Welcome to Core Concepts in Electrophysiology. My name is Gordon Tomaselli, and I'll be talking about basic electrophysiology principles for the clinician today. These are my disclosures, none of which are relevant to this talk. So the outline for the test, so the map for the test, is shown in the URL at the bottom of this slide. What I'll be talking about today are basic principles, including ion channels, currents, and action potentials, as well as arrhythmia mechanisms, and how drugs interact with these arrhythmia mechanisms. So this is the setup for recording ionic currents and action potentials. It's the activity of the former that inscribes the action potential, which is nothing more than the change in transmembrane voltage over the cardiac cycle. One impales a cell and measures the electrical potential inside the cell, you'll find it's very negative. With stimulation of that cell, you get a stereotypic and regenerative response that is the cardiac action potential. It is the both active membrane properties, as well as the network properties of the heart, action potentials, currents, and tissue structure that inscribe the features, the distinct features of the electrocardiogram. So for example, the PR duration reflects activation of atrial myocytes, inscription of the P wave, and slow calcium-dependent conduction through the AV node. The QRS is really the summation of phase zero of the action potentials throughout the ventricle and his Purkinje system. And the QT interval reflects the time from activation to recovery of the ventricle and really is dominated by the plateau of the action potential, which is a balance between repolarizing potassium currents and inward depolarizing calcium currents. So before we move on, a few definitions. The ion channels in the heart are really categorized by the biological stimulus that causes them to open. The majority are voltage-gated channels, and these are sodium, calcium, and potassium channels. There are chemicals like acetylcholine and ATP that affect the function of potassium channels, and these are ligand-gated channels. There are several biophysical features of these channels that are important, including permeation, which is the method by which ions selectively go through the pore of these channels. Now the driving force for any given ion happens to be the combination of the electrical and chemical gradient. And in fact, when the electrical gradient and chemical gradient are balanced, this is called the zero current or reversal potential. Gating is a process that describes the opening and closing of these channels. And rectification, like an electronic rectifier, is the process by which a channel will promote conduction in one direction preferentially to another direction. This could be mediated by gating processes or features of the pore and permeation itself. Current magnitude is the product of the number of specific ion channels in the membrane, the probability that any one of them is opened, and the single-channel current amplitude. So again, action potentials are inscribed by ionic currents, which vary regionally in the heart. Both depolarizing and repolarizing currents vary regionally in the heart and result in a difference of appearance of the action potential in different regions of the heart, like in the ventricle and the atrium here. So let's go over the phases of the action potential using the ventricular action potential as an exemplar. Phase 0 of the action potential is the result of sodium entering the cell and producing a rapid change in membrane voltage. Phase 1 is due to closure of sodium channels and actually activation of some potassium channels. Phase 2 and phase 3 are the plateau. And they are the result of the balance between inward calcium current and outward potassium current. Ultimately, fortunately, potassium currents win. The cell completely repolarizes and is poised to be reactivated again. Again, there are differences here between atrium and ventricle, but there are also differences in these ionic currents and action potentials within chambers of the heart. So for example, in the right atrium around the SA node, the action potential is mediated by slow calcium-dependent upstroke and has a relatively rapid phase 4 diastolic depolarization. It is the predominant pacemaker in the heart. Compare that to the AAV node, where upstroke's a little bit faster, but diastolic depolarization is slower. It is a subsidiary pacemaker. In the atrial muscle itself, the upstroke of the action potential is mediated by sodium current. There's also regional heterogeneity across the wall of the ventricle in the following fashion. Action potentials on the endocardial side are longer than on the epicardial side. This creates a situation where activation is in the polar opposite direction as recovery. So the heart is activated from endocardium to epicardium, but recovers from epicardium to endocardium. What this creates is a polarity of the QRS complex and the T wave in the electrocardiogram that are in the same direction normally. Now altered transmural heterogeneity of the action potentials can be seen in several pathologic conditions, including ST changes in ischemia, in Brugada syndrome, there's a dramatic difference in early repolarization in the endocardium and in the epicardium. And this heterogeneity can also produce morphologic changes in the ST and T waves in congenital and drug-induced long QT syndrome. So the fundamental drivers of all electrical activity in the heart are ion channels. These are transmembrane glycoproteins that are comprised of four internally homologous repeats. Each of one has a highly charged S4 domain that actually serves as a voltage sensor. These alpha or main subunits or alpha one subunits for sodium and calcium channels respectively wrap around an ion-selective pore and combine with other subunits in the membrane to create the holochannel. And the holochannel has distinct functional and pharmacologic features that we'll come back to later. Potassium channels are similar, except the alpha subunits are one quarter the size of the subunits for sodium and calcium channels. They tetramerize again with subsidiary subunits to form the intact potassium channel. One of the unique features of these channels is that there's a lot of diversity in structure, particularly in the potassium channel side, but it seems that all of these channels derive from the same progenitor. That is a two-membrane spanning repeat inwardly rectifying channel, which perhaps by a couple