1. ECG Abnormalities
  2. Summary of ECG Abnormalities
  3. Understanding ECGs


  1. ECG Abnormalities

Conduction Problems

Always remember the pattern of conduction:
SA node ? AV node ? His Bundle ? bundle branches
When looking at conduction problems, you are best to look at whichever lead shows p waves most clearly. This is usually lead II or V1.
The PR interval the time taken for the depolarisation to spread from the SA node to the ventricular muscle. This should not be greater than 0.2s – i.e. 1 big square.

First degree Heart block

If the PR interval is greater than 0.2s, then we call it first degree block. All the waves will still be present, there will just be a gap between the p wave and QRS complex.
First degree heart block is not in itself very important – it can be a sign of coronary artery disease, acute rheumatic carditis, digoxin toxicity or electrolyte disturbance.

Second degree Heart block

This is where there is an intermittent absence of QRS complexes – and thus an indication that there is a blockage somewhere between the AV nodes and the ventricles.
There are three types of this:
  • Mobitz type 2 phenomenon – this is where there is a regular rhythm, and a fairly constant PR interval, but every now and again there is an absent QRS (pictured above). basically for every QRS, there are 2 or 3 p waves.
  • Wenckebach phenomenon (aka Mobitz type 1) – progressive lengthening of the PR interval followed by an absence of the QRS, then a shortened PR interval and normal QRS, and the cycle begins again. The cycle is variable in length, and the R-R interval shortens with the lengthening of the PR interval
  • 2:1 and 3:1 conduction – there is one normal cycle, then one cycle with an absent QRS (2:1) or there is one normal cycle, then two cycles without a QRS (3:1) – pictured below
  • Acute – MI
  • Chronic – heart disease (CHD)
  • Mobitz type 2 and Weckenbech don’t require and specific treatment
  • X:1 block may require a pacemaker (temporary or permanent), especially if the ventricular rate is slow

Third degree Heart block – complete heart block

This occurs when atrial contraction is normal, but no beats are conducted to the ventricles.
The ventricles are still excited by their own internal ‘ectopic pacemaker’ system! Thus the definition of complete heart block is: