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Charle's Law (T x P = V)

*Henry's Law ( Gas in fluid directly proportional to pp of gas to which the fluid is exposed)

Dalton's Law (PT = (P1+P2+P3) - Pn)

General Gas Equation (P1V1/T1 = P2V2/T2) Boyle's + Charles' Laws

Pascal's Law (P = Force/Area)

Partial pressure of a gas:

• Is determined by the concentration of the gas and ambient pressure
• Reflects the increasing pressure and compression of the gas, the concentration remains the same
• the concentration of O2 in air is about 21%, and the partial pressure of O2 in air at surface (1 atm abs) is about 0.21 atm.
• at 2 atm abs, the number of O2 molecules per unit volume is twice what it is at the surface, and the partial pressure is double

• are related to their partial pressure
• change according to depth.

• appear as the partial pressure of O2 increases.
• causes lung damage with extended exposure to a PO2 above 0.6 atm (equivalent to 60% O2 at surface or 30% O2 at 33 ft).
• causes convulsions, especially in working dives, if the PO2 approaches or exceeds 2 atm (eg, 100% O2 at 33 ft or 50% O2 at 99 ft).

• produce nitrogen narcosis, a condition resembling alcohol intoxication.
• becomes noticeable at 100 ft or less.
• is generally incapacitating at about 10 atm abs (300 ft)
• produces an anesthetic effect resembling that of 30% nitrous oxide at sea level.
• is replaced by helium as the diluent for O2 in deep diving due to the lack of anesthetic effect.

Diving to a significant depth during the breath-hold complicates the situation by elevating the PO2 and permitting extended O2 uptake at depth. A diver who has "pushed the limits" under those circumstances may lose consciousness when alveolar PO2 falls to a low level on ascent. This phenomenon is probably responsible for many unexplained drownings among spearfishing competitors and others who do extensive breath-hold diving. The term shallow-water blackout is sometimes applied, but it is best reserved for its original meaning: unconsciousness from CO2 buildup in rebreathing types of scuba. (Hypoxia is also a potential problem in rebreathing units if O2 is displaced by excess N2).

Carbon dioxide retention:

• In normal individuals on land, hyperpnea or breathlessness usually provides ample warning of increased CO2 in inspired gas.
• Such a response may be more the exception than the rule under water, especially where high PO2 and exertion are also factors.
• Some individuals develop spontaneous CO2 retention through an inadequate increase in pulmonary ventilation during exertion.
• Whatever the source, abnormally high PCO2 per se can cause loss or impairment of consciousness at depth
• can also increase the likelihood of O2 convulsions
• can augment the severity of nitrogen narcosis.

Nitrogen

• is an inert gas existing in largest quantity in the atmosphere, 79% in air.
• is inert, meaning that it does not take part in energy transformations.
• is the gas that causes nitrogen narcosis through the effect of Dalton's law
• is the gas that causes decompression sickness on ascent from depth with reduction of pressure, (Boyle's Law).
• is the gas that determines decompression schedules.

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Nitrogen narcosis

• occurs when divers go below 100 FSW, due to the laws of partial pressures.
• Complex reasoning decreases 33% and manual dexterity decreases 7.3%.
• The condition causes loss of motor function and decision-making ability and can be more clearly defined as causing one to become "drunk", as with alcoholic beverages.
• The comparison to having had "three Martinis" is apt, and it has been stated that one should consider the narcotic effect of one Martini for every 50 feet of sea water.
•  is potentiated by increased CO2 levels.

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The law is stated as:

p ATA=pO2 + pN2 + p other gases

thus: pN2= fN2 x ATA

'Critical volume hypothesis '

• an increased volume of nitrogen in the membranes
• this volume relates to solubility.
• explains the pressure reversal of anesthetics.

• Cold
• Stress
• Heavy work and fatigue
• CO2 retention

Treatment of nitrogen narcosis

• immediate controlled ascent to the surface,
• the buddy or divemaster observing the diver for unusual behavior
• administration of O2
• temporary cessation of diving
• Prevention should be the best treatment, with no further diving below 100 feet.

