Two and three dimensional display of myocardial ischemic "border zone" in dogs

Anisotropic shortening in the wavelength of electrical waves promotes onset of electrical turbulence in cardiac tissue: An in silico study

Several pathological conditions introduce spatial variations in the electrical properties of cardiac tissue. These variations occur as localized or distributed gradients in ion-channel functionality over extended tissue media. Electrical waves, propagating through such affected tissue, demonstrate distortions, depending on the nature of the ionic gradient in the diseased substrate. If the degree of distortion is large, reentrant activity may develop, in the form of rotating spiral (2d) and scroll (3d) waves of electrical activity. These reentrant waves are associated with the occurrence of lethal cardiac rhythm disorders, known as arrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF), which are believed to be common precursors of sudden cardiac arrest. By using state-of-the-art mathematical models for generic, and ionically-realistic (human) cardiac tissue, we study the detrimental effects of these ionic gradients on electrical wave propagation. We propose a possible mechanism for the development of instabilities in reentrant wave patterns, in the presence of ionic gradients in cardiac tissue, which may explain how one type of arrhythmia (VT) can degenerate into another (VF). Our proposed mechanism entails anisotropic reduction in the wavelength of the excitation waves because of anisotropic variation in its electrical properties, in particular the action potential duration (APD). We find that the variation in the APD, which we induce by varying ion-channel conductances, imposes a spatial variation in the spiral- or scroll-wave frequency ω. Such gradients in ω induce anisotropic shortening of wavelength of the spiral or scroll arms and eventually leads to instabilitites.

Reentry via high-frequency pacing in a mathematical model for human-ventricular cardiac tissue with a localized fibrotic region

Localized heterogeneities, caused by the regional proliferation of fibroblasts, occur in mammalian hearts because of diseases like myocardial infarction. Such fibroblast clumps can become sources of pathological reentrant activities, e.g., spiral or scroll waves of electrical activation in cardiac tissue. The occurrence of reentry in cardiac tissue with heterogeneities, such as fibroblast clumps, can depend on the frequency at which the medium is paced. Therefore, it is important to study the reentry-initiating potential of such fibroblast clumps at different frequencies of pacing. We investigate the arrhythmogenic effects of fibroblast clumps at high- and low-frequency pacing. We find that reentrant waves are induced in the medium more prominently at high-frequency pacing than with low-frequency pacing. We also study the other factors that affect the potential of fibroblast clumps to induce reentry in cardiac tissue. In particular, we show that the ability of a fibroblast clump to induce reentry depends on the size of the clump, the distribution and percentage of fibroblasts in the clump, and the excitability of the medium. We study the process of reentry in two-dimensional and a three-dimensional mathematical models for cardiac tissue.

3D IMAGING OF THE MITOCHONDRIAL REDOX STATE OF RAT HEARTS UNDER NORMAL AND FASTING CONDITIONS

The heart requires continuous ATP availability that is generated in the mitochondria. Although studies using the cell culture and perfused organ models have been carried out to investigate the biochemistry in the mitochondria in response to a change in substrate supply, mitochondrial bioenergetics of heart under normal feed or fasting conditions has not been studied at the tissue level with a sub-millimeter spatial resolution either in vivo or ex vivo. Oxidation of many food-derived metabolites to generate ATP in the mitochondria is realized through the NADH/NAD+ couple acting as a central electron carrier. We employed the Chance redox scanner - the low-temperature fluorescence scanner to image the three-dimensional (3D) spatial distribution of the mitochondrial redox states in heart tissues of rats under normal feeding or an overnight starvation for 14.5 h. Multiple consecutive sections of each heart were imaged to map three redox indices, i.e., NADH, oxidized flavoproteins (Fp, including flavin adenine dinucleotide (FAD)) and the redox ratio NADH/Fp. The imaging results revealed the micro-heterogeneity and the spatial distribution of these redox indices. The quantitative analysis showed that in the fasted hearts the standard deviation of both NADH and Fp, i.e., SD_NADH and SD_Fp, significantly decreased with a p value of 0.032 and 0.045, respectively, indicating that the hearts become relatively more homogeneous after fasting. The fasted hearts contained 28.6% less NADH (p = 0.038). No significant change in Fp was found (p = 0.4). The NADH/Fp ratio decreased with a marginal p value (0.076). The decreased NADH in the fasted hearts is consistent with the cardiac cells' reliance of fatty acids consumption for energy metabolism when glucose becomes scarce. The experimental observation of NADH decrease induced by dietary restriction in the heart at tissue level has not been reported to our best knowledge. The Chance redox scanner demonstrated the feasibility of 3D imaging of the mitochondrial redox state in the heart and provides a useful tool to study heart metabolism and function under normal, dietary-change and pathological conditions at tissue level.

Regulation of ion gradients across myocardial ischemic border zones: a biophysical modelling analysis

The myocardial ischemic border zone is associated with the initiation and sustenance of arrhythmias. The profile of ionic concentrations across the border zone play a significant role in determining cellular electrophysiology and conductivity, yet their spatial-temporal evolution and regulation are not well understood. To investigate the changes in ion concentrations that regulate cellular electrophysiology, a mathematical model of ion movement in the intra and extracellular space in the presence of ionic, potential and material property heterogeneities was developed. The model simulates the spatial and temporal evolution of concentrations of potassium, sodium, chloride, calcium, hydrogen and bicarbonate ions and carbon dioxide across an ischemic border zone. Ischemia was simulated by sodium-potassium pump inhibition, potassium channel activation and respiratory and metabolic acidosis. The model predicted significant disparities in the width of the border zone for each ionic species, with intracellular sodium and extracellular potassium having discordant gradients, facilitating multiple gradients in cellular properties across the border zone. Extracellular potassium was found to have the largest border zone and this was attributed to the voltage dependence of the potassium channels. The model also predicted the efflux of [Formula: see text] from the ischemic region due to electrogenic drift and diffusion within the intra and extracellular space, respectively, which contributed to [Formula: see text] depletion in the ischemic region.

Mechanisms for initiation of reentry in acute regional ischemia phase 1B

BACKGROUND

During phase 1B of acute regional ischemia, the subepicardial and subendocardial layers coupled to the inexcitable midmyocardium remain viable.

OBJECTIVE

The purpose of this study was to examine how the degree of hyperkalemia in the surviving layers, the lateral width of border zone between the normal tissue and the central ischemic zone, and the degree of cellular uncoupling between the surviving layers and the midmyocardium contribute to initiation of reentry.

METHODS

Simulations were conducted on the state-of-the-art model of rabbit ventricles with realistic representation of the spatial distribution of the ischemic insult.

RESULTS

Hyperkalemia in the surviving layers led to induction of reentry by increasing refractoriness and slowing conduction in the layers. Such reentries were formed solely in the subepicardium. A minimal level of hyperkalemia was required for induction of reentry. Progress increase in hyperkalemia led to a biphasic change in vulnerability to reentry. For each level of hyperkalemia, increased cellular uncoupling between subepicardium and midmyocardium increased inducibility of reentry by restoring subepicardial tissue excitability via blocking midmyocardial electrotonic effect. In addition, increased lateral width of the border zone prevented inducibility of reentry as conduction block occurred in the central ischemic zone when the wave propagated across the border zone from the normal zone.

CONCLUSION

The degree of hyperkalemia in the surviving subepicardium, the lateral width of border zone, and cellular uncoupling between the subepicardium and midmyocardium determine dispersion of refractoriness, conduction velocity, excitability, and, therefore, inducibility of reentry during phase 1B.

Mechanistic investigation into the arrhythmogenic role of transmural heterogeneities in regional ischaemia phase 1A

AIMS

Studies of arrhythmogenesis during ischemia have focused primarily on reentrant mechanisms manifested on the epicardial surface. The goal of this study was to use a physiologically-accurate model of acute regional ischemia phase 1A to determine the contribution of ischaemia-induced transmural electrophysiological heterogeneities to arrhythmogenesis following left anterior descending artery occlusion.

METHODS AND RESULTS

A slice through a geometrical model of the rabbit ventricles was extracted and a model of regional ischaemia developed. The model included a central ischaemic zone incorporating transmural gradients of I(K(ATP)) activation and [K+]o, surrounded by ischaemic border zones (BZs), with the degree of ischaemic effects varied to represent progression of ischaemia 2-10 min post-occlusion. Premature stimulation was applied over a range of coupling intervals to induce re-entry. The presence of ischaemic BZs and a transmural gradient in I(K(ATP)) activation provided the substrate for re-entrant arrhythmias. Increased dispersion of refractoriness and conduction velocity in the BZs with time post-occlusion led to a progressive increase in arrhythmogenesis. In the absence of a transmural gradient of I(K(ATP)) activation, re-entry was rarely sustained.

CONCLUSION

Knowledge of the mechanism by which specific electrophysiological heterogeneities underlie arrhythmogenesis during acute ischaemia could be useful in developing preventative treatments for patients at risk of coronary vascular disease.

Fluorescence spectroscopy and imaging of myocardial apoptosis

Fluorometry is used to detect intrinsic flavoprotein (FP) and nicotinamide adenine dinucleotide (NADH) signals in an open-chest rabbit model of myocardial ischemia-reperfusion injury. Myocyte apoptosis has been shown clinically to contribute to infarct size following reperfusion of ischemic myocardium. A noninvasive means of assessing apoptosis in this setting would aid in the treatment of subsequent ventricular remodeling. We show that in vivo fluorometry can be useful in apoptosis detection in open-chest surgeries. Specific changes in myocardial redox states have been shown to indicate the presence of apoptosis. Two main mitochondrial intrinsic fluorophores, NADH and FP signals, were measured during normoxia, ischemia, and reperfusion experimental protocol. Ischemia was induced by occlusion of the largest branch of the circumflex coronary artery and fluorescence signals are collected by applying two different fluorescence techniques: in vivo fluorometry and postmortem cryoimaging. The first technique was employed to detect FP and NADH signals in vivo and the latter technique uses freeze trapping and low-temperature fluorescence imaging. The heart is snap frozen while still in the chest cavity to make a "snapshot" of the metabolic state of the tissue. After freezing, the ischemic area and its surrounding border zone were excised and the sample was embedded in a frozen buffer for cryoscanning. These two data sets, in vivo fluorometry and low-temperature redox scanning, show consistent extreme oxidation of the mitochondrial redox states (higher redox ratio) suggesting the initiation of apoptosis following reperfusion. This represents the first attempt to assess myocyte apoptosis in the beating heart.

Simultaneous fluorometry and phosphorometry of Langendorff perfused rat heart: ex vivo animal studies

Fluorescence imaging of intrinsic fluorophores of tissue is a powerful method to assess metabolic changes at the cellular and intracellular levels. At the same time, exogenous phosphorescent probes can be used to accurately measure intravascular tissue oxygenation. Heart failure is the leading cause of death in America. A rat heart can potentially model the human heart to study failures or other abnormalities optically. We report simultaneous fluorescence and phosphorescence measurements performed on a rat heart. We have used two different optical systems to acquire fluorescence signals of flavoprotein and nicotinamide adenine dinucleotide--the two intrinsic fluorophores of mitochondria--and the phosphorescence signal of an intravascular oxygen probe to extract intracellular and intravascular metabolism loads, respectively.

Regional cardiac tissue oxygenation as a function of blood flow and pO2: A near-infrared spectroscopic imaging study

Near-infrared spectroscopic imaging (NIRSI) is useful to assess cardiac tissue oxygenation in arrested and beating hearts, and it shows potential as an intraoperative gauge of the effectiveness of bypass grafting. The purpose of this study was to determine whether NIRSI can reliably differentiate among a range of cardiac oxygenation states, using ischemia and hypoxia models independently. An ischemia-reperfusion model was applied to isolated, beating, blood-perfused porcine hearts, in which the left anterior descending (LAD) artery was cannulated. LAD flow was decreased stepwise to approximately 50, 20, and 0% of normal flow and was completely restored between ischemic episodes. Upon completion of the ischemia-reperfusion protocol, the hearts were further subjected to periods of increasingly severe global hypoxia. Regional oxy- and deoxy-hemoglobin (myoglobin) levels were derived from spectroscopic images (650 to 1050 nm) acquired at each step. Oxygenation maps vividly highlighted the area at risk for all degrees of ischemia. Oxygenation values differed significantly for different LAD flow rates, regardless of whether intermediate reperfusion was applied, and oxygenation values during progressive hypoxia correlated well with blood oxygen saturation. These results suggest that NIRSI is well suited, not only to identify ischemic or hypoxic regions of cardiac tissue, but also to assess the severity of deoxygenation.

Regional myocardial mechanics: integrative computational models of flow-function relations

Many cardiac disorders result in regionally altered myocardial mechanics. Although myocardial strain distributions can be measured experimentally and clinically, regional wall stresses must be computed from computational models. Combining these approaches can provide insight into the structural basis of regional dysfunction under conditions such as acute myocardial infarction and ischemia-reperfusion. Recently, 3-dimensional computational models have helped to elucidate the structural basis of the functional border zone adjacent to acutely ischemic myocardium. They have also shown that heterogeneous dysfunction in ischemic-reperfused stunned myocardium does not necessarily imply heterogeneous myofilament injury. Now that computational models are able to reproduce many complex features of the 3-dimensional patterns of regional myocardial deformation observed experimentally, we suggest possible roles for such integrative models in clinical diagnosis.

Laser induced fluorescence attenuation spectroscopy: detection of hypoxia

The development of a new laser-induced fluorescence (LIF) spectroscopy technique for the measurement of the attenuation spectrum of tissue is described. The technique, termed laser-induced fluorescence attenuation spectroscopy (LIFAS), has been applied to study the effects of hypoxia on the in vivo optical properties of renal and myocardial tissue in the 350-600-nm band. Excimer laser (Xe-Cl) is used to excite a small volume of the tissue (rabbit model, N = 20) and induce autofluorescence. The emitted LIF is monitored fiberoptically at two locations that are unevenly displaced about the fluorescing volume. The optical attenuation of the tissue is calculated from the dual LIF measurements by assuming an exponential decay of the fluorescence with distance. The results indicate that hypoxia modulates the attenuation spectrum leading to characteristic changes in its shape. Primarily, the spectral profile becomes more concave between 455 nm and 505 nm and two spectral peaks at about 540 and 580 nm disappear leaving in their place a single peak at about 555 nm. The attenuation spectra of normoxic and hypoxic tissue are used to train partial least squares multivariate model for spectral classification. The model detected acute renal and myocardial hypoxia with an accuracy greater than 90% (range: 90%-96%) and 74% (range: 74%-90%), respectively.

Beneficial effect of amrinone on the size of acute regional ischemia in isolated rabbit hearts

The effect of inotropes on myocardial ischemia is difficult to predict because they may influence the determinants of myocardial O2 demand and O2 supply differently. Several PDE-inhibitors have been reported to possess antiischemic properties related to their hemodynamic and O2-sparing effects. To assess whether PDE-inhibitors also possess direct cardioprotective properties, the effects of amrinone (2.5 x 10(-5) mol/L) in comparison to isoproterenol (5 x 10(-9) mol/L) and ouabain (1.5 x 10(-7) mol/L) were studied in isolated rabbit hearts perfused according to Langendorff at a constant pressure (70 cmH2O) and electrically driven at a constant pacing rate. Regional ischemia was induced by coronary artery ligation and quantified by epicardial NADH fluorescence. All substances significantly increased the actively developed left ventricular pressure to a similar extent (+20%) (P < 0.05). Coronary flow was significantly decreased by ouabain (-15%) and significantly increased by isoproterenol (+25%) and particularly by amrinone (+50%) (P < 0.05). Neither ouabain nor isoproterenol significantly changed the intensity or the distribution pattern of NADH fluorescence, whereas the size of the ischemic zone was significantly reduced by amrinone (-25%) (P < 0.05). The PDE-inhibitor amrinone was shown to possess a direct cardioprotective effect by improving myocardial perfusion and O2 supply in isolated rabbit hearts.

Demonstration of functional border zone with myocardial contrast echocardiography in human hearts. Simultaneous analysis of myocardial perfusion and wall motion abnormalities

BACKGROUND

Although the presence of a functional border zone (FBZ), defined as the nonischemic but asynergic myocardium adjacent to the ischemic area, has been demonstrated in animal hearts, it is not known whether this zone exists in humans.

METHODS AND RESULTS

Myocardial contrast echocardiography (MCE) was performed before and during balloon inflation in the area of coronary stenosis by injecting contrast medium through the guiding catheter in 13 patients with effort angina who underwent successful coronary angioplasty. The area showing MCE defect during balloon inflation was determined with reference to the preangioplasty MCE and was regarded as an ischemic area. The size of the FBZ was assessed by measuring the length of the endocardium that showed asynergy in the echo-enhanced (nonischemic) area. The FBZ measured was 13 +/- 4 mm in the short-axis view (n = 5) and 16 +/- 9 mm in the long-axis view (n = 8).

CONCLUSIONS

Nonischemic contractile dysfunction exists even in human hearts. The presence of an FBZ may limit the use of wall motion analysis in assessing the risk or ischemic area in patients with myocardial infarction. MCE appears to be a unique technique for assessing the risk or ischemic area.

Monitoring myocardial reperfusion injury with NADH fluorometry

Using NADH fluorometry to monitor myocardial metabolism, the mechanism of reperfusion injury was investigated after the delivery of an experimental reperfusate. Using an isolated working heart preparation, rat hearts underwent 15 min of global ischemia at 37 degrees C. Following the ischemic insult, an oxygenated enriched reperfusion solution was given for 5 min. The hearts were then returned to a working state and aortic flow recorded to evaluate recovery. NADH levels were monitored throughout the experiment with a fluorometer and glycogen, AMP, ADP, and ATP were measured biochemically pre- and postischemia, after reperfusion and after recovery. In this study, reperfusion injury was best abated by an enriched reperfusate. Our results indicate the mechanism for this amelioration is not high-energy phosphate replenishment. Rather, as indicated by NADH fluorescence, the hearts attain an intermediate level of metabolism that permits glycogen to be restored and functional recovery to be improved.

Discrepancies between myocardial blood flow and fiber shortening in the ischemic border zone as assessed with video mapping of epicardial deformation

Myocardial function around the border of ischemia was investigated in eight open-chest dogs using video mapping of epicardial deformation. With this method, 40-60 white markers attached to the left ventricular epicardium were traced in time automatically. Before and 5-10 min after coronary artery occlusion, blood flow and epicardial deformation were determined in 30-40 regions with a spatial resolution of about 5 mm. Epicardial deformation was expressed as subepicardial fiber shortening and surface area decrease during the ejection phase. The latter indicates local contribution to stroke volume. The absolute values of these variables were normalized relative to the central ischemic (= 0%) and remote non-ischemic area (= 100%). The 50% contour line of a variable was defined as its border. The average distance between the borders of perfusion and function was not significantly different from zero, due to considerable variation in this distance both within one heart (+/- 5.7 mm) and between mean distances for different hearts (+/- 4.4 mm). The width of the transition zone (distance between the 20% and 80% contour lines) of surface area decrease and subepicardial fiber shortening was significantly larger (20.5 and 15.0 mm, respectively) than those of transmural and subepicardial blood flow (8.5 and 9.5 mm, respectively). The present results demonstrate that in a 20-mm zone around the border of ischemia, major discrepancies are present between perfusion and deformation.

Cardiac ischemia and endothelial function in the isolated rabbit heart

Truly effective prevention of reperfusion myocardial damage is precluded in part by a lack of understanding of the earliest events which accompany ischemia. The purpose of this study was to assess the coronary endothelial response to two forms of ischemic injury in an isolated crystalloid perfused rabbit heart. Global cardiac ischemia, confirmed by NADH fluorescence photography, was induced either by mechanically reducing coronary flow by 90% (MRCF, N = 11) or by an infusion of N-formyl-methionyl-leucyl-phenylalanine (fMLP, N = 11), a known stimulus for leukotriene synthesis and coronary vasospasm. Compared with control, MRCF resulted in an increase in effluent concentrations of both prostacyclin (152 +/- 22 pg/ml vs 951 +/- 214 pg/ml, P less than 0.05) and plasminogen activator (0.8 +/- .3 IU/ml vs 1.4 +/- 0.5, P less than 0.05) but no detectable increase in effluent thromboxane B2 or leukotriene C4 concentrations. fMLP infusion resulted in an immediate reduction in coronary flow coincident with diffuse myocardial ischemia. In contrast to MRCF, however, fMLP-induced ischemia resulted in a significant but smaller increase in effluent prostacyclin concentration (210 +/- 47 pg/ml vs 606 +/- .55 pg/ml, P = 0.05) and a marked increase in both thromboxane B2 (less than or equal to 33 +/- 4 pg/ml vs 1141 +/- 375 pg/ml, P less than 0.05) and leukotriene C4 (less than 0.25 ng/ml vs 3.3 +/- 1.2 ng/ml, P less than 0.05) concentrations. Additionally, fMLP caused a reduction in effluent plasminogen activator activity (0.5 +/- 0.1 IU/ml vs 0.39 +/- 0.1 IU/ml, N = 4).(ABSTRACT TRUNCATED AT 250 WORDS)

