First Minutes Of Ischemia Research Articles

Border zone repolarization duration as a predictor of early ischemic ventricular fibrillation

Ventricular fibrillation (VF) is a dangerous complication of acute myocardial infarction. Within an episode of acute ischemia, VFs are clustered into early (1A) and delayed (1B) phases based on different mechanisms. Our previous work demonstrated that prolonged repolarization duration in the border zone predisposed to VF development. A prolongation of action potential duration in the affected area is a frequent though not inevitable change at the very beginning of ischemia. However, the role of this phenomenon in arrhythmogenesis is unclear. The objective of the present study was to evaluate an association between early (1A phase) VF with repolarization duration in the border zone in an experimental porcine myocardial infarction model.Myocardial ischemia was induced in 19 anesthetized domestic pigs by ligation of the left anterior descending (n=9) or left circumflex (n=10) coronary arteries. Epicardial mapping was done with a 64‐lead epicardial array placed on the affected area with adjacent regions. Recordings were done in the baseline state and during ischemia with a custom‐designed system (16 bits; bandwidth 0.05 to 1000 Hz; sampling rate 4000 Hz). Local activation time (AT) and end of repolarization time (RT) were measured in each epicardial lead as the instants of a dV/dt minimum during QRS‑complex and dV/dt maximum during T‑wave, respectively. Activation‐repolarization interval (ARI, a surrogate for action potential duration) was taken as a rate‐corrected difference between RT and AT. Dispersion of repolarization (DOR) was calculated as a difference between the latest and earliest RTs throughout all recorded epicardial leads.Ten early VF episodes developed during first minutes of ischemia (1‐8 min). In logistic regression analysis, VF incidence was associated with ARI duration in the border zone (regression coefficient 1.02, 95% CI 1.003‐1.037, p=0.023), but not DOR (regression coefficient 1.005, 95%CI 0.990‐1.021, p=0.508) during first ten minutes of ischemia. In multivariate logistic regression analysis (baseline ARI duration vs the fact of ARI prolongation), VF incidence was associated only with the baseline ARI duration (regression coefficient 1.025, 95%CI 1.007‐1.044, p=0.005).Our data demonstrated the role of the prolonged repolarization duration in the development of 1A phase VF, which was independent of DOR and related to its baseline duration. Since the border zone is by definition at least partly accessible for blood flow, it may present a target for VF prevention therapy in a setting of acute myocardial ischemia.

C0-C1f region of cardiac myosin binding protein-C induces pro-inflammatory responses in fibroblasts

Abstract Background Cardiac myosin binding protein-C is a protein expressed in the myosin thick filament backbone that was recently described as a novel cardiac biomarker. Its N-terminal region, C0-C1f, is released within the first minutes of ischemia and plays a crucial role in the initiation of inflammation in bone marrow-derived macrophages. Long-term C0-C1f exposure induces cardiac fibrosis in transgenic mice; however, the mechanism by which C0-C1f causes fibrosis is unclear. The aim of the study was to investigate the effects of C0-C1f on fibroblasts, which are the main contributor to cardiac fibrosis, in vitro. We determined whether C0-C1f directly activates fibroblasts and causes a transdifferentiation to myofibroblasts or induces inflammatory responses. Moreover, we clarify whether other cell types could be involved in inducing fibrosis, i.e. by the release of pro-inflammatory cytokines upon C0-C1f interaction. Methods A novel human fibroblast cell line (huFib) was treated with C0-C1f, C0-Linker, TGF-β, or LPS for different time periods. Inflammatory and fibrotic responses were evaluated at the RNA and protein level using different techniques including microarray, qRT-PCR, and immunofluorescence imaging. For signalling pathway analysis, TLR4 and NFκB were inhibited using chemical compounds TAK-242 or Bay11–0785 respectively. Results C0-C1f treatment induced an increase in mRNA corresponding to pro-inflammatory genes in huFib cells (i.a. CXL1 upon 24 hours treatment: 29 fold, p<0.001 and CCL2 4-fold, p<0.001). The mRNA expression levels of pro-fibrotic genes such as ACTA2 or COL1A1, which were upregulated by TGF-β, were not reduced by C0-C1f (ACTA2 induced by TGF-β: 3,8 fold, p<0.001, co-stimulation with C0-C1f: 1,8 fold, p=0.11 compared to control; COL1A1 induced by TGF-β: 2,94 fold, p<0.001, co-stimulation with C0-C1f: 2.09 fold (p<0.01) compared to control. Interestingly, co-stimulation of fibroblasts with C0-C1f and TGF-β led also to markedly lower inflammatory response compared to C0-C1f treatment alone (CXCL1 induction upon co-stimulation: 2,0 fold, p<0.001, CCL2: 1,9 fold, p=0.001, which is a reduction by 27 fold, p<0.001 or 2 fold, p=0.002, respectively). Inhibition of TLR4 or NFκB signaling diminished C0-C1f-mediated inflammatory responses. Conclusion C0-C1f induces inflammation in fibroblasts via TLR4/NFκB signalling pathway. Downregulation of C0-C1f mediated inflammatory responses upon co-stimulation with TGF-β suggests crosstalk between the two signaling pathways. Contrary, C0-C1f reduced TGF-β mediated pro-fibrotic responses reflected by conversion of fibroblasts into myofibroblasts was not observed. Taken together, these data are consistent with the idea that C0-C1f might play a key role in the early initiation of inflammation upon myocardial infarction, also in fibroblasts, and that TGF-β acts as a counterpart at later stages of cardiac remodeling. Funding Acknowledgement Type of funding sources: Foundation. Main funding source(s): William G. Kerckhoff Stiftung für wissenschaftliche Forschung und Fortbildung