Subjective feelings to alcohol and nitrogen narcosis
HPNS and nitrogen narcosis
Nitrogen narcosis attenuates shivering thermogenesis.
Nitrogen narcosis and diver adaptation.
Perception of thermal comfort during narcosis.
Repeated hyperbaric exposures on susceptibility to nitrogen narcosis.
Hyperbaric air, ethyl alcohol and dextroamphetamine
Nitrogen narcosis and ethyl alcohol
Lithium effects: protection against nitrogen narcosis
Diving experience and emotional factors 

The effects of oxygen are increased at depth so that the maximum PO2 in diving is 1.6 ATA:

• achieved at 218 fsw breathing air
• 132 fsw breathing 32% O2
• 20 fsw breathing 100% O2.
• the limiting factor in the use of nitrox (increased O2 percentages) in increasing the bottom time of "tech" divers.
• causes all O2 treatments using 100% O2 to be given at 60 feet or shallower, except for gas gangrene and CO poisoning.
• creates the elevated pO2 that causes the convulsions of O2 toxicity

The effect on the central nervous system ( the Paul Bert effect), results in:

Convulsion at depth in water

• results in drowning
• results in arterial gas embolism
• is prevented by not using oxygen breathing with SCUBA
• is prevented by limiting oxygen exposure with hyperbaric oxygen therapy 100% O2 greater than 60 FSW.

CO2 retention with it's attendant dangers of death from convulsions and hypoxia (low oxygen level) is primarily of caused by

• "skip breathing"
• breath-hold diving
• breathing in a sealed environment
• using contaminated air.

Signs and Symptoms of CO2 retention include

• rapid respiration in 4-6%
• rapid pulse rate
• shortness of breath in 7-10%
• headache
• excessive sweating
• mental impairment
• convulsions and unconsciousness in 11-20%.

Elevated CO2 levels play a significant role in shallow water blackout and in nitrogen narcosis.

The acceptable CO2 levels

• for diving operations is 1.5% surface equivalent (10.5 mmHg)
• for hyperbaric chamber operations is one that allows a vent schedule of 4scfm/person displacement.

With the increased usage of closed circuit scuba diving, mainly by the military-but recently by more and more civilian divers, there is the possibility of hypercarbia (high CO2 levels), among other medical considerations.

This increased CO2 due to malfunction of the CO2 absorbent canisters can be avoided by:

• decreasing the exercise rate
• watch out for the operating limits of the canister
• checking for leaks at the start of the dive
• not reusing the absorbent.

SHALLOW-WATER BLACKOUT (Latent hypoxia)

Shallow-water blackout (SWB) is the sudden loss of consciousness caused by oxygen starvation following a breath holding dive. This was first described by S. Miles as "latent hypoxia", shallow water blackout is the term he ascribed to unexplained loss of consciousness in divers using closed-circuit oxygen breathing apparatus at shallow depths. Unconsciousness strikes most commonly within 15 feet (five meters) of the surface, where expanding, oxygen-hungry lungs literally suck oxygen from the divers blood. Once you lose consciousness you die. The blackout occurs quickly, insidiously and without warning. Mercifully, the victims of this condition die without any idea of their impending death.

There are about 7000 drownings in the U.S. annually-many of whom are good swimmers. Craig, in 1976 reported interviews of survivors of near drowning. All had hyperventilated prior to the swim, had the urge to breathe, and had no warning of the impending unconsciousness. Hyperventilation is used by divers to reduce the concentration of CO2 and extend the length of breath-holding.

Beginning breath-hold divers, because of their lack of adaptation, are not generally subject to this condition. It is the intermediate diver who is most at risk. He is in an accelerated phase of training, and his physical and mental adaptations allow him to dive deeper and longer with each new diving day- sometimes too deep or too long. Advanced divers are not immune.

Conditions that produce latent hypoxia (Shallow water blackout)

Hyperventilation

Hyperventilation is the practice of excessive breathing with an increase in the rate of respiration or an increase in the depth of respiration, or both. This will not store extra oxygen. On the contrary, if practiced too vigorously, it will actually rob the body of oxygen. The magical benefit of hyperventilation is what it does to carbon dioxide levels in the blood. Rapid or deep breathing reduces carbon dioxide levels rapidly. It is high levels of carbon dioxide, not low levels of oxygen, that stimulate the need to breathe.

