ECG device:

The ECG device is one of the basic medical devices that must be available in clinics and hospitals, and the ECG device records the electrical signals of the heart ECG, which are captured from the surface of the body using electrodes and displayed on the display screen or printed on paper tapes, and doctors conducted the first electrocardiograms in 1901 AD

the heart:

The heart is the main organ in what is known as the circulatory system, the heart muscle forms the functionally effective tissue in the heart, where its contraction secures the transfer of blood and pumping from the heart to the rest of the organs, which makes the heart the main pumping station for blood from the heart to the organs to provide it with oxygen loaded with blood coming from the lungs, and then the heart pumps blood coming from the organs and loaded with carbon dioxide to the lungs to purify it and load it again with oxygen The heart contains four separate chambers called: the right and left ears and the right and left ventricles The basic gathering of blood contained in the title occurs in the right atrium to then move to the right ventricle, where guarded by a valve does not allow the return of blood from the ventricle to the atrium. Its ability to contract lies in its muscles, called the heart muscle, and it consists of muscle cells that have the ability to.

Poles and connections:

The wave of electrical change in the heart muscle can be recorded by placing biopoles on the surface of the body. The electrodes are flat metal probes that have the property of good signal conduction. The electrodes are placed on the skin of the person whose heart signal is to be drawn using cream (jell gel) to increase the efficiency of signal conduction. For standard recording of the heart signal, we need five electrodes installed on different places of the body.

Colors that distinguish wires and their positions:

Right arm... its color is white.
Left arm... its color is black.
Right foot... its color is green.
Left foot... its color is red.
Chest.. its color is brown.

These electrodes connect to the input of my team's operations amplifier through the connection picker. The ECG signal obtained by two different pairs of electrodes has different waveform and height depending on the delivery method chosen. The right foot uses RA as a common pole and the junction picker connects the appropriate poles to the two inputs of the band operation amplifier.

Electrode connections are divided into three types:

Bipolar terminal connections and this type represents the most famous positions of the poles on the body and is called the Einthoven triangle. It symbolizes:

lead I: The left arm is connected to the non-reflective end of the amplifier inlet and the right arm is connected to the reflective end.
lead II: The left foot is connected to the non-reflective end of the amplifier inlet, while the right arm is connected to the reflective end, and the right arm is shortened with the left arm.
lead III: The left foot is connected to the non-reflective end of the amplifier inlet, the left arm is connected to the reflective end, and the right foot is shortened with the right arm.

2.Unipolar terminal connections:

AVR connection: The right arm is connected to the non-reflective input, while the left arm and left foot are combined and connected to the reflective input.
AVL connection: the left arm is connected with the non-reflective input, while the right arm and the left foot are combined and connected with the reflective input.
AVF connection: the left foot connects with the non-reflective inlet while the right arm and left arm combine and connect with the reflective input.

3.Unipole thoracic connections:

This type is symbolized by (V1-V6), and it consists of six locations distributed over the rib cage, and one of them is connected to the non-reflective entrance of the amplifier, while the electrodes of the three parties (the left foot, the left arm, and the right arm) are gathered by means of a network of Wilson’s resistors, and the total is connected amplifier inverting input.

Major waves in the ECG signal:

P wave: It is the first positive wave in the schema, and represents the excitement of the atria and the depolarization in them, and begins at the beginning of pumping blood through the heart. A positive P wave at all poles except at the AVR pole is negative, less than 3 mm high and has a duration of 0.11 seconds.
QRS wave: represents the excitement of the ventricles and the depolarization in them, and its duration ranges between 40 and 80 milliseconds, and the QRS component consists of the following waves:
T wave: represents the repolarization of the ventricle, i.e. represents the diastole of the ventricles, when blood flows to the arteries, while atrial diastole does not appear in the planning for the QRS overcoming.

Sometimes an additional wave called the U wave appears, which is a small wave that comes after the T wave and towards it, and if it flips against, it is an indication of myocardial infarction.

The distances between the waves have diagnostic indications:

P-R distance: It is measured from the beginning of the P wave to the beginning of the QRS wave and the time period required for the electrical change to travel from the ears to the ventricles is 0.12-0.20 seconds.
S-T piece: Comes immediately after the QRS, measured from the end of S to the beginning of T and is on the horizontal line at the level of T-P. And it begins to gradually convex. If the time is above or below the horizontal line, it means myocardial ischemia.
Q-T segment: Measured from the beginning of the QRS wave to the end of the T wave, the duration of ventricular contraction varies according to heart rate, sex and age.
Distance R-R: by which the heart rate HR is calculated.

Components of the ECG device:

Calibration: This part effectively adjusts and calibrates the device properly before starting the ECG process, as it makes a square pulse with a height of 1 mv showing that the device is in good condition.
Sensitivity point: This part is very important in maintaining the sensitivity of the device.
Site Officer: His entire work is to adjust the thermal indicator.
Mark: This wave is used at an abnormal wave in planning so that the doctor can know the disease.
Thermal indicator: It draws the wave on paper, which is accurately a thermal resistance within which a limited current passes that raises the temperature of the plotter, to carry out the required drawing process.
Speed determination: The ECG device contains two speeds (25-50) mm / s and each speed is used according to the existing condition and determined by the doctor back to the heart, if the patient is old, his pulse is weak, so we use the low speed (25) mm / s.If the patient is young and has a rapid pulse, we use high speed (50) mm/s to get up to date with the patient's planning.
Divider: One of the protection circuits in the device, as it uses a protection circuit from high currents and voltages.
Magnifiers: Their function is to magnify the ECG signals captured by the electrodes, because that signal is slightly high and difficult to process without magnification.
Filters: Their work is limited to filtering the wave of external influences that can affect the ECG, because these effects have a major role in obtaining the wrong planning.
Ground: Used to leak excess charges and protect against electric shocks.
Electrodes:It was mentioned in a previous article, to read it, click here.
C.R.O. Screen: This is when the doctor dispenses with paper for a continuous reading of the heart.

