Hyperbaric Physiology

Free diving

Swimming underwater on a single breath

1 ATA = 760mmHg

Each 10m you descend you add 1 ATA 

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Boyles Law

At a constant temperature, the volume of gas in inversley proportional to the pressure to which it is exerted

Not good for air spaces in the body

Due to changes in pressure and volume

Depth, pressure and volume:

0m, 1 ATA, 1L

10m, 2 ATA, 0.5L

20m, 3 ATA, 0.33L

30m, 4 ATA, 025

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Implications of Boyle's Law

Ear pressure:

Pressure on outside of eardrum increases, pushes inwards, and can rupture unless diver equalises pressure via valsalva manouvre (forcing air into middle ear)

Mask Squeeze:

Pressure changes in airspace of mask can **** or push on the eyeballs causing damage - equalised by breathing into mask through nose

Lung squeeze:

Volume of airspace decreases with depth, can collapse lungs and cause rupture of capillaries and internal bleeding

Blood shift also occurs, shifting blood from other parts of the body to the lungs to resist collapse

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Dalton's Law

The pressure exerted by a mixture of gases is equal to the sum of pressures which would each exert if it alone occupied the space filled by the mixtures 

This is equal to partial pressure

PO2 at sea level = 160mmHg

Partial pressure = Total pressure x % of total pressure

PO2 will decrease as diver descends - if falls below level required for consciousness the diver will pass out

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Mammalian Diving Reflex

Found in all mammals and humans

Bradycardia most common manifestation

When face is immersed the cardiorespiratory centre of the brain slows the heart rate to conserve oxygen

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Factors affecting maximum breath hold time

World record is 22 minutes

Main stimulant is high CO2

Other factors: low O2, cold shock, swallowing, larger lung volume, chest wall afferent, diving response, breathing movements

Hyperventilation - increases breath hold time but is dangerous

Reduces initial arterial PCO2 and raises cerebrospinal fluid pH but has little effect on arterial PO2

This means it takes longer to reach stimulus for breath

The urge to breath will be triggered by low PO2 but this may be after PO2 has fallen below level to support consciousness

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SCUBA Diving

Self contained underwater breathing apparatus

Allows for longer submission times than free diving

Air is compressed to 200ATA - and delivered at ambient pressure on demand

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Henry's Law

The greater the pressure, the more gas will dissolve into the liquid

So when pressure is reduced, more gas will come out of the system

There are several problems associated with breathing gases at pressure 

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Nitrogen Narcosis

Nitrogen dissolves into the blood and tissues 

Has a similar effect to an anaesthetic on the nervous system 

At deeper depths can cause stupor and unconsciousness

This response is influenced by conditions 

For example - darkness, cold, nervousness, fatigue

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Air embolism

Air expands as a diver ascends 

Unless the diver breathes out during ascent the lungs will be damaged

Air embolism = excessive stretching of alveolar membrane forcing microbubbles into circulation

These can aggregate and lodge in the brain or vital organs

Spontaneous pneumothorax - tearing of alveoli due to excessive expansion, then air escapes into lung and causes them to collapse and through ruptured plural membrane to accumulate in tissues and organs

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Decompression sickness

At depth nitrogen dissolves into the blood and tissues so the greater the depth and duration, the more dissolved in

It dissolves more easily into fat than to muscle

If ascent too rapid - dissolved N2 comes out too quickly and forms bubbles in body tissues and fluid

Symptoms usually appear 4-6 hours after dive

Dizziness, itchy skin and joint pain and can result in paralysis and death in minutes

Treatment - oxygen therapy on site, decompression chamber

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Oxygen Toxicity

At sufficient pressure and exposure time - oxygen can cause functional impairment and ultimately chemical destruction of cells

Partial reduction of oxygen by one electron to form superoxide or two to form hydrogen peroxide

i.e. free radicals is the basis for oxygen toxicity

Inspiring gas with PO2 of more than 2 ATA increases chances of oxygen toxicity

Divers should not breathe pure oxygen at depths of more than 7m

At extreme depths special gases mixtures are needed to avoid it

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Mixed gases

Helium is most common inert gase to substitute nitrogen with in deep diving

It doesnt induce narcosis and is more easily eliminated from body

Low density reduces breathing resistance at depth 

Changes voice characteristics and increases heat loss

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Thermal Considerations

Water conducts heat away from the body

There is a risk of hypothermia

Passive systems - wet/dry suits slow the heat loss

Active systems - for longer dives:

Electrically heated suits, hot water suits

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Saturation Diving

Divers live in steel surface chambers pressurised to that of the depth they are working at

The nitrogen in the air they breath is replaced with helium - so the body is saturated with helium gas

Divers essentially remain at depth for over a month

At end of working period the chamber is brought back slowly to 1 ATA

In order to allow gas to diffuse out safetly, it takes about 6 days from 170m

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