Characterization of shock-induced action potential extension during acute regional ischemia in rabbit hearts

Development of device therapy for ventricular arrhythmias

The past 25 years have seen the implantable cardioverter defibrillator emerge as the treatment of choice for ventricular arrhythmias with reduction in size but increased therapeutic options. Understanding the complex mechanisms of ventricular arrhythmias and defibrillation in normal and diseased hearts has been the focus of many research teams including that of John Uther at the Westmead Hospital Department of Cardiology. Marked improvements in capacitor and battery technologies, arrhythmia discrimination, pacing algorithms, shock waveforms and monitoring capabilities enable wider use and patient acceptance. Emergence of cardiac resynchronisation therapy and the implantable defibrillator for treatment of chronic heart failure is not only giving quality of life and extended survival for heart failure patients but has also cast new light on the evolution of heart failure.

Measurements of Electrophysiological Effects of Components of Acute Ischemia in Langendorff-Perfused Rat Hearts Using Voltage-Sensitive Dye Mapping

INTRODUCTION

This study was carried out to evaluate optical mapping in the presence of cytochalasin-D as a method for measuring electrophysiological responses in general, and in particular the responses to acute ischemia in the Langendorff-perfused rat heart. Cytochalasin-D is commonly used to reduce contraction for the purpose of suppressing motion artifacts in voltage-sensitive dye recordings of cardiac membrane potential.

METHODS AND RESULTS

Observations using optical mapping were complemented by recordings of the surface electrogram to provide information independent of the optical measurements. Perfusion of Langendorff-perfused rat hearts with 3 microM cytochalasin-D resulted in a 24% prolongation of the QT interval of surface electrograms indicating that cytochalasin-D prolongs the rat ventricular action potential. Individual components of the electrophysiological response to acute ischemia were globally induced as follows: (1) opening of K(ATP) channels was induced by perfusion of 2 micro M P-1,075, (2) accumulation of extracellular K(+) was simulated by increasing perfusate [K(+)] to 12 mM, and (3) acidosis was simulated by reducing perfusate pH to 6.5. The responses to these interventions could be reliably documented using optical recordings, as well as from surface electrograms. Whole-cell patch clamp measurements on isolated rat ventricular myocytes indicate that cytochalasin-D produces an approximately 2.5-fold increase in P-1,075-induced I(K,ATP).

CONCLUSION

These results provide the necessary background information for interpreting electrophysiological measurements during acute ischemia in the presence of cytochalasin-D.

Effect of acute global ischemia on the upper limit of vulnerability: a simulation study

The goal of this modeling research is to provide mechanistic insight into the effect of altered membrane kinetics associated with 5-12 min of acute global ischemia on the upper limit of cardiac vulnerability (ULV) to electric shocks. We simulate electrical activity in a finite-element bidomain model of a 4-mm-thick slice through the canine ventricles that incorporates realistic geometry and fiber architecture. Global acute ischemia is represented by changes in membrane dynamics due to hyperkalemia, acidosis, and hypoxia. Two stages of acute ischemia are simulated corresponding to 5-7 min (stage 1) and 10-12 min (stage 2) after the onset of ischemia. Monophasic shocks are delivered in normoxia and ischemia over a range of coupling intervals, and their outcomes are examined to determine the highest shock strength that resulted in induction of reentrant arrhythmia. Our results demonstrate that acute ischemia stage 1 results in ULV reduction to 0.8A from its normoxic value of 1.4A. In contrast, no arrhythmia is induced regardless of shock strength in acute ischemia stage 2. An investigation of mechanisms underlying this behavior revealed that decreased postshock refractoriness resulting mainly from 1) ischemic electrophysiological substrate and 2) decrease in the extent of areas positively-polarized by the shock is responsible for the change in ULV during stage 1. In contrast, conduction failure is the main cause for the lack of vulnerability in acute ischemia stage 2. The insight provided by this study furthers our understanding of mechanisms by which acute ischemia-induced changes at the ionic level modulate cardiac vulnerability to electric shocks.

Spatial heterogeneity of action potential alternans during global ischemia in the rabbit heart

