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.