What Is an ECG Amplifier?

The electrocardiogram machine can automatically record the bioelectrical signals (ECG signals) generated by the myocardial excitement during cardiac activity. It is a medical electronic instrument commonly used in clinical diagnosis and scientific research.

Before the heart beats, the myocardium is excited first, and a weak current is generated during the excitation. This current is transmitted to various parts through human tissues. Due to the different tissues of each part of the body, and the distance between each part and the heart are different, different potential changes are shown in various parts of the human body surface. ECG. An electrocardiograph is an instrument that records these physiological electrical signals. ^[1]

According to the number of synchronous output channels of the recorder in China, it is divided into single-channel, three-channel, six-channel and twelve-channel ECG machines.

Chinese name: ECG machine
Features: Recording heart activity

Input resistance: More than 2M
Insulation resistance: Not less than 20M

ECG machine components

1.Input section

2.Enlarge part

3.Control circuit

4. Display section

5.Record section

6.Power supply

ECG important parameters

ECG input resistance

This is the input resistance of the preamp. The larger the input resistance, the smaller the waveform distortion caused by the different electrode contact resistance, and the higher the common-mode rejection ratio. Generally requires more than 2M, internationally greater than 50M.

ECG common mode rejection ratio

ECG machines generally use a differential amplifier circuit. This circuit can suppress the same phase (also known as common mode signals, such as interference signals generated by surrounding electromagnetic fields), and out of phase signals (also called differential mode signals, which need to be collected). ECG signals (differential mode signals) have amplification effects. Common mode rejection ratio (CMRR) refers to the ratio of the amplification factor Ad of the electrocardiograph to the differential mode signal (ECG signal) and the amplification factor Ac of the common mode signal (interference and noise). Requirement is greater than 80dB, internationally greater than 100dB.

ECG anti-polarization voltage

A polarization voltage is generated between the skin and the surface electrode due to polarization. This is mainly due to the voltage retention phenomenon formed after the cardiac current flows, the polarization voltage has a great impact on the ECG measurement, and it will produce baseline drift and other phenomena. Polarization voltage can reach tens of millivolts or even hundreds of millivolts. If the polarization voltage is not handled well, the interference will be very serious.

Although the electrodes used in the electrocardiograph have been made of special materials, due to temperature changes and the effects of electric and magnetic fields, the electrodes still generate polarizing voltages, typically 200-300mV. This requires the electrocardiograph to have a polarization-resistant voltage. Amplifier and recording device. Requires greater than 300mV, internationally greater than 500mV.

ECG sensitivity

It refers to the amplitude of the recorded waveform when a standard voltage of 1mV is input. It is usually expressed in mm / mV, which reflects the size of the amplifier's magnification. The standard sensitivity of the electrocardiograph is 10mm / mV. The purpose of the standard sensitivity is to facilitate the comparison of various electrocardiograms.

ECG internal noise

It refers to the noise caused by the electronic thermal movement when the internal components of the electrocardiograph work, not the noise caused by improper external interference. This kind of noise makes the electrocardiograph still have a small clutter wave output when there is no input signal. This kind of noise If it is too large, it will not only affect the aesthetics of the graphics, but also affect the normality of the ECG. Therefore, the smaller the noise, the better. Noise waveforms should not be seen in the trace. The amount of noise can be calculated by the magnitude of the effect reduced to the input. Generally, it is required to be lower than the effect of adding a signal from a few microvolts to a few tens of microvolts at the input. International regulations 10V.

ECG time constant

During DC input, the amplitude of the signal traced by the electrocardiograph will gradually decrease with time, and the time required for the output amplitude to decrease from 100% to about 37%. The general requirement is greater than 3.2s. If it is too small, the amplitude drops too quickly, and even the input square wave signal becomes a sharp wave signal, which cannot reflect the true situation of the ECG waveform.