Electron paramagnetic resonance monitoring of ischemia‐induced myocardial oxygen depletion and acidosis in isolated rat hearts using soluble paramagnetic probes

A new low-field electron paramagnetic resonance approach for noninvasive measurements of myocardial oxygen tension and tissue acidity was developed. The approach was applied to monitor myocardial pO(2) and pH in a model of global no-flow ischemia (30 min) and reperfusion in isolated perfused rat hearts. The myocardial oxygen measurements were performed using deuterated Finland trityl radical probe. A rapid decrease in myocardial pO(2) from 160 mmHg to about 2 ± 1 mmHg was observed within the first minute of ischemia followed by incomplete restoration of pO(2) to 50 mmHg during 30 min of reperfusion. The lower oxygen concentration after ischemia was attributed to the 50% reduction in coronary flow after ischemia as a consequence of myocardial ischemia and reperfusion damage. Myocardial pH measurements using a specially designed imidazoline pH-sensitive nitroxide showed severe myocardial acidification to pH 6.25 during 30 min of ischemia. Preconditioning of the hearts with two 5-min periods of ischemia significantly reduced the acidification of myocardial tissue during sustained ischemia. Noninvasive electron paramagnetic resonance monitoring of myocardial oxygenation and pH may provide important insights into the mechanisms of ischemia and reperfusion injury and a background for development of new therapeutic approaches.

Effect of Magnesium on the Monophasic Action Potential During Early Ischemia in the In Vivo Human Heart

This study sought to examine the effects of magnesium on epicardial action potential duration in patients during early myocardial ischemia. Magnesium has been shown to reduce arrhythmias in experimental models of myocardial ischemia. Experimental and clinical observations suggest an effect on repolarization. Patients undergoing elective coronary artery bypass surgery were randomized (double blind) to receive intravenous magnesium (n = 10) or placebo (n = 10). Patients were placed on cardiopulmonary bypass and paced at 600 ms, and stable monophasic action potentials were obtained. Ischemia was achieved by aortic cross-clamping for 2 min while normothermia was maintained. Serum magnesium levels increased from 0.60 +/- 0.03 to 1.69 +/- 0.07 mmol/liter (mean +/- SEM) in the magnesium group, with no change in the placebo group. Epicardial temperature was identical in the two groups and did not alter during ischemia. At 90% repolarization, initial action potential prolongation was observed in the placebo group over the first minute of ischemia (282.0 +/- 6.0 to 294.0 +/- 4.8 ms) but not in the magnesium group (278.3 +/- 5.9 to 274.5 +/- 7.4 ms). At 2 min of ischemia, action potential duration was shorter in the magnesium group than in the placebo group (258.1 +/- 5.5 vs. 281.3 +/- 5.9 ms, respectively, p < 0.05). Intravenous magnesium infusion altered the epicardial action potential response to ischemia in patients. These findings may have important implications in the pathogenesis of arrhythmias in ischemic myocardium.

