Basics of Ventilator Systems
This application note provides an overview of ventilator systems with a focus on system design for precision measurement and reliable protection. It discusses the four key components of a pneumatic ventilation system: gas supply, gas flow mixture, inhalation and exhalation systems. Within each of the components are a variety of sensors including pressure, flow, humidity, and temperature sensors. These sensors provide feedback to accurately control solenoid valves to ensure the air volume, pressure, and air-oxygen mixtures are delivered to the patients at specified intervals. Included in the system design are several fail-safe features and alarms to ensure reliable operation in any condition.
The modern ventilator is an electromechanical device designed to deliver a precise mixture of air-oxygen, at a specified pressure and volume at set intervals. Ventilators provide four different modes to assist and support with breathing. These modes range from controlled mandatory ventilation for patients that rely on the ventilator for life support to continuous positive airway pressure for patients that can breathe independently. Basic modes include:
- Positive End-Expiratory Pressure (PEEP)
- Continuous Positive Airway Pressure (CPAP)
- Controlled Mandatory Ventilation (CMV)
- Synchronized Intermittent Mandatory Ventilation (SIMV)
To deliver the precise amount of air-oxygen mixture at a set volume and interval, a ventilator system needs precise measurement of pressure, air flow, and temperature for each of the four modes. In addition to precise airflow, ventilators are also required to run continuously and reliably. To do this, ventilator systems need to handle any type of system failure. This includes power failures, system malfunctions, and gas supply shortage.
This application note describes a ventilator system block diagram for the four subsystems, which include the gas delivery system, pneumatic gas mixture system, inhalation system, and exhalation system.
Gas Delivery System
The gas delivery system consists of an oxygen tank and air tank, with an optional compressor to pressurize the air tank. Individual tank pressure is monitored using a pressure sensor to ensure proper gas delivery. This requires a robust signal chain solution that is optimized for accurate pressure sensor reading. External components provide the most flexibility and accuracy for pressure sensor measurement. For example, the MAX11128, a multi-channel 12-bit SAR ADC with 1Msps, paired with the MAX4430 low distortion op amp and a low-noise reference like the MAX6126, provides a very accurate pressure measurement.
This precision signal chain ensures that if either the gas supply falls below a set pressure, the system will sound an alarm to have the tank changed. For a ventilator system with an optional compressor, the system can control the compressor to pressurize the air tank to a specified pressure. See Figure 1.
Figure 1. Ventilator gas delivery system.
Pneumatic Gas Mixture System
The high-pressure gases move through an air regulator that reduces gas pressure to system specifications. The oxygen content in the air is about 21 percent. To increase the oxygen content, based on the individual patient's needs, requires accurate control of the solenoids to precisely manage the percentage of air-oxygen mixture. This requires a high-accuracy 12-bit DAC such as the MAX5702.
The correct percentage of oxygen in the air is one aspect of the air preparation. The air mixture also needs to be heated and humidified for maximize comfort. Air temperature must be accurately measured using a temperature sensor IC like the MAX31875. See Figure 2.
Figure 2. Pneumatic gas mixture system.
Inhalation and Exhalation Systems
A dual-tube system attached to the patient, along with pressure sensors, are used to detect breathing patterns and measure pressure during inhalation and exhalation. During inhalation, a pressure sensor detects the negative pressure as the patient is taking a breath. This activates the solenoids to release the air-oxygen mixture at a set rate and pressure. If the patient does not take a breath in the set interval, the ventilator will mechanically provide the breath. A real-time clock (RTC), like the MAX31342, provides an accurate low-power solution for timekeeping.
During exhalation, the ventilator controls the exhalation valve to control the pressure during the exhalation process. Through the use of the sensor readings and solenoid valve controls, the mechanism generates the four basic modes of a ventilator system. See Figure 3.
Figure 3. Inhalation and exhalation systems.
Four Basic Modes of a Ventilator System
Positive End-Expiratory Pressure (PEEP) – In this mode, the exhalation pressure is maintained by controlling a valve when the patient exhales to slightly increase the alveolar pressure at the end of expiration.
Continuous Positive Airway Pressure (CPAP) – In this mode, the system delivers constant pressure which keeps airways unobstructed and is used for a patient that can breathe without assistance.
Controlled Mandatory Ventilation (CMV) – In this mode, breathing is completely controlled by the ventilator. A set volume and pressure of air volume is provided at a set interval to the patient.
Synchronize Intermittent Mandatory Ventilation (SIMV) – In this mode, the ventilator detects a breath and delivers the set volume and pressure to the patient. If a breath is not detected with a set interval, the ventilator will provide the air to the patient automatically.
In addition to a high level of accuracy needed to measure and control air delivery, fail-safe systems are also required to ensure continuous reliable operation.
Designing for reliable protection requires independent system monitoring. Many modern microcontrollers integrate several protection features including brownout detection, watchdog timers, and other types of protection mechanisms. However, the more robust solution deploys external supervisory and watchdog timer devices like the MAX16155 to independently monitor power rails and ensure the software is operating properly.
By using external essential analog ICs, designers can build ventilators with a high degree of precision and reliable operation. Today's medical ventilators systems need precise measurement of pressure, air flow, humidity, and temperature to optimize the four basic modes—CPAP, PEEP, CMV, and SIMV. In addition, ventilators are also required to run continuously, reliably, and handle any type of system failure. To do this, designers need to utilize reliable protection products including supervisory and watchdog products.