- ECG Recording Setup Definition: The ECG recording setup is defined as a system to measure the electrical activity of the heart using various components and circuits to produce a clear ECG waveform.
- ECG Block Diagram: The block diagram of an ECG recording setup includes essential components like the defibrillator protection circuit, lead selection logic, calibration circuit, pre-amplifier, power amplifier, feedback network, and output display unit.
- Depolarisation and Repolarisation: Depolarisation is the contraction of heart muscle fibers, while repolarisation is the relaxation of these muscles, both crucial for heart function.
- ECG Waveform Components: Key parts of the ECG waveform include the P wave, QRS complex, and T wave, each representing different phases of heart activity.
- Output Display Unit: The output display unit, such as a CRO or pen chart recorder, is used to visualize the ECG waveform.
ECG Waveform
An ECG recording setup measures the heart’s electrical activity and draws the ECG waveform below. The same chain of protection, lead selection, amplification and display is what produces that trace.
Depolarisation
When electrical activity spreads through the heart, muscle fibers then contract and move blood. That electrical change is depolarisation. When the ventricles pump, arterial pressure during that beat is systolic pressure.
Repolarisation
The electrical recovery of heart muscle is called electrical repolarisation. Relaxed heart chambers sit in diastole, when arterial pressure is 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 Block Diagram
Defibrillator Protection Circuit
One end of the electrode leads connects to the patient’s RA, LA, chest and LL. The other end passes through the defibrillator protection circuit, which typically includes overload voltage protection and, in many designs, a buffer amplifier.
Lead Selection Logic
This block selects the electrode lead system. A 12-lead recorder can choose bipolar limb leads, augmented limb leads or chest leads.
Calibration Circuit
Calibration injects a known 1 mV pulse so the technician can set the gain. Changes in the lead selection circuit can cause artifacts in the ECG output. The calibration unit lets the technician check those errors against a known step.
Pre Amplifier
An instrumentation amplifier and a differential amplifier with high gain and high CMRR are used as pre-amplifiers.
Power Amplifier
The output unit is driven by a power amplifier. After the signal is amplified by the pre-amplifier, it goes to the power amplifier. A paper recorder attaches the output to a pen motor, which needs high electrical power. High power-gain amplifiers are used for that pen. Digital monitors do not need a pen motor.
Feedback Network
A feedback network is used to provide damping to the pen motor on a paper recorder.
Output Display Unit
A paper chart recorder or, in a teaching lab, a CRO can act as the output device. Clinical machines now more often use a digital screen and a printout.
