Up to date, expert answers to frequently asked questions (FAQ) about oxygen supply systems, respiratory care and pulse oximetry written by OCC & collaborators.
Non-invasive ventilation (NIV)
What is the correct mask and circuit setup to deliver CPAP/NIPPV?
It depends on several factors. Not all masks, circuits or devices are compatible or able to deliver CPAP/NIPPV.
The basis of non-invasive CPAP/NIPPV is a constant gas flow against an expiratory resistor that creates positive “back pressure” in the circuit (i.e. Pressure = Flow x Resistance). Back pressure is increased either by increasing gas flow against the resistor or increasing the resistance to gas flow.
There are 2 circuit options for non-invasive CPAP (or NIPPV/BiPAP):
- A “true” single limb circuit (commonly found in home NIPPV/BiPAP devices) has 1 hose directing gas from the device to the mask and no active expiratory valve. There are two common setups for NIPPV/CPAP with a true single limb circuit. (Note: circuits with both an inspiratory limb and a short expiratory limb with expiratory valve, while commonly referred to as ‘single limb’ are not ‘true’ single limb circuits)
- “Vented Mask” setup – The exhaled breath has to get out of the patient/circuit somewhere, therefore, the mask must act both as the resistor and the exhalation port. A vented mask has holes that allow for fixed resistance. Because the resistance is fixed, CPAP is increased by increasing flow through the circuit.
- Fixed leak valve circuit setup – An alternative option to using a vented mask, is to use a non-vented mask and to place the fixed/intentional leak into the circuit by adding a ‘leak valve,’ which contains holes that allow fixed resistance.
- A dual limb circuit (commonly found in ICU and transport ventilator) has 2 hoses connected by a “Y” (or “wye”) adapter where the expiratory limb directs gas flows to an active expiratory valve that controls expiratory resistance. The expiratory valve either is clearly visible in the expiratory hose (“expiratory manifold”) or the expiratory hose attaches directly back into the ventilator (i.e. the expiratory manifold is built into the ventilator). To function properly (ie to create/adjust CPAP, or pressure-support/ BiPAP) the circuit must only be used with a non-vented mask (i.e. the ventilator will control both gas flow and resistance).
- Troubleshooting Note: Using a vented mask with any dual limb ventilator circuit will result in the inability to effectively create sufficient positive pressure, increased aerosolization and likely set-off alarms.
Some ventilators can utilize both true single limb and dual limb setups. Another approach to determine the correct mask type for NIPPV/CPAP is to determine if the system (ventilator or circuit) has an an active exhalation valve:
- System with active exhalation valve – non-vented mask required
- System without active exhalation valve – vented mask required (or fixed leak adapter in the circuit)
When access to optimal equipment is not readily available, alternative solutions (https://link.springer.com/article/10.1007/s41782-020-00092-7) have been proposed and reported with uncertain outcomes. These are most often not in line with manufacturers’ recommendations and are not endorsed here. Always check manufacturer recommendations for all devices to confirm correct equipment and mode settings.