• What you will learn ?
  • What is ECG ?
  • Why ECG is important ?
  • Conduction System
  • How many waves in ECG ?
  • How do the waves comes ?
  • How to read ECG ?
  • Normal ECG.

What is ECG ?

The electrocardiogram (ECG) is the recording of the electrical activity generated during and after activation of the various parts of the heart. It is detected by electrodes attached to the skin.

Why it is Important ?

The ECG provides information on:
1.The heart rate or cardiac rhythm.
2. Position of the heart inside the body.
3. The thickness of the heart muscle or dilatation of heart cavities.
4. Origin and propagation of the electrical activity and its possible aberrations.
5. Cardiac rhythm disorders due to congenital anomalies of the heart.
6. Injuries due to insufficient blood supply (ischemia, infarction, …)
7. Malfunction of the heart due to electrolyte disturbances or drugs.

How many waves in ECG ?

  • The P Q, R, S and T deflections are all called waves.
  • The Q, R and S waves together make up a complex; and the interval between the S wave and the beginning of the T wave is called the ST ‘segment’.
  • In some ECGs an extra wave can be seen on the end of the T wave, and this is called a U wave. Its origin is uncertain, though it may represent repolarization of the papillary muscles.
  • If a U wave follows a normally shaped T wave, it can be assumed to be normal. If it follows a flattened T wave, it may be pathological.

How do waves come ?

How to read ECG ?

  1. Rate
  2. Rhythm
  3. P wave morphology
  4. PR interval
  5. QRS interval, QRS complex morphology
  6. ST segment
  7. T wave
  8. Electrical axis
  9. U wave, and QT duration.


Confirm, if the rhythm is sinus, that the RR intervals are equidistant, that the P wave is positive in lead II, and that the PP intervals are equidistant and equal to the RR interval. Do an arrhythmia assessment if the rhythm is abnormal.

How to Calculate Rate ?

For regular rhythm: start with a complex that lies on a bold vertical grid line.
Rate = 300 bpm ÷ number of large boxes (0.2 second) in one RR interval.
Normal rate is between 60 bpm (five boxes) and 100 bpm (three boxes); therefore, no need to calculate exact rate.


Rate = 1500 ÷ Number of small (1 mm, 0.04 second) squares in one RR interval.

Normal ECG Intervals and Parameters

  • P waves <3 small squares (0.12 second) in duration, and amplitude <3 mm. Upright in lead I, inverted in aVR (if opposite, suspect reversed arm leads or dextrocardia).
  • PR interval – 0.12 to 0.2 second (up to 0.22 second in adults).
  • Q waves Normally present in aVR; occasionally in V1 or in aVL (vertical heart) Often present in lead III: should be ≤0.04 second duration. Other leads except lead I: <0.04 second duration and ≤3 mm deep; lead I ≤1.5 mm in patients older than age 30. Q waves may be up to 5 mm deep in several leads in individuals age <30.
  • R waves V1: 0 to 15 mm, age 12 to 20. 0 to 8 mm, age 20 to 30. 0 to 6 mm, age >30. V2: 0.2 to 12 mm, age <30. V3: 1 to 20 mm, age >30.
  • QRS duration 0.05 to 0.1 second; ≥0.1 second, consider incomplete LBBB, incomplete RBBB, or WPW syndrome.
  • ST segment Isoelectric or <1 mm elevation in limb leads and <1 mm in precordial leads except for normal variant.
  • T wave Inverted in aVR; upright in I, II, and V3 through V6. Variable in III, aVF, aVL, V1, and V2. Axis O degrees to +110 degrees age <40. −30 degrees to +90 degrees age >40.

P- Wave Morphology

  • The P wave represents the depolarization of both right and left atria.
  • It should be upright in leads I and II, as well as in the precordial leads V3 through V6.
  • It is variable in leads III, aVL, V1, and V2: upright, inverted, or diphasic.
  • The best two leads to examine the P wave are leads II and V1 as they look at the atria in opposite directions (lead II looks along the axis of the atria, while V1 looks across the atria). Lead V1 allows the easy separation of the two components of atrial depolarization.
  • The P wave consists of two components: the depolarization of the right atrium (P1) and the depolarization of the left atrium (P2).

Right Atrial Abnormalities
Right atrial (RA) abnormality refers to delayed activation of the right atrium as a result of dilatation, hypertrophy, scarring or a conduction abnormality. RA abnormality is also known as P pulmonale because it is often the result of severe lung disease. An RA abnormality is reflected in the early portion of the P wave.

Left Atrial Abnormalities
The P wave becomes broadened (P wave duration ≥ 0.11 s) because of prolonged total atrial activation time. P wave amplitude generally remains unchanged. The P wave may be notched, or double-peaked (M shape pattern) with an interpeak interval ≥ 0.04 s due to the delay in left atrial activity which also causes wider separation of right and left atrial depolarization.

PR Interval

The PR interval

begins at the onset of the P wave and ends at the onset of the QRS complex. This interval represents the time taken by the cardiac impulse to reach the ventricles starting from the sinus node and high right atrium. It is called PR interval because the Q wave is frequently absent. Normal values are between 0.12 and 0.20 s ; prolongation defines 1st-degree atrioventricular block.

Q wave

  • In general, a Q wave that is wider than 0.03 second is considered abnormal, except in leads III, aVR, and V1, in which Q waves may be wide and deep in normal individuals.
  • Lead aVR normally records a negative QRS, QS, or QR complex
  • Normal QS complexes occasionally are found in leads III and V1 and rarely in V2.

QRS Complex

The QRS complex represents the duration of ventricular depolarization. The short duration of the QRS complex indicates that ventricular depolarization normally occurs very rapidly (0.06 to 0.10 s). The QRS complex begins at the onset of the Q wave and ends at the endpoint of the S wave. The deflections are still termed QRS complexes even if one or more of the 3 waves (Q, R, S) are not visible.

Hence the traditional use of the term RR interval to indicate the time between two QRS complexes regardless of their configurations. The ventricles have a much larger muscle mass compared to the atria, causing the QRS complex to exhibit a much larger amplitude than the P wave. The amplitude of the QRS complex is increased secondary to a larger myocardial mass in left ventricular hypertrophy.
If the QRS complex is prolonged (> 0.1 s) conduction is impaired within the ventricles.

ST Segment

The ST segment begins at the endpoint of the S wave and ends at the onset of the T wave, lasting 0.08 to 0.12 s. During the ST segment, the atria are relaxed and the ventricles are contracting but no electricity is noted. Electrical activity is not visible so that the ST segment is normally isoelectric but ST elevation with a slight upward concavity may also be normal thereby complicating the diagnostic value of the ST segment.
The length of the ST segment shortens with increasing heart rate. A change from baseline producing ST segment depression or elevation occurs in pathological situations.

T wave

The T wave is the most variable wave in the ECG. T wave changes including low amplitude T waves and abnormally inverted T waves may be the result of many cardiac and non-cardiac conditions. The normal T wave is usually in the same direction as the QRS except in the right precordial leads. In the normal ECG the T wave is always upright in leads I, II, V3 to V6, and always inverted in lead aVR. The other leads are variable depending on the direction of the QRS and the age of the patient. The shape of the T wave is normally rounded and smooth. Also, the normal T wave is asymmetric with the first half moving more slowly than the second half.

U Wave

The U wave is not always seen. It is typically low amplitude, and, by definition, follows the T wave. The U wave may be seen in some leads, especially the right precordial leads V2 to V4. U waves are associated with metabolic disturbances, typically hypokalemia. Additionally, it may be seen closely following the T wave and can make interpretation of the QT interval especially difficult.