3.2 The Triple H’s
Noticeable fluctuations within the breathing loop gas composition can be defined by the Triple H’s: hypoxia, hyperoxia, and hypercapnia. This book is not intended to be the sole source for rebreather physiology, so I will note only a short definition of these conditions as they apply to recognition of a potentially hazardous condition that requires diver response.
Hypoxia
Cause
Hypoxia can occur should the oxygen supply fail, and the diver breathe down the oxygen partial pressure to below normoxic levels. Hypoxia can also occur by ascending with too low a pO2 in the breathing loop, or when breathing a hypoxic gas on open circuit.
Signs and Symptoms
The diver loses consciousness with little warning apart from behavioral patterns, which may be hard to identify. The onset will be rapid and, in the case of a complete lack of oxygen, can be fatal.
Treatment
Provide and allow the diver to breathe ample diluent bailout and air or oxygen as soon as he is recovered to surface. If the occupant is not breathing and or is having difficulty breathing, artificial respiration needs to be applied immediately. Seek the advice and attendance of trained medical professionals.
Prevention
Ensure the oxygen supply is supplied from a qualified source with documentation. Analyze the oxygen prior to use. Pre-dive and test the oxygen supply equipment: cylinders, regulators, etc. Ensure the correct flow rate of oxygen is being used. Properly maintain and test the oxygen regulators on a regular basis. Do not allow the onboard oxygen supply pressure and volume to be depleted below the operating guidelines.
Hyperoxia
Cause
Oxygen toxicity refers to the harmful effects of breathing oxygen at high partial pressure levels where the oxygen becomes toxic to the lungs and other body tissues. The result of breathing elevated concentrations of oxygen is called hyperoxia. Elevated oxygen partial pressure would be caused by a major failure of the oxygen supply, delivering too much oxygen to the diver’s atmosphere. This would displace the desired atmospheric fraction of oxygen and thus elevate the partial pressure. Hyperoxia is also dose dependent, meaning that lengthy exposure to elevated pO2s may result in adverse effects.
Signs and Symptoms
Signs and symptoms may vary from day to day in the same individual and between individuals.
Symptoms may include:
● Convulsions
● Visual disturbances or impairment
● Ears ringing
● Nausea
● Twitching
● Irritability
● Dizziness
Treatment
Oxygen toxicity is managed by reducing the elevated oxygen levels in the breathing atmosphere.
Provide the diver with fresh air/diluent as soon as possible. Prevent the diver from injuring himself during a seizure, but do not severely restrict his movements. Keep the diver under observation for a minimum of 12 hours. Seek the advice and attendance of a medical professional.
Prevention
Maintenance of the oxygen supply system mitigates risks of hyperoxia. The diver should also rehearse shutting down the high-pressure oxygen supply to stop delivery of oxygen to a failed oxygen supply system. Dives should be planned such that the cumulative oxygen dosing is within recommended limits and personal tolerances.
Hypercapnia
Cause
The saturation of the carbon dioxide absorbent, water intrusion into the absorbent, or channeling within the canister due to improper filling may cause the scrubber to fail and thus lead to high carbon dioxide levels. Prolonged exertion causes heavy breathing that exceeds the carbon dioxide scrubbing capabilities of the system and may also cause a high carbon dioxide level.
Signs & Symptoms
Mild symptoms include:
● Muscle twitching
● Reduced neural activity
● Flushed Skin
● High blood pressure
Elevated symptoms include:
● Headache
● Lethargy
● Elevated or irregular heartbeat
● General distress, panic, sweating
● Convulsions and unconsciousness
Treatment
Reduce physical activity and increase the supply of oxygen. Diver should breathe open-circuit gas until atmosphere is adequately flushed of CO2 concern. Return the diver to the surface or other shelter in order to supply fresh air/gas or oxygen. Recovery is generally rapid, although a headache may persist. Seek the advice and attendance of a medical professional.
Prevention
Ensure the carbon dioxide absorbent is fresh, dry, packed correctly, and do not exceed the endurance of the canister. The scrubber should be refilled completely for each and every dive.