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ECG Made Easy(Part-2)



P wave:

P wave denotes atrial contraction. Look for P wave in lead II.  Normal P wave measures 2.5mm tall and 3mm wide.
  • Absent P wave with an irregularly irregular rhythm may indicate Atrial Fibrillation.
  • Tall peaked P wave is called P pulmonale and indicates Right Atrial Enlargement.
  • Broad Biflid P wave is called P mitrale and indicates Left Atrial enlargement.
  • Inverted P wave indicates Atrial ectopic
  1. If PR interval is less than 3 small boxes, the origin of ectopic is AV node
  2. If PR interval is more than 3 small boxes,the origin is elsewhere from the atrium.

PR Interval:

It is measured between beginning of P wave to beginning of QRS complex. It represents spread of impulse from SA node to the ventricular tissues. It measures about 3-5 small boxes. The significance of PR interval is diagnosis of heart blocks.

Heart Blocks:

1st Degree Heart block:

Prolonged PR interval more than 5 small boxes indicate 1st degree Heart block.

2nd Degree Heart block:(Mobitz Type 1):

This is characterised by progressive lengtheing of PR interval with each beat, untill a QRS complex is dropped and then the cycle repeats.

2nd Degree Heart block:(Mobitz Type 2):

Unlike Mobitz Type 1, here the PR interval is constant, but a QRS complex is dropped occasionally.

3rd Degree/ Complete Heart Block:

Complete heart block occurs when the atrium and ventricle beats independent of each other. Atrium contracts regularly but the impulse is not conducted to the ventricles.So ventricles contract at their their own pace of about 40/min. P waves and QRS shows no relationship


QRS Complex:

QRS complex is normally less than 3 small boxes wide(120ms). In V1, S wave is larger than R wave. In V5, V6, height of R wave is less than 25mm

  • Wide QRS is found in case of bundle branch blocks and and beats originating from ventricle.

Right Bundle Branch Block(RBBB):

Remember, "Always Right comes first". Look for RBBB in Lead V1. It shows RSR pattern.

Left Bundle Branch Block(LBBB):

Look for LBBB in Lead V6. It shows M pattern.

  • Increase in height of QRS complex denotes increased muscle mass of the ventricle.LVH or RVH

Right Ventricular Hypertrophy:

It is characterised by abnormal architecture of V1 i.e, R wave exceeding the height of S wave and deep S wave in V6.
It may be accompanied by RAD and P pulmonale.

Left Ventricular Hypertrophy:

It is characterised by tall QRS complex in V5, V6 more than 25mm and deep S wave in V1. It may b accompanied by T wave inversion and LAD.

QT interval:

It is measured between the beginning of Q wave to the end of T wave. It is usually between 400-440ms. If QT interval is nore than 440ms, it is said to be prolonged. QTc interval or corrected QT interval is calculated at a standard heart rate of 60bpm.

Premature Ventricular Contraction:

Look at the ECG as a whole for any bizzare, oddly placed beat. PVC is characterised by
  • Wide QRS
  • Premature-occurs before the next expected beat
  • ST-T changes
  • Compensatory pause


The ST and T wave changes are discussed in the Never Miss an MI chapter. Do Check it out here. "Expertise comes with Practise"
Keep Medizening..!!

Dr.Praful JK, MBBS

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