Theme 2: Cardiac Electricity and Arrhythmia Flashcards

Question

Describe how data from a voltage clamp experiment is presented.

Answer 1

* A baseline voltage is decided, at which the cell is held * The voltage is then changed to a series of values in turn * At each voltage, the current resulting from this change is recorded * Because these are very fast events, they are superimposed on a graph with the same time base

Answer 2

* Each ion current can be eliminated using a substitution: * To eliminate Na⁺-currents replace Na⁺ with N-methyl-d-glucamine (NMDG) or choline. * To eliminate Ca²⁺ currents remove Ca²⁺ and add a chelator e.g. EGTA * To eliminate K⁺ currents replace K⁺ with Cs⁺ * If you substitute the substituted line from the overall line, you have a graph for the current of only that one ion * Alternatively, you can also use pharmacology to block certain types of channels (e.g. TTX)

Answer 3

* The equation I = (V_clamp-E_reversal) x g can be rearranged to give: * g = I / (V_clamp-E_rev) * The conductance can then be plotted against voltage

Answer 4

* A voltage step can be applied to a channel * At each voltage, the current (or conductance) is measured over time * The rate at which inactivation occurs can be determined by fitting an exponential function to the declining phase of the current (or conductance). * The exponential constant derived from this fit is called a time constant and is a measure of the speed of inactivation (long time constant = slow; short time constant = fast).

Answer 5

* The voltage is clamped at a holding potential * It is then increased to a pre-pulse potential (this is the independent variable) -\> This may inactivate some of the channels * Then a test pulse occurs, which is a more depolarised potential which is of sufficient amplitude to fully activate the current * The process is then repeated several times using increasingly more positive pre-pulse potentials and the peak current recorded during the test pulse plotted as a function of the pre-pulse voltage. * In the diagram below, you can see that the peak current is higher after the pre-pulse at more negative voltages. The graph shows how the relative peak height changes with voltage.

Answer 6

* Graphs of channel activation and inactivation agaisnt voltage can be combined on the same axes * This shows the proportion of channels that are activated and inactivated at any given voltage * In some cases, there may be an area of overlap, known as a window current -\> This is a situation where there may be a sustained current because channels can go through a cycle of being activated, inactivated and then activated again

Answer 7

* A double pulse protocol is used. * The first pulse must be of sufficient amplitude and duration to fully inactivate the channel. * A second pulse then follows after a variable time delay and the amplitude of the current evoked by the second pulse is then plotted against the time delay between the first & second pulse.

Answer 8

It's a type of voltage clamp where you use a patch electrode to isolate a small patch of membrane with only 1 or a few ion channels.

Answer 9

* Cell attached * Inside out * Outside out * Perforated vesicle

Answer 10

* Studying the length of time a channel spends in the open or closed state (dwell times) can yield important information about channel kinetics. * Measuring the duration of open, or closed, events requires the use of two thresholds set at sufficiently different current amplitudes to prevent noise in the recording from being mistaken for a real transition between open & closed states. * Where there is only one kinetic state corresponding to the open (or closed) state the frequency distribution of dwell times can be described by a single exponential function. * Where there are multiple kinetic states, as is often the case with closed states, the frequency distribution of closed times has several exponential components. These are best revealed by plotting the data using a logarithmic time base (and logarithmic time bins). Distinct kinetic states are then revealed as multiple peaks in the frequency distribution.

Answer 11

1. Generating rhythm (i.e. pacemaker) 2. Transmission across electrical synapses 3. Coupling onto second messenger cascades (i.e. EC coupling)

Answer 12

* A current can be triggered by changing either the driving force or the conductance * The conductance is the easier factor to change * It can be changed by changing the channel expression and targeting, pore permeability and selectivity, time, ligands and V_m

Answer 13

* The action potential is a combination of sodium, calcium and potassium currents. * However, these all have different temporal patterns.

Answer 14

* The mouse action potential is similar but has different currents. * It is also much faster since the mouse heart beats much faster than the human heart. * These differences are important to consider in animal studies.

Answer 15

* DNA from a cell is extracted and inserted into the model cell (e.g. a xenopus oocyte) * This DNA is eventually expressed and the channel is expressed on the surface of the cell * This means it can now be studied using patch clamp techniques

Answer 16

* Are the modulatory influences present? (e.g. does phosphorylation happen in the model cell?) * Are the channels expressed in the right environment? * Are the channels anchored correctly? * Are all relevant accessory sub-units also included? (e.g. beta subunits)

Answer 17

Inward rectifier potassium channels (3 types)

Answer 18

* The inward rectifier potassium channels have a very large conductance around the resting membrane potential * This means that, at more negative voltages they produce a large inward current, while at more positive voltages they produce a large outward current -\> This serves to very rapidly return the membrane to the resting membrane potential when there are small unwanted disturbances (i.e. noise) * However, at much more positive voltages, the conductance falls and the channels are blocked, which is important in allowing action potentials to be fired

Answer 19

* Voltage-gated sodium channels * Can be blocked by TTX and local anaesthetics

Answer 20

Voltage-clamp fluorimetry: * A single leucine is replaced with a cysteine * This cysteine is reactive and can be tagged with rhodamine, which is fluorescent * If the cysteine is in the voltage sensor, depolarisation drags the rhodamine into the membrane, so that the fluorescence is reduced * The location of the cysteine can be changed to see where the voltage sensor is

Answer 21

(Armstrong, 1973): * Administered pronase * This cuts the inactivation loop in the ball and chain model of inactivation and prevents inactivation of the voltage-gated sodium channels In long QT syndrome (LQT3), there is a mutation in the linker between the 3rd and 4th domain, which means that sodium channel inactivation does not fully happen and there is a persistent sodium current that elongates the QT interval.

