Connie L. Engle

The Ib Phase of Ventricular Arrhythmias in Ischemic In Situ Porcine Heart Is Related to Changes in Cell-to-Cell Electrical Coupling

BACKGROUND This study was designed to test the hypothesis that the loss of cell-to-cell electrical interaction during ischemia modulates the amplitude of ischemia-induced TQ-segment depression (ie, the injury potential) and the occurrence of ventricular fibrillation (VF) during the so-called Ib phase of ventricular arrhythmias. METHODS AND RESULTS Regional ischemia was induced by 60 minutes of mid-left anterior descending coronary artery ligation in open-chest swine (n = 10). Cell-to-cell electrical uncoupling was defined as the onset of the terminal rise in whole-tissue resistivity (Rt). Local activation times and TQ-segment changes (injury potential) were determined from unipolar electrograms. Extracellular K+ ([K+]e) and pH (pHe) were measured with plunge-wire ion-selective electrodes. VF occurred in 6 of 10 pigs during regional no-flow ischemia between 19 and 30 minutes after the arrest of perfusion. The occurrence of VF was positively correlated to the onset of cell-to-cell electrical uncoupling (R2 = .885). Cell-to-cell electrical uncoupling superimposed on changes of [K+]e and pHe contributed to the failure of impulse propagation between 19 and 30 minutes after the arrest of perfusion. During ischemia, maximum TQ-segment depression was -10 mV at 19 minutes, after which TQ-segment depression slowly recovered. The onset of the TQ-segment recovery was correlated to the second rise in Rt (R2 = .886). CONCLUSIONS In the regionally ischemic in situ porcine heart, loss of cell-to-cell electrical interaction is related to the occurrence of VF and changes in the amplitude of the injury current. Cellular electrical uncoupling contributes to failure of impulse propagation in the setting of altered tissue excitability as a result of elevated [K+]e and low pHe. These data indicate that Ib arrhythmias and ECG changes during ischemia are influenced by the loss of cell-to-cell electrical interaction.

Effect of graded coronary flow reduction on ionic, electrical, and mechanical indexes of ischemia in the pig.

This study was performed to determine the relative sensitivities of ionic, electrical, and mechanical indexes of myocardial ischemia. We used ion-selective and bipolar plunge electrodes, epicardial unipolar electrodes, a suction electrode, and ultrasonic crystals to determine the changes in intramyocardial extracellular potassium ([K+]e) and extracellular pH (pHe), local activation, epicardial TQ-ST segment, monophasic action potential duration (MAPD), and regional contractility during graded coronary flow reduction in open-chest pigs. A carotid-to-coronary shunt was created to perfuse the left anterior descending coronary artery via a roller pump. The shunted coronary flow was reduced in a stepwise fashion at 5-min intervals. In 25 pigs, the approximate myocardial blood flow associated with the initial changes in each variable was as follows: midmyocardial [K+]e, pHe, and TQ-ST segment, 0.7 to 0.8 ml/min/g; subepicardial [K+]e and TQ-ST segment, 0.6 to 0.7 ml/min/g; segmental shortening, 0.5 to 0.6 ml/min/g; local activation and epicardial TQ-ST segment, 0.3 to 0.4 ml/min/g; epicardial MAPD, 0.15 to 0.2 ml/min/g. Our results indicate that changes in [K+]e, pHe, and TQ-ST segment provide the most sensitive means of detecting myocardial ischemia and of determining the effect of interventions capable of influencing the ischemic process.

Ability of Activation Recovery Intervals to Assess Action Potential Duration During Acute No‐Flow Ischemia in the In Situ Porcine Heart

Activation Recovery Intervals During No‐Flow Ischemia. Introduction: The ability to assess transmural changes in action potential duration during acute no‐flow ischemia is essential to an understanding of the tachyarrhythmias that occur in this setting. The purpose of this study was to determine if activation recovery intervals determined from unipolar electrograms would provide this information.

Magnitude and time course of extracellular potassium inhomogeneities during acute ischemia in pigs. Effect of verapamil.

