Basic Structure of a Ventilator
The basic structure of a ventilator, which combines an electronic circuit with a mechanical air circuit, generally consists of nine main components: an oxygen source, solenoid valve, air mixing device, pressure-limiting valve, humidifier and temperature control circuit, airway resistance meter, breathing valve, signal box, and solenoid valve control circuit.
- Oxygen Source
The oxygen source is a storage device designed to supply the patient with the required oxygen for inhalation. It is typically provided by a high-pressure oxygen cylinder or a compressed air source.
- Solenoid Valve
The solenoid valve acts as an on-off switch for the oxygen source. Unlike a mechanical regulating valve, it is electrically controlled. By regulating the current to the solenoid valve coil, the valve can open or close to control the airflow. Its operation is similar to that of a standard relay.
- Air Mixing Device
The air mixing device utilizes the principle of high-speed fluid pressure reduction. Oxygen is sprayed through a high-speed nozzle, creating a negative pressure zone that draws air from the sides, mixing it with the oxygen flow.
- Pressure-Limiting Valve
This valve limits the gas pressure to ensure a consistent output pressure. It is typically set at 6 kPa (60 cmH₂O) during manufacturing. If the output pressure exceeds this limit, the valve automatically releases excess gas to protect the patient.
- Humidifier and Temperature Control Circuit
This is a constant-temperature device that humidifies, filters, and warms the inhaled air to prevent lung irritation from cold air and dehydration of the respiratory tract. The temperature control circuit adjusts the water temperature in the humidifier to maintain a stable environment.
- Airway Resistance Meter
The airway resistance meter measures the resistance in the patient’s airway. From this resistance and the oxygen source pressure, the oxygen concentration (excluding ambient oxygen) and tidal volume can be calculated.
- Breathing Valve
The breathing valve uses pneumatic control to manage three functions: inhalation, exhalation, and negative pressure ventilation. When the inspiratory valve opens, the machine provides airflow to the patient. During exhalation, the inspiratory valve closes, allowing the exhaled air to exit through a dedicated pathway.
- Signal Box
The signal box consists of a pair of sealed electrical contacts driven by a sensitive elastic diaphragm. When the suction reaches a set negative pressure, the contacts disconnect, outputting a pulse signal. This signal, which corresponds to the patient’s inhalation pressure, is then sent to the solenoid valve control circuit to help manage the solenoid valve.
- Solenoid Valve Control Circuit
This logic-based control system, composed of pulse digital circuits, controls the solenoid valve’s operation. By adjusting the air circuit, it regulates the breathing rate, inspiratory-to-expiratory ratio, synchronized breathing, and automatic switching between active and passive breathing modes.