Heart blocks and types of heart blocks

HEART BLOCK

The atrioventricular junction is the only passage available for the atrial current to travel to the ventricles is the rest of the AV ring and AV valves (mitral tricuspid) are made af non-conducting fibrous tissue. Thus, the AV junction very important in atrio-ventricular conduction. Therefore, heart block is called atrioventricular block (AV block) as the conduction delay or block the atrioventricular junction (AV node or His bundle). A delay in conduction through this junction may be due to degeneration, fibrosis, destruction.inflamation etc. Thus, heart blocks may be caused by drugs (Digitalis, beta blockers, calcium channel blockers, etc.), inflammation (Diphtheria, Rheumatic disease, etc.), ischemia, infarction and very rarely by other conditions.

From the ECG heart block or atrioventricular block is diagnosed in three degrees of severity.

First degree heart block 

Here, every atrial activity (P wave) is ducted through the AV junction to the ventricles but with a delay of more than 0.20 seconds (PR interval greater than 5 mm). Every P wave has a QRS comp following it but the PR interval is prolonged.

First degree heart block-PR interval>0.20 seconds

Second degree heart block

Here, some of the atrial currents are conducted through the AV junction to the ventricles, while some are not. There fore,some P Waves will have QRS complexes following them and others will not.This degree of heart block, which is more serious than the first degree.

Second degree heart block is two types-

1. The Wenckebach type or the Mobitz type I

Second degree heart block Wenckebach type (Mobitz type 1) Progressive prolongation of PR intervals (measurements given in seconds). ending in a dropped beat (shown by an arrow)

In which the PR interval is progressively prolonged until one P wave does not have a QRS complex following it (the so called dropped beat). This is easy to diagnose by carefully observing successive PR intervals starting with a normal value and progressively increasing to abnormal values and ending in a dropped beat with a pause (P without QRS following it).

2. Mobitz type II 

Second degree heart block Mobitz type  II - Intermittently. P waves do not have QRS complexes following them

In Mobitz type 2 is no progressive increase in the PR intervals. Some P waves have QRS complexes following them while others do not for no apparent reason.This second type is the more serious variety of second degree heart block as it may lead to a higher degree of block or serious complications (eg, very slow heart rate).

second degree heart block (Mobitz type II)

Depending on the ratio of the number of P Waves to the number of QRS complexes following them, second degree blocks.

Third degree heart block or complete heart block 

This is the most severe form of AV block, where there is no relation between the P waves and the QRS complexes. No P is conducted and there is no fixed PR interval as the P and QRS waves are entirely independent of each other. In this condition,the QRS complexes are produced by the ventricles themselves, at a slow rate with long R-R intervals (the ventricles have their own independent rhythmicity at a slow rate of 30-40 beats/min). The atria, being controlled by the SA node, have a higher rate of about 60-100 beats/min. Obviously, this is the most serious and severe type of heart block and it requires careful observation and treatment, usually requiring the insertion of a permanent artificial pacemaker.

Complete heart block-dissociated P waves and QRS complexes with an atrial rate of 104/ min ventricular rate of 36/min and varying PR intervals

trifascicular blocks may also lead to heart blocks of varying degrees of severity by blocking conduction at the level of the fascicles instead of at the AV node or His bundle. In fact, most of the adult complete heart blocks are due to tri-fascicular block rather than AV nodal blocks.

The ECG is invaluable in the diagnosis of all conduction disturbances including heart blocks and trifascicular blocks Heart blocks with narrow QRS complex located at the AV node or the His bundle whereas, heart blocks with wide QRS complexes are usually fascicular blocks below the His bundle).

ACCELERATED CONDUCTION AROUND THE AV JUNCTION

There are a few conditions where, due to the presence of bypass fibres around the AV junction, there is accelerated conduction from the atria to the ventricles (instead of the normal slow conduction through the AV node). These conditions are diagnosed by short PR intervals (of less than 0.12 seconds or 3 mm) and some abnormal notches on the QRS complexes called delta waves. These occr in at least two well-known conditions - the Wolff-Parkinson-White (WPW) syndrome and the Lown-Ganong-Levine (LGL) syndrome which are associated with clinical and electrocardiographic features of tachyarrhythmias.

Wolff-Parkinson-White-Syndrome-,short PR intervals with delta Waves

Wolff-Parkinson White syndrome is an example of accelerated conduction around the AV junction where impulses are conducted to the ventricles through an accessory path called the bundle of Kent. The electrocardiographic features of this condition include a shortened PR interval (of less than 0.12 seconds) and a prolonged QRS complex of more than 0.11 seconds) due to the presence of delta wave on its upstroke or down-stroke. It is often associated with supraventricular tachyarrhythmias.

The Lown-Ganong-Levine syndrome differs from the Wolff-Parkinson-White Syndrome in that there are no delta waves and the ORS duration is normal unlike in the former. The other features are similar.

Lown-Ganong-Levine syndrome - short PR interval without delta waves

CLINICAL THINKING SUMMARY

1. AV blocks or heart blocks occur in three degrees of increasing severity - first, second or third. These blocks are diagnosed by studying the P waves, the QRS complexes and the relation between the two.
2. Third degree or complete heart block is the most serious and requires urgent attention and treatment. This is diagnosed from the ECC by Variable PR intervals with a slow ventricular rate of 30-40/minute P waves occurring at a higher rate independent of the ventricular rate.
3. Second degree heart block, where the impulses are blocked intermittently, are of two types - the benign Wenckebach type or Mobitz Type first (with progressive increase in the PR intervals) or the more serious Mobitz Type II (with irregular blockade of P wave conduction and no progressive increase in the PR interval.
4. First degree heart block is diagnosed by prolonged PR intervals >0 20 seconds) only there are no dropped beats).
5. There are two syndromes of accelerated conduction around the AV junction called the Wolff-Parkinson-White and the Lown-Ganong-Levine syndromes associated with short PR intervals and tachyarrhythmias. Delta waves are present in the WPW syndrome.