 15 minutes of severe ischemia in vivo, were studied. Changes in the adenine nudeotide pool, cell volume regulation, myocardial calcium, and ultrastructure were studied at the end of 15 minutes of ischemia and after 0.5, 3.0, and 20 minutes of reflow. Before reperfusion, adenosine triphosphate and the adenylate pool decreased by 63% and 44% of control, respectively, and the adenylate charge was reduced to 0.65. After 3 minutes of reperfusion, the adenylate charge was restored to control by the rephosphorylation of adenosine mono- and diphosphate, but adenosine triphosphate was still reduced by 45%. Mild tissue edema was detected after 0.5 minute of reflow and persisted throughout 20 minutes of reperfusion. The increased tissue water was accompanied by a slight increase in sodium and a marked increase in tissue potassium. Although massive calcium accumulation develops when irreversibly injured tissue is reperfused, no calcium overload was detected during early reperfusion of reversibly injured myocytes. Reperfusion for 3 minutes exaggerated the mitochondrial swelling induced by 15 minutes of ischemia but after 20 minutes of reperfusion, myocardial ultrastructure was essentially normal except for rare swollen, or disrupted, mitochondria. Thus, the cellular abnormalities associated with brief periods of ischemia persist for variable periods of time after reperfusion of reversibly injured myocytes. First: although adenine nudeotide repletion occurs very slowly, the adenylate charge was restored after 3 minutes, indicating rapid resumption of mitochondrial adenosine triphosphate production. Second: caldum overload was not detected, but myocardial edema and increased potassium persisted throughout the 20 minutes of reperfusion. Third: the ultrastructural consequences of ischemia were nearly reversed after 20 minutes of reperfusion.

Animal models for protecting ischemic myocardium: results of the NHLBI Cooperative Study. Comparison of unconscious and conscious dog models.

The Animal Models for Protecting Ischemic Myocardium Study was undertaken for the purpose of developing reproducible animal models that could be used to assess interventions designed to limit infarct size. This paper describes the results obtained in an unconscious dog model and in a conscious dog model, developed in three participating laboratories. The unconscious dog model, involving reperfusion after 3 hours of ischemia in open-chest dogs, was intended to determine whether therapy followed by early reperfusion would limit infarct size more than reperfusion alone. The conscious dog model used chronically instrumented dogs and permanent coronary occlusion to better mimic myocardial infarction in man. In both models, the proximal circumflex artery was occluded, and the primary experimental endpoint was infarct size, as measured by histological techniques 3 days after the initial occlusion. Infarct size was analyzed in relation to baseline variables including the anatomic area at risk, collateral blood flow to the subepicardial zone of ischemia and hemodynamic determinants of myocardial metabolic demand. Most of the variation in infarct size in control dogs could be related to variation in the area at risk, collateral blood flow, and rate pressure product. Using multivariate analysis and groups of 15 dogs, an intervention that limited infarct size by 10–13% of the area at risk would have been detected 50% of the time. Larger treatment effects would be detected more readily, and smaller effects often would be missed, unless group sizes were larger. Two drugs, verapamil and ibuprofen, were evaluated in both models, with experimental group sizes averaging 13 and 20 dogs, in the unconscious and conscious models, respectively. Three of 15 verapamil-treated dogs in the unconscious model study had much smaller infarcts than expected from baseline parameters. With these exceptions, neither drug limited infarct size in either model.

Prolonged Depletion of ATP and of the Adenine Nucleotide Pool due to Delayed Resynthesis of Adenine Nucleotides following Reversible Myocardial Ischemic Injury in Dogs

Abstract After 15 min of severe ischemia induced by circumflex artery occlusion in open-chest dogs, 65% of the ATP and 50% of the total adenine nucleotide (ΣAd) pool is lost from the subendocardial myocardium [ 12 ]. Nevertheless, this injury is reversible if the affected tissue is reperfused with coronary arterial blood. 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 ΣAd. The circumflex artery was occluded for 15 min and reperfused for 20 or 60 min, or 24 or 96 h. Ten seconds prior to excision of the heart, the circumflex artery was reoccluded and the fluorescent dye thioflavine S was injected intravenously in order to identify the ischemic or the reperfused tissue which had been ischemic. The mean ATP after 15 min of ischemia was reduced 62% from 5.42 ± 0.33 to 2.08 ± 0.21 μmol/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 h because of the initial depletion of adenine nucleotides and because minimal salvage or de novo synthesis 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 h. These observations indicate that the marked depression of ATP and adenine nucleotides and the slow recovery of these metabolites occurred in myocardium which nevertheless was reversibly injured in terms of cellular viability.

