Dive theory study guide



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Carotid Sinus Reflex:
The carotid sinus receptors monitor the pressure of arterial blood reaching the brain through the carotid arteries.
Low blood pressure triggers a higher heart rate (tachycardia).

High blood pressure triggers a lower heart rate (bradycardia).


Receptors will interpret the pressure from an excessively tight hood, wet suit neck seal or dry suit neck seal as high blood pressure.
This reduces blood flow to the brain as the heart slows causing the diver to feel light headed if this continues it can lead to unconsciousness.

Carbon Monoxide poisoning:
This is caused by contaminated air from using the wrong lubricants or improper maintenance of the compressor system. Smoking is another source of carbon monoxide, it takes 8-12 hours to flush the CO from you body after smoking just one cigarette!! Smoking also destroys the surfactant in you lungs. This is the lubricant that prevents the walls of the airways in the lungs from sticking together. If this happens you could have a lung over expansion injury without ever holding your breath!!

Carbon monoxide bonds with the hemoglobin 200 times more readily than oxygen but doesn’t release as easily. When air contaminated with CO is breathed at depth the hemoglobin carries less and less oxygen as the CO bonds with it. However, at depth, the blood still carries enough oxygen dissolved in the plasma by the high partial pressure to meet tissue demands.

As the diver ascends and surfaces the plasma cannot carry enough oxygen and the diver blacks out due to hypoxia.

Signs and symptoms include headache, confusion, narrow vision, bright red lips and fingernail beds. Give Oxygen and get to medical care.



Oxygen Toxicity:
If you dive on air to greater than 57m or much shallower when using enriched air nitrox mixes. You can have problems with the high partial pressure of oxygen.
There are two types:


  1. Central nervous system (CNS) toxicity – caused by exposure to oxygen partial pressures greater than 1.4 bar. (57m on air, 33m on EANx32, 29m on EANx36 and just 4m on pure Oxygen.

Signs and symptoms include: visual disturbances, ear ringing, nausea, twitching, irritability and dizziness. But the most serious symptom/sign is a convulsion usually without warning which can cause the diver to lose his mouthpiece and drown.


  1. Pulmonary toxicity – caused by a long exposure to high partial pressures of oxygen. Usually only occurs after a series of multiple dives using enriched air or in technical diving using high concentrations of oxygen for long decompression schedules.

Signs and symptoms include burning in the chest and an irritated cough.

It usually resolves itself if the diver ceases diving for a few days.

It is avoided by following NOAA and DSAT oxygen exposure tables.


Nitrogen Narcosis:
Almost any gas can cause a narcotic effect under pressure.

It appears to be related to nerve impulse blockage due to gas dissolved in the nerves.

Oxygen has about the same potential so don’t expect to be less susceptible when using Nitrox mixes.

Expect narcosis to be noticeable at around 30m, it varies from one diver to the next and is not predictable.

Helium is not narcotic under high pressures which is why it is used in deep technical diving.
Ascending a few meters usually relieves the symptoms which are not directly hazardous – the danger comes from impaired judgment and co-ordination which may lead to bad decisions.
Other symptoms can included euphoria (feeling happy), anxiousness, panic, dizziness, tunnel vision.

Decompression sickness:
When under pressure gases will dissolve into liquids, our body is mostly water so when we breathe air at depth it goes into solution in body tissues.
Oxygen is consumed metabolically, but nitrogen is physiologically inert and dissolves into the blood and tissues.
This dissolved gas still exerts pressure which is known as tissue pressure.

Different tissues absorb and release nitrogen at different rates.


