Comprehensive Guide to Sinus Rhythms and Junctional Arrhythmias

In this lecture video, I am going to discuss  about Sinus Rhythms and Bradyarrythmias.   I am going to discuss about sinus rhythm,  sinus bradycardia, sinus tachycardia and   bradyarrythmias. At the end of the lecture  video, you will be able to identify sinus  

rhythms and how to differentiate sinus rhythm  from junctional rhythms and how to identify and   differentiate between sinus pause, sinus arrest  and sinoatrial exit block. What is an arrhythmia? An abnormality in cardiac rhythm is called an  arrhythmia. SA node is the predominant pacemaker  

of the heart and it depolarises spontaneously.  It is a modified cardiac muscle situated at the   superior part of lateral wall of right atrium. The  fibres of a SA node are continuous with fibres of   atrial muscle. So, once the impulses are  generated, it spreads rapidly through the  

atrial muscle. The rate of sinus node discharge  is controlled by autonomic nervous system. There are conditions which lead to sinus  node dysfunction, such as fibrosis,   degenerative condition or ischemia of  the sinus node. This leads to variety of  

arrhythmias commonly found in elderly. And they  may be asymptomatic or present with symptoms   such as dizziness, palpitation or syncopal attack.  Management depends on the patient's symptoms and   maybe by drugs to improve the heart rate or with  permanent pacemakers. The point I want to stress  

upon here is, it is vital to differentiate  these rhythm abnormalities based on the ECG   and to intervene early to prevent  any life threatening conditions. Next point I I am going to discuss is, what  happens if the SA node is dysfunctional  

or suppressed? And, what are subsidiary  pacemakers and what is their role? If SA   node is dysfunctional or suppressed,  in order to continue heart beating,   other subsidiary pacemakers in the AV  node, specialised conducting system  

and muscle may initiate electrical activation; but  typically, subsidiary pacemakers discharge at a   slower rate. These are the various  pacemaker sites and rate of depolarisation. SA node depolarises at a rate of 60 to 100 beats  per minute; AV node depolarises at a rate of 40  

to 60 beats per minute; and ventricles  at a rate of 20 to 40 beats per minute.   Before discussing about sinus bradycardia  and sinus tachycardia, let me explain   about sinus rhythm. What is sinus rhythm? If the  depolarisation begins in the SA node of the heart,  

it is said to be in sinus rhythm. In the  ECG, P wave characterised by positive P   waves in lead 2, 3, aVF; negative P  wave in AVR; and biphasic P wave in V1. Next, about bradycardia and how to differentiate  sinus bradycardia with junctional rhythms. So,  

bradycardia is defined as heart rate less than  60 beats per minute in adults during wakeful   state. Bradycardia results from either failure  of impulse initiation or impulse conduction.   Depressed automaticity leads to failure of  impulse initiation. Asymptomatic bradycardia  

can be seen in people with high vagal tone, such  as in athletes. So, it is vital to differentiate   bradycardia as whether physiological or  pathological to intervene accordingly. So, there are many causes for bradycardia. There  are some extrinsic causes and intrinsic causes.  

Extrinsic causes such as hypothyroidism, drugs,  etcetera; and intrinsic causes are ischemia and   infarction of the sinus node, fibrosis of the  sinus node or sick sinus syndrome. Bradycardia may   be due to sinus bradycardia, junctional rhythms  or atrioventricular blocks. Now, how to recognise  

sinus bradycardia and how to differentiate  sinus bradycardia from junctional rhythm? So, in this ECG, let us first calculate the  rate. To do that, first trace the R waves. So,   here is an R wave; here is an R wave; in  between there are 1, 2, 3, 4, 5, 6; 6 big boxes.  

So, 300 divided by 6 is equal to 50. The rate  here is less than 60. So, it is bradycardia.   Now, how to say sinus? There is a P wave which  precedes the QRS complex, and here is a P wave   which precedes the QRS complex, and there is a  fixed PR interval. So, to say sinus, there is  

an upright P wave preceding the QRS complex and  followed by a uh P wave preceding a QRS complex. So, this is an example of sinus bradycardia. If  there is no P wave preceding the QRS complex,   then it is called junctional rhythm.  Now, moving on to sinus tachycardia.  