Renal sympathetic nerve activity during cardiac ischemia and reperfusion in rats.

We studied the role played by prostaglandins and oxygen-derived free radicals in mediating reflex changes in renal sympathetic nerve activity (RSNA) during myocardial ischemia and reperfusion. Ligation of the left coronary artery for 20 min and reperfusion for 10 min were performed in anesthetized rats with sinoaortic denervation and with intact cardiac afferent nerves (control, n = 7), with cardiac sympathetic denervation (SD, n = 6), with vagal denervation (VD, n = 7), and with combined SD + VD (n = 6). In control rats, RSNA decreased by 10 +/- 3% from baseline (P < 0.05) during the first minute of ischemia and increased above baseline after 5 min of ischemia, with the maximum increase at the first minute of reperfusion. In rats with SD, RSNA decreased by 19 +/- 4% from baseline (P < 0.05) at the first minute of ischemia and remained depressed during the entire ischemic and reperfusion periods. In rats with VD, RSNA increased by 26 +/- 5% from baseline (P < 0.05) at the first minute of ischemia, and the increase in RSNA at the end of the ischemic period and at reperfusion was greater than in control rats. No changes in RSNA during ischemia and reperfusion were observed with combined SD + VD. Reflex changes in RSNA that occurred at the onset of ischemia in both VD (n = 7) and SD (n = 7) rats were abolished by indomethacin (5 mg/kg i.v., 20 min before ischemia). Reflex changes in RSNA after prolonged ischemia (> 10 min) and during reperfusion in both VD (n = 7) and SD (n = 7) rats were abolished by the antioxidant deferoxamine (20 mg/kg i.v., 20 min before ischemia). Deferoxamine also diminished the increase of RSNA at the onset of ischemia in VD rats. Thus, in rats, the vagal afferent reflex predominates during early ischemia and the sympathetic afferent reflex predominates during prolonged ischemia and reperfusion. Reflex changes in RSNA that occur at the onset of ischemia are mediated by activation of vagal and sympathetic afferent endings by prostaglandins. Reflex changes in RSNA after prolonged ischemia and during reperfusion are mediated by activation of vagal and sympathetic afferent endings by oxygen-derived free radicals.

Crucial role of intracellular effectors on glycogenolysis in the isolated rat heart: potential consequences on the myocardial tolerance to ischemia.

The role played by glycogenolysis in the ischemic heart has been recently put into question because it is suspected that a slowing down of this process could be beneficial for the tolerance of the myocardium to ischemia. The role of the intracellular effectors that control the rate of glycogenolysis has therefore regained interest. We aimed to understand the role played by those intracellular effectors which are directly related to the energy balance of the heart. To this end, we review some of the previously published data on this subject and we present new data obtained from P-31 and C-13 NMR spectroscopic measurement on isolated rat heart. Two conditions of ischemia were studied: 15 min global no-flow and 25 min low-flow ischemia. The hearts were isolated either from control animals or from rats pre-treated with isoproterenol (5 mg.kg-1 b.w. i.p.) 1 h before the perfusion in order to C-13 label glycogen stores. Our main results are as follows: (1) the biochemically determined glycogenolysis rate during the early phase of ischemia (up to 10-15 min) was larger in no-flow ischemia than in low-flow conditions for both groups, (2) direct measurement of the glycogenolysis rate, as determined by C-13 NMR, after labelling of the glycogen pool in the hearts from isoproterenol-treated rats, confirms the estimations from the biochemical data, (3) glycogenolysis was slower in the hearts from pre-treated animals than in control hearts for both conditions of ischemia, (4) the total activity of glycogen phosphorylase (a + b) increased, by 50%, after 5 min no-flow ischemia, whereas it decreased by 42% after the same time of low-flow ischemia. However, the ratio phosphorylase a/a + b was not altered, whatever the conditions, (5) the concentration of inorganic phosphate (Pi) increased sharply during the first minutes of ischemia, to values above 8-10 mM, under all conditions studied. The rate of increase was larger during no-flow ischemia than during low-flow ischemia. The concentration of Pi was thereafter higher in controls than in the hearts from isoproterenol-treated animals. The calculated cytosolic concentration of free 5'AMP increased sharply at the onset on ischemia, reaching in a few minutes values above 30 microM in controls and significantly lower values around 15 microM, in the hearts from isoproterenol-treated rats. (6) The hearts from isoproterenol-treated rats displayed a reduced intracellular acidosis, when compared to controls, under both conditions of ischemia. We conclude that the intracellular effectors, mainly free AMP, play an essential role in the control of glycogenolysis via allosteric control of phosphorylase b activity. The alteration in the concentration of free Pi, the substrate of both forms of phosphorylase, can be considered as determinant in the control of the rate of glycogenolysis. The attenuation of ischemia-induced intracellular acidosis in the hearts from isoproterenol-treated rats could be a consequence of a reduced glycogenolytic rate and is likely to be related to a better resumption of the mechanical function on reperfusion.