If the diver stays at depth eventually his body will saturate meaning the gas pressures have reached an equilibrium and he can’t absorb any more nitrogen.
Calculating different tissue absorption and release is the basis of decompression models for tables and computers.
Recreational dives are too short to reach saturation, but upon ascent from any dive the nitrogen pressure in the tissues is greater than the surrounding pressure, this is called supersaturation.
If the difference between the tissue pressure and the surrounding pressure (the pressure gradient) is kept within limits the nitrogen dissolves harmlessly out of the body through respiration.
After some dives Doppler ultrasound flowmeters detect silent bubbles in divers – these are harmless in themselves but could join up to form larger bubbles if there’s too many of them.
If the diver ascends too quickly or misses required decompression stops these small bubbles accumulate and form larger bubbles causing decompression sickness (the bends).
Signs and symptoms depend on where the bubbles form.
Type 1 DCS pain only usually in the joints and limbs and skin bends (rashes on the shoulders and upper chest).
Type 2 DCS life threatening involving the nervous system, numbness, tingling, paralysis, weakness, fatigue, unconsciousness and death.

There are several physiological factors that may predispose a diver to DCS,

In other words make it more likely he will get bent even when sticking to tables or a computer especially if near the limits.

These are:



  • Fat tissue: Fat releases nitrogen slowly so a fat diver may have more nitrogen after a dive.

  • Age: As we get older our circulatory systems become less efficient slowing down the gas exchange.

  • Dehydration: This reduces the blood in circulation, slowing nitrogen elimination. Always drink plenty of water while diving.

  • Injuries/illness: These could alter or restrict circulation to localized areas where nitrogen isn’t eliminated as quickly.

  • Alcohol before or after diving: This alters circulation patterns, dilates capillaries and promotes dehydration, all of which alter nitrogen elimination and bubble formation.

  • Carbon Dioxide excess: Raised CO2 levels will alter circulation and gas exchange.

  • Cold Water: When the diver gets cold circulation to the extremities is reduced hindering nitrogen release.

  • Heavy Exercise: During the dive this can raise CO2 levels and accelerate circulation so more nitrogen is absorbed. After a dive it accelerates circulation altering nitrogen elimination.

  • Altitude/Flying: Tables and computers are based on surfacing at sea level, if we go to altitude after a dive this increase the pressure gradient and bubbles may form – returning to sea level will not alleviate the bubbles once formed!

Treatment for DCS


First aid Treat all cases as serious, give the patient oxygen, keep the patient lying level on the left side with the head supported, provide primary care and arrange transport to the nearest medical facility.

DCI stands for decompression illness and covers all injuries caused by a rapid ascent or coming up too soon.
DCS refers only to the condition caused by dissolved nitrogen in the system.

Heat and Cold:
Overheating can cause problems. First heat exhaustion where the body works at full capacity to cool. Signs and symptoms include: weak rapid breathing, weak rapid pulse, cool clammy skin, profuse sweating, dehydration and nausea. Treatment is to get the diver to a cool area, remove exposure suit, and give nonalcoholic fluids, rest until cool.

Second heat stroke, if heat exhaustion is not treated promptly it can lead to heat stoke which is life threatening. The body’s cooling system has now completely failed. Signs and symptoms include: strong rapid pulse, no perspiration, skin flushed, hot to touch. Treatment: Cool by whatever means available and get the diver to emergency medical care.


The body responds to heat loss by vasoconstriction (reduced blood flow to the extremities) causing finger and toe numbness. Then shivering as the body tries to generate heat by muscle movement. This signals a losing battle against the cold and is termed mild hypothermia. Get the diver to a warm place as soon as possible. If heat loss continues the condition gets worse; advanced hypothermia.

Shivering stops. Vasoconstriction stops. The diver may feel warm as blood rushes to the skin. The core temperature drops, mental processes slow, the diver becomes drowsy, then unconsciousness, coma and death. Advanced hypothermia requires emergency medical care as soon as possible.



The Ear:

The ear is divided into the outer, middle and inner ear.



  • The outer ear consists of the external ear and ear canal, it is open to air/water pressure and channels sound to the eardrum.

  • The middle ear is separated from the outer ear by the ear drum and is sealed against air/water. The ear drum vibrates and passes sound to the ossicles, small bones that conduct sound to the inner ear.

  • The inner ear consists of the vestibular canals that control balance and the cochlea which turns vibrations into nerve impulses sent by the auditory nerve to the brain.