When the sinus rate is accelerated more  than 100 beats per minute, then it is called   sinus tachycardia. Sinus tachycardia may be  due to intrinsic sinus node abnormalities   such as enhanced automaticity, or due to  abnormal autonomic regulation of the heart  

such as enhanced sympathetic tone  or depressed parasympathetic tone. It is a secondary phenomenon and  underlying causes to be investigation.   There can be acute or chronic causes; acute  causes such as exercise, pain, anxiety, fever,  

acute heart failure, pulmonary embolism,  etcetera; and chronic causes such as anaemia,   thyrotoxicosis, catecholamine excess, etcetera.  So, in this ECG, let us calculate the rate here.   So, this is an R wave; this is an R wave;  in between there are 1, 2, 3, 4, 5, 6, 7,  

8, 9; there are 9 small boxes. So, let us  take 10. 1,500 divided by 10 is equal to 150. The rate here is more than 100; so, it is called  tachycardia. Now, how to say sinus? There is a P   wave which precedes the QRS complex followed  by a T wave; then a P wave which precedes  

the QRS complex followed by a T wave. So,  this is an example of sinus tachycardia.   Then, moving on to junctional rhythm. Junctional  dysrhythmias originate in the AV junction;   that is, area around AV node and bundle of His.

Junctional rhythm is identified based  on the rate, which is usually less,   that is around 40 to 60 beats per minute.  As it originates from other than sinus node,   based on the QRS width, which is usually normal,  as it follows the usual conduction system;  

and importantly, based on the morphology of  P waves. This point I want to stress upon   again that sinus P waves in lead 2 is upright or  positive; and junctional P waves may be inverted,   which comes before or after the QRS  complex; or it may be a hidden P wave.

This picture explains about normal sinus  conduction and about junctional conduction. So,   based on the sequence of depolarisation, there are  several possible outcomes. So, these are some of   the complexes here. In that, we can see various P  wave morphologies. So, this is a P wave morphology  

which is negative, which comes before the QRS  complex and there is shortened PR interval.   And here, you cannot able to find P  wave, and it is a hidden P wave here.   And in this complex, you can see a P  wave which comes after the QRS complex.

So, this P wave which is negative  and before the QRS complex   indicates that atria is depolarised from bottom  up. And the hidden P wave indicates that atria   and ventricles are depolarised simultaneously.  And P wave which comes after the QRS complex  

indicates that AV node initiated depolarisation,  but ventricles depolarised just before the atria.   Now, I am going to discuss about junctional  rhythms, such as junctional escape rhythm,   accelerated junctional rhythm, as  well as junctional tachycardia.

Junctional escape rhythm arises from AV junction  at a rate of 40 to 60 beats per minute. It is   characterised by heart rate around 40 to 60  beats per minute; rhythm which is regular;   and P waves which are inverted may appear before,  during or after the QRS complex. And QRS complexes  

are relatively narrow as is it usually follows the  usual conduction system. If P wave is present, PR   intervals will be shorter than normal. Let us take  this example. First let us calculate the rate. Here is an R wave; here is an R wave; in between  there are 1, 2, 3, 4, 5; there are 5 big boxes.  

So, 300 divided by 5 is equal to 60. The rate here  is 60. And the rhythm is regular rhythm. And you   cannot able to see a P wave here. So, there is a  hidden P wave in this ECG. And the QRS is normal,   as it follows the usual conduction system.  This is an example of junctional escape rhythm.  

Next, about accelerated junctional rhythm,   which arises from the AV junction at  a rate of 60 to 100 beats per minute. It is due to increased automaticity of the AV  node or decreased automaticity of the SA node,  

which is characterised by rate around 60  to 100 beats per minute; a regular rhythm;   P waves may be inverted, appear before,  during or after the QRS complex;   QRS complexes are relatively narrow; and P waves  if present, and the PR interval will be shorter  

than normal. Let us take this example. This is  an R wave over here; this is an R wave over here;   there are on 1, 2, 3 big boxes. So,  300 divided by 3 is equal to 100. So, the rate is 100. The  rhythm is a regular rhythm.  

And P wave which comes before the  QRS complex, and it is inverted,   and there is a shorter PR interval. And the QRS  complex is normal, as it usually follows the   normal conduction system. This is an  example of accelerated junctional rhythm.  

Next, about junctional tachycardia.  It is a fast ectopic rhythm,   arises from bundle of His at a rate more than  100 beats per minute. It is characterised by   rate more than 100 beats  per minute; regular rhythm;  

P waves are inverted, may appear before, during or  after the QRS complex; and QRS complex is normal. In this example, you can see R wave here,  and an R wave here; and you can see 1, 2;   2 big boxes between 2 R waves. So, 300 divided  by 2 is equal to 150. The rate is more than 100,  

and the rhythm is a regular rhythm. And if you  see the P wave, it has come after the QRS complex,   and the QRS complex is normal. This is  an example of junctional tachycardia.   Next, I am going to discuss about sinus pause,  

sinus arrest and how to differentiate sinus  pause, sinus arrest from SA exit block. Sinus node consists of P cells or pacemaker  cells which forms the impulses, and T cells   which transmit the impulses to the atrial muscle.  Sinus pause is the failure of atrial activation  

due to inability of SA node to form impulses.  This result in pause without visible P waves.   The differentiating point between sinus pause   versus sinoatrial exit block is that, the pause  is not an exact multiple of preceding PP interval.  

