Basic principles and main types of ventilators
Respiratory support is one of the most critical means to save the lives of acute and critical patients. Therefore, ventilators have been widely used in clinical treatment. It has become an indispensable device; it is widely used in the fields of first aid, anesthesia, ICU and respiratory therapy; mastering the basic knowledge and basic operation methods of ventilators is the basic knowledge and skills necessary for clinicians. This article briefly summarizes some common sense of the clinical application of ventilator:
The basic principle of ventilator: inspiratory action produces negative thoracic pressure during spontaneous ventilation, alveolar and airway negative pressure occurs in passive lung expansion, Thus, the pressure difference between the airway orifice and the alveoli is formed to complete the inhalation; after inhalation, the thorax and the lungs elastically retract, resulting in an opposite pressure difference to complete the exhalation. Therefore, normal breathing is due to the “active negative pressure difference” between the alveoli and the airway opening generated by the body through the breathing action to complete the inhalation. , to meet the needs of physiological ventilation. The ventilator ventilation is driven by the external mechanical drive to generate a positive pressure difference between the airway orifice and the alveoli, and exhalation is when the thoracic and lung elastic retraction produces a passive positive pressure difference between the alveoli and the airway opening after the removal of the external mechanical driving pressure. That is, there is a “passive positive pressure difference” in the breathing cycle to complete the breathing.
According to the working characteristics of the ventilator, it can be divided into the following types:
1. Constant pressure ventilator
When inhaling, the ventilator pumps a certain pressure of gas into the airway to expand the alveoli, and the airway pressure gradually rises. When the predetermined pressure is reached, the airflow is terminated and the expiratory phase is turned. The tidal volume of this type of ventilator is related to the preset pressure, inspiratory time, flow rate, etc. of the ventilator. If the flow rate is low, the inspiratory time is short, the predetermined pressure is low, the tidal volume will be small, and vice versa, the tidal volume will increase. Tidal volume can be reduced if lung compliance decreases or bronchospasm increases airway resistance.
The disadvantage of this type of ventilator is that when the airway pressure increases, the tidal volume cannot be guaranteed. In other words, this type of ventilator maintains pressure but not capacity.
2. Constant-volume ventilator
The ventilator pumps a fixed volume of gas into the patient’s airway and lungs to generate inhalation and exhalation. action. The advantage of this type of ventilator is that within the safe pressure range, a certain tidal volume can be guaranteed in a closed airway state. The disadvantage is that airway leakage cannot be compensated, and excessive airway pressure can also cause hypoventilation. In short, this type of ventilator maintains capacity but not pressure.
3. Timed ventilator
It is a timed, pressure-limited and constant-flow ventilator. The ventilator generates airflow and enters the airway for a predetermined time. Inhalation stops and exhalation occurs. During exhalation, there is still low-pressure airflow passing through the airways. Its inspiratory time, respiratory rate, inspiratory/expiratory ratio, and inhaled oxygen concentration can be adjusted.
The above classifications are based on inspiratory and expiratory transitions. There are also classifications by control method (electric, pneumatic) and use. There is also a type of high-frequency ventilation ventilator, which is characterized by high respiratory rate, low tidal volume, and non-closed air circuit operation.
Working process
The pressure of the gas injected into the patient is generated by the turbo pump inside the machine. Working process: The air enters the safety valve through the filter. The opening size of the safety valve and the speed of the pump are controlled by the CPU. The pressure and volume of the ventilation are set by the doctor according to the needs of the SARS patients, and the appropriate amount of gas is adjusted through the one-way valve. Enter the human body mask and enter the human body, that is, positive pressure is inhaled; when the one-way valve is closed, the inhalation pressure is reduced, and the positive inhalation pressure in the patient’s lungs automatically flows out, that is, it is exhaled through the mask.
The pressure of the gas injected into the patient, the oxygen pressure of the oxygen cylinder and the positive pressure air are generated.