A new synthetic antiarrhythmic peptide reduces dispersion of epicardial activation recovery interval and diminishes alterations of epicardial activation patterns induced by regional ischemia

Common antiarrhythmic agents affect ionic membrane channels and thereby alter cellular electrical activity. Since this accounts for the proarrhythmic effects as well we tried to find new substances with different profiles of actions. A new antiarrhythmic peptide, H2N-Gly-Ala-Gly-4 Hyp-Pro-Tyr-CONH2 (AAP 10), was synthetized using the Fmoc-strategy. This peptide was analyzed for its electrophysiological profile of action in normal isolated rabbit hearts perfused according to the Langendorff technique either under control conditions or after induction of a regional ischemia. For this purpose 256 channel epicardial mapping was employed allowing the determination of the timepoints of activation at each electrode thus identifying the origins of epicardial activation (socalled breakthrough-points, BTP). Epicardial spread of activation was then described mathematically by activation vectors which gave direction and velocity of the epicardial activation wave at each electrode. Single heart beats were analyzed under control conditions and under treatment with AAP 10 or under regional ischemia with or without AAP 10-pretreatment (10(-8) mol/l). We calculated the percentage of similar vectors (VEC) with unaltered direction (deviation < or = 5 degrees) and the percentage of identical breakthroughpoints (deviation < or = 1 mm) compared to control conditions. In addition, apparent epicardial velocities, total activation time of a given region and activation-recovery interval (ARI) as well as dispersion of ARI (i.e. standard deviation of ARI) and distribution of ARI were analyzed. Under control conditions treatment with AAP 10 (10(-10) to 3 x 10(-7) mol/l) led to a significant decrease in ARI-dispersion without alteration of any of the other parameters under investigation. Left ventricular regional ischemia resulted in a marked alteration of the activation patterns (a significant decrease in vectorfield- and breakthroughpoint-similarity) which could be significantly inhibited by pretreatment with AAP 10. In addition, we found that AAP 10 depressed the increase in ARI-dispersion during the first minutes of ischemia and accelerated normalization of ARI-dispersion during reperfusion. In additional experiments, it could be shown that AAP 10 did not alter action potential duration, maximum dU/dt, amplitude or resting membrane potential of isolated guinea pig muscles using a common intracellular action potential recording technique.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of short-term myocardial hibernation. Its limitation by the severity of ischemia and inotropic stimulation.

