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ECG İnterpretation

To interpret an ECG it is very important to combine your findings on patient history and physical examination with ECG findings.

The 12-lead ECG only has 10 leads( electrodes). These comprise 4 limb electrodes and 6 chest electrodes. With these 10 electrodes we derive 12 leads on an ECG paper.

When electrical activity (or depolarisation) travels towards an ECG lead, the wave is net positive. When the activity travels away from the ECG lead the wave is net negative. If it is at 90 degrees then the complex is ‘isoelectric’ i.e. positive and negative waves are of same length. This can often be seen in V4 lead. Isoelectric complexes may be seen in leads other than V3-4 in different diseases like chronic obstructive pulmonary disease, emphysema.

The heart areas represented by the ECG leads are summarized below:

  1. V1, V2 = Right ventricle
  • V3, V4 = İnterventricular septum
  • V5, V6 = Left and lateral side of the heart
  • I, aVL = Left and lateral side of the heart
  • II, III and aVF = Inferior territory (remember ‘F’ for ‘feet’)
  • aVR = R side of the heart

1

During ECG interpretation we usually look at lead II and break it down into different waves, segments and calculate durations. The usual paper speed is running at 25mm/sec, therefore 1 large square is equivalent to 0.2 secs and a small square to 0.04 secs.

Waves, Segments and Intervals in an ECG

  1. P-wave: Atrial Contraction
  • PR interval: Represents the time taken for an electrical wave to spread from the sino-atrial (SA) node across the atrium and down to the ventricular muscle via the Bundle of His.
  • QRS: Ventricular Contraction
  • ST segment: Ventricular Relaxation
  • T-wave: Ventricular Repolarization

Normal Duration of ECG Segments:

  1. PR interval: 0.12 – 0.2 secs (3-5 small squares)
  • QRS: <0.12 secs (3 small squares)
  • QTc: 0.38 – 0.42 secs

How to Read an ECG?

There are many different systems to interpret the ECG. If you read in a systematic way then the chance you miss something important is very low.

  • Patient details( History and Physical examination)
  • Situation details( How the patient has arrived, how the leads are connected, paper speed in ECG etc)
  • Rate

10.Rhythm

11.Axis

12.P-wave and P-R interval

13.Q-wave and QRS complex

14.ST segment

15.QT interval

16.T-wave