  • The ossicles connect to the cochlea at the oval window which flexes in and out with the vibrations. The round window on the cochlea flexes out when the oval window flexes in, like a water filled balloon.

The middle ear is connected by the Eustachian tube to the throat to maintain equilibrium with outside pressure.


As the diver descends pressure pushes in on the eardrum causing discomfort, by equalizing the diver forces air up the Eustachian tube to equalize pressure in the middle ear alleviating the discomfort. Expanding air normally exits from the middle ear through the Eustachian tube easily on ascent.
If the diver does not equalize sufficiently hydrostatic pressure forces blood and fluid into the middle ear until equilibrium is restored. The ears feel full and hearing is reduced – should be checked by a doctor. This is called middle ear squeeze.
If the diver does not equalize and descends faster than fluids can fill the middle ear the eardrum tears due to the pressure. The diver feels a sharp pain, and then relief as the pressure is relieved. When cold water enters the middle ear the diver will experience dizziness or vertigo until the water warms to body temperature. This is because is causes convection in the fluids of the vestibular canals effecting balance.
If the diver delays equalization, then tries to equalize forcefully using the Valsava maneuver (blowing against pinched nostrils from the diaphragm and lungs) pressure on the ear drum presses in on the ossicles which press in on the oval window on the cochlea; the round window flexes out in response. The Valsalva maneuver raises pressure in the chest, which causes an increase in pressure in the cochlea (connected by fluid as part of the nervous system). This, plus the transmitted pressure bursts the round window outwards.

This is serious and requires medical treatment; the diver may never be able to dive again! To avoid this always use the Frenzel maneuver (blowing against pinched nostrils but just using the throat muscles to push the air up the Eustachian tube). Bet that’s the way you do it anyway!

This injury is known as round window rupture.
If the air cannot escape from the middle ear through the Eustachian tube on ascent a phenomenon known as reverse squeeze occurs. This is usually caused by diving with a cold or using a decongestant that has worn off. Stop or slow ascent and wait for trapped air to work its way out. Swallowing may help or inhaling against pinched nostrils. If ascent continues the eardrum will rupture outward.
Earplugs or a tight wet suit hood create an air space between the plug/hood and the eardrum. When the diver descends the eardrum flexes out towards the plug/hood and can rupture if descent continues. Feels like middle ear squeeze.

Avoid by never wearing earplugs and pulling away hood momentarily when equalizing on descent.


Let’s have some definitions before we continue….
Barotrauma means pressure injury. Baro = pressure. Trauma = injury.

It can happen on descent or ascent if an air space is not equalized.

An unequalized air space is also called a squeeze.
Other air spaces:
Sinuses are spaces in the head connected to the nose that filter and moisturize air before it reaches the lungs. They normally equalize with the ears with no problems. Sinus squeeze is usually caused by diving with a cold or congestion. The unequalized sinuses fill with blood and fluid to equalize during the dive – may feel like a sharp pain above the eyes. Upon ascent expanding air pushes blood and fluid into the nasal cavity and the diver surfaces with blood in his mask. This is usually not serious and heals on its own.
Mask space: This normally equalized by the diver exhaling into the mask during descent. (That is why you can’t dive using goggles). If this is not done the tissues swell, forced into the unequalized mask space, capillaries in the skin and eyes rupture. Looks terrible but usually clears by itself.
Dry suit space: Squeeze can be caused by not adding enough air to the suit or descending too quickly. Can constrict breathing and caused welts and pinches where the dry suit squeezes.

And finally the biggest air space:


The lungs:
Lung squeeze is caused by a breath hold descent that reduces lung volume below the residual volume. (This would happen if you went down to over 30m on a breath hold dive). It can occur shallower if you descend with partially full or empty lungs. It causes fluid to accumulate in the lungs and can be life threatening – remember Enzo in the Big Blue. Not likely in recreational diving.

Lung over expansion injuries:
This what happens if you hold your breath on ascent when using scuba. They can also be caused by lung congestion if diving with a chest cold, or by a local blockage due to loss of surfactant due to smoking.

There are four types of injury that can occur.


