Coupon Accepted Successfully!


Electrical Events

The conducting system is made up of modified cardiac muscle. Though there are ‘latent pacemakers’ in other portions of the conducting system, the SA node is the normal pacemaker of the heart because its prepotential (refer- cardiac muscle) is the steepest. The atrial and ventricular muscle normally do not show prepotential. The SA node is situated at the junction of superior vena cava and right atrium.

(The AV node is situated in the right posterior portion of the inter atrial septum)

Innervation of SA node & AV node


SA node

AV node


Right vagus

Left vagus


Right stellate ganglion

Left stellate ganglion


Stimulation of SA node & AV node

Right vagus

Inhibits SA node

es heart rate

Left vagus

Inhibits AV node

Slows A-V conduction

Right stellate ganglion

Stimulates Sa node

es heart rate

Left stellate ganglion

Stimulate AV node

Shortens AV conduction time and refractoriness


Conductions speeds & Frequency


Rate (m/s)

Impulse frequency

SA node



AV node

0.02- 0.05 Q  slowest


Bundle of His



Bundle branches



Purkinje system

4 Q      Fastest




Note: Conductions speeds in Internodal artial pathwaysatrial & Venticular muscles is 1m/sec


Spread of cardiac excitation–(Depolarization begins in SA node)

Ventricular depolarization- the first part of the ventricle to get depolarized is the left endocardial surface of the interventricular system; then the right endocardial surface of the interventricular system. It then passes down and through the Purkinje system depolarizes the ventricles from endocardium to epicardium.

The top of the interventricular septum and the base of the heart are the last to be depolarized.

Ventricular repolarisation- The apical epicardial surface is the first to repolarise; the base endocardial surface is the last to repolarise.

ECG – The 12 lead ECG consists of

1. 3 bipolar limb leads viz lead I, II, III (also called standard limb leads) :record activity in vertical plane


2. 3 unipolar (augmented) limb leads viz avR, avL, avF :record activity in vertical plane


3. 6 unipolar chest leads viz V1 to V6: record activity in Horizontal plane(anteroposterior)


4. Unipolar limb leads – the connection of the bipolar limb leads is :                 

Example :  Lead I is between LA and RA, with the LA ‘positive’ and RA ‘negative’. The direction of the lead axis is taken from negative to positive e.g the arrow indicates the direction of lead II.


The basic electrical recording principle are

  1. If the direction of the cardiac impulse is towards the recording electrode, a positive (upward) deflection is recorded; if it is moving away from the recording electrode, a negative (downward) deflection is recorded.
  2. The height of deflection depends on

i. The strength of the cardiac impulse vector.


ii. How the vector is oriented to the lead axis. If it is parallel, it records maximum deflection; if it is perpendicular, it records minimum deflection.   

iii. Calculation of axis- The connections of the bipolar limb leads can be represented in another way;



If one goes ‘clockwise’ from point ‘ A’ to point ‘B’ it is from 00 to +1800;

if one goes ‘anticlockwise’ from point ‘A’ to point ‘B’ it is from 00 to –1800.

Illustrative example:


The vector ‘V’ can be taken as –300 or as +3300.

Similarly, direction of lead II is +600 or - 3000



  • The normal direction of the mean QRS vector is generally between- 300 to +1100.
  • Normally, the maximum deflection is recorded in lead II because the direction of the mean QRS vector is most parallel to lead II.      


  • If there is a left ventricular hypertrophy, the vector will ‘shift’ in the direction shown by dotted arrow; in which case, the vector would become most parallel to lead I. So, if one wants to know the value of vector, the vector can be ‘superimposed’ on the triaxial system.



                                        Value of V = 1500                         Value of V = 600 or +3000                                                        

  • Einthoren’s law : Mean deflection is lead II = Mean deflection in lead I + Mean deflection in lead III i.e II = I + III
  • Augmented unipolar limb leads – the unipolar limb leads are VR (right arm), VL (left arm) and VF (left foot); the augmented limb leads are aVR, aVL and aVF. The ‘augmentation’ is in terms of amplitude of deflection i.e aVR amplitude is 1½ times the amplitude in VR (the configuration remains the same). In the unipolar leads, one electrode that is kept at the point where the potential is to be measured is called the exploring electrode. The other electrode (called indifferent electrode) is kept at near zero potential by connecting 3 wires from the right arm, left arm and left leg, through a resistance (of say 5 kilo ohm). This is also called the Wilson’s terminal.                                                         


  • Note that the bipolar leads measure the potential difference whereas the unipolar leads measure the actual
  • potential at that point.
  • Pricordial chest leads

   The leads can be divided as lateral leads(left ventricle), anterior,inferior and septal.


