When you start looking at ECGs, it's best to have a methodical approach – you can go in any order you like, so long as you make sure you catch all of the following steps.

1. Identify and standardise

Check the patient name, age, date of birth, hospital number and the time and date at which the ECG were taken. Also check the scale on the ECG is at the standard levels – 1cm = 1mV and speed = 25mm/sec.

2. Check rhythm

Is the rhythm regular or irregular? Look for a change in the R-R interval for ventricular rhythm or P-P interval for atrial rhythm: the easiest way to do this is to lay a piece of paper alongside your ECG trace, mark where about 4 R waves occur then move the paper along the trace and see if waves still line up with marks.

3. check rate

Remember – when calculating times from an ECG trace, 1 small square (1mm) represents 0.04s, 1 large square (5mm) represents 0.2s and 5 large squares (25mm) represent 1 second.

There are several ways of calculating rate:
If your ECG is regular, then you may use one of the following equations:
60 divided by R-R interval in seconds
or
300 divided by R-R interval counted in big squares

4. p-wave

Should be smaller than 0.25mV (25mm) and upright in leads II, III and aVF and there should be one P-wave per QRS. An absence of P-waves denotes Atrial Fibrillation or a Junctional/Nodal rhythm.

5. P-R Interval

This is measured from the beginning of the P-wave to the beginning of the Q-wave. The normal range is 0.12 to 0.2s – an interval of greater than 0.2s implies delayed AV conduction.

6. QRS

This is measured from the beginning of the Q-wave to the end of the S-wave. The normal duration is less than 0.12s. A QRS of more than 12s indicates ventricular conduction defects such as bundle branch block.
Low-voltage (smaller than 5mm) QRS complexes can indicate hypothyroidism, chronic obstructive airways disease, myocarditis, pericarditis and pericardial disease.
Normal Q-waves should last less than 0.04 seconds and be less than 2mm deep – they become greater than this approximately 20 hours after MI; if present in lead III consider PE.
Normal axis is between -30 ° and +90 ° (see here for axis estimation)

7. Q-T Interval

Measured from the start of Q to the end of T.
Corrected QT (QT C ) =

QT ÷ vRR

normal is between 0.38 and 0.42
Prolonged in acute MI, myocarditis, bradycardia, head injury, hypothermia, Urea/electrolyte imbalance, congenital disorders, various drugs.

8. ST segment

The section running from the end of ventricular depolarisation (S) to the start of ventricular repolarisation (T).
Elevation of >1mm = infarction.
Depresstion of >0.5mm = ischaemia.

9. T-Wave

Normally inverted in VR and V1, inverted in V2 in some young people and V3 in some black people.
Abnormal if inverted in I/II/V4-V6 – indicates ischaemia/infarction.
T-waves become tall and tented in hyperkalaemia and small in hypokalaemia.