Revision as of 14:50, 11 March 2018

INTRODUCTION

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.

With a canula delivery, the approximate correlation between oxygen flow rate and FiO2, is similar to the table below:

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 40 to 80 L/min of air volume in the absence of large leaks. (our example will assume 60 L/min)

What is the oxygen concentration in the diluted CPAP circuit?

The equation to calculate concentration from flow is:
(oxygen flow rate (litres) at 100% O2) + (air volume (litres) at 21% O2)) / Total air volume = oxygen concentration Percent in circuit
therefore:
at 1 litres/minute oxygen addition at 100% and assuming a total CPAP flow of 30 L/min:
(1-L/min x 100) + (30-L/min x 21) = 730/31-L/min = 23.55% Oxygen.

at 3 litre/minute oxygen at 100% and assuming a total CPAP flow of 30 L/min:
(3-L/min x 100) + (30-L/min x 21) = 930/33 L/min = 28.18% 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 be lower.

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.

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