Answer 22

* L-type calcium current -\> Enables calcium entry for contraction * Transient outwards potassium current (I_TO) -\> Allows a very small repolarisation right before the plateau phase, which ensures a sufficient driving force for calcium entry. * Ultra-rapid potassium current (I_Kur) -\> Enables repolarisation * hERG current (potassium) -\> Accelerates repolarisation right at the end of the action potential

Answer 23

* It occurs right after full depolarisation * It allows some slight repolarisation (after which the channels close and inactivate) * This repolarisation increases the driving force for the calcium current, so more calcium can enter

Answer 24

(Cooper, 2010): * Studied the importance of I_TO by removing the notch in the action potential that it creates * Did this by voltage clamping rat ventricular myocytes with human action potentials and recording intracellular Ca²⁺ with fluorescent dyes. * Loss of the notch resulted in about a 50% reduction in the initial phase of the Ca²⁺ transient due to reduced ability of the L-type Ca2+ channel to trigger release. * There was also desynchronisation of myocyte contraction. * Loss of the notch is observed in heart failure, so this is a possible mechanism.

Answer 25

* It is activated during the plateau and continues until repolarisation * It enables slow repolarisation, which is important for calcium entry

Answer 26

* The hERG current is small throughout the plateau and then becomes very large right at the end of the action potential * This enables very rapid termination of the action potential, protecting against delayed after depolarisations * The hERG channel allows this by having a very large conductance at intermediate voltages

Answer 27

1. They are sensitive to extracellular [K⁺] -\> This means that hypokalemia can lead to prolonged action potentials 2. They are targets for many drugs (if this is unwanted, the drug may not be viable) 3. Mutations can lead to channelopathies: e.g. LQT2

Answer 28

* Top = Normal * Middle = Ventricular tachycardia * Bottom = Ventricular fibrillation

Answer 29

(Nash, 2006): * Used an electrode sock to study the electrical activity of the heart during heart surgery (when the heart is often placed in ventricular fibrillation to allow operation) * Recordings often showed that normal activity became ventricular tachycardia (resulting from a single reentrant circuit) and then ventricular fibrillation (when there were several reentrant circuits)

Answer 30

Rare monogenetic disorders: * Structural cardiomyopathies: * Hypertrophic cardiomyopathies * Dilated cardiomyopathies * Arrhythmogenic right ventricular cardiomyopathies (due to desmosome proteins) * Primary electrical disease (due to mutations in ion channels) Genome wide association studies (GWAS): * SNPs

Answer 31

(Herring, 2019): * Suggest a probabilistic model for arrhythmogenesis, where each event must occur in sequence in order for the arrhythmia to be established * First, a trigger is required, which can be a delayed/early afterdepolarization or abnormal automaticity * This then requires a substrate for propagation, which could be an electrical/structural heterogeneity that is static or dynamic * Next, a re-entry mechanism is required (or else it is just a single irregular heartbeat) -\> This leads to ventricular tachycardia * If wavebreak occurs, this is ventricular fibrillation * The reason this model is difficult to study is because each event may be influenced by a huge number of factors

Answer 32

(Marban, 1986): * Found that ouabain, isoprenaline and increased [Ca²⁺]_o increased the likelihood of DADs, which can subsequently lead to the ectopic firing of another action potential or the formation of an arrhythmia after stimulation is stopped * These DADs can be prevented by ryanodine or BAPTA (calcium buffer), which suggests that DADs may be generated by SR calcium overload, leading to increased release and subsequent increased activity of the NCX (which is electrogenic)

Answer 33

(Marban, 1986): * Used drugs to induce a prolonged action potential * These prolonged action potentials frequently had an EAD, which can lead to arrhythmias (January, 1989): * Used voltage-clamping to mimick an action potential -\> Depolarised the cell, held it at the plateau voltage, and then repolarised around the time an EAD would occur * At the time of the EAD, an inward current was measured * This current was increased by a calcium channel activator, stopped by a calcium channel blocker, and unaffected by TTX * This suggests that EADs occur when the calcium channels have time to re-activate and produce a calcium current

Answer 34

* The right substrate may allow re-entry of an action potential into an area of the heart that would usually be refractory * This can occur due to abnormal conduction velocity, refractory period or size of the circuit * For example, a model of how this could happen in the heart is where there is an area of anatomical or functional conduction block. This causes the action potential from limb A to loop back round limb B through a zone of slow conduction or unidirectional block. This circuit can then continue.

