Comprehensive Guide to AV Blocks and Bundle Branch Blocks Explained

Hello friends. Myself, Dr. Durga; I am from  Chettinad Hospital and Research Institute.   In this lecture, today we are going to discuss  about the AV blocks, that is the atrioventricular   block which is also known as heart blocks and  bundle branch block. In the next 2 slides,  

I am going to discuss about the conduction system  and the normal ECG, just to revise fast. See,   this is the SA node. From the SA node, there is   initiation of the electrical impulses  which leads to start the contraction.

Now, these electrical impulses from the  bundle of Thorel, Bachmann and Wenckebach   via atrial myocardium goes to the AV node. In the;  what is the unique property of this AV node? The   AV node has a decremental response or delays.  Wa what is important of this delay? Actually,  

this protects the ventricle from the ah  fast atrial arrhythmias. After this AV node,   this is the bundle of His. Via the bundle of His,  it goes to the left and right bundle branch. And   in the left bundle branch, it goes to the left  anterior division and left posterior division.

Both these bundle branch ultimately divides  into a network of conducting uh system which   is known as Purkinje fibres. So, this is how  there is the activation which starts from the   atria and comes to the ventricle. Okay. Here  we are going to discuss about the blocks which  

occurs in the AV node and the bundle branch.  See, this electrical impulse, generation of   these electrical impulses and traversing through  the atrial and ventricular mechanic produces a   changes in the action potential, which can be  seen in the ECG, uh through the different waves.

We all know that we see the 3, 4  waves that P, Q, R, S, T. Okay. So,   this P wave actually represents the atrial  depolarisation. QRS complex represent the   ventricular depolarisation. This PR segment  represents the AV nodal, that is atrioventricular  

nodal conduction time. And then, T wave  represents the ventricular repolarisation. So,   these are the features of the normal ECG. The  rate is usually between 100 to 60 beats per   minute with regular rhythm. P wave is usually  upright in lead 1 and 2, and negative in aVR.

Each QRS complex is preceded by a P wave. And  these QRS complex is usually less than 100   millisecond wide. PR interval remains constant.  PR interval remains constant. Okay. So,   these are the features of the normal ECG.  Here you can see, this is the P wave, this is  

PR segment, and this is QRS, and then this is T  wave. This is how the normal complex looks like.   Okay. Now, we will go to the our topic, that is  the atrioventricular nodal blocks or heart blocks. So, what is this atrioventricular  nodal blocks or heart blocks?  

Whenever there is a disturbance in conduction  of the impulses through the AV node,   there it leads to AV blocks. Okay. Whenever there  is a disturbance of conduction through the heart,   which is particularly occurring at the AV node.  It can be because of any disease or damage of  

the AV node. Either there is a delay or there is a  total block of impulses to get passed through the   AV node. These conduction defects  can be detected from the ECG. See, this AV nodal conduction time, as I  told you, is represented in the ECG via PR  

segment. But usually, we do not measure  PR segment, we measure PR interval. So,   what is the difference between this PR  segment and PR interval? Look at this ECG.   PR segment is, starts from the end of P wave to  the start of R wave, but the PR interval is from  

start of P wave to the start of R wave. Okay.  This is what is the difference between the PR   segment and PR interval. Usually, we do not  measure PR segment, we measure PR interval. So, when uh; so, this heart blocks can  be divided into 3 types, that is the  

first degree heart block, second degree  heart block and third degree heart block.   This third degree heart block is  also known as complete heart block.   And then, again this second degree heart block  is divided into other 3 subtypes; that is the  

Mobitz type 1, Mobitz type 2 and advanced  second degree heart block. In this advanced   second degree heart block we will be just  looking at the 2 is to 1 block. Okay. So, first is the first degree heart block.  Actually, to call it as a block is a misnomer.  

There is no block; there is a delay in the  conduction through the AV node. There is a   delay in the conduction. So, actually, the AV node  conducts more slowly than the normal. So, there   is occurrence of the prolonged PR interval. PR  interval is prolonged, but it is usually constant.  

PR interval is greater than 0.2 seconds or  more than 5 small squares, but it is constant. So, P wave will be con, P wave will be normal, QRS  will be normal, there will be only prolongation of   the PR interval more than 5 small boxes. So,  you can see in this, the rhythm given below.  

