ECG Interpretation

Below is a systematic approach to the interpretation of the ECG.

  1. Check Patient Details
    1. Name, DOB (and age), patient number
  2. Rate
    1. Calculate either by
      1. 300 divided by the number of large squares between each QRS complex (if the rhythm is regular)
      2. or by multiplying the total number of QRS complexes in a rhythm strip (usually 10 seconds) by 6
  3. Rhythm
    1. Is the rhythm regular or irregular?
      1. Regularly irregular (heart block)
      2. Irregularly irregular (Atrial fibrillation)
  4. Axis
    1. Hexaxial-Reference-systemThe most accurate way of determining the axis is by REMEMBERING THIS DIAGRAM
      1. I.e. lead I (0°); lead II (60°); lead III (120°); aVF (90°); aVR (-150°) and aVL (-30°).  Also -30-90° is normal; >90° is RAD; <-30° is LAD
      2. You then can calculate the axis by finding the iso-electric lead and +/- 90°.  Depending on the positive leads, you can determine the axis.
        1. e.g. if the isoelectric lead (biphasic lead with similarly sized positive and negative deflections) is lead aVL (-30°), the axis will either be 60° or -120°.  If leads I (0°), II (60°) and aVF (90°) are all positive, we know it must lie within this range, so it has to be 60° (normal axis).
      3. In short the axis is always in the direction of positive leads, away from negative leads and 90° to isoelectric leads.
    2. Alternatively, alcalc
    3. RAD suggests right ventricular hypertrophy e.g. Pulmonary hypertension; mitral stenosis; PE (note there can also be the classic prominent S wave in lead I and prominent Q wave in lead III and inverted T wave in lead III (S1Q3T3)); cor pulmonale; right ventricular cardiomyopathy.
    4. LAD can be due to left ventricular hypertrophy but is more commonly seen in LBBB, inferior MIs and other conduction defects e.g. WPWS
  5. Go through the waveform systematically
    1. P-waves: Present/Absent (Atrial fibrillation/flutter)
    2. PR interval (0.12-0.2 secs/3-5 small boxes): Normal/prolonged (see heart block)
    3. QRS complex (<0.12 secs): Narrow (normal) or broad (Ventricular arrhythmias; bundle branch block)
    4. ST-segment: elevated/normal/depressed (ischaemic changes (see ACS))
    5. T waves: normal/inverted/tall (old ischaemic changes or acute MI/hyperkalaemia)

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