Heart Block

1st Degree Heart Block

There's prolonged conduction of electrical activity through the AV node, therefore leading to its characteristic prolonged PR interval (>200ms/5 small/1 big square) on ECG.

Causes include:

• High vagal tone (e.g. athletes)

• Inferior MI

• Electrolyte abnormalities (e.g. hyperkalaemia)

• Drugs - non-DHP CCBs, b-blockers, digoxin

N.B. This isn't always pathological, as athletes/very active patients can have it.

Management - Is benign, therefore doesn’t require any specific management. Any underlying cause should be treated.

2nd Degree Heart Block

Some atrial impulses don’t make it through the AV node to the ventricles, therefore there are instances where the p waves don’t lead to QRS complexes. It's classified into Mobitz I (Wenckebach’s phenomenon) and Mobitz II.

Mobitz I (Wenckebach’s phenomenon) - ECG shows progressive lengthening of PR interval, which eventually results in a p wave that fails to conduct a QRS complex. Patients are usually asymptomatic and don't require any management as the risk of complete heart block and asystole is low.

Mobitz II - ECG shows a set ratio of P waves to QRS complexes e.g. 2:1 or 3:1 - Constant PR interval. Pts will need a permanent pacemaker as the risk of complete heart block and asystole is high.

N.B. Patient's must notify DVLA and stop driving for at least 1 week after a pacemaker is inserted.

3rd Degree/Complete Heart Block

Atrial impulses fail to be conducted to the ventricles, therefore leading to severe bradycardia and dissociation between the p waves and QRS complexes. There's a huge risk of asystole with these patients, so they'll need a permanent pacemaker.

N.B. Patient's must notify DVLA and stop driving for at least 1 week after a pacemaker is inserted.

If the patient has adverse effects (e.g. HF, MI, syncope, shock) with a bradyarrhythmia, the 1st thing to do is give IV Atropine!

N.B. Pacemakers are categorised into external and internal. The external type (i.e. external pacing, like that used in Mobitz II or CHB) is almost always used for temporary stabilisation of the patient before definitive management with an implantable (internal) pacemaker can be done.