In the past, the decision to opt for a particular flow sensing technology in respirators and ventilation devices was a painstaking and complex process. More recently, however, flow sensor solutions have emerged that provide a fully calibrated and temperature-compensated output signal.
Proximal flow sensors are widely used in respiratory devices for intubated patients and noninvasive ventilation patients in hospitals, home care situations, and emergency rooms (see Figure 1). With applications ranging from neonatal to adult care, the associated requirements for proximal flow sensors are both diverse and challenging. Sensors must be reliable and cost-effective while offering long-term stability in addition to a host of other characteristics. Proximal flow sensors also have particularly high requirements with regard to hygienic sterilization due to the patient’s contact with air, which can potentially be infected with pathogens.
Flow Sensing Technology Used for Proximal and Expiratory Sensing
There are already a number of sensors on the market that can be sterilized with autoclaving or other methods. All of these sensors use one of two different measurement principles: the hot-wire anemometry principle, or differential pressure measurement via an orifice or a variable orifice to increase sensitivity at low flow ranges. Both measurement principles have specific benefits. However, all sensors — regardless of which principle is applied — pose difficulties with regard to sterilization, and great care is required during the cleaning and sterilization process to prevent damage to the sensors.
One safe and cost-effective alternative (in terms of total cost of ownership) for reusing sensors might be a microelectromechanical systems (MEMS)- based single-use proximal flow sensor. In contrast to existing technologies, the sensors are fully calibrated whereas hot wire sensors need to be calibrated before use. A fully calibrated sensor helps to save time for the hospital staff so that they can focus on other tasks.
Function of Proximal and Expiratory Flow
Beyond pressure sensing, flow sensing poses one of the major challenges for respiratory devices. For the sake of simplicity, the following focuses on positive pressure ventilators, where the patient is connected to the device either through a mask (noninvasive) or through intubation or tracheostomy (both invasive). Modern ventilators have a wide range of application modes, including pressure controlled, flow controlled, and many more. For patients who cannot breathe on their own, the trigger for the next inhalation can be set by a timer. But things become more complicated if a patient is breathing spontaneously. In the latter case, the patient’s breathing effort must be detected as rapidly as possible to achieve good synchronization between the device and patient. This article looks at the position of the sensor for this purpose and discusses the effect of sensor position on trigger sensitivity.
In addition to the triggers to start the next breath, the end of the inspiratory phase also needs to be determined based on a value: the volume, flow, time, or pressure. This is called the limit variable. It is also necessary to establish the flow control between the trigger and the limit based on one of the values.
Triggers for patient-triggered ventilation can be set based either on the pressure or the flow signal. If pressure signals are used as the trigger, achieving the desired sensitivity is difficult. This is mainly due to the fact that pressure sensors tend to drift over time. Frequent offset correction is therefore required to ensure reliable trigger sensitivity without false triggers.
Thanks to their excellent stability, flow sensors placed in the proximal configuration yield a very fast response and high sensitivity. With expiratory positioning of the flow sensor, the stability and sensitivity are the same, but signal detection is delayed by the travel time of the flow through the expiratory tube. Expiratory flow sensing also has other advantages over a proximal configuration such as minimizing the likelihood of contamination with mucus. For its part, proximal sensing is less affected by leaks further down the breathing circuit.
As noted earlier, the inspiratory phase may be terminated based on volume, flow, pressure, or even time. The same applies to the flow control between the trigger and the limit of the inspiratory phase. For example, a ventilator setting chosen by the hospital staff might use time-triggered inspiration as well as a time-triggered limit. In this case, the flow needs to be controlled between those two points. Another setting might use different parameters for the trigger, limit, and control. The combination of parameters might be chosen for medical reasons or simply based on the preference of the staff member setting up the ventilator. Monitoring the pressure, flow, and volume values over time provides an opportunity to observe changes in the patient’s condition, such as reduced lung capacity.
Pressure-triggered limits can be greatly affected by the compliance of the breathing circuit, which can change if the circuit is exchanged or if the tubes and hoses are positioned differently. A more extreme bend in the hose, for example, can affect the circuit.
The compliance of the breathing circuit has little effect on measurement and integration of the flow signal if the flow sensor is placed in the proximal configuration. This is not the case for expiratory flow sensing, however. In such cases, having the pressure and flow signal helps illuminate the influence of compliance. Proximal flow sensing is less affected by leaks — due to connecting humidifiers and nebulizers, for example — further away from the patient.
While there is undoubtedly a trend toward more intelligent and adaptive ventilation modes, the basic underlying modes will still be based on the pressure-, flow-, volume- and time-based values discussed above.
Placement of Flow Sensors: Proximal Versus Expiratory Placement
It is essential to distinguish between dual-limb and single-limb circuits (see Figure 2). In a dual-limb circuit, the inspiratory path and the expiratory path each have separate tubes. The inspiratory tube and the expiratory tubes meet at the y-piece, and the last few centimeters to the patient pass through a single tube. During inspiration, the air flows through the inspiratory tube to the y-piece and from there to the patient. During exhalation, the air flows to the y-piece, closes a flap that prevents the air from flowing back through the inspiratory tube, and opens the expiratory tube. In a single-limb breathing circuit, there is only one tube from the ventilator to the patient. Before the patient, there is an expiratory valve that lets the air from the ventilator pass through to the patient during inhalation. During the expiration phase, the same valve opens and allows the air to be released to the ambient.
In both cases, the flow sensor measuring the inspiratory flow can be placed in the machine, where the sensor will not come into contact with wet or contaminated air. In the case of single-limb circuits, the expiration flow can only be measured when a proximal flow sensor is used; otherwise only the inspiratory flow is known, and the number of available ventilation modes is limited. With a dual-limb circuit, either a proximal or expiratory flow sensor solution might be used. Proximal flow sensing has some advantages in terms of trigger sensitivity due to the close proximity to the patient. On the other hand, that close proximity also brings additional challenges, such as contamination with mucus, which might be more easily controlled with an expiratory placement.