Medical ventilation technology has come far since the original iron lung was first used more than 90 years ago. Using negative pressure to ventilate, the patient was inserted into an airtight chamber looking like a prop from a Sci-Fi movie. The iron lung mimicked the physiological act of breathing. Pumps controlling airflow periodically increased and decreased air pressure within the chamber and thus indirectly in the patient’s chest. Lungs were inflated by applying negative pressure to the patient’s body and then when the pressure inside the chamber increased, it compressed the patient’s chest and forced them to exhale. But the machines were bulky and restricted a patient’s ability to move. So, when positive pressure ventilators were developed in the 1950s, the technology caught on. Patients were not restricted, and caregivers could better examine them. Modern ventilators use either a tracheal tube or mask to apply positive pressure and inflate the lung during inspiration.

The Role of Sensors

Figure 1. Emergency ventilation requires a very robust sensor that is easily set up for the emergency medical services.

The development of ventilators is closely linked to the available sensor technology. Continuous airflow measurements during anesthesia monitoring, intensive care treatment, and in clinical and ambulatory environments provide crucial information to assess cardiorespiratory and breathing circuit behavior.

A variety of measurements are used in modern ventilators. Flow sensing technology plays a significant role, along with concentration, altitude, and pressure. Air and other gases — primarily oxygen — are used for ventilation and the quantity and mixture of these gases must be precisely controlled.

Three Application Areas

Ventilation devices are essentially used in three areas: emergency treatment, intensive care, and at home.

  • For emergency treatment, the size and weight of the device is crucial; medical providers need to carry it to the patient immediately and it needs to be extremely easy to use. Therefore, ventilators in emergency settings are typically battery or pneumatically powered.

  • Ventilation devices for intensive care have a greater functional scope than devices used in emergency treatment. When it comes to intensive care, performance capability is most important.

  • Devices used at home need to ensure patients or caregivers can effectively use them to maintain proper breathing.

Hospitals may use invasive ventilation, which makes use of a tracheal tube, however non-invasive ventilation — interfacing the patient with a mask — is also used in formal medical settings. It is the primary ventilation type for homecare patients where ease of use is usually the main focus, since patients’ conditions are typically uncritical in this environment.

Flow Sensing Technology Requirements

Each of the three medical application areas has its own set of needs:

Figure 2. Ventilation devices in an intensive care setting require sensors with high dynamic range and resolution in order to meet the varying needs of patients from infants to athletes.

Emergency ventilation requires a very robust sensor that is easily set up for the emergency medical services. In an ambulance, seconds can mean the difference between life and death. Easy mechanical and electrical connections are crucial in those moments. Also, ambulances do not have controlled climates for the storage of their equipment. Temperatures can drop below freezing, down to -20°C (-4°F), which would cause humid exhaled air to potentially ice up and inhibit or even block the flow to the patient, so the sensors must be accurate at these temperatures.

Intensive care has additional requirements for the flow sensing technology. Ventilation devices in this setting typically require sensors with high dynamic range and resolution in order to meet the varying needs of patients from infants to athletes. These flow sensors should also be able to measure flow volumes bi-directionally and cover a range of -250 liters to +250 liters per minute with high precision and very high sensitivity, especially in the important range around zero flow. Fast sensor response times are needed to allow the ventilator to immediately react and adjust the different ventilation modes.

Sensors used in home ventilation need to measure different flow rate ranges depending on the type of patient while ensuring accuracy of measurement. Robustness and ability to clean the sensor are valued, so the patient won’t have to maintain a large stock of single-use sensors.

Across all three of these applications, sensors are required to be robust and stable for long periods of time, to minimize the need for frequent recalibration and maintenance.

Inspiration and Expiration

Figure 3. Placement of flow sensors in a respiratory device.

In ventilation systems, air needs to be measured both on the inspiration side — inhaling, and on the expiration side — exhaling. The inspiration and expiration sensors have traditionally been installed inside the ventilator device, typically distant from the patient. The gas is clean and dry when leaving the ventilator device and often conditioned by a humidifier on the way to the patient. The air exhaled by the patient is always moist because it comes directly from the patient and has been humidified by the lungs. These differences must be considered in the design of the sensors.

While modern positive pressure ventilation better reflects the pathophysiology of the patient and allows medical providers to observe changes in the condition of a patient over time, the technique comes with challenges in ensuring the highest accuracy in measurement. Complex breathing circuits vary widely in terms of the types of tubing, humidifiers, and adapters used in the device. Leaks and imperfections are ever-present, which means the flow rate measured within the ventilator can differ significantly from the actual flow reaching the patient. Errors in airflow measurements can be caused by changes in air temperature, humidity, and breathing gas composition, as well as other factors. These errors, especially for small flows, are corrected using complex — and often inaccurate or error prone — compensation algorithms. These technical challenges can be addressed through proximal flow sensing: measuring respiratory flow as close to the patient as possible.