AV Block Flashcards
(36 cards)
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
