In the present global situation emergency ventilation is one of the most successful techniques for treating severe cases of COVID-19. Due to the rapid scaling of ventilator production world-wide, combined with the barriers to international trade created by quarantine measures critical components for ventilators are in extremely short supply. The Praxis Emergency Ventilator is designed using only off the shelf parts already available in Canada. These parts are in lower demand and will allow a large scaling of production due to existing stock already deployed throughout corporate supply chains.
The praxis emergency ventilator is designed to fill the gap between manual ventilation and commercial medical ventilators. It is sufficient to keep a patient breathing and have controlled pressure, volume, and breathing rate. The system is compact, and can be left to run automatically freeing medical personnel to perform other tasks where previously manual ventilation would be the only recourse.
Praxis Prototype and Design Ltd. in partnership with Ontario Tech University, and in consultation with VentureLab, and gapWireless will make use of our pre-existing cross-functional engineering, design, and production teams to rapidly develop this system in 3 phases.
Proof of concept (complete)
The design is refined and robustness is improved. A working prototype is constructed by the end of this phase.
Field Testing & Consultation (in progress)
The design is sent to consulting partners for feedback and collaborative iteration to improve manufacturability. Additionally tests with a computer “patient” and undertaken.
Production run
The design is made open source and free to use and construction of units in canada begins.
Statement of NeedIdeally all patients would be immediately placed on commercial ventilators however with the extreme shortage of supplies we are seeing cases where civilian doctors are forced to triage patients due to limited access to ventilators. This new design is intended to bridge that gap, providing sufficient ongoing mechanical ventilation while patients wait for commercial ventilators to become available. The simplicity of the design coupled with a focus on using only available components means that these units can be in hospitals within weeks rather than months helping to add overflow capacity to medical systems.
In the current global situation resources for traditional Ventilation equipment are in extremely short supply. Compounding this the restrictions placed on international trade due to a variety of factors. Since there is extremely limited time to provide essential ventilation equipment we propose a ventilator based on readily available off the shelf components. This ventilator can be constructed using primarily non-medical components to minimize the impact on the medical supply chain.
Other emergency ventilator designs such as the proposed design showcased by Tesla require external air sources and as such are only suitable for already well supplied hospitals. The proposed design features self-contained air pumps and can operate with no external supplies other than electricity. This makes this new design suitable for use in field hospitals and in under-developed and under-supplied medical facilities such as those in Canada's north. Further, due to the robust design these units will travel well as they are self-contained, stackable, and resistant to damage due to shocks and movement.
Project Activity, Methodology and OutcomesThe primary focus of this collaboration is to develop a robust, constructable ventilator to provide autonomous mechanical ventilation to patients while commercial ventilators are unavailable due to the ongoing extraordinary demands for such units. We focus on three principal design requirements during the development.
Design PrinciplesSafety
Safety is the primary concern of any medical device. To this end the emergency ventilator has full redundancy of design. There are no single points of failure in the software or electrical systems and the design features only 5 mechanical moving parts. The system is self-monitoring and will raise alarms if any pressure or timing aberrations occur.
Accessibility of Components
The main goal of this design is to be manufacturable with no specialized equipment and minimal expertise. As such all components are sourced from outside of the standard medical supply chain. All components are already commercially available and exist in large quantities in existing supply chains. In order to avoid using medical grade air bellows and complex stepper motor constructions we have designed around using air/water displacement in a sealed reservoir to provide positive pressure ventilation action.
Simplicity
There is no time for extensive calibration and testing of new technologies in the current emergency. As such this design is kept as simple as possible. Wherever additional non-safety related features introduce complexity these features have been reduced or removed to ensure the system is quick and simple to source, build, and train users on. The system features only 4 buttons and a display to adjust breaths per minute and volume per breath.
Mechanical Design OverviewThe PRAXIS Emergency Ventilator operates by pumping water between two reservoirs. The reserve reservoir is a soft container of variable volume. This container acts as a reserve of water and provides no resistance to flow in and out of this container. The soft reservoir contains at least one, and preferably two, water pumps attached by water hose to the rigid container. These pumps will pump water into the rigid container during the inhale, positive pressure, phase. The rigid reservoir contains at least one (preferably two) similar water pump(s) which act to return water to the soft container during exhale (low pressure) phase. The rigid container is airtight and has 4 check valve assemblies attached to it two are input valves, one is an output valve and one is an over-pressure preventions safety valve.
