WHAT IS SHALLOW-WATER BLACKOUT?
As has been pointed out, diving without compressed air
(breath-hold
diving, skin diving) is very different from scuba diving, since the
lungs compress on descent and decompress on ascent. Water
pressure
squeezing the lungs during a breath-hold dive is usually not great
enough to cause problems from compression of the lungs (most
breath-hold divers don't go deep enough to experience significant
lung squeeze). Middle ear discomfort is a more common problem,
and the breath-hold diver must swallow or blow against a pinched
nose to equalize ear pressures.
Since, in a breath-hold dive, air compressed on descent merely
expands back to its original volume on ascent, there is no danger of
pulmonary barotrauma. But breath hold diving is not without
hazard. Perhaps the most serious potential hazard for the
breath-hold diver is "shallow-water blackout."
Shallow-water blackout is a sudden unconsciousness from lack of
oxygen during a breath-hold dive. (The term was originally applied,
in the 1940s, to blackout from CO2 buildup seen with re-breathers;
over the years the term's definition has been changed.)
Shallow-water blackout doesn't always occur in shallow water; it can
occur at any depth. However, for reasons which will be explained,
the
breath hold diver is at greater risk for blacking out during ascent,
near the surface.
To appreciate shallow-water blackout, consider the air hunger you
feel during a breath-hold dive. When you hold your breath two
things happen in the blood; CO2 increases and O2 decreases. The
principle reason you feel air hunger is the increase in CO2, not the
decrease in O2. Without the slight increase in CO2 from breath
holding, your sensation of air hunger would be delayed and you
would stay down longer, even while your oxygen level is falling.
This
is an example where CO2 buildup is a good thing!
The risk of shallow-water blackout is increased from excessive over
breathing (hyperventilation) just prior to the dive. Hyperventilation
lowers blood CO2. At most one should take three to four deep
breaths before a breath-hold dive; more than that can lower CO2
sufficient to delay its buildup and therefore the urge to breathe and
surface for air. In other words, blood CO2 may be lowered so much
by pre-dive hyperventilation, that it takes a relatively long time for
CO2 to build up under water and cause "air hunger." The dive is
prolonged but at the diver's peril. Blood oxygen will fall relatively
quickly under water compared to the buildup of CO2. A critical
hypoxic state can be reached before there is any drive to breathe,
i.e.,
before there is any sensation of air hunger. This critical hypoxia is
often reached on ascent, near the surface, hence the term
"shallow-water blackout." However, it can occur at any depth and
lead to sudden unconsciousness and drowning.
There is another factor that contributes to hypoxia, one that helps
explain why blackout tends to occur near the surface. Even though
the body utilizes oxygen throughout the breath hold dive, at depth
the water pressure effectively increases oxygen pressure in the lungs
and the blood. All the while, of course, the diver is metabolizing
oxygen, so the total amount available is steadily declining.
Paradoxically, however, being deep is somewhat protective, because
the pressure of oxygen in the lungs and blood is higher than it
would
be at the surface with the same breath holding time. However, as the
breath hold diver rises toward the surface, the pressure of oxygen in
his lungs falls precipitously, not only because his body continues to
utilize oxygen, but also because the surrounding pressure falls. Near
the surface the breath-hold diver's blood oxygen pressure falls to a
critical level and he blacks out. (Ambient pressure falls on ascent
from a scuba dive as well, but the oxygen supply is continuously
replenished with fresh air from the tank).
A CASE OF "SHALLOW-WATER BLACKOUT"
A young scuba instructor working on a liveaboard dive boat, and
one
of the boat's male guests, decide to go breath-hold diving one
afternoon. They, and two other boat guests along just for the ride,
take a dingy out to the site of a famous wreck.
Each of the breath-hold divers carries four pounds of lead weight to
assist in descent, and while one dives the other stays in the water as
a
spotter. While the non-diving guests remain in the dingy, the divers
each make a breath hold plunge. The first dive for each lasts about
1.5
minutes, at a depth of 60 to 70 feet.
On the scuba instructor's second breath-hold dive, he goes a little
deeper and stays on the wreck a little longer. Over 2 minutes into
his
dive, he is seen to ascend quickly from the wreck, then stop at 10
feet
from the surface; at that point he shows no movement. The spotter
dives down and drags the unconscious diver to the surface. The
rescuing diver provides in-water mouth-to-mouth resuscitation
and,
with the aid of the two other people, lifts the by now
semi-conscious
diver into the dingy. The rescued diver fully regains consciousness
but remembers nothing about what happened.
A few minutes later they are back on the liveaboard. The dingy
observers reveal what an awful sensation they felt as the limp
instructor was pulled to the surface; they thought he might be dead.
The rescued scuba instructor only complains of having some chest
discomfort and feeling fatigued. He is also observed to have blue
nail
beds (cyanosis). He is given 100% O2 and, when he claims to feel
better, goes to lay down in his cabin. A few hours later he feels
worse
and has a fever; the captain decides to motor to the nearest town,
where the diver is hospitalized. Diagnosis: pneumonia (presumably
from aspiration of some sea water.) He is given antibiotics and the
next day is released; he eventually recovers fully.
Youth, diving experience and excellent physical condition allowed
the scuba instructor to stay down much longer than the average
person; this was also his (almost fatal) undoing. What happened is
that his delayed urge to breathe made him attempt an ascent too
late;
just 10 feet from the surface he blacked out from lack of oxygen.
Had
there not been an experienced spotter on the surface the instructor
would surely have drowned.
