The Physics of Diving

Tracking Oxygen Exposure

When diving, avoid CNS toxicity by staying within a maximum pp of about 1.4 ata.

Pulmonary oxygen toxicity:-
NOAA Oxygen Limit for a Single Exposures
PO2 in ata              Time in minutes
0.6                               720
0.7                               570
0.8                               450
0.9                               360
1.0                               300
1.1                               240
1.2                               210
1.3                               180
1.4                               150
1.5                               120
1.6                               45

example:-                   1.2ata = 210 minutes
so 30% exposure   = 210 / 100  x  30  =  63 minutes.

Calculating Nitrox Fills (not required for IDC)

For an empty tank:-

O2 required  –  O2 in top up gas
——————————————    x  Final pressure of fill
N2 in top up gas

32  –  21
————   x  230  =   32.02 Bar
79

Henry’s Law:-

1.         The quantity of gas dissolved in a liquid is proportional to the partial pressure of the gas.
2.         The amount of gas that will dissolve into a liquid at a given temperature is directly proportional to the partial pressure of that gas.
3.         Gas dissolves into a liquid until the gas balances with ambient pressure. This is called saturation. The gas entering the liquid is called gas tension.
4.         If ambient pressure rises, more gas dissolves into the liquid until reaching saturation at the higher pressure. The difference between the partial pressure of gases in contact with a liquid and the gas tension within the liquid is referred to as the pressure gradient.
5.         If pressure drops and gas tension becomes greater than ambient pressure, the liquid is said to be super saturated and gas will come out of solution until equilibrium is reached again.
6.         The higher the temperature of a liquid the less gas can be held by that liquid.

7.         Nitrogen absorption by the body is a direct consequence of Henry’s Law.

More Physiology

Blood
Plasma:-        Carries nutrients, chemicals, dissolved gases including some waste carbon         dioxide and nitrogen. Plasma accounts for half the weight of blood.

Red blood cells (erythrocytes):-
Carries most of the oxygen required via haemoglobin, a protein that bonds and unbonds with oxygen. Without haemoglobin, the blood would have to circulate the plasma 15 to 20 times faster to supply a resting body with sufficient oxygen. Red blood cells make up about 45% of the blood.

Haemoglobin:-
Carries and releases oxygen efficiently because variations in oxygen partial pressure influences its capacity to remain bonded with oxygen.
5% of carbon dioxide the blood carries is in solution in the plasma, 20% combines with haemoglobin and 75% transports in the form of bi-carbonates.

Cardiovascular Structure:-
The heart, arteries, veins, and capillaries make up the cardiovascular structure. The heart moves blood through arteries, arteries branch into tiny capillaries, the capillaries lead to veins that return the blood to the respiratory system and heart to begin again.

The heart:-
A 4 chamber pump. The upper chambers on each side are called the atria.
The atrium receives blood and then pumps it into the ventricles below. The ventricles then pump blood away from the heart.
Left side of the heart is larger than the right side.
Left side supplies the whole body with blood.
Oxygen poor blood returns to the right side of the heart.
The carotid arteries branch off the aorta immediately and supply blood to the brain.
Arteries carry blood away from the heart.
Veins carry blood toward the heart.
Capillaries are microscopic vessels between arteries and veins; gas exchanges occurs in the capillaries.

Blood pressure:-
The pulse rate measures the frequency of the heart rate. An average adult at rest is 60 to 80 beats per minute. Blood flows into the arterial system in pressure surges that coincide with heart contractions. The systolic pressure is the surge, or high point as the heart contracts, and diastolic pressure is the lull after the surge, or low point when the heart relaxes.

Adrenaline:-
During stress or fright, the adrenal gland releases adrenaline into the circulatory system as part of the survival response of the body. Adrenaline stimulates the heart, constricts blood vessels, and increases the breathing rate to prepare the body to flee or fight a threat.

The Respiratory System

1.         The respiratory system brings oxygen to the body and carries waste carbon dioxide out.

2.         The process of using Oxygen is oxidative metabolism.

3.         Breathing starts when the body detects a carbon dioxide increase and / or an oxygen decrease in the blood.

4.         The reflex respiratory centres in the brain have primary command of breathing.

5.         The peripheral chemoreceptors monitor oxygen levels and signal the respiratory centres when the partial pressure of oxygen declines below a specific level.