Answer 35

STRUCTURAL (Arenal, 2004): * Obtained left ventricular electroanatomic voltage maps using a catheter in patients who had had a myocardial infarction that left scar tissue * Studied the scar tissue in more depth and found that there were often re-entrant circuits within the scar tissue that produced arrythmias such as ventricular tachycardia * Clinically, the exit points of these circuits could be sealed off to prevent these circuits, but this is not usually successful FUNCTIONAL (Nash, 2006): * The closer in time an ectopic action potential is to the normal action potential before it, the shorter its duration * However, this effect has heterogeneity in different cardiac myocytes -\> This can be illustrated an a polar map that shows that different parts of the heart are heterogenous * This heterogeneity can result in wavefronts breaking and causing circuits around an area where there is a large different in action potential duration (Tomek, 2019): * Studied calcium handling at the edge of a myocardial infarct in rats by using Langendorff perfused hearts * When the time between action potentials is shortened, alternans (alternating short and long action potential duration) begins to occur at the edge of the infarct -\> This is due to differences in calcium handling which alter action potential duration * This effect could be reversed by norepinephrine, which increased the amplitude/duration of each action potential so that the alternans is reduced (Gardner, 2015): * Studied the idea that sympathetic innervation of a myocardial infarct affects the susceptibility to arrhythmias * Compared control mice heterozygous for protein tyrosine phosphatase receptor σ (PTPσ) with PTPσ knockout mice -\> The KO increases innervation * These mice were then implanted with ECG telemetry transmitters and then subjected to sham or MI surgery. * Ten days after surgery, the mice were injected with 10 μg of the beta agonist isoproterenol (ISO), which is used to induce arrythmias. * The KO mice (innervated) had less heterogeneity in action potential duration and were less susceptible to arrhythmias The take home message is that any heterogeneity in the myocardium can result in arrythmias.

Answer 36

* Defibrillation is used to depolarise the whole heart, in the hope that the SAN will take over again * The chance of this being successful reduces by 7-10% with each minute

Answer 37

(Echt, 1991): * Compared the effectiveness of encainide/flecainide (the first anti-arrhythmic drugs) against placebo in preventing cardiac events * The drugs showed much higher rates of cardiac events and the trail was ended early

Answer 38

Beta blockers (e.g. Bisoprolol, Metoprolol)

Answer 39

* Implantable cardioverter defibrillators * These monitor for the signs of VT/VF and can use a variety of strategies to save the patient

Answer 40

* In general, the patients at highest risk of sudden cardiac death are: * Those with a prior myocardial infarction * Survivors of cardiac arrest and VT/VF * Those with an ejection fraction below 35% * These therefore benefit most from an ICD * Patients who should not be given an ICD include: * When there is also bypass surgery * Immediately//shortly after a heart attack * Older patients without CHD The high-risk groups form a small part of the population so ultimately they only make up a small percentage of total sudden cardiac deaths, but the frequency of events in the high-risk population make it worth getting an ICD.

Answer 41

(Schwartz, 2009): * Discussed the cutting of the sympathetic nerves supplying the heart as prevention against cardiac arrhythmias (Mahajan, 2005): * Case report of using epidural anaesthesia of the stellate ganglia to reduce sympathetic stimulation of the heart These are thought to work by reducing the sympathetic stimulation of the heart. However, they also work when the patient is already taking beta blockers. This can be explained by the fact that sympathetic nerves also release other modulators, such as NPY.

Answer 42

(Kalla, 2020): * Studied an isolated Langendorff perfused rat heart * Prolonged high frequency stimulation of the right stellate ganglia or left stellate ganglia caused the release of neuropeptide Y into the perfusate. * This shows that sympathetic stimulation releases not only noradrenaline but also NPY onto the heart. * Set up optical mapping of voltage and calcium transients. * Even in the presence of full beta blocking, sympathetic stimulation led to larger calcium transients. This effect was blocked by Y₁ receptor antagonism. * This suggests that NPY is involved as a product of sympathetic stimulation and it leads to changes in intracellular calcium handling * Also used a ventricular fibrillation induction protocol to induce arrhythmias. * This required less energy during sympathetic stimulation and beta blocker compared to a just beta blocker. However, there is no difference between sympathetic stimulation and no sympathetic stimulation when there is a beta blocker and NPY blocker present. * This suggests the importance of NPY in arrhythmias. * Studied 78 patients shortly after myocardial infarction and found that those with high NPY had significantly higher incidence of VT and VF. * In rats after myocardial infarction, VT and VF occur significantly more with NPY administration compared to control. This increase is blocked by a Y₁R blocker. Overall, this shows the importance of NPY as a marker of arrhythmia risk and also as a target for prevention.

Answer 43

Death following instantaneous collapse due to unexpected circulatory arrest.

Answer 44

* Accounts for around 50% of cardiovascular deaths * An electrical disturbance precedes SCD * Most (\>90%) SCD has some underlying pathology

Answer 45

* Re-entry -\> When an action potential can re-enter a part of the myocardium and re-excite it before the next SAN action potential arrives * Afterdepolarisation (early and delayed) -\> When a cell has depolarisations occurring before the next action potential arrives. Usually caused by channelopathies, which are inherited, or by excessive sympathetic drive. * Automaticity -\> When the dominant pacemaker sits in the ventricle, which does not allow enough filling time.

Answer 46

* Ventricular fibrillation -\> Asynchronous contraction of ventricles with no output. Responsible for around 80% of SCD. * Asystole -\> Impaired initiation and propagation of AP from pacemaker regions to ventricle. * Electromechanical dissociation -\> Normal electrical activity but mechanical dysfunction.

Answer 47

* Channelopathies can lead to alterations in currents that lead to abnormal action potentials: * Long QT syndrome -\> Commonly caused by minK mutations that affect I_Ks or by hERG mutations that affect I_Kr -\> These prolong the action potential * Catecholaminergic polymorphic ventricular tachycardia (CPVT) -\> Caused by mutations in the RyR2 receptor on the SR, leading to a leak of calcium * These abnormalities may be aggravated by emotional or physical stress, due to sympathetic activation -\> This leads to cardiac arrhythmias

Answer 48

* Long QT syndrome features a prolonged action potential due to altered delayed rectifier potassium current * This current is the sum of two currents: I_Kr (HERG channel) and I_Ks (minK). A mutation in either of these channels can lead to long QT. * Long QT features increased calcium entry and sequestration, which means that more calcium must be removed by the NCX between action potentials. * The NCX is electrogenic so it can lead to early after depolarisations (EADs), which can lead to arrhythmias. * A series of the EADs can present as Torsade de Pointes.

