The Science Journal of the American Association for Respiratory Care

2007 OPEN FORUM Abstracts

PERCEPTIONS OF NON-REBREATHING MASK FIO2 VS. ESTIMATED FIO2 USING THE REVERSE ALVEOLAR AIR EQUATION

K. Moody1, C. Trippe1, D. Pursley1, A. Light1

Background: We surveyed 38 randomly selected RRTs and RNs and asked them the following question: “What FIO2 does a non-rebreathing mask (NRBM) deliver to a patient’s airway at 12-15 L/m?” The 20 RRTs had an average of 20 years of experience, while the 18 RNs had a mean of 12 years of experience. On average, the RNs said the NRBM gave 96% while the RRTs believed that it gave 84%, with 32 of the respondents answering “90-100%”. However, the reality is that “modern disposable non-rebreathers normally do not provide much more than approximately 70% oxygen” (Wilkins, et al., 2003, p. 840). Because the disposable NRBM is actually a low flow oxygen system, we agree that it delivers a more moderate FIO2 and thought it would be interesting to calculate the estimated FIO2 from the PaO2 of persons breathing oxygen from this device.

Method: We recruited ten healthy, non-smoking, normothermic volunteers between 21-29 years of age and had them breathe oxygen from a NRBM at either 12 L/m or 15 L/m. The subjects were told to relax and breathe normally for a period of fifteen minutes. At the end of the fifteen minute period, we performed a radial artery blood gas and measured PaCO2 and PaO2 using an IL GEM 3000 blood gas analyzer. Assuming our subjects had normal cardiopulmonary anatomy and physiology, we estimated PAO2 by dividing PaO2 by a normal a/A ratio of 0.9 to reflect a 10% higher partial pressure of oxygen in the alveolus than in the arterial blood (Wilkins, et al., 2003, p. 231). Knowing approximate alveolar partial pressure of oxygen, we then calculated FIO2 based on what we call the reverse alveolar air equation: FIO2 = [(PaO2 ÷ 0.9) + (PaCO2 • 1.25)] ÷ (PB - 47).

Results: Our results indicate that the NRBM delivered an average FIO2 of 0.60 in young, healthy non-smokers. FIO2 varied from 0.53 to 0.71 in our ten subjects.

Conclusion: Contrary to the opinion of the healthcare professionals in our survey, the NBRM tended to deliver more moderate concentrations of oxygen in our young, healthy volunteers. Patients with high minute ventilation placed on a NRBM will theoretically receive an even lower tracheal FIO2 due to additional dilution of room air with oxygen. In this situation, high flow devices which meet or exceed the patient’s inspiratory flow demand may provide a more suitable method of delivery to give high concentrations of oxygen.

Wilkins, et al. (2003). Egan's Fundamentals of Respiratory Care, 8th Edition. Mosby.

Age/Gender L/M Height VE
(L/m)
VT
(mL)
f PB
(mmHg)
PaO2
(mmHg)
PaCO2
(mmHg)
Estimated
FIO2
28/Female 12 5'8" 6.41 712 9 730 292 33 0.53
23/Female 12 5'7" 8.10 810 10 730 299 37 0.55
29/Male 12 6'2" 6.40 582 11 730 301 40 0.56
22/Male 12 5'7" 6.55 595 11 730 316 42 0.59
25/Male 12 5'9" 7.45 497 15 730 293 42 0.55
25/Female 15 5'6" 6.17 560 11 730 396 39 0.71
26/Female 15 5'1" 11.2 800 14 730 397 40 0.71
21/Male 15 6'1" 12.8 853 15 730 304 34 0.55
26/Female 15 5'9" 8.90 890 10 730 335 37 0.61
25/Male 15 6'2" 11.6 967 12 730 349 41 0.64



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