Does infarct size influence loss of embolised 15-micrometer microspheres from ischaemic myocardium?

The relationship between myocardial infarct size and loss of 15-micron microspheres from ischaemic tissue was investigated in anaesthetized cats. Radioactive microspheres were injected in the left atrium before and 5 h after left anterior descending coronary artery occlusion. Left ventricular hypoperfused zone (HZ) averaged 36.6% and infarct size (IS) 31.6%. Thus, 86% of HZ evolved into necrosis. Preocclusion blood flow was lower in ischaemic (1.62 ml/min per g) compared with non-ischaemic myocardium (2.09, p = 0.002), indicating 22% microsphere loss. In ischaemic subendocardium, oedema (3.7%) could account for the apparent loss. In ischaemic subepicardium, oedema was less pronounced and 18% physical sphere loss occurred. Subepicardial loss increased in proportion to IS and IS/HZ ratio (r2 = 0.71; p less than 0.005). Non-entrapment of 15-micron spheres in coronary circulation averaged 0.6%, and preocclusion spheres appeared in coronary sinus blood throughout the ischaemic period. In systemic circulation, non-entrapment during injection of preocclusion spheres was 7.8%, but only 1.8% 5 h later. Release of postocclusion spheres took place during KCl injection. Thus, myocardial ischaemia is associated with alterations in microvascular function allowing release of entrapped 15-micron spheres. Also, the magnitude of microsphere loss per gram tissue is related to infarct size.     

Moderate normovolaemic haemodilution increases regional myocardial blood flow but has no effect on experimental infarct size

Regional myocardial blood flow was measured with radioactive microspheres before and 1, 3, 6, and 12 h after occlusion of the left anterior descending coronary artery (LAD) in anaesthetized cats. In one group of cats, 1 h after LAD occlusion, moderate normovolaemic haemodilution was started with a dextran 40 solution, which reduced the haematocrit by 20-25% throughout the rest of the experiment. In the other group, the haematocrit was kept at the original level. Infarct size was measured planimetrically 12 h after the coronary ligation, using the tetrazolium staining technique for disclosing the ischaemic myocardium. Haemodilution increased blood flow in the non-ischaemic myocardium and in the so-called 'border zone' comprising both normal, and ischaemic tissue. However, in the severely ischaemic myocardium dependent on collateral blood flow, where there was no admixture of non-ischaemic myocardium, the blood perfusion was not significantly affected by the haemodilution. After 12 h of ischaemia the infarct size in the control group was fully comparable with that in the haemodiluted group: 28 +/- 3 and 27 +/- 4 (percentage of the left ventricle, mean +/- SEM), respectively.     

Local myocardial function following coronary occlusion in cats,Effect on non-ischaemic regions

Summary. The mechanical function and perfusion in ischaemic and non-ischaemic myocardium after coronary occlusion was studied in 10 cats using pressure-length loop analysis and radiolabeled microspheres. Measurements in three regions—ischaemic, adjacent normal and remote normal myocardium—all showed different responses to coronary occlusion. In the ischaemic region loop area, segment shortening and tissue flow were markedly reduced. In the adjacent normal region, both loop area and segment shortening as well as flow increased. In the remote normal region, neither loop area, segment shortening nor flow showed consistent changes. End-diastolic segment length increased in all regions, most in the ischaemic region and least in the remote region. The increased end-diastolic segment length in all regions after coronary occlusion indicates activation of the Frank-Starling mechanism as an attempt to maintain stroke volume. However, the end-diasolic segment length did not increase uniformly for all normal myocardium: it depended on the proximity to the ischaemic region. Increased contractile function in the adjacent normal myocardium due to non-uniform distribution of the Frank-Starling effect is the most likely mechanism behind the left ventricle's ability to partially compensate for loss of contractile mass during acute regional ischaemia in anaesthetized cats.     

Renal blood flow during acute ischaemic heart failure in dogs: effects of dopamine and high doses of insulin

The effects of acute ischaemic heart failure on renal blood flow and the influence of dopamine at low dose range and high doses of insulin were examined. Acute left ventricular (LV) failure was induced in dogs by injection of 50-micron plastic microspheres into the left main coronary artery. The dogs showed signs of severely depressed LV function. Cardiac output was decreased to a significantly greater extent than renal blood flow, and while total peripheral resistance was significantly increased, there were no significant changes in renal vascular resistance. The results indicate different sympathetic discharge to the various vascular beds during acute ischaemic heart failure. Dopamine at low dose range and high doses of insulin were found to improve myocardial contractility and to reduce renal vascular resistance and increase renal blood flow.     