Answer 49

(Winbo, 2021): * Co-cultured stem cell-derived sympathetic neurons along with myocytes from long QT patients. * Current-clamped the myocytes and stimulated the neurons with nicotine (to release noradrenaline) * This led to increased action potential duration and signs of early after-depolarisations.

Answer 50

(Chang, 2008): * CAPON is a NOS adaptor protein -\> Its role is to translocate neuronal NO around the cell * It has previously been identified as containing one of the only SNPs shown to increase the risk of sudden cardiac death -\> This is because it can lead to long QT syndrome * Found that overexpression of CAPON in ventricular myocytes leads to shortening of the action potential, which shortens the QT interval * This is thought to be because CAPON-mediated translocation of NO leads to decreased calcium channel activity and increased I_Kr CAPON is also present in sympathetic neurons, where overexpression leads to reduced exocytosis of neurotransmitter.

Answer 51

* Developmental disorders can lead to uneven innervation of the heart, which causes arrhythmias. * Scar formation after myocardial infarction may involved hyperinnervation around the scar, leading to heterogeneity in innervation and therefore arrhythmias. * Diabetes mellitus may involved denervation of the heart, so there is no feedback from the heart. This can lead to silent ischaemia (where no angina is felt), which can lead to arrhythmias.

Answer 52

* Awakening and activity * Mental stress * Physical exertion

Answer 53

* Stress leads to the release of catecholamines * This leads to stimulation of beta adrenergic receptors on cardiac myocytes * (Lubbe, 1992): * Plotted VFT threshold and tissue cAMP against perfusate adrenaline * VFT threshold decreased and tissue cAMP increased as adrenaline increased * This effect was exacerbated by theophylline (phosphodiesterase inhibitor) and blocked by atenolol * Beta adrenergic receptors are also found on sympathetic neurons, meaning that catecholamines can cross the blood brain barrier and stimulate the neurons to release noradrenaline onto the cardiac myocytes.

Answer 54

* (Somers, 1993): * Used microneurography to study sympathetic nerve activity during the wake state, stages 2, 3 and 4, and REM sleep * Found that REM sleep involved very high sympathetic activity, which could in part be triggered by the dreams in REM sleep * This could explain why there are high rates of cardiac events during REM sleep * (Wallens, 1972): * Studied a woman with long QT syndrome, who suffered cardiac arrhythmias frequently after a sudden auditory stimulus during sleep * After propranolol therapy, no ventricular ectopic activity followed the auditory stimulus, allowing recovery * This suggests that sudden awakening can cause sudden cardiac death, which is due to catecholamine release

Answer 55

* Takotsubo cardiomyopathy -\> The ’broken heart syndrome’ * (Wittstein, 2005): * Found that emotional stress can precipitate much higher concentrations of adrenaline and noradrenaline than at rest or during a myocardial infarction. * Emotional stress can precipitate severe, reversible left ventricular dysfunction in patients without coronary disease. * Exaggerated sympathetic stimulation is probably central to the cause of this syndrome. * (Meisel, 1991): * Observed that the incidence of myocardial infarction and sudden cardiac death increased significantly in Israel during the first days of the Gulf war (when there was the perceived threat of death) * However, despite the continuing missile threat, the incidence of acute MI reverted to normal after the initial phase of the Gulf war. * This suggests the importance of emotional stress in cardiac events. * (Kovach, 1994): * Induced anger in dogs * Acutely, the anger led to an increase in heart rate, MAP and coronary blood flow * However, it also produced a post-anger state ("stewing"), where three parameters returned to normal but coronary vascular resistance increased * This led to an increase in arrhythmias that could be blocked using beta blockers

Answer 56

(Vanoli, 1991): * Studied normal awake dogs and dogs with vagal stimulation * They subjected each dog to a period of ischaemia (where they are vulnerable to arrhythmias) * Vagal stimulation reduced the heart rate and protected against arrhythmias (Hull, 1994): * Compared dogs who were under cage rest for 5 weeks with those that were exercise trained for 6 weeks * The exercise trained dogs were more resistant to arrhythmias during a period of acute myocardial ischaemia This suggests that exercise is protective in the long term, but extreme exercise can lead to arrhythmias occasionally.

Answer 57

(Herring, 2019): * Stellectomy (removal of the stellate ganglia) is a preventative measure * Compared unilateral stellectomy with bilateral stellectomy * Bilateral stellectomy led to improved survival at 12 month follow-up * The problem with this operation is that Horner's syndrome may occur if the surgeon is not skilled

Answer 58

* The most common sustained arrhythmia in humans, characterised by irregularly irregular atrial electrical activity, resulting in asynchronous atrial contraction. * Since the atrial cannot contract normally, less blood is pumped into the ventricles before they contract.

Answer 59

* P wave is absent and f waves are present instead * RR intervals vary in length (irregular ventricular rhythm) * QRS complex is normal

Answer 60

* AF is the most common sustained cardiac arrhythmia, affecting more than 33 million people globally. * There is a 1 in 4 lifetime risk (Patel, 2019). * AF treatment costs 5-fold more than patients without AF (Andrade, 2014) with more than £1 billion spent per year in the UK. * The number of AF patients is growing due to the ageing population, since age is the biggest risk factor for AF.

