Oxygen Treatment
Oxygen Therapy For Diving Accidents: At Atmospheric and
Hyperbaric Pressure WHAT IS OXYGEN THERAPY?
Oxygen, of course, is part of the air we breathe. Oxygen is also the
most widely prescribed "drug" in hospitals; about a quarter of all
patients entering an acute care hospital will receive inhaled oxygen
at
some point in their stay. Since air already contains 21% oxygen,
what
doctors prescribe is more accurately known as supplemental oxygen,
i.e., an inhaled oxygen concentration greater than the 21% in
surrounding air. Sometimes the gas mixture prescribed is called
"enriched" air, to distinguish it from "ordinary" air.
In hospitals, 100% oxygen is piped into each patient's room, ready
for delivery at whatever concentration needed. The actual percentage
of oxygen delivered is determined by the type of appliance used to
bring the pure oxygen from the wall source to the patient's face,
e.g.,
nasal prongs or various types of face mask. These appliances serve to
mix the 100% oxygen from the wall source with the 21% oxygen
from
ordinary air; depending on the appliance used, the percentage of
oxygen delivered to the patient can range between just above 21% to
over 90%.
The oxygen percentage a doctor orders depends on the clinical
condition of the patient. Generally, the lower the patient's oxygen
level, the higher the O2 concentration. Pure or undiluted (100%)
oxygen is only used rarely, and then only in an intensive care unit.
To minimize the risk of oxygen toxicity physicians try to keep the
oxygen concentration at 40% or lower.
WHAT IS OXYGEN TOXICITY?
Oxygen toxicity is damage to some part of the body from inhaling
too much oxygen. It is impossible to get oxygen toxicity by
inhaling
air (21% oxygen) at sea level or lower pressures. A significantly
higher percentage of oxygen than the normal 21%, or inhalation at a
significantly higher ambient pressure than sea level, can cause
oxygen toxicity. As used in hospitals, outside of a hyperbaric
chamber, the main risk is lung damage. Under water, inhaling too
much oxygen (usually a result of the higher ambient pressures), the
main risk is seizures.
WHAT IS THE DIFFERENCE BETWEEN OXYGEN THERAPY FOR
BUBBLE DISEASE AND OTHER MEDICAL CONDITIONS?
Supplemental oxygen is widely employed to improve a patient's low
oxygen level. Virtually any condition affecting the lungs can lead to
a low oxygen level: asthma, bronchitis, emphysema, pneumonia,
heart failure, etc. Apart from patients with "bubble disease" - DCS
or
AGE - if the oxygen level is not reduced there is usually no need to
prescribe oxygen therapy.
By contrast, oxygen is used in DCS and AGE to shrink bubbles that
have formed in the blood and tissues. It is usually not given to
improve a low oxygen level. A low blood oxygen level is not the
problem in DCS or AGE (unless the lungs are clogged with bubbles,
or there is some other direct effect on the lungs, e.g., aspiration
from near drowning). In any case, the initial blood oxygen level
doesn't matter. For all cases of DCS or AGE the first aid goal is to
administer 100% inhaled oxygen.
Once extra oxygen is inhaled the gas does not accumulate for later
use; it is metabolized right away. The fact that oxygen is not stored
by the body is why people die quickly when air is cut off; all
available
oxygen at that point is completely exhausted in about four minutes.
This is also the reason why, if a patient with pneumonia is low on
oxygen, it does no good to prescribe "30 minutes of oxygen twice a
day." Intermittent dosing may be appropriate for most other drugs,
because they stay in the body for some time, but not for
supplemental oxygen. To be effective for patients whose blood is low
in oxygen, supplemental oxygen has to be supplied and inhaled
continuously, until the underlying problem is corrected. (Football
players who come off the field for a few whiffs of oxygen on the
sidelines are not really benefited in any physiologic sense; by the
time they get back in the game the supplemental oxygen they
inhaled
has been completely used up.)
WHY IS 100% OXYGEN RECOMMENDED FOR ALL VICTIMS OF A
SCUBA DIVING ACCIDENT?
Scuba accident victims may be low on oxygen, especially if they have
aspirated water or developed some other lung complication, but the
main reason for 100% oxygen therapy is to shrink any gas bubbles
formed during ascent. Treatment of gas bubbles requires 100%
oxygen regardless of the blood oxygen level, and for as long as is
practical; just how long is governed by the risk of oxygen toxicity.
(Note. Examination of the patient is not a reliable guide to the
blood oxygen level. A patient can be in distress with a normal
oxygen
level, or appear calm and peaceful while succumbing from hypoxia.
Neither respiratory rate and nor the skin color are reliable guides to
hypoxia. Hospitals can measure the patient's oxygen level in various
ways. It is unlikely that any test for blood oxygen will be available
at
the site of a diving accident but, for reasons stated, none is really
needed.)
Unlike the typical patient with a low oxygen level (from pneumonia,
heart failure, asthma, etc.), the victim of bubbles from diving
should
benefit from even a short period of 