Effect of changes in perfusion pressure on the position of the electrophysiologic border zone in acute regional ischemia in isolated perfused dog and pig hearts

Distribution of extracellular potassium and its relation to electrophysiologic changes during acute myocardial ischemia in the isolated perfused porcine heart

An experimental approach is described to quantitate inhomogeneity in extracellular K concentration ([K+]out) in the presence of ischemia and to relate this inhomogeneity to the electrophysiologic changes. Extracellular potassium concentration and local direct-current electrograms from the same sites were measured in isolated perfused pig hearts with the use of multiple electrodes. Dispersion of [K+]out is described under three conditions: (1) during regional ischemia in the "central zone" and the "borderzone", (2) during global ischemia, and (3) during perfusion of the heart with a high-K perfusate. Inhomogeneity was greatest during regional ischemia, especially in the borderzone, where generally lower concentrations were measured. When during regional ischemia the normal zone was perfused with a high-K perfusate, dispersion in the ischemic borderzone diminished, and higher concentrations than in the central zone were measured. During global ischemia inhomogeneity was slightly larger than during high-K perfusion. Dispersion during the latter was considered due to experimental error. A decrease in [K+]out during regional ischemia after the initial increase was closely correlated with electrical recovery of the electrograms. This decrease occurred earlier in the borderzone than in the central zone. During ischemia [K+]out was not related to the occurrence of monophasic electrograms, which are indicative of the absence of local regenerative responses. For every single electrode position a linear relationship between TQ depression and [K+]out was found, the slope of which varied with the position of the electrode. When all sites were taken together, there was no correlation between TQ depression and [K+]out. We conclude that: (1) inhomogeneity of K+ is largest in the borderzone, (2) potassium flows from the ischemic zone into the normal zone, (3) transient electrical recovery is related to a decrease (after an initial increase) in [K+]out, which is at least partly due to a flow of K+ toward the normal zone, (4) monophasic ("block") electrograms can be recorded from intrinsically excitable tissue, (5) for every single site in the ischemic region there is a linear relationship between local [K+]out and local TQ segment depression, and (6) the degree of TQ depression at a particular site is not a reliable index of the degree of ischemic injury at that site.

Extracellular potassium dynamics in the border zone during acute myocardial ischemia in a canine model

Bifunctional intramyocardial potassium ion (K+)-sensitive and bipolar wire electrodes were used to evaluate extracellular K+ dynamics and electrophysiologic changes during acute myocardial ischemia in the border zone, ischemic zone (5 to 7 mm from the border), central ischemic zone (15 to 25 mm from the border) and normal myocardium in 11 open chest dogs during a 30 min ligation of the left anterior descending coronary artery. At the end of this period, the hearts were injected with rhodamine dye and quickly frozen. Ultraviolet NADH (nicotinamide adenine dinucleotide) rhodamine fluorescence photography was used to localize the border between normally perfused and ischemic tissue and determine the site of electrodes in relation to this border. Before coronary ligation, extracellular K+ ranged from 4.0 +/- 0.3 to 4.3 +/- 0.3 mM in these four zones. After ligation, extracellular K+ accumulated in the ischemic and central ischemic zones in a pattern characterized by an initial rapid increase for approximately 5 min, followed by a slowly rising plateau phase, reaching maximal levels of 9.8 +/- 2.0 and 14.4 +/- 4.4 mM, respectively. In contrast, K+ dynamics in the border zone showed a biphasic response, with an initial rapid increase to a maximal level of 7.5 +/- 2.4 mM at approximately 9 min after coronary ligation, followed by a gradual decrease to a level of 5.3 +/- 1.2 mM by the end of the 30 min ligation period. No significant changes in K+ occurred in the normal zone throughout the ischemic period. The correlation of K+ electrode, electrophysiologic and postmortem NADH-rhodamine fluorescence data indicated the existence of a well defined border zone.(ABSTRACT TRUNCATED AT 250 WORDS)

Early changes in collateral blood flow to ischemic myocardium and their influence on bimodal vulnerability during the first 30 min of acute coronary artery occlusion in dogs

Coronary collateral blood flow and its changes during the first 30 min of acute coronary artery occlusion were studied in 34 anesthetized dogs to clarify its influence on the vulnerability of the heart in this first arrhythmic phase. Collateral flow was determined with 9-micron tracer microspheres (TM) injected at 1, 10 and 30 min after acute proximal occlusion of the left circumflex (LCX, n = 30) or left anterior descending (LAD, n = 4) coronary artery. The post-mortem selective retrograde coronary angiography was used to evaluate the functional extent of preexisting coronary collaterals. After acute LCX occlusions collateral flow increased significantly (p less than 0.05) from 17.8 +/- 2.7 ml.(min.100 g)-1 (mean +/- SE; n = 13) at 1 min to 29.3 +/- 5.1 ml . (min . 100 g)-1 at 30 min of occlusion in animals with well-formed preexisting coronary collaterals (group 1), whereas it was very low (4.7 +/- 0.6 ml . (min . 100 g)-1; n = 13) and changed only slightly in dogs with poor collaterals (group 2). Most of these animals died by ventricular fibrillation (VF) before a second measurement could be carried out. In four dogs with poor collaterals but an unusually small LCX area (group 3), flow remained almost constant (11.0 +/- 4.7 versus 11.0 +/- 4.4 ml . (min . 100 g)-1) during this period. Similar results were obtained in animals with acute LAD occlusion (group 4). Ventricular extrasystoles (VES) mainly developed in animals with poor collaterals and respective low and almost unchanging collateral flow (group 2,3,4) while none or only a few VES occurred in dogs in which flow initially was clearly higher and increased significantly (group 1). The animals of group 1, 3 and 4 all survived the first arrhythmic phase, whereas all the animals of group 2 died by VF, either in the arrhythmic subphase Ia (n = 7) or Ib (n = 6). The results of the present study show that only the initial magnitude of flow is decisive for the development of VES and VF during the first arrhythmic phase, whereas the subsequent increase in flow is of no importance to the bimodal vulnerability in the first arrhythmic phase (subphases Ia and Ib). The in vivo measured critical minimal collateral flow of about 5 ml . (min . 100 g)-1 corresponds well with the survival limit (border between collateral state III and IV) previously determined by post-mortem retrograde coronary angiography.