The beginning diver is very sensitive to carbon dioxide levels. These levels build even with a breath-hold of 15 seconds, causing the lungs to feel on fire. The trained diver has blown off massive amounts of carbon dioxide with hyperventilation, thus outsmarting the brain's breathing center. Normally metabolizing body tissues, producing carbon dioxide at a regular rate, do not replace enough carbon dioxide to stimulate this breathing center until the body is seriously short of oxygen.
 

Hyperventilation causes some central nervous system changes as well. Practiced to excess, it causes decreased cerebral blood flow, dizziness and muscle cramping in the arms and legs. But moderate degrees of hyperventilation can cause a state of euphoria and well-being. This can lead to overconfidence and the dramatic consequence of a body performing too long without a breath: blackout.
 

Pressure changes in the freediver's descent-ascent cycle conspire to rob him of oxygen as he nears the surface by the mechanism of partial pressures. Gas levels, namely oxygen and carbon dioxide, are continuously balancing themselves in the body. Gases balance between the lungs and body tissues. The body draws oxygen from the lungs as it requires. The oxygen concentration in the lungs of a descending diver increases because of the increasing water pressure.

As the brain and tissues use oxygen, more oxygen is available from the lungs while he is still descending. This all works well as long as there is oxygen in the lungs and the diver remains at his descended level. The problem is in ascent. The re-expanding lungs of the ascending diver increase in volume as the water pressure decreases, and this results in a rapid decrease of oxygen in the lungs to critical levels. The balance that forced oxygen into the body is now reversed. It is most pronounced in the last 10 to 15 feet below the surface, where the greatest relative lung expansion occurs. This is where unconsciousness frequently happens. The blackout is instantaneous and without warning. It is the result of a critically low level of oxygen, which in effect, switches off the brain.

Dalton's Law of partial pressures applies. (Pb - PO2 + PN2 + Pother gases.)

As Pb decreases, the partial pressures of all component gases decrease in the same ratio. The hypoxia of predive hyperventilation is corrected by an increased PO2 during descent.

During descent, the lung volume decreases due to chest compression, resulting in increased lung PO2, PCO2 and PN2.

• In the lung there is an increased breathing rate and a reduced PCO2. Lung volume is reduced to one-half, lung PO2 is increased, lung PCO2 increases initially, but is followed by lowered PCO2 due to reversed gradient.
• The blood reacts by developing a respiratory alkalosis and a right shift of the HbO2 (oxyhemoglobin) curve. The reaction is CO2 + H2O - H+ + HCO3.
• Carotid body chemoreceptors cause a slow-down of the heart and permit longer breath holding.
• There is vasodilatation of the brain vessels with hypoxia (low oxygen). There is rapid O2 usage, the arterial PCO2 is lowered so that respiration is not stimulated until )2 drops s low that the breath hold breakpoint is reached. The breakpoint (PCO2/PO2) in a trained person is less sensitive to increased PCO2 or lowered O2. The act of consuming oxygen rapidly (as in chasing a large fish), delays the breakpoint because of the higher PCO2 and the exercise per se. The diver becomes lightheaded, dizzy, has tingling, air hunger, muscle rigidity and unconsciousness.
• While at depth, increased lung PO2 provides a favorable gradient for O2 transfer from the lung to blood, occurring more rapidly than if the diver were on the surface.
• Because alveolar PCO2 increases with compression, CO2 does not leave the blood to enter the lung. Arterial CO2 rises rapidly (especially with exercise) initially, then the tissues store CO2. Trained divers use a timed bottom time ( 1.5 minute maximum) to avoid unconsciousness on return to the surface.

• The lung re-expands to normal, the PCO2 becomes elevated as more diffuses into the lung and the PO2 drops dramatically.
• In the blood the PCO2 elevates depending on the depth of the dive and the amount of exercise. Deep dives drive more CO2 from the lungs into the tissues and increases the problem. There is a right shift of the HgbO2 curve.
• When the break point is reached , the chemoreceptors are stimulated by CO2, thus stimulation of respiration. Low O2 also stimulates respiration.
• In the brain:
• CO2 stimulates respirations
• Vasodilation encourages O2 consumption
• Latent hypoxia occurs
• Unconsciousness ensues
• On ascent the lungs re-expand reducing the favorable diffusion gradient for oxygen. Shallower depths cause this gradient to approach zero, the diver reaching a critical state of hypoxia.
• Hypoxia causes unconsciousness, possibly before the diver reaches the surface.