Air embolism: This is also called arterial gas embolism (AGE). The alveoli and pulmonary capillaries rupture allowing air to enter the bloodstream and flow into the arteries. This is serious and immediately life threatening, the bubbles can lodge anywhere, but usually flow through the carotid arteries straight to the brain causing stroke like symptoms, dizziness, confusion, shock, paralysis, personality change, unconsciousness and death..
With all lung over expansion injuries always expect air embolism as this is the worst case scenario, it often occurs with the other types of lung injury.
Pneumothorax: The air from the rupture goes between the lung and chest wall, causing the lung to collapse. Also serious the diver will have chest pain and may cough up blood.
Mediastinal emphysema: The air from the rupture accumulates in the center of the chest over the heart and interferes with circulation; the diver may feel faint and short of breath, still serious.
Subcutaneous emphysema: The air from the rupture accumulates in the soft tissues at the base of the neck. The diver’s voice may change and the skin may crackle to the touch.

First aid treatment for these injuries is the same as for DCS, hence the common term DCI to encompass both types of injury.

Lie the patient down and administer oxygen, apply primary care and get to a medical facility ASAP.
Note that symptoms of lung over expansion injury usually occur immediately after the dive whereas symptoms of decompression sickness are delayed and can appear 15 minutes after surfacing or even up to 24 hrs before the diver notices symptoms. Micro bubble maximums occur around 35 -40 minutes after surfacing this is the time when most bends will become apparent. But remember this is not an exact science.

DECOMPRESSION THEORY AND THE RDP SECTION THREE

The Haldanean Decompression Model:
Virtually all dive tables and dive computers calculate no decompression limits and decompression stops (when needed) based on a Haldanean decompression model.

This is named after John Scott Haldane who developed the first such mathematical decompression model and based on it the first dive tables in 1906.


Modern decompression models are based on the same ideas.
When the diver descends to a given depth, the nitrogen pressure in his breathing air is higher than the nitrogen tissue pressure in his body, so more nitrogen dissolves into the body tissues.
With enough time the nitrogen pressure equalize and the body cannot take on any more nitrogen. This is called saturation.
When the diver ascends the nitrogen tissue pressure in the body becomes higher than the nitrogen pressure in his breathing air, causing the tissues to release nitrogen to equalize the nitrogen pressure again.
The difference between the dissolved nitrogen tissue pressure and the nitrogen pressure in the breathing air is called the pressure gradient. Whether the diver is descending or ascending.
When the diver ascends the tissues can tolerate some gradient of high tissue pressure without causing decompression sickness.
If the pressure gradient exceeds acceptable limits, bubbles may form and cause decompression sickness.
Decompression sickness can be avoided by keeping the gradient within acceptable limits.
This means the diver must stay within the limits dictated by his table or computer and maintain a slow ascent rate as indicated by his tables or computer.

Haldane discovered that different parts of the body absorb and release dissolved nitrogen at different rates.


To account for these differences he constructed a model consisting of five theoretical tissues.
These theoretical tissues do not directly correspond to any particular body tissue so they are called compartments or tissue compartments.

The RDP has 14 compartments.