The functional border zone in conscious dogs

Studies focusing on the functional border zone have been performed largely with anesthetized, open-chest preparations. Therefore, we instrumented 14 dogs at sterile surgery with sonomicrometers arrayed to measure systolic wall thickening across the perfusion boundary produced by circumflex coronary occlusion. We fitted sigmoid curves to the data to model the distribution of wall thickening impairment as a function of distance from the perfusion boundary, which was delineated with myocardial blood flow (15 micron diameter microspheres) maps. Using this approach, we defined the functional border zone as the distance from the perfusion boundary to 97.5% of the sigmoid curve's nonischemic asymptote. The lateral extent of the functional border zone, measured 10 min and 3 hr after occlusion, was 32 and 28 degrees of circumference, respectively. To evaluate the severity of nonischemic dysfunction, we measured average systolic wall thickening within the functional border zone. It was reduced from 3.69 +/- 1.10 (mean +/- SD) mm to 2.98 +/- 1.07 mm (p less than .01) and 2.74 +/- 1.12 mm (p less than .01) early and late after coronary occlusion. Thus, a narrow functional border zone was evident during circumflex coronary occlusion in conscious dogs. Its lateral extent was limited to approximately 30 degrees (similar to findings in open-chest, anesthetized dogs), severe dysfunction was restricted to the immediate vicinity of the perfusion boundary, and the average severity of nonischemic dysfunction within the functional border zone was mild.

Three-dimensional distribution of collateral blood flow within the anatomic area at risk after circumflex coronary artery occlusion in dogs

It is generally accepted that occlusion of a major coronary artery in the dog results in a transmural gradient of collateral blood flow, with the subepicardial region receiving the greatest perfusion. The lateral and base to apex distribution of collateral blood flow and of metabolic and functional consequences of ischemia have been more difficult to define. One reason for such difficulties has been the failure to define the anatomic boundaries of the ischemic vascular bed so that uncontaminated samples of ischemic and non-ischemic tissue could be selected for study. In the present study, the three dimensional distribution of myocardial blood flow during occlusion of the circumflex artery was mapped in seven dogs. At the end of the study, the boundaries of previously ischemic and non-ischemic regions were identified by simultaneous coronary perfusion with red and blue dyes. Left ventricular slices were separated into ischemic and non-ischemic vascular beds based on the dye boundaries, with 1-2 mm of tissue trimmed from this interface to eliminate visually apparent admixture. The ischemic vascular bed of each cross sectional slice then was cut into five transmural wedges, each 3-5 mm wide; each wedge was further subdivided into subendocardial, middle, and subepicardial thirds. The results of blood flow measurements in these samples indicate that the dye injection technique identifies a real interface with a sharp lateral transition in blood flow between ischemic and non-ischemic vascular beds. Within the ischemic vascular bed, there is a transmural gradient of collateral blood flow, but within a given mural layer, there is no consistent gradient from the center to lateral edge or from base to apex of the ischemic region. Thus, in studies designed to characterize the properties of myocardium on either side of the ischemic/non-ischemic interface, reasonable resolution can be achieved by coronary dye infusions to permit visual identification of this interface. On the other hand, in studies in which collateral blood flow is measured as a baseline predictor of infarct size, measurements can be made in a central ischemic block which will be representative of most or all of the ischemic region. Borderzone samples can be excluded to avoid contamination of ischemic samples with non-ischemic tissue.

Changes in contractility fail to alter the size of the functional border zone in anesthetized dogs

The functional border zone is nonischemic myocardium that exhibits reduced function adjacent to an ischemic area. To determine if the functional border zone can be modified by pharmacologic interventions that alter contractility, we infused isoproterenol (0.04-0.10 micrograms/kg/min) or administered propranolol (2 mg/kg) during circumflex coronary occlusion in nine anesthetized, open-chest dogs. We measured systolic wall thickening on both sides of the perfusion boundary, which was delineated with myocardial blood flow (microsphere) maps constructed from small tissue samples. By fitting sigmoid curves to the composite systolic wall thickening data after coronary occlusion, we modeled the distribution of functional impairment across the perfusion boundary. Defined as the distance from the perfusion boundary to 97.5% of the nonischemic asymptote of the sigmoid fits, the functional border zone was 31 degrees of circumference after coronary occlusion alone. Isoproterenol increased +dP/dt by 58% and augmented nonischemic systolic wall thickening without changing the lateral extent of the functional border zone (32 degrees). Propranolol reduced +dP/dt by 24% and depressed nonischemic systolic wall thickening, but the size of the functional border zone remained limited to 28 degrees. Within the functional border zone, wall thickening was significantly but only moderately reduced (-28%) compared with thickening in nonischemic myocardium more than 10 mm away from the perfusion boundary. The ratio of nonischemic border zone to central nonischemic area wall thickening remained the same with each intervention. We conclude that the dimensions of the functional border zone are fixed early after coronary occlusion and that inotropic interventions do not modify the extent or relative severity of nonischemic regional dysfunction.

Right ventricular tissue PO2 in dogs. Effects of hemodilution and acute right coronary artery occlusion

Right ventricular (RV) epicardial tissue oxygen pressure (PtO2) was measured polarographically by means of a platinum multiwire surface electrode on the in situ beating heart of ten anesthetized dogs prior to and after moderate (Hct 28%) normovolemic hemodilution (HD) with dextran 60. In five dogs the effect of acute occlusion of the right coronary artery (RCA) on PtO2 was analyzed. The PtO2 histograms at baseline revealed a bell-shaped configuration and a mean PtO2 of 46.2 +/- 7.1 mm Hg which coincides with the PtO2 on the left ventricle (LV) reported by others. After HD mean PtO2 increased to 51.4 +/- 8.5 mm Hg (P = 0.02) without alterations of the histogram's configuration. Hemodynamics and blood gas analyses were unchanged after HD. RCA ligature was followed by non-uniform changes in the PtO2 pattern. Thus, despite marked differences in external work and O2 consumption, the PtO2 in both RV and LV myocardium are similar at rest. The increase of PtO2 in the RV at reduced Hct values, which is not seen in the LV myocardium, can be related to differences in functional capillary density between both ventricles.

Reversible and irreversible changes in the dog heart during acute left ventricular failure due to experimental multifocal ischaemia

Acute left ventricular (LV) failure was induced in closed-chest pentobarbital anaesthetized dogs (n = 15), by injection of 50 micron plastic microspheres into the main left coronary artery. There were marked reductions of cardiac output and peak LV dp/dt after the embolization, while LV end-diastolic pressure (LVEDP) increased markedly. Biopsies were taken 1, 2, 3, 4, 6, 8, 10, 36, 48 hours, 7 and 14 days after injection of microspheres. Histological examination of the left ventricle revealed multiple ischaemic lesions distributed throughout the are supplied by the left main coronary artery. Oilred O staining revealed deposition of fine lipid droplets in the ischaemic cardiac muscle cells bordering on the necrotic areas. Ultrastructurally the lipid-containing cells showed numerous vacuoles localized in association with the mitochondria. The vacuoles could be observed already 1 hour after embolization and increased in number up to 48 hours. At the end of the observation period at two weeks, the number of lipid droplets was markedly reduced and the heart regained its functional activity. It is concluded that the myocardial lesions induced by coronary embolization of plastic microspheres were associated with acute left ventricular pump failure and consist of multiple foci of damaged myocardium with a central core of necrotic tissue. In the periphery of these lesions there were myocytes with lipids and other signs of light and moderate ischaemic injury. Our suggestion is that these cells represent a region of intermediate injury of "border zone" cells that are a potentially salvageable myocardium.