of rounds of gene duplication form twin pore channels and voltage-gated potassium as well as sodium and calcium currents. And again, as a reminder, these channels tetramerize, associate with ancillary subunits to form the intact channel. Now there's another set of ion channels that are very important in the heart. These are part of the network property of the heart, and these are gap junction channels, which are formed by proteins called connexins of various types. These are transmembrane proteins with four-membrane spanning repeats that hexamerize to form an intact hemichannel or channel in one membrane, also called a connexon. This connexon looks for another connexon in an adjacent membrane, and together, they form an intact gap junction channel. These have a very specific appearance on electron microscopy as shown here. It's a pentolaminar membrane, which reflects the apposition of two adjacent membranes. These are also pretty diverse and heterogeneous, and they're heterogeneous because there are a number of connexin proteins, which can combine in various ways to produce different types of gap junction channels. When it's all one protein, they're homoameric and homotypic. When it's all one protein in each hemichannel, it's homoameric heterotypic, and when they promiscuously recombine, they're heteroameric and heterotypic gap junction channels. Now I think it's important to consider the membrane physiology that these channels live in. And as I mentioned, the driving force for ion flow in the heart is generally the electrochemical and concentration gradient for ion channels. That is passive transport. Transporters will actively transport ions, but in the circumstance of ion channels, it's the electrochemical gradient that's important. And there are differences in the chemical concentration of various ions in the resting heart cells. For example, at rest, sodium is very low inside the cell and high outside the cell, and by the way, the cell is, as we saw previously, electronegative on the inside, electropositive on the outside. This creates a concentration gradient for flow from high to low for sodium, as well as an electrical gradient from positive to negative. So there's a strong force for sodium channels to flow from the outside to the inside, but at rest, sodium channels are closed, so there really isn't any current flow. In contrast, potassium is high on the inside and it's low on the outside, and in fact, this creates a concentration gradient for flow from in to out, which is balanced by an electrochemical gradient because of the electronegativity inside for flow from out to in. And it turns out that in fact there are potassium channels open at rest, the inward rectifier potassium channel, which actually governs the resting membrane potential, or the Nernst potential, again, which is determined by the ratio of potassium ions both inside and outside the cell. Now, if we change the extracellular potassium, what we can do is bias the membrane potential in a way to depolarize the cell. This depolarization of the cell results in channel opening, and in particular, the sodium channel opens, and there's a rush of sodium into the cell. And in working myocardium-like muscle, the action potential upstroke, or phase zero, is inscribed. Now, there are a number of distinct time-dependent behaviors of ionic currents, and in order to measure this, we need to be able to control the membrane potential, and we can do that by a process called voltage clamp. Here, we control the transmembrane voltage of the cell, the experimentalist does, and we can measure the current, both inward and outward current, as a result of this change in membrane voltage. A couple of conventions here first. Currents that are upward represent outward flow of cationic current from the cell interior to the outside. Currents that are downward represent cationic current flow into the cell. Now, there are a variety of different time-dependent behaviors of these currents, and in fact, in this example, this represents a sodium current, which opens quite rapidly, and actually closes despite the stimulus to remain open, that is, the depolarized membrane voltage. This kind of closure is called inactivation, current waning despite the maintained depolarized voltage, and occurs not only in sodium currents, but also calcium currents and transient outward currents. And by the way, this activation really is quite rapid in onset. Compare this to this potassium current, in which opening is really quite delayed, and is relatively slow after activation. And what's not shown here is that this channel tends to conduct current in the outward direction, so it's a delayed rectifier current, and the rectification is in the outward direction. There are other currents that are not shown here that are relatively time-independent, and actually change their activity when the voltage changes, and do so almost instantaneously. An example is the inward rectifier potassium current. Now, potassium currents are probably the most diverse set of voltage-dependent currents in the heart, and they're diverse because of functional differences, including the rate at which they activate, and the degree and type of rectification. So IKS is the slow component of the delayed rectifier, and the time base here is one second, so it turns on really quite ponderously, quite slowly, and it tends to conduct current in the outward direction, so therefore an outward rectifier. It's encoded by a gene called KVLQT1, and a protein called KVLQT1, and a gene, KCNQ1 and mutations in KCNQ1 cause type 1 long QT syndrome. These currents are responsive to adrenergic stimulation. Another delayed rectifier is the rapid component of the delayed rectifier, encoded by a gene called KCNH2, or the HERG protein, and HERG mutations cause long QT syndrome type 2, and virtually all drugs that cause torsades de plantes are IKR blockers. And finally, there's the rapid, the ultra rapid delayed rectifier potassium current, which is encoded by a gene called KCNA5, and this one's distinctive because it's detected in the atrium and not the ventricle, and has been a long sought target for therapeutics for atrial fibrillation, and again, turns on really quite rapidly here. The time base here is 10 milliseconds versus a second in these other two cases. So the other kind of electrogenic feature of the heart is calcium handling, and what happens after electrical stimulation of a heart cell has to do with the changes in intracellular calcium, which mediate contraction. So