Evaluation of a QRS scoring system for estimating myocardial infarct size. II. Correlation with quantitative anatomic findings for anterior infarcts

The ability of an independently developed QRS point score to estimate the size of infarcts predominantly within the anterior third of the left ventricular was evaluated by quantitative pathologic-electrocardiographic correlation. The study was limited to 21 patients with a single infarct documented by postmortem examination, for whom an appropriately timed standard 12 lead electrocardiogram was available that did not exhibit signs of left or right ventricular hypertrophy, left or right bundle branch block or anterior or posterior fascicular block. At necropsy the heart was cut into five to seven slices. The location and size of the infarct was quantitated by computer-assisted planimetry of the slices. The electrocardiogram of 19 (90 percent) of the patients exhibited either a Q wave or an R wave of no more than 20 ms in lead V2. The infarct in the two patients without this electrocardiographic finding was small, occupying 2 and 3 percent of the left ventricle, respectively. The percent infarction of the left ventricle correlated with the QRS point score (r=0.80). Thus in patients without complicating factors in the electrocardiogram and with a single infarct, the electrocardiogram provides a marker for infarction in the anterior third of the left ventricle and permits estimation of infarct size.

Total ischemia in dog hearts, in vitro. 1. Comparison of high energy phosphate production, utilization, and depletion, and of adenine nucleotide catabolism in total ischemia in vitro vs. severe ischemia in vivo.

This study was done to compare rates of high energy phosphate (HEP) utilization and depletion, as well as the production and distribution of catabolic products of adenine nucleo tides in dog heart during total ischemia in vitro and severe ischemia in vivo. Both HEP production from anaerobic glycolysis and HEP utilization occurred much more quickly during the first 15 mmtuet of severe ischemia in vivo than in total ischemia in vitro. HEP utilization exceeded production in both types of ischemia and tissue HEP decreased progressively. Much of the creatine phosphate (CP) was lost within the first 1–3 minutes; adenosine triphosphate (ATP) depletion occurred more slowly than CP and more slowly in vitro than in vivo. ATP was reduced from control contents of 5–6 μmol/g to 1.0 μmol/g by 75 minutes of total ischemia in vitro, but reached a similar level within only 30 minutes of severe ischemia in vivo. HEP utilization and production during ischemia were estimated from the rate of accumulation of myocardial lactate and essentially ceased when the ATP reached 0.4 μmol/g wet weight. At this time, more than 80% of the HEP that had been utilized in ischemia in vivo or in total ischemia in vitro had been derived from anaerobic glycolysis. ATP depletion was paralleled by dephosphorylation of adenine nucleottdeg. The lost nucleotides were recovered stoichiometrically as adenosine, inosine, hypoxantbine, and xanthine in both models of ischemia, a finding which demonstrates that the low collateral flow of severe ischemia allows little washout of nucleosides, bases, or lactate to the systemic circulation. These results Indicate that total ischemia in vitro can be used as a model of severe ischemia in vivo in that the pathways of energy production and depletion and of adenine nucleotide degradation generally are similar.

Effect of the calcium antagonist verapamil on necrosis following temporary coronary artery occlusion in dogs.

Calcium metabolism may play an important role in the pathogenesis of myocardial ischemic injury. The effect of the sarcolemmal calcium flux inhibitor, verapamil, on myocardial necrosis was studied in dogs subjected to temporary coronary artery occlusion. One group of dogs was untreated. A second group was given 0.8 mg/kg verapamil intravenously over a 30 min period beginning 10 min prior to coronary occlusion. In a third group, the dose of verapamil was increased until complicated by hypotension or conduction abnormalities. Cardiac necrosis was produced in all dogs by 40 min of left circumflex coronary artery occlusion followed by 2-4 days of reperfusion. At the end of the experiment, animals were sacrificed and necrosis was quantitated histologically in transmural slices through the posterior papillary muscle. Pre-treatment with the lower dose of verapamil resulted in significantly less necrosis (14% treated vs 35% untreated) with minimal hemodynamic consequences. Higher doses of verapamil were even more effective in limiting cardiac necrosis despite the development of hypotension and varying degrees of heart block.