In the initial versions of the Breathney prototype ventilator an ‘Ambubag’ was used to create the necessary airflow and pressure. One-time use (SpurII) and reusable ambubags (MkIV) were both used in the prototypes (both seen on picture 1).
Although the use of these ambubags seems a no-brainer certain expectations, set by the MHRA (Medicines and Healthcare products Regulatory Agency) and the WHO (World Health Organization), are to be met in order to obtain the necessary certification to use this newly developed medical equipment on real patients. These objectives, consisting of system and ambubag lifetime requirements, can be seen in table 1. For the Breathney prototype, we aim to meet the MHRA expectations.
|Standard||Explicit system requirements||Ambubag design specs|
|MHRA||VTmax = 800ml|
RRmax = 35 BPM
t = 14 days
|WHO||VTmax = 1000*/2000**ml|
RRmax = 60 BPM
t = 2 years warranty (replaceable parts?)
|N = 63072000|
V = 1000/2000ml
The ‘N’ in the specifications stands for the no. cycles the bag needs to outlive, at the bare minimum. This number is the translation of the cycles that the machine would do when working non-stop at an output volume (‘V’) of 800 ml during 14 days (maximum hospitalization time of a COVID-19 patient).
Therefore tests were done to check whether or not the bags were fit for the job. Testing was done by running the plunger system, in three different versions, until a failure would occur in the bag. A failure was defined as damage to the integrity of the bag causing the inability to pump air or to create sufficient pressure. The three different setups used for testing: a wide plunger version, a narrow plunger version and a symmetrical plunger version (all seen on pictures 2-4). Both types of ambubags (one-time and reusable) were tested in the three setups
The bags were tested using the setup seen on picture 5. A piston is responsible for actuating the bags simultaneously. The results of the tests can be seen on table 2.
|Test ID||Bag Type||Plunger Type||Displaced Volume||Peak Pressure||No. Of Cycles Until Failure||Failure Mode|
|C1||MkIV||WV2||800||27-33||133000||tear near large end cap|
|C2||MkIV||WV2||800||27-33||5700||tear near large end cap|
|D1||MkIV||NV1||800||27-33||53700||tear in middle of bag|
|D2||MkIV||NV1||800||27-33||200000||tear in middle of bag|
|E1||SpurII||NV2||800||27-33||<30000||bag no longer inflates|
|E2||SpurII||NV2||800||27-33||<30000||bag no longer inflates|
|F1||MkIV||NV2||400||27-33||<942000||small tear in the middle|
|F2||MkIV||NV2||400||27-33||<942000||small tear in the middle|
|G1||SpurII||NV2||400||27-33||<30000||bag no longer inflates|
|G2||SpurII||NV2||400||27-33||<30000||bag no longer inflates|
NV, WV and SV
Respectively narrow version, wide version and symmetrical version.
Two reusable `Ambu Mark IV’ bags are simultaneously tested. Both bags show a large tear near the large end cap suggesting the material has to be stretched beyond the fatigue limit. Inspecting the bags shows that the tear coincides with the sharp edges on the indenter.
This setup uses the significantly narrower indenter to combat the high amount of stretch. Two reusable `Ambu Mark IV’ bags are simultaneously tested. An improvement can be seen in the actuation cycles before failure. Interestingly the tear didn’t form close to the end cap as seen in the previous tests but closer to the middle of the bag. This might suggest that the material is either over-stretched in a specific spot or the material fails due to friction wear. The propagation of the tear also happens slower than seen in the previous tests making the detection more difficult.
This setup tests two non-reusable `Ambu Spur II’ bags.The hypothesis being that the smaller end caps would decrease the amount of stretch.The actuation rate was reduced to allow the bag to fully inflate. Both bags fail to inflate after a small amount of cycles. This might be due to `crazing’ or the formation of microscopic cracks changing the material’s properties locally. This type of failure is probably due to the different material used, SEBS plastic for the non-reusable bags compared to silicone for the reusable bags.
F & H
This setup tests a reduced displacement to check the hypothesis that the over-stretching only happens at the highest displacement and reducing the displacement might significantly reduce wear. Four reusable `Ambu Mark IV’ bags are simultaneously tested. It is clear that the bags last signi_cantly longer with similar failure mode.
The same reduced volume test has been conducted using 2 non-reusable`Ambu Spur II’ bags.The same failure mode was observed making it clear this type of failure predominates in this type of bag.
I & J
Four reusable `Ambu Mark IV’ bags are tested at a displacement of 800ml with symmetric actuation. The symmetric actuation essentially halves the amount of stretch which clearly extends their lifetime. However, it should be noted that as a side-effect of this type of actuation the bags started rotating (roughly 10°/ actuation). This distributes the wear equally around the bag possibly greatly increasing the lifetime.
Two reusable `Ambu Mark IV’ bags @ displacement of 800ml symmetric actuation. In this setup the bags are not allowed to rotate to observe the effect of the rotation.