Basics of ECG- Interpretation of waves and intervals

A normal ECG is electrical representation of a normal heart beat or sinus rhythm. The cardiac action potential causing deporalization and repolarization of various cardiac tissues gives a pattern of rhythic change is Electrocardiograph which can be used to diagnose different diseases of the CVS.



Normal Rate

The beats per minute is commonly the ventricular rate. If atrial and ventricular rates differ, as in a 3rd-degree block, measurement both rates is required.

A Normal adult rate is between 60–100 bpm.

Bradycardia – <60 bpm
Tachycardia- > 100 bpm

Normal Rhythm

Measure the R-R intervals and P-P intervals.

Regular rhythm: Intervals consistent

Regularly irregular rhythm ( sinus arrhythmia): Repeating pattern.

Irregularly irregular: No pattern as in Atrial fibrillation.


Waves in ECG

P wave

A normal P wave originates from the Sinoatrial Node , SA node. It represents atrial depolarization.Normal P wave has a

  1. Height is < 2.5 mm (2.5 small squares)
  2. Width is  <0.08

Significance of normal P wave-

  1.  impulse originating in SA node
  2. normal atrial conduction and a normal atrium

Abnormality of P waves

  1. Tall P wave- >2.5mm – seen in Right Atrial Enlargement.   “P pulmonale” tall and tented P wave as seen in Right Atrial enlargement. eg in Cor pulmonale.
  2. Wide P – >0.08 sec- Left Artrial Enlargement. ” P Mitrale” -broad and bifid P wave as seen in Left Atrial enlargement. eg in Mitral stenosis.
  3. Inverted: AV Junctional Rhythm, Normal in aVR, Arm lead reversal, Coronary sinus rhythm, Dextrocardia, Left arterial rhythm.
  4. Not followed by QRS: Mobiz type I & II AV block, Third degree AV block
  5. Occuring on T wave: Atrial ectopic, AV reentry tachycardia, AV nodal reentry tachycardia, AV junction
  6. rhythm, Ventricular ectopic with retrograde conduction
  7. Absent: Atrial fibrillation, Atrial flutter, Hyperkalaemia, Mild AV junction rhythm, Sinus arrest or sinoatrial block
  8. Abnormal shape: Atrial ectopic, Multifocal atrial tachycardia, Wandering atrial pacemaker

Q wave

Q wave is normally seen in lead V5,V6. It is produced due to septal depolarization. Height > 25% of R wave, Width < 0.04 (1 small squares).

Pathological Q-

  1. If seen in lead II, V1,V2 or if >5mm in V5,V6. Pathological Q as seen in old MI.

QRS Complex

It represents depolarization of ventricular muscles and is most prominent wave in ECG. R wave has a gradual normal increase in height through lead V1 to V6.

Width < 0.12 (3small squares)

Abnormality of QRS complex

  1. Large QRS: Calibration set to 20 mV, Dextrocardia, LVH, RVH, MI, WPW syndrome
  2. Small QRS: Calibration set to 5 mV, Dextrocardia, Emphysema, Obesity, Pericardial effusion
  3. Absent: Ventricular stand still, Asystole during cardiac arrest
  4. Wide: Caliberation set to 50mm/sec, Burgada syndrome, Hyperkalaemia, LBB, RBB, WPW syndrome
  5. Varying size:Pericardial effusion
  6. Poor progression R wave – Left Ventricular Hypertrophy, Anteroseptal MI

 J point

The J point is the the junction between the termination of the QRS complex and the beginning of the ST segment. Abnormal J (slurring elevation)  is seen in Hypothermia -” Osborne Wave.”

T wave

It represents end of repolarization of the ventricles. Normally it is less than 2/3 height of R wave.

Height Abnormality of T waves

  1. Tall: Hyperkalaemia- Tented (concave outside) T wave, MI ( convex outside)
  2. Small: Hypokalemia, Hypothyroidism, Pericardial effusion
  3. Inverted: Hyperthyroidism, Hyperventillation, Left Bundle Branch Block,Right Bundle Branch Block, Mitral valve prolapsed, Myocardial Infarction, Subarachnoid hemorrhage.

Intervals and Segments of ECG

PR Interval 

It represents conduction of impulse from atrium to ventricles.
Height 0.08 sec & 1 mm elevation in limb lead, >0.08 sec & 2 mm elevation in chest lead

  1. Elevated: Acute MI, Pericarditis, Left Bundle Branch Block, Left Ventricular aneurysm, Printzmentals angina Brugada syndrome
  2. Depressed: Myocardial ischaemia, MI (NSTEMI), Right Ventricular Hypertrophy, Left Ventricular Hypertrophy, Drugs(Antiarrhythmic)
  3. Flat, Downsloping, Depressed: MI

ST Segment

Isoelectric segment in ECG originates with beggining of ventricular repolarization.

Normally 0.08 sec in duration,Slightly upward concavity

  1. ST Elevation:>0.08 sec & 1 mm elevation in limb lead, >0.08 sec & 2 mm elevation in chest lead
  2. Elevated: Acute MI, Pericarditis, LBBB, LV aneurysm, Printzmentals angina Brugada syndrome
  3. Depressed: Myocardial ischaemia, MI (NSTEMI), RVH, LVH, Drugs(Antiarrhythmic)
  4. Flat, Downsloping, Depressed: MI

QT Interval

Normal:0.42 sec, Corrected: (QTc) = QT/√RR

  1. Long: Cardiomyopathy, Drugs(antiarrhythmic), Hypocalcaemia, Hypothyroidism, Acute MI, Myocarditis
  2. Short: Hypercalcaemia, Drugs (Digoxin)

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