Oxygen Bleed with CPAP
Many CPAP users also require supplemental oxygen. The air that we inhale on a day to day basis is made up of 21% of oxygen, 78% of nitrogen and 1% of trace elements such as argon, carbon dioxide, neon, helium and methane. For the purposes of this article, fractions and percentages will be used interchangeably for ease of explanation.
Sometimes, 21% of oxygen may not be enough to maintain adequate oxygen saturations. In these situations, supplemental oxygen can be administered via various oxygen delivery devices. It may be important to increase the "fraction of inspired oxygen" (FiO2). Introducing oxygen into a respiratory ventilation circuit allows the concentration of oxygen to be increased, potentially increasing the FiO2 to 100%. In non-invasive CPAP and BiPAP applications the delivered oxygen is much lower due to dilution from the CPAP venting rate that exceeds respiratory requirements.
This article will show how to calculate the final oxygen concentration in a CPAP/BPAP circuit at the mask, and the effect of PAP pressure on flow rate for any rate and concentration of oxygen added to the circuit.
Oxygen Bleed Adapter
The Oxygen Bleed Adapter is inserted into the CPAP/BPAP circuit to allow oxygen to be mixed into the respiratory circuit. Oxygen is always provided at higher pressure at a set flow-rate, so the dose can be calculated. Several bleed adapters are available, and they are inexpensive. The images below show oxygen bleed adapters for both standard and heated tubing.
Physical Characteristics of an Oxygen Bleed
Concentrated oxygen is generally delivered at pressures in excess of 15 psi, and is metered by Flow. CPAP air pressure is delivered at pressures less than 30 cm H2O. 1-PSI is 70 cm-H2O, so the oxygen concentrator flow is fully delivered to the CPAP circuit. A typical oxygen bleed circuit delivers between 1-liter/minute (L/min) to 3-L/min. The typical CPAP or Bilevel respiratory circuit flows 30 to 80 L/min of air volume in the absence of large leaks. (our example will assume 30 L/min). Higher CPAP flow rates will decrease the FiO2 for any oxygen flow rate. So the use of a full face mask with its higher leak rate, will require more oxygen bleed than a nasal mask at the same pressure to achieve the same results.
What is the oxygen concentration in the diluted CPAP circuit?
The equation to calculate concentration from flow is a simple mass balance:
(oxygen flow rate (litres) at 100% O2) + (CPAP air volume (litres) at 21% O2)) / Total air volume = oxygen concentration Percent in circuit
Note that the design leak rate for any mask at a given pressure does not increase with an oxygen bleed, so CPAP air volume (Vcpap) must be reduced by the amount of flow from the oxygen generator in L/min, which gives us the following:
Determination of FiO2 (fraction of inspired oxygen) with CPAP:
FiO2=((Oxygen Flow L/min x 100%) + (Vcpap L/min - Oxygen Flow L/min) x 21% O2)) / Vcpap therefore:
at 1-litres/minute oxygen addition at 100% and assuming a total Vcpap of 30 L/min:
(1-L/min x 100% O2) + (30-1 L/min x 21% O2) = 709/30-L/min = 23.63% Oxygen.
at 3-litre/minute oxygen at 100% and assuming a total CPAP flow of 30 L/min:
(3-L/min x 100%O2) + (30-3 L/min x 21% O2) = 867/30 L/min = 28.9% oxygen.
Any variable can be changed based on available data. If the flow rate is 40 L/min from the CPAP, the oxygen concentrations will be lower. If leaks are present or pressure results in a higher average vent-rate, the oxygen concentrations will also be lower. If a mask with less vent rate is used, then concentrations of oxygen will increase.
For oxygen concentrators, it is important to use the oxygen percent output by the concentrator. While bottle oxygen is pure at 100% oxygen, a concentrator may put provide a concentration of 75% to 80%. The proper value can be determined from the machine specifications and input to the formula.
xPAP Flow Rates vs Pressure
The following chart shows the expected flow rate for several Resmed mask models and types at different flow rates. These flow rates are for a perfect circuit without unintentional mask leaks or mouth breathing, and so are a conservative estimate of the total flow in an xPAP circuit for different types of masks. It is clear that different mask types have different expected flow rates. Nasal masks have the lowest designed leak rate, followed by nasal pillows, and finally full-face masks. Most new masks include an owner's manual that provides the exact flow/pressure relationship at the back of the manual.