Some important parameters of the electrocardiograph

Input resistance:
That is, the input resistance of the preamplifier. The larger the input resistance, the smaller the waveform distortion caused by the different contact resistance of the electrodes, and the higher the common mode rejection ratio. It is generally required to be greater than 2MΩ, and greater than 50MΩ internationally.
Common mode rejection ratio:
The electrocardiograph generally uses a differential amplifier circuit, which has a suppressive effect on the same phase (also known as the common mode signal, such as the interference signal generated by the surrounding electromagnetic field) and an amplifying effect on the out-of-phase signal (also known as the differential mode signal, the ECG signal to be collected is the differential mode signal). Common mode rejection ratio (CMRR) refers to the ratio of the differential mode signal (ECG signal) amplification factor Ad of the electrocardiograph to the common mode signal (interference and noise) amplification factor Ac, indicating the size of the anti-interference ability. It is required to be greater than 80dB, and greater than 100dB internationally.
Anti-polarization voltage:
Polarization voltage will be 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 through. The polarization voltage has a great influence on the ECG measurement, and will produce baseline drift and other phenomena. The highest polarization voltage can reach tens of millivolts or even hundreds of millivolts. If the polarization voltage is not handled properly, 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 influence of electric and magnetic fields, the electrodes still generate polarization voltage, generally 200-300mV, which requires the electrocardiograph to have an amplifier and recording device that can withstand polarization voltage. The requirement is greater than 300mV, and internationally greater than 500mV.
Sensitivity:
Refers to the amplitude of the recorded waveform when the standard voltage of 1mV is input. It is usually expressed in mm/mV, which reflects the size of the amplifier magnification of the whole machine. The standard sensitivity of the electrocardiograph is 10mm/mV. The purpose of specifying the standard sensitivity is to facilitate the comparison of various electrocardiograms.
Internal noise:
It refers to the noise generated by the thermal motion of electrons when the internal components of the electrocardiograph are working, rather than the noise formed by external interference due to improper use. This noise causes the electrocardiograph to still have a tiny chaotic wave output when there is no input signal. If this noise is too large, it will not only affect the beauty of the graph, but also affect the normality of the electrocardiogram. Therefore, the noise should be as small as possible, and the noise waveform should not be seen in the trace curve. The noise size can be calculated by converting it to the input end. It is generally required to be lower than the effect of adding a few microvolts to tens of microvolts below the input end. The international standard is ≤10μV.
Time constant:
When the DC input is used, the amplitude of the signal recorded by the electrocardiograph will gradually decrease with the increase of time. The time required for the output amplitude to drop from 100% to about 37%. It is generally required to be greater than 3.2s. If it is too small, the amplitude drops too fast, and even the input square wave signal will become a sharp wave signal, which cannot reflect the true situation of the electrocardiogram waveform.
Frequency response:
The ECG waveform of the human body is not of a single frequency, but can be decomposed into sine wave components of different frequencies and different proportions, that is, the ECG signal contains rich high-order harmonics. If the ECG machine has the same gain for signals of different frequencies, the waveform recorded will not be distorted. However, the amplifier's amplification ability for signals of different frequencies is not necessarily exactly the same. When the ECG machine inputs signals of the same amplitude and different frequencies, the relationship between the amplitude of its output signal and the frequency change is called the frequency response characteristic. The frequency response characteristics of the ECG machine mainly depend on the frequency response characteristics of the amplifier and the recorder. The wider the frequency response, the better. Generally, the amplifier of the ECG machine is easier to meet the requirements, and the recorder is the main factor determining the frequency response. Generally, it is required to be between 0.05 and 150Hz (-3dB).
Insulation:
In order to ensure the safety of medical staff and patients, the ECG machine should have good insulation. Insulation is often expressed by the resistance of the power supply to the casing, and sometimes by the leakage current of the casing. Generally, the insulation resistance of the power supply to the casing is required to be no less than 20MΩ, or the leakage current should be less than 100μA. For this reason, electrocardiographs usually use "floating technology".
Safety:
Electrocardiographs are electronic devices that are directly connected to the human body, and their safety to the human body must be carefully considered. From a safety perspective, electrocardiographs can be divided into three types: Type B, Type BF, and Type CF (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 part of medical electrical equipment that is directly connected to the patient is called the "applied part". In order to further ensure patient safety, the applied part of medical electrical equipment is often also isolated.
According to the degree of isolation of the applied part, medical electrical equipment is divided into Type B, Type BF, and Type CF.
Type B: The applied part is not isolated.
Type BF: The applied part is isolated with floating ground and can be used both in vitro and in vivo, but cannot be used directly on the heart.
Type CF: The applied part is isolated with floating ground and has a high degree of protection against electric shock and can be used directly on the heart.