Relationship between myocardial adenine nucleotide catabolism and tissue blood flow rate in experimental ischaemia

A method was developed for tissue preservation and evaluation of the adenine nucleotide metabolism in small samples of myocardium after 45 min of ischaemia. Ischaemia was produced by coronary artery occlusion in anaesthetized cats. Adenine nucleotides and their metabolites were measured by isocratic liquid chromatographic systems which allow quantitative analysis of the nucleotides and their metabolites inosine, hypoxanthine and xanthine in biopsies of 5-20 mg tissue. Regional myocardial blood flow was measured in the tissue surrounding the biopsies by the distribution of 15 micron radiolabelled microspheres. In central ischaemic regions the ATP level was approximately 1 mumol/g wet weight, whereas in normally perfused myocardium the ATP level was approximately 5 mumol/g tissue. In tissue with intermediate flow values, intermediate ATP levels were found. Energy charge, which summarizes all adenine nucleotide concentrations, was reduced from 0.88 to 0.50, and the molar concentrations of inosine, hypoxanthine and xanthine increased in ischaemic tissue. We conclude that this method provides reliable characterization of the local cellular energy status in cat hearts with ischaemic regions.     

Effect of practolol on blood flow distribution in the myocardium during acute regional ischaemia in cats

The β-adrenergic blocking agent practolol was given to 11 cats with acute coronary artery ligation, and regional myocardial tissue blood flow was measured by the distribution of 15 μm labelled microspheres. Practolol reduced heart rate and cardiac contractility, but left ventricular end-diastolic pressure rose in eight animals. In three animals, however, the haemodynamics were essentially unchanged and these are referred to as non-responders. No changes in regional myocardial blood flow were observed after practolol administration, either in ischaemic, border or normal areas of the left ventricle. This indicates less serious imbalance between oxygen demand and delivery in ischaemic tissue. There was no endocardial/epicardial redistribution of tissue flow. Practolol did not appear to improve coronary perfusion, and beneficial clinical effects of practolol are therefore probably related to reduction of myocardial oxygen demands.     

Regional myocardial tissue blood flow during β-adrenergic blockade in cat hearts with acute ischaemia

Summary. Selective β₁ or β₂-adrenergic blockade was achieved by practolol or IPS 339, respectively, in cats with acute ligation of a coronary artery. During blockade, heart rate was kept constant by atrial pacing and blood pressure reduction was prevented by aortic clamping. Regional myocardial blood flow was measured by the distribution of 15 μm labelled microspheres. Practolol slightly reduced epicardial blood flow in ischaemic myocardium, while blood flow in border and normally perfused myocardium remained unchanged. Following IPS 339, myocardial tissue flow increased in normally perfused myocardium, on average by 37% in the endocardium and 30% in the epicardium. No changes occurred in the other regions. The flow changes brought about by IPS 339 were unrelated to haemodynamic changes, and the coronary vascular resistance was reduced. These results are indicative of coronary vasodilation related to β₂-adrenergic receptor blockade and was confined to well-oxygenated areas surrounding the acutely ischaemic zone.     

Effects of isoproterenol on adenine nucleotide catabolism in cat hearts with acute regional ischaemia

Previous studies have demonstrated that beta-adrenergic agonists cause extension of ischaemic lesions in the myocardium. In the present study the effects of isoproterenol on adenine nucleotide metabolism were investigated in cat hearts with regional ischaemia following permanent coronary occlusion for 45 min. Adenine nucleotides and their metabolites were measured by high performance liquid chromatography and regional myocardial blood flow was measured by 15-microns radiolabelled microspheres in a total of 255 paired tissue samples. Compared with untreated control cats, ATP and the energy charge were more reduced in ischaemic myocardium of isoproterenol-treated cats. Increased amounts of the degradation products inosine and hypoxanthine/xanthine were also found in these regions. These results could be ascribed to increased cardiac performance caused by isoproterenol, which augments the imbalance between energy production and oxygen supply in ischaemic myocardium.     