Answer 61

* AF itself is rarely life-threatening * If left untreated for a long time, it can lead to: * Stroke * Heart failure * Heart attack * Sudden cardiac arrest * Dementia

Answer 62

Atrial fibrillation arises due to a combination of triggers that induce the arrhythmia and the substrate that sustains it: * Triggers -\> Triggers include sympathetic or parasympathetic stimulation, bradycardia, atrial premature beats or tachycardia, accessory AV pathways, and acute atrial stretch. Recently identified as triggers are ectopic foci occurring in “sleeves” of atrial tissue within the pulmonary veins or vena caval junctions. * Substrate -\> These are either (1) structural or (2) electrical changes in the atria (termed remodelling).

Answer 63

* A short action potential is a hallmark of AF * This can be due to reduced influx of calcium and increased efflux of potassium during the action potential

Answer 64

Atrial fibrosis is commonly seen in patients with chronic AF.

Answer 65

* Yes, because electrical remodelling and structural remodelling are self-reinforcing, as well as driven by AF itself. * This is the concept that "AF begets AF".

Answer 66

(Wijfells, 1995): * Studied awake chronically instrumented goats * Subjected the goats to a period of burst pacing (high energy stimulation) to induce atrial fibrillation * 24 hours of burst pacing produced atrial fibrillation, but 2 weeks of burst pacing produced sustained atrial fibrillation * This is evidence for the idea that AF begets AF

Answer 67

* Electrical remodelling leads to structural changes and further AF, which means that it is not usually sufficient to treat the electrical remodelling. * This is the concept that AF begets AF.

Answer 68

* AF can be sustained by either re-entry mechanisms or rapid focal ectopic firing (where firing spreads from a site, such as the pulmonary vein). * Remodelling creates a substrate that is prone to re-entry due to: * Shortened refractory period * Slowed conduction * Increased atrial size * Paroxysmal (short-lasting) AF is usually caused by ectopic firing from the pulmonary vein, while progression to permanent AF is usually caused by remodelling of the atria to allow re-entry mechanisms

Answer 69

Ectopic firing in the pulmonary veins

Answer 70

* Enhanced automaticity -\> When there is some slight depolarisation during diastole due to decreased I_K3 and increased I_f * EADs -\> When there is prolonged action potential duration due to slowed repolarisation (I_K), allowing channels to reactivate and depolarisation to occur * DADs -\> When there is calcium overload or RyR dysfunction, so that there is excess calcium release from the SR and the electrogenic NCX has to deal with it. This leads to depolarisation. These situations leave the cell prone to early action potential firing and thus arrhythmias.

Answer 71

* Oxidative stress * microRNAs

Answer 72

(Reilly, 2011): * Studied how superoxide (a measure of oxidative stress) varies with duration of atrial fibrillation * Used a goat model of pacing-induced atrial fibrillation * Found that superoxide was increased in the left atria after 2 weeks and in both atria after 6 months * After 2 weeks, the major source of superoxide in the left atria was NADPH oxidase * After 6 months, the major sources of superoxide in the right atria were mitochondrial oxidase and uncoupled nitric oxide synthase, and mitochondrial oxidase in the left atria * The superoxide can affect electrical remodelling and structural remodelling (Zheng, 2016): * Carried out a meta-anlysis of the use of peri-operative rosuvastatin in patients undergoing elective cardiac surgery * Statins reduce the concentration of superoxide radicals * Perioperative statin therapy did not prevent postoperative atrial fibrillation or perioperative myocardial damage. * This suggests that free radicals may not contribute to atrial fibrillation as much as the previous study would suggest.

Answer 73

(Reilly, 2016): * Studied the idea that miR-31 (microRNA-31) may be involved atrial fibrillation by depleting dystrophin and neuronal nitric oxide synthase. * miR-31 leads to decay of the mRNA that encodes nNOS. It also represses dystrophin translation, which means that nNOS subcellular localization is altered. * In nNOS knockout mice, the probability of AF induction by atrial burst pacing and duration of the induced AF episodes was greater than in wild type mice. * Immunostaining for nNOS and dystrophin in atrial myocytes showed that the dystrophin-nNOS complex is disrupted in atrial myocytes in AF.

Answer 74

Yes, atrial dysfunction can lead to ventricular dysfunction and vice versa.

Answer 75

* Specific trigger (e.g. hyperthyroidism, alcohol) * Post-operative AF * Paroxysmal (\< 7 days) * Persistent (\> 7 days / cardioversion) * Longstanding persistent (\> 12 months) * Permanent

Answer 76

Treatment falls into 3 categories: * Rate control * Rhythm control * Stroke prevention

Answer 77

* Rate control attempts to limit the heart rate to a safe range (60-90bpm). It does not attempt to return the sinus rhythm. It is usually chosen when the patient does not have any symptoms. * Rhythm control attempts to restore sinus rhythm. It is usually chosen when there are symptoms. It may help to avoid the side effects and toxicity of anti-arrhythmic drugs.

