• An electrocardiogram (ECG) is the recording of small electric waves being generated during heart activity.
• Thus the electrical activity of heart starts at the top of the heart and spreads down.
• In the heart there are specialized cells called pacemaker cells. These cells produce small electrical signals by changing their electrical charge from positive to negative and back.
• For the cardiovascular system to work properly, the atria and ventricles must operate in proper time relationship.
• It is necessary that to initiate the heart beat, the action potentials generated by the pacemakers propagate in all directions along the surface of atria.
• There are two types of leads used for the ECG measurement.
• The unipolar electrode have only one true pole(positive pole).The bipolar leads have one positive and one negative pole. In the standard ECG recording, there are five electrodes connected to the patient. Right Arm (RA), Left Arm (LA), Left Leg (LL), Right Leg (RL) and Chest(C).
• The stretch between two limbs (arm or leg) is electrodes are called a lead.
• Einthoven proved that at any instant of the cardiac cycle, the frontal plane representation of the electrical axis of the heart is a two dimensional vector. It is called Einthoven triangle.
• A typical ECG representation of the normal cardiac cycle consists of a P-wave, a QRS complex and a T- wave.
• Baseline is the horizontal segment of the waveform before the P-wave is called baseline or iso-potential line.
• Simply the P- wave represents the depolarization of the atrial muscles.
• The QRS complex is a recording of a single heart beat on the ECG that corresponds to the depolarization of both ventricles.
• The T-wave represents the recovery or re-polarization of ventricles.
• U- wave is the result of after potentials of the ventricular muscles.
• In an ECG machine an appropriate lead pair can be selected by a lead selector switch which can be switched to different pairs according to the type of waveform needed.
• The ECG signal is having very weak amplitude levels. So it is necessary that for proper analysis and plotting purpose, the waveform is to be amplified.
• The PMMC (Permanent Magnet Moving Coil) galvanometer is a special type of device, where the deflection of the coil depends on the amplitude and the polarity of the signal applied to its input.
• In most common ECG recording techniques, we use hot tip stylus for thermal writing.
• Usually we use thermal recorders for the representation of ECG waveform.
• Cardiac memory describes the phenomenon whereby the T-wave abnormalities that results from a change in the direction of cardiac activation as during ventricular pacing.

Author Bio: The Author of this article, Sreejith is writing articles on Electrocardiogram of the Heart and Trivandrum Portal