The first check valve on the rigid reservoir allows air to pass into the unit as water is pumped out of the rigid reservoir. The second check valve is the same but has attachments to allow an oxygen hose to be attached. Optionally a solenoid can be attached here to increase control over oxygen levels. The third valve allows air to flow to the patient during the high pressure phase. Finally the safety valve is present to provide a final, mechanical, protection against overpressures which pose a threat of inducing barotrauma in the patient.
The system features mechanical push buttons and an LED display showing the breaths per minute and volume per breath. These are read continuously by the MCUs and allow mechanical control over the patient's breathing profile.
Design Block DiagramSoftware Design OverviewThe PRAXIS Emergency Ventilator control unit orchestrates the intake and outtake water pumps and solenoid valves to achieve the appropriate breathing rate according to the user-specified (1) breaths per minute and (2) breath volume. It also monitors system pressure to prevent barotrauma or patient-ventilator breathrate mismatches.
Reliability and safety are the primary considerations in the software design. As a result, a number of features were included to ensure maximization of these objectives.
When inputs are read, the value is checked against the previously read value and a sanity check is performed. If the values differ significantly, the software will determine whether to perform additional readings. This is intended to prevent incorrect readings caused by potential electrical fluctuations.
To increase redundancy in the system design the microcontroller unit (MCU) is physically duplicated as well. There are two complete control units arranged in a master/slave configuration. The master unit is the active default, while the slave unit is idle. Both MCUs machine state and timing is perpetually synchronized, so a failover can happen and not disrupt the breathing rhythm. The master MCU is constantly sending data to the slave MCU. As a result, when a disruption in incoming data is detected by the slave MCU, it takes over control of the peripherals. In the case of a failover, an audible buzzer is triggered to alert the user of the event.
In addition, each MCU has a dedicated watchdog timer (WD) to detect and recover from malfunctions. The WDs constantly monitor a timing signal from their respective MCUs and can issue a power reset if a timing discrepancy or drop in voltage is detected.
For additional resilience the MCUs are running significantly underclocked providing a much wider acceptable operating voltage before a brown-out occurs. The MCUs are rated to 16MHz but are running at 1MHz which corresponds to an acceptable voltage level of 1.4-5.5V.
The software providing the timing is based of extremely low-level operations on timing registers and uses interrupt based timers which, due to having the lowest niceness level possible, ensure that the breath timing is the #1 task for the MCU at all times.
All program code is MISRA C++ compliant.
Electrical Subsystem Design OverviewPRAXIS Emergency Ventilator is intended for rapid development and rapid manufacturing, with wide available components. As this device is to be used in life supporting activities, care must be taken to design with the highest level of reliability possible. Redundancy is the key factor for systems with rapid development, and synergy among mechanical, software and hardware is essential. The hardware development is divided in two parts, first the energy management system (EMS), and second the control and actuators.
The EMS is used to power all the elements on the unit with within their specifications, and the main element on this design is the VMS-60 AC-DC internal power supply unit (PSU), which is compliant to IEC 60601-1 edition 3.1 safety standards for 2 x MOPP applications and 4th edition EMC requirements. Furthermore, the AC-DC PSU is backed up by a second PSU with the same model, for redundancy.
In case the patient needs to be moved to another area of the hospital, and the mains needs to be disconnected, the EMS has a battery backup which is automatically switched. The battery used in this case is an 60Ah lead-acid battery, which is capable of powering the system for up to 5 hours. Furthermore, there is a battery charger to ensure the battery is always full. The EMS is compatible with any car battery, and it will accept lithium-ion batteries with minimum adjustments via software.
After ensuring reliable sources of energy, the EMS produces their other power rails to feed all the components. A dedicated 3.3V is generated through a low noise buck converter, and it is used to power the MCU and its WD. A dedicated 5V buck converter is used to power all the 5V logic and ICs necessary in the system.
As the design provides reliability by introducing redundancy, all blocks in bold are redundant, in other words, they are doubled. For example, the buck converts are doubled and dedicated to each MCU and WD used in the design. The same is valid for the EMS and AC-DC PSU.