Myocardial adenine nucleotide depletion within 1 h of acute coronary artery occlusion.

In anaesthetized open-chest casts with occlusion of the left anterior descending coronary artery (LAD), adenine nucleotides and degradation products were studied in small myocardial tissue samples (10-20 mg) with high-pressure liquid chromatography, and tissue blood flow was measured with radioactive microspheres 5, 10, 20, 40, and 60 min after LAD occlusion. There was a rapid and parallel decrease of myocardial ATP and accumulation of adenosine, inosine, hypoxanthine, and xanthine both in epicardial and endocardial half-layers of the ischaemic myocardium within the first 20 min of coronary occlusion. After 40 and 60 min, myocardial ATP content decreased and degradation products accumulated further in the endocardium but stabilized epicardially. Analysis of covariance showed that the slightly higher blood flow in ischaemic epicardial layers, did not explain the transmural difference in ATP content after 40 and 60 min. Adenosine decreased after 40 min of ischaemia in both wall layers reaching negligible amounts after 60 min. It is concluded that breakdown of energy stores is less severe in epicardial than in endocardial wall layers during the first hour after acute coronary occlusion in the cat heart. This transmural difference cannot be explained entirely by less severe epicardial ischaemia. Therefore, transmural heterogeneity in metabolic function during severe ischaemia may also be important.     

Does α1-adrenergic blockade influence regional blood flow regulation in hearts with coronary artery occlusion?

Summary. The effects of selective α₁-adrenergic blockade with doxazosin on regional myocardial tissue blood flow was studied in anaesthetized cats with acute coronary artery occlusion. Reflex tachycardia was prevented by selective β₁-adrenergic blockade with atenolol and coronary perfusion pressure was kept constant by partial stenosis of the descending aorta. Administration of atenolol reduced cardiac mechanical work-load by its negative inotropic and chronotropic effects, and reduced myocardial tissue blood flow in normally perfused myocardium. This reduction was most pronounced in the endocardial half-layer of the myocardium adjacent to the ischaemic region. Administration of doxazosin in this situation clearly reduced peak systolic and coronary perfusion pressure. But when coronary perfusion pressure was raised to pre-administration values, measurements of regional blood flow revealed no changes either in ischaemic or non-ischaemic myocardium. Also, there was no sign of redistribution of blood flow between endocardial and epicardial tissue in any area. This study, therefore, indicates that α₁-adrenoceptors play a minor role in the regulation of coronary blood flow in normal myocardium as well as ischaemic myocardium.     

Effect of hyaluronidase on substrate exchange and blood flow in the ischaemic myocardium of the dog

Summary. The mechanism for reduced myocardial ischaemic injury by hyaluronidase was studied in open chest anaesthetized dogs. Repeated coronary artery occlusions were performed and the effect of hyaluronidase (225 NF units per kg) was studied during infusion of noradrenaline 0·125 mg/kg · min. Ischaemic injury was measured as the sum of ST-segment elevations (S?ST) at 10–15 sites. Regional myocardial blood flow was determined by tracer microspheres. Blood for metabolic studies was sampled from a local coronary vein draining ischaemic tissue and from the coronary sinus draining predominantly non-ischaemic tissue. Hyaluronidase reduced S?ST and increased subepicardial and transmural blood flow in ischaemic myocardium, but flow was not significantly changed in the ischaemic subendocardium or in non-ischaemic myocardium. Hyaluronidase had no significant effect on arterio-local venous differences of oxygen, glucose, lactate or free fatty acids across the ischaemic myocardium. In conclusion, reduction of myocardial ischaemic injury by hyaluronidase can be explained by increased collateral blood flow and not by an effect on fluxes of substrates across the ischaemic myocardium.     

