I've received several questions on twitter about this. There are a variety of ways to do it, depending on available materials…
(1) Filter attached directly to the mask
The simplest way to achieve this is to attach the filter directly to the mask, as shown below.
This should work well – provided that the filter and mask are adaptable to one another.
This staved off some intubations for us and saved us time to preoxygenate for those needing it. Bipap mask -> viral filter -> etco2 tubing -> BVM bag -> PEEP valve on expiration port -> wall O2. #medtwitter pic.twitter.com/4Wz7S0eM5g
— Michelle Romeo, MD (@doctormromeo) April 1, 2020
(2) Two-limb closed system with a full-featured mechanical ventilator
This involves attaching the BiPAP mask to a traditional mechanical ventilator (with both inflow and outflow tubing). Filters are placed on both the inflow and outflow limbs, generating a completely closed system.
The advantage is that this generates a 100% closed system (assuming no mask leak, of course).
The disadvantage of this approach is that it involves using a full-featured mechanical ventilator to perform CPAP or BiPAP. Since the ventilator isn't designed for this purpose, it may have inferior leak compensation and trigger sensitivity, when compared to a dedicated BiPAP unit (e.g. a Respironics V60 units).
(3) One-limb system with a specialized noninvasive machine (e.g. Respironics V60)
This involves the following:
- BiPAP mask is connected to a specialized noninvasive machine.
- One viral filter is attached to the inflow limb.
- A second viral filter is attached to the exhaust port, as shown below:
Here's a similar setup (image from tweet by Armstrong Medical).
- A dedicated noninvasive machine (e.g. Respironics V60) may have superior triggering capacity and leak compensation, compared to a traditional mechanical ventilator.
- Using a V60 for non-intubated patients will free up a full-featured ventilators for use with intubated patients.
- Some BiPAP devices do have a built-in one-way valve (example below). This is designed to allow air into the tubing if there is negative pressure. This valve is intended to prevent the patient from asphyxiating in the event of catastrophic device failure (e.g. if the BiPAP machine stopped functioning or the tubing became occluded).
- So this isn't a 100% closed system. However, as long as the BiPAP machine is working, there should be positive pressure within the tubing – so this one-way valve should remain closed.
And make sure you're using the right filters!
- Not everything that looks like a viral filter is a viral filter (heat and moister exchangers shouldn't be confused with viral filters).