Infarct size reduction: a review of the clinical trials

The most important finding to emerge from this review of experimental and clinical studies is that the earlier therapy is begun after the onset of symptoms of acute MI, the greater the potential for reduction of infarct size and possibly mortality. It is difficult to define a precise time after which therapy would not have an effect, since the clinical trials for each drug group vary significantly in respect to time of therapy initiation. In experimental studies, major salvage of ischemic myocardium occurs when the drug is given within two hours of coronary artery occlusion. If drug therapy is begun four to six hours postocclusion, then only minor or no reductions in infarct size will occur. The ability of any drug or intervention to reduce infarct size in humans would be optimized if therapy were begun less than four hours of onset of symptoms. With the realization of the wavefront phenomenon and the potential salvage of myocardium at risk with reperfusion, the introduction of reperfusion in the clinical setting with thrombolytic agents or other procedures becomes highly desirable. Clot-selective thrombolytic agents, such as tissue plasminogen activator, diminish the adverse effects and high costs of intracoronary thrombolytic therapy or PTCA. Consequently, it is probable that the initial procedure of choice would be the use of clot-selective thrombolytic therapy. Thrombolytic therapy only lyses thrombi and does not affect the underlying causes of the coronary artery occlusion. Therefore, therapy to reduce the chances of reinfarction and death must also be initiated. Percutaneous transluminal coronary angioplasty, in selected patients, should reduce the reocclusion rate. Beta-adrenoceptor blocking agents appear to be an excellent therapy for reducing mortality when administered chronically; these agents reduce myocardial oxygen consumption and reverse the imbalance between oxygen supply and oxygen demand caused by activation of the sympathetic nervous system and actions of catecholamines. Since thrombus formation has occurred at least once in patients who survive an MI, it is probable that the conditions for thrombus formation still exist. Therefore, institution of antiplatelet aggregating drugs, such as aspirin, would seem to be an appropriate prophylactic regimen. Beta blockers and possibly nitroglycerin have desirable effects when thrombolysis is unavailable. The efficacy of calcium-channel blocking agents on reduction of infarct size appears to be limited, although in the setting of stable and unstable angina postinfarction, these agents can play an important role.(ABSTRACT TRUNCATED AT 400 WORDS)

The distribution of functional impairment across the lateral border of acutely ischemic myocardium

To evaluate the degree and lateral extent of dysfunction in nonischemic myocardium adjacent to ischemic muscle, we measured systolic wall thickening with sonomicrometers during circumflex coronary occlusion in 12 anesthetized, open-chest dogs. The locations of the wall thickness measurements relative to the perfusion boundary were determined with myocardial blood flow (microspheres) maps constructed from multiple, small tissue samples. Five minutes after circumflex occlusion, systolic wall thickening in the central ischemic zone decreased from 3.00 +/- 0.61 (mean +/- SD) mm to -0.61 +/- 0.36 mm (P less than 0.01). In nonischemic myocardium greater than 10 mm from the perfusion boundary, systolic wall thickening increased from 2.56 +/- 0.57 to 3.24 +/- 0.72 mm (P less than 0.01). In nonischemic myocardium within 10 mm of the perfusion boundary, systolic wall thickening was slightly but significantly reduced compared with control (2.72 +/- 0.80 to 2.44 +/- 0.79 mm, P less than 0.05), supporting the concept of regional dysfunction in nonischemic myocardium at the lateral borders of an ischemic area. Sigmoid curves were fitted to the data to model changes in wall thickening as a continuous function of distance from the perfusion boundary. This allowed estimation of the extent of dysfunction into nonischemic myocardium which averaged less than 8 mm (approximately 30 degrees of endocardial circumference) at one border. The level of functional impairment in this zone was relatively modest, and systolic wall thickening in the immediate border area was reduced more than 50% from control only in tissue characterized by a blood supply of mixed ischemic and nonischemic origin. We conclude that a functional border zone exists lateral to an acutely ischemic area, but measurement of regional function produces relatively small exaggeration of the size of the acutely ischemic zone if severe reduction in mechanical performance is used to define the extent of the ischemic area.

Relation between left ventricular global and regional function and extent of myocardial ischemia in the canine heart

To develop a quantitative relation between the overall severity of acute ischemia and left ventricular global and regional function, two minor axis internal diameters and myocardial wall thickness were determined using ultrasonic crystals in 10 open chest dogs with carotid-left anterior descending artery cannulation. The overall extent of ischemia produced by graded stenosis of the cannulation system was estimated by total myocardial blood flow deficit, calculated using radioactive microspheres and a balloon-reservoir perfusion technique permitting precise separation of ischemic from nonischemic tissue. Although cardiac output and left ventricular stroke work were maintained through chamber enlargement until total myocardial blood flow deficit was about 10%, ejection indexes of left ventricular function decreased progressively with increasing ischemia and correlated inversely with total myocardial blood flow deficit (r = -0.55 to -0.73). Ejection indexes of left ventricular global function correlated directly with regional function in the ischemic zone (r = 0.67 to 0.83), although global function decreased at a far slower rate than regional contraction during progressive coronary stenosis with an ischemic region comprising about 25% of total left ventricular weight. During myocardial ischemia, regional dysfunction resulted in progressive global contractile dysfunction; left ventricular hemodynamic status was maintained until ischemia was severe.

Interaction between a normoxic and a hypoxic region of guinea pig and ferret papillary muscles

Papillary muscles were mounted in a three-compartment bath. The tip of the muscle was exposed to hypoxic and glucose-free solution. The other parts of the preparation were superfused with Tyrode's solution, building a free-flow border between hypoxic and normoxic superfusates. The normoxic part of the bath was subdivided by a rubber membrane so that current pulses could be applied between segments of the preparation. Signs of electrotonic interaction between normoxic and hypoxic parts were observed a few minutes after the onset of hypoxia. Transmembrane action potentials in the normoxic part retained their plateau, but progressively shortened. Those in the hypoxic tip showed an early phase of rapid repolarization followed by a plateau phase near the resting potential. Terminal repolarization in the two parts coincided for many minutes. After 35 minutes, fast propagated activity ceased in the tip and was replaced first by conducted slow responses, then by decremental conduction. At 50 minutes, cells near the borderline had resting potentials of either -76 +/- 7 mV (SD, n = 9) in normoxic tissue or -16 +/- 3 mV (SD, n = 9) in hypoxic tissue. Concurrently, subthreshold potentials no longer appeared to spread into the tip. Unipolar electrograms remained diphasic over the normoxic part but lost their negative deflection near the borderline, implying the absence of axial current flow into the hypoxic part. Furthermore, electrotonic potentials generated by current flow across the rubber membrane did not spread beyond a line of demarcation. Reduced nicotinamide adenine dinucleotide fluorescence increased in the hypoxic part, and appeared to correlate with the development of electrical decoupling.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary venous perfusion of the ischemic myocardium during acute coronary artery occlusion in isolated rat hearts

