Heart block

From Ganfyd

Heart block has three grades of disturbance:

First degree heart block

1st degree heart block

Occurs because of:

1. Almost always conduction delay in atrio-ventricular node
2. Rarely intra-His conduction delay

Criteria are:

• PR interval (onset P wave to onset QRS complex) > 0.21 sec with normal atrio-ventricular node conduction.
• Criteria are::
  • P waves present and have normal morphology for sinus depolarisation
  • QRS complexes present and have normal morphology for patient/activity
  • There is one P wave for each QRS complex
• PR interval > 0.21 seconds

Second degree heart block

This can occur because of:

1. Partial failure of atrio-ventricular node conduction
2. Partial failure of bundle of His conduction
3. Simultaneous partial failure of left and right bundle branches (very rare)
4. Partial failure of one bundle branch in the presence of complete failure of conduction in the other bundle branch

Type I second degree heart block - Wenckebach - Mobitz type I

One way of associating the ECG pattern with the name is that the QRS complexes appear to be 'walking back' towards the P-waves, which sounds a bit like Wenckebach.

Progressive increase in the duration of the PR-interval, culminating in failure of conduction of the atrial impulse.

Classic Wenckebach, Type I 2nd degree heart block

Criteria are:

• P waves present and have normal morphology for sinus depolarisation
• QRS complexes present and have normal morphology for patient/activity
• There must be progressive prolongation of the P-R interval with each succeeding heart beat until one P wave occurs without a QRS complex in a repeated n P waves to n-1 QRS complexes.
• The P-R intervals progressively increase (and R-R intervals decrease) until longest P-R interval preceeds the dropped QRS complex and the shortest P-R interval of the next pattern is immediately after the dropped beat.

Type II second degree heart block - Mobitz type II

Unpredictable non-conduction of atrial impulses. Between the non-conducted atrial impulses, the rate is otherwise regular.

• Criteria are:
  • P waves present and have normal morphology for sinus depolarisation
  • QRS complexes present and have normal morphology for patient/activity
  • The P-R interval may be normal or long but is constant for all conducted beats. One P wave occurs without a QRS complex in a repeated n P waves to n-1 QRS complexes with n > 3.
  • Although failure of A-V conduction may occur more than once in a period of observation, it is never seen for at least two or more consecutive P waves.
  • The QRS morphology does not change for non-conducted beats

2:1 second degree heart block

Inferred Type I 2:1 2nd degree heart block as Wenckebach 3:2 at one stage

Criteria as for Types I and II block above but only every second beat has A-V conduction

• Can only be differentiated by surface ECG if intermittent with higher block thus:

Type I

• Inferred if P-R intervals incremental when not 1:2 block

Type II

• Inferred if P-R intervals fixed when not 1:2 block

High grade atrio-ventricular block

• Criteria are::
  • P waves present and have normal morphology for sinus depolarisation
  • QRS complexes present and have normal morphology for patient/activity
  • The P-R interval may be normal or long but is constant for all conducted beats.
  • Some P waves are followed by QRS complexes and some are not
  • The atrio-ventricular conduction ratio is 3:1 or higher.

Complete heart block

Complete heart block with low ventricular escape

This can occur because of:

1. Complete failure of atrio-ventricular node conduction
2. Complete failure of bundle of His conduction
3. Failure of left and right bundle branches to conduct simultaneously

Complete heart block with high level escape

• Criteria are::
  • No consistent or meaningful relationship between atrial and ventricular activity
  • QRS complexes present and may be of normal morphology but more often are abnormal in shape, duration and axis
  • The form of the QRS complex is usually constant
  • The QRS rate is usually constant and is in the range 15-70 beats/minute
  • Any form of or no atrial activity is possible