ECG frequency response

The human ECG waveform is not a single frequency, but can be decomposed into sine wave components with different frequencies and different proportions, which means that the ECG signal contains rich higher harmonics. If the electrocardiograph has the same gain for signals of different frequencies, the traced waveform will not be distorted. But the amplifier's ability to amplify signals of different frequencies is not necessarily the same. When an electrocardiograph inputs signals of the same amplitude and different frequencies, the relationship between the output signal amplitude and frequency changes is called the frequency response characteristic. The frequency response characteristics of the electrocardiograph mainly depend on the frequency response characteristics of the amplifier and recorder. The wider the frequency response, the better. Generally, the amplifier of ECG is easier to meet the requirements, and the recorder is the main factor that determines the frequency response. The general requirement is 0.05 ~ 150Hz (-3dB).

ECG insulation

In order to ensure the safety of medical staff and patients, the electrocardiograph should have good insulation. Insulation is usually expressed by the resistance of the power supply to the case, and sometimes by the leakage current of the case. Generally, the insulation resistance of the power supply to the case is not less than 20M, or the leakage current should be less than 100A. For this reason, electrocardiographs usually use "floating technology".

ECG machine safety

The electrocardiograph is an electronic device that is directly connected to the human body, and must pay great attention to its safety to the human body. From the perspective of safety, the electrocardiograph can be divided into three types: B type, BF type and CF type (see the National Standard of the People's Republic of China GB-10793-2000 ^{[2] for} details). According to the general rules of the International Electrotechnical Commission (IEC): The direct connection between medical electrical equipment and patients is called the "application part". In order to further ensure the safety of patients, the application part of medical electrical equipment is often also provided with isolation measures.

According to the isolation degree of the application part, the medical electrical equipment is divided into B, BF, and CF types.

Type B: The application part is not isolated.

Type BF: It is applied in floating isolation and can be used in vitro and in vivo, but not directly in the heart.

CF type: Floating isolation is applied, which has high protection against electric shock and can be directly used in the heart.

ECG machine classification

The heart is the powerhouse of human blood circulation. It is precisely because the heart automatically and constantly performs rhythmic contraction and relaxation activities that makes blood flow in a closed circulatory system and maintains life. Before and after the heart beats, the myocardium becomes agitated. During the agitation process, a weak biological current is generated. In this way, every cardiac cycle of the heart is accompanied by changes in bioelectricity. This bioelectrical change can be transmitted to various parts of the body surface. Due to the different tissues of the body and the distance from the heart, the electrical potential of the ECG signal in different parts of the body is also different. For a normal heart, the direction, frequency, and intensity of this bioelectrical change are regular. If the electrical signals of different parts of the body surface are detected by electrodes, then amplified by an amplifier, and recorded by a recorder, an ECG pattern can be obtained. Doctors can diagnose heart disease based on the recorded ECG waveform shape, amplitude, and relative time relationship between the waves, and then compare it with normal ECG. Such as arrhythmia, myocardial infarction, pre-constriction, hypertension, cardiac ectopic beats and so on.

First, the classification of ECG

ECG machines have different classification methods. Such as:

Classified by machine function

According to the function of the machine, the electrocardiogram machine can be divided into ordinary electrocardiograph (analog electrocardiograph) and electrocardiograph (digital intelligent electrocardiograph) with graph tracing and analysis function.

Classification by recorder

The recorder is the tracing element of the electrocardiograph. For analog electrocardiographs, the early recorders were mostly dynamic coil recorders with disc springs returning to zero torque, and position feedback recorders were mostly used after the 1990s. For digital ECG machines, the recorder is a thermal or dot-matrix printer.

1. Dynamic coil recorder: The structural principle of the dynamic coil recorder is a fixed magnetic circuit and a rotatable coil composed of magnetic steel. The output signal of the electrocardiograph power amplifier is applied to the coil of the recorder, and a stylus is fixed on the coil. When an ECG signal is output, the power amplifier outputs a current to the coil, and the coil rotates. When the deflection angle of the coil is the same as the return torque of the disc spring, stop the deflection. In this way, the recording pen driven by the coil traces the ECG waveform on the recording paper.