Answer 78

RATE CONTROL * Beta blockers (preferably beta-1 selective) * Non-dihydropyridine calcium channel blockers (diltiazem, verapamil) * Digoxin If pharmacological treatment is challenging, you can also perform catheter ablation of the AV node and implant a permanent pacemaker. RHYTHM CONTROL * Pharmacological control involves anti-arrhythmic drugs that are given regularly in the long-term or taken when needed (e.g. on onset of palpitations) * Catheter ablation may also be done

Answer 79

1. What is the true prevalence of AF? 2. Is there an alternative mechanism for stroke in AF (rather than just thrombus formation in the atria)? 3. Does AF ablation improve prognosis? 4. Is AF a cause or consequence of an underlying pathology?

Answer 80

(Healey, 2012): * Conducted a study of 2580 patients, 65 years of age or older, with hypertension and no history of atrial fibrillation, in whom a pacemaker or defibrillator had recently been implanted. * Monitored the patients for 3 months to detect subclinical atrial tachyarrhythmias and followed them for a mean of 2.5 years for the primary outcome of ischemic stroke or systemic embolism. * Subclinical atrial tachyarrhythmias, without clinical atrial fibrillation, occurred frequently in patients with pacemakers and were associated with a significantly increased risk of ischemic stroke or systemic embolism. * So yes, subclinical atrial fibrillation is dangerous.

Answer 81

* Classically, strokes in AF are thought to occur after turbulent blood flow in the atria leads to thrombus formation * However, there is now some evidence that vascular risk factors (such as endothelial dysfunction) may underlie the AF and also lead to non-AF related stroke mechanisms

Answer 82

(Marrouche, 2018): * Compared ablation and medical therapy in terms of probability of survival free of hospital admission, with 60 month of follow up * Ablation was shown to be worse in terms of survival * However, criticism of the study included: * Underpowered and highly selected population * Exclusion of patients and events post-randomisation * High rate of loss to follow-up * Small number of events (Packer, 2019) - CABANA trial: * Compared ablation and medical therapy in 2204 patients in terms of cardiovascular events and death, with 60 month of follow up * Among patients with atrial fibrillation, catheter ablation, compared with medical therapy, did not significantly reduce the primary composite outcome. So overall there is contradictory evidence about whether ablation is superior to anti-arrhythmic therapy in preventing cardiovascular events and death.

Answer 83

(Wijesurendra, 2016): * Proposed the idea that "lone" AF may be the consequence of subtle left ventricular dysfunction, rather than its cause. * Ablation does not lead to normalisation of the LV dysfunction. * These findings suggest that AF may be the consequence (rather than the cause) of an occult cardiomyopathy, which persists despite a significant reduction in AF burden after ablation.

Answer 84

* Right coronary artery * The consequence of this is that problems, such as ischaemia and MI associated with the right coronary artery, can lead to SAN and AVN disease

Answer 85

* SAN disease -\> When the SAN is unable to consistently produce a regular sinus rhythm * AVN disease * Second-degree heart block, type 2 mobitz -\> Where there is usually a fixed number of non-conducted P waves for every successfully conducted QRS complex (e.g. 2:1 block) * Third-degree heart block -\> Where there is complete heart block, so no SAN impulses reach the ventricles and therefore the AVN generates the rhythm for the ventricles (much slower)

Answer 86

* In 1958 * Inventor: Rune Elmqvist (Engineer), Surgeon: Dr Ake Senning (Karolinska Institue)

Answer 87

* It is implanted into the chest * It has two leads: * One lead runs to the SAN, in case its function needs to be corrected * One lead runs to the ventricles, to assist with their function

Answer 88

1. Risks of implant procedure: bleeding, infection, pneumothorax, tamponade, lead displacement, driving restriction 2. Lack of physiological pacing via His-Purkinje system 3. Risk of device malfunction 4. Battery longevity 5. Lead longevity: risk of fracture, insulation break, loss of capture

Answer 89

* Membrane clock -\> The idea that pacemaking is due to membrane currents, mostly the funny current (I_f) but also other currents * Calcium clock -\> The idea that pacemaking is due to a calcium clock in the SR (periodic calcium release from the SR) * Coupled clock -\> The idea that the membrane clock and calcium clock co-exist

Answer 90

* Funny current (I_f) -\> HCN * Sustained inward current (sodium!) -\> Ca_V1.3 * Lack of inward rectifier potassium current (I_K1) -\> Kir2.1 * L-type calcium current * Delayed rectifier potassium current

Answer 91

Note that the sustained inward current is not shown here.

Answer 92

* (Kodama, 1997) carried out block of I_CaL (L-type calcium channels) by nifedipine * (Lei, 2011) carried out block of I_Kr (delayed recitifer potassium channels) by E-4031 * Blocking each of these channels results in cessation of pacemaker activity * However, the membrane potential does not stay at the most negative potential reached between beats -\> This is evidence for the lack of inward rectifier potassium channels present in the SAN (since these are what typically make the resting membrane potential so negative) * (Maike, 2002): * Used an adeno-associated viral vector to suppress I_K1 (inward rectifier potassium channels) in VENTRICULAR myocytes * This caused them to start spontaneously contracting, similar to SAN cells * This suggests that the lack of inward rectifier potassium channels in the SAN is important for function

