Cardiovascular System
Third-degree (complete) AV block and escape rhythms
In third-degree (complete) atrioventricular (AV) block, no impulses pass from the atria to the ventricles, so a slow backup pacemaker, called an escape rhythm, drives the ventricles.
Remember
Complete heart block means the atria and ventricles beat independently, a slow escape rhythm drives the ventricles, and most patients need pacing.
In third-degree AV block, no impulses pass from the atria to the ventricles. The sinoatrial node (the node that keeps the atria beating at their own rate) still paces the atria, so the P waves (the atrial activity on the ECG) carry on regularly at their own rate, which is faster than the rate of the ventricles.
The atria no longer drive the ventricles, so a backup pacemaker below the block takes over. This is the escape rhythm, a slow backup rhythm that starts when the usual conduction fails. The QRS complexes (which show the ventricles being activated) therefore have no consistent relationship with the P waves, and the P wave rate is faster than the QRS rate. This is called AV dissociation.
The width of the QRS tells you where the escape pacemaker sits. A narrow QRS means the ventricles are being activated through the normal conduction pathway, so the escape pacemaker (also called the escape focus) is in the atrioventricular junction (the AV node and the bundle of His, parts of that pathway) and the block lies at the level of the AV node above it. That gives a rate of about 35 to 50 beats per minute. A wide QRS means the escape focus is in the ventricular muscle or Purkinje fibres, which points to a block lower down, in the His-Purkinje system (the fast conduction fibres of the ventricles). Here the rate is slower and less reliable, at about 20 to 40 beats per minute.
The slow rate lowers cardiac output, so patients may have dizziness, near-syncope (almost fainting) or collapse, and there is a risk of sudden death. That is why urgent admission, monitoring and usually a permanent pacemaker are needed, with temporary pacing that may be used first.
Memory aid
the atria and ventricles are like two people who have stopped talking, each keeping their own pace
Picture two people who no longer talk to each other, and you will remember that the atria and ventricles each keep their own rate.
Key facts
- Block
- no impulses pass from the atria to the ventricles
- ECG
- regular P waves and regular QRS complexes with no consistent relationship, and a P wave rate faster than the QRS rate
- Narrow QRS
- an escape rhythm from the atrioventricular junction, a block at the level of the AV node above it and a rate of about 35 to 50 beats per minute
- Wide QRS
- an escape rhythm from lower down, a slower and less reliable rate of about 20 to 40 beats per minute
- Symptoms
- patients may have dizziness, near-syncope or collapse, because the slow rate lowers cardiac output
- Management
- urgent admission, monitoring and usually a permanent pacemaker, with temporary pacing that may be used first
Exam traps
- A junctional escape rhythm does not fit a block within the His-Purkinje system, because a junctional escape comes from above the level of that block, and a block there leaves only ventricular tissue below it as the escape pacemaker, which gives wide QRS complexes.
- Loss of coordinated atrial and ventricular contraction alone does not make complete block an emergency, because it reduces filling a little and the slow ventricular rate is what turns complete block into an emergency.
- A ventricular rate faster than the atrial rate does not fit complete block, because a slow backup rhythm paces the ventricles and the P wave rate is the faster one.
- Alternate P waves failing to conduct, giving two P waves for each QRS complex, is 2:1 block, not complete block, because every second P wave still reaches the ventricles and the P waves and QRS complexes stay related.
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