6.         Carbon dioxide primarily controls breathing.

7.         If the peripheral chemoreceptors detect low oxygen, but the reflex respiratory centre finds the carbon dioxide acceptable, the respiratory centre may not stimulate breathing. If the oxygen level is normal, but the carbon dioxide level is high, this will cause the respiratory centre to start breathing.

8.         When a breath is required, the respiratory centre stimulates the diaphragm to flex downward causing a pressure decrease, causing in intake of breath.

9.         The air passes the mouth, nose, and sinuses, which moisten and filter the air.

10.       Air passes the epiglottis in the throat into the trachea (windpipe). The epiglottis acts as a valve between the trachea and oesophagus (which leads to the stomach). The trachea branches into the right and left bronchi, which lead to the lungs. Within the lungs the bronchi branch down to smaller passages, called bronchioles. The bronchioles terminate in the alveoli, which are sacs surrounded by pulmonary capillaries. This is where gas exchanges with the blood.

11.       Blood at no point comes into contact with the air.

12.       The respiratory system normally uses only 10% of the available oxygen in each breath.

13.       Breathing rate is normally 10 to 20 times per minute.

14.       The amount of air breathed in and out is called the tidal volume.

15.       Vital capacity is the maximum volume that can be inhaled after total expiration.

16.       Residual volume is the air left in the lungs after complete exhalation.

17.       Dead air spaces are the portion of the diver’s tidal volume that plays no direct part in gas exchange.

18.       Without equipment, dead air spaces consist of the sinuses, trachea, and bronchi.

19.       There is a 15% to 20% reduction in tidal volume caused by water pressure compressing the thorax.

20.       At depth, air becomes denser. Dense air is less smooth at a given speed. i.e. a poor lamina flow. The air becomes turbulent in the trachea and bronchi with air moving faster in the centre leading to swirls that rotate away from the direction of travel at the sides increasing resistance.

21.       This resistance has a benefit in that it causes a slight pressure increase in the bronchioles and alveoli during exhalation. This increase helps keep the air passages from collapsing after exhalation.

22.       Resistance causes more problems than benefits. The faster the air moves, the denser the air, and the more irregular the air passage, the more energy must be consumed to overcome resistance due to turbulence. Therefore, conserve energy and breath slowly to minimise resistance caused by turbulence in the airways.

23.       An involuntary response to breathing is called bradycardia, or slowing of the heart. Apneaic bradycardia is triggered when by cold moisture on the face, which slows the heart rate.

Breath Hold Diving

1.         During breath hold diving the circulatory system uses oxygen stored in the lungs, muscles and blood to supply tissues.

2.         Accumulating carbon dioxide creates urges to breathe and eventually it is too great to ignore, forcing the diver to surface and breathe.

3.         Hyperventilating (3 to 4 times) can increase breath hold time. Doing so reduces circulatory carbon dioxide so it takes longer to accumulate enough to stimulate breathing. Excessive hyperventilation may lead to shallow water blackout.

Shallow Water Blackout

1.         Caused by excessive hyperventilation, depleting a divers carbon dioxide level.

2.         At depth, a divers body consumes oxygen faster than carbon dioxide accumulates to stimulate breathing.

3.         Depleted oxygen causes no problem at depth because high pp allows haemoglobin to bond with the remaining supply.

4.         The diver ascends, the pp drops and haemoglobin can no longer bond with oxygen; the diver blacks out without warning due to hypoxia (insufficient oxygen).

Carotid-Sinus Reflex

Arterial blood reaches the brain through the carotid arteries, which branch up each side of the neck. Carotid-sinus receptors monitor blood pressure within the carotid arteries and send impulses to the cardioinhibitory centre in the brain to regulate the heart.

When the carotid-sinus receptors detect high blood pressure, it stimulates the cardioinhibitory centre, which slows the heart rate. When the carotid sinus receptors detect low blood pressure, it stops stimulating the cardioinhibitory centre, which in turn stops sending signals to the heart. The heart rate then increases.

Wearing anything tight around the neck could constrict the carotid sinus. This could lead to unconsciousness preceded by light headiness and discomfort.

Hypercapnia

1.         Hypercapnia (also called hypercarbia) excessive carbon dioxide, can result from several causes:-
2.         The most common is when a diver fails to breathe slowly and deeply enough. Resulting in headache, confusion and accelerated breathing.
3.         Some forms of closed and semiclosed scuba (rebreathers) have been associated with hypercapnia.
4.         Heavy work underwater can produce carbon dioxide faster than the respiratory system can eliminate it.
5.         Skip-breathing. (Holding the breath while scuba diving in an attempt to extend the air supply).
6.         High carbon dioxide levels due to contaminated air.

