Welcome everyone. This is the basic electrophysiology principles for the clinician lecture. My name is Gordon Tomaselli from the Albert Einstein College of Medicine. These are my disclosures and they have no impact on this presentation. The blueprint for this talk is that we're going to cover topics in basic electrophysiology, including basic principles as they relate to ion channels, currents, and action potentials. We'll consider arrhythmia mechanisms, and we'll also consider how drugs interact with arrhythmia mechanisms. You'll get a more full pharmacology lecture later in this course as well. The detailed blueprint for this part of the test can be found at the website at the bottom of this slide. One of the things I won't cover that are important for this test are things like mechanisms of defibrillation, the physics of ablation, electronics, recording, filtering, and definitions that you'll learn in the EP laboratory. These are important, but they'll be covered elsewhere. What I want to talk a little bit about is how to best understand clinical electrophysiology from the cellular and tissue manifestations of electrical activity in the heart. One of the ways to do that is to measure the electrical activity in heart cells using a microelectrode recording. The schematic for doing this is shown in this particular slide. What this slide will show is a measurement of the difference in voltage between the tips of two microelectrodes, one that's in the bath and one that impales a cardiac cell. When one impales a cardiac cell, one finds that the cell's interior is electronegative with a voltage in a ventricular myocyte of about minus 80 millivolts. The difference between the bath and the inside of the cell is about minus 80 millivolts. Now, if a stimulus is delivered, this generates a regenerative stereotypic response that's called the ventricular action potential in this case, or the action potential in this case. It's got several phases that we'll talk about a little bit later. Action potentials and the currents that drive them are really the foundation of all the measurements that you'll make in the clinical electrophysiology laboratory. They vary regionally within chambers of the heart and also within particular heart chambers. The action potentials in the ECG, for example, the PR duration on a surface electric cardiogram, is the time from initial activation of the atrium to the ventricle. It really is a reflection of the summation of all of the action potentials in the atrium and what is electrically silent at the body surface as conduction through the AV node, which is dependent upon slow calcium-dependent AV nodal conduction. QRS duration, on the other hand, is the summation of all of the upstrokes of the action potential in the ventricle. These upstrokes depend on fast sodium current-dependent conduction. As I said, are the result of the summation of action potential upstrokes throughout the ventricle. The QT interval is the time required for the summation of ventricular repolarization. Ventricular repolarization occurs as a result of a balance between depolarizing currents that tend to raise the membrane potential and repolarizing currents. Ultimately, the repolarizing currents win, the action potential repolarizes, and it's a balance between calcium depolarizing currents and repolarizing potassium currents that result in repolarization of the action potential and inscription of the QT interval. Before we start, a little bit of an update on definitions. Ion channels are named for the biological stimulus that causes their opening. Voltage-gated ion channels are opened by changes in membrane voltage, and they constitute the majority of channels in the heart. They include sodium, calcium, and potassium channels. Ligand-gated channels, on the other hand, are activated by ligands like acetylcholine or ATP, and acetylcholine and ATP-gated potassium channels are ligand-gated channels. Some biophysical features that you should know about include permeation. This is the ability of an ion channel to conduct and selectively conduct current. It's the current that occurs through an ion channel is driven by the opening and closing of the channel, but once the channel is opened by the electrochemical gradient for flow of the specific ion that's permeant in a particular channel, so for a sodium channel, the electrochemical gradient for sodium in an open sodium channel, and the current at which there is zero current flow. It's called the zero current potential or the reversal potential. This is where the chemical gradient for flow down a concentration gradient is exactly balanced by the electrical gradient that is the relative intra and extracellular positivity or negativity. Gating refers to the opening and closing of ion channels, and this is done by several mechanisms that we'll talk about. Rectification, like the rectification of an orthodox, if you will, electrical rectifier, is the ability of the channel to pass current preferentially in one direction. Now, the current magnitude in any given cell or tissue is a function of the number of channels that are conducting that current, the probability at any point in time that any one of those channels are open and the single channel current amplitude. These are concepts that we'll go over as we proceed through the talk. Action potentials are sculpted by the concerted activity of a number of different depolarizing and repolarizing ionic currents. The example I've shown in this slide is a ventricular action potential, which is particularly long with four phases. Phase 0 is the result of inward sodium flux through voltage-dependent sodium channels and results in a rapid upstroke of the action potential which mediates conduction. Phase 1 is due to closure of the sodium channels, but also opening of a repolarizing current, the transient outward potassium current. Phase 2 or the plateau of the action potential is due to the concerted activity of inward currents, L-type calcium currents which tend to hang the membrane voltage up at positive voltages, and the delayed activation of repolarizing potassium currents, which tend to repolarize the action potential duration back to resting levels. In working myocardium, resting membrane potential is flat. In pacemaking tissue, there is spontaneous depolarization of phase 4, so-called diastolic depolarization. These currents basically are underlied by a number of different ion channel and transporter genes, which we'll have a chance to talk about in just a bit. Now, action potentials vary regionally in the heart, and they vary regionally because the ionic currents in the heart vary regionally. Certainly, there's a dramatic difference in the action potential in the atrium, as well as the ventricle, and that is because of regionally varying, repolarizing, and depolarizing currents that we mentioned on the previous slide. But there's also variability within chambers in the heart, and within the ventricle, there's a transmural across-the-wall variability in action potential duration, with the action potentials on the endocardial side of the ventricular wall being longer than on the epicardial side. This results in activation being from endocardium to epicardium, but because the epicardium recovers faster, recovery occurs from epicardium to endocardium. Despite the opposite polarity, the waves of the electrocardiogram, QRS, and T waves are generally at the same polarity because the sequence of activation and inactivation and