When electrical activity occurs, heart muscle fibers are contracted and produces a movement. This condition is refered as Depolarisation. When blood is pumped around the body, it results in contraction. When chambers are contarcted, it is called systolic.
The realxation of heart muscles are called electrical repolarisation. Relaxed heart chambers are called diastolic.
|Wave||Origin||Amplitude (mV)||Duration (seconds)|
|P wave||Due to depolarization of atria||0.25||0.12 to 0.22 (PR interval)|
|R wave (QRS Complex)||Due to repolarization of atria and ventricle contraction||1.60||0.07 to 0.1|
|T wave||Due to relaxation of myocardium||0.1 to 0.5||0.05 to 0.15 ST interval|
|S T interval||Contraction of ventricles||–||–|
|U wave||Due to slow expansion of purkinje fibers||< 0.1||0.2 (T – U interval)|
ECG Recording Setup
Defibrillator Protection Circuit
The one end of the electrode leads are connected along RA, LA, chest and LL of the patient. The other end of electrode passes through defibrillator protection circuit. The protection circuit has buffer amplifier and over-load voltage protection circuit.
Lead Selection Logic
This block helps to select the type of electrode lead system. We can choose either bipolar or augmented electrode system.
Calibration is a process that helps to eliminate errors in the system. Here, any changes in the lead selection circuitresults in artefacts in the ECG output. Therefore, the calibration unit helps the technician to correct the error in ECG output.
An instrumentation amplifier and a differential amplifier with high gain and high CMRR is used as pre amplifier.
The output unit is driven with power amplifier. After the signal is amplified using pre-amplifier it goes to the power amplifier. The output unit is attached with a pen motor. A high electrical power is required to initiate recording with pen recorder. Hence, a high power gain amplifiers are used as power amplifier.
Feedback network is used to provide damping to the pen motor.
Output Display Unit
Either a CRO or a pen chart recorder acts as the output device.