Thus see, if you count the PR interval, this is 1,  2, 3, 4, 5, 6 here; 1, 2, 3, 4, 5, 6; 1, 2, 3, 4,   5, 6. So, it is prolonged; more than 5 boxes, but  it is constant in each of the, in all the rhythms.   Okay. So, so, what are the points you should  remember about the first degree heart block?

There is a prolonged AV conduction which  leads to prolongation of the PR interval.   uh Usually, the QRS complex is narrow. And if  QRS complex is narrow, it suggests that the   block is in the AV node. The site of  involvement can be atria, can be AV node,  

can be His Purkinje. Usually, we find out by  the QRS. So, the QRS is usually if it is narrow,   then it says, the origin is in the AV  node. Clinically, uh the patient does   not have any symptoms, the prognosis  is good and no treatment is required.

Then comes the second degree heart block. This  second degree AV block is diagnosed when one or   the more impulses of the atria is not transmitted  the ventricles. When some of the atrial impulses   are not transmitted to the ventricles, then it is  known as second degree heart block. As I told you,  

the second degree heart block is of 3 types; type,  Mobitz type 1, that is, Wenckebach; Mobitz type 2;   and advanced second degree heart  block. So, first is Mobitz type 1. So, what occurs in Mobitz type 1 is, there  is progressive increase in the PR interval or  

progressively delay in the conduction of impulses,  which leads to increase in the PR interval until   one of the atrial beat or P wave is not  transmitted to the ventricle, until one of the   atrial impulse is not transmitted to  the ventricle. See in the rhythm below;  

sorry. So, this is PR interval. If you see,  this PR interval is progressively increased.   This says that the conduction through  the AV node is progressively increasing. And then, this is the P wave which is not getting  transmitted. This is what is drop beat. This is  

what is drop beat. Okay. Progressive increase  in the conduction time followed by one of the   atrial impulse will not be transmitted. So, what  you have to remember about the Mobitz type 1 is,   the PR interval is not constant and it is  increasing progressively till one of the beat is  

not transmit; that is drop beat. The PR interval   is longest just before the drop beat  and shortest just after the drop beat. If you look at the difference between  the longest and the shortest PR interval,  

the, it is more than 100 milliseconds. The  ratio of P is to QRS is usually 3 is to 2   or it can be 4 is to 3 or 5 is to 4, commonly  being is 3 is to 2. So, what are the symptoms   which you get? Usually, lethargy, fatigue  or lightheadedness and sometimes confusion.  

This rhythm is usually benign, does not produce  any hemodynamic changes. If at all the patient   is symptomatic, you can just give atropine  or needed you can go with the pacemaking. This can be transmitted to  third degree heart block,  

but the risk of transmit of this heart block is  low. So, here I am giving you one scenario. Look,   a 70-year-old female cames with a history of  hypertension and diabetes and dyslipidemia,   present to the casualty with a history of  intermittent palpitations and breathlessness.  

She also state that she has uh dyspnoea  on exertion. On enquiring, she does not   give any history of chest pain or syncope or  near syncope, cough or any other complaints. And while looking at the monitor, we get  the following rhythm in the ECG. Let us see.  

Here, just look at the PR interval. Take the  last rhythm, look at the PR interval. See,   PR interval is progressively increasing  till one of the P wave is not transmitted.   So, this is drop beat. Okay. The PR  interval just before the drop beat is  

longest and the PR interval just after the drop  beat is shortest. If you take the difference   of the shortest and the longest PR interval,  it is definitely more than 100 milliseconds. You can count the small boxes and subtract,  and you will get the difference is more than  

100 milliseconds. So, with these criteria, we  can say in this ECG, the patient is having a   block, Mobitz type 1 block. Okay.  And this phenomenon is known as,   this progressive increase is known as Wenckebach  phenomenon. Next come is the Mobitz type 2 block.  

