Minimalist Pressure-controlled Emergency Ventilator

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.  

Figure 1 – Schematic representation of the proposed emergency ventilator.  

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.