Effects of verapamil on intracellular lipid accumulation in cat hearts with 3 h of regional-ischaemia

In the regionally ischaemic heart lipid droplet accumulation is found in the ischaemic area and is most pronounced in the periphery. The purpose of the present study is to explore the potential effects of the calcium-channel-blocker verapamil on this accumulation. The drug is known to reduce the intensity of myocardial ischaemic injury. The myocardial ultrastructure was studied in anaesthetized open chest cats with 3 h of coronary artery occlusion. Biopsies were taken from the ischaemic, border and normally perfused myocardium defined in vivo injections of fluorescein and verified by blood flow measurements using radiolabelled microspheres. Arterial concentration of non esterified fatty acids (NEFA) was measured during the ischaemic period. A higher accumulation of lipid droplets was found in the central ischaemic myocardium of verapamil-treated cats than in control animals (p less than 0.05). The normally perfused and borderline areas were unaffected by verapamil as far as lipid accumulation was concerned and showed the same pattern as in the untreated group. The increased accumulation of lipid droplets in the ischaemic myocardium, after treatment with verapamil, may reflect a preserved metabolic activity in the ischaemic tissue or result from a higher supply of fatty acids due to increased perfusion of the central ischaemic tissue.     

The ultrastructure of the myocyte in different regions of experimental infarcts in the cat heart

The ultrastructure of myocytes was studied in the left ventricular myocardium of the cat heart after 3 h of LAD ligation. The ischemic, borderline, and normally perfused myocardium was defined by in vivo injection of fluorescein and by regional myocardial blood flow measurements with 15.5 microns radio-labeled microspheres. A semiquantitation of the number of irreversible injured cells in per cent of total counted in the three different zones showed that 53%-63% were irreversibly injured in the ischemic zone, 7%-26% in the borderline area, while none were irreversibly injured in the normally perfused myocardium. The interior excitation-contraction couplings in the normally perfused myocardium comprise interior dyads, triads, reversed triads, and encircling couplings. While the couplings in general were structurally resistant to ischemia, injured interior couplings were apparent in severely damaged cells of the hypoperfused tissue. Such injuries comprise a widening of the junctional gap and a disintegration of the junctional processes.     

Deleterious influence of hypothyroidism on evolving myocardial infarction in conscious dogs.

To study the influence of hypometabolism on evolving myocardial infarction in a model with intact autoregulation, we investigated 53 awake dogs after coronary artery occlusion. Severe hypothyroidism was induced by the intravenous administration of 131I. Animals were instrumented to obtain hemodynamic measurements, and regional myocardial blood flow was measured with radioactive microspheres. Infarct size was determined by the creatine kinase depletion method, and dysrhythmia analysis was performed from 24-h Holter monitor tapes in animals matched for infarct size. The microarchitecture of hypothyroid myocardium was determined by the electron microscope. Before coronary occlusion, mean systemic pressure in hypothyroid dogs was reduced by 14% and cardiac output reduced by 32%, with no change in left ventricular end-diastolic pressure, first derivative of left ventricular pressure rise, (dP/dt), or heart rate. After coronary occlusion, there was deterioration in hemodynamic measurements in both groups, with lower absolute levels of mean systemic blood pressure and cardiac output obtained in hypothyroid dogs. Hypothyroidism was detrimental to evolving infarction with a 36% increase in infarct size present in hypothyroid dogs (30 +/- 2%) compared to euthyroid controls (22 +/- 3%), P less than 0.05. Dysrhythmias were more severe in hypothyroid dogs. There were no changes in the relationship between regional myocardial blood flow and the extent of infarction after coronary occlusion. Abnormalities in microarchitecture were present in hypothyroid dog myocardium. Severe hypometabolism in this model was associated with alterations in hemodynamics, more severe dysrhythmias and changes in microarchitecture. The combined effect of these alterations resulted in an overall detrimental influence of hypothyroidism on evolving myocardial necrosis in this model.     

Regional perfusion in hearts with acute coronary artery occlusion and subsequent β2- and α1-adrenergic blockade

Summary. Blockade of cardiac adrenoceptor subtypes, coronary or myocardial, might elicit compensatory interaction from remaining unblocked subtypes. An attempt to explore this interplay was made by studying regional myocardial blood flow alterations associated with β₂-adrenergic blockade followed by β₁-adrenergic blockade in anaesthetized cats with acute coronary occlusion. In order to maintain constant needs for perfusion, atrial pacing was established and the aortic blood pressure was kept constant. In myocardium remote from the ischaemic region, β₂-adrenergic blockade produced higher endocardial blood flow whereas no flow changes were observed close to the ischaemic region. With subsequent β₁-adrenergic blockade, blood flow increased endocardially in non-ischaemic regions, but remained unchanged in epicardial tissue. Control experiments without coronary ligation revealed no increase in left ventricular oxygen consumption during the experiments and support the theory that the observed blood flow increase in the coronary ligation group, following drug interventions, was not caused by increased cardiac work. This study indicates that combined β₂- and α₁-adrenergic blockade alters the balance between receptor subtypes. Unopposed β₁-mediated vasodilation is the most likely candidate to explain why endocardial flow was increased.     