Effects of retrograde coronary venous perfusion on oxygen supply and energy metabolism of ischemic myocardium of the isolated perfused rat heart were examined by means of NADH fluorescence photography. Occlusion of the left coronary artery produced regional ischemia of the left ventricular free wall, as evidenced by the sharply demarcated increase in NADH fluorescence. During ischemia, a narrow area of minimal fluorescence (140 +/- 10 microns), indicating sufficient oxygenation for oxidative phosphorylation, was observed around the epicardial coronary veins in the ischemic lesion. Retrograde perfusion was introduced through the coronary vein (left cardiac vein) that drained off the ischemic area, which resulted in a marked reduction of the area of increased NADH fluorescence in the epicardial surface. In the cross-sectional view, although the myocardium of the entire ischemic area induced by left coronary artery occlusion could be perfused by venous retroperfusion, the effect on reduction of the area of increased NADH fluorescence was seen only in the epicardial half of the myocardium. Retrograde coronary venous perfusion also resulted in a small increase in tension development (P less than 0.05), a decrease in resting tension (P less than 0.01), and partial preservation of myocardial high energy phosphate content (P less than 0.01). We propose that coronary venous retroperfusion improves oxygenation, partially preserves oxidative phosphorylation in the epicardium, and improves contractile function in the ischemic region.

Structural and electrophysiological changes in the epicardial border zone of canine myocardial infarcts during infarct healing

Structural and electrophysiological properties of the epicardial muscle which survives on the surface of transmural infarcts of the canine heart (epicardial border zone) were studied at different times after occlusion of the left anterior coronary artery (LAD). Isolated preparations were superfused in vitro, transmembrane potentials recorded, and impulse propagation mapped. In preparations from subacute infarcts (1 and 5 days), resting potential, action potential amplitude, upstroke velocity, and duration were all significantly reduced. Well-defined directional differences in propagation occurred. Propagation was more rapid in the direction perpendicular to the left anterior coronary artery than in the direction perpendicular to the base of the heart, because of the uniform anisotropic structure of the surviving muscle fibers which were arranged in tightly packed bundles oriented perpendicular to the left anterior coronary artery. The only ultrastructural abnormalities found in these muscle fibers was an accumulation of large amounts of lipid droplets. As the infarcts healed, resting potential, action potential amplitude, and upstroke velocity returned to normal by 2 weeks, although action potential duration decreased further. Lipid droplets had disappeared, and connective tissue had invaded the epicardial border zone, separating the muscle bundles. By 2 months, action potentials were normal, but the muscle fibers were widely separated and disoriented by the connective tissue (parallel bundles no longer were found). In these regions with a nonuniform anisotropic structure, the well-defined directional differences in impulse propagation were lost. However, activation was very slow, perhaps because of diminished connections between cells. The persistence of slow conduction in healed infarcts may contribute to the occurrence of chronic arrhythmias.

The border zone of the early myocardial infarction in dogs; its characteristics and viability

In open-chest dogs, the left anterior descending coronary artery was ligated for 150 min. The heart was semiserially cut on a cryomicrotome and areas of ischemic damage were visualized by means of glycogen depletion (PAS reaction) and tissue acidosis (a "sandwich" technique with pH indicator dispersed in a layer of gel). The extent of myocardial damage was determined morphometrically. The mass of the glycogen-depleted heart muscle was greater than the mass of the ischemic tissue detected by means of decreased pH (p less than 0.01). The border zone was characterized by glycogen depletion without acidosis. Circulation studies using intravital fluorescein staining have shown that perfusion is partially retained in the border zone; it is assumed that the hypoperfusion triggers glycogenolysis. Nevertheless, the level of perfusion suffices to wash out the acidic end products. Comparison of contrapulsation-treated dogs and untreated dogs shows that the amount of damaged tissue comprising the border zone can be reduced by this therapeutic intervention (p less than 0.02) - in contrast to the acidotic tissue, the amount of which is not significantly influenced. Therefore the border zone contains damaged but still viable muscle cells.

The functional implications of scintigraphic measures of myocardial ischemia and infarction

To compare serial functional and perfusion scintigraphic changes after myocardial infarction, we performed left ventricular (LV) cineangiograms and thallium (TI)-201 myocardial perfusion scintigrams before and 1 hour, 2 days, 9 days, and 1 month after closed chest coronary occlusion in 14 dogs as survival permitted. Survivors were studied with technetium-99m (stannous) pyrophosphate (TcPYP) scintigrams at 48 hours, and at postmortem examination infarction was documented and measured after nitroblue tetrazolium (NBT) staining. The TcPYP image was abnormal in 10 dogs, each of which had infarcts on NBT staining measuring 3 to 23 gm. In all 14 dogs, perfusion scintigrams became abnormal and LV ejection fraction (EF) fell when measured within 48 hours of occlusion. In the nine late survivors studied over 1 week after the event, perfusion scintigrams and EF improved in those which developed infarcts and normalized in those without infarction. The decrement in LVEF after coronary occlusion generally showed serial improvement and correlated with the size of the defect in the accompanying TI-201 scintigram (r = 0.74). TI-201 defect size seen in late studies correlated well with NBT infarct size (r = 0.89) and TcPYP image infarct size (r = 0.82), as it did with the decrement in LVEF noted in late studies (r = 0.86). The results suggest that early perfusion scintigrams together with TcPYP images may be useful for estimating the amount of reversible dysfunction after coronary occlusion.

Temporal and spatial characteristics of evolving cell injury during regional myocardial ischemia in the dog: the "border zone" controversy

An open chest dog heart with multiple coronary ligations was used to define the temporal and spatial characteristics of injury evolving during regional ischemia. With the use of a multiple (40 sample) biopsy device, adjacent transmural biopsy specimens were obtained from the transition zone between normal and ischemic tissue after 5, 30, 45, 60 and 120 minutes of ischemia. The first 1.8 mm of epicardial tissue was taken for the analysis of flow and metabolites. The results confirmed the existence of a sharp interface of flow and metabolism in the epicardial lateral plane at the boundary of the ischemic zone. There was no significant zone of intermediate injury (flow and metabolism being depressed uniformly throughout the ischemic area). Comparison of the distribution of flow determined by radiolabeled gadolinium-153 at onset of ischemia with that indicated by radiolabeled tin-113 microspheres given at the end of various periods of ischemia revealed no change in the position or steepness of the flow interface at any time during the first 2 hours of ischemia. This observation, together with the absence of any major redistribution or enhancement of residual flow to the ischemic zone, indicated that there was little or no significant collateralization between 5 and 120 minutes. Analysis of the adenosine triphosphate (ATP) content revealed a rapid depletion during the first 5 minutes of ischemia; the content then remained essentially unchanged until 30 minutes, after which time a second phase of accelerated ATP depletion was observed until 45 minutes. ATP content then remained relatively constant up to 2 hours.