recovery are in different directions. Now, there can be altered transmural heterogeneity of the action potential duration that underlies several pathologic conditions. In particular, ST changes in ischemia may be the result of a profound effect on action potential duration in the endocardium. Rugada syndrome is the result of a difference in action potential duration between the endocardium and the epicardium, particularly in the right ventricular cap, in the right ventricular wall, and right ventricular outflow tract. Torsade de Point, polymorphic ventricular tachycardia, can be due to an exaggerated dispersion of this action potential repolarization that occurs both in congenital and drug-induced long QT syndromes. This dispersion in action potential duration is thought to underlie the U. All of the current flow and the inscribed changes in membrane voltage or the action potential are the result of the activity of voltage-gated cardiac ion channels primarily. There are some non-voltage independent ion channels as well, but these are transmembrane glycoproteins that contains, in the case of sodium and calcium channels, six membrane-spanning repeats with an unusual fourth membrane-spanning repeat in each domain that's highly charged yet in the membrane that serves as a voltage sensor. The domains between the fifth and the sixth membrane-spanning repeat are folds of protein that form the ion-selective core of the channel. Now, the Alpha or the Alpha 1 subunit of each of these channels is sufficient to produce an ionophore in the membrane. However, the function of these Alpha and Alpha 1 subunits is modulated by ancillary subunits in the case of calcium channels, Alpha 2, Delta, Gamma, and Beta subunits. In the case of potassium channels, we'll come back to this in just a minute, rather than a covalently linked tetramer, each membrane domain is a separate subunit, potassium channels, and those potassium channels tetramerize to form the intact Alpha subunit, and heteromultamerize with Beta subunits to form the intact channel. Now, one of the things that's happened over evolution is these channels have evolved from a common precursor. This common evolutionary precursor are the inward rectifier potassium currents. These are two membrane-spanning repeat potassium currents that by several rounds of gene duplication produced twin pore potassium currents like TWIC and TASC, and the more garden variety voltage-dependent potassium channels, potassium channel subunits that underlie transient outward ultra-rapidly delayed rectifier, and rapid and slow component of the delayed rectifier potassium currents. Now, this common evolutionary parent has a couple of implications for key structures within these channels, like the voltage sensors are similar in all of these channels, but also the permeation pathway and where drugs bind, may also be similar so that antiarrhythmic drugs that block channels in particular are very promiscuous in terms of the channels that they block. They're not very selective for one particular channel over another in general. As I mentioned, potassium channels tetramerize with the Alpha subunits tetramerize, and heteromultamerize with accessory Beta subunits, and wrap around the central water-filled pore that selectively and rapidly conducts ions of a particular species in this case, potassium ions. Again, these Beta or ancillary subunits can significantly modify channel function. Another type of ion channel that's important in the heart and that mediates conduction are so-called intercellular ion channels or gap junction channels. Gap junction channels are formed by proteins called connexins. There are several types of connexins in the heart. These have four membrane-spanning repeats, and these four membrane-spanning repeats subunits hexamerize to form a channel subunit in one membrane. It's called a connexon. This connexon looks for another connexon in an adjacent membrane to form two hemi channels that come together to form an intact intercellular ion channel, conducting ions from the cytoplasm of one cell to another cell. The electron microscopic appearance of the junction between cells, where these connexions live, has this penta-laminar appearance of electron-dense, electron-light, electron-dense, electron-light, so on. These hexameric channels have the ability to be even more diverse because as I mentioned, connexin 43 is one connexin, but there are several other connexins that are expressed in the heart. These connexins can promiscuously come together to form different types of connexin or gap junction channels. If one connexin forms both the channel subunits in each half of the hemichannel, this is called a homomeric homotypic channel. If a connexin of one type forms one hemichannel and another type forms another hemichannel, these are homomeric heterotypic channels, and diversity can be increased even further, enhanced even further by the different connexins forming different connexon or hemichannels. It further exaggerates diversity of the gap junction channels. Let's see how all of these channels function physiologically in the context of a heart cell. Now remember, in addition to channels, they are transporters that maintain concentration gradients of important ions in the heart cell. At rest, the heart cell has, for example, in the case of sodium, has a relatively low intracellular sodium concentration and a high extracellular sodium concentration. The inside of the cell is electronegative compared to the outside of the cell, and there's a steep concentration gradient. The ion wants to run down its concentration gradient from outside to inside. In parallel, an electrical gradient, it's a positively charged ion that wants to run down its electrical gradient from outside to inside. Now, why doesn't sodium current flow at rest? That's because sodium channels are closed at rest. Despite the fact that there's both a chemical and electrical driving force, there's no net flow of sodium current across the cardiac membrane at rest. In contrast, the chemical gradient for potassium, concentration gradient is greater on the inside compared to the outside, and the concentration gradient therefore is from inside to outside. Remember, the cell is electronegative inside, so the electrical gradient is in the opposite direction. In fact, at rest, one type of potassium channel, particularly the inward rectifier potassium channel, is open, and the voltage at which the cell is at rest is determined by the balance between concentration gradient and the electrical gradient, the so-called reversal potential for potassium or Nernst potential for potassium. Now, if we depolarize the cell in this particular thought experiment by raising the level of potassium, that's done quite slowly, sodium channels will open up and sodium will run down its electrical and chemical gradient. Fortunately, sodium channels don't stay open for very long, so the gradient does not run down extensively. Again, the reversal potential and the potential at any given time across the membrane is a function of the ion channels that are open and the current that's flowing. At rest, the resting membrane potential is close to the Nernst potential, the reversal potential for potassium, with the outside potassium concentration being lower than the inside. Therefore, this number creates a negative log and a negative resting membrane potential. The thought experiment that we just did changed the levels of potassium. That's not how things work in an electrically active part where there's much more instantaneous changes in membrane voltage that drives the function of ion channels. In this modification of a microelectrode experiment, we use what's called voltage clamp. Voltage clamp allows us to control the membrane potential of the cell in any way we'd like. In doing so, we can change the membrane voltage and elicit the opening and or the closing of a variety of different ion channels. Couple of conventions here as well. By convention, a positive or upward current is the outward movement of cations or positive ions. And that's for example, an outward or repolarizing potassium current. A downward current is the inward flux of a cationic or depolarizing current, such as sodium current in this case. So when a cell is activated, sodium currents are on sodium channels are opened and the current flows. This is a robust current. This is the most numerous channel in ventricular myocyte. And this, despite the fact that this biological stimulus for this current to be open, that is this depolarization occurs, the sodium channel closes on its own, fortunately to prevent, as I said, the run down of current. And this type of closure is called inactivation. This is current waning despite a maintained depolarizing voltage. And this occurs not only in sodium currents, but calcium currents and transient outward potassium current. So despite the fact that depolarization remains, the channel closes. Potassium currents on the other hand will activate and will activate with a bit of a delay. Hence, some of them are called delayed rectifiers. So this delayed or slowed activation occurs a period of time after depolarizing pulse. And there are several delayed rectifier currents that are present in the heart. And we'll talk a little bit about those in just a second. But I also wanted to mention that there are other currents that are not shown here that are relatively time independent. And that change and that the current flow through those, depending upon what they're permeable to, changes instantaneously. And this includes inward rectifier potassium currents that we mentioned on the last slide. So there are three important delayed rectifiers in the human heart. There are a number of them, but three that I'd like to highlight. One is something called IKUR, the ultra-rapid delayed rectifier, which is encoded by KD1.5. It's found in the human heart with a preference in the atrium and there are aspirations for producing an atrial-specific antiarrhythmic drug that blocks KD1.5. But again, here's where the common ancestor comes into place. Any drug that's designed to block a voltage-dependent channel of any type is likely to have off-target effects on voltage channels of other types. This may be a target in atrial fibrillation if a good pharmacophore can be found. The second type of delayed rectifier is the so-called rapid component of the delayed rectifier. It also opens with a delay. The ultra-rapid opens up with a delay of tens of milliseconds. This opens up with a delay of tens to hundreds of milliseconds. It's encoded by a gene called HERD or KCNH2. And mutations in HERD are one of the commonest causes of congenital long QT syndrome or long QT syndrome too. And almost all drugs that produce this torsade depliant side effect are IKR blockers. Finally, IKS is a slow component of the delayed rectifier. Again, opens up with a delay. In this case, the delay is hundreds of milliseconds to seconds. It is encoded by KDLQT1 and a subancellary subunit called MINK. Mutations in KCNQ1 are a common cause of long QT syndrome type 1. These currents will increase with adrenergic stimulation and all of these currents exhibit outward rectification. That is very little to no inward current flow but substantial outward current flow. So potassium is flowing out of the cell to hyperpolarize the cell membrane. One caveat, many of these channels are studied in expression systems, not in the native cell. Problem with that is that in order to produce a high fidelity replicate of a current, ancillary subunits are required. So for example, KCNQ1, which underlies IKS doesn't look anything like IKS when expressed alone in the expression system. However, if you add the beta subunit expression and activation is delayed, and this looks much more like the genuine current that's expressed in a cardiac myocyte by the combination of KCNQ1 and KCNU1. This is also important when studying in vitro the pharmacology of drugs and their effects on some of these channels. You need to try and replicate the highest fidelity possible the channel itself in the native membrane. So finally, another important electrogenic component of the heart cell is what happens to calcium during electrical activation. Now, electrical activation will drive calcium release and contraction within the heart cell. So the sequence of events starts with sodium running in and rushing into the sodium channel producing the phase zero upstroke of the action potential. This phase zero upstroke of the action potential depolarizes the cell and activates L-type calcium current. This L-type calcium current serves as a trigger for calcium-induced calcium release in the dyad through the ryanodine receptor. This trigger results in a massive release of calcium. This calcium binds to troponin, this troponin that allows for contraction. And this produces the upstroke of the cytosolic calcium transient as well as the plateau of the action potential. Now, this calcium has to be taken back up into intracellular calcium stores or through the sodium-calcium exchanger extruded from the cell. SERCA actually takes up calcium, restores it so it can be ready to be released during the next activation and contraction and restores the baseline level of cytosolic calcium as well as stores the restoration of the action potential to phase four or resting levels. Now, in failing heart cells, the relative activity of the SR calcium uptake and sodium-calcium exchanger may be preserved so that SR calcium with rapid rates may be compromised in the failing heart cell. We'll come back to that in just a minute as well. So there are a number of important tissue features that are the result of the activity of lyme channels and of action potentials and action potential propagation in tissues. And those include excitability and conduction. Pacemaker activity reflects, as I mentioned previously, phase four diastolic depolarization in non-muscle tissue, particularly in the sinus node, some parts of the AV node and intranodal conduction. There is spontaneous depolarization of the membrane potential of these cells and that phase four depolarization allows the membrane potential to get to threshold for activation in the case of the conducting system, activation of L-type calcium channels to initiate an upstroke of the next action potential and produce spontaneous fire. The mechanism by which this net inward diastolic depolarizing current occurs is through the activity of pacemaker currents, so-called hyperpolarization activated cyclic nucleotide gated channels, a particular type of calcium channel, the T-type calcium channel, and sodium calcium exchanger. This might be particularly important in failing tissue where upregulation of the exchanger may lead to abnormally automatic tissue or cells and we'll come back to that in a minute as well. In pathologic conditions, that is muscle cells that shouldn't be automatic, the same mechanisms are present but they may be exaggerated in their density. And there may be other mechanisms that may be active to produce diastolic depolarization including abnormalities in intracellular calcium. So excitation is a function of pacemaker activity as is firing rate in spontaneously active tissues. And there's a number of ways to modulate the firing rate in pacemaker tissues. Remember phase 4 diastolic depolarization, cyclic nucleotide gated channels, calcium currents and sodium calcium exchange. One can change the firing rate by changing the threshold for excitation, in this case, in pacemaker tissues for excitation of the L-type calcium current and with this can be done using calcium channel blockers. One can depress the slope of phase 4 and the slope of phase 4 can be depressed by hyperkalemia, by the presence of beta blockers blocking some of the calcium currents and by the presence of an HCN4 blocker evaporative. If you make phase 3 much more negative, that is you increase the negative maximum diastolic potential of the action potential without a change in phase 4 diastolic depolarization, this will also increase the time that it takes to get to threshold and decrease the firing rate. Finally, prolongation of the action potential duration itself by, for example, blocking potassium currents through the use of potassium channel blocking drugs in and of itself can also delay the start of diastolic depolarization and depress the firing rate. So another important feature is excitation and is the correlative excitation refractoriness. Excitation in ventricular myocytes is mediated by phase 0 of the action potential, the rapid upstroke, which is due to the activity of sodium currents. Decrease phase 0 slope can result from decreased activity of the sodium current and can depress conduction velocity. Sodium currents recover from inactivation as a function of voltage. So the more depolarized a cell is, the more inactivated are the sodium currents. And sodium currents, like many currents, channel through three different phases from closed to open, the only form of the channel that can conduct current to the inactivated state. This is a special form of state closure which requires transit through the closed state to reopen. And the population of the inactivated state is very dependent upon the membrane voltage. So if one plots the inactivation curve, that is the percentage of channels available to be excited as a function of membrane voltage, this is a very steep curve. And at relatively depolarized potentials, minus 60, minus 50 millivolts, a minority of this current is available to open and to produce phase zero upstroke. So if one uses extra stimulation, like you do in the EPI laboratory, and approaches on the prior action potential, you may be able to get an upstroke, but that upstroke will require larger stimulus to produce and will be a slowed upstroke because of less sodium current available. On the other hand, if one tries to stimulate during the plateau at, let's say, minus 30 millivolts, no stimulus, regardless of the strength, will produce a second action potential. The tissue is absolutely refractory and cannot be stimulated. As one gets to more negative potentials, you will be able to recruit some sodium current. You will be able to produce an upstroke that's slow and perhaps conduction that's aberrant. This is the relative refractory period. And whether or not there's a period of supernormal activity where enough sodium current has been recovered to produce a robust upstroke, but we're closer to threshold, is really a matter of debate. So the other property, once there's excitation, is conduction of the excitation wave through the tissue. This has to do with network properties of the tissue, the tissue structure, and its connectivity, the connection of one cell to another. And tissue structure and wave propagation really are profoundly influenced by the shape of a wave as it transits through the heart. So for example, when compared to a linear wave, a convex wave of activation, the source of that wave has to activate a larger sink of non-excited tissue so the conduction velocity is slower. In contrast, a concave wave, which is activating a sloth of tissue that's smaller than the activation wave from itself, has a conduction velocity that's faster than the linear conduction velocity. And in the extremes of circumstances, there are places in the heart where there are what are called source-sink mismatches. So a very small source has to activate a very large sink. These are areas where block might occur. And think of the junction between the Purkinje fiber and ventricular muscle where block can occur, and the inverse where a large source can activate a sink and influence conduction as well. Think about ventricular myocardium or atrial myocardium converging on an accessory pathway. So the way that a particular tissue conducts a wavefront is determined by the membrane properties of the action potentials in that tissue. So the so-called fast-response tissue, regardless of the degree of prematurity, conducts at roughly the same conduction velocity. It's also called all-or-none tissue, generally mediated by sodium current-based conduction. On the other hand, there are tissues that have what are called the slow response or so-called decremental response. These are tissues that mediate excitation by calcium current. Think about kind of the AV node and think about what happens to the PR interval as the degree of prematurity of an activation wavefront increases. Think an extra stimulus. So an atrial extra stimulus that's more premature will be conducted with a longer PR interval, for example. So in summary, there are fast-response tissues. The fast-response tissues are generally in the ventricular muscle and His Purkinje system. They are sodium-channel dependent, or recover from inactivation in a voltage-dependent fashion and exhibit fast conduction, generally speaking, regardless of the coupling interval or so-called all-or-none conduction. In the extremes, this isn't exactly true, but it's generally true when compared to slow-response tissue, which are calcium-dependent, which exhibit slow conduction due to both voltage and time-dependent recovery of calcium currents. And this response is seen in the sinus node, the AV node, and some diseased depolarized tissue. So electrical propagation in the heart also has a couple of different features and metrics that determine rate of conduction. So electrical propagation in the heart is considered to be anisotropic and discontinuous. It's anisotropic because conduction in one direction, that is, along the long axis of the cell, is faster than conduction across the long axis of cells. And the conduction velocity determinants are the connections that we talked about previously. The major ones in the heart are connection 43. This is the major one in the ventricle, ventricular myocardium, and in specialized conductive systems, connection 40 and connection 45. These are highly aggregated at cell ends, hence one of the reasons for anisotropy. In addition to connections, sodium currents as well as cell shape and alignment are determinants of conduction velocity. And again, the distribution as well as cell shape, distribution of connections and currents towards cell shape result in a conduction velocity that end-to-end is much faster than side-to-side. Macroscopically, conduction appears to be continuous, although Matty Spock showed us many years ago that actually it's discontinuous in the activation