As has been pointed out, diving without compressed air
(breath-hold
diving, skin diving) is very different from scuba diving, since the
lungs compress on descent and decompress on ascent. Water
pressure
squeezing the lungs during a breath-hold dive is usually not great
enough to cause problems from compression of the lungs (most
breath-hold divers don't go deep enough to experience significant
lung squeeze). Middle ear discomfort is a more common problem,
and the breath-hold diver must swallow or blow against a pinched
nose to equalize ear pressures.
Since, in a breath-hold dive, air compressed on descent merely
expands back to its original volume on ascent, there is no danger of
pulmonary barotrauma. But breath hold diving is not without
hazard. Perhaps the most serious potential hazard for the
breath-hold diver is "shallow-water blackout."
Shallow-water blackout is a sudden unconsciousness from lack of
oxygen during a breath-hold dive. (The term was originally applied,
in the 1940s, to blackout from CO2 buildup seen with re-breathers;
over the years the term's definition has been changed.)
Shallow-water blackout doesn't always occur in shallow water; it can
occur at any depth. However, for reasons which will be explained,
the
breath hold diver is at greater risk for blacking out during ascent,
near the surface.
To appreciate shallow-water blackout, consider the air hunger you
feel during a breath-hold dive. When you hold your breath two
things happen in the blood; CO2 increases and O2 decreases. The
principle reason you feel air hunger is the increase in CO2, not the
decrease in O2. Without the slight increase in CO2 from breath
holding, your sensation of air hunger would be delayed and you
would stay down longer, even while your oxygen level is falling.
This
is an example where CO2 buildup is a good thing!
The risk of shallow-water blackout is increased from excessive over
breathing (hyperventilation) just prior to the dive. Hyperventilation
lowers blood CO2. At most one should take three to four deep
breaths before a breath-hold dive; more than that can lower CO2
sufficient to delay its buildup and therefore the urge to breathe and
surface for air. In other words, blood CO2 may be lowered so much
by pre-dive hyperventilation, that it takes a relatively long time for
CO2 to build up under water and cause "air hunger." The dive is
prolonged but at the diver's peril. Blood oxygen will fall relatively
quickly under water compared to the buildup of CO2. A critical
hypoxic state can be reached before there is any drive to breathe,
i.e.,
before there is any sensation of air hunger. This critical hypoxia is
often reached on ascent, near the surface, hence the term
"shallow-water blackout." However, it can occur at any depth and
lead to sudden unconsciousness and drowning.
There is another factor that contributes to hypoxia, one that helps
explain why blackout tends to occur near the surface. Even though
the body utilizes oxygen throughout the breath hold dive, at depth
the water pressure effectively increases oxygen pressure in the lungs
and the blood. All the while, of course, the diver is metabolizing
oxygen, so the total amount available is steadily declining.
Paradoxically, however, being deep is somewhat protective, because
the pressure of oxygen in the lungs and blood is higher than it
would
be at the surface with the same breath holding time. However, as the
breath hold diver rises toward the surface, the pressure of oxygen in
his lungs falls precipitously, not only because his body continues to
utilize oxygen, but also because the surrounding pressure falls. Near
the surface the breath-hold diver's blood oxygen pressure falls to a
critical level and he blacks out. (Ambient pressure falls on ascent
from a scuba dive as well, but the oxygen supply is continuously
replenished with fresh air from the tank).
A CASE OF "SHALLOW-WATER BLACKOUT"
A young scuba instructor working on a liveaboard dive boat, and
one
of the boat's male guests, decide to go breath-hold diving one
afternoon. They, and two other boat guests along just for the ride,
take a dingy out to the site of a famous wreck.
Each of the breath-hold divers carries four pounds of lead weight to
assist in descent, and while one dives the other stays in the water as
a
spotter. While the non-diving guests remain in the dingy, the divers
each make a breath hold plunge. The first dive for each lasts about
1.5
minutes, at a depth of 60 to 70 feet.
On the scuba instructor's second breath-hold dive, he goes a little
deeper and stays on the wreck a little longer. Over 2 minutes into
his
dive, he is seen to ascend quickly from the wreck, then stop at 10
feet
from the surface; at that point he shows no movement. The spotter
dives down and drags the unconscious diver to the surface. The
rescuing diver provides in-water mouth-to-mouth resuscitation
and,
with the aid of the two other people, lifts the by now
semi-conscious
diver into the dingy. The rescued diver fully regains consciousness
but remembers nothing about what happened.
A few minutes later they are back on the liveaboard. The dingy
observers reveal what an awful sensation they felt as the limp
instructor was pulled to the surface; they thought he might be dead.
The rescued scuba instructor only complains of having some chest
discomfort and feeling fatigued. He is also observed to have blue
nail
beds (cyanosis). He is given 100% O2 and, when he claims to feel
better, goes to lay down in his cabin. A few hours later he feels
worse
and has a fever; the captain decides to motor to the nearest town,
where the diver is hospitalized. Diagnosis: pneumonia (presumably
from aspiration of some sea water.) He is given antibiotics and the
next day is released; he eventually recovers fully.
Youth, diving experience and excellent physical condition allowed
the scuba instructor to stay down much longer than the average
person; this was also his (almost fatal) undoing. What happened is
that his delayed urge to breathe made him attempt an ascent too
late;
just 10 feet from the surface he blacked out from lack of oxygen.
Had
there not been an experienced spotter on the surface the instructor
would surely have drowned.
 |  |  |  |  |  |