Hypocapnia

1.         Hypocapnia (also called hypocarbia) inefficient carbon dioxide, can cause physiological problems:-
2.         Excessive voluntary hyperventilation or unintentional hyperventilation due to stress or fright.
3.         Symptoms are light-headiness which can be followed by fainting.
4.         Hypocapnia during breath holding may lead directly to shallow-water blackout without warning.
5.         Shallow-water blackout gets its name because it occurs on ascent near the surface following a breath holding dive. Due to a low carbon dioxide, level there is no stimulus to breathe. The diver stays underwater longer using more oxygen. The divers body depletes the oxygen supply, but increased partial pressure of oxygen at depth in the alveoli allows haemoglobin to continue using oxygen even after dropping below a level that would cause unconsciousness at the surface. When the diver ascends the oxygen partial pressure in the lungs falls rapidly, so the haemoglobin can no longer bond with oxygen and supply the body. Consequently, hypoxia results instantly, and the diver blacks out.

Carbon Monoxide Poisoning or Contaminated Air

1.         Generally, carbon monoxide poisoning occurs from contaminated air. It usually occurs with various oil-derived hydrocarbons that has a strong smell or taste.
2.         Haemoglobin’s bond with carbon monoxide more than 200 times more readily than with oxygen, but will not unbond as easily. It can take up to 12 hours for the body to eliminate it.
3.         Carbon monoxide bonds with enzymes in the blood.
4.         When carbon monoxide poisoning symptoms occur – headache, confusion, and narrowed vision.
5.         Breathing air contaminated by carbon monoxide at depth results in the haemoglobin carrying less and less oxygen as the carbon monoxide bonds with it. However, at depth blood still carries sufficient oxygen dissolved in plasma by high pp to meet tissue demands, when the diver surfaces, plasma no longer can carry enough dissolved oxygen and the diver blacks out from hypoxia.
6.         Contaminated air may cause lips and nail beds to turn bright red, headaches, nausea, dizziness, and unconsciousness.
7.         Smoking raises carbon monoxide levels in the blood 3 to 12 time’s normal which impairs oxygen transport and carbon dioxide elimination.
8.         It takes 10 to 12 hours for gas exchange to return to normal after smoking.
9.         Treatment:- Fresh air, oxygen if available. Medical attention is needed in all cases.

Nitrogen Absorption and Decompression

Gases dissolve into liquids proportionately to the pressure. This is the basis for decompression.

Oxygen is consumed metabolically, but nitrogen gas is physiologically inert and is therefore a concern.

At the surface the body is saturated with nitrogen, as the diver descends the body isn’t saturated any more because more nitrogen from the breathing gas can go into solution in the tissues, and begins to do so according to Henry’s law. The pp in alveolar air increases. If the diver were to stay at depth long enough (12 or more hours) his body would reach saturation once again.

Saturation occurs exponentially, it take less time to saturate to 50% than to go from 50% to 100%. To reach 50% is called the tissue half time. Different tissues have different half times. Blood absorbs rapidly and contains little nitrogen before it saturates. These are called fast tissues. Fat absorbs a lot of nitrogen and has a long half time, these are slow tissues.

When a diver ascends the pressure decreases on the body. As the diver has been absorbing, nitrogen during the dive the tissue pressure exceeds the surrounding pressure. The tissues are then said to be supersaturated. That is they contain more nitrogen than they can hold at the ambient pressure. So nitrogen begins to dissolve out (exsolve) of the tissues and the body begins to eliminate it, which is where potential decompression sickness problems arise.

The introduction of the Doppler ultrasound bubble detector in the 1970s discovered small bubbles in the bloodstream. These bubbles are called silent bubbles because they don’t produce DCS. Smaller bubbles are called microbubbles. Microbubbles tend to combine, forming larger bubbles, eventually reaching the size of harmless, but detectable silent bubbles.

Factors Predisposing Divers to Decompression Sickness

Fat tissue:-
1.         Fat is a slow tissue.
2.         Fatter people are more likely to get DCS.