Microvascular function at the margins of early experimental myocardial infarcts in isolated rabbit hearts

Injection of low-viscosity resin was used to identify in situ functional blood vessels at the margins of developing regional myocardial infarcts. The ventral interventricular branch (VIB) of the left coronary artery was occluded for 0-240 min in 20 isolated perfused rabbit hearts. After perfusion fixation with glutaraldehyde, resin was injected into the coronary arteries--that injected into the VIB contained dispersed lead dioxide and that injected into the remainder of the heart contained Fat Red 7B dye. This allowed macroscopic and microscopic identification of functional blood vessels. Following transmural freeze fracture, left ventricles were examined using back-scattered electron imaging in a scanning electron microscope. Close to 60% of capillaries in nonischemic myocardium allowed the passage of resin. Thirty minutes of ischemia produced a hyperemic increase to 80%-90% in the proportion of filled vessels. After 60 min, however, a severe reperfusion defect corresponding to the "no-reflow" phenomenon had developed, with virtually all vessels collapsed and less than 10% functional. Among the structurally normal myocytes adjacent to the infarct margin there was a significant reduction (to 30%-40%) in the proportion of functional capillaries. This was due to groups of dilated vessels which were not accessible to arterial supply. Although these marginal "low-flow" regions were of small volume at any one point in time, they seem likely to contribute to the progression of ischemic necrosis, and are probably nonfunctional due to the compression of their venous drainage traversing the infarct.

The development and progression of myocyte injury at the margins of experimental myocardial infarcts

Distinct differences in the extent and progression of the lateral and epicardial boundaries of evolving regional infarcts were demonstrated in isolated rabbit hearts. Ischemia was produced by interrupting (0-240 minutes) flow in the ventral interventricular branch of the left coronary artery, whilst the remainder of the heart was continuously perfused with oxygenated Krebs-Henseleit bicarbonate buffer. Perfusion fixed blocks were freeze-fractured then examined using back-scattered electron imaging in a scanning electron microscope. Control myocytes showed relatively smooth, continuous internal fracture faces. After 30 min of ischemia myocytes showed evidence of mild, probably reversible, injury in the form of prominence of pits and channels. Severe injury, characterized by separation of organelles and prominent intracellular spaces, developed after 60 or more min of ischemia, first in the subendocardial two thirds, and after 120 min across the full thickness of the ventricular wall. At the lateral margins of infarcts there was a distinct cell-to-cell boundary between control and severely injured myocytes, with only a few scattered mildly injured cells within 30 mu of the infarct. Although transmural progression of necrosis provides the potential for recovery of the external aspect of the myocardium in the ischemic zone by reperfusion, corresponding regions of salvageable myocytes at lateral infarct margins are very narrow.

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

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

Effects of unilateral stellate ganglion stimulation and ablation on electrophysiologic changes induced by acute myocardial ischemia in dogs

We recorded direct-current extracellular electrograms simultaneously from 60 left ventricular epicardial sites in 38 alpha-chloralose-anesthetized dogs during repeated, 5 min coronary arterial occlusions. In each dog recordings made during control occlusions were compared with those made in occlusions after, or during, the following interventions on the sympathetic nervous system: left stellate ganglion stimulation, left stellectomy, right stellectomy, and clamping the abdominal aorta with intact sympathetic nerves to induce a rise of blood pressure equal to that present during left stellate stimulation. Heart rate was kept constant. Measurements included determination of TQ segment potentials and times of local activation. After 2 min of ischemia, the degree of TQ segment depression was increased by left stellate ganglion stimulation and was decreased by both left stellectomy and clamping the aorta. Also, the area showing negative TQ potentials, indicating decreased resting membrane potentials, was enlarged by both left stellate stimulation and right stellectomy and reduced by left stellectomy. No differences were found in the results of experiments in which the left anterior descending coronary artery was occluded and those in which the circumflex branch was occluded. Left stellate stimulation significantly improved conduction within the ischemic zone. No evidence was found to suggest that the arrhythmogenic effects of left stellate stimulation and of right stellectomy, confirmed in the present study, resulted from an increased likelihood for reentry in the subepicardium of the ischemic zone.

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.