|Positive Inspiratory Pressure (PIP)||10-40||cmH2O|
|Positive End Expiratory Pressure (PEEP)||0-20||cmH2O|
|Pure oxygen operation|
|Safety pressure relief valve||0-45||cmH20|
|Breathing rate||12-25||breaths per minute (bpm)|
|Inspiration/expiration time ratio (I/E)||1:1 to 1:3|
The development of this project is possible due to the contribution of LIP, UNIDEMI, FCT NOVA, Nova Medical School, ICNAS, Harvard University and two Portuguese engineers working for the Fórmula 1 teams.
The design tries to minimize the use of technical components and those used are common in industry, so its construction may be possible in times of logistical disruption or in areas with reduced access to technical materials and at a moderate cost. Most of the device can be manufactured with modest technical means.
It should be clear that the devices presented are meant to be for proof-of-concept only and it is not implied that they may be suitable as bedside instruments. Further engineering will be required for that purpose, adapted to the local conditions. As the components proposed are not medical-grade, the use of a ventilator made on this basis should be considered only as a last resort solution.
Principle of operation
The schematic representation of the proposed emergency ventilator is shown in Figure 1. Pure oxygen at the standard pressure of 4 bar (400 kPa) is fed from the hospital supply to an adjustable pressure regulator with output range of 10 to 40 mbar, allowing the PIP pressure to be set by just turning a knob. The regulator output is fed to the inspiration electrovalve V1. This valve should have enough of an aperture for the air to pass through easily at normal breathing flows.
A closed deposit (R1) may be placed on this line to provide a reserve of pressurized air for faster pressurization of the lungs upon inspiration. This helps to make the pressure profile more rectangular in shape, which is clinically desirable. The output of V1 connects to the C1 water column, to the M1 water manometer and to the patient inspiration tube, which is a consumable item normally provided by the hospital. The C1 water column provides a safety purge to the atmosphere in case of any malfunction that may cause a dangerous overpressure of oxygen to the patient. It can be set by adjusting the water level H1 to be slightly above the maximum intended PIP. In normal operation there should be no gas flowing through this column. To fulfill its safety purpose, the connecting and inner tubes should have sufficient diameter for a considerable gas flow to pass through without overpressure.
The M1 U-tube water manometer measures the PIP and PEEP pressures directly in cmH2O by mere observation of the water height difference H3. The tube length should comfortably exceed the intended maximum PIP. Optionally, for more precise measurement, near critical damping of the water column oscillations can be achieved by adjusting the needle valve V3. Even then, it was observed that the column only reached a stationary state in each breathing stage for the 12 bpm rate, so the absolute pressure adjustments should be made at such rate or lower. A more viscous liquid would probably also improve this characteristic.
If filled with a conductive liquid (e.g. salted water), the sustained presence or absence of the liquid at certain levels is perceptible (with the help of simple electronics) by electrodes placed across the tube. Both high and low PIP and PEEP alarms can be implemented.
From the patient "Y-piece" the expiration tube connects to the expiration electrovalve V2, similar to V1.
Finally, the expiration air is vented to the atmosphere through the water column C2. The water level H2 directly determines the PEEP pressure.
The valves must be electrically commanded with adjustable rate and duty-cycle in the ranges stated in the introduction. There are several electronical solutions for this functionality, adjustable to the availability of components.
Two slightly different devices were built independently for cross-check purpose.
A note is in order here to justify the extensive use of water columns in this design: The water column is a device with many interesting flow-control characteristics: (a) it has no moving parts except the water itself, so it is very reliable and easy to build; (b) it regulates air pressure in the tens of cmH2O range accurately, adjustably, and quite independently of the flow; (c) if made of transparent materials the observation of the difference in water levels between the two vessels provides direct information of the differential pressure; and (d) it provides check-valve functionality up to a certain reverse pressure.
Besides being bulky, the main drawback of the water column is the water itself, as its level must be adjusted/monitored and may become bacterially contaminated, requiring anti-bacterial treatment. The water may be replaced by other suitable substances such as low viscosity oils.
It is clear that the functionality provided by the water columns can be performed more conveniently by traditional pneumatic components, if available.
The details have been systematized in a scientific paper that is now available for consultation: Proof-of-concept of a minimalist pressure-controlled emergency ventilator for COVID-19. The patent has been registered in the name of Humanity, so that no one can benefit from this innovation.
This research paper describes two designs of the emergency ventilator.
Homepage of the volunteer project that formed the basis for the Minimalist Pressure-controlled Emergency Ventilator.