2. Position feedback recorder: The position feedback recorder is a recorder without mechanical return spring. The special electronic circuit can play the role of return spring. When the machine is powered off, the stylus of the position feedback recorder can be arbitrarily turned.

3. Dot-matrix thermal recorder: The thermal recorder uses semiconductor heating points sintered on a ceramic substrate to iron out graphics and characters on thermal paper that develops color when exposed to heat.

Classification by power supply

According to the power supply method, it can be divided into DC type, AC type, and AC and direct-type ECG machine. Among them, cross and straight dual-use type are mostly. DC-powered type usually uses rechargeable batteries for power supply. The AC-powered type uses an AC-DC conversion circuit. The AC is first converted to DC, and then stabilized by a highly stable voltage regulator circuit before being supplied to the ECG machine.

Divide by the recordable signal derivative

According to the signal derivatives that can be recorded at one time, the ECG is divided into single-lead and multi-lead types (such as three-lead, six-lead, and twelve-lead). The ECG signal amplification channel of the single-lead ECG machine has only one channel, and the ECG waveforms of each lead should be recorded one by one. That is, it cannot reflect the changes of the lead ECG at the same time. Multi-lead ECG machines have multiple amplification channels. For example, six-lead ECG machines have six amplifiers, which can reflect the simultaneous changes of the six-lead ECG signals at a time.

ECG machine instructions

1. Set the power switch to "ON".

2. The power switch is set to "AC (Alternating Current), '" at this time, "LINE", "TBST", "PA PER SPEED (25mm / s)", "SENSITIVITY (l)", and "STOP", and the crystal lamp emits light.

3 Adjusting the baseline control travel button should change the position of the stylus so that it stops near the center of the recording paper.

4 Press the "CHECK" key. At this time, the "STOp" light is off and the "CHECK" light is on.

5. Press the calibration key "lmV" to make the stylus swing correspondingly as the calibration key is pressed.

6. Press "START", the "CHECK" light is off, the "START" light is on, and the recording paper moves at 25mm / sec.

7. Continue to press the calibration key, you can see a clear calibration square wave on the moving recording paper, and its amplitude should be 10mm.

8. Press the "LEAD SELECTOR" key to switch from "TEST" to "I" and "II".

9. After obtaining a clear record on the ECG chart, you can continue to press the "LEAD SELECTOR" key to switch from "I" lead to "" lead, and so on. Repeat the above operation to complete 12 leads. ECG record.

10 After using the instrument, cut off the power and do the cleaning work. And do a good job of instrument registration.

ECG machine routine maintenance

The electrocardiograph should be regularly maintained and maintained to extend the life of the electrocardiograph and its various components. Specific requirements are as follows.

1 After taking the electrocardiogram every day, keep the electrodes clean. If rust spots appear on electrodes made of copper alloy, the rust spots can be wiped off with fine sandpaper and then soaked in physiological saline overnight to form a stable film on the surface. The silver-plated electrode can be washed and dried with water to avoid scratching the silver-plated layer.

The core wire or shield of the lead cable is easily broken and damaged, especially near the joints at both ends. Do not pull or twist with force. When storing, it should be coiled into a large diameter ring or hanging to avoid excessive distortion or acute angle folding.

3 AC and DC electrocardiograph should be charged in time according to the instructions to extend the battery life.

4 The electrocardiograph should avoid high temperature, sun, moisture, dust or impact, and cover the dust cover with a cover.

5 Open the cover every six months for dust removal, dehumidification and inspection. Remove the dust in the circuit board in time to ensure that the inside of the machine is dry to avoid short circuit caused by moisture or dust and damage to the circuit board.

6 The performance of the electrocardiograph is regularly checked by the medical instrument maintenance department. The hot pen recording type electrocardiograph should adjust the pressure and temperature of the hot pen according to the thermal sensitivity and paper speed of the recording paper. The electrocardiogram machine often has artifact interference faults in use. When it is serious, it will affect the normal tracing of the electrocardiogram. It should be checked in time.

