Unlocking the Mystery: A Comprehensive Guide to Understanding How a Multi-Parameter Monitor Works.The multi-parameter monitor is a device that can monitor and record a variety of patients’ basic vital signs parameters in real-time, hereinafter referred to as monitor, generally including electrocardiogram, non-invasive blood pressure, pulse oximeter, end-carbon dioxide and other various parameters. Multi-parameter monitors, as integrated medical measuring instruments, are widely used in hospital wards and Intensive care unit(ICU). The multi-parameter monitor is one of the most common critical medical monitoring equipment.What can a multi parameter monitor can do.

1) Non-invasive Blood Pressure Monitoring (NIBP)

The vibration method is a method of measuring blood pressure. Its principle is to use the cuff to inflate to a certain pressure. When the force is fully compressed, the arterial blood vessel is blocked and the arterial blood flow is blocked. Then, as the cuff pressure decreases, the arterial blood vessel will appear: the process of complete obstruction-gradually opening and fully releasing. In the whole process, the pulsation of the arterial blood vessel wall will generate gas oscillations in the gas in the cuff.This oscillation has a certain corresponding relationship with the arterial systolic pressure, diastolic pressure, and average pressure. Therefore, the systolic pressure, average pressure, and diastolic pressure of the measured part can be obtained by measuring, recording, and analyzing the pressure vibration wave in the cuff during deflation.

2) Electrocardiogram (ECG) Monitoring

The electrochemical activity of the excitable cells in the heart muscle causes the heart muscle to become electrically excited so as to cause the heart to contract mechanically. The closing action current generated by this exciting process of the heart flows in the volume conductor of the human body and propagates to various parts of the body, thereby causing changes in potential differences at different surface parts of the human body.Electrocardiogram (ECG) is to record the potential difference on the body surface in real-time. The monitor can generally monitor 3 or 5 leads, can display the waveform of one or two leads at the same time, and can directly display the heart rate. The powerful monitor can monitor the 12-lead ECG; The waveform is further analyzed to extract the ST segment waveform and arrhythmia event.

3) Temperature TEMP Monitoring

The temperature monitoring of the monitor is based on the thermistor principle. Generally, the measurement of multi-parameter monitors for body temperature uses the thermistor with a negative temperature coefficient as the temperature sensor, which is obtained according to the characteristic that the resistance value of the thermistor changes with temperature. Monitors provide single-channel body temperature but high-end monitors can provide dual-channel body temperature. There are two types of body temperature probes: surface probes, and intracavitary probes.

4) Blood Oxygen SPO2 Monitoring

The probe is clamped on the finger. The measurement is based on the different light absorption characteristics of hemoglobin and oxyhemoglobin in the blood, by using two different wavelengths of red light (660nm) and infrared light (940nm) through the tissue, and then converted into an electrical signal by the photoelectric receiver.Two light-emitting diodes (LEDs) placed side by side are fixed on the upper wall, and the emission wavelength is 660nm red light and 940nm infrared light. There is a photoelectric detector on the lower wall, which converts the red light and infrared light transmitted through the blood vessels of the finger arteries into electrical signals. The weaker the photoelectric signal it detects, it means that when the light signal penetrates the probe site, the more the signal is absorbed by the blood, tissue, bone, etc.The absorption coefficients of these two types of light for skin, muscle, fat, venous blood, and pigment are constant, so they only affect the magnitude of the DC component in the photoelectric signal. However, the concentration of HbO2 and Hb in the blood changes periodically with the pulsation of the blood, so their absorption of light also changes in a pulsating manner, which leads to the signal intensity output by the photodetector fluctuating with the concentration of HbO2 and Hb in the blood. As a result, we get the value of SPO2.

5) Respiratory RESP Monitoring

In multi-parameter monitors, most of the breath measurement uses the chest impedance method. The thoracic motion of a person during breathing will cause a change in the body resistance of the human body, the amount of change is about 0.10~30, which is called breathing resistance.The monitor generally uses two upper electrodes of the ECG lead to inject a safe current of 0.5mA~5mA into the human body with a constant frequency of 10kHz~100kHz carrier frequency sine wave, so that we can pick up electrical signals of changes in breathing impedance from the same electrodes. This change in breathing impedance describes the dynamic waveform of breathing and can extract breathing frequency parameters. The movement of the rib cage and the non-breathing movement of the body can cause changes in the electrical resistance of the body.When the frequency of this change is the same as the frequency band of the amplifier of the breathing channel, it is difficult for the monitor to determine which is the normal breathing signal and which is the interference signal. When the patient is experiencing severe sustained physical activity, the measurement of the respiration rate will be inaccurate.

6) Carbon Dioxide at the End of Breath (PetCO2) Monitoring

End-respiratory carbon dioxide is an important monitoring indicator for anesthetized patients and patients with diseases of the respiratory metabolic system. The main measurement method of CO2 is the infrared absorption method, which is mainly based on the different absorption procedures of different concentrations of CO2 for specific infrared light.There are two main types of CO2 monitoring: main-stream and side-stream.The mainstream type is to directly place the gas probe in the patient’s respiratory airway catheter, directly convert the concentration of CO2 in the respiratory gas, and then send the electrical signal to the monitor for analysis and processing to obtain the PetCO2 parameter. The sidestream optical sensor is placed in the monitoring instrument, and the patient’s breathing gas is drawn by the gas sampling tube into the monitor for concentration analysis in real-time.

7) Pulse Rate

Photoelectric volumetric pulse measurement is the most common monitoring measurement. The sensor consists of two parts: a light source and a photoelectric transducer. It is clamped on the fingertip or auricle of the patient. The light source selects light of a certain wavelength that is selective to the oxygenated hemoglobin in the arterial blood. This light passes through the human peripheral blood vessels. When the arterial pulsation congestion volume changes, the light transmittance of this light is changed, and it is received by the photoelectric transducer.The light transmitted or reflected by the tissue is converted into an electric signal and sent to the amplifier for amplification and output, thereby reflecting the volume change of the arterial blood vessel. The pulse is a signal that changes periodically with the beating of the heart, and the volume of the arteries also changes periodically. The period of the electrical signal change of the photoelectric transducer is the pulse rate.