Cardiac ischemia causes beat-to-beat fluctuation in action potential duration (APD) alternans, which leads to T wave alternans and arrhythmias. Occurrence of APD alternans that is out of phase at two sites is especially important, but most APD alternans studies have involved rapid pacing of normal myocardium rather than ischemia. To determine the spatial features of APD alternans during ischemia, blood-perfused rabbit hearts were stained with 4-[beta-[2(di-n-butylamino)-6-napthyl]vinyl]pyridinium (di-4-ANEPPS) and imaged with a high-resolution camera. Hearts were perfused with oxygenated Tyrode solution at 37 degrees C for staining and then switched to a 50:50% blood/Tyrode mixture. Hearts were paced from the right ventricle at 3/s, and made ischemic by stopping flow for 6 min. Images of 10,000 pixels were obtained at 300 frames/s. Motion artifact was controlled by immobilization and by manual selection of undistorted single-pixel records. Upstroke propagation and conduction isochrones were displayed by computerized image processing. APD alternans was demonstrated in six of seven hearts, and was out of phase in different regions of the image in three hearts. The largest spatial variation in the onset of depolarization to 50% repolarization (APD50) was 155%. This caused beat-to-beat reversal of repolarization. An alternans map could be constructed for well-immobilized portions of the image. There were discrete regions of APD alternans separated by a boundary, as occurs with intracellular Ca2+ concentration alternans. Pixels as close together as 1.1 mm showed an APD alternans that was out of phase. The out-of-phase APD alternans was not due to conduction alternans, as shown by upstroke intervals and conduction isochrones. This contrasts with rapid pacing, where a causal relationship appears to exist. These new observations suggest distinct mechanisms for the genesis of arrhythmias during ischemia.

Calcium and cardiac arrhythmias: DADs, EADs, and alternans

Rapid progress has been made in understanding the molecular mechanisms by which calcium ions mediate certain cardiac arrhythmias. Principal advances include imaging of cytosolic calcium in isolated cells and in intact tissues, use of fluorescent indicators and monophasic action potentials to record membrane potentials in isolated tissue, and sequencing of the genes that encode critical ion channel proteins. In this review, five types of arrhythmias are discussed where calcium ion currents, or currents controlled by calcium, appear to be responsible for arrythmogenesis. These include: (1) the delayed afterpotential that occurs in conditions of intracellular calcium overload such as digitalis toxicity; (2) the early afterdepolarization that occurs when action potential duration is prolonged; (3) the slowly conducted calcium-dependent action potential (the slow response) in the SA and AV nodes; (4) the phenomenon of calcium transient alternans during ischemia, which is related to action potential duration alternans and t-wave alternans; (5) catecholamine-induced cardiac arrhythmias in families with mutations of the sarcoplasmic reticulum calcium-release channel. For each type of arrhythmia, the clinical implications of emerging knowledge are discussed. An especially important issue is whether ventricular fibrillation during acute coronary artery occlusion is due to calcium transient alternans. Ventricular fibrillation due to acute ischemia is an important subset of the 400,000 sudden cardiac deaths that occur annually in the U.S. Certain drugs, including beta blockers, fish oils, verapamil, and diltiazem, seem to specifically prevent ventricular fibrillation in this setting, and in most cases an effect of the drug on cytosolic calicum appears to be involved.

Myocardial dysfunction after electrical defibrillation

We hypothesized that electrical shocks that defibrillate hearts successfully also produce myocardial injury, but only in settings in which the myocardium is underperfused. Myocardial function was measured in isolated, conventionally perfused or underperfused rat hearts during sinus rhythm and conventionally perfused or underperfused hearts during ventricular fibrillation (VF) after delivery of a sham, a 0.4 J, or a 0.7 J shock. In underperfused hearts, the dP/dt, negative dP/dt, left ventricular diastolic pressure and left ventricular pressure-volume relationships demonstrated significant impairment in myocardial function. Impairment increased with the higher energy shocks. This contrasted with normally perfused hearts, whether in sinus rhythm or during VF, in which shocks resulted in no significant impairment. Electrical shocks therefore produce myocardial injury but only when myocardial perfusion is reduced.

Modelling induction of a rotor in cardiac muscle by perpendicular electric shocks

A strong, properly timed shock applied perpendicularly to a propagating wavefront causes a rotor in the canine myocardium. Experimental data indicate that the induction of this rotor relies on the shock exciting tissue away from the electrodes. The computational study reproduced such direct excitation in a two-dimensional model of a 2.7 x 3 cm sheet of cardiac muscle. The model used experimentally measured extracellular potentials to represent 100 and 150 V shocks delivered through extracellular electrodes. The shock-induced transmembrane potential was computed according to two mechanisms, the activating function and the unit-bundle sawtooth potential. The overall process leading to initiation of a rotor was the same in model and experiment. For the 100 V shock, the directly excited region extended 2.26 cm away from the electrode; the centre of the rotor ('critical point') was 1.28 cm away, where the electric field Ecr was 4.54 Vcm(-1). Increasing the shock strength to 150 V moved the critical point 1.02 cm further and decreased Ecr by 0.39 Vcm(-1). The results are comparable with experimental data. The model suggests that the unit-bundle sawtooth is responsible for the creation of the directly excited region, and the activating function is behind the dependence of Ecr on shock strength.

Cytochalasin D as excitation-contraction uncoupler for optically mapping action potentials in wedges of ventricular myocardium

INTRODUCTION

Cytochalasin D in tissue bath superfusate inhibits the contraction of isolated thin trabeculae from canine right ventricle without affecting the intracellular action potential recorded with glass microelectrode. The purpose of this study was to test whether cytochalasin D could also be used to immobilize perfused wedges of ventricular muscle without affecting the action potential duration or propagation, and also to determine the optimal concentration and time duration of drug in the perfusate.

METHODS AND RESULTS

Using a membrane potential sensitive dye, di-4-ANEPPS, and a high-resolution photodiode optical mapping system at a rate of 1,000 frames/sec, we recorded action potentials on the transmural surface of arterially perfused wedges of muscle from the canine left ventricular free wall. We also recorded arterial pulse pressure as a surrogate for tissue contraction. Cytochalasin D at > or = 20 micromol/L in the perfusate for > or = 6 minutes reduced the arterial pulse pressure to approximately one tenth of its initial value and significantly reduced or eliminated motion artifacts in the action potentials. A sustained concentration of 10 micromol/L cytochalasin D in the perfusate prevented contraction from recurring after the tissue was immobilized with an initial concentration of 25 micromol/L. Cytochalasin D had little effect on the action potential duration and on its transmural gradient, and did not slow the transmural velocity of excitation propagation.

CONCLUSION

Cytochalasin D can be used to uncouple excitation and contraction in perfused canine cardiac muscle for the fluorescent-optical mapping of action potentials without affecting action potential duration or slowing transmural propagation.

Progressive depolarization: a unified hypothesis for defibrillation and fibrillation induction by shocks

Experimental studies of defibrillation have burgeoned since the introduction of the upper limit of vulnerability (ULV) hypothesis for defibrillation. Much of this progress is due to the valuable work carried out in pursuit of this hypothesis. The ULV hypothesis presented a unified electrophysiologic scheme for linking the processes of defibrillation and shock-induced fibrillation. In addition to its scientific ramifications, this work also raised the possibility of simpler and safer means for clinical defibrillation threshold testing. Recent results from an optical mapping study of defibrillation suggest, however, that the experimental data supporting the ULV hypothesis could instead be interpreted in a manner consistent with traditional views of defibrillation such as the critical mass hypothesis. This review will describe the evidence calling for such a reinterpretation. In one regard the ULV hypothesis superseded the critical mass hypothesis by linking the defibrillation and shock-induced fibrillation processes. Therefore, this review also will discuss the rationale for developing a new defibrillation hypothesis. This new hypothesis, progressive depolarization, uses traditional defibrillation concepts to cover the same ground as the ULV hypothesis in mechanistically unifying defibrillation and shock-induced fibrillation. It does so in a manner consistent with experimental data supporting the ULV hypothesis but which also takes advantage of what has been learned from optical studies of defibrillation. This review will briefly describe how this new hypothesis relates to other contemporary viewpoints and related experimental results.

Transmembrane potentials during high voltage shocks in ischemic cardiac tissue

Transmembrane, voltage sensitive fluorescent dye (TMF) recording techniques have shown that high voltage shocks (HVS), typically used in defibrillation, produce either hyper- or depolarization of the transmembrane potential (TMP) when delivered in the refractory period of an action potential (AP) in normal cardiac tissue (NT). Further, HVS produce an extension of the AP, which has been hypothesized as a potential mechanism for electrical defibrillation. We examined whether HVS modify TMP of ischemic tissue (IT) in a similar manner. In seven Langendorff rabbit hearts, recordings of APs were obtained in both NT and IT with TMF using di-4-ANEPPS, and diacetylmonoxime (23 microM) to avoid motion artifacts. Local ischemia was produced by occlusion of the LAD, HVS of either biphasic (5 + 5 ms) or (3 + 2 ms) or monophasic shapes (5 ms) were delivered at varying times (20%-90%) of the paced AP. Intracardiac ECG and TMF recordings of the TMP were each amplified, recorded, and digitized at a frequency of 1 kHz. The paced AP in IT was triangular in shape with no obvious phase 3 plateau, typically seen in NT. There was normally a reduced AP amplitude (expressed as fractional fluorescence) in IT (2.6% +/- 1.79%) compared to 3.8% +/- 0.66% in NT, and shortened AP duration (137 +/- 42 vs 171 +/- 11 ms). One hundred-Volt HVS delivered during the refractory period of paced AP in IT in five rabbits, elicited a depolarization response of the TMP with an amplitude up to three times greater than the paced AP. This is in contrast to NT where the 100-V HVS produced hyperpolarization in four hearts, and only a slight depolarization response in one heart. These results suggest that HVS, typically delivered by a defibrillation shock, modify TMPs in a significantly different manner for ischemic cells, which may influence success in defibrillation.