• Signs and symptoms of latent hypoxia (Shallow water blackout)
• Extreme weakness, trembling, unconsciousness in the water, amnesia of the event, drowning.

Link:  'Scuba Diving Explained'., Lawrence Martin, MD

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In addition to the changes due to the Physics of Dalton's Law, there are other physiological changes that take effect during shallow water blackout and free diving.

Diving Reflex

The human body is capable of remarkable adaptations to the underwater environment. Even untrained divers will show a dramatic slowing of the heart when immersed. This is commonly referred to as the diving reflex. Immersion of the face in cold water causes the heart to slow automatically. Chest compression can also slow the heart. Untrained divers can experience up to a 40 percent drop in heart rate. Trained divers can produce an even lower heart rate some can slow to an incredible 20 beats per minute.

Spleen Effects

Trained freedivers develop several other physiological adaptations that lead to deeper and longer dives. The spleen, acting as a blood reservoir, assists trained divers in increasing their performance. Apparently their spleen shrinks while diving, causing a release of extra blood cells.

According to William E. Hurford M.D., and co-authors writing in The Journal of Applied Physiology, the spleens of the Japanese Ama divers (professional women shellfish free divers) they studied decreased in size by 20 percent when they dove. At the same time their hemoglobin concentration increased by 10 percent (Volume 69, pages 932-936, 1990).

This adaptation, similar to one observed in marine mammals (the Weddell seals' blood cell concentration increases by up to 65 percent), could increase the divers ability to take up oxygen at the surface. It could also increase oxygen delivery to critical tissues during the dive.

Interestingly, the spleens contraction and the resultant release of red cells is not immediate- it starts taking effect after a quarter-hour of sustained diving. This spleen adaptation, as well as other physiologic changes, probably take a half-hour for full effect. This might account for the increased performance trained free divers notice after their first half-hour of diving, and also may be one of the causes of unexplained heart failure in the diver with a border line heart condition.

Other adaptations

There are other known adaptations: blood vessels in the skin contract under conditions of low oxygen in order to leave more blood available for important organs, namely the heart, brain and muscles. Changes in blood chemistry allow the body to carry and use oxygen more efficiently. These changes, in effect, squeeze the last molecule of available oxygen from nonessential organs. Most importantly, the diver's mind adapts to longer periods of apnea (no breathing). He can ignore, for longer periods of time, his internal voice that requires him to breathe.

PREVENTION OF SHALLOW-WATER BLACKOUT

Factors that can contribute to this condition.

• hyperventilation
• exercise
• a competitive personality
• a focused mind-set
• youth.

The use of hyperventilation in preparation for freediving is controversial. No one disagrees that prolonged hyperventilation, after minutes of vigorous breathing accompanied by dizziness and tingling in the arms and legs, is dangerous. Some diving physicians believe that any hyperventilation is deadly because of the variation in effects among individuals and on one person, from one time to another. Other physicians, studying professional freedivers such as the Ama divers of Japan, found that they routinely hyperventilated mildly and took a deep breath before descending. Their hyperventilation is very mild; they limit it by pursed lip breathing before a dive.

Probably the best approach can be found in the U.S. Navy Diving Manual (Volume 1, Air Diving), which states: Hyperventilation with air before a skindive is almost standard procedure and is reasonably safe if it is not carried too far. Hyperventilation with air should not be continued beyond three to four breaths, and the diver should start to surface as soon as he notices a definite urge to resume breathing.

Learn the deadly effects of exercise underwater and plan to deal with this situation.

Freedivers learn to prolong their dives by profoundly relaxing their muscles (see the section on deep diving). Most divers make minimal use of their muscles except when they fight a fish or free an anchor. A physician writing in an Australian medical journal found a common scenario for diving deaths in Australia is the experienced diver with weight belt on, speargun fired.