So, what happens, some but not all the sinus P  waves is transmitted to the ventricle. So, there   is a, there is a drop beats, but this conduction  failure, this conduction failures occur suddenly. See, in the last, what we said, that there will  be progressive increase in the PR interval,  

and then one of the P wave will be dropped. Here,  what happens is, suddenly 1 or 2 P waves will not   be transmitted to the ventricle. So, there is a  sudden failure of conduction of some but not all   sinus P waves to ventricle; that is drop beats,  but the PR interval is constant. Look at this  

rhythm strip. This P wave is not transmitted.  Here this P wave is normally transmitted;   it here also, here also; but suddenly,  again this P wave is not transmitted. Okay. So, there is a sudden failure,  without any preceding increase in the  

PR interval. And if you see the PR interval,  it is 1, 2, 3. Here also it is 1, 2, 3. In   this beat also it is 1, 2, 3. So, the PR interval  is constant. So, constant PR interval with sudden   failure of conduction of the atrial impulse to the  ventricle, that gives to Mobitz type 2 block. So,  

what are things we should remember in the Mobitz  type 2 block is conduction failed suddenly and   unexpectedly without any proceeding change in the  PR interval, and usually the QRS complex is wide. The PR interval is constant, both before and after  the occurrence of a drop beats. Mostly the problem  

is in the infranodal conduction system. So, there  is one important clue. The important clue is,   when you want to differentiate between the Mobitz  type 1 block and type 2 block. So, in the Mobitz   type 1 block what happens, there is progressive  increase in the PR interval, then a drop beat.  

Here, the PR interval will be constant and  then sudden appearance of a drop beats. This is how you differentiate  between the Mobitz type 1 and   type 2 block. Okay. Here also, the symptoms  is, maybe fatigue or dizziness or pre-syncope  

and syncope. There is high chances that it  may land up in third degree heart block. So,   permanent pacemaking is the treatment of choice.  So, here I am giving you one more scenario. The   70-year-old female known case of diabetes and  hypertension presented with lightheadedness,  

dizziness and pre-syncope since last 2 days,  no history of any chest pain on examination. You found that the patient is having  regularly irregular pulse, the BP 90,   50, and the uhm heart rate is round about 47.  And when you look at the ECG monitor, you got  

the following rhythms. So, look at the rhythm.  This is QRS, which is wide. And this is P wave,   which is not transmitted. Again, this is a P wave  which is transmitted with a wide QRS. This P wave   is transmitted with a wide QRS again. These are  two P waves which are not being transmitted. Okay.

So, there is a sudden appearance of a, or sudden  failure of the impulses to get transmitted to the   ventricles. And then, these are all drop beats.  Okay. If you look at the; this is drop beat;   if you look at the PR interval, the PR interval  will be constant even before and after. There  

is no change in the PR interval. So, with these  features, you can say that the patient is having a   Mobitz type 2 block. Okay. Now, comes  the advanced second degree block. When we are not able to categorise these blocks  into Mobitz type 1 and type 2, then we classified  

it as a advanced second degree heart block.  In this, we will just see a 2 is to 1 block.   For every 2 P waves, there will be 1 QRS. See, in  this rhythm below, this P wave is not transmitted;   this P wave along with a QRS. So, 1, 2 P waves;  for 2 P waves, there is 1 QRS. Again 1 and 2 P  

waves; for 1 P waves, for 2 P waves, there is 1  QRS. Okay. So, what are the things occurring here? There is every alternating P waves  are not conducted or blocked.   It is difficult to diagnose the level of lesion  in this case. Narrow QRS complex may be nodal  

or infranodal, but wide QRS is almost always  infranodal. This can also progress to third degree   heart block. Usually, symptoms are uncommon,  but the same symptoms, some lightheadedness,   dizziness, lethargy can be there. See, if the  lesion is at the level of AV node and the patient  

is asymptomatic, then usually the prognosis  is good, you do not have to treat the patient. But if the patient is symptomatic,  then irrespective of the node or AV   or infranodal, you have to treat the  patient with the permanent pacing.  