Short-term hibernating myocardium is characterized by a decrease in contractile function in proportion to the reduced myocardial blood flow. Myocardial creatine phosphate content, initially decreased during the first minutes of ischemia, returns to near-control values, the ischemia-induced net lactate production is attenuated, and the myocardium remains viable despite ongoing hypoperfusion and contractile dysfunction. Hibernating myocardium after 85 minutes of ischemia maintains an inotropic reserve and responds to short-term intracoronary dobutamine infusion with increased work; however, this inotropic response is at the expense of metabolic recovery. We therefore hypothesized that the development of myocardial hibernation is a delicate process that is easily disturbed by unfavorable alterations in the oxygen-supply demand balance. To study the impact of prolonged inotropic stimulation on the development of myocardial hibernation, the left anterior descending coronary artery was cannulated and hypoperfused at constant flow in 12 enflurane-anesthetized swine. The reduction of coronary inflow was followed by a reduction of regional myocardial work (sonomicrometry) from 248 +/- 59 mm Hg.mm to 73 +/- 35 mm Hg.mm (P < .05) at 5 minutes of ischemia. Dobutamine (2.5 +/- 1 micrograms/min) was then infused for an additional 85 minutes. Work was increased at 5 minutes of dobutamine to 139 +/- 34 mm Hg.mm (P < .05 versus 5 minutes of ischemia). However, this increase was only transient, and after 85 minutes of dobutamine, work was decreased below the initial ischemic value (42 +/- 34 mm Hg.mm). At 5 minutes of ischemia, creatine phosphate content was reduced from 8.80 +/- 1.97 to 6.21 +/- 3.87 mumol/g wet wt, and myocardial ATP content was decreased slightly from 4.75 +/- 0.92 to 4.12 +/- 1.29 mumol/g wet wt (both, P = NS). After 5 minutes of dobutamine, further reductions in creatine phosphate content to 3.11 +/- 0.76 mumol/g wet wt and in ATP to 3.14 +/- 0.81 mumol/g wet wt were observed (both, P < .05 versus control). During the remainder of the continuous dobutamine infusion, creatine phosphate content remained unchanged, whereas ATP further decreased significantly to 1.68 +/- 0.96 mumol/g wet wt. The beta-adrenoceptor density of the left anterior descending coronary artery-perfused myocardium was 36.5 +/- 5.8 fmol (-)-[125I]iodocyanopindolol/mg protein under control conditions, and this was unchanged during ischemia and the subsequent dobutamine infusion. Following 90 minutes of ischemia with 85 minutes of dobutamine and 2 hours of reperfusion, infarct size (triphenyl tetrazolium chloride staining) was 26.3 +/- 7.5% of the area at risk. With constant hypoperfusion, dobutamine redistributed blood flow away from the subendocardium (0.20 +/- 0.08 versus 0.11 +/- 0.04 mL.min-1.g-1) toward the subepicardium (0.45 +/- 0.13 versus 0.51 +/- 0.21 mL.min-1.g-1) as well as to the right ventricle (0.26 +/- 0.08 versus 0.32 +/- 0.09 mL.min-1.g-1). Therefore, in two other groups of six and five swine, the severity of ischemia was increased to achieve an 80% or a 90% reduction in regional function, respectively, and the importance of the severity of blood flow reduction per se for the development of myocardial infarction was studied. The infarct size in the animals undergoing 85 minutes of dobutamine (26.3 +/- 7.5%) was increased above the level expected from the blood flow reduction alone (6.3 +/- 6.4%, P < .01). Both the increased severity of ischemia and the enhanced energy expenditure induced by dobutamine impair the development of myocardial short-term hibernation and precipitate myocardial infarction.

Correlation dimension of heartbeat intervals is reduced in conscious pigs by myocardial ischemia.

A reduced standard deviation of RR intervals (SDRR) predicts increased mortality in groups of survivors of myocardial infarction. Like SDRR, the correlation dimension (D2) describes variation within a sampled time series, but uniquely it reveals 1) the epoch's geometric structure and 2) the degrees of freedom of the generator. These unique features may be more sensitive predictors of mortality than SDRR. We developed a new algorithm for estimating D2 (i.e., the "point-D2"), tested it with known data, and found that it had greater accuracy for finite data than other published algorithms. Analysis of RR intervals from eight conscious pigs undergoing acute occlusion of the left anterior descending coronary artery revealed a drop in the point-D2 from a control mean and standard deviation of 2.50 +/- 0.81 to 1.58 +/- 0.64 during the first minute of ischemia (p less than 0.01) and to 1.07 +/- 0.18 during the last minute preceding ventricular fibrillation (p less than 0.01). Partial occlusions (50-90% reduction of coronary blood flow) evoked point-D2 reductions only 25-30% of control (p less than 0.01). The point-D2 means were correlated between pigs with the magnitude of the respiratory sinus arrhythmia (p less than 0.01), but during ischemia this correlation was replaced by one between the standard deviation of the point-D2s and SDRRs. Because the simultaneous reduction in the mean point-D2 and its standard deviation to 1.07 +/- 0.18 occurred in every case, was unique to the few minutes preceding ventricular fibrillation, and never reached these low values during other conditions in which it was reduced, we conclude that the point-D2 may be an accurate prospective predictor of mortality within the individual subject.