Age:-
1.         The circulatory and respiratory systems work less efficiently in older people. This interferes with the gas exchange.
2.         Remaining fit and eating less fat can help.

Dehydration:-
1.         Dehydration reduces the amount of blood available for gas exchange, slowing nitrogen removal from the body.
2.         Consumption of diuretics, (like caffeine) profuse perspiration and dry scuba air all tend to dehydrate.
3.         During metabolism of alcohol, a divers body uses a great deal of water, thus a diver with a hangover is likely to be partially dehydrated.

Injuries and illness:-
1.         Any condition that affects normal circulation can potentially affect nitrogen elimination.
2.        Healed injuries may produce local circulation difficulties and illness can produce a general reduction in circulatory efficiency.

Alcohol:-
1.         Before a dive, even small quantities tend to accelerate circulation and can cause tissues to carry higher amounts of nitrogen.
2.         Following a dive alcohol dilates capillaries, possibly increasing the rate of nitrogen release and contributing to bubble formation.
3.         Alcohol is also a diuretic.

Carbon dioxide:-
1.         Elevated carbon dioxide from skip breathing or improper breathing may interfere with gas transport by the circulatory system by dilating capillaries and increasing nitrogen uptake under pressure.

Cold water:-
1.         Diving in cold water or with an inadequate exposure suit changes normal circulation as the body takes heat-conserving measures.
2.         At the start of a dive, nitrogen goes to all parts of the body, but as heat conservation reduces circulation, there is less blood to carry away dissolved nitrogen during the ascent.

Heavy exercise:-
1.         Exercise while diving accelerates circulation, Upon surfacing and resting there is no corresponding circulation to accelerate nitrogen elimination.

Altitude and flying after diving:-
1.         Reduced atmospheric pressure at altitude increases the pressure gradient between tissue and ambient pressure.
2.         If DCS occurs due to altitude returning to sea level does not usually alleviate it.

Effects:-
1.         Paralysis, shock, weakness, dizziness, numbness, tingling, difficulty breathing, joint, and limb pain. In most severe cases unconsciousness and death.

Treatment:-
First aid:- Treat for shock, oxygen, CPR. Re-pressurisation.

Recreational Dive Planner / Wheel made improvement over Navy tables

1.         They are designed for recreational diving and use conservative limits to minimise bubble formation, even at levels that do not cause DCS.
2.         They were tested using Doppler technology not available when the Navy tables were developed.
3.         They contain more surface interval times for higher precision.
4.         They base surface interval elimination on half time that creates a realistic credit for surface intervals.
5.         The wheel makes it possible to calculate multi-level dive times by giving credit for slower nitrogen uptake in shallow water.
6.         The wheel allows the diver to more closely calculate time and depth profiles.

Types of Decompression Sickness

1.         DCS may take up to 36 hours after the dive to manifest.

2.         About half of all cases, appear within 30 minutes. DCS gets worst with time unless treated.

3.         The most common symptom is excessive fatigue.

4.         Grouped into:-
Type I             –           Skin and pain only symptoms.
Type II            –           More significant symptoms. Both may be present together.

Cutaneous decompression sickness:-
Bubbles coming out of solution in skin capillaries, characterised by a red rash in patches, usually on the shoulders and upper chest. Not serious by itself.

Joint and limb pain decompression sickness:-
Joint or limb pain occurs in about 76% of DCS. Bi-symmetrical symptoms are unusual. Considered serious.

Neurological decompression sickness:-
Can affect movement, touch and life support functions. May become paralysed from the neck down. Considered most serious.

Cerebral decompression sickness:-
Bubbles travelling to the brain can cause a stroke as they block blood flow. Symptoms similar to arterial gas embolism, including blurred vision, headache, confusion, unconsciousness and death. Considered most serious.

Pulmonary decompression sickness:-
DCS manifesting itself in lung capillaries signals the possible onset of life-threatening symptoms. It’s rare. Silent bubbles and microbubbles reaching the pulmonary capillaries normally diffuse into the alveoli or in rare cases may travel into the arterial system and cause neurological DCS. The heart rate will rise and blood pressure will drop. Without treatment, the circulatory system may fail completely. Pulmonary decompression sickness creates breathing pain, a short irritating cough. The victim feels air-starved, hence the nickname ‘the chokes’. Symptoms progress rapidly and may lead to shock.