wave, transits through myocardial tissue, looking for the path of least resistance. Usually that's determined by the place where connections are most dense. So with this foundation, let's consider arrhythmia mechanisms because alterations in some of these foundational properties driven by ion channels produce abnormalities of cardiac rhythm, and they fall into two large categories. Those that are due to abnormalities of impulse initiation of which there are two types, enhanced automaticity and triggered rhythms, and those that are due to abnormalities in impulse propagation. These mechanisms, these classes of mechanisms, are not mutually exclusive. Triggered beats may engage a pre-existing re-entrant circuit and create arcs of functional block generating substrate for re-entry. Any given arrhythmia may have initiation and a maintenance mechanism that differs. You in the electrophysiology laboratory will be preoccupied with re-entry, and we'll talk a lot about the different types of re-entry. Automaticity may occur in normally automatic tissue, such as the sinus node or even the AV node, and this is enhanced normal automaticity, or automaticity may occur in a tissue that's not normally automatic, such as the atrial or ventricular myocardium. This is abnormal automaticity. Warm-up, if it's present, that is an increase in the rate over time, is suggestive of an automatic focus. The ability to transiently suppress with overdrive pacing and then re-warm-up is also suggestive of automaticity. The rates may increase with sympathetic nervous system stimulation, stretch, ischemia, or hypokalemia due to an increase in the phase 4 diastolic depolarization that's driving enhanced automaticity. The other type of impulse initiated abnormal heart rhythms are triggered rhythms. Stimulation of a myocyte or the heart may, under non-physiologic conditions, give rise to a second non-driven action potential upstroke. These come in two different varieties. One shown here is a so-called early after depolarization. Early after depolarizations are depolarizations that occurred before the action potential is fully recovered. Early action potential, early after depolarization can be incited by drugs that prolong the Q-tangible. They occur more frequently at slow rates and in the presence of hypokalemia, hypomagnesemia, so repleting both potassium and magnesium can reduce EADs. The mechanism is a net inward plateau current, oftentimes calcium current, but not always. It produces clinical arrhythmias, in particular, TORSAD, the plant polymorphic PT, and may be involved in some ischemic ventricular arrhythmias. The other type of after depolarization, other type of triggered arrhythmias is triggered by what's called a delayed after depolarization or DAD. These are incited by digitalis or catecholamines. They occur more frequently at rapid rates and they can be blunted by calcium channel blockade to reduce calcium loading, particularly during rapid rates. They are the result of SR calcium load and leak through leaky ryanodin receptor channels. Clinically, DAD-mediated arrhythmias may be seen in digitalis toxicity and in ischemia. As I mentioned, you'll be preoccupied by reentry. Reentry is the commonest form and mechanism of arrhythmia that you see in the electrophysiology laboratory. Reentry as a mechanism has been known for over 170 years. One of the first descriptions of reentry was by John McWilliam at the University of Aberdeen, who described what was the mechanical manifestation of reentry that is peristaltic contraction and propagated waves in the ventricle of the heart. Alfred Mayer at the Carnegie Institute defined electrically indefinite conduction in rings of electrically active jellyfish and described this as a form of electrical reentry, not in the heart, but in another electrically excitable tissue, the jellyfish. George Ralph Mines really was the first to describe reciprocating rhythms as a mechanism of circulating excitation and arrhythmia in heart cells. Mines from both Cambridge and McGill University in 1912. Tragically, George Mines died at age 29. He was found dead in his laboratory, thought to be the result of self-experimentation. AV node reentry is one of the more common forms of reentry, and it's exemplar that I'll show here to define what the basic requirements are for reentry. The requirements are that there are two pathways to conduct electrical activity in the heart, that these pathways have heterogeneous electrical properties, allowing for block in one pathway and generally, propagation but slow propagation in the other end. An AV node reentry tachycardia, premature beat can block in the so-called fast pathway, that's got a longer refractory period and conducts more slowly along the slow pathway. This can activate the ventricle via oval vihiscous Purkinje system, and can find the slow pathway available for retrograde excitation of this pathway. Retrograde excitation of this pathway will allow for completion of this reentry circuit, activation in a retrograde fashion of the atrium, and continuation of this electrical wavefront back down to the ventricle. So reentrant tachycardia over the AV node, requiring two pathways with heterogeneous properties. Why is it that you can train an arrhythmia? Well, this has to do with the fact that that an activation wavefront may fit very comfortably into a pathway which hosts that activation wavefront. This is an example of an activation wavefront, and these are the action potentials at the head and at the tail of this activation wavefront. So at the tail, the action potentials are fully recovered, this ought to be excitable again. And in fact, when there's a form of reentry that includes this large excitable gap, pacing anywhere in this gap will find enough working myocardium, sodium channels available to be able to, in an anterograde and a retrograde fashion, activate this circuit and then train the circuit. This introduces the concept of the wavelength of the tachycardia, which is a product of the conduction velocity, speed with which the activation wave transits the myocardium and the refractory period and the index of the action potential duration. In order for tachycardia to exist in the reentry circuit, the wavelength must be smaller than the path length of the tachycardia or the head of the activation wave will bump into the tail and extinguish itself. So reentry is characterized in the node reentry tachycardia, AV reentry tachycardia, ventricular tachycardia in and around the scar. And real circuits are heterogeneous, that is conduction velocity, refractoriness can produce wobble or changes in velocity in these reentrance circuits. So in addition to excitable gap reentry, there's another type of reentry called functional reentry, where there is no fixed pathway, no excitable gap. These reentry circuits may be continuously variable as an atrial fibrillation or ventricular fibrillation, or they may settle into specific pathways. In leading circle reentry, the center of the reentry circuit is continuously activated and refractory or inexcitable. There may in fact under some circumstances be very small transient excitable gaps in atrial fibrillation and ventricular fibrillation, but these are uncommon. Another type of functional reentry is something called spiral wave reentry. Spiral wave reentry is a bit different in that, again, a spiral wave is a circuit that's as small as it can possibly be. It's characterized by something called a phase singularity where all phases of the action potential meet, and it's where the wave tail and the wave head meet. This has got a continuously excitable core and refractory tissue in the spiral wave itself. Unlike leading wave reentry, the conduction velocity of a spiral wave depends upon where you are in the spiral wave. With high degree of curvature, so-called convexity, conduction velocity is slower indicated by the smaller arrows. With a more planar portion of the conduction of the spiral wave, conduction velocity is faster. So the conduction velocity of the spiral wave depends upon where you are in the spiral. This might be best seen in a movie. This is a spiral wave reentry that's been produced by a co-culture of neonatal rat ventricular myocytes and fibroblasts. And what you can see is a spiral wave, the head of the spiral wave that collides with another spiral wave, extinguishing the second wave here, but also resulting in a planar wave that conducts faster, that activates this dish, this culture dish at about anywhere from 12 to 3 o'clock at a much more rapid rate. So again, spiral wave reentry, another form of functional reentry. One of the ways to deal with spiral wave reentry is to change the relationship between the wavelength and the path length. And one can do that best in many cases by increasing the wavelength of the tachycardia, increasing the refractory period leading to extinction of reentry. So there are a number of mechanisms that in the clinical circumstance can contribute to the development of arrhythmia. There are a number of predisposing factors, but in fact, most arrhythmia generation requires a conspiracy, and I'll show you an example here, Torsade de Plante. So normally, there's a rate-dependent divergence, and we spoke about this before, in the duration of the action that happens between the epicardium and the endocardium, endocardium being longer than the epicardium. This can be exaggerated at slower rates and can be even more exaggerated by the presence of some drugs that actually specifically prolong action potential duration differentially, and those drugs with class III anterarrhythmic action are characteristic of this type of an effect. But even that is not enough generally to produce an arrhythmia. In addition to an action potential prolonging drug, several other features may be needed, including, for example, things that would further exaggerate dispersion of repolarization, including, for example, hypokalemia and slow rates. And sometimes in the presence of structural heart disease, there are other arrhythmogenic mechanisms, including alterations of L-type calcium current and an upregulation of sodium-calcium exchanger that can produce further depolarizing current, leading to exaggerated heterogeneity of repolarization and intramural reentry, first led in by some of the persistent bigeminy, that is, PDCs, after each normally conducted beat with a long, short initiation sequence of polymorphic ventricular epicardia and torsadipline. This self-terminated in this particular case, but if not, if left to its own devices, it can cause sudden cardiac death. Now, how do we deal with this clinically? Well, because of the limitations of anterrhythmic drugs, most often we deal with this type of event by using devices like internal defibrillators. However, there is a role for anterrhythmic drugs, and it's useful to consider the anterrhythmic drug mechanisms of action, and there are several. Classifications have been, schemes have been developed, but have been modified over time in a number of different ways that we'll talk about in just a second. But classically, anterrhythmic drugs have been ion-channel blocking drugs. So, for example, sodium channels, when blocked, slow conduction, and that slowing of conduction and blockade occurs in an exaggerated fashion at fast rates. It's a so-called forward use of this. Calcium channels, when blocked, can produce avianodal conduction slowing and avianodal conduction block. Calcium channel blockers have the effect of generally increasing the rate, the duration of recovery or refractory period or action potential duration, and the Q-T interval, and has a particularly unhelpful feature called reverse use dependence, where, in fact, blockade is more exaggerated at slow rates than at fast rates, particularly for blockers of the IKR channel, the rapid component of the delayed rectifier. The slow component of the delayed rectifier similarly increases the refractory period and Q-T interval, but does so if blocked independently of the IKR with little to no reverse use dependence, this unhelpful feature. And there are other drugs that are being developed that have been developed with not as much selectivity as we'd like to block specific channels that exist in specific tissues, like the atrial-specific IKBR channel. In addition, drug modification of arrhythmias may be done with beta-adrenergic blocking drugs in circumstances where sympathetic nervous system activation is a trigger or a maintenance mechanism for some arrhythmias, for example, those mediated by CPVT. And I would also hasten that all the action of antirrhythmic drugs are modulated by ancillary drug properties, that is, what effects they have on the autonomic nervous system, what effects they may have on heart rate, and other components of the extracellular environment, including differences in extracellular potassium as well as extracellular pH. Now, again, we talk about the chamber-specific differences in action potential profiles being driven by different ionic occurrences, and this was ionic occurrence, and this drives differences in pharmacology of different tissues by different drugs. So, for example, in the atrium, amiodarone and fluconide may have variable effects depending upon the degree to which particular occurrence in the atrial tissue, transient outward potassium occurrences, are blocked. Pure IKR blockers, like they block, like they affect ventricular action potentials, tend to prolong the action potential duration. In the ventricle, there's more robust expression of IKR and IKS, and a number of blockers of IKR and IKS, but blockers of other channels tend to prolong action potential duration. Again, sodium channel blockers tend to slow the rate of rise of phase zero of the action potential, and drugs that tend to open potassium channels, like nicorando, which opens KTP channels, will shorten ventricular action potential duration. In the setting where the late-sodium current is active, ranolazine, a late-sodium channel blocker, at low doses can also shorten action potential duration. So drug effects also depend upon the occurrence that are expressed. In addition to drug effects, static drug effects, drug block, and in particular, sodium channel block, as I mentioned, is state-dependent and exhibits something called forward-use dependence. But state-dependent block means that drugs tend to want to bind to particular conformation-level channels. Sodium channel blockers interact with the open state of the channel, phase zero, and the inactivated state of the channel, phases two and three. But usually, they don't bind with high-availability to closed states of the channel. So if one plots the fraction of sodium channel blocked over time, what you can observe is that anything that slows the recovery of the channel from inactivation, either a pathologic condition like ischemia, or drugs that avidly bind to the inactivated state and therefore prevent recovery from the inactivated state to