This system will mix supplied oxygen with ambient air to deliver the desired oxygen concentration .
The system consists of five parts:
- a solenoid valve
- a Sensirion SDP31 flow sensor
- A 2 liter latex buffer bag inside the base of the machine
- A 10cmH2O overpressure valve is used to protect the bufferbag from overinflation
- A flow limiter on the oxygen intake line, that is adjusted according to to the pressure of the oxygen source
A one-way valve is used on the air intake line. Between the bellow and the oxygen system another one way valve is placed. This way, the oxygen system doesn’t influence the bellow.
The flow limiter is added to limit the oxygen flow to a maximum of 130 lpm. This flow rate has been tested to be safe in case of an open failure of the valve.
FLOW RESTRICTOR CALIBRATION TABLE
| Pressure (bar) | Flow restrictor thickness (mm) | Flow (l/min) | Flow (l/min) |
| 5 | No restrictor | 250 | 325 |
| 3 | 220 | 285 | |
| 2,5 | 156 | 176 | |
| 2,4 | 150 | 169 | |
| 2,3 | 129 | 130 | |
| 2,2 | 109 | 117 | |
| 2,1 | 82 | 85 | |
| 2 | 11 | 33 | |
| 2 | No restrictor | 120 | 195 |
| 3 | 110 | 154 | |
| 2,5 | 77 | 90 | |
| 2,4 | 75 | 82 | |
| 2,3 | 66 | 71 | |
| 2 | 5 | 13 | |
| 1.5 | No restrictor | n/a | 80 |
OPERATION
As the inhalation starts, the solenoid valve opens to let oxygen into the bufferbag.
The desired volume is injected into the buffer bag, measured with the flow sensor and the valve closes.
Because the bellow is being pressed down at this moment in the cycle, the pressure in the bellow is higher than in the buffer bag (40 vs 10cmH2O) and there won’t be any flow from the solenoid valve to the patient at this point.
Once exhalation starts, the bellow inflates and the oxygen is sucked out of the buffer bag. Once the buffer bag is emptied, air will be sucked in through the second one-way valve.
If the oxygen system fails, this is detected in software by comparing measured flow rate vs expected flow.
A 10cmH2O overpressure valve prevents possible overinflation of the buffer bag. This valve has a high enough flow rate to dump the inflowing oxygen in case the solenoïd valve fails open.
The oxygen inlet valve is only activated during inhalation to prevent direct contact of the patient with the high flow oxygen.
The flow sensor constantly measures the flow through the oxygen valve.
If there is no flow during valve opening, an alarm is triggered. If the valve should be closed and there is still flow detected, an alarm is also triggered.
The main disadvantage of this system is the longer period needed to adjust to the desired value, as only one correction is made in one breathing cycle.
A key factor for this setup is the duration of the inhalation. During this period, the oxygen needs to fill the buffer bag sufficiently before the exhalation starts. This limits the possible inlet pressures. The current system design allows for inlet pressures between 1,5 and 5 bar with the needed flow limiters.
In these conditions 100% Fi02 can be achieved at the minimum interval (35 BPM at I/E 1/2).
