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Description of Preliminary Layout for Open Source Pandemic Ventilator Design PDV1 070319
The design consists of a pressure regulator, 3 valves, a bellows unit, a manometer pressure sensor unit, some switches and alarms, a PLC and a PC.
This design is optimized to be able to be built with readily available components. Other than the tubing that connects to the patient, (which would be sterilized and reused) none of the components are specifically “medical parts”. It is expected that supplies of components specifically designed for medical devices will be in very short supply in a pandemic situation, and would not be available. Later on I will try a version using a commercial pressure sensor instead of the manometer system described. This will allow greater control flexibility of the device. The design philosophy is such that the physical components can be constructed fairly easily. All the parts are commodity items and easily available (you could steal em from factories if you had to) even if all production and supply ceases in the pandemic. The software design for the PLC and PC will be more complex, but will be freely available as open source including source code.
The Pandemic Ventilator is designed to meet AARC guidelines for Acquisition of Ventilators to Meet Demands for Pandemic Flu for alarms and for ease of use. I have not yet added humidification, oxygen concentration control or PEEP to this design but these additions will probably not affect the final design too much. In concept it is a system that essentially takes something similar to an ambu-bag manual ventilator and adds enough automation and safety control systems to bring it up to the AARC acquisition guidelines.
Description of Components (see drawing)
- A bellows unit is constructed from plywood or sheet plastic using a standard door hinge at one end. More exact plans for the bellows will be published once I build the first prototype.
- A plastic bag (possibly a drainage bag) is placed in the lower portion of the cylinder, with the opening of the bag connected to the system tubing with Tuck tape. The bag supplies the sealing properties for the bellows, and may have to be changed on a regular basis if it wears out.
- The bellows will have a magnet attached at the top to activate the position sensors.
- MS1 and MS2 are magnetically activated sensors (either Hall effect or magnetic switch) that sense when the piston has reached the top and bottom of its travel. MS2 is a fixed sensor, and MS1 can be adjusted to allow for adjustments in volume per cycle.
- A weight is placed on the top of the bellows in order to allow the bag to deflate and push air into the patient’s lungs during the inspiration phase. The amount of weight on the bellows divided by the area in contact with the bag determines the maximum pressure that can be developed.
- V1(NC), V2(NC), and V3(NO) are ¾” or ½” solenoid valves (or similar).
- The manometer is a U shaped tube filled with water. It’s height in relation to the measuring point and the length of vertical tubing are adjusted in order to be able to measure the limit maximum and minimum pressures expected. A magnet is inserted into some foam in order to make it float at the water surface. The water level limits (which indicates the pressure limits) is monitored by magnetic sensors clipped to the outside of the tubing. The pressures can also be marked on the tubing so that the operator can have a visual indication of the maximum pressure achieved. I will publish more exact specifications as to the height and length of the tubing once I build and test the prototype.
- An outlet filter is connected to the exhalation line in order to prevent infection of staff with flu virus.
- Interconnections are with standard plastic pipe and flex hosing with gear clamps.
- The PLC should have a minimum of 7 inputs and 5 outputs. The PLC should also have a communications port (RS232) in order to communicate with the PC. Software to operate various popular brands of PLC will be developed and made freely available. Various controls and alarms are connected directly to the PLC.
Description of PLC Control Algorithm (see drawing)
(Starting from the pressurized air source)
- The pressure regulator reduces the supply pressure to a reasonable working pressure
- The PLC opens V1 and closes V2. V3 is open to let the patient exhale.
- Air enters the bellows and raises the piston. The PLC will sense MS2 opening and begin a timer. If the allowable window of time interval between MS2 opening and MS1 closing is not achieved, a malfunction alarm is generated, stopping the cycle.
- When the magnet reaches MS1, V1 closes.
- When the low pressure limit of the manometer is triggered or enough time has elapsed since the last cycle based on the selected inhalation to exhalation ratio or set time, V2 opens and V3 closes to inflate the patient’s lungs. The pressure limit points on the manometer are monitored by the PLC. If the pressure rises too high, an overpressure alarm is generated and V2 is closed and V3 is opened to protect the patient from overpressure.
- The piston begins to drop and MS1 is opened. The PLC starts a timer. If the time interval between MS1 opening and MS2 closing is too short a line disconnection alarm is generated. If the time interval is too long, an obstruction alarm is generated. When the piston drops and MS2 is closed again, V1 opens, V2 closes and V3 opens to allow the patient to exhale again.
- The PLC also monitors the cycle rate over time and generates an alarm if the selected control parameters are exceeded.
- The PLC generates audible and visual alarms as required. Audible alarms can be set to sound different for different alarm conditions. The Stop, Mute and Reset/Resume keys allow for control of the ventilator.
Description of PC function (see drawing)
- The PC is connected to the PLC via an RS232 connection. This connection transfers status data from the PLC to the PC and also allows the PC to input set points and controls. The PLC does not absolutely require the PC to maintain operation, but will generate an audible alarm if the communication is interrupted.
- The PC will have visual interface software that mimics popular existing ventilators in appearance and operation. Non supported operations will be crossed out on the interface. This allows for easier retraining and higher comfort levels among operators of the equipment. The supported equipment type is selected when the software is installed.
- The relevant operating and alarm parameters are entered into the PC and they are transferred to the PLC registers.
- The PC can also perform diagnostics on the operation of the PLC in order to enhance total safety. The PC can also generate it’s own visual and audible alarms.
- The UPS generates an alarm at power failure and supplies backup power as well. If the PC is a laptop with a battery, it will also be able to supply power failure alarms.
Still to be done
Oxygen, Humidification, PEEP design
Build and test a prototype with PLC only.
Add PC control functionality
I could use some help, so far it’s me, my son, and some friends at work (in dialysis) developing this.
Please be patient if I do not have all the terminology right, and comment or email tell me if I am making some drastic mistakes or you have some insight that would let me make this thing much better. I knew almost nothing about ventilators three weeks ago