Each compartment has a halftime for the rate at which it absorbs and releases nitrogen.
Halftime is the time, in minutes, for a compartment to go halfway from its beginning tissue pressure to complete saturation.
After one halftime the tissue would be 50% saturated
After two halftimes the tissue would be 75% saturated
After three halftimes the tissue would be 87.5% saturated
After four halftimes the tissue would be 93.75% saturated
After five halftimes the tissue would be 96.875% saturated
After six halftimes the tissue would be 98.4375% saturated
It would never reach 100% using the halftime concept, so after six halftimes the tissue compartment is considered full or empty.
Haldanes original halftimes ranged from 5 to 75 minutes.
The RDP’s halftimes range from 5 to 480 minutes split over 14 compartments.
They are 5, 10, 20, 30, 40, 60, 80, 100,120, 200, 240, 300, 360 and 480 minutes.
Sometimes tissue pressure is expressed in meters of seawater (gauge) msw.
Example: A 5 minute halftime compartment will have a tissue pressure of 9msw after 5 minutes in 18meters of seawater.
Example: A 20 minute halftime compartment will have a tissue pressure of 18msw after 40 minutes in 24m of seawater.
Example: A 60 minute halftime compartment will take 360 minutes (6 hours) to saturate to a given depth. (60 x 6 halftimes).
Besides different halftimes each compartment has a different M-value.
This is the maximum tissue pressure allowed in the compartment when surfacing to prevent exceeding the acceptable gradient.
There are actually different M-values for each compartment at different depths, these are used to calculate decompression schedules. In no decompression diving we only use the one that applies to the surface
The slower the compartment, the lower the M-value.
The faster the compartment, the higher the M-value.
The M-value is determined by test dives showing what does and what does not result in Doppler detectable bubbles.
Remember that the M-values are calculated for surfacing at sea level which is why you need to apply special procedures when diving at altitudes above 300m.
When any compartment reaches its M-value the dive ends or it becomes a decompression dive.
On deeper dives faster compartments will reach there M-values first, hence deeper dives have short no decompression limits.
On shallower dives, the depth is not enough for the faster compartments to reach there M-values. Therefore a slower compartment controls the dive and the model allows more no decompression time.
The compartment that reaches its M-value first is called the controlling compartment.
These models are mathematical extrapolations; there is no direct relationship between the decompression model and the human body. This is why divers learn that there is always some risk of DCS even within table/computer limits and are asked to dive conservatively within the limits.
The actual risk of getting DCS within the limits is 0.04% - so don’t panic and hang up your fins!

US Navy tables:
The first dive tables to be widely used and adapted to recreational diving where the U.S.Navy tables designed in the 1950’s.
Six compartments were used with a slowest halftime of 120 minutes.
While at the surface all compartments would lose nitrogen at different rate depending on their halftime. Any compartment could control a repetitive dive, depending on the first dive, the surface interval and the second dive.
To solve this problem the U.S.Navy designed its surface interval credit on the worst case scenario, the slowest compartment (120 mins). This is why it takes 12 hours (720 mins. 6 x 120) to be “clean” when using their tables.
These tables were tested with US Navy divers, subjects were all male in their 20’s and 30’s and reasonably fit. The test criteria were bends/no bends.

The Recreational Dive Planner (RDP):
In the mid-1980’s, Dr. Raymond Rogers recognized that the USN tables were not ideal for recreational diving.
The 120 minute half time used for surface interval credit, while appropriate for decompression diving seemed excessively conservative for recreational divers making only no decompression dives.
The test group the USN used didn’t reflect recreational divers who include females and people of all ages.
New technology in the shape of Doppler ultrasound flow meters had come into being ; these showed that silent bubbles often formed at USN table limits, suggesting lower M-values would be more appropriate for recreational divers.
With the help of DSAT (Diving Science & Technology), Rogers developed the RDP. It was tested in 1987/88 at the Institute of Applied Physiology & Medicine with Dr. Michael Powell as the principal investigator.
A 60 minute gas washout tissue was used. Multi level diving was tested. Big range of test subjects, like recreational divers. Limited to Doppler detectable bubbles instead of bends/no bends. Tested to the limits for 4 dives per day for 6 days. Though more conservative diving practices are recommended.

Dr. Rogers found that the old 120 minute gas washout tissue was too conservative for recreational diving and adopted a 60 minute gas washout tissue.

This means you get twice as much credit for surface intervals and are clean in 6 hours. The WXYZ rules make sure the slower compartments stay within limits.

Dr. Rogers also lowered the M-values to match recent Doppler data. These are sometimes called Spencer limits after the physician who first proposed them.


They produced different versions of the RDP…

The table version, (because that’s what divers were familiar with) and the multilevel electronic planner eRDPML version (originally the Wheel), to enable you to calculate multi level profiles.

DSAT have also produced four tables for enriched air diving.

Tables for using EANx32 and EANx36 an Equivalent air depth table and an Oxygen exposure table.

The pressure groups from all versions of the RDP are interchangeable.




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