Hyperbaric Physiology
- Created by: anniecritchlow
- Created on: 13-05-19 15:15
Free diving
Swimming underwater on a single breath
1 ATA = 760mmHg
Each 10m you descend you add 1 ATA
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
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
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
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
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
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
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
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
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
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
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
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
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
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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