with stimulation and the upstroke of the action potential, sodium rushes into the cell. As sodium rushes into the cell, it depolarizes the cell membrane and actually causes a number of other changes. The number of other changes that occur are a catalytic increase in calcium current, which produces a large release of calcium into the sarcoplasmic reticulum, and the inscription of the calcium transient. This large increase in calcium into the cytosol binds to the contractile apparatus causing contraction of the cell. Now, you have to get rid of this calcium in some way to allow the cell to be poised to be reactivated and contract again, and the way that happens is one of two mechanisms, either reuptake of calcium into the sarcoplasmic reticulum to be available for the next catalytic release of calcium, or extrusion of calcium from the cell through an electrogenic sodium-calcium exchanger. In structural heart disease, this is remodeled so that more of the calcium is extruded from the cell than under normal conditions, and this results in a couple of consequences, including a loss in what's called a positive force-frequency relationship, that is, contraction tends to increase as the cardiac cell is contracting. This force-frequency relationship is no longer positive, for example, in a structurally diseased heart, but may be flat or, in fact, negative. The other thing that calcium handling does is it influences the rate of diastolic depolarization and, therefore, automaticity, which, again, is a reflection of pacemaker activity and phase IV diastolic depolarization. So under normal conditions, and under normal conditions in areas of the sinus node and some parts of the AV node, as well as the infranodal conduction system, but not normally in working muscle, atrial or ventricular muscle, phase IV, the transmembrane potential gradually increases during diastole or during phase IV, and this is the result of activity of pacemaker channels, or HCN4 channels, as well as a specific type of calcium channel and sodium-calcium exchanger. Under pathologic conditions, these currents may be altered, and abnormal intracellular calcium cycling may produce a change in the rate of phase IV diastolic depolarization, and, therefore, the firing rate of these action potentials. What are the other things that can change the firing rate of these action potentials in pacemaker tissue? Well, one of the things that can happen is that the threshold for excitation of calcium current, which are the currents that lead to the upstroke of this type of action potential, can be increased by things like hypercalcemia or calcium channel blockers. The rate of phase IV diastolic depolarization, or the slope, can be depressed by things like hyperkalemia, beta blockers, and evabradine, slowing the rate of firing. Phase III of the action potential can be more negative, with or without a change in the slope of phase IV, and we see this with activation of acetylcholine and adenosine-activated potassium currents. And finally, prolongation of the action potential itself can delay repolarization such that in automatic tissue, it can also slow the rate of firing. So let's talk about a couple of other properties of excitable tissue. As you recognize from the electrophysiology laboratory, with increasing prematurity of an S2 compared to an S1, there's less likely to be activation of that S2 and block. And what might happen is that stimulus strength might be needed to be increased. And in fact, at extremes, the phase zero of the stimulated beat may be slowed. So as you encroach on phase III of the action potential and get closer and closer to the previous action potential, there's a point at which a stimulus, irregardless of strength, is unable to elicit another action potential. This defines the absolute refractory period. The relative refractory period is a period of time where you can actually stimulate a second action potential, but it requires an increase in stimulus strength and sometimes may be reflected by a slowed-up stroke. There's some debate about whether or not there's supernormality, that is, a stimulus of less amplitude can stimulate an action potential at times immediately after repolarization when the voltage might be closer to the threshold for activation of the sodium current. This is all a function of the fact that sodium channels recover from inactivation as a function of voltage. And that's described by this curve, which is a steady state inactivation curve, which describes a fraction of channels available for excitation as a function of voltage. And as you get to more depolarized voltages, indicated by the star here and here, far fewer sodium channels are available when an insufficient number are available, no stimulus of any strength will elicit another action potential. So the other feature that we need to talk about is conduction. And conduction is not only a function of active membrane properties of the heart, like we talked about action potentials and ion channels, but it's also a function of the network properties of the heart. That is, the structure of the heart, the connectivity between cells through gap junction, the presence of fibrosis, and the presence of edema. So in the heart, there are actually very few places where an activation wave is linear. And in this case, the conduction velocity equals theta 0. More often than not, activation is curvilinear. And in the case of a convex activation wave, the conduction velocity compared to a planar wave is slower. That is, the source of the activating current is smaller than the sink. In contrast, a concave activation wave has a conduction velocity that's faster than a planar wave. And that's because the source of the activation is larger than the sink in this case. There are a couple of places of transition in the heart that are important to consider, where the source and the sink are a bit mismatched. Let's consider the Purkinje fiber network and the transition from a Purkinje fiber to a ventricular muscle. This is a small source activating a larger sink. Tendency for block is high. And in fact, this is sometimes a target for ablation in the case of PVCs that are initiated in the Purkinje system, and in fact, a strategy to denetwork the Purkinje fiber from ventricular muscle in the case of ablation of polymorphic ventricular tachycardia and ventricular fibrillation. Just the opposite is seen in the input into an accessory pathway from atrial or ventricular muscle, where a large source activates a smaller