Data Interpretation
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ECG Interpretation

The key to ECG interpretation is practice. The way to use this guide is to print it out and use it on the wards or with a book of examples. Some good testbooks are recommended in the Things To Buy page.

Normal values

Big square: 0.2s. Small square: 0.04s.

A system for looking at ECGs

  1. Patient details
  2. Rate: 300 divided by the number of big squares between R-R values.
  3. Rhythm: regular? Irregular? Irregularly or regularly irregular?
  4. Axis
    • Normal: QRS complexes are ↑ in I, II
    • Right deviation: ↓ in I
    • Left deviation: ↓ in II, III
  5. P waves
  6. PR interval: less than 1 big square
  7. QRS complexes: less than 3 small squares
    • Q waves?
    • LV hypertrophy?
    • Predominant R waves in V1?
  8. ST elevation or depression
  9. T waves normal?
  10. QT interval
  11. U waves: Hypokalaemia

Specific Abnormalities

Here are some common abnormalities that you would be expected to recognise in finals, listed roughly in order of importance. Beware that some of these are rules of the thumb, and that it is necessary to practice interpretation to be any good at it.

A more comprehensve list can be found in the cardiovascular section of the Oxford Handbook.

ST elevation

Indicates myocardial infarction (MI). The leads in which the ST segment is raised give an indication of the area of heart infracted and which vessels are blocked:
Inferior MI
ST elevation in II, III, VF
Right coronary artery
Anterior MI
ST elevation in V3 to V4
Left anterior descending artery
Lateral MI
ST elevation in V5 to V6
Circumflex artery
Posterior MI
ST depression in V3 to V4
This is illustrated in the following diagram:

Q Waves

Indicates an old MI.

True Q waves must be 2 small squares deep, 1 small square wide. Alternatively more than 1/3 of QRS height. Q waves can be normal in leads I and AVR.

Heart Block

First Degree
Long PR interval (>1 big square).
Second Degree
Third Degree (Complete)
No relationship between P waves and QRS complexes (although both are present).

Atrial Fibrillation

  1. No P waves
  2. Irregularly irregular rhythm

Atrial Flutter

  1. Ventricular rate of about 150 usually
  2. Between QRS complexes "sawtooth" complexes representing a fluttering atrial contraction
  3. Irregularly irregular

Ventricular Tachycardia

  1. Broad QRS complexes
  2. No P waves
  3. Tachycardia

Ventricular Fibrillation

A random appearing rhythm. Beware of mistaking it for VT.


Saddle shaped ST segment elevation in all leads without reciprocal ST depression

Ventricular Hypertrophy

Tall R waves in V1-V3 and deep S waves in V4-V6 indicate right ventricular hypertrophy.

Tall R waves in V4-V6 and deep S waves in V1-V3 indicate left ventricular hypertrophy.

Bundle Branch Block

The mnemonic MaRRoW helps to recall the features of right bundle branch block. RR stands for "Right." M indicates a M shape in V1 (an RSR shape). s

The WiLLiaM indicates left ("LL") bundle branch block. W indicates an SRS pattern in V1 and a RSR pattern in V6.


Can be lethal so important to identify.
  1. Tall tented T waves
  2. Flat p
  3. increased PR interval

Wolf Parkinson White Syndrome

Wide and initially slurred QRS complex (delta wave) and a short PR interval.

Digoxin Effect

Reverse tick (concave downwards) in all leads, T wave inversion.

Trifascicular block

  1. Right Bundle Branch Block
  2. Left axis deviation (Left Anterior Hemiblock)
  3. 1st degree HB (long PR interval)

Internet Resources

NYU ECG guide
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