From Action Potentials to Arrhythmias

[Music] [Music] greetings to one and all on this forum i am dr sanjay andrew professor of physiology from chetna hospital and research institute in chennai my topic for the discussion is from action potentials to arrhythmias so what i will be doing over the next half an hour is i will be using these objectives to update you on certain core principles of electrocardiography i shall begin with the electrophysiology of the heart giving you giving you an overview of what has already been discussed o

n this forum and then i will go on to certain electro physiological hallmarks first i will talk to about the electro talk to you about the electro physiological hallmarks seen in the ecg associated with the sinus rhythms and then i will follow it up with the same in certain conduction disorders of the heart and then i will give you a primary classification of arrhythmias from where my clinicians will take over and give you further inputs on the same so we shall have a recap on the electrophysiol

ogy of the heart the human heart is an electromechanical pump that primarily has five core physiological properties automaticity is the ability of the heart to spontaneously generate an impulse conductivity is the ability of the heart to generate the simple throw of the conducting system rhythmicity is the ability of the heart to make sure that this impulse is conducted in a regularly regular fashion contractility is the actin myosin interaction within the cardiac myocyte and this allows the hea

rt to contract as a whole or as a sensitive there is another electrical property known as refractiveness now this is the duration of an action potential where a second stimulus will not be able to generate another impulse now as far as understanding the ecg is concerned you should know that most of the cardiac arrhythmias are because of either an increase or a decrease in the atomicity and conductivity of the heart refractiveness is also essential to understand how arrhythmias develop i will be

talking to you about it later in the presentation these are the five core physiological properties which we have already discussed automaticity is the ability of the heart to spontaneously generate an impulse rhythmicity is the inherent regularly regular discharge of a cardiac impulse conductivity is a transmission of these impulse throughout the conducting system contractility is contraction of the cardiac muscle as a whole and refracting refractiveness is the inability of cardiac muscle to res

pond to electrical stimulation during a particular interval in its action potential so this sludge gives you an overview of the conducting system which you might already know so you can see the sa node is the primary pacemaker of the heart where your impulse is spontaneously generated throughout the life of a person from the sa node you have three internal tracks which link up the sa node to the av node and the av node continues as the bundle office which in turn terminates as the purkinje fiber

s the bundle office is thrown into a left bundle branch and a right bundle branch and in this diagram you can see the left bundle branch has an anterior division and a posterior division this slide shows two important electrical properties of the heart namely rate and visibility and velocity of conduction of a cardiac impulse on this side we can see that the rate and rhythmicity of a cardiac impulse is greatest at the primary pacemaker or the cyanoatrial node and on this side we can see that the

fastest velocity of cardiac impulse conduction occurs in the bundle of fist and the purkinje system and the slowest velocity of cardiac impulse conduction occurs at the atroventricular node this is an interesting slide which gives you an historic historical update about the discovery of the conducting system so you can see that even though the contacting system terminates with the purganji fibers the purkinje fibers were the were the part of the conducting system that were first discovered in 1

845 and the internal tracks were the last the anterior middle and posterior internal tracts they were the last to be discovered in 1963. now apart from the normal conducting system which i just described quite a proportion of the population have certain abnormal bypass fibers also known as the paranormal tracks and here in this diagram you can see the av node and the bundle of his which i will be referring to as the fascicle now five of these abnormal or paranoral tracts have been described and

here at 1 you have the james atriophasical bundle which extends between the atrium and the bundle of his at 2 you have the in intranodal bundle which is present in the av node and at 3 you have mahem's fascicle ventricular bundle which extends between the bundle of his and the ventricle and at 4 you have mahem's nodo ventricular bundle which extends between the av node and the ventricle and at five you have kent's atrioventricular bundle which extends between the atrium and the ventricle so thes

e paranormal tracts are sometimes responsible for certain abnormal rhythms which may be picked up in the ecg in fact one abnormal rhythm which is known as accelerated conduction is due to these paranormal tracts and i will be describing about it later in this presentation now this slide shows the action potentials of the various conducting tissues of the heart in physiology whenever we describe the classical cardiac action potential we describe the ventricle action potential with its four faces