Sinus pause is characterised by absence of P  waves for a duration of more than 1.5 seconds. If the delay is more than 3 seconds, then it is  called sinus arrest. Let us take this example.   In this ECG, there is a pause. And if you  take this PP interval, preceding PP intervals,  

the pause is not an exact multiple of preceding  PP interval. So, if we calculate the number of   boxes between the R waves, so, 1, 2, 3, 4, 5, 6,  7, 8, 9, 10; around 10 big boxes; so, 10 into 0.2   is equal to 2 seconds. So, the delay is around 2  seconds. So, this is an example of sinus pause.

Seen in normal individuals with a increased  vagal tone; hypersensitive carotid sinus; drugs   such as digitalis; and inferior wall myocardial  infarction. Next, about sinoatrial exit block.   As the name indicates, sinoatrial exit  block is due to intermittent failure in  

conduction of sinus impulses into atria. The  differentiating point between sinoatrial exit   block from sinus pause is that, the pause here  is an exact multiple of preceding PP interval. Sinoatrial exit block is graded into 3 degrees,  first degree SA block, second degree SA block and  

third degree SA block. First degree SA block:  In this, time for propagation of sinus impulse   into atria is significantly prolonged.  This cannot be ditected on a surface ECG.   Let us take this example. Top one is normal ECG  and below is first degree SA block. If you compare  

the arrow marks of both ECGs, instead of atrial  depolarisation over here, it has been delayed. So, there is a delay in atrial depolarisation.  This is an example of first degree SA block.   Second degree SA block is further classified as  Mobitz type 1 SA block and Mobitz type 2 SA block.  

Mobitz type 1 SA block is characterised by  progressive prolongation of SA conduction   with intermittent failure of impulses to  conduct to surrounding atrial tissue. No P   waves appear for 1 beat, and thus, sequence begins  again. There is a phenomenon called group beating.

That means, grouping of P QRS complex  with progressively shortened PP interval,   followed by a pause, and the pause is   less than twice the shortest cycle. If you take  this as an exa example, this is a PP interval.  

And compared to this PP interval, that is  around 23 small boxes, next PP interval   is around 21 small boxes and followed by a pause,  and then the sequence begins again. So, this pause   is less than twice the shortest PP interval.  So, this is an example of Mobitz type 1 block.

Mobitz type 2 block is characterised by fixed PP  interval; that is, no change in SA node conduction   before and after the pause. The pause  is multiple of preceding PP interval.   Let us take this example. So, compared  to this PP interval, this PP interval  

is equal, followed by a pause over here,  and the pause is an exact multiple of   the preceding PP interval. So, this is  an example of Mobitz type 2 SA block.   Third degree or complete SA  block, which is manifest as  

no P waves with long pauses leading to  ectopic, atrial or ventricular rhythms. So, if you see, there is a P wave over  here, and next P wave comes over here. So,   there is a long pause over here. This is an  example of third degree or complete SA block.  

Summary: Electrical activation of the  heart normally originates in the SA node.   If SA node is dysfunctional or suppressed,  other subsidiary pacemakers may initiate   electrical activation. Subsidiary  pacemakers discharge at a slower  

rate. If the depolarisation begins in the SA node  of the heart, it is said to be in sinus rhythm. Bradycardia is defined as heart  rate less than 60 beats per   minute and results from either failure of  impulse initiation or impulse conduction.  

Tachycardia occurs when the heart rate is more  than 100 beats per minute. Junctional rhythm   originates in the AV junction and it is identified  based on the rate, QRS width and morphology of P   waves. Junctional P waves can be inverted P wave,  and it comes before or after the QRS complex,  

or it may be a hidden P wave. Sinus pause or  arrest is the failure of SA node to discharge. SA exit block is due to intermittent  failure in conduction of sinus impulse   into atria. SA block is graded into 3 degrees;  first degree SA block, second degree SA block  

which is further classified into  type 1 Mobitz block and type 2   Mobitz block, and third degree SA block.  These are my references. Thank you.