Potassium accumulation in the globally ischemic mammalian heart. A role for the ATP-sensitive potassium channel.

We investigated the contribution of opening of the ATP-sensitive K+ channel to extracellular accumulation of K+ during ischemia with the use of glibenclamide, a specific blocker of this K+ channel. To characterize the electrophysiological effects of glibenclamide during metabolic inhibition (by either application of dinitrophenol or hypoxia) we performed patch-clamp studies in isolated membrane patches of guinea pig myocytes and in intact guinea pig myocytes and studied action potential parameters in isolated superfused guinea pig papillary muscle. We studied the effect of glibenclamide on extracellular accumulation of K+ and H+ in isolated retrogradely perfused globally ischemic hearts of rat, guinea pig, and rabbit. Experimental evidence is presented that supports the conclusions that glibenclamide 1) effectively blocks open K+ATP channels, 2) reverses the dinitrophenol-induced increase of the outward current and prevents the hypoxia-induced shortening of the action potential, 3) decreases the rate of K+ accumulation during the first minutes of ischemia in stimulated hearts, an effect which was entirely absent in quiescent hearts, and 4) does not influence the rate and extent of ischemia-induced extracellular acidification.

Dynamics of skeletal muscle energetics during ischemia and reperfusion assessed by in vivo31P NMR

The dynamics of rat skeletal muscle energy metabolism in response to ischemia and reperfusion have been investigated by in vivo 31P NMR spectroscopy. The time course of changes in the phosphocreatine, inorganic phosphate, ATP peaks and in intracellular pH during 2 and 4 h of tourniquet ischemia followed by up to 24 h of tissue reperfusion have been determined. Furthermore, the ATP and IMP concentrations in the soleus and tibialis muscles have been determined by high performance liquid chromatography analysis in response to ischemia and subsequent reperfusion. The results demonstrate an initial overshoot in the pH during the first minutes of ischemia. It is also shown that the muscles recover completely after 2 h of ischemia whereas the energy state of the muscle cell is not restored after 4 h of ischemia followed by up to 24 h of reperfusion. However, the soleus muscle recovers better than the tibialis. The results are discussed in terms of oxygen availability, reperfusion injury, IMP accumulation and different response between muscles with different fibre composition.

Differentiation between cortical and subcortical lesions following focal ischemia in cats by multimodality evoked potentials

Regional ischemia was induced in cats by occluding the middle cerebral artery. Evoked and spontaneous electrical activity as well as regional cerebral blood flow (rCBF) were recorded with platinum depth macroelectrodes in three primary cortical areas: the auditory cortex (A, middle ectosylvian gyrus) and the front and hind limb somatosensory cortex (SF and SH, lateral and medial posterior sigmoid gyrus). To distinguish among the various evoked potentials after click, median or tibial nerve stimulation, electrical field interactions had to be eliminated using a multiplex stimulation and analysis system. Spontaneous electrocortical activity was evaluated by power spectral analysis. In all areas, evoked potentials were abolished 10 min after arterial occlusion. However, rCBF behaved differently in these regions: it was severely reduced in A, decreased moderately in SF and remained unchanged in SH. The graded reduction of rCBF in the three cortical areas was related to changes in electrophysiological activity during the first minutes of ischemia. In A, auditory potentials were abolished within 3 min after occlusion, whereas in SH, the decrease of somatosensory responses started after about 5 min. In SF, two components of the EP changes were found: an early decrease immediately and a later decrease about 5 min after occlusion. The different rates of EP impairment possibly correspond to two types of ischemia. The fast EP abolishment seems to be caused by local cortical damage whereas the delayed EP decrease probably reflects impairment of subcortical white matter structures. Thus, this method may be useful for distinguishing between gray and white matter ischemia.