Answer 93

* Sustained inward current is a sodium-based current that is passed through Ca_v1.3 channels (along with the L-type calcium current) * (Mangoni, 2003): * Knockout of the Ca_v1.3 channel in the SAN results in a much slower cycle length, so the heart rate is reduced compared to wild-type * Voltage clamping of the wild-type and knockout mice shows that the Ca_v1.3 channel enables a small inward current at the range of diastolic depolarisation * (Toyoda, 2017): * Compared wild-type and Ca_v1.3 knockout mice SAN cells * In both the wild-type and knockout mice, lowering extracellular calcium close to 0 reduced the total current at the range of diastolic depolarisation * However, nifedipine had no further effect in the knockout mice but it did in the wilf-type mice * This is evidence for the idea that the sustained inward current is via Ca_v1.3 channels (and is therefore sensitive to nifedipine) but is not calcium-mediated

Answer 94

* The SAN has Ca_V1.3 and Ca_V1.2 channels, while the ventricle has Ca_V1.2 channels * Both allow L-type calcium currents, but Ca_V1.3 also allows the sustained inward current, which is sodium based and contributes to pacemaking

Answer 95

* It is a hyperpolarisation-activated mixed sodium–potassium current that activates at voltages in the diastolic range. * It is classically thought to be the pacemaking current that enables pacemaking. * (Brown, 1979): * First described the hyperpolarisation-activation of the funny current * Showed that I_f was increased by adrenaline * (DiFrancesco, 1991): * Showed that I_f is increased by cAMP (not PKA) * (Mesirca, 2014): * Used a knockout mouse model where doxycycline is used to induce the knockout of HCN4 * This caused progressive bradycardia and atrioventricular block * This suggests that the HCN4 channel plays an important role in pacemaking in the SAN and also in some way in the AVN * However, since the heart does not stop completely, it would be a stretch to call I_f **the** pacemaking current, since it is not necessary for pacemaking * (Qu, 2003): * Used adenoviral transfer to transfer HCN2 and GFP into a localised area of the left atrium in dogs * They also produced a control with just the GFP * Vagal stimulation led to bradycardia in the controls, but did not in the HCN2 group, since the left atrium pacemaker takes over * This shows the importance of HCN2 in pacemaking

Answer 96

HCN4 and HCN2

Answer 97

* Ivabridine is a HCN channel blocker * (BEAUTIFUL trial, 2008): * Showed the effectiveness of ivabridine in treating angina * (SHIFT trial, 2010): * Showed the effectiveness of ivabridine in treating heart failure in patients who were already on a beta blocker but still had a heart rate above 75

Answer 98

* Traditionally, pacemaking in the SAN has been thought to be due to cell membrane ion channels. * However, there is increasing evidence that in reality pacemaking may be due to spontaneous calcium-release events from the SR. * There is contradictory evidence about what drives this release.

Answer 99

(Rigg, 1996): * Used ryanodine and cyclopiazonic acid to alter the ability of the sarcoplasmic reticulum (SR) to store calcium (i.e. reduce calcium release). * Both of these interventions significantly reduced the rate of spontaneous beating of guinea-pig sino-atrial node preparations. (Huser, 2000): * Used confocal microscopy to study calcium release events in SAN cells * Noticed that during the end of diastole, there are calcium sparks from the SR, which precede the upstroke of the action potential * Blocking the T-type calcium current (I_CaT) using nickel caused reduced pacemaker activity and reduced calcium sparks during diastole * This suggests that the T-type calcium current drives calcium sparks, which could in turn drive the NCX and thus drive the potential up to threshold (Bogdanov, 2001): * Used voltage-clamping and slowly depolarised the membrane to mimic diastolic depolarisation * Inhibited the NCX using lithium * This caused the current during depolarisation to be smaller and almost completely blocked pacemaker activity * This suggests that calcium sparks drive the NCX, which causes the membrane potential to rise to threshold (Vinogradova, 2004): * Voltage-clamped a cell while performing calcium imaging * They suddenly clamped the cell at a polarised voltage (so that no more action potentials could fire) * The calcium sparks continued for some time with periodicity that matched the frequency of the previous action potentials * This is evidence for the idea that the SR has inherent clock activity without the need for the T-type calcium current (I_CaT) to trigger it (Lakatta, 2010): * Plotted a graph of the action potential cycle length against the local calcium release period (i.e. how frequently calcium sparks happen) * Plotted points with different drugs that alter heart rate * Found a correlation between LCR period and cycle length * This suggests that the period of calcium sparks is what drives pacemaking

Answer 100

* It is a theory that combines the membrane clock and calcium clock mechanisms * It suggests that SR calcium release regulates the funny current (I_f) * This would explain why both of these factors appear to affect pacemaking

Answer 101

(Mattick, 2007): * Administered BAPTA to chelate the calcium in the cell -\> This led to a reduction in I_f * This reduction could be reversed when forskolin (adenylate cyclase activator) was also administered * This showed that I_f is calcium-dependent and this could be due to calcium-activated adenylate cyclases in the cell (Wu, 2008): * Genetically encoded an inhibitor of CaMKII * Compared to controls, the inhibition of CaMKII led to a blunting of the chronotropic response to isoprenaline * This supports the idea that calcium can feedback on itself by activating CaMKII, which modulates calcium release at the membrane and SR, thereby also modulating pacemaking. (Sirenko, 2016): * Inhibited CaMKII using AIP (without isoprenaline) * This led to a reduction in heart rate Other groups have suggested that the coupled clock does involve both I_f and local calcium release from the SR, but the end point of both of these is the NCX, which is the crucial exchanger that brings everything together: (Groenke, 2013): * Produced an atrial-specific knockout of NCX * This led to profound bradycardia where there was a lack of P waves (so pacemaking was maintained by the AVN) * However, I_f and calcium sparks were intact * This suggests that the NCX is a crucial component of the coupled clock (Sanders, 2007): * Rapidly switched a cell to low sodium as a way of stopping NCX function * This led to almost complete cessation of pacemaking activity and progressively lower calcium levels in the cell * Eventually there were some signs of spontaneous pacemaking activity again

Answer 102

Explanation of final point: * When there is more than 100nm calcium in the cell, there is a baseline of NCX activity that maintains the cell slightly more depolarised than it would otherwise be * This is important because pacemaking would not work otherwise

Answer 103

The diagram shows a membrane clock and a calcium clock that are highly interlinked. Both of these clocks may also be modulated by the autonomic nervous system.