Medical researchers feel that many pool deaths, classified as drownings, are really the result of shallow-water blackout. Most occur in male adolescents and young adults attempting competitive endurance breath-holding, frequently on a dare. Drowning victims, especially children, have been resuscitated from long periods of immersion in cold water 30 minutes or more. The same is not true for victims blacking out in warm-water swimming pools. Warm water hastens death by allowing tissues, especially brain tissues, to continue metabolizing rapidly; without oxygen, irreversible cell damage occurs in minutes.

SUMMARY

• Do not hyperventilate to excess no more than three or four breaths.
• Reduce exercise at depth.
• Recognize the danger of focusing.
• Don't hesitate to drop your weight belt.
• Avoid endurance dives.
• Adjust your weight belt so that you will float at 15 feet.
• Don't practice breath-holding in a swimming pool. Always have an observer standing by to assist.

Reference: Hong, SK. 1990. Breath-Hold Diving. In: Bove and Davis, Diving Medicine, 2^nd ED., Philadelphia, PA: WB Saunders, pp 59-68.
Gas Pressure:Mt Sinai Books
From a lecture by Paul Sheffield, PhD
Medical Seminars, Bonaire, 1996

Carbon monoxide poisoning is a rare cause of problems when diving, it does occur when there is
contaminated air in recreational diving tanks. CO poisoning is the leading cause of poisoning deaths in the
U.S.(about 8600 deaths per year) and is easily missed unless health care providers are especially vigilant.

The most commonly observed result related to CO poisoning is neurological dysfunction; abnormalities in
the cardiac, pulmonary and renal organ systems do occur. About 14% of patients sustain permanent brain
damage, and delayed neurological sequelae do occur 3-21 days later in about 12% of people.

CO risk factors include:

     Pre-existing cardiovascular disease
     Age greater than 60 years
     An interval of unconsciousness (longer the higher the risk)
     Little association with COHgb (carboxy hemoglobin)

Carbon Monoxide signs:

     Tachycardia (rapid pulse)
     Tachypnea (rapid breathing)
     Retinal venous engorgement (as seen through an ophthalmoscope)
     Ekg conduction defects
     COHgb greater than 20%

Carbon monoxide in diving is the product of incomplete combustion of hydrocarbons and is usually from compressors. In addition to the effect on the hemoglobin molecule, it has a toxic effect on the cytochrome A3 system. Prevention requires periodic air sampling. The maximal allowable level is 20 ppm (0.002%) 

Several agencies have begun training recreational divers with oxygen enriched compressed air ("Nitrox", EAN). Recreational nitrox diving has in common with traditional compressed air diving the use of nearly all the same equipment and the use of only one gas mix per tank per dive.

Advantages

Advantages accrue for the user of nitrox in that he/she enjoys a prolonged no-stop time on the basis of "equivalent air depth", or a safer decompression if one sticks to the air tables-but not both. The advantages are of a prolonged no-stop time are also decreased because at depths shallower than 75 feet the dive is shortened by the limited capacity of the tank and oxygen toxicity safety limits maximum depths to around 120 feet.

Disadvantages

Disadvantages of using nitrox include explosive risks and the need to have dedicated equipment that is grease free. Because mistakes can be made in mixing the O2 and air to get the appropriate mixture, each tank needs to be analyzed for O2 content in the presence of the proposed user. Another mistake can be in selection of an inappropriate mixture for the depth of the particular dive. The greatest disadvantage is the risk of drowning from O2 neurotoxicity-the regulator and mouthpiece used by sports divers falling out during a convulsion.

Commercial divers regularly use nitrox but are much safer due to the better control exerted from the surface with the mixture being delivered via a hose and the diver wearing an oro-nasal mask with a helmet or a band-mask and an open-circuit demand regulator, none of which is likely to be lost during a convulsion caused by oxygen neurotoxicity.

Nitrox will be used by the recreational training agencies in open circuit breathing apparatus but it can also be used in closed and semi-closed apparatus. These rebreathers are now being introduced to the recreational diving industry and bring with them the hazards of "soda lime cocktail" and dilutional hypoxia which can lead to unconsciousness without warning and death by drowning.

65 Links to Nitrox Diving