however stimulation of the different parts of the conducting system give different types of waveforms for example at the sa node you have the classical pacemaker potential which begins to evolve as we go down the conducting system into the classical ventricular action potential so this must be kept in mind whenever we go about understanding the electrophysiology of the heart this slide shows a classical ventricular action potential and you can see that the ventricular action potential has four p

hases the first phase is the face of depolarization of phase zero and this is followed by the early repo repolarization of phase one and this is followed by a plateau which is phase two and then we have late repolarization which is phase three and finally we have phase four which is returning back to the resting membrane potential the ionic bases of these phases are described on this side so you can see that phase 0 is due to opening of the fast sodium channels and phase 1 is due to closing of t

he first sodium channels phase two is opening of the calcium channels and phase three is due to closing of the calcium channels and finally we have the return back to the resting membrane potential of phase four now antiarrhythmic pharmacotherapy is widely used to manage cardiac arrhythmias and this sludge shows there are four classes of drugs which are used to manage cardiac arrhythmias and on the far corner there are some examples of each but what i would like you to understand is this section

of the tabular column so you can see that each gra each group of drug tends to act on the cardiac action potential and try to control the arrhythmias more on this will be told to you by a pharmacologist in subsequent sessions this slide correlates a ventricular action potential with a normal electrocardiogram and what we can see here is the p wave has no correlation because this is the ventricle action potential the p wave is due to atrial depolarization now the qrs complex corresponds with pha

se 0 of the ventricular action potential and phase 2 or the plateau corresponds with the st segment of the electrocardiogram and the phase of repolarization corresponds with the t wave of the electrocardiogram now earlier in my presentation i had described five core properties of the heart one of them was refractoriness so we shall try to understand refractiveness of the ventricular action potential because it is important for for our understanding of how arrhythmias are generated so the refract

ive period as you might be knowing is the period of the action potential where the second stimulus will not be able to generate another impulse and there are three types of refractory periods namely the effective or absolute refractory period and the relative refractory period and another phase known as the supernormal phase the absolute refractory period extends from phase 0 up to the mid portion of phase 3. during this period even a strong second stimulus will not be able to elicit an action p

otential the relative refractory period follows the absolute refractive period in the ventricular action potential and during this period a very very strong stimulus can bring about a second action potential now it is the last portion of the refractory period also known as the supernormal phase which is present in the phase four of the action potential it is this phase from which most of the arrhythmias are generated so during the super normal phase the cells of cardiac muscle are hyper excitabl

e with a single stimulus capable of producing multiple responses i repeat it is a supernormal phase from which most of the cardiac arrhythmias are generated there is another classification of cardiac muscle cells namely the automatic cardiac muscle cells and the non-automatic cardiac muscle cells the automatic cardiac muscle cells are located in the sa node and the av node and these cells spontaneously generate impulses while the non-automatic cells are located lower down in the conducting syste

m and these cells depend on being excited by the sa node and the av node however in certain abnormal states the non-automatic cells can become this slide shows the core differences between the automatic cells and the non-automatic cells so here you can see the pacemaker potential which is primarily due to the automatic cells and the ventricle action potential which is primarily due to the non-automatic cells now here you see the pacemaker potential has a slowly rising phase four and phases one a

nd two merge with each other each other and this uh electrical activity is predominantly calcium dependent and here you have the ventricular action potential which is sodium dependent predominantly and you can see that different phases phase 0 1 2 3 and 4 are very clearly defined so having told you about the core electrophysiological properties of the heart i am going to go on to certain electrophysiological hallmarks of the ecg associated with the sinus rhythms now a sinus rhythm is refers to r

hythm in which the heart beats sequentially and normally described as regularly and regular the sinus rhythm usually exhibits a normal rhythm with or without altered rates so there are three important sinus rhythms which you should be aware of namely sinus tachycardia sinus bradycardia and sinus arrhythmia so sinus tachycardia is a heart rate of regular rhythm with a rate greater than 100 per minute so here you can see this is a tracing of a sinus tachycardia and you can see the rate has markedl

y increased and functionally or physiologically a sinus tachycardia is seen during exercise and periods of stress while pathologically there are states associated with the sinus tachycardia and a few examples of that is fever anemia and hyperthyroidism the sinus bradycardia is a heart rate of regular rhythm with a rate less than 60 per minute so the sinus bradycardia is usually seen when the vehicle tone is increased and functionally it is typically seen in well-trained athletes and there are ce

rtain pathological states where a sinus bradycardia may occur and disorders of the conducting system as well as hypothyroidism are two examples of sinus bradycardia so here you can see a classical sinus bradycardia where the heart rate has markedly decreased now the next sinus rhythm is the sinus arrhythmia which is a heart rate of regular rhythm with alternating phases of fast and slow rates from a functional point of view or physiologically a sinus arrhythmia is usually seen in children and du

ring the different phases of the respiratory cycle during inspiration the heart rate increases and during expiration the heart rate decreases sinus arythmia is also associated pathologically with disorder generation of impulses in the sa node here you have a tracing of a sinus arrhythmia and you can see the heart rates being fast and then slowing down and then becoming fast again so having told you about the three sinus rhythms now i will go through an update on certain electrophysiological hall

marks which are seen in the ecg in certain conduction disorders of the heart so we shall begin with aberrant conduction so this is actually a refractoriness or a delay in conducting a supraventricular impulse into the ventricles and it should be kept in mind when differentiating a supraventricular tachycardia from a ventricular tachycardia so here you can see an example of aberrant conduction so here's the ecg strip in which you see the p wave occurring after the qrs complex because of apparent

conduction that is seen in this tracing next we have the accelerated connection previously i told you about the paranormal tracks and accelerated accelerated conduction is usually seen when whenever there is a parent nodal tract in which through which an impulse bypasses the av node so the classical finding of a accelerated connection is a shortened pr interval and this is seen in two important syndromes which may be described later namely the wolf parkinson white syndrome and the long long gano

ng 11 syndrome so here you can see a tracing from a patient with long ganong level syndrome and a accelerated conduction and you can see the pr interval being shortened because of accelerated conduction then we have another disorder known as the atroventricular dissociation so this is actually a functional block in the av node functional block of conduction in the av node so as a result the ventricles fire at a faster rate than the atria since the av is refracted every node is refractory to the

passage of impulses from the sa node so here we usually see the p waves marching towards and overtaking the qrs complexes and occasionally there will be a normal rhythm known as the capture beat so here you can see a tracing of atrial ventricular dissociation you can see the p waves marching towards the qrs complexes and eventually overtaking the chiaris complexes and this may be followed by a normal rhythm or isorhythmic pattern known as the capture beat there is another electrophysiological di

sorder known as acrocede synchrony here what happens is two adjacently situated cardiac tissues may fire at the same rate even though they are stimulated at different rates with this marginal difference now this may be seen in av dissociation which i just now told you and usually across state synchrony is characterized by a positive wave following the qrs complex as you can see in this tracing now we come on to the phenomenon of electromechanical dissociation or also known as pulseless electrica

l activity this is a phenomenon which usually precedes death and here what happens is the mechanical contraction of the heart does not occur in spite of electrical activity being recorded and usually the rtl pulse cannot be palpated in such patients and there are quite a few causes of electromechanical dissociation which is listed out in this slide so now we have another rhythm where there is electromechanical dissociation known as the agonal rhythm so here we what we have is a slow rhythm with

wide and bizarre qrs complexes and as i told you it precedes cardiac arrest and it's a classical example of electromechanical dissociation that is agonal rhythm is a classical exam example of electromechanical dissociation so this is a classical tracing of a agonal rhythm now we come to two closely related electrophysiological abnormalities namely the ventricular by germany and the ventricular trigeminy so the ventricular bichumny is a electrophysiological phenomenon in which a sinus beat altern

ate alternates with an ectopic beat so here you can see sinus beat alternating with a ectopic beat and this is a classical tracing of a ventricular by germany so usually these electrophysiological changes are associated with certain arrhythmias which the clinicians will be talking to you about and this is the ventricular trigeminy which i told you so here you see two sinus beats may alternate with an ectopic beat or two ectopic beats may alternate with the sinus bit here you have two ectopic bea

ts alternating with the sinus beat in this tracing now the next electrophysiological abnormality which i would like to tell you is the blocked atrial ectopic now this occurs as a consequence of digital digital toxicity as you all know digital is the drug used to manage cardiac failure so what happens in the block data lectopic is the ital premature beats they are noted in the ecg as a single entity and are also known as isolated p waves so you can see this in this tracing now we come on to the c

ongenital complete heart block it's a block and conduction of the impulses at the upper part of the atrioventricular junction so usually the ecg is characterized by a normal rate with a narrow qrs complexes that are dissociated from the p waves so you can see the qrs complex they narrow down and the p waves are actually dissociated from the qrs complexes now another type of electrophysiological disorder is a concealed conduction what happens here is certain impulses that are conducted may not be

picked up on the ecg at the point where they are supposed to be picked up but they can be made out by analyzing subsequent complexes atrial fibrillation is an example of concealed conduction and what happens here is the pr interval following a ventricular premature beat is usually longer than normal so here you can see the pr interval forming a ventricular ectopic can is longer than normal so this is a example of concealed conduction now decremental increment is noted to occur in the second deg

ree av nodal block here what happens is there is a delayed and conduction of impulses at the av node and the pr interval is initially widened but gradually begins to narrow down that is why it is known as a decremental increment and here you can see this tracing the pr interval is initially increased and as we go down it tends to narrow down or decrease now dissociated beat refers to a block and conduction of impulses at the av node because of a ventricular premature beat being generated distall

y so the ecg shows a positive p wave with a shortened pr interval so this tracing you can make out a positive p wave with a very very short pr interval and that is known as a dissociated beat an ectopic beat is an abnormal beat that arises outside the sa node this could be either atrial junctional or ventricular so the tracing on top is a tracing of a atrial ectopic so here you have abnormal p waves you can see the p waves being abnormal then we have the junctional ectopic where the p waves are

retrograde and then the ventricular topic where the qrs complexes are said to be bizarre now an escape beat occurs when the primary pacemaker of the heart that is the sa node fails to fire as a result usually a secondary pacemaker namely the av node takes over with a single beat known as the escape beat this can be seen in this tracing these are all sinus rhythms and here you have the essay note failing to fire and the secondary pacemaker taking over and this is the escape beat now wenky back ph

enomenon is a commonly described entity and it usually occurs during hard blocks what happens here is there is grouping of beats with an interval between the group beats so here is the tracing of winky backs phenomenon here you can see three beats with an interval and then grouping of beats again and the last electro physiological abnormality that i will be describing to you is known as the toss at the points which refers to ventricular arrhythmia with ventricular complexes of varying shapes so

here you can see a tracing of toss at the points you can see the ventricular complexes are of different shapes and usually this electrophysiological disturbance is usually seen following the use of antiarrhythmic drugs so now that i have given you an update about certain electrophysiological disturbances which can be picked up in the ecg i conclude this presentation by giving you a primary classification of arrhythmias so it is useful to classify arrhythmias cyanonic science i know atrial node a

rrhythmias atrial arthritis junctional or nodal arithmeos and ventricular arrhythmias so you can see i have listed out few examples at each group and this will be described by my clinicians in the subsequent sessions and as i conclude this session i would like to thank certain people without whom this wouldn't have been possible so my heartfelt thanks goes out to the flag borders of niptil for giving us this opportunity to share our knowledge with one and all on this forum the flag bearers of ch

etna hospital and research institute for always motivate motivating us to excel in our endeavors of teaching and research my teachers and students all over the world for empowering me with the wisdom of physiology and medicine and last but surely not the least i'd like to thank all the participants of this learning exercise thanks to one and all