Supplementary Oxygen
Lets now look at factors that can affect how the administered oxygen reaches the lungs.
In a patient with respiratory failure, the inspired oxygen concentration (FiO2) that is administered does not accurately reflect the concentration of oxygen that reaches the alveoli. The final concentration reaching the lungs depends upon the patient’s peak inspiratory flow rate (PIFR), respiratory rate, respiratory pattern and how well the mask fits the patient’s face.
Of these factors, PIFR is particularly important.
In health, the PIFR is ~ 40 – 60 litres/minute. Remember lung volumes we discussed in Week 1? In respiratory failure, however, there is a marked increase in PIFR of up to 200 litres/minute. The significance of this is that when the PIFR exceeds the flow rate of supplemental oxygen provided, then extra gas will be entrained. This extra gas is from room air around the mask. As room air contains only 21% oxygen, the effective concentration of oxygen reaching the lungs is diluted.
The exception would be in oxygen delivery devices with a reservoir bag (non-rebreather mask). In this case, the extra gas entrained would be oxygen from the reservoir bag and not the atmosphere. The drop in oxygen concentration reaching the lungs is therefore minimised by these devices.
Click next to move on to Oxygen Delivery Systems
Introduction to Acute Respiratory Failure
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