sink. Conduction block less likely here. On the other side, when the sink is activating muscle on the other side of the accessory pathway, block is more likely analogous to the Purkinje fiber ventricular muscle transition. So rate or prematurity dependence of conduction in the heart can tell you a bit about what's mediating that conduction. So in fast response tissue, that is tissue where conduction is mediated by sodium current, the response over a range of rates and prematurity tends to be all or none. It's there or it's not there. In contrast, slow response tissue, where conduction is mediated by calcium currents, tends to have rate and prematurity dependence, being faster at slower rates and longer premature intervals and slower as the rate gets faster and the coupling interval gets shorter. And that's reflected, for example, in the PR interval lengthening with faster heart rates. So in summary, the ionic mechanisms of fast and slow responses are shown here. Fast response tissue has action potentials with a rapid sodium channel dependent upstroke, fast all or none conduction, and voltage dependent recovery from inactivation. Slow response tissue generally has phase 4 diastolic depolarization, slower upstrokes, and decremental conduction, and has, in addition to voltage dependent, time dependent recovery from inactivation. Again, seen here in the SA node, the AV node, and depolarized tissue, as opposed to fast response tissue, which are seen in atrium, ventricles, and the Hesper-Kinji system. The other feature about conduction is that it's anisotropic and discontinuous. What does that mean? It means there's a directionality to conduction. That is, electrical activity tends to be conducted more rapidly along as opposed to across the myocardial cell and cell structure. This is because the determinants of conduction velocity connections tend to be distributed at cell ends, as does sodium current. And cell shape and alignment actually determines conduction velocity. Conduction throughout a cell is much faster than conduction across cells, where the activation wave encounters multiple high resistance membranes. Again, the consequence here is side-to-side conduction is much slower than end-to-end conduction on the order of two to three times faster along versus across fibers. The other feature of conduction is that it appears on the macroscopic level to be continuous. But in fact, the activation wave, when transiting through myocardial tissue, is trying to find the pathways of least resistance. And the pathways of least resistance really are encountering as many of these cell ends as possible, where there's a high concentration of connections, intercellular channels, and sodium currents that mediate conduction. Armed with this, let's think a little bit about arrhythmias and arrhythmia mechanisms. There are three main arrhythmia mechanisms, two of which are the result of abnormalities of impulse initiation. One is a result of an abnormality in impulse propagation. So enhanced automaticity and triggered arrhythmias are the result of impulse initiation. And reentry is the result of an abnormality in impulse propagation. I would say here that these are not mutually exclusive mechanisms. So for example, triggered beats may engage a pre-existing reentrance circuit and create arcs of functional block generating the substrate for reentry, for example. One considers automaticity. There are several types of automaticity. Enhanced normal automaticity occurs when a normal pacemaking tissue, like the sinus node, is firing at a more rapid rate, or can be abnormal automaticity. This is enhanced normal or abnormal automaticity when a subsidiary pacemaker takes over and fires at a faster rate than the sinus node. Warm up, that is a gradually increasing rate, is highly suggestive of an automatic focus. And you may transiently suppress, with overdrive pacing, an automatic rhythm, which then will gradually rewarm. The rate of these automatic rhythms may increase with sympathetic stimulation. They may also increase with stretch, ischemia, and hypokalemia, which increase the rate of phase four diastolic depolarization. In the situation of triggered automaticity, stimulation of a myocyte of the heart may, under non-physiologic conditions, give rise to pathologic non-driven upstrokes that are the result of after depolarizations. And after depolarizations occur in a couple of different types, either early during the repolarization phase of the action potential, or delayed after the action potential is fully repolarized. There are different exciters, inciters, exaggerators, and blunters of each of these types of early after depolarizations. And in fact, there's some evidence that there may be convergence of these mechanisms under some conditions. So for early after depolarizations, QT prolonging drugs will incite these. Slow rates and hypokalemia will exaggerate them. And they can be blunted by increasing the rate, hence overdrive pacing in these types of arrhythmias, increasing potassium, and increasing magnesium. In the case of delayed after depolarizations, digitalis and catecholamines tend to lead to increased loading of SR calcium and release, leaky release of that SR calcium is the thing that inscribes DADs. Rapid rates exaggerate these. Calcium channel block may alleviate them. The mechanism here in EADs tends to be an increase in inward plateau current, often L-type calcium current. And the DAD is, again, a consequence of SR calcium overload and leaky calcium release channels or ryanodine receptor channels. These may be seen in DIG toxicity and ischemia and may be seen, EAD-based arrhythmias may be seen in TORSA. So reentry is probably the arrhythmia mechanism that's going to preoccupy you most in the electrophysiology laboratory. And as an exemplar here, I'm using AV node reentry tachycardia. And what are the requirements? Well, the requirements are that there be two pathways for conduction of an activation wave, that those pathways have heterogeneous electrical properties. There's block in one and propagation in the other. So for example, in the case of AV node reentry tachycardia, an atrial premature beat may be conducted to the ventricle but will block in one pathway, that is, the fast pathway with longer refractory periods, and then be conducted down to the ventricle over the slow pathway that has a shorter refractory period. It can then activate the ventricle. And if it finds the fast pathway excitable, can reenter the fast pathway in the retrograde direction and reactivate the atrium over the fast pathway in a retrograde direction and continue multiple times. So again, two pathways, heterogeneous properties, block in one, conduction retrograde in the other. What this really implies is that this type of reentry or anatomic reentry has an excitable gap. And what does that mean? It means that the activation wave itself has tissue that's in various stages of recovery from recently activated to nearly recovered. And this creates a large section of tissue or a section of tissue that has yet to be activated or has previously been activated. And now, stimulation at this point will find enough in muscle sodium channels available to initiating a propagating impulse that will activate both in the orthodromic and antidromic direction and reset the tachycardia. This raises the concept of the wavelength of the tachycardia in anatomic reentry, which is the product of the conduction velocity and the refractory period. And in order for anatomic reentry to continue, the wavelength of the tachycardia has to be shorter than the path length or the anatomic circuit in which it lives. There are a number of circumstances in which the wavelength of a tachycardia can live, including AV node reentry, as we discussed, AV reentry, and scar-related ventricular tachycardia. These real circuits are highly heterogeneous in terms of their electrophysiologic properties, induction refractoriness. So these things can actually show a fair degree of wobble in the rate of the tachycardia as well as differences in variability in the excitable gap. The other form of reentry is something called functional reentry, and there are a couple of varieties of functional reentry, including leading circle reentry, which I won't talk about in detail, but suffice it to say, in both cases, leading circle reentry and spiral wave reentry, the circuit is as small as it can possibly be. That is, the wavelength of the tachycardia roughly approximates the path length or the path over which the activation wave travels. In spiral wave reentry, the center or the core is continuously excitable, and there's a place at the tip of a spiral wave where the wave back, in dotted lines here, and the wave front, in solid lines here, meet, and where all phases of the action potential exist in one place. These activation waves or spiral waves have differences in conduction velocity based upon the curvature, and this is a function of the tissue itself, the curvature of the wave itself. More curved portion of the spiral wave has a slower conduction velocity. Again, as we talked about previously with conduction, this curvature slows the conduction velocity. That that's more linear has a bit faster conduction velocity. These spiral waves may undergo collision and wave breaks, and in this sense may contribute to fibrillatory conduction. They may also be continuously variable or settle into specific pathways producing blutter-like arrhythmias. There are, in fact, transient, small, excitable gaps that may be present in spiral wave reentry, but entrainment is generally not possible, in this circumstance. Now, it might be easier to look at this, and I'm not sure that this is gonna play. Oh, it does, in the form of a movie, and this is a co-culture of neonatal rat ventricular myocytes, as well as fibroblasts that are exhibiting two spiral waves which collide and merge in the center of the culture and activate the entire culture itself. So this is not collision and fibrillatory conduction, but this is collision and merger and activation. Now, how does one approach a spiral wave therapeutically? Well, one of the things that might be possible is to increase the wavelength of the tachycardia so it no longer fits into the path length, and this could result in extinction of the reentry mechanism. Class I antiarrhythmic drugs, which actually slow conduction, will decrease the wavelength of the tachycardia and may allow for it to fit in the same path length and be proarrhythmic, albeit perhaps with a bit slower tachycardia rate. So the right approach or the preferred approach would be to try and change the wavelength to path length relationship in such a way that reentry is no longer possible. So those are mechanisms that contribute to the genesis of arrhythmias. Well, what needs to happen to really create an arrhythmia? And in fact, usually it's not a single mechanism, but multiple mechanisms. And I'm using here torsades de pointes as an example of what creates an enhanced arrhythmia susceptibility and the development of a tachycardia. So we talked about the normal difference in action potentials duration across the wall of the heart. So an exaggeration of this dispersion of repolarization may create some susceptibility, but generally that's not sufficient in and of itself to create an arrhythmia. Instead, some other things have to happen. So for example, exposure to an antiarrhythmic drug that differentially prolongs the action potential duration in different layers of the heart enhances dispersion of repolarization or and or hypokalemia and or slow rates can prolong recovery and change the morphology of the ST and the T waves. This may result in an arrhythmogenic mechanism in and of itself, a triggered beat and PVCs, and these can actually continue persistently creating something called persistent bigeminy, or as I mentioned previously, they may engage a re-entry circuit and produce a tachycardia. So on this side of the tracing is persistent bigeminy, perhaps triggered in nature. Ultimately after a long short initiation sequence, polymorphic ventricular tachycardia, which may be intramural re-entry based on this heterogeneity of repolarization. So a number of things, there has to be a conspiracy, in fact, that comes together to create the situation where an arrhythmia would occur. The other thing that this does is it actually raises a question about the limitation of limitations of antiarrhythmic drugs. In this case, it was a contributor to the genesis of an arrhythmia, not the prevention of an arrhythmia. And I think it's important that we understand the mechanism by which these drugs work, not only for their beneficial effect, but for the potential for them to cause mischief. So antiarrhythmic drug actions, really primarily in terms of the ones that are used most prominently, are their mechanism of action is ion channel block. Drugs that block sodium channels are considered to have class one action, they slow conduction, and their effect is exaggerated at faster rates. This is something called forward use dependence, a useful property for treating tachyarrhythmias. Calcium channel blockers are classified as having class four action. And again, antiarrhythmic drugs have multiple actions, so you classify their actions, not the drugs themselves. This can cause avianodal conduction slowing. Potassium channel blockers will prolong the action potential duration in class three action and do so with different kinetics. So IKR will prolong the QT interval on refractory periods that's exaggerated at slow rates. This is something called reverse use dependence, which is not a particularly useful property for a drug. IKS blockers will do the same thing with less reverse use dependence. And again, IKUR tend to be atrial specific and their targets for drugs that are aspiring to pharmacologically convert atrial fibrillation. Adrenergically dependent arrhythmias may be treated by drugs with class two action that is sympathetic blockers. And it's important to remember that all of these actions are modulated by ancillary drug properties, their effect on the autonomic nervous system, as well as other features of the arrhythmia and heart function, including heart rate, extracellular potassium, pH, and a constellation of ion channels that exist in different regions of the heart. So this heterogeneity is not only important for the shape of the action potential, but for the response to antiarrhythmic drugs. So amiodarone and flecainide have a number of different effects, such that in the atrium, what they do to the atrial action potential is a little bit uncertain and condition dependent, as opposed to purer IKR blockers like dafetalide and dronadarone, which tend to prolong the action potential duration in the atrium. Now, more robust expression of IKR, these delayed rectifiers, and IKS in the ventricle really means that amiodarone and all the IKR blockers more consistently prolong the action potential duration. There are drugs like the potassium channel opener, nicorandal, and ranolazine that can shorten the action potential duration under particular conditions, ranolazine when there's an increase in the late sodium current. And sodium channel blocking drugs, again, can, under the right conditions, slow the rate of rise of phase zero of the action potential. This is particularly true for flecainide and propafenone, but at usual rates, not so much lidocaine. So this leads to the concept of phasic or state-dependent block. And state-dependent block occurs in the case of sodium channels for the following reason. Sodium channel blockers tend to interact with open or inactivated states of the channel represented by red and blue here and tend to unbind when the channel is closed. What this means is that the percentage of channels that are blocked at fixed rates increases with increasing stimulation. This is due to relatively slow recovery from drug block during diastole. These recovery rates can be decreased even further and the extent of block increased under various conditions like ischemia and the intrinsic property of some of these drugs have a slower off-rate like those like flecainide and propofenone. The other concept here is the rate dependence of block, so changing rate, what is the impact on block by sodium channel blockers? Now, at resting rates, the percentage of the duty cycle in which the sodium channels occupy either the open or the inactivated state is shown here. At increasing rates, the fraction of the duty cycle occupied in which the sodium channel exists in the open or the inactivated state increases substantially. What this means is that there's less time for the channels to recover from block at faster heart rates and there's accumulation of block during tachycardia. The extent of block really produces the extent of use dependence or rate-dependent block. Now, this varies. Rate-dependent block is determined primarily by the unbinding rates of various drugs. As I've alluded to previously, the rate of unbinding of sodium channel blockers from the channels varies. This is a table of drugs that block sodium channels. We'll come back to the von Willem classification in just a minute. Again, sodium channel blockers are considered to have class one action. This class one action exists in several subclasses and the subclasses are determined primarily by the rate of unbinding of these drugs from the channel. So class 1a drugs like quinidine, procainamide, and disopiramide have an intermediate off rate and consistently will prolong the action potential duration. This is due to block of delayed rectifier potassium currents. They also have other ancillary properties like being alpha blockers, ganglionic blockers, and anticholinergics. Drugs like lidocaine and maxillitine have class 1b actions and they have very fast off rates and in fact fast enough so that they really don't affect either excitability or action potential upstroke, even attack of cardio rates. Drugs with class c action have a ponderously slow off rate like propofenone and flaconide and also other ancillary effects that contribute to their anti-arrhythmic and pro-arrhythmic effects, quite frankly. Class 1d drugs are a new classification. Again, slow off rates, only one agent here, ranolazine, class 1d because in addition to everything else it does it's a relatively preferential, and this is all relative, late sodium channel blocker. Drugs with class reaction like amiodarone also block sodium currents with intermediate off rates and there are other medications that are used that have putative effects on channels including amitriptyline, the tricyclics like amitriptyline and imipramine which bind to sodium channels with relatively fast off rates and have a number of other effects. These are all important not because necessarily of their anti-arrhythmic properties, but it's important to recognize that they may also have pro-arrhythmic effects on cardiac tissue. So, the other feature that anti-arrhythmic drugs modulate is refractoriness or excitability. It's easy to understand, for example, why drugs with class 3 action that prolong the action potential duration increase refractoriness, but sodium channel blockers, even if they don't prolong the action potential, will decrease membrane responsiveness and increase refractoriness. That's because sodium channel blocking drugs will shift the steady state inactivation curve in the left direction to more negative voltages. What does this mean? It means that at any given voltage, the number of sodium channels available to open in the absence of drug is greater than that in the presence of drug. If one considered this along the trajectory of the action potential in the absence of drug, at this point, if this represents this fraction of sodium channels available, the cell has to repolarize even further to create the same fraction of sodium channels available. So, decreasing excitability, increasing refractoriness because of the need for the drug to unbind from the channel. So, let's go back to the antiarrhythmic drug classification scheme. There are a number of classification schemes that have been proposed. The one that's used most is the Vaughn-Williams classification, which was first introduced in 1970. At that time, there were four classes of drug action, sodium channel block one, beta block two, action potential prolonging class three, and class four was calcium channel blockers. It's become apparent that there are a number of antiarrhythmic drugs that have actions outside of these four classes, and now the class, Vaughn-Williams classification system includes classes zero to class seven. Drugs with class zero action block phase four diastolic depolarization. These are HCN blockers. Vabradine is one that's released. Class five drugs block the transient receptor potential channel. There aren't any that are FDA approved right now, and by the way, there hasn't been a new antiarrhythmic drug approved since 2008, so we're still working on this. Drugs with class six action affect gap junction channels. Again, none of these are released for use in the United States, and class seven drugs really are a large class of variable actions of drugs that impact upstream modulatory effects, including things like fibrosis and things like inflammation. There's been a thought to expand some of these classifications. For example, class two drugs to include anything that modulates the autonomic nervous system, either beta blockade or sympathetic activation, and class three, it's been proposed, should include anything that modulates the action potential duration, including things that might shorten the action potential duration, like IKCH blockers. So, as I mentioned, no new antiarrhythmic drugs approved in the U.S. for nearly a couple of decades now, and this really kind of motivates new strategies, targets, and preparations for antiarrhythmic drugs, and one of the big and more recent targets, upstream targets, like tissue structure, inflammation, and fibrosis, and this is particularly notable in the use and in the prevention of atrial fibrillation, so a number of things have been evaluated, including ACE inhibitors, ARBs, ARNIs, anti-inflammatory drugs, inhibitors of the inflammasome itself. ROS inhibitors and cam kinase inhibitors have been thought to be useful in some ischemia-associated arrhythmias, and again, the search for specific targets and currents, particularly when we're targeting an atrial arrhythmia, so IKUR, there's a drug called Vernaculant, but this also has sodium channel blocking effects. Targeting the late current with rinolazine has been looked at in studies of ventricular tachycardia, and in the RAID series of studies of atrial fibrillation, again, as I mentioned, connects in modulation through the use of gaptides, and targeting overloaded and leaky calcium release channels in the SR with flecainide and carvedilol has been useful in things like catecholaminergic polymorphic VT. Again, another target is channel remodeling and trafficking pathways. The other thing that's been looked at are different preparations of drugs, particularly for use in acute onset situations like inhaled calcium channel blockers in the NODE studies to treat PSVT, and inhaled flecainide in the instant studies for treatment of atrial fibrillation. Now, there's another group of compounds that have been evaluated for their effect to control arrhythmias, and these are drugs that have been FDA approved for other purposes and are being looked at in a repurposed manner. These include antineoplastic drugs like the CDK inhibitor riscovatein, which blocks calcium channels, a number of drugs that are used in treatment of neurologic disease, the gabapentins, fulnarazine, ralulazole, which blocks sodium and calcium currents, and recently, drugs that have been used to treat obesity and type 2 diabetes, the SGLT2 inhibitors, which also inhibit late sodium current. There are drugs that are used in pulmonary disease and respiratory stimulant, which blocks twin pore potassium channels, and again, a number of upstream targets that aspirationally want to affect inflammation and fibrosis like botulinum toxin, colchicine, which inhibits the inflammasome, canakinumab, which is a monoclonal antibody to IL-1 beta, and metformin, which is proposed to have some antifibrotic properties. Again, important to recognize because these may be agents to use to treat arrhythmias, but even more importantly, because you should recognize the potential proarrhythmic effects of these drugs in patients being treated with them. So, one of the historic properties of antiarrhythmic drugs is the problem of the toxic-to-therapeutic window. The EC50 curves for these drugs and the side effect curves, under some circumstances, when efficacy may appear in many patients, is reasonable with manageable separation and manageable toxicity. Unfortunately, all too often, the efficacy curve is blunted and shifted to the right, so higher concentrations are needed for even less efficacious treatment of arrhythmias, and this starts to impinge upon the side effect profile curve, narrowing the toxic- to-therapeutic margin, narrowing the number of patients that might benefit, and increasing the potential for serious and dose-related toxicity. Now, there are a number of other features of high- risk pharmacokinetics that hamper some antiarrhythmic drugs and other drugs, but one of them is the need to convert a prodrug into an active metabolite, and if somebody is a poor metabolizer, efficacy of these drugs may be compromised because of the lack of an active metabolite. Drugs like a drug that we no longer use, Enkinide, is a prodrug that needed to be metabolized, and clopidogrel, which is an antiplatelet drug, also needs to be metabolized. The other liability is when there's failure of an excretory organ, and failure of an excretory organ, for example, liver failure, increases the risk of side effects from drugs like propofenone and warfarin, which are metabolized by CYP isoenzymes in the liver. Finally, when a drug uses a single pathway for elimination, like sotalol and dofetilide in the kidney, and perhaps the most egregious example, digoxin, which is eliminated by P-glycoprotein, as is a number of other drugs, this creates a situation where clogging up of these pathways, particularly the P-glycoprotein pathway with digoxin, can interfere with the elimination of a number of other drugs, hence the extremely long list of drug-drug interactions with digoxin. Now, there are also clinically important genetic variants that impact not only the baseline electrophysiology itself, active membrane properties, and we'll talk about that in just a minute, but also clinically important genetic variants that hamper drug clearance, particularly those that are hypofunctioning. For example, hypofunctioning alleles of cytochrome P450 2C9 will impair the metabolism of warfarin. Similarly, hypofunctioning alleles of 2D6 or dibrisiquin can impact the metabolism of propafenone, virtually every liver-metabolized beta blockers, and other drugs as well. This is phase one of metabolism, which is oxidation reduction, and phase two may be similarly affected. Phase two represents the phase of metabolism involving conjugation. N-acetyltransferase type two also has a number of clinically important allelic variants. The number of allelic variants measured for this is well over 90 at this point, and this drug acetylates procainamide, and in doing so, creates N-acetylprocainamide, which has action potential prolonging effects. Now, variants of N-acetyltransferase fall into several categories. Patients with these alleles may be slow acetylators, may be fast acetylators, or intermediate acetylators. Fast acetylators create a lot of NAPPA and have the risk of action potential prolongation. Slow acetylators metabolize procainamide slowly, and these patients tend to have an increased risk of procainamide-induced lupus. There are a number of other metabolizing enzymes for which there aren't, or very rarely, clinically significant variants. However, many drugs are metabolized by these systems, hence the large capacity for drug-drug interactions, and I'll focus on just cytochrome P453A4, which metabolizes a whole host of drugs and has substrates that include amiodarone, calcium channel blockers, trevenidine, and others, but also has a number of drugs that aren't necessarily metabolized but inhibit this system, including ketoconazole, and others that can influence the efficacy of drugs by being inducers, such as diphenylidantoin. So, again, lots of genetic variants that influence phase I and phase II metabolism of antiarrhythmic drugs, but there are also genetic variants in the targets of these drugs in ion channels, and these occur, and the effect of these drugs is really quite variable, from variants that have a very large individual variant effect to those that have a very small individual variant effect but are more common. So, the circumstance of a monogenic or Mendelian inherited trait is that of a trait or a variant that is very, very rare but produces strong functional effects such that, in fact, in isolation, in and of themselves, can create the circumstance to produce an arrhythmia. Examples of that are mutations in the ryanodine receptor that is associated with CPVT and mutations in sodium and potassium channels that produce Long QT syndrome, types I, II, and III. There may be a situation where there are weakly penetrant variants with moderate effects on function and, in and of themselves, don't have the ability to produce arrhythmia, but there may be polygenic effects or, in fact, environmental effects that can result in the production of arrhythmia. And most commonly, acquired traits and susceptibility to those acquired traits are the result of the action of multiple common variants that, in and of themselves, have very weak effects on the phenotype and do not produce disease in isolation. Arrhythmias like atrial fibrillation fall into this category. Now, this has implications for the role of genetic testing in arrhythmias. And in this study of the yield of genetic testing in idiopathic ventricular fibrillation across five different countries, the most common outcome of a genetic test was something called a VUS, a variance of uncertain significance. Much less commonly was a variant that was discovered, considered to be pathogenic or likely pathogenic. So both the provider and the patient, particularly in circumstances in arrhythmias like ventricular fibrillation, need to be prepared for an ambiguous genetic test result. And this kind of polygenic nature of even putatively inherited arrhythmias is really a consistent theme and one that was observed, for example, using an endophenotype of polymorphic VT, the QT interval, and the association of genetic variants in a genome-wide association study of the QT interval in over 75,000 patients. And what was found was there were common variants in a number of genes that produced small changes in the QT interval. Again, polygenic in nature. Much rarer were variants that were found that produced congenital long QT syndrome, large effect from a single variant. Now, one of the reassuring aspects of this study was that the common variants and the rare variants were found in the same genes and the genes that have been described to produce long QT syndrome. So again, this lends biological plausibility to the notion that both common and rare variants contribute to overall arrhythmia susceptibility through, in fact, this endophenotype of QT interval. In addition, a number of variants were found in calcium signaling proteins that were associated with prolongation of the QT interval. And this is consistent with what we know about acquired forms of long QT syndrome in the setting of structural heart disease that's associated with abnormalities in calcium handling. Again, lending credibility to the biological plausibility argument of the role of this endophenotype in producing polymorphic ventricular tachycardia and arrhythmia is both heritable and acquired. So let me stop here. There are a couple of things that I have not covered that will be covered in subsequent lectures, including mechanisms of defibrillation, the physics of ablation, the fundamentals of electronics, recording and filtering, and definitions. And you should be familiar with all of these things. So let me thank you, and I'll conclude here.

electrophysiology

ion channels

action potentials

arrhythmia mechanisms

electrocardiogram

cardiac rhythms

antiarrhythmic drugs

refractory periods

gap junction channels

genetic variations