Okay. This is what is advanced heart block.  Now, we will move on to the; so, here 1 ECG.   Here also you can see, in the last rhythm strip,  one P wave is not transmitted, another is;   so, all the alternating P waves is not being  transmitted. Okay. This P wave, not transmitted;  

this is with QRS. This again is not transmitted.  So, every alternating P wave is not transmitting. This, the patient is having a fixed  ratio, 2 is to 1 block. Okay. Now,   this is the third degree heart block which  is also known as complete heart block. So,  

there is a complete absence of atrioventricular  conduction. None of the impulse from the atria   is transmitted to the ventricles. There is  a complete AV dissociation, with the atria   and ventricles is having their independent  rates. No relation between the P and QRS;  

and usually, the patient is having a severe  bradycardia. There is a escape pacemaker. This escape pacemaker can be in the AV node  or it can be infranodal in the His bundle,   bundle branches or distal to it. Even the  myocardium itself which has a automaticity of  

10 to 15 beats per minute. Okay. Usually, the  patient is symptomatic; dizzy spell, uh there   can be pre-syncope, syncope, frank syncope,  even sudden cardiac arrest can also occur.   Now, this, the level of block can be at the level  of AV node or at the level of His Purkinje system.  

When it is at the level of AV node, the it  shows a junction rhythm with narrow QRS. When it is level of Purkinje, it shows ventricular  escape rhythm with wide QRS. So, what are the   points we should remember for saying the patient  is having a complete heart block? P wave rate is  

normal. Complete dissociation between the P waves  and QRS complexes. P can fall on ST-T segment   and usually the atrial rate will be faster  than the ventricular rate. You can just see.   This is the P wave. This shows the atrial rate.  

Here the P wave is here, then here, here,  here, here. So, P is going on its own. There is no connection between the P and the  QRS. This QRS is going on its own. If you see   the rate difference between the uh RR and  PP, see, the atrial rate is higher than the  

ventricular rate. Okay. So, these are the  features of a complete heart block. There   is a AV dissociation. And then, P wave  rate is faster than the ventricular rate.   So,you are getting an old lady, female,  brought to the casualty. You observe  

her skin is cyanotic. It is blue; she is  sweaty. She is having GCS of only 8 bar 15. She is only responding to painful stimuli. And  then, the paramedic says that her BP is only 44   systolic with a heart rate of round about 34 beats  per minute. And in the ECG, the following rhythm  

is seen. And look at the monitor, this is the  rhythm. So, here, with this you can say, this is   P wave. So, somewhere here, this is P wave, this  is P wave, this is P wave, this is P wave. So,   P wave, it is going on its own and QRS is going  on its own. So, there is a AV dissociation.

You can see the atrial rate is more than  the ventricular rate. Here in this ECG,   it is also visible that the patient is having  an ST elevation. Okay. So, patient is having an   acute MI. So, acute MI is also one of the cause  for the complete heart block. So, in with these  

findings in this ECG, we can say patient is  having a acute MI with a complete heart block.   Now, we will move on to the  bundle branch block. Okay. So, first is the right bundle branch block. There  are 3 criterias to say the patient is having a  

right bundle branch block. First is the  QRS duration will be greater than equal to   120 milliseconds. The QRS duration should  be greater than equal to 120 milliseconds.   There is rSR dash pattern or notched R wave  in V1. See, rSR dash, small r S and then dash,  

okay, big R. So, rSR dash pattern in lead  V1 along with a terminal S wave in lead   1, aVL and V6. If you look at the lead 1, aVL  and V6, there will be wide terminal S wave,   because the 3 criterias by which you can say the  patient is having a right bundle branch block.

Now, the the there are 2 types, like complete  and incomplete. There is not much difference   between complete and incomplete. If you want to  say the patient is having an incomplete right   bundle branch block, this is only the QRS duration  differs. The QRS duration in those patients is in  

between 110 to 120 milliseconds. It will be more  than 100 but less than 120 milliseconds to say the   RBBB of incomplete type. But you remember these  3 criterias to say if the patient is having   right bundle branch block or not. Okay.