Answer 104

* Intrinsic heart rate gradually falls with age -\> A constant resting heart rate is maintained by a degree of parasympathetic tone that decreases with age and then eventually a degree of sympathetic tone takes over * By switching off the parasympathetic tone and maximising sympathetic tone, a maximum heart rate can be achieved -\> This decreases with age due to the falling intrinsic heart rate

Answer 105

(D'Souza, 2014): * Studied the heart rate of rats and mice, both in vitro and in vivo (with autonomic block) * Compared these before and after training -\> Training caused the heart rate to fall * The effect of this training normalised after applying blockers of I_f * Patch clamping also showed reduced I_f after training (Yavari, 2017): * Studied the idea that γ2 AMPK is the metabolic sensor in exercise * Activation of γ2 AMPK in the SAN is suggested to lead to decreased membrane clock and calcium clock activity

Answer 106

It is the idea that mechanical changes in the heart (e.g. physical stimuli) can feed back onto the electrical activity of the heart.

Answer 107

Mechano-sensitive ion channels

Answer 108

* Cation-Nonselective Stretch-Activated Channels (SAC_NS) * Since these are non-selective, they tend to show a linear I-V relationship * This makes it easy to model their effects on membrane potential However, there are also other types of SACs, such as SAC_K which carry a potassium current and have a much more negative reversal potential.

Answer 109

* Between 0 and -30mV * Since the resting membrane potential is usually around -70mV, if the cell is stretched at rest, then the cell will be slightly depolarised due to SAC_NS * If the cell is stretched when depolarised, the SAC_NScurrent will try to repolarise the cell to 0/-30mV

Answer 110

There are both SAC_NS and SAC_K present. Each will try and drag the membrane potential towards its reversal potential (SAC_Kis much more negative), so the effect on the action potential depends on which channels are open at any given time. This is affected by: * Stretch target (affected by environment) -\> For example, ischaemia leads to favoured opening of SAC_K. * Stretch timing -\> The timing at which stretch happens determines what the membrane potential is at that given time and therefore how the membrane potential will change. * Amplitude of stretch -\> A high enough amplitude may lead to action potential firing.

Answer 111

* If the amplitude is small, no action potential will be fired. * The effect of stretch is depolarisation, which shows that SAN_K are not open under these conditions. * If the amplitude is large enough, an action potential may be fired.

Answer 112

Stretch alters cardiac myocyte calcium handling by the SR (and by binding to troponin C).

Answer 113

Commotio cordis (Commotio refers specifically to disturbances where the heart itself is not damaged by the mechanical stimulus)

Answer 114

* There is a very narrow time window just before the peak of the T wave where the heart is vulnerable to stretch-induced VF * This is because this is when the repolarising myocytes are susceptible to EADs and DADs -\> This is the **trigger** for an arrhythmia * The heterogeneity in the repolarisation of myocytes acts as the **sustaining mechanism** * The combination of the trigger and sustaining mechanism means that an arrhythmia can arise

Answer 115

The electrical window is much longer than the mechanical window, since an electrical stimulus can reach much more of the heart than a localised mechanical stimulus.

Answer 116

* Stretch-induced arrhythmogenesis requires supra-threshold stimulus overlapping with trailing edge of preceding excitation. * In other words, the stretch must produce a depolarisation that catches up with the wave of repolarisation after the last action potential. * This produces a region of functional block between the areas of depolarisation and repolarisation. * In turn, this creates the possibility for re-entrant circuits (i.e. arrhythmias). * Timing is very important in order for this areas of functional block to be produced and a re-entrant circuit to form. (Quinn, 2017): * Applied mechanical stimuli to isolated rabbit hearts during optical voltage mapping and combined this with pharmacological block of SAC_NS and SAC_K channels. * Showed that local mechanical stimulation reliably triggers premature ventricular excitation at the contact site. This premature ventricular excitation was diminished by pharmacological block of SAC_NS. * In hearts where electrocardiogram T waves involve a well-defined repolarization edge traversing the epicardium, PVEM can reliably provoke ventricular fibrillation if, and only if, the mechanical stimulation site overlaps the repolarization wave edge. * SAC_K block hadno effect on PVEM inducibility, but shifts it to later time points by delaying repolarization and prolonging refractoriness.

Answer 117

1. Macro-to-micro -\> Our current models are very poor at mapping cell-level changes onto macro-level changes in the whole heart. If we cannot do this, then we also cannot predict the effects of mechanical inputs on the heart. 2. Micro-to-nano -\> There is very new evidence that cardiac myocytes may act as their own pumps to drive fluid exchange in the T-tubules during the contraction-relaxation cycle (Rog-Zielinska, 2021). The possibility of such nano changes adds another component to how mechanical deformation may affect the heart, making the situation more difficult to understand.

Theme 2: Cardiac Electricity and Arrhythmia Flashcards

(158 cards)