Commit df880a02 authored by Rahul's avatar Rahul
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Added common problems in Ambu bag respirators

parent 89c4b194
......@@ -87,13 +87,44 @@ The team is currently converting the design into a manufacturable version that c
## Mechanical Design
#### Common problems assosiated with Ambu Bag Automation devices
**1. Life of Ambu bag (BVM)**
- Bag Valve masks are designed to be used for a short time like in an emergency or when moving the patient. They are not designed for continous operation over long periods. We might get a few hundred thousand cycles of operation before it fails completely. The average time of a pateient in the hospital is about 14 days, of that he amy need ventilation for about 7-10 days. A 20bpm rate equals 1,200 cycles an hour and 28,800 cycles a day.
*Possible Solution* - Using good quality bags, and stress releiving all the hoses in the breathing circuit can help increase the life of the bag. The machine should also not put too much pressure on one part of the bag.
**2. Mechanism of Actuation**
- Many creative designs for actuating Ambu bags have come up lately, a few ground principles are, use mechanisms and materials that are simple and robust. If we are really trusting the health of a patient using the device, we don't want our Hobby servo's to fail or our Rasberry Pi's freezing. A minimalist approach, means less things to break. Keep in mind the million of cycles it has to work continously.
- Ventilation is not a fixed state, the parameters of opeartion need to be continously adjusted, so strict CAM based mechanism with only one setting cannot be used in a live setting. The compression of the Ambu bag must be controlled, both position and velocity.
- Steppers make for excellet open loop controls, but care must be taken so that acompaniying electronics are robust and can survive the continous use.
- Mechanisms with lots of moving components are troublesome as, it is only as strong as the weakest link.
*Possible Solution* - Using industrial grade components that are rated for continous operation are a good starting point. In our mechanism, we choose a Car wiper motor, as they are simple and rated for 3-5 million cycles of operation, but there is a problem that it is not backdrivable.
**3. Dead space in the Breathing circuit**
- All BVM based designs suffer from a common problem, their limited Tidal volumes,they are designed to be used at or near the patients mouth, if we connect an Ambu bag based machine with a long tube to the patient, there is a serious problem called 'Dead space' in which the patient breaths in the same air that was expelled, for a 1m tube, this might be as big as 300ml.
*Possible Solution* -One way to solve it is to keep exhaust valve as close to patient as possible. Another way is to use a dual limb ciruit made out of two **'Y-connectors'** the Y- connectors have one way valves in them. this means the BVM constantly delivers air to the patient using one tube and the exhaust tube is fed via another Y-connetor to the 'Peep' valve.
**4. Measuring Pressure in the breathing circuit**
- There is host of ventilator induced injury of which Barotrauma is a main concern, we cannot raisethe pressure in the sytem beyond a certain value, most comonly 40CmH20. Provisions must be made to measure the pressure and to ensure that the system does not over pressure the lung.
- To main pressures, we need ot measure and display are, Peak inspiratory pressure and Plateau pressure.
- Peak inspiratory pressure is the max pressure reached in the inspiration cycle. Plateau pressure is the pressure in the circuit when the inspiration has finished. They both are vital sources of information indicating the state of the patient.
**5. Filtration of exhaust**
- Due to the contagious nature of COVID 19, filtration of the breathing circuit and exhaust is highly recomended. Using a HEPA filter near to the patient is recomended to keep the system and exhaust free from containation.
**Note 1-** We have seen some designs online, which have wide plates and compress the Ambu Bag fully. From our testing, we found that this may not be the best approach. The Ambu bag has a cylindrical middle section where the bag can be depressed. There are two ring like structures which holds the bag's shape and help it return to bak to the original position. When the whole bagis compressed, the ring structures wil get fatigued and the bag will fail to return to its original shape, this is prominent at higher bredths per minutes, and tidal volumes.
Hence we opted to model our mechanism after the human hand, with the thumb being the arm that the machine moves. The bag is supported in a convex shape by the base structure. Though this does create two concentration points on the bag, we believe this will be a better approach for the life of the bag.
**Note 2-** All BVM based designs suffer from a common problem, their limited Tidal volumes,they are designed to be used at or near the patients mouth, if we connect an Ambu bag based machine with a long tube to the patient, there is a serious problem called 'Dead space' in which the patient breaths in the same air that was expelled, for a 1m tube, this might be as big as 300ml. One way to solve it is to keep exhaust valve as close to patient as possible.
Another way is to use a dual limb ciruit made out of two 'Y-connectors' the Y- connectors have one way valves in them. this means the BVM constantly delivers air to the patient using one tube and the exhaust tube is fed via another Y-connetor to the 'Peep' valve.
### Version 3 with Clamp type Arm, DC WormGear Motor, feedback sensors and enclosure
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