Effects of exposure to high pressure (hyperbaria)

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Effects of exposure to high pressure (hyperbaria)

• dangers stem from changes in gas volumes within
  enclosed spaces and increased solubility of gases
• Pressure increases 1 atmosphere (760 mm Hg) for
  every 10 m (33ft)

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Barotrauma

• tissue injury caused by changing pressure
• human body has limited ability to distend and
  compress
• trauma comes from exceeding those limits

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• Boyles Law applies here vol. of a gas decreases
  or increases as a diver goes up or down
• pressure is not allowed to equalize with air from
  outside the space

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Gas Toxicity

• gases like CO, O2, CO2, N2 and He can be
  dangerous under certain circumstances

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CO dangerous for all life forms

• compressed air may have been contaminated by
  exhaust fumes
• danger due to high affinity for hemoglobin 240
  more times the affinity than O2
• problem is compounded if diver is a smoker or
  exposed to air pollution

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O2 Toxicity

• O2 at high pressure is toxic to all life forms --
  depends on its concentration and length of
  exposure
• physical exercise speeds up development of
  toxicity
• principle sites of O2 toxicity are lungs and CNS

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• Pulmonary symptoms include
• substernal distress with soreness in chest
• airway resistance on inspiration
• histological changes in alveoli
• pulmonary edema
• flushing of the face
• cough that starts out dry and gets wet

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• CNS symptoms include nausea
• contraction of the field of vision
• Convulsions
• lack of sphincter control
• Unconsciousness
• death
• can also cause arrhythmias
• mechanism that causes it is unknown

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• hyperbaric O2 may interfere with CO2 transport
• at high pressure, more O2 is dissolved into the
  blood
• hemoglobin doesnt desaturate thus isnt
  available for O2 transport
• increased CO2 vasodilates cerebral blood vessels
  causing acidosis and increased PO2 in the brain
• cell function may also be disrupted as well as
  neural transmission in CNS

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CO2 Toxicity

• most common in closed-circuit scuba systems and
  hose-supplied helmets
• inadequate respiratory exchange leads to
  hypercapnia with heavy exercise at high pressures
  
• happens if diver tries to suppress their Ve in
  order to conserve air, CO2 builds up

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Symptoms of CO2 Toxicity

• Uncomfortable breathing
• Headache
• Mental deterioration
• Violent respiratory distress
• Unconsciousness
• Convulsions

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Nitrogen Narcosis

• some gases exert a narcotic or anesthetic effect
  at high pressure
• effects depends on the partial pressure of the
  gas and its solubility in the bodys tissue and
  fluids
• nitrogen can cause condition nitrogen narcrosis
  30 m (100ft)

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Progression of symptoms

• Euphoria
• Impaired performance
• Weakness
• Drowsiness
• Unconsciousness

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• caused by interference in the transfer of signals
  across the neural synapses
• this is why the use of compressed air is limited
  to 50 m (165 ft)
• replace nitrogen with helium
• below 150 m (500ft) can cause neuromuscular
  disorder called high-pressure nervous syndrome
  (HPVS) tremors, vertigo and nausea

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• slowing compression rate during dive and adding
  nitrogen to O2 mixture can help prevent this
• nitrogen narcosis is a limiting factor during
  deep dives
• almost impossible to avoid in commercial dives
• slows down information processing in the brain,
  but does not distort perception
• slowing down activity level can help

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Decompression Sickness (the Bends)

• caused by a nitrogen bubble formation in the
  tissue due to too rapid of an ascent
• symptoms include itchy skin, fatigue, pain in the
  muscles, joints, and bones, perspiring, nausea

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• more serious ones include respiratory distress,
  ataxia (loss of muscle coordination), vascular
  obstruction, paralysis, unconsciousness, and
  death
• called the chokes when it affects the lungs and
  the staggers when it affects the CNS
• symptoms appear about 1 hour after surfacing but
  can occur either immediately or up to 12 hours
  post

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• if decompression is too rapid, N2 returns to
  gaseous state and bubbles form in blood and
  tissues
• decompression tables help divers figure
  absorption levels
• decompression time increases with depth and
  length of dives
• must also consider body fat, age, physical
  condition, gas mixtures, alt of dive

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Hyperbaric exercise

• Studies are conducted in hyperbaric chamber or
  underwater
• Chamber provides opportunity to isolate variables
  like partial pressures, temperature, and gas
  mixtures
• can simulate ocean dives too, to help understand
  differences

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Biological measurements difficult in hyperbaric
environment

• expensive equipment and facility
• technically exacting
• open water measurements are complex b/c of
  restraints of aqueous environment

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Factors adding difficulty to exercising underwater

• increased air density
• cold
• decreased efficiency
• CO2 retention
• inert gas narcosis

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Ve may be limiting factor b/c maximal voluntary
Ve decreases with depth

• results in progressively smaller difference
  between exercise Ve and max capacity

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• higher densities of air increase flow resistance
  in scuba equipment and airways, causes
  hyperventilation which leads to retention of CO2,
  increased work to breathe
• ability to increase expiratory flow rate is
  limited
• after reaching max flow rate, further effort
  results in partial airway collapse

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O2 consumption increases with submax work with
increasing depth

• increased energy cost of breathing
• maintaining body temp.
• movement in higher hydrostatic pressures

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Experienced divers can achieve 91 of their
land-measured max O2 but work efficiency is
reduced

• max capacity is dictated by tolerance to high
  levels of CO2 and of max O2 consumption
  attained before reaching critical PCO2

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Swimming angle and drag produced by scuba
equipment greatly affects energy cost of
underwater work and individual differences in
swimming efficiency
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Diving bradycardia

• HR decreases as water temp. decreases and
  pressure increases
• divers should not use land-measured relationship
  between HR and O2 consumption, dangerous
• HR can be used to estimate energy cost in diving
  only when HR/VO2 relationship is know for a
  certain diver at a certain depth

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Strength decreases with muscle temperature below
25C
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Commercial Diving Methods

• Scuba is most widely known, but not used much
• Surface demand diving commonly used at depth 50m
  (164 ft)
• Diver connected to reinforced hoses, air supplied
  form surface
• Atmospheric diving (submarine with robot arms)

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Saturation diving exposure to hyperbaria

• uses He-O2 gas mixtures
• most commonly used method below 50 meters
• divers become totally saturated with inert gases
  after 24-36 hours
• after that, further exposure doesnt require
  additional decompression time
• divers live in decompression chamber when not
  working