The subendocardial border zone during acute ischemia of the rabbit heart: an electrophysiologic, metabolic, and morphologic correlative study.

Isolated preparations of rabbit interventricular septum were perfused through the coronary arteries with oxygenated Tyrode's solution and placed in a tissue bath where they were superfused as well. Transmembrane potentials were simultaneously recorded from the subendocardium with two flexibly mounted microelectrodes, one from a superficial cell, and the other from a deep cell. Ischemia was produced by stopping coronary flow while superfusion with oxygenated Tyrode's solution was maintained. After a 7 to 12 min ischemic period, the preparation was fixed by coronary perfusion with fixative while the microelectrodes remained in place. After fixation, the microelectrodes were withdrawn. Appropriate tissue blocks were cut in 4 micron serial sections and the microelectrode track was followed until the tip position was identified. Transmembrane potentials during ischemia were divided into two categories: "border zone" potentials (resting membrane potential [RMP] 73 +/- 3 mVe, action potential amplitude [APA] 81 +/- 13 mV, action potential duration [APD] 116 +/- 48 msec, n = 12) and "ischemic" potentials (RMP 53 +/- 4 mV, APA 44 +/- 11 mV, APD 102 +/- 42 msec, n = 8). Ischemic potentials were recorded from cells at depths greater than 560 micron below the endocardial surface and border zone potentials were recorded in a layer at between 130 and 650 micron below the surface. In a separate series of experiments, extracellular concentrations of K+ and pH were measured with ion-sensitive electrodes at different depths and, after a 10 min period of ischemia, part of the septum was placed in liquid nitrogen to allow determination of phosphocreatine (PC) levels in successive 50 to 100 micron layers. After 10 min of ischemia, extracellular K+ gradually increased from 4 to 9 mM in endocardium to a depth of 600 micron, pH fell from 7.4 to 6.6 over the same distance, and PC decreased to very low, stable levels at only 800 micron. It is concluded that in the first 10 min of acute ischemia, an endocardial border zone exists of 40 to 60 cell layers in which transmembrane potentials are affected relatively little by ischemia. Within this electrophysiologic border zone extracellular K+ was lower than 9 mM, pH was higher than 6.6, and tissue content of PC was not lower than 40% of normal. In layers deeper than 600 micron, with further development of a metabolic gradient, action potentials became markedly depressed. This electrophysiologic inhomogeneity within the ischemic subendocardium could be a factor in arrhythmogenesis during the first minutes of ischemia.

Autonomic mechanisms in ventricular fibrillation induced by myocardial ischemia during exercise in dogs with healed myocardial infarction. An experimental preparation for sudden cardiac death.

The relationship between activity of the autonomic nervous system, myocardial ischemia, and malignant arrhythmias has been investigated in a new experimental preparation for sudden death. Fifty-seven dogs were chronically instrumented and studied under control conditions (n = 15) and 1 month after production of an anterior myocardial infarction (n = 42). The protocol consisted in occluding the left circumflex coronary artery for 2 min, commencing at the last minute of an exercise stress test and extending through the first minute after cessation of exercise. With this protocol, ventricular fibrillation was observed in 40% of normal dogs and 66% of dogs with infarction. In 14 dogs with infarction, left stellectomy reduced the incidence of ventricular fibrillation to zero (p less than .001). The reflex changes in heart rate elicited within the first minute of ischemia during exercise in the animals that survived (from 204 +/- 14 to 198 +/- 31 beats/min, -6) were opposite those in animals that had ventricular fibrillation (from 208 +/- 24 to 229 +/- 30 beats/min, +21) (p less than .05). The ischemia-induced reduction in heart rate despite continuation of exercise suggests the presence in the dogs that survived of active vagal reflexes that may have played an important role in the maintenance of cardiac electrical stability. This preparation has the potential to induce ventricular fibrillation consistently in conscious animals by the interaction of a few clinically relevant factors (acute myocardial ischemia, submaximal exercise and its cessation, sympathetic and vagal reflexes, and heart rate) and offers the possibility of acquiring further insights into the mechanisms of malignant arrhythmias and evaluating novel strategies for targeted prevention.