Role of asynchronous activation of the ventricular fibres by an ectopic pacemaker in the accidents, especially fibrillation, caused by Ic antiarrhythmic drugs

Class Ic antiarrhythmic drugs, which are known to slow down conduction in the ventricular muscle, are likely to impair synchrony in activity of the ventricular fibres. Asynchronous activation was first investigated between an ischaemic and a normal area by the simultaneous recording in anaesthetized, open-chest pigs of two left ventricular monophasic action potentials (MAPs) under ventricular pacing at a high rate of 180 beats.min-1. Asynchronous activation was then investigated in the intact myocardium according to the distance separating the recording from the pacing electrode. Furthermore, mechanical effects of left ventricular systole were observed by recording dP/dt(max) and mean arterial blood pressure during the pacing periods. Ischaemia was produced by transient complete occlusion of the left anterior descending coronary artery near its origin; as a result, activation time reached 85 ms in the ischaemic area under flecainide administered iv in a 2.5 mg.kg-1 dose instead of approximately 60 ms in the normal area for fibres equi-distant from the pacing electrode. Similar delays in activation were observed in the intact myocardium, depending on whether the explored region was close to or far from the pacing electrode. In the latter case, the difference in activation time may become markedly greater if the distance or the dose of flecainide are increased. This difference, which possibly exceeds one-third of the MAP duration (practically unchanged by flecainide), may account for the occurrence of fibrillation or the sudden loss of systole mechanical efficacy.     

Cardiac function in open-chest dogs after left to right ventricular shunting and right coronary artery occlusion

Summary. Patients with acquired ventricular septal defect (VSD) after myocardial infarction have a particularly bad prognosis if right ventricular function is severely impaired. The significance of an ischaemic right ventricular free wall on cardiac function during interventricular shunting was examined in open-chest dogs. An external interventricular shunt could be opened and closed at will, and by occlusion of the right coronary artery (RCA), a part of the right ventricular free wall was rendered ischaemic. Aortic flow decreased by 8±2% when the shunt was opened in the presence of a normal right ventricle, and by 16±2% (difference: P<0.05) in the presence of right ventricular ischaemia. Aortic flow fell by 19±3% when the RCA was occluded. Right ventricular dyskinesia was demonstrated after occlusion of RCA, by recording segment lengths in the right ventricular free wall. The dyskinesia was aggravated when the shunt was opened. The left ventricle exerted a ‘negative’ work on the ischaemic right ventricular free wall. Retention of blood in the right ventricle, with a subsequent decline in left ventricular filling and an almost unchanged interventricular shunting of blood, explain why aortic flow fell more when the shunt was opened in the presence of right ventricular ischaemia.     

Effects of nitroglycerin on transmural myocardial blood flow in the unanesthetized dog.

This study was designed to determin the effect of nitroglycerin upon transmural distribution of myocardial blood flow in the awake dog during normal conditions and in the presence of ischemia-induced coronary vasodilation. Studies were performed in chronically prepared dogs with electromagnetic flowmeters and hydraulic occluders on the left circumflex coronary artery. Regional myocardial blood flow was estimated by using radionuclide-labeled microspheres, 7-10 mum in diameter, injected into the left atrium. During control conditions endocardial flow (0.86 plus or minus SEM 0.05 ml/min per g) slightly exceeded epicardial flow (0.72 plus or minus 0.03 ml/min per g, P smaller than 0.05), and this distribution of flow was not significantly altered by nitroglycerin. After a 5-s coronary artery occlusion, reactive hyperemia occurred with excess inflow of arterial blood effecting 360 plus or minus 15% repayment of the blood flow debt incurred during occlusion. When arterial inflow was limited to the preocclusion rate during coronary vasodilation after a 5-s total coronary artery occlusion, flow to the subepicardial myocardium was increased at the expense of underperfusion of the subendocardial myocardium, and the delayed reactive hyperemia was markedly augmented (mean blood flow debt repayment =775plus or minus 105%, P smaller than 0.01). Tese data suggested that subendocardial underperfusion during the interval of coronary vasodilation in the presence of a flow-limiting proximal coronary artery stenosis caused continuing subendocardial ischemia which resulted in augmentation of the reactive hyperemic response. In this experimental model both the redistribution of myocardial blood flow which occurred during an interval of restricted arterial inflow after a 5-s coronary artery occlusion and augmentation of the subsequent reactive hyperemic response were returned toward normal by nitroglycerin. This effect of nitroglycerin may have resulted, at least in part, from its ability to vasodilate the penetrating arteries which deliver blood from the epicardial surface to the subendocardium.     

