Free Diving and Shallow Water Blackout
Physics and Physiology
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
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. .
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.
On Ascent 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
THE
PHYSIOLOGY OF SHALLOW-WATER BLACKOUT
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
Shallow-water blackout was a hot research topic for
diving physicians in the 1960s, when they worked out the basic physiology
described above. They also studied the case histories of SWB victims,
identifying several factors that can contribute to this condition. These
include hyperventilation, exercise, a competitive personality, a focused
mind-set and 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
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
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.
Learn the basics of CPR and
think about adapting them to your diving arena, whether diving from shore,
board or boat.
Reference: Hong, SK. 1990. Breath-Hold Diving. In:
Bove and