AV Block Flashcards
What is AV block?
Cardiac electrical disorder defined as impaired (delayed or absent) conduction from the atria to the ventricles.
The severity of the conduction abnormality is described in degrees: first-degree; second-degree, type I (Wenckebach or Mobitz I) or type II (Mobitz II); and third-degree (complete) AV block.
What is first degree heart block?
First-degree heart block occurs where there is delayed atrioventricular conduction through the AV node. Despite this, every atrial impulse leads to a ventricular contraction, meaning every p waves results in a QRS complex. On an ECG this presents as a PR interval greater than 0.20 seconds (5 small or 1 big square).
How does first degree heart block appear on an ECG?
The PR interval is prolonged and unchanging; no missed beats.
PR interval >0.2 seconds (or >200 milliseconds).
What is second degree heart block?
Second-degree heart block is where some of the atrial impulses do not make it through the AV node to the ventricles. This means that there are instances where p waves do not lead to QRS complexes.
There are 2 types of second degree heart block:
- Mobitz type 1 (Wenckebach’s phenomenon)
- Mobitz type 2
How does Mobitz type 1 (second degree heart block) appear on an ECG?
The PR interval becomes longer and longer until a QRS is missed, then the pattern resets.
How does Mobitz type 2 (second degree heart block) appear on ECG?
Fixed, unchanging PR intervals with regular loss of QRS.
Example:
P-QRS-P–P-QRS-P–
This would be an example of Mobitz type 2 with 2:1 conduction block. This is a dangerous rhythm as it can progress to complete heart block.
Then, occasional loss of AV conduction for 1 beat (during sinus rhythm, excluding premature atrial beats).
Finally, fixed, unchanging PR intervals.
What is third degree heart block?
This is referred to as complete heart block. This is no observable relationship between P waves and QRS complexes. There is a significant risk of asystole with third-degree heart block.
How does third degree heart block appear on an ECG?
No impulses are passed from atria to ventricles so P waves and QRS appear independently of each other.
Why is third degree heart block a medical emergency?
As tissue below the AVN paces slowly, the patient becomes very bradycardic, and may develop haemodynamic compromise. Therefore, urgent treatment is required.
What are the causes of first degree and second degree heart block?
- Normal variant
- Athlete
- Sick sinus syndrome
- IHD (esp. inferior MI)
- Acute myocarditis
- Drugs (digoxin and beta-blockers)
What are the causes of third degree heart block?
- IHD (esp. inferior MI)
- Idiopathic (fibrosis)
- Congenital
- Aortic valve calcification
- Cardiac surgery or trauma
- Digoxin toxicity
- Infiltration (abscess, granulomas or tumours)
What are the signs of heart block?
- HR <40 bpm
What are the symptoms of heart block?
- Syncope
- Pre-syncope
- Episodic lightheadedness
- Progressive exertional fatigue and/or dyspnoea
What investigations should be ordered for heart block?
- 12 lead ECG
- Serum troponin
- Serum potassium
- Serum calcium
- Serum pH
Why investigate using ECG?
Diagnosis of the various degrees of AV block is frequently made incidental to the work-up for any number of presenting conditions.
On ECG:
- First-degree AV block: fixed PR interval >0.210 seconds (or >210 milliseconds)
- Second-degree AV block:
- Type I: progressive PR interval prolongation, eventual loss of AV conduction for 1 beat, return to normal PR interval, then progressive PR prolongation with eventual loss of AV conduction; second-degree AV block,
- Type II: occasional loss of AV conduction for 1 beat preceded and followed by fixed, unchanging PR intervals
- Third-degree AV block: no consistent PR relationship
Why investigate using serum troponin?
Because acute ischaemia should be aggressively managed and presents a potentially reversible cause of AV block, an indicative history and physical examination should prompt the obtaining of serum cardiac enzymes.
May be elevated.
Why investigate serum potassium?
Severely abnormal values may represent reversible causes of AV block.
May be very low or very high.
Why investigate serum calcium?
Severely abnormal values may represent reversible causes of AV block.
