Step-by-step ECG interpretation

1. Verify patient details - name and DOB
2. Check date and time ECG was taken
3. Check calibration of the ECG paper
   1. Usually paper speed 25mm/sec, voltage calibration 1cm/mV
4. Determine the axis
   1. Lead I positive and lead II positive = normal cardiac axis
   2. Lead I positive and lead II negative = left axis deviation
      1. Common finding after inferior MI
   3. Lead I negative and lead aVF positive = right axis deviation
5. Work out the rate and rhythm
   1. Is electrical activity present?
   2. Is the rhythm regular or irregular
   3. What is the heart rate?
      1. 300/number of large squares between beats if regular
      2. Count number of QRS complexes in 30 large squares and multiply by 10 if irregular (or 15 large squares and multiply by 20)
   4. P-waves present?
   5. What is the PR interval? (normally 0.12-0.2s)
   6. Is each P-wave followed by a QRS complex?
   7. Is the QRS duration normal? (normally <0.1s)
6. Look at individual leads for voltage criteria changes or any ST or T-wave changes

Calibration of ECG trace

• Heart rate = 300/no. of large squares between beats (use no. of squares between R-R interval)

ECG patterns

Locating an MI

Atrial fibrillation

• Atrial rate >300 bmp

• Irregularly irregular rhythm

• No P waves - irregular baseline

• Narrow QRS

  

Atrial flutter

• Rate: atrial 300 bmp, ventricular usually 150 bmp