So, these are some of the causes. And then, if  you say about the clinical significance, RBBB is   commonly seen and including in adults and usually  benign. But when this right bundle branch block   occurs in setting of MI, acute myocardial  infarction, then the patient is usually has  

a poor prognosis. And it indicate that the patient  is having a proximal LAD occlusion. Then comes the   left bundle branch blocks. So, these are the  5 criterias for a left bundle branch block. If you want to see the patient is having a left  bundle branch block, that is LBBB, QRS duration  

should be greater than equal to 120 milliseconds.  There should be a broad R wave with a   initial slurring. Can see, there is a broad R wave  with initial slurring in lead 1, aVL, V5 and V6;   broad R wave with slurring in lead  1, aVL, V5 and V6. See, there will be  

prominent QS or small r and S pattern in lead  V1, prominent QS or rS pattern in lead V1.   Then, there is absence of absence  of small q wave in lead V6. Normally, there is a small q wave present in  the lead 6, which will be absent if the patient  

is having a LBBB. And then, the prolonged R wave  peak time is greater than 60 milliseconds in lead   V5 and V6. This is, R wave peak time is greater  than 60 milli. Usually, normally it is less than   35 seconds, but in LBBB it becomes more than 60  seconds in lead V5 and V6. So, you have to take  

the point at; this is R wave and this is, from  here to here you you have to, see this difference. This difference is more or this duration  is more than 60 milliseconds to say R   wave peak time is prolonged. So, these  criteria says that the patient is having a  

left bundle branch block. First is QRS duration,  more than 120 milliseconds. Then there is a   slurring of the R wave in the lead 1, aVL, V5  and V6. So, there is a broad R wave in V1, aVL,   in lead 1, aVL, V5 and V6. There  is a QS or rS pattern in lead V1.  

There is a QS or rS pattern in lead V1. Then  there is an absence of small q wave in lead V6. And the R wave peak time is  more than 60 milliseconds. Okay.   These are few causes of the  left bundle branch block.  

Then comes the clinical sigma. It is, this LBBB is  almost always pathological and generally a marker   of the underlying left ventricular heart disease.  New onset LBBB says the patient might have an   underlying ischemia and infarction  and needs a thrombolytic therapy.  

Then LBBB in the setting of  MI, acute MI deteriorates the   prognosis also if we want to uh sa uh say  about the left ventricular hypertrophy. If the LBBB is present, then the normal  LVH criteria is not applicable. And the  

presence of this LBBB says there is a  ventricular dyssynchrony in presence of   heart failure. So, the the ventricle,  both the right and left vertical should   contract in synchronised way. The patient  is having LBBB in presence of heart failure,  

then it says the patient is having a  dyssynchrony, ventricular dyssynchrony.   This is about LBBB. Then, a small a  few things about the Holter monitoring. Holter monitoring is small wearable device that  keeps track of heart rhythm. A holter monitor uses  

electrodes and recording device to track the heart  rhythm for 24 to 48 hours. It is also known as   ambulatory electrocardiography. Here we can  record the ECG or get the ECGs or rhythms for a   extended period of time. So, how it works?  This, there is a small monitor, just a size  

of the playing card, and there are several leads  and wires which are attached to these monitor. There are 3 surface electrodes and these surface  electrodes are connected on the chest wall with   the help of a gel. To this, the metal electrodes  are connected and this conducts the heart's elect  

electrical activity through the wire to the  halter monitor, where it is recorded. So, when   you should go for a holter monitoring? When you  want to diagnose and assess the cardiac arrhythmia   and conduction abnormality; whether the patient  is symptomatic or asymptomatic, you can use this.

If the patient is having recurrent syncope,  near syncope or episodic dizziness,   or if the patient is having a recurrent  palpitations unexplained, then you can   go with the holter monitorings. So, how it helps?  The advantages is: It gives you a 24 to 48 hours  

of full disclosure is available in that, and  that you get a heart rate graphs and atrial   fibrillation, burden graphs, all those things.  Disadvantages is: 24 to 48 hours is a short   duration, and sometimes the patient may not have  the symptoms at that specified period of time.

Then it is difficult to diagnose. If the  symptoms are very much intermittent, then   the yield of the test is low. But say if you get  these type of differences in Holter monitoring,   it is alarming. If you get a second Mobitz type  2 or third degree AV block, that is alarming.  

Sinus pauses, if it is more than 3 milliseconds,  3 seconds, then it is alarming. Marked bradycardia   during waking hours or tachyarrhythmias, all  these things if present, this says the patient   is having an alarming arrhythmias. Okay. With  this, I am finishing my topic. Thank you so much.