First degree heart block is defined as the prolongation of the PR interval (on the ECG) to >200ms (normally 120-200ms; marked 1st-degree block >300ms). In contrast to other heart blocks, every P wave is transmitted to a QRS complex (and ventricular contraction i.e. no ‘missed’ beats, just slowing).
1st degree heart block rarely shows any abnormalities of the ECG other than prolonged PR interval (i.e. no QRS widening, no absent QRS etc). This is a result of dysfunction, almost always, at the level of the AV-node. Rarely, 1st degree heart block is accompanied by widening of the QRS complex, signifying block at the level of the His-purkinje complex.
- Ischaemic heart disease (inc coronary heart disease)
- AV block is common post MI (in particular inferior MI)
- Other heart disease e.g. infection (endocarditis, rheumatic fever, diphtheria, TB) may also be a cause of AV block
- Several drugs (particularly antiarrhythmics e.g beta blockers, calcium channel blockers etc)
- Electrolyte disturbances
- hypokalaemia, hypomagnesemia
- More rarely, intrinsic AVN disease may be the cause
- Most patients are asymptomatic
- May have reduced exercise tolerance
- May also present with pre-syncope/syncope (particularly on exertion)
- PR interval exceeds 200msec (1 big square)
The risk is that these patients will develop 2nd degree heart block. However, this risk is pretty small and, unless they are symptomatic, pharmacological treatment is not usually necessary (aside treatment of an underlying cause)