Nitrox Physiology

1. Nitrox Physiology

2. Introduction • We will cover • Partial pressure • Pressure ‘T’ • Oxygen use • Oxygen toxicity • Nitrogen

3. Partial Pressure • Pressure each gas exerts in a mix (Dalton’s Law) • At constant temperature, in a mixture of gases each constituent gas exerts a partial pressure in proportion to its percentage in the mixture • On surface pressure 1 Bar • Air is 21% oxygen, 79% nitrogen • ppO2 is 0.21 bar • ppN2 is 0.79 bar • EAN36 is 36% oxygen, 64% nitrogen • ppO2 is 0.36 bar • ppN2 is 0.64 bar

4. 0 Metres 1 Bar Absolute Pressure x 2 Volume x 1/2 10 Metres 2 Bar Absolute Pressure x 3 Volume x 1/3 20 Metres 3 Bar Absolute Pressure x 4 Volume x 1/4 30 Metres 4 Bar Absolute Boyle’s Law

5. Partial Pressure Pressure at depth: P = (D / 10) +1 where P = Pressure (Bar) & D = depth (metres) Air breathed at ambient pressure Example: On air at 25m ppO2 is 0.21 x 3.5 = 0.74 bar On EAN36 at 25m ppO2 is 0.36 x 3.5 = 1.26 bar

6. ppg Fg P The Pressure ‘T’ Three variables: • Partial pressure of gas – ppg • Decimal fraction of gas in mix – Fg • Pressure – P • Use Pressure ‘T’ ppg = Fg x P P = ppg/Fg Fg = ppg/P

7. ppg Fg P The Pressure ‘T’ for nitrox ppg = Fgx P P = ppg / Fg Fg = ppg / P Partial pressure of gas – ppg Fraction of gas in mix – Fg Pressure – P

8. ppg 1.4 Fg 0.32 P P The Pressure ‘T’ for nitrox • Examples…  • On a dive holiday you are given a 32% mix (EAN32) What depth is this safe to dive to? • To find the MOD you need to use the ppg of 1.4 bar for O2 and the % of the mix in your cylinder ~ 0.32 P= 1.4 / 0.32 = 4.4 Bar therefore the max depth for this mix is 34m

9. ppg 1.4 Fg Fg P 4 The Pressure ‘T’ for nitrox • Examples…  • You want to dive to 30m what is the best nitrox mix to use for this depth? • To find the mix you need to use the ppg of 1.4 for O2 and the P of the dive depth 30m (4 bar)  Fg= 1.4 / 4 = 0.35% therefore a 35% mix is good for a 30m dive

10. ppg ppg Fg .28 P 5 The Pressure ‘T’ for nitrox • Examples…  • You have a 28% Nitrox mix. You want to dive to 40m.Is this a good mix for this depth? • You know percentage of O2(28%) & P of the dive depth 40m (5 bar), need to find the ppg ppg= .28 x 5 = 1.4 therefore a mix of 28% is good for a 40m dive (just about)

11. Oxygen Use • Survive ppO2 of 0.16 to 0.50 • ppO2 0.16 and under – hypoxia • ppO2 0.50 and over – hyperoxia • High ppO2 and time – toxicity • Oxygen Toxicity hit • Central Nervous System (CNS) • Quick hit • Pulmonary Toxicity • Slow hit

12. Table 1Oxygen Limits for Divers Partial Pressure (bar) Effect <0.10 Coma or Death 0.10 Unconsciousness 0.12 Serious Hypoxia 0.16 First signs of Hypoxia 0.21 Normal air environment 0.35 Normal saturation exposure 0.50 Maximum saturation exposure 1.40 CFT Recreational MOD Limit 1.60 CFT Decompression Gas Limit 2.40 60/40 Nitrox therapy gas at 6 Bar 3.00 50/50 Nitrox therapy gas at 6 Bar

13. Oxygen Toxicity Limits • CFT recommendations: • 1.4 bar – maximum partial pressure of oxygen for recreational diving. • Used to determine MOD

14. NOAA Table 2National Oceanic and Atmospheric Administration • Depth and time • Safe exposure times - maximum • Higher ppO2 – shorter exposure • Rules • 2 hour break (single exposure) • 12 hour break (daily exposure) • Breaks on air • CFT recommend that dives are calculated to remain within 80% of the limit as a precautionary measure

15. Table 2 NOAA

16. Oxygen Toxicity Two types – Pulmonary ~ Slow hit – Central nervous system ~ Quick hit Pulmonary toxicity • Whole body toxicity • Long term exposure to oxygen • Irritate lungs, dry cough • Not considered problem for recreational diving

17. Oxygen Toxicity • Central Nervous System (CNS) • Toxicity increases with ppO2 • Acute symptoms – no warning • Limits exceeded – CNS toxicity • Predisposing factors - fatigue-workload-temperature-hydration • Symptoms as per ConVENTID

18. CNS Symptoms Con = Convulsions V = Visual disturbances E = Ears - e.g. tinnitus (ringing in ears). N = Nausea T = Twitching of muscles, especially facial I = Irritability D = Dizziness, lack of coordination

19. Oxygen Convulsions • Tonic phase – rigid • Clonic phase – spasms • Breathing ceases • Ascend when spasms cease • Expel air from lungs • Risk of drowning • No lasting damage • Keep within recommended levels

20. CNS O2 Hit ~ Treatment • Surface may not be first option • Ensure regulator is in victim’s mouth • Allow convulsion to cease • Ascend at safe rate • Treat as unconscious diver • Embolism poses greater risk over drowning and DCI

21. Oxygen Toxicity • Factors affecting oxygen toxicity • ppO2 • Depth • Time • Carbon dioxide retention (dilate blood vessels) • Exercise • Temperature • Fatigue • Dehydration • Cannot build tolerance to oxygen

22. Nitrogen • Narcotic effect • Decompression sickness • EANx mixtures – less nitrogen • Balance oxygen & nitrogen

23. Safety • Stay within 1.4 bar oxygen limit • Stay within maximum exposure time • Stay in thermal balance • Breathe normally • Keep exercise to a minimum • Don’t dive when fatigued • Hydrate properly • Use decompression schedules correctly • Reset nitrox computer before each dive

24. Summary • Covered Nitrox physiology: • Partial pressure • Pressure ‘T’ • Oxygen use • Oxygen toxicity • Treatment • Nitrogen

25. Questions?