Delayed cardioprotective effects of exercise in dogs are aminoguanidine sensitive: possible involvement of nitric oxide

Dogs were subjected to exercise on a treadmill, using a protocol in which the speed and slope were increased every 3 min, and which elevated both heart rate (to a mean of 198+/-14 beats.min(-1)) and mean arterial blood pressure (to 150+/-4 mmHg). Then, 24 or 48 h later, the dogs were anaesthetized with a mixture of alpha-chloralose and urethane and subjected to a 25 min occlusion of the left anterior descending coronary artery. The control dogs (instrumented but not exercised) were subjected to the same procedure. In some dogs the nitric oxide synthase inhibitor aminoguanidine (50 mg.kg(-1); intravenous) was administered 30 min before occlusion. Baroreflex sensitivity (BRS) was determined by the rapid bolus injection of phenylephrine 60 min before, and again 3 min after, the onset of occlusion. Exercise markedly reduced the consequences of coronary artery occlusion 24 h (but not 48 h) later, without modifying myocardial tissue blood flow. In the exercised dogs there were reductions in arrhythmia severity [ventricular fibrillation (VF) during occlusion, 0%; survival from the combined ischaemia/reperfusion insult, 70%] compared with controls (VF during occlusion, 36%; survival, 9%). BRS was preserved during occlusion in the exercised dogs (before occlusion, 1.60+/-0.54 ms.mmHg(-1); 3 min after occlusion, 1.37+/-0.4 ms.mmHg(-1)), but not in controls (before occlusion, 1.28+/-0.29 ms.mmHg(-1); 3 min after occlusion, 0.45+/-0.12 ms.mmHg(-1); P<0.05), and other ischaemic changes (inhomogeneity of electrical activation and changes in the ST-segment, recorded over the ischaemic region) were also less marked in the exercised dogs. Exercise-induced cardioprotection was abolished by aminoguanidine (VF during occlusion, 25%; survival, 0%). The results show that even a single period of exercise affords delayed protection against ischaemia/reperfusion-induced VF and other ischaemic changes. Since this protection is abolished by aminoguanidine, and since (inducible) NO synthase activity was elevated 3-fold in left ventricular samples 24 h after exercise, we suggest that this protection is mediated by nitric oxide.     

Distribution of coronary blood flow during acute coronary occlusion in dogs. Effect of nicotinic acid and sodium salicylate

The effects of the antilipolytic agents nicotinic acid (NA) and sodium salicylate (SS) on the distribution of coronary blood flow during acute myocardial ischaemia were studied in open chest dogs. Fifteen min following experimental coronary artery occlusion, blood flow in the ischaemic myocardium was on average 28% of flow in the non-ischaemic myocardium. The reduction in blood flow in the ischaemic mycardium was more pronounced in the endocardial than in epicardial halves of the myocardium. No significant change in blood flow was observed after administration of NA or SS in either the ischemic or nonischemic part of the myocardium. Both drugs reduced the extent of myocardial ischaemic injury as shown by reduced epicardial ST-segment elevations. Arterial concentrations of fatty acids were lowered by NA or SS, whereas the mechanical activity of the heart remained unchanged. It is concluded that the reduction of acute myocardial ischaemic injury effected by NA or SS is not due to changes in myocardial blood flow, but more likely to lower myocardial oxygen demand related to reduced fatty acid utilization.