ECG interpretation made simple

Dr Andrew Money-Kyrle outlines a straightforward approach for GPs to confidently interpret ECGs.

When confronted with an ECG it is best to start off with the basics: rate, rhythm and axis

Many general practices have an ECG machine and it is a useful screening test for heart problems, but confident analysis of ECGs requires time to develop interpretation skills.

This is not always easy for busy physicians. This article outlines a few pitfalls for the non-specialist ECG interpreter.

When confronted with an ECG it is best to start with the basics: rate, rhythm and axis. Then consider the PQRST complexes in all the leads with some basic questions in mind:

  • Is the PR interval normal (three to five little squares or 120 to 200ms)?
  • Are there pathological Q waves?
  • Is the QRS complex more than 120ms (>three little squares = bundle branch block [BBB])? Right BBB has a positive R wave in V1, left BBB is negative in V1 and positive in lead V6.
  • Do the R waves increase gradually and normally across the chest leads (the R wave should be bigger than the S wave by V4)?
  • Is there abnormal ST depression or T inversion?
  • Is the QT interval normal (a QTc >450ms may be associated with ventricular tachycardia [VT])?

Do not rely on the ECG computer analysis as it is only a guide.

Rates between 60 and 100bpm are within normal limits. The rate is calculated by dividing 300 by the number of centimetre squares between complexes; a patient with two big squares between complexes has a heart rate of 150bpm (a common rate for AF with 2:1 block), three big squares would be 100bpm and six big squares would be 50bpm (a little on the slow side). Slow rates may be normal in athletes.

Sinus rhythm should have clear P waves. The commonest tachycardia is poorly controlled AF - look for the variable RR interval with the loss of clear P waves.

When the rate is fast it can be difficult to see the irregularity but careful measurement will show it. AF is often asymptomatic and is increasingly common with age. Other common fast narrow complex tachycardias include supraventricular tachycardias (SVTs) or atrioventricular nodal re-entrant tachycardias. These are usually fast, regular rhythms associated with palpitations generally in a younger age group.

Important slow heart rhythms include complete heart block (total loss of association between P waves and QRS complexes), 2:1 block and slow AF. Patients who feel faint or black out are likely to need permanent pacing.

The axis is normal if leads I and II are both positive (this is from -30 to +90 degrees).

Left axis deviation occurs if lead I is positive and II is negative. Right axis deviation is negative in I and positive in II. Left axis deviation may occur in left ventricular hypertrophy (LVH), along with ST depression in lateral leads. Right axis deviation occurs in situations where there is right heart strain, such as pulmonary embolisms and atrial septal defect.

Isolated ventricular ectopic beats are common and do not usually need specific treatment. Patients often complain they keep 'missing a beat' followed by a heavy heartbeat. They usually notice this when resting and it is a good sign if it disappears during activity.

Broad complex runs, particularly if associated with black outs or severe dizziness, are more sinister. Although rhythms, such as fast AF and SVT, may cause BBB, it is safest to assume all broad complex runs (>three beats) are likely to be due to VT.

Pacemakers give a BBB pattern and the pacing spike is usually visible. Pacemakers will only pace if the natural heart rate is too slow (usually <60bpm), so intermittent pacing is common.

Other findings
ST segment shifting and T inversion are frequent findings and do not always indicate ischaemia. Other causes include LVH, digoxin effects (such as 'reversed tick'), 'high take-off' (a normal finding in young patients), pulmonary emboli, myocarditis and pericarditis.

  • Dr Money-Kyrle is a consultant cardiologist at Stoke Mandeville Hospital, Buckinghamshire.

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