Prior studies have demonstrated the presence of inhomogeneities in myocardial [K+]e after serial 10-minute occlusions of the left anterior descending coronary artery in the pig, even within restricted locations of an ischemic zone. These inhomogeneities are thought to underlie the electrophysiological abnormalities responsible for lethal ventricular arrhythmias through reentrant and nonreentrant pathways, but a clear association has not been demonstrated. As a prerequisite to establishing this association, these studies were performed to establish measurement standards for [K+]e inhomogeneity, to quantify the magnitude and time course of these inhomogeneities, to determine whether the inhomogeneities are greater in the ischemic border where lethal ventricular arrhythmias are known to originate, and to assess the effect of a known antifibrillatory drug on [K+]e inhomogeneities. [K+]e (expressed as the change in potassium equilibrium potential, dEK [mV]) was measured in 15 preparations using an average of 17 closely spaced, critically calibrated K(+)-sensitive electrodes having stable response characteristics. A series of four 10-minute occlusions each separated by a 50-minute reperfusion period were performed in each study. In half of the studies, intravenous verapamil (0.2 mg/kg bolus followed by 0.0065 mg/kg/hr) was administered before the fourth occlusion. In nine studies (five control and four verapamil), electrodes were placed in the marginal ischemic zone (from 2 mm outside to 5 mm inside the visible cyanotic border). In six other studies (three control and three verapamil), electrodes were placed in the central ischemic zone (10-20 mm within the ischemic region). We determined that the standard deviation is the best measure of inhomogeneity and that 12 equivalent measurement sites are required to estimate it with a satisfactory degree of statistical confidence. We found that after 10 minutes of ischemia, mean dEK was 1.6 times greater in the central than in the marginal ischemic zone, whereas mean standard deviation at the same time was 1.5 times greater in the marginal than in the central ischemic zone. Verapamil reduced mean dEK and mean standard deviation in both ischemic zones for most of the occlusion by delaying the rise in [K+]e and the inhomogeneity of that rise by 3-5 minutes. Comparisons of mean dEK with mean standard deviation revealed a steep linear relation in the marginal zone and a curvilinear relation in the central zone where higher mean dEK values were not accompanied by higher values for mean standard deviation. Furthermore, we determined that these relations were not altered by verapamil.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of verapamil on ischemia-induced changes in extracellular K+, pH, and local activation in the pig.

In experimental animals, the calcium channel-blocking agents lessen the arrhythmogenic, ionic, metabolic, and electrical changes that occur during acute myocardial ischemia. To date, these effects have been studied separately, and the effects of these agents on local activation have not been correlated with ionic or metabolic effects. In open-chest, anesthetized swine, we used bipolar and ion-selective plunge electrodes to simultaneously measure ischemia-induced changes in left ventricular local activation, extracellular K+ ([K+]e), and extracellular pH (pHe). The effects of verapamil (0.2 mg/kg) on these variables were studied during a series of 10 min occlusions of the left anterior descending coronary artery. Compared with control occlusions, verapamil (1) slowed the rise in [K+]e at the center of the ischemic zone and at its lateral margin and decreased the peak [K+]e by 0.9 mM at the center (p less than .05) and by 0.1 mM at the margin (p = .10); (2) slowed the development of acidosis and decreased the peak level of acidosis beyond that expected solely as a result of serial occlusions by 0.19 pH units at the center (p less than .05) and by 0.07 pH units at the margin (p = .10); and (3) slowed the development of local activation delay and often prevented the local activation block that was observed during control occlusions. Effects on local activation became less marked at [K+]e levels greater than 9.0 mM, and the effects of verapamil on local activation were not explained solely by its effects on the local rise in [K+]e or fall in pHe. A possible mechanism for this additional effect on local activation is suggested by preliminary results showing a diminution by verapamil of ionic inhomogeneity.

Comparison of the Effects of Regional Ischemia and Hyperkalemia on the Membrane Action Potentials of the In Situ Pig Heart

APD During Ischemia. Introduction: This study was designed to determine the role of increased extracellular potassium [K+]e on action potential duration (APD) in the in situ porcine heart during acute regional no‐flow ischemia.

Marked activation delay caused by ischemia initiated after regional K+ elevation in in situ pig hearts.