the closed state can result in cumulative block of the sodium channel and cumulative slowing of conduction as a result of this block. Again, if one increases the rate of activation in the presence of a sodium channel blocking drug, what happens is that at faster rates, more of the duty cycle of the sodium channel, so at rates there compared to there, in black compared to the rate in blue, a larger percentage of the duty cycle spent in phase 0 and phase 2 and 3. Therefore, there will be tachycardia-dependent cumulative block. This is so-called forward use dependence, and it's a useful feature for treating tachycardias, which will allow for cumulative sodium channel block in tissues that use the sodium current for activation. The differences in the kinetics of blockade resulted in one of the first modifications of the drug classification scheme that's most commonly used, and this is the so-called Vaughn-Williams classification scheme. In its first iteration, described four classes of action, class 1, sodium channel block, class 2, which is not on this slide, beta blockers, class 3, potassium channel block and extra potential collongation, and class 4, calcium channel block. We'll come back to that in just a minute. But it became very apparent that sodium channel blockers were really quite heterogeneous, and it required a sub-classification of class 1 drugs into 1a, b, and c. 1a, b, and c have intermediate, fast, and very slow kinetics of off-rates in addition to multiple different off-side or off-target, or ancillary drug effects including action potential prolongation due to blockade of potassium channels, beta blockade, abrogation of the phase one notch due to transient network potassium channel blockade. Amiodarone has an intermediate off-rate of the sodium channel, but profoundly prolongs the action potential duration of sympatholytic and has many other potentially anterogenic actions. It's useful to consider drugs used in other circumstances, which can also block sodium channels. These drugs like amitriptyline and imipramine used in psychiatry have off-rates like lidocaine, very fast off-rates, but also other effects that might produce effects on the action potential duration, therefore, it could produce prorogative. The inverse of excitability is refractoriness in drugs. Certainly, anterogenic drugs that block potassium channels and prolong the action potential duration, will certainly increase refractoriness just because of prolongation of the action potential duration per se. But drugs that block sodium currents also will produce refractoriness. The reason they produce refractoriness is they produce a hyperpolarizing shift in the steady-state inactivation. A channel that has a drug bound to it is much less likely to go from the inactivated to the closed state. It's got to unbind the drug first. At any given voltage in the presence of drug, there are fewer sodium channels shown in green here compared to the absence of drug. In order to get the same level of sodium channel availability, one needs to unbind the drug to move the activation curve back to the depolarized direction. Again, sodium channel availability is profoundly affected by the presence of the drug. It may take more in the way of hyperpolarization to get to a point where, in fact, enough sodium channels are available to mediate upstroke of the subsequent action potential and to relieve refractoriness. In addition to the sub-classification based on sodium channel blockers, it became very clear that there were other mechanisms, other antiarrhythmic mechanisms of action of these drugs. These have now been added to the classification scheme and other classifications have been added, and they've been added because of different ion channel targets, for example. As I mentioned, evabradine is a drug that blocks the pacemaker current, HCN4 current, and this is now referred to as a class 0 antiarrhythmic drug, class 1 sodium channel blockers, class 2 are beta blockers, class 3 are things that alter the action potential duration, mostly prolonging the action potential duration, but some K-channel openers that shorten the action potential have been considered class 3 in this classification scheme. Class 4 antiarrhythmic drugs block calcium channels. These are non-dihydropyridine calcium channel blockers like borapamil and diltiazin. Class 5 drugs are TRIP channel blockers using these twin core calcium channel blockers. There are a number of agents that have been developed to block connections, and in blocking connections will block conduction to gaptides as well. Finally, there's a whole class of antiarrhythmic drug action that does not involve channels at all, but instead involves upstream modulatory effects. Some of those are shown here. It's been very clear that antiarrhythmic drugs alone have been relatively ineffectual for treatment of many arrhythmias. Preventing the tissue remodeling that occurs that predisposes to arrhythmias has been the main target for development of therapeutic strategies. These include alterations in structure and fibrosis in a salutary way by ACE inhibitors, ARBs, and MAP kinase inhibitors in a fit prevention. Reactive oxygen species, particularly in ischemia, and blockers of ROS as well as cam kinase inhibitors are also strategies that are being explored. As I mentioned in the previous slide, there are specific currents or current components that have been previous or ongoing targets for antiarrhythmic drug development. In fact, now understanding the way in which ion channels remodel and traffic within the cell has also been another area of active investigation in trying to produce antiarrhythmic drug strategies. In addition to the channel targets themselves, there are platforms for delivery of drugs, particularly useful for paroxysmal arrhythmias like paroxysmal SVT and paroxysmal atrial fibrillation where rapid onset and rapid offset drug in inhaled form like intrepamil and flecainide have been studied previously. In addition, there are a number of compounds that have been used to treat other conditions that are on the market or were on the market that have electrophysiologic effects. These compounds have been oftentimes repurposed to try to be used as antiarrhythmic drugs in the arrhythmia context. For example, riscobutane is a purine-based cyclin-dependent kinase inhibitor. It's an antineoplastic drug which alters inactivation of calcium channels and is being considered for use as an antiarrhythmic. Gabapentin and pregabulin bind to calcium channel subunits, and it inhibits those channels as well. Flunarazine is a mixed calcium and sodium channel blocker. Phenoxyrene is used in Parkinson's disease, and its primary effect is a dopamine reuptake inhibitor, but it also blocks several channels on the study in the so-called restored SR single-dose AF study, which again highlights the liability of these strategies, was stopped due to poor arrhythmia. Rulazole is also a benzothiazole that's used for ALS, that blocks the neuronal sodium channel, which actually populates the dyad in ventricular myocytes and has been used in some patients with CPVT and in some cases the AF. The most important thing to point out here is that all of these drugs, whether used for an arrhythmia or other purposes, have the potential to make arrhythmias work worse or have proarrhythmic effects. It really is the driver for why there's been so much investigation in developing new antiarrhythmic drugs. Ideally, what you want to have is a drug