Patterns of cellular injury in myocardial ischemia determined by monoclonal antimyosin

The development of cellular injury in the rat left ventricle resulting from left coronary artery occlusion was examined by immunofluorescence after intravenous injection of monoclonal antimyosin. Cardiac muscle cells that bound antimyosin during ischemia were localized by staining sections with fluorescein-conjugated anti-mouse IgG. Fluorescent staining was detectable within the ischemic region of the left ventricle 3 hr after occlusion and injection of antimyosin. After 6 hr of ischemia, the highly irregular margin of the ischemic zone was clearly outlined by fluorescent cells. At 3-6 hr after occlusion, marked heterogeneity in cellular staining was observed in the epicardial half of the ischemic area, with intensely fluorescent cells intermixed with cells of markedly lower fluorescence. By 24 hr, a homogeneous pattern of staining was observed throughout the ischemic zone. In nonischemic regions of the heart and in rats treated for 24 hr with antimyosin without occlusion, there were only background levels of staining. We conclude that: (i) visualization of ischemic cells via antimyosin provides a sensitive means for examining developing patterns of injury; (ii) the heterogeneity of staining during early ischemia may reflect variation in cellular resistance to deprivation; and (iii) the pattern of fluorescence at the margin of the occluded region indicates that the "border zone" is composed of interdigitating ischemic and nonischemic tissues.

Pathophysiology of irreversible ischemic injury. The border zone controversy

At the present time our experimental findings plus the weight of other experimental evidence suggest that there is unlikely to be a quantitatively significant border zone in the lateral plane. The transition from normal to ischemic tissue is likely to be accomplished over a distance of 1.0 mm or less and possibly in as little as the dimensions of one cell. The situation in the transmural plane is less well established, but if the same situation occurs, then the absence of a spatially indentifiable border zone of intermediate injury will require a major reappraisal, although not an abandonment, of concepts for the therapeutic limitation of infarct size. Any extrapolation of the observations, comments, and conclusions made in this paper to the human heart should be made with extreme caution. Major species differences exist, particularly in relation to the characteristics of collateral flow. Most experimental studies have involved single or multiple coronary artery ligation, a situation that generates large areas of sharply demarcated ischemia. These areas are very severely ischemic and short of reperfusion, which is hardly a practical consideration in the early phases of evolving myocardial infarction; the affected tissue is inevitably condemned to cell death and necrosis. The situation prevailing in man with partial coronary artery occlusion or diffuse ischemic heart disease may well be be very different and is clearly in urgent need of investigation.

Myocardial function in areas of heterogeneous perfusion after coronary artery occlusion in conscious dogs

Regional myocardial function and blood flow in endocardial layers were correlated in myocardial segments subtending severely ischemic and adjacent, normally perfused myocardium in conscious dogs 1--3 weeks after recovery from coronary artery occlusion. With coronary artery occlusion induced by a hydraulic occluder, endocardial blood flow (measured with radioactive microspheres) and function (determined with an ultrasonic dimension gauge) in homogeneously nonischemic segments increased slightly but not significantly. In homogeneously ischemic segments, blood flow and function decreased (p less than 0.01) by 96 +/- 1% and 98 +/- 4%, respectively. In segments subtending zones of unequal perfusion, endocardial blood flow increased nonsignificantly in the myocardium surrounding the nonischemic crystal, while decreasing by 93 +/- 2% (p less than 0.01) in myocardium surrounding the ischemic crystal. Surprisingly, these segments behaved like homogeneously ischemic segments, i.e., endocardial shortening decreased by 92 +/- 6% (p less than 0.01). Thus, the failure to detect shortening despite normal perfusion of the myocardium surrounding one of the transducers suggest a potential problem with interpretation of regional function measurements or an inability of the apparently nonischemic myocardium to contract.

The microcirculation of the human heart: end-capillary loops with discrete perfusion fields

We studied 10 autopsied human hearts by perfusing colored Microfil into separate coronary arteries to define organization of capillaries at the borders between two perfusion fields. Sections of "cleared" myocardium were examined with epiillumination at the grossly identified borders of Microfil perfusion. In two- and three-color-injected hearts, the capillaries were arrayed in a pattern of arcades and loops without connections between separately perfused capillary beds. In hearts perfused through only one coronary artery, the capillaries were organized into tufted loops at the border. These findings contrast with the microcirculatory pattern in canine skeletal muscle and brain, in which heterologous capillaries are focally interconnected. We conclude that the human microcirculation is composed of end-capillary loops that supply discrete perfusion fields. This pattern of unconnected heterologous capillary beds suggest that there is no obvious anatomic arrangement of the microcirculation that could account for a significant ischemic lateral border zone in human myocardial infarctions.

Platelet-mediated cardiac ischemia

Although platelets have been associated with angina pectoris, myocardial infarction, and sudden death, the platelet's capacity for induction and propagation of cardiac ischemia remains incompletely defined. We therefore evaluated the effects of platelet activation occurring within the coronary circulation and tested the hypothesis that inhibition of platelet function would prevent platelet-induced cardiac ischemia. Human platelets were isolated from blood obtained from normal donors by Sepharose 2B column chromatography, resuspended in Hepes buffer, and added to the perfusate of a Langendorff rabbit heart (platelet counts greater than 10,000/microliters). Without, and with low dose (10 microM) prostaglandin E1 (PGE1), a reversible inhibitor of platelet function, immediate and irreversible global cardiac ischemia, as monitored by NADH fluorescent photography, ensued (N = 4) following platelet activation with thrombin (0.1 to 1 U/ml). Higher concentrations of PGE1 (0.1 to 1 mM, N = 2) or aspirin ingestion (1000 mg taken approximately 12, 4, and 1 hr prior to experiment, N = 2) completely prevented this platelet-induced myocardial ischemia. Aspirin, unlike PGE1, was effective despite its inability to block thrombin-induced platelet aggregation in our in vitro gel-filtered system. We conclude that activation of platelets within the coronary circulation is sufficient for induction of irreversible cardiac ischemia. The efficacy of aspirin, a cyclooxygenase inhibitor, further suggests that the products of arachidonate metabolism (e.g., thromboxanes) have a fundamental role in the genesis of platelet-mediated myocardial ischemia.

Electrophysiology of the normal-to-hypoxic transition zone

The "normal-to-hypoxic" transition zone was modeled after the chamber technique for perpendicular and parallel orientation with respect to the direction of fiber axis. The evidence obtained from the recording of the transmembrane action potentials suggests the presence of a preservation phenomenon in the hypoxic area, based on the utilization of energy stores of the normoxic area. Better intercellular coupling in longitudinal than in transverse fiber direction results in the anisotropic properties of the preservation phenomenon. The preservation phenomenon provides a basis for the existence of critical size of the viable hypoxic area compared to the size of the transitional zone. The crucial role of electrotonic coupling was demonstrated, as well as the possible contribution to the preservation phenomenon mechanism of the cell-to-cell diffusion of metabolites.

