Oxygen - effects during diving when descending and at depth

• Oxygen
  • oxygen toxicity, involving convulsions, becomes a problem when oxygen partial pressure is too high
    • the NOAA Diving Manual recommends a maximum single exposure of 45 minutes at 1.6 bar absolute, of 120 minutes at 1.5 bar absolute, of 150 minutes at 1.4 bar absolute, of 180 minutes at 1.3 bar absolute and of 210 minutes at 1.2 bar absolute
  • oxygen toxicity becomes a risk when these oxygen partial pressures and exposures are exceeded. The partial pressure of oxygen determines the maximum operating depth of a gas mixture
  • high-pressure oxygen has several effects on the body
    • leads to an increase in ventilation and a decrease in alveolar and arterial carbon dioxide buffering tension due to a rise in central venous carbon dioxide
      • this in turn is due to the reduction in carbon dioxide capacity of haemoglobin overcoming the decreased carotid body excitation
    • a vagally mediated bradycardia and vasoconstriction of the intracranial and peripheral vessels
    • there is a small fall in cardiac output
    • central nervous system (CNS) toxicity can be seen in the diving situation when closed and semi-closed rebreather sets are used
      • divers may be exposed acutely to a high partial pressure of oxygen either through equipment failure or from going too deep and thereby raising the partial pressure of oxygen in the breathing gas to an unacceptable level
      • also can occur in sport divers using high partial pressures of oxygen (PO2) in order to decrease decompression time for a particular dive profile
      • the diving physician is most likely to encounter CNS toxicity when treating decompression illness in the recompression chamber using a high PO2 (typically 2.8 atm)
• symptoms of CNS toxicity, can be remembered using the acronym VITBEND:
• V, Visual abnormalities, such as tunnel vision.
• I, Irritability. Can include anxiety, confusion or fatigue
• T, Twitching, usually in the lips or other facial muscles
• B, Breathing. Difficulty in taking a full breath
• E, Ear problems, including tinnitus and vertigo
• N, Nausea, sometimes intermittent
• D, Dizziness, including clumsiness and incoordination
• most dramatic manifestation is the grand mal type convulsion
  • consciousness is maintained right up to the time of the convulsion - the convulstion does not give rise to any electroencephalographic (EEG) changes (1)
    • exercise and immersion in water are two conditions that can lower the threshold to CNS toxicity
    • third potentiating factor has been suggested to be carbon dioxide buildup, often occurring due to the "skip" breathing pattern adopted by many divers in order to conserve breathing gas, in which the diver takes a breath, holds it for as long as possible, and then slowly breathes out

Notes:

• atmospheric pressure at sea level is 1 atm (equivalent to 101.3 kPa) and is the pressure that is experienced by all parts of the human body at sea level
• 1 atm = 1.01325 bar = 101.3 kPa

Reference:

• 1) Edge CJ. Recreational diving medicine.Current Anaesthesia Critical Care 2008; 19 (4): 235-246.