The main stages of the planning process:
Motor speed regulation stage: The motors in ECG devices are usually connected to resistors and transistors respectively for the purpose of feedback, when the load on the motor increases or decreases, it may cause an increase or decrease in voltages, and in this way we maintain the speed of the engine during the operating period.
Time management stage: This stage is very important in the work of the device and its principle of work is that the engine does not work directly at the beginning of the operation of the device and delays a time of (2.2) seconds to allow the plotter to reach the required temperature.
Power Processing Stage: The work of this stage is limited to converting the voltages that reach the device from (220V) to (12V) through a transformer and a bridge or through a battery, which is usually rechargeable.
Voltage regulation stage: The voltage regulation circuit generates an oscillation of (30-40) KHz from the existing and converted transesters that distribute the voltages to the plotter with a value of (7V) or more according to the type of device and on the rest of the electrical parts.
Signal magnification stage: This stage is considered one of the most important stages in the planning process. The process of magnifying the signal begins after it is picked up by the patient by electrodes, but in the second stage, the signal will enter the buffer amplifier and then the other amplifiers.
types of amplification:
Insulating amplifier: Sometimes and randomly occurs a shortness in the circuit of the ECG device, this shortening may lead to the flow of current to the patient's body through wires and electrodes connected between the device and the patient, which causes an electric shock to protect the patient from exposure to an electric shock, the ECG device fed from the electrical network must contain the insulating amplifier.
The spinning magnifier provides protection for the patient because its resistance reaches 10¹² ohms, which prevents the arrival of current from the feeding network to the patient's body.
There are three types of insulating amplifiers:
photodiode.
load current.
current transmission.

2. Initial amplifier:

The ECG heart signal is considered one of the weak signals in two respects: its height is low (3mv) and its frequency is low, ranging in its range (0.01 to 200) Hz, so the signal needs to be magnified, the initial amplifier is the first stage of magnification of the heart signal and the initial amplifier is a bio-amplifier and this amplifier amplifies the difference between the two input signals.

3. Dynamic Operational Amplifier:

To obtain a higher magnification, the dynamic operational amplifier is often used to magnify the vital signal, which also works to amplify the difference between the two input signals.

6- Signal filtration stage:

There are four main sources of interference when recording the ECG signal:

Muscle movement: It emits noise at high frequencies that intersect with the frequency of the ECG signal, and in order to reduce the impact of this type of distortion, the patient must be in a relaxed position.
Patient breathing: The movement of the patient's chest during breathing changes the position of the electrodes relative to the position of the heart, and to get rid of this type of confusion, the patient is asked to stop breathing for several seconds each time the planning is performed.
Electrode movement: The electrodes are not fixed well or the conductive gel is dry and creates noise at low frequencies.
Interference caused by the electricity network and nearby devices: The frequency of this type is the frequency of the network.

To obtain a pure ECG signal and minimize the impact of interference on it, the following must be ensured during the planning procedure:

The set of wires connected to the patient is installed correctly and in its location in the device.
The presence of a ground wire connected to the device in order to discharge excess charges in the device.
The presence of the gel under the electrodes and not drying it out.
Make sure that there are no other electrical appliances near the planning device.
Avoid using metal beds and when necessary, connect the bed with a ground wire.
Avoid fluctuating with electric current.

Types of filters:

Low-frequency filter at cut-off frequency (fc=200Hz): This filter passes the signal with frequencies below the fc cut-off frequency and magnifies it by ACL.

2. High-frequency filter at cut-off frequency (fc=0.1Hz): This filter passes the signal above the fc cut-off frequency and magnifies it by ACL.

3.Cut-off filter cuts off the network frequency (fn=60Hz): A filter that prevents the passage of a signal with certain frequencies such as the network frequency.

7. Signal processing and pulse detection: Signal processing and pulse detection are in continuous monitoring devices. This stage comes after the elimination of noise by means of filters, and this stage consists of a differential amplifier circuit, a narrow-band pass filter and contains nonlinear circuits. The importance of this stage lies in detecting the occurrence of the pulse and thus calculating the heart rate, when the rate is less or more than the specified limit, the device gives an audible alarm.

How does the device work:

Install the electrodes safely on the patient's skin in the designated places after hair removal in those places with the use of a conductive gel after which the ECG device is turned on, where the standard voltage value equal to 1mV is recorded.
The electrical activity of the heart is recorded by measuring the potential difference between the electrodes that are placed on the specified places.
The change in the potential difference between the electrodes is recorded and this results in the heart signal of those electrodes, which travels through the wires into the device.
The signal to be measured is selected by the selector key.
This signal is magnified because it is very weak and this is considered the first stage of magnification.
Filter the signal by filters to get rid of noise and get a pure heart signal.
The signal is then passed through two parallel paths:

Signal processing circuit to detect the heartbeat and calculate its rate of change and heights.
Magnifier ability to magnify the signal again to display it on the screen (CRT) or draw it on the blue with the plotter.

The principle of the painter's work:

It works on the principle of a galvanometer, which consists of a magnet with a coil wire in the middle, known as when an electric current passes through a wire, it produces a magnetic field around that wire. There is a force on the wire that moves it, and this force is perpendicular to this magnetic field, and this is the principle of a galvanometer.

When a current passes through this coil, a force is created that moves the column, and even if there is a severe deviation in the column, there is a spring that balances this column, and this column is connected to an indicator where there is a space inside it to draw the wave on the printing paper.