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
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:
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
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.
Sensor setups close to the patient on the other hand are directly exposed to the patient, which brings its own challenges. One example of the technical challenges for proximal sensing is hotwire sensors. They rely on the measurement of small changes in electrical current or resistance values over a platinum wire 1/10th of a human hair thick, heated to several hundred degrees. These values are then relayed back to the ventilator, amplified, and processed into a flow value. A second challenging example is the use of differential pressure elements in which two tubes transport the flow-dependent pressure drop from the patient back to the pressure sensor inside the ventilator. This measurement, more often than not, suffers from clogging of the tubes due to water droplets originating from condensed humidity. The proximal flow solution developed by Sensirion AG (Staefa, Switzerland) performs all measurement and processing operations on a single chip inside the flow sensor and thus avoids these problems.
Advantages of Proximal Sensing
Patient-side sensors measure both the patient’s inspiration and expiration. If patient ventilation extends over a long period of time, weaning (removing the patient from the ventilator supporting their breathing) creates challenges for the sensor system, as it is important that spontaneous breathing is detected quickly and reliably. Due to the exposure of the sensor to the patient’s exhaled air, the proximal sensors are required to be suitable for reconditioning (cleaning and autoclaving) or be intended for single use.
Measuring airflow, volume, and pressure as close to the patient as possible — proximally — is the most accurate measurement possible. It allows the patient to be ventilated with a highly accurate tidal volume, eliminating most of the issues that can arise from flow measurement within the ventilator. It allows detection of small respiratory signals and is robust against leakage in the breathing circuit. Proximal sensors insensitive to high humidity conditions have proven helpful in reducing the causes of monitoring and triggering problems leading to ventilator errors.
Proximal sensing originated in neonatal care and has become the undisputed standard when ventilating infants. Ventilated volume is critical for such a small body and non-proximal approaches can lead to significant errors. However, a majority of non-neonatal ventilators still use solely inspiratory and expiratory measurements. Proximal ventilation, however, is on the rise because it does not require the tedious and error-prone compensation of leakage paths and breathing circuit accessories.
A New Way Forward Using MEMS Technology
From early-day rotameters to flow measurements with differential pressure sensors over orifices or hot wire anemometers, sensor measurement technology has evolved to keep pace with increasing requirements for ventilation. Inspiratory sensing is used for accurate and instant fan control and inspiratory airflow monitoring.
Expiratory sensing is used for balancing the air exhaled by the patient with inspiratory ventilated air.
Proximal sensing is used to measure inhaled and exhaled air with the highest accuracy, directly at the patient.
The Sensirion hot wire anemometer is used for a range of flow sensors based on MEMS technology (micro-electromechanical system). These sensors utilize an on-chip micro-heater with temperature sensors placed up and downstream of the heater. With no flow, both sensors provide the same temperature reading, which then shifts to a gradient reading depending on the amount and direction of flow and the thermal properties of the gas. This relationship is established during calibration and enables extremely high sensitivity in the low-flow regime and around zero flow. It provides highly robust calibrated digital flow readings with no offset.
The Sensirion sensor range covers both adult and neonatal ventilation applications with both single-use and reusable sensors. Since the proximal sensors encounter humid or potentially contaminated patient air, replacement or cleaning is essential. Reusable sensors can be cleaned using various methods, from washing to autoclaving.
The Benefits of Next-Generation Flow Sensing Technology
Hot-wire anemometer sensor technology does not require zero-point or offset adjustment, does not drift over time, or need to be calibrated during the lifetime of the sensor. Since the measurement signal is processed inside the sensor and the output is digital, components like A/D convertors or external circuitry are obsolete. Medical staff can safely, quickly, and reliably manage patient ventilation.