Electrotonically mediated delayed conduction and reentry in relation to "slow responses" in mammalian ventricular conducting tissue

A narrow zone of block in isolated false tendon preparations was created by perfusion of the central compartment (gap) of a three-compartment tissue bath with either an isotonic sucrose solution or a solution designed to mimic the extracellular milieu in ischemic tissue. Driven responses on the proximal side of the gap were transmitted to the distal side after long delays. The characteristics of the "ischemic" gap model were found to be qualitatively similar to those of the sucrose gap model in which impulse transmission is electrotonically mediated. In both models, the effects of driven action potentials were mimicked by electrotonic displacement of membrane potential by current pulses passed across the gap. Foot-potentials representative of electrotonic potentials bringing the distal membrane to threshold were present in all cases and were found to be largely unaffected by the slow channel-blocking agent, verapamil. Transmembrane activity recorded from the central portion of the gap segment was shown to be electrotonic in nature. Ectopic activity in the form of reflected reentry was readily demonstrable in the ischemic gap model in the presence or absence of verapamil as well as in the sucrose gap model. When propagation across the gap was mediated by "slow" responses, transmission was relatively prompt and reentry did not occur. Our observations suggest that very slow conduction through ischemic areas may result from step delays imposed by electrotonic transmission of impulses across inexcitable segments of cable rather than from uniform slow conduction of propagated action potentials with slow upstrokes.

Relation between conduction delay and ventricular fibrillation: characteristics of conduction of premature impulses during acute myocardial ischemia

Critical conduction delay has been shown to be the important factor in reentrant arrhythmias. To determine the causal relation between conduction delay and spontaneous ischemic ventricular fibrillation, conduction delay of induced premature ventricular impulses in the ischemic and the border zones of ventricular myocardium was investigated in 23 dogs. There were 8 control dogs, 9 dogs that manifested ventricular fibrillation within 30 minutes of ligation of the distal left anterior descending coronary artery (group I), and 6 dogs that manifested ventricular fibrillation between 30 and 60 minutes of ligation of the proximal left anterior descending coronary artery (group II). Conduction delay was measured as the change of conduction time from preligation levels to postligation levels in epicardial and endocardial sites in both base to apex (anterograde) and apex to base (retrograde) directions. Conduction delay in all four directions was compared in the control group and groups I and II (ventricular fibrillation). By means of continuous recordings on tape, the origin of ventricular fibrillation was determined to be in the ischemic zone (13 of 15 dogs) or in the border zone (2 of 15 dogs). Conduction delay in all directions was greater in the group with ventricular fibrillation whether the fibrillation occurred after the first or second ligation. Specifically, epicardial anterograde conduction (in the border zone) and retrograde conduction (in the ischemic zone) were significantly delayed in group I. Conduction in group II was significantly delayed in both the ischemic and the border zones in three of four directions at 35 minutes. The rate of change of conduction time in all four directions was significantly greater in the group with ventricular fibrillation than in the control group. Pending further work, this model may provide a reliable marker for the development of spontaneous ventricular fibrillation during acute myocardial ischemia and may permit assessment of various interventions as specific therapy for acute reentrant ischemic tachyarrhythmias leading to ventricular fibrillation.

Linear relationship between perfusion area and infarct size

The purpose of this study was to develop a technique measuring the perfusion area of the coronary artery preocclusively and to study the relationship between the perfusion area and infarct size. 125I tracer microspheres were selectively injected into the left circumflex coronary artery (LCX) preocclusively, and then the LCX was ligated. 48 hours later the heart was removed, rapidly frozen, and 50-mu transverse sections were obtained from base, middle and apex of the canine left ventricle, and used for autoradiography to measure perfusion area and for tetrazolium staining to measure infarct size. Dogs were divided into 2 groups: group 1 in which the main trunk of the LCX was occluded to produce large infarct (n = 10) and group 2 in which the distal branch of the LCX was occluded to produce small infarct (n = 10). There was a linear correlation between the perfused and infarcted area regardless of a size or location of the perfusion area involved. These results indicate that the extent of infarction is directly proportional to the perfusion area and is not altered by the location in the ventricle.

Characterization of the lateral interface between normal and ischemic tissue in the canine heart during evolving myocardial infarction

A new nonrotating multiple biopsy device has been developed to allow the rapid, simultaneous and contiguous sampling of cardiac muscle in the large mammalian heart. Each cutter obtains 40 adjacent transmural left ventricular biopsy samples, each of 4 mm section. The epicardial 1.8 mm of each biopsy section was analyzed for flow, adenosine triphosphate, adenosine diphosphate, adenosine monophosphate, creatine phosphate and lactate. Use of this procedure in the dog heart 30 minutes after coronary arterial ligation permitted characterization of the nature of flow and metabolic gradients as the sampling site moved from the core of an areas of regional ischemia to the surrounding normal tissue. These studies of metabolic and flow geometry in the lateral plane indicate the existence of a sharp interface of flow and metabolism between normal and ischemic tissue. The absence of intermediate levels of flow and metabolism indicate that, in the lateral plane at least, a quantitatively significant and spatially identifiable "border zone" region does not exist. However, these findings, do not preclude the existence of such a zone of jeopardized tissue in the transmural plane or the occurrence of a temporal border zone to which interfaces of flow and metabolism may migrate with time.

The "border zone" in evolving myocardial infarction: controversy or confusion?

Considerable controversy has arisen over the existence of a "border" zone of intermediate injuries during regional myocardial ischemia. Much of this controversy arises from the inappropriate use of terminology and the misquotation or misinterpretation of previously published studies. This article considers the nature of the interface between normal and ischemic tissues; proposes a series of definitions based on current knowledge of etiology of tissue injuries; reviews critically the existing evidence for an against the "border zone" concept; and considers the quantitative significance of a "border zone" of salvageable tissue if it exists.

The histological lateral border of acute canine myocardial infarction. A function of microcirculation

Studies from this laboratory have shown that the border of a 24-hour canine infarct is histologically sharp and is composed of numerous interdigitating peninsulas of necrotic and normal tissue. To see if this sharp boundary is spatially related to the capillary beds of occluded and non-occluded arteries, the left anterior descending artery (LAD) was ligated in five mongrel dogs. Twenty-four hours later, white silicone rubber (Microfil) was injected into the LAD distal to the ligature; simultaneously and under the same pressure, red Microfil was injected into the left main coronary artery (LMCA). In hematoxylin and eosin sections from the border of the infarct, capillaries supplied by the LAD (white) were either in areas of necrosis, in normal epicardium or, rarely, in normal tissue along the lateral boundary; those supplied by the LMCA (red) were almost always in normal regions. Quantitative evaluation of this relationship revealed that the majority of the vessels in the normal and necrotic tissue were concordant (i.e., that normal tissue was supplied by the LMCA, and necrotic tissue by the LAD). However, a small zone of vascular discordance, averaging approximately 30 micrometers in width, was present along the infarct boundary, possibly representing a narrow border zone of little consequence. Hence, the complex interdigitation of normal and necrotic tissue in the lateral border of an infarct is predominantly a function of the interdigitation of the capillary beds supplied by the occluded and nonoccluded arteries.

End-capillary loops in the heart: an explanation for discrete myocardial infarctions without border zones

Separate perfusions of canine coronary arteries with colored silicone-rubber compound reveal that in the region where two microcirculations abut, capillaries derived from individual large vessels are discrete, with no interconnections. Terminal homologous capillaries from loops rather than anastomosing with heterologous capillaries. This anatomic arrangement may account for discrete myocardial infarctions without ischemic border zones.