ECG machine troubleshooting

In the daily use of the electrocardiograph, there are often faults such as excessively thick or thin thermal pen tracing graphics, interference, excessive baseline drift, and abnormal damping, which seriously affect the use of the electrocardiograph. The causes and troubleshooting methods are summarized below.

1 The trace graph is too thick or too thin

Cause: The temperature is too high or too low.

Remedy: Adjust the temperature of the hot pen, and adjust the potentiometer to make the temperature of the thermo pen appropriate.

2 interference

When the ECG machine records the paper, the ECG is superimposed with a certain amplitude and a regular sine wave or an irregular burr, which is interference. Divided into the following:

Interference when lead switch is set to "0"

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First, determine whether the interference is 50 Hz or low frequency. If it is 50Hz interference, the lead input part is disconnected or disconnected, that is, the lead is disconnected; the lead switch to the input board of the amplifier board is broken, disconnected or disconnected. If it appears as low frequency interference, first check whether the bristles on the brush are neat; whether there is electrical contact with the place that should not be in contact, and then check whether the motor coil is broken.

Interference at work

Interference can be caused by broken lead wires, broken wires or leakage. However, the interference is generally caused by the lead wire being broken, and the interference is only introduced when the relevant lead state is present. The easiest and most thorough method is to replace the lead wire, but if it has repair value, it is best to cut it from the break or leakage, and then re-solder. Poor contact of lead switches, disconnection of wires, desoldering, etc. will all introduce interference. Contact of the recorder or hot pen coil with the case may also introduce interference.

Other interference

The power supply ripple is too large, the filter capacitor is damaged, the internal should not be connected to the ground, the internal wiring is unreasonable or the shielding is poor.

3 baseline drift

When the lead switch is in the "0" position, the baseline depicted by the recorder is not horizontal and slowly rises or falls, which is the baseline drift.

Remedy: Wipe the plugs and sockets on the amplifier board with alcohol to prevent leakage. After it is completely dry, observe whether the baseline drift is still too large. Then check whether the coupling capacitor between the preamplifier and the voltage amplifier is leaking. Then, check the closed relay circuit. Use the line division method to disconnect the closed relay circuit. If the drift meets the standard requirements, the fault is that the closed relay is damaged or leaks and should be replaced. Finally, check the FET. First use the substitution method to replace the front-stage FET with the same type of FET. If the fault is eliminated, the input resistance of one FET (or two) in the previous stage is unstable and should be replaced.

But two field effect tubes with similar parameters must be replaced by the original two, otherwise, if the field effect tube is not symmetrical, it will have a great impact on other indicators of the electrocardiograph. Such as drift is still too large. Generally, the input resistance of a post-stage FET is unstable, and it can be replaced in the same way.

4 Damping error

When the 1mV calibration voltage waveform is traced, the waveform has no overshoot and rounded corners, which means that the damping is too large. Remedy: Adjust the damping adjustment potentiometer to make the damping moderate.

When the 1mV calibration voltage waveform is traced, the overshoot on the waveform is too large, that is, the damping is too small. elimination method:

Adjust the damping adjustment potentiometer to make the damping moderate. If the damping is too small and cannot be adjusted, first check whether the damping adjustment potentiometer is de-soldered, damaged or has poor contact. Replace if damaged. If it is not damaged, the fault is caused by demagnetization of the record. The recorder should be replaced or re-magnetized.

(3) If the damping is not uniform, generally the hot pen is placed unevenly. There is a large gap between the hot pen positioning clip and the guide rail. When adjusting, adjust it.

The electrocardiogram machine is a relatively precise instrument. When using it, avoid places such as humidity, vibration, strong electric fields, and magnetic fields. The electrocardiogram room should be kept as far away as possible from the X-ray room, physiotherapy room, elevator, etc. to reduce and avoid interference.

Because of the mature, reliable, easy-to-use, and affordable price of the electrocardiograph diagnostic technology, it does not harm patients, and has become one of the most popular medical electronic instruments in hospitals at all levels.