Nitrogen Narcosis

1.         When breathing air, nitrogen narcosis develops with an increase in nitrogen partial pressure, typically at about 30 m. The effect of a gas depends upon gas solubility in nerve cells.
2.         Most gases can cause anaesthesia under high partial pressure. Helium is not narcotic even under very high pressure. (Helium used for tech diving).
3.         Drugs used to counter such as:- travel sickness, hay fever and diarrhoea other medicine such as sleeping pills and barbiturates all have an anticholingeric effect, meaning they block transmissions of nerve signals and increase nitrogen narcosis. Narcoses can occur at depths of 18m.
4.         Alcohol is the worst of all, any medicine that makes the mouth dry may increase narcosis at depth.
5.         Some seasickness pills have caused extreme narcosis at 18 m.
6.         Effects:- Impaired judgement and co-ordination, false sense of security, lack of concern for safety, may also feel anxious or uncomfortable.
7.         Treatment:- Ascend.

Responses to Thermal Changes

1.         Human core temperature is 37 C or 98.6 F
2.         If it deviates more than a few degrees it can be life threatening.
3.         As the body heats up:-
i)          Initially skin capillaries dilate, allowing heat from the blood to radiate.
ii)         Perspiration follows, cooling the skin.
iii)        The pulse accelerates to circulate blood more rapidly.
iv)        Breathing increases.
4.         Heat exhaustion:-
i)          Is the condition when the body works at maximum capacity to keep the core temperature cool.
ii)         Symptoms:- Weak and rapid breathing, weak pulse, cool and clammy skin, sweaty skin, dehydration, nausea and weakness.
5.         If the body continues to heat up heat stroke, will follow:-
i)          Symptoms:- Strong and rapid pulse, perspiration stops, the skin is flushed  and hot. At this point, the core temperature rises because the body’s cooling mechanisms have failed.
6.         Brain and systemic damage are likely, without medical attention, even death.
7.         Heat stroke is considered a medical emergency.
8.         As the body cools down:-
i)          Temperature centres in the hands, feet and head trigger heat-conserving responses that start with a change in circulation.
ii)         Blood flow to the extremities slows through vasoconstriction (constriction of blood vessels) to reduce heat loss.
iii)        This results in numbness in the fingers and toes after a long exposure.
iv)        Then the body reacts by shivering, which generates heat through muscle activity and accelerated metabolism.
v)         The head loses considerable heat because the body can’t reduce blood supply to the brain.
9.         Hypothermia, if the diver ignores shivering, the core temperature will drop, leading to hypothermia.
10.       As the condition worsens shivering and vasoconstriction cease. The diver suddenly feels comfortable as warm blood rushes to the skin. This is dangerous because the body now loses heat at an uncontrolled rate, and the core temperature drops rapidly. The diver then becomes drowsy, uncoordinated, and forgetful. If unchecked, the diver loses consciousness, followed by coma and death.
11.       Water conducts heat 20 times faster than air.
12.       Water must be over about 27 C to enjoy a dive without insulation.

Physiological Responses to Pressure Changes on Body Air Spaces

1.         Any air space can suffer barotrauma (pressure injury) during ascent or decent.

2.         Descending barotrauma are called squeezes and ascending barotraumas are called reverse squeezes or reverse blocks.

Ear and Sinus Barotrauma

1.         The outer ear traps sound waves and channels them down the auditory canal to the eardrum (tympanic membrane) of the middle ear.

2.         Sound waves vibrate the eardrum that is attached to 3 bones that conduct and magnify vibrations, the ossicles.

3.         Vibrations go from the ossicles into the cochlea, an inner ear organ filled with liquid called perilymph.

4.         The ossicles attach to the oval window of the cochlea, which flexes in and out in response to ossicle vibration.

5.         As the oval window flexes, it pushes against the perilymph, causing pressure waves to oscillate through the cochlea.

6.         The pressure waves stimulate auditory nerve endings in the cochlea, turning the pressure waves into nerve impulses the brain understands.

7.         The round window on the cochlea acts as a pressure compensator, flexing outward when the oval window flexes inward against the incompressible perilymph, and flexing inward when the oval window flexes outward. Without this action, hearing would not take place.

8.         Also in the inner ear are the vestibular canals that control balance. Pressure on the ear can not only affect hearing, but balance too.

9. The diver compensates for increase pressure as he descends by blowing additional air through the Eustachian tube in the middle ear, restoring equal pressure to both sides of the eardrum.