May be very low or very high.
Why investigate serum pH?
If severe acidosis or alkalosis is suspected, a serum pH should be obtained. Severely abnormal values may represent reversible causes of AV block.
May be very low or very high.
Give examples of common AV-node blocking drugs
The most common AV-nodal blocking medications include beta-blockers, non-dihydropyridine calcium-channel blockers and digoxin.
How is asymptomatic heart block treated?
No specific treatment is required. Patients are at low risk for progression to higher-degree AV block. ECGs may be re-checked if symptoms develop, but do not need to be re-checked on a routine basis.
How is symptomatic heart block treated?
Medications blocking the AV node should be stopped first.
In the absence of a reversible cause, these patients should undergo permanent pacemaker (PPM) implantation. Biventricular pacemaker, with or without an implantable cardioverter-defibrillator (ICD) placement, may be considered when the left ventricular ejection fraction is <35%
What are the complications of heart block?
- Progression to complete heart block in type 2 second degree heart block
- Pacemaker implantation sequelae
What differentials should be considered in heart block?
- Supraventricular tachycardia
- Atrial fibrillation or multifocal atrial tachycardia
How does heart block and supraventricular tachycardia differ?
Differentiating signs and symptoms:
- In patients with profound first-degree AV block and very long PR intervals, the P wave may encroach so closely on the QRS complexes that they appear to be retrograde P waves, suggesting SVT
Differentiating investigations:
- Variations in the heart rate and the PR and RP (QRS to P) intervals suggest against the diagnosis of SVT
- Comparison of numerous ECGs may be very useful
How does heart block and atrial fibrillation/ multifocal atrial tachycardia differ?
Differentiating signs and symptoms:
- The irregularity of the RR intervals in type I second-degree AV block may lead to the incorrect diagnosis of atrial fibrillation or MAT
- The presence of distinct P waves and the grouped pattern of the RR intervals are characteristic of type I second-degree AV block and are not seen in atrial fibrillation or MAT
Differentiating investigations:
- Careful inspection of the ECG should allow distinction between these diagnoses
How can the different types of heart blocks be remembered?
If the R is far from P, then you have a First Degree.
Longer, longer, longer, drop! Then you have a Wenkebach.
If some P’s don’t get through, then you have Mobitz II.
If P’s and Q’s don’t agree, then you have a Third Degree.
Which heart block is shown in the picture?
First degree heart block.
PR interval > 300 ms, P waves are buried in the preceding T wave.
Which heart block is shown in the picture?
Sinus bradycardia with 1st degree AV block.
PR interval > 300 ms.
Which heart block is shown in the picture?
Normal sinus rhythm with 1st degree AV block.
PR interval 260 ms.
Which heart block is shown in the picture?
Mobitz I AV block.
Progressive prolongation of PR interval, with a subsequent non-conducted P wave.
Repeating 5:4 conduction ratio of P waves to QRS complexes.
Relatively constant P-P interval despite irregularity of QRS complexes.
Which heart block is shown in the picture?
Mobitz I AV block.
QRS complexes clustered in groups, separated by non-conducted P waves.
The P:QRS conduction ratio varies from 5:4 to 6:5.
Note the difference in PR interval between the first and last QRS complex of each group.
Which heart block is shown in the picture?
Mobitz type 2.
Arrows indicate “dropped” QRS complexes (i.e. non-conducted P waves).
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, triple for two dropped beats, etc).
Which heart block is shown in the picture?
Complete Heart Block.
Atrial rate is ~ 85 bpm.
Ventricular rate is ~ 38 bpm.
None of the atrial impulses appear to be conducted to the ventricles.
Rhythm is maintained by a junctional escape rhythm.
Marked inferior ST elevation indicates that the cause is an inferior STEMI.
Which heart block is shown in the picture?
Complete Heart Block.
Atrial rate is ~ 60 bpm.
Ventricular rate is ~ 27 bpm.
None of the atrial impulses appear to be conducted to the ventricles.
There is a slow ventricular escape rhythm.