File:Contiguous leads.svg

Normal ECG

  1. The P wave is due to atrial depolarization, upright in II, III, and aVF inverted in aVR
  2. The PR interval is the interval from the beginning of the P wave to the Q wave(0.12-.20 Sec)
  3. The Q wave is the beginning of ventricular depolarization( - ve Wave)
  4. The QRS complex represents the depolarization of the ventricles (0.1sec)
  5. The QT interval is the interval from the beginning of the Q wave to the end of the T wave (0.4 sec)
  6. The ST segment is the segment from the end of the S wave to the beginning of the T wave (0.3 sec)
  7. The T wave represents ventricular repolarization.
  8. U wave : a small positive wave which may be seen following the T wave . This wave represents the last
  9. remnants of ventricular repolarization. Inverted or prominent U waves indicates underlying pathology or conditions affecting repolarization.
  10. Q-T interval: The Q-T interval represents the time for both ventricular depolarization and repolarizationto occur, and therefore roughly estimates the duration of an average ventricular action potential.  This interval can range from 0.2 to 0.4 seconds depending upon heart rate.  At high heart rates, ventricular action potentials shorten in duration, which decreases the Q-T interval.  Because prolonged Q-T intervals can be diagnostic for susceptibility to certain types of tachyarrhythmias, it is important to determine if a given Q-T interval is excessively long.  In practice, the Q-T interval is expressed as a "corrected Q-T (QTc)" by taking the Q-T interval and dividing it by the square root of the R-R interval (interval between ventricular depolarizations).  This allows an assessment of the Q-T interval that is independent of heart rate.  Normal corrected Q-Tc intervals are less than 0.44 seconds.

Some generalizations in normal ECG




All the deflections are negative

aVL /  aVF

Predominantly positive or biphasic

V1, V2

No Q wave; deep S wave

V3, V4


V5, V6

Small Q wave, Tall R wave

Lead I, II, III

All positive deflection; largest in lead II

ECG in some abnormal condition

  1. Accelerated A-V conduction
    Wolff- Parkinson White Syndrome : (Here, the abnormal connection is between atria & ventricle, it is called the bundle of Kent)

a. Short P- R interval

b. Prolonged QRS deflection, which is slurred on the upstroke

c. P- J interval is normal

Lown- Ganong- Levine syndrome : (Here, the abnormal connection is between atria and bundle of his; if is called James bundle)

d. Short P - R interval

e. Normal QRS         

f. P – J interval is decreases
  1. M.I

a. Changes due to current of injury – ST segment elevation. This is most noticeable in chest leads just over the infarcted area (current of injury: the infarcted area is negative (extra cellularly) relative to the surrounding area; this results in flow of current into the infarcted area from the surrounding areas.)

The ST segment elevation is because of 3 basic abnormalities of cardiac muscle in    M.I

  i. Rapid repolarization (seconds after the infarct)

  ii. in RMP (minutes after the infarct )

  iii. Delayed depolarization (half an hour after infarct)

Changes due to electrical silence –(after days/ weeks, the infarct becomes electrically silent)

i. Q wave changes

ii. R wave changes (failure of progression)

Changes due to conduction abnormalities

Heart block

  1. Atrioventricular block is blockage of the conduction from the atria to the AV-node.
  2. The first-degree AV block is a prolongation of the PR-interval (above 0.2 s) implying a delay of the conduction - not a real block. All beats are conducted 1:1 ratio.
  3. The second-degree AV block occurs when some signals are not conducted so 2:1 or 3:1 pattern. Mobitz I heart block is characterized by progressive prolongation of the PR interval on the electrocardiogram (ECG) on consecutive beats followed by a blocked P wave (i.e., a 'dropped' QRS complex). After the dropped QRS complex, the PR interval resets and the cycle repeats.also called Wenckebach phenomenon. Mobitz II heart block is mostly a disease of the distal conduction system (His-Purkinje System). It is characterized by intermittently nonconducted P waves not preceded by PR prolongation and not followed by PR shortening.
  4. The third degree AV block (complete AV-block) is a total block of the conduction between the SA node and the ventricles. A latent AV- or ventricular pacemaker (Mainly bundle of His) maintains Cardiac output with a spontaneous escape rhythm around 40-50 bpm (Adam-Stokes syndrome)

Test Your Skills Now!
Take a Quiz now
Reviewer Name