conduction blocks- AV blocks

Question 1

First degree AV block

Answer 1

rate: identify underlying rhythm
rhythm: Regular
P wave: Present
P:QRS ratio: 1:1
Pr-interval: Above 200ms (>0.20s)
QRS width: <120ms (0.12s)

Causes:
Increased vagal tone
Athletic training
Inferior MI
Mitral valve surgery
Myocarditis (e.g. Lyme disease)
Electrolyte disturbances (e.g. Hyperkalaemia)
AV nodal blocking drugs (beta-blockers, calcium channel blockers, digoxin, amiodarone)
May be a normal variant

No treatement is usually needed.

Question 2

Second degree AB block

Mobitz type 1 second degree AB block which is also called Wenckebach block)

Answer 2

rate: identify underlying rhythm
rhythm: Regularly irregular (which means theres a usual pattern)
P wave: Present
P:QRS ratio: Variable
Pr-interval: Varialbe (going…going.. gone)
QRS width: <120ms (0.12s)

• Not every impulse is able to pass through the AB node into the ventricles (which is why P:QRS ratio is greater than 1)
• May either disappear after a while or actually progress to a worse type of block.
• Don’t confuse sinus block to AV block, where the latter has a P wave with no ensuing QRS complex.

Mobitz I is usually due to reversible conduction block at the level of the AV node.
Malfunctioning AV node cells tend to progressively fatigue until they fail to conduct an impulse. This is different to cells of the His-Purkinje system which tend to fail suddenly and unexpectedly (i.e. producing a Mobitz II block).

Question 3

Mobitz type 2 av block

Answer 3

rate: identify underlying rhythm
rhythm: Regularly irregular (which means theres a usual pattern)
P wave: Present
P:QRS ratio: X:X - 1
Pr-interval: Normal ***DROPPED QRS
QRS width: <120ms (0.12s)

*** FAR MORE DANGEROUS THAN MOBITZ TYPE 1. THIS CAN EASILY DEVELOP TO THIRD DEGREE HEART BLOCKS DUE TO ITS DISTAL LOCATION (Occasionaly).

Definition:
Intermittent non-conducted P waves without progressive prolongation of the PR interval (compare this to Mobitz I).
The PR interval in the conducted beats remains constant.
The P waves ‘march through’ at a constant rate.
The RR interval surrounding the dropped beat(s) is an exact multiple of the preceding RR interval (e.g. double the preceding RR interval for a single dropped beat, treble for two dropped beats, etc).

Mechanism:
Mobitz II is usually due to failure of conduction at the level of the His-Purkinje system (i.e. below the AV node).
While Mobitz I is usually due to a functional suppression of AV conduction (e.g. due to drugs, reversible ischaemia), Mobitz II is more likely to be due to structural damage to the conducting system (e.g. infarction, fibrosis, necrosis).
Patients typically have a pre-existing LBBB or bifascicular block, and the 2nd degree AV block is produced by intermittent failure of the remaining fascicle (“bilateral bundle-branch block”).
In around 75% of cases, the conduction block is located distal to the Bundle of His, producing broad QRS complexes.
In the remaining 25% of cases, the conduction block is located within the His Bundle itself, producing narrow QRS complexes.
Unlike Mobitz I, which is produced by progressive fatigue of the AV nodal cells, Mobitz II is an “all or nothing” phenomenon whereby the His-Purkinje cells suddenly and unexpectedly fail to conduct a supraventricular impulse.
There may be no pattern to the conduction blockade, or alternatively there may be a fixed relationship between the P waves and QRS complexes, e.g. 2:1 block, 3:1 block.

Question 4

2:1 conduction block…

Which mobitz?

Answer 4

It is not always possible to determine the type of conduction disturbance producing a fixed ratio block, although clues may be present.
Mobitz I conduction is more likely to produce narrow QRS complexes, as the block is located at the level of the AV node. This type of fixed ratio block tends to improve with atropine and has an overall more benign prognosis.
Mobitz II conduction typically produces broad QRS complexes, as it usually occurs in the context of pre-existing LBBB or bifascicular block. This type of fixed ratio block tends to worsen with atropine and is more likely to progress to 3rd degree heart block or asystole.
However, this distinction is not infallible. In approximately 25% of cases of Mobitz II, the block is located in the Bundle of His, producing a narrow QRS complex. Furthermore, Mobitz I may occur in the presence of a pre-existing bundle branch block or interventricular conduction delay, producing a broad QRS complex.
The only way to be certain is to observe the patient for a period of time (e.g. watch the cardiac monitor, print a long rhythm strip, take serial ECGs) and observe what happens to the PR intervals. Often, periods of 2:1 or 3:1 block will be interspersed with more characteristic Wenckebach sequences or runs of Mobitz II.

Question 5

Third degree av block

Answer 5

rate: Atrial and ventricular rates differ.
rhythm: Regularl.
P wave: Present
P:QRS ratio: variable
Pr-interval: no pattern
QRS width: normal or wide

Definition:
In complete heart block, there is complete absence of AV conduction – none of the supraventricular impulses are conducted to the ventricles.
Perfusing rhythm is maintained by a junctional or ventricular escape rhythm. Alternatively, the patient may suffer ventricular standstill leading to syncope (if self-terminating) or sudden cardiac death (if prolonged).
Typically the patient will have severe bradycardia with independent atrial and ventricular rates, i.e. AV dissociation