A brief discussion on the application of SM58 series pressure sensors in ventilators

A ventilator is a device that can replace, control or change a person's normal physiological breathing, increase lung ventilation, improve respiratory function, reduce respiratory work consumption, and save heart reserve capacity. At present, the sales of China's medical device industry only account for 2% of the world's medical device sales, which is far from meeting the needs of domestic health and wellness. The sustainable development of the medical device industry is an inevitable product of the development of human history. It is not only a sunrise industry, but also an eternal industry. Under the current situation, China has ushered in the best period for the development of medical devices: Recently, China has proposed to vigorously promote community medical care, establish a new rural cooperative medical system, and strengthen the construction of rural health infrastructure. It is estimated that during the "Eleventh Five-Year Plan" period, China's investment in rural health infrastructure construction will reach 20 billion yuan. The equipment standards and related policies of grassroots hospitals will also be introduced one after another, which indicates that the ventilator industry faces excellent market opportunities.

1. Overview

Oxygen is an indispensable substance for human survival. Humans breathe in and out air through the respiratory tract and respiratory organs (nose, pharynx, larynx, trachea, bronchi and lungs) by contracting and relaxing the respiratory muscles. Through the oxidative metabolism of human cells, they inhale oxygen and expel carbon dioxide gas in a continuous cycle. This is the respiratory function of humans. A healthy adult breathes (16-20) times per minute when calm, and the amount of air inhaled each time is about 500 ml, and the amount of air passing through the lungs is about 10 liters per minute. The total oxygen storage capacity of the human body, including blood and body fluids, is about 500 ml. When the human body is at rest, the oxygen consumption per minute can be (200-300) ml, and during strenuous exercise, the oxygen consumption per minute can reach 5500 ml. In this way, if a person does not breathe for a few minutes, it will be dangerous to his life. Therefore, the human body must breathe continuously and replenish oxygen continuously to maintain normal life activities.

Under normal circumstances, the oxygen that a healthy person takes in from the air through breathing activities can meet the needs of oxidative metabolism of various organs and tissues in the human body. However, if the physiological function of the respiratory system encounters obstacles, such as chemical poisoning, neuromuscular paralysis, drowning shock, suffocation, respiratory failure after surgery, etc., oxygen supply and artificial respiration are required for rescue treatment to increase the patient's ventilation, quickly relieve the problem of hypoxia and carbon dioxide retention, and improve the ventilation function. For patients with respiratory failure and cessation of spontaneous breathing, ventilators are indispensable equipment.

2. The basic principle of ventilators

The human inhalation function is: respiratory muscle contraction - chest volume expansion - alveolar expansion (forming negative pressure) - inhalation of air from the outside; the exhalation function is: respiratory muscle relaxation - alveolar elastic contraction - increased pressure in the lungs - exhalation of gas to the outside. It can be seen that the respiratory function is formed by the pressure difference between the expansion and contraction of human alveoli and atmospheric pressure. The basic principle of the ventilator is to establish this pressure difference by intelligent detection methods, thereby realizing the forced artificial respiration process. Therefore, the ventilator must have four basic functions, namely, inflating the lungs, switching from inhalation to exhalation, discharging alveolar gas, and switching from exhalation to inhalation, in a cycle. Therefore, it must have: ⑴ the power to deliver gas to replace the work of human respiratory muscles; ⑵ the ability to generate a certain respiratory rhythm, including respiratory frequency and inhalation-exhalation ratio, to replace the function of the human respiratory central nervous system to control the respiratory rhythm; ⑶ the ability to provide appropriate tidal volume (VT) or minute ventilation (MV) to meet the needs of respiratory metabolism; ⑷ the supplied gas is preferably heated and humidified to replace the function of the human nasal cavity, and can supply an amount of O2 higher than that contained in the atmosphere to increase the concentration of inhaled O2 and improve oxygenation.

Pressure sensors play an important role in respiratory equipment. They are responsible for converting physical values ​​(such as airway pressure and flow) into differential signals. The signals generated by air and oxygen flow sensors can help the microprocessor adjust the motor to adjust or maintain the air pressure required for the patient's inhalation and exhalation. That is, the pressure sensor monitors the patient's breathing signal at all times to control the action of the ventilator so that it is synchronized with human breathing. Therefore, the selection of pressure sensors is crucial. Usually, these cost-effective linear sensors have large offsets and offset drifts, resulting in signals that exceed or fall below proportional temperature changes. The SM58 series of high-precision linear pressure sensors produced by SMI of the United States combine the most advanced pressure sensor technology in the United States with CMOS digital signal processing technology to produce pressure sensors with signal amplification, calibration and temperature compensation. Multi-level pressure nonlinear correction, calibration voltage output is 0.5V~4.5V; calibration pressure signal and digital temperature can be obtained through the I2C interface; directly output the amplified and calibrated analog signal. The SM58 series has gauge pressure, differential pressure and absolute pressure configurations, and has micro-pressure and low-high pressure ranges. Among them, the micro-differential pressure and low-pressure sensors are unique in the market and are the best choice for pressure sensors in ventilators.

SM58 Series Pressure Sensor

3. Medical ventilator principle diagram