BackgroundConduction mediated by the slow inward (Ca2+) current occurs in vitro under specific experimental conditions but has not been documented in ventricular muscle in vivo during regional myocardial ischemia, perhaps because certain constituents of ischemia (including hypoxia and acidosis) may inhibit the Ca2+ current in this setting. We hypothesized that slow conduction mediated by the Ca2+ current could occur during acute ischemia in situations in which the extracellular K+ rise was more marked relative to the degree of acidosis, as may occur at ischemic boundaries. Methods and ResultsIn open-chest, anesthetized swine, an arterial shunt from the carotid artery to the mid-left anterior descending coronary artery was created through which a solution of KCl was infused to raise extracellular K+ ([K+]e) to approximately 9.4 mmol/L before the initiation of ischemia, which we termed “K+-modified ischemia.” Ischemia initiated at a normal [K+]e (“unmodified ischemia”) resulted in a mean activation delay in the center of the ischemic zone of 55 ± 26 milliseconds after 5 minutes of ischemia and a decrease in epicardial longitudinal conduction velocity from 53 to 21 cm/s before the onset of conduction block. K+- modified ischemia resulted in a mean activation delay in the center of the ischemic zone of 181 ± 8 milliseconds and a decrease in epicardial longitudinal conduction to less than 10 cm/s. K+-modified ischemia was associated with ventricular fibrillation in 85% of episodes compared with 28% of episodes of unmodified ischemia (P < .01). Verapamil prevented the occurrence of marked activation delay during K(+)-modified ischemia, producing local activation block following a maximum activation delay of 74 ± 25 milliseconds. In two experiments, responses mediated by the slow inward current were produced by regional K+ elevation to 15 to 16 mmol/L, followed by concomitant regional administration of epinephrine (10−7 mol/L). Regional [K+]e elevation alone to this level resulted in local activation block following a maximum activity delay of 70 to 80 milliseconds, whereas administration of epinephrine in combination with high [K+]e resulted in return of local activation with an activation delay of 160 to 180 milliseconds (ie, similar to that during K+- modified ischemia). ConclusionsCompared with unmodified ischemia, K+-modified ischemia resulted in marked activation delay and a high incidence of ventricular fibrillation. Based on measurements of longitudinal conduction velocity, the inhibitory effect of verapamil, and the results of experiments with high [K+]e plus epinephrine, we conclude that the marked activation delay during K+-modified ischemia represents conduction mediated by the slow inward current. Because the conditions produced by K+-modified ischemia (high [K+]e with minimal acidosis) are similar to conditions in and near ischemic border regions, we hypothesize that responses mediated by the slow inward current may occur in such regions during unmodified ischemia and may participate in the development of reentrant arrhythmias.

Unanticipated lessening of the rise in extracellular potassium during ischemia by pinacidil

BACKGROUND The efflux of potassium (K) through the ATP-sensitive K channel is considered an important cause of the rise in extracellular K ([K+]e) during no-flow ischemia. We postulated that agents that enhance K conductance in this channel would enhance the rise in [K+]e. METHODS AND RESULTS We studied the effects of 10 and 25 mumol/L pinacidil, and ATP-sensitive K channel opener that provides metabolic protection to the ischemic myocardium, on the rise in [K+]e recorded by K-sensitive electrodes, the change in action potential duration (APD) recorded by microelectrodes, and the changes in activation during ischemia in in situ pig hearts and Tyrode-perfused rabbit interventricular septa. Pinacidil 25 mumol/L unexpectedly lessened the rise in [K+]e and the activation delay in both preparations. Pinacidil 10 mumol/L had no effect in the rabbit and only a slight effect in the pig. Both concentrations significantly exaggerated the APD shortening induced by ischemia. By varying stimulation frequency, we demonstrated that the rise in [K+]e during ischemia, both before and after pinacidil, correlated with the time that the action potential was at its plateau voltage. CONCLUSIONS Our results indicate that the rise in [K+]e during ischemia is due to multiple factors, including K conductance across membrane channels, K driving force as reflected by the time that the action potential is at its plateau voltage, and the metabolic effects of ischemia. The unanticipated lessening of the rise in [K+]e by pinacidil reflects the balance of its effects on these several parameters.

Effect of Rate on Changes in Conduction Velocity and Extracellular Potassium Concentration During Acute Ischemia in the In Situ Pig Heart

Rate Effects on Conduction and [K+]e During Ischemia. Introduction: The purpose nf our study was to determine if the slowing oflongitudinal intraventricular conduction in the in situ porcine heart during acute regional no‐flow ischemia was rate dependent. Further, we investigated whether any rate dependence could he correlated to a rate‐dependent component of the ischemiainduced rise in extracellular potassium concentration. [K+]e.

Effects of verapamil and propranolol on changes in extracellular K+, pH, and local activation during graded coronary flow in the pig.

The beta-adrenergic and calcium channel blocking agents are known to reduce heart rate and alter myocardial contractility. More recent evidence suggests that both agents affect the metabolic consequences of ischemia, independent of their effects on heart rate and contractility. We used a low-flow model of ischemia in swine with heart rate held constant by atrial pacing. Blood was shunted from the carotid artery to the left anterior descending coronary artery through a controlled-flow roller pump to assess the threshold flow for the rise in extracellular potassium ([K+]e) and fall in extracellular pH (pHe) associated with ischemia during control situations and after the administration of either propranolol or verapamil. We also measured the changes in activation delay and contractility associated with graded flow reductions in the presence and absence of these drugs. We found that when heart rate is held constant, 1) verapamil shifts the threshold flow for [K+]e and pHe to lower levels, but propranolol does not; 2) verapamil lessens activation delay, while propranolol aggravates the delay; and 3) verapamil reduces afterload and selectively depresses contractility in the reperfused ischemic zone. We conclude that the calcium channel blockers and the beta-adrenergic-blocking agents have different effects and possibly different modes of action and should not be considered interchangeable when evaluating therapeutic options for patients with ischemic heart disease.