The novelty of this design is the control of the air flow and pressure created by the movement of water between two tanks, using conventional water pumps and valves. The figure below represents the control and actuators portion of the design, the system is redundant with all the essential subsystems doubled. If the master system fails, the slave system will take action and will keep operation through different physical paths, ensuring reliability and non-stop operation. The WD is used to detect MCU errors and to monitor MCU voltage, and if a failure is detected, the WD resets the MCU and the slave MCU assumes control over the system seamlessly, until the master MCU restores its operation.
The controllable actuators are the water pumps and the solenoid valves, and they are driven by dedicated ICs. All of these elements are connected to the MCU GPIOs and special peripherals, such as PWMs.
The user interface is provided by GPIO controlled buttons to adjust the breaths per minute, and air flow/pressure necessary.
There are two main focuses for evaluation on this design. First and foremost is the medical validation that the design is sufficient in both safety and capacity for medical needs. The design is focused on providing mechanical ventilation with pressures-to-patient no higher than 40cm H2O at a rate of at least 40 breaths per minute and 500 mL per breath under normal conditions. To evaluate this the proof of concept will be attached to a virtual “patient” computer which will monitor flow rates and pressure for no less than 24 hours. During this period the design must provide continuous mechanical ventilation with the following parameters within 2% of expected.
Breaths per minute
- Breaths per minute
Litres per breath
- Litres per breath
Pressure to patient
- Pressure to patient
Breathing profile (length of inhale, exhale, and pause)
- Breathing profile (length of inhale, exhale, and pause)
The initial proof of concept has been completed and is undergoing confirmation testing; it will be submitted to our consulting partners at ventureLab and Ontario Tech University for review once initial results are available. Once appropriate validation has been completed the system will be submitted to health canada for review via the fast-tracked review process.
Test ResultsCurrent RunTime: 97hrs continuous -(May 1 @1:30)Flow and Pressure CharacteristicsFlow (SLPM) and Pressure(PSIx30) graph. This data was taken after 48hrs of continuous runtime with the exhaust tube terminating in a vat of water > 40cm deep. The noise at peak flow and the spike in initial pressure corresponds to air bubbles releasing through the liquid. Peak pressure is 0.9psi and average pressure is ~0.45PSI breathing rate is ~30BPM. 📷
Long Run Test ResultsThe following results are taken from the alpha test of the praxis emergency ventilator on april 30 after approximately 82 hours of continuous runtime. The dataset is 1809 breaths over the course of approximately 60 minutes. These histograms show the reliability and precision of the system over this representative period.
Data is available for review at: https://docs.google.com/spreadsheets/d/11xvEwokMxB-Rx4pOc2TGkDajsjyQg5As9EAra3SKVyg/edit?usp=sharing
Average pressure outputNo over-pressure relief valve was featured in this testing apparatus to facilitate measurement of the pressure characteristics. Much of the noise in the average pressure output can be attributed to the stochastic nature of the exhaust bubbling through the water column used as a surrogate lung.
As the graph below shows the average volume per breath is ~425 ml. This is a point of improvement being currently worked on. This volume increases to ~550ml at atmospheric pressure. As such we can conclude a sufficient amount of water is being displaced and that it is the pressurization of the over-large testing pressure-chamber (solid reservoir) which is causing the low flow volume. Additionally a new pump-pipe configuration is being trialed to improve flow rates during breaths.
Here we see the Inhale and exhale times for the system. The settings on this initial test unit are analog and as such are not highly accurate to a given time due to human error (corrected in the next version by using digital control). These results show how accurate the system is. The standard Deviation for the inhale time is ~42ms and for the exhale it's ~46ms.
The central objective of our ventilator project is to get as many stop-gap ventilators to hospitals and medical professionals as quickly as possible. In order to achieve this we have undertaken to provide ventilators at cost and to provide all production designs free of cost including software, parts lists, and blueprints. We currently store our developing software on github at https://github.com/praxispd/EmergencyVentilator. Additionally once hardware testing is completed we will post the above listed information both there and to our website praxispd.com.
In addition to this we are tapping into our regional network as well as the networks of our consulting partners to ensure the device can be properly evaluated by medical professionals and to facilitate the rapid approval of our device by Health Canada. We will work with these partners to make any changes deemed necessary and to ensure the product can still be quickly built and deployed.
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