ECG Basics

The electrocardiogram (ECG) is a diagnostic tool that measures and records the electrical activity of the heart in detail. Being able to interpretate these details allows diagnosis of a wide range of heart problems.

ECG Electrodes

Skin Preparation:

Clean with an alcohol wipe if necessary. If the patients are very hairy – shave the electrode areas.

ECG standard leads

There are three of these leads, I, II and III. 
Lead I: is between the right arm and left arm electrodes, the left arm being positive. 
Lead II: is between the right arm and left leg electrodes, the left leg being positive. 
Lead III: is between the left arm and left leg electrodes, the left leg again being positive.

Chest Electrode PlacementElectrode Placement
V1: Fourth intercostal space to the right of the sternum.
V2: Fourth intercostal space to the Left of the sternum.
V3: Directly between leads V2 and V4.
V4: Fifth intercostal space at midclavicular line.
V5: Level with V4 at left anterior axillary line.
V6: Level with V5 at left midaxillary line. (Directly under the midpoint of the armpit)

Click on Picture to view large image

ECG Leads - Views of the Heart

Chest Leads

V1 & V2

V3 & V4

V5 & V6


Right Ventricle

Septum/Lateral Left Ventricle

Anterior/Lateral Left Ventricle

The ECG records the electrical activity that results when the heart muscle cells in the atria and ventricles contract.

PQRST of the ECG

ECG Interpretation

The graph paper that the ECG records on is standardised to run at 25mm/second, and is marked at 1 second intervals on the top and bottom. The horizontal axis correlates the length of each electrical event with its duration in time. Each small block (defined by lighter lines) on the horizontal axis represents 0.04 seconds. Five small blocks (shown by heavy lines) is a large block, and represents 0.20 seconds. 

Duration of a waveform, segment, or interval is determined by counting the blocks from the beginning to the end of the wave, segment, or interval.

P-Wave: represents atrial depolarization - the time necessary for an electrical impulse from the sinoatrial (SA) node to spread throughout the atrial musculature. 

P-R Interval: represents the time it takes an impulse to travel from the atria through the AV node, bundle of His, and bundle branches to the Purkinje fibres.

QRS Complex: represents ventricular depolarisation. The QRS complex consists of 3 waves: the Q wave, the R wave, and the S wave.

Q-T Interval: represents the time necessary for ventricular depolarization and repolarization.

T Wave: represents the repolarization of the ventricles. On rare occasions, a U wave can be seen following the T wave. The U wave reflects the repolarization of the His-Purkinje fibres.

S-T Segment: represents the end of the ventricular depolarization and the beginning of ventricular repolarization.

The ECG and Myocardial Infarction

During an MI, the ECG goes through a series of abnormalities. The initial abnormality is called a hyperacute T wave. This is a T wave that is taller and more pointed than the normal T wave. 

Hyperacute T Wave

Hyperacute T Wave

The abnormality lasts for a very short time, and then elevation of the ST segment occurs. This is the hallmark abnormality of an acute MI. It occurs when the heart muscle is being injured by a lack of blood flow and oxygen and is also called a current of injury.

ST Elevation

An ECG can not only tell you if an MI is present but can also show the approximate location of the heart attack, and often which artery is involved. When the ECG abnormalities mentioned above occur, then the MI can be localized to a certain region of the heart. For example, see the table below:

ECG leads Location of MI Coronary Artery
II, III, aVF Inferior MI Right Coronary Artery
V1-V4 Anterior or Anteroseptal MI Left Anterior Descending Artery
V5-V6, I,aVL Lateral MI Left Circumflex Artery
ST depression in V1, V2 Posterior MI Left Circumflex Artery or Right Coronary Artery
Further Reading
  1. - An online training resource for all healthcare practitioners involved in conducting and interpreting Electrocardiographs (ECG)

  2. ECG/EKG Links

  3. What's so difficult about ECG's--a bundle of what?

  4. Mind your P's and Q's

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ECG Interpretation

  • Look at the entire strip. Identify P-QRS-T complexes.
  • Do these complexes seem to occur regularly?
  • Do you see any abnormal beats?
  • Does the heart rate appear to be fast, slow, or normal?
  • Does the rhythm appear to be regular or irregular?


When no signal or a poor signal is observed the following should be considered:
  • Have the cables been correctly connected?
  • Is the equipment functioning correctly?
  • Could external electrical equipment interference be a problem?
  • Was skin preparation adequate?
  • Could the electrodes suffer from:
    • gel dry out?
    • Poor adhesion?

P - Wave

  • Are P waves present?
  • Do P waves have a normal shape - usually upright and rounded on the rhythm strip?
  • Are all the P waves similar in size and shape?
  • Do all the P waves point in the same direction?
  • Do you see a P wave with each QRS complex?
  • Is each P wave the same distance from its QRS complex?

P-R Interval

  • What is the duration of the P-R interval? (Normal 0.12 to 0.20 seconds/ 2.5-5.0 squares)
  • Is the P-R interval constant?

QRS Complex

  • Are all the complexes the same size and shape?
  • What is the duration of the QRS complex? (Normal duration is no more than 0.10 seconds/1.5-2.5 squares; if greater, it indicates a Bundle Branch Block (BBB))
  • Are all the QRS complexes the same distance from the T waves that follow them?
  • Do all the QRS complexes point in the same direction?
  • Are any QRS complexes present that appear different from the other QRS complexes on the strip? If so, measure and describe each one individually.

S-T Segment

  • Is the S-T segment on the baseline or is it above or below it?
  • Could a P wave be hidden in the S-T segment?

T Wave

  • Are T waves present?
  • Do all the T waves have the same size and shape?
  • Could a P wave be hidden in the T wave?
  • Do the T waves point in the same direction as the associated QRS complex?