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Electricity Of The Heart & ECG?
                                          
Electrocardiography measures the electrical activity of the heart.  The heart has specialized tissue which is capable of generating electrical impulses.  These electrical impulses are capable of inducing the heart muscles to contract, thus propelling blood out of the heart.  The sinus (also called sinoatrial) node is a collection of cells located in the right atrium at the junction of the superior vena cava to the right atrium.  The sinus node is capable of spontaneously every fraction of a second to generate an electrical impulse.  This impulse then travels through the atrial muscles, causing them to contract.  This will cause the blood to be squeezed out of the atria and into the ventricles through the tricuspid and mitral valves.  The electrical impulse then reaches another specialized conduction tissue known as the AV node (atrioventricular).  As the name implies, this node is located at the junction of the atria to the ventricles.  The AV node is the only pathway for the electricity to travel to the ventricles.  It conducts electricity slower than in any other site in the heart, and this fact is very important, since this delay in electrical conduction from the atria to the ventricles allow the ventricles to receive all the blood from the atria before it contracts and eject its contents out of the heart.  Early or late conduction from the atria to the ventricles through the AV node will disturb this balance and cause reduction in the amount of blood being ejected out of the heart (reduced cardiac output).   The electrical changes in the atria and ventricles could be recorded through leads attached to the body surface, and this is the basis of electrocardiography (ECG).
Historical Facts
ECG was invented by Einthoven, a  mathematician and scientist in the late nineteenth century.  He recorded the electrical impulses produced by the heart and labeled them as P, Q, R, S and T.  The choice of these letters stems from his mathematical background, where the center of a circle is typically labeled "O" and various points on the circle's circumference as P, Q, R, S and T.
 
The Normal ECG:    P, QRS & T waves of ECG
The "P" wave represents the contraction of the atrial muscles.

The "QRS" complex of waves represent the contraction of the ventricles.

The "T" wave represents the electrical changes during the relaxation phase of the ventricles.

The electrical changes of the heart are recorded through special wires contacting the body through an electrically conducting patch which adheres to the skin.   Electrodes recording electrical changes  placed on each limb  provide six different leads, termed I, II, III, aVF, aVr, aVL and aVF.  In addition six electrodes are placed on the chest in front of the heart providing six additional leads, termed: V1, V2, V3, V4, V5 and V6.

Since the atria contract before the ventricles, there should be a P wave preceding each QRS complex.

 
 
How does an ECG help?

Many information about the heart function and anatomy could be obtained from the ECG recording, this include:

I.  Heart rate and rhythm:  The heart rate could be calculated from the frequency of the P, QRS waves frequency.  In addition the heart rhythm could be determined.  Normally a P wave precedes each QRS complex since the atria contract before the ventricles.  In certain abnormal heart rhythms the atria and ventricles contract together or the ventricles contract independently of the atria, these abnormalities will manifest itself in an abnormal relationship of the P to the QRS waves.   Finally, the regularity (or lack of) could be seen on the ECG, normally the heart beats regularly or with slight variation in heart rate depending upon respiration (faster heart rate when breathing in and slower when breathing out), however, irregularity of heart rhythm will manifest itself with a variable distance between on P, QRS & T waves and the next.

 
Samples of normal & abnormal heart rhythms:

Normal sinus rhythm (NSR)
 

Supraventricular tachycardia (SVT)
 

Premature ventricular contractions (PAC)
 

II.  Heart chambers' size:

The size of the various heart chambers can also be determined from the ECG:
RAE
Enlargement of the right atrium manifests itself as tall P waves.
LAE
Enlargement of the left atrium manifests itself as wide P waves.
RVH
Enlargement of the right ventricle manifests itself as tall R wave in lead V1 and deep S waves in lead V6
LVH
Enlargement of the left ventricle manifest itself as tall R wave in V6 and deep S waves in lead V1.
 

III.  Damage to the heart muscles:
Heart muscle damage could be detected on ECG, this may be changes in the T-wave, raised or depressed ST segments or presence of deep Q-waves.

     

 

IV.   Abnormal blood salts (electrolytes):

Abnormal levels of certain blood electrolytes such as potassium, calcium and others could result in abnormal ECG findings. examples of this include:

 
High blood potassium level results in tall T-waves.

 Low blood calcium level results in prolongation of the QT-interval (distance from beginning of the Q-wave to end of T-wave

 

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