that's got a very wide toxic to therapeutic window. In fact, that turns out not to be the case with antiarrhythmic drugs. Antiarrhythmic drugs tend to have less efficacy and efficacy at higher concentrations that are impinging upon the toxic effect of these drugs. There will continue to be an ongoing search for better drug strategies for treatment of arrhythmias because of this liability of antiarrhythmic drugs. There are a couple of other drug liabilities that one needs to consider, and those include the pharmacokinetics of these drugs. In some cases, a drug is delivered as a pro-drug that requires an active metabolite. Previously used drug called Ankinite was such a drug, and if you had liver disease or were a poor metabolizer, it did not activate this drug. It wasn't particularly effective. In addition, there are parent drugs that are not well metabolized, and when not well metabolized, active compound or active metabolites can accumulate and can produce pro-arrhythmic side effects, and this can occur in the case of expiratory organ failure, usually liver or kidney. Finally, drugs that are particularly troublesome are those that have a single pathway for elimination. Largely because that single pathway for elimination is usually gummed up with other drugs that patients are taking that also use that same pathway. Perhaps the most egregious example is Digitalis. Digitalis is excreted by a key glycoprotein. Key glycoprotein excretes a number of drugs. Therefore, the list of drugs that interact with Digitalis is extensive because of its single elimination pharmacokinetics. Sotilol and dofetilide, similarly in patients with kidney disease, primarily eliminated by the kidney and not metabolized, can also produce pro-arrhythmic effects. There are, in addition to clinically important genetic variants in channel proteins that alter function, clinically important genetic variants that alter the metabolism of antiarrhythmic drugs, both phase 1 metabolism of these drugs and phase 2 conjugation of these drugs. There are significant genetic variants that alter, for example, the metabolism of warfarin, as well as beta-blockers. Dibrisiquin is one of those that contains significant variants that alter the ability of patients to metabolize many of these drugs. In addition, there are alterations in drugs like inasapiltransferase that alters the ability of a drug to be conjugated and conjugated in anticipation of excretion. There are other CYP isoforms that don't have a lot of genetically significant variants but are very active in terms of their ability to be used by lots of drugs for excretion. CYP3A4, CYP5, CYP6, and CYP7, for example, are used for metabolism of many different drugs or many different interactions in metabolism of drugs that one should be cautious about when using antiarrhythmic drugs. Indeed, P-glycoprotein, as I mentioned with digoxin, is a pathway for its elimination of many drugs and therefore can inhibit the elimination of digoxin, leading to toxic levels of digoxin and the proarrhythmic effects of those toxic levels of digoxin. So let's finally consider the genetic architecture of a number of conditions that are associated with an increase in arrhythmia. There is a continuum of complexity here. There are a number of genetic variants that are admittedly very, very rare that have an oversized effect and on their own can produce an arrhythmic phenotype. More often than not, there are less rare variants that have weakly penetrant effects that have some effect but require either rare or more common genetic variants to produce an arrhythmic or other kind of phenotype. And most commonly acquired arrhythmias really have inputs from a number of common variants with very weak effects. So, for example, atrial fibrillation appears to be like this. And things that are more monogenic or Mendelian in terms of cardiac channelopathies are things like certain forms of Long QT syndrome and certain forms of CPVT. Brugada syndrome is less and appears to require not just rare monogenic variants but a host of weakly penetrant variations that produce the phenotypic effect of not only electrocardiographic change but the arrhythmic consequence of that electrocardiographic change. Genetic architecture of these diseases has significant implications when it comes to genetic testing and really creates some limitations of genetic testing as indicated here by the yield of genetic testing in a particular presumed to be heritable arrhythmia, idiopathic ventricular fibrillation. In a number of series, genetic testing in patients with idiopathic CF in the minority of cases, overwhelming minority of cases in some circumstances, identified a pathogenic or likely pathogenic variant. Far more common were variants of uncertain significance, that is, buses, that is, changes in disease genes and changes in disease genes that we don't really know what they mean. And also there are changes in genes that we don't know if they have any relevance to the phenotype at all. These are called buses, genes of uncertain significance. But the overwhelming majority of cases are no variations at all in something that's considered to be heritable. That means we just don't understand enough about the genetics at this point in many of these heritable forms of arrhythmias. And you'll hear more about this in subsequent lectures. Now, one of the ways to discovery and hypothesis generate is to do something called broad-based genetic association studies. And in this case, this is a genome Manhattan plot because of the peaks here of a genome-wide association study trying to associate a gene variant with the length of the QT interval, and this was done in over 75,000 subjects, and a number of different variants that reach statistical significance. And statistical significance is substantial here. It's 10 times 10 to the 7th or greater. Now, again, these are probably common variants. There are most likely common variants in each of these genes that are weakly penetrant and weakly associated with, in this case, the phenotype of QT interval prolongation. But reassuringly, there are a number of these variants that have been identified that harbor ultra-rare mutations that actually produce congenital long QT syndrome like SCN5A, KCNH2, KCNQ1. And there are also rare variants that have been associated with alterations in calcium signaling that have effects on the QT interval, including a number of ATPases and a number of calcium transport proteins. So reassuringly, there's biology here that may help to identify new variants that might be associated with either arrhythmias themselves or arrhythmogenic endophenotypes as shown in this slide. So let me sum up and finish here by doing a couple of things. We've gone over channels, action potentials, arrhythmia mechanisms, drug effects. There are a number of other suggested topics that will be covered later in core concepts, including mechanisms of defibrillation, the physics of ablation, electronics, recording, and filtering, and definitions of refractory periods, anterograde, retrograde, and response to stimulation. You should also be familiar with all of these concepts in preparation for the EP examination. So let me end there. And thank you for your attention. Thank you.

basic electrophysiology

ion channels

action potentials

arrhythmia mechanisms

drug interactions

pharmacology of antiarrhythmic drugs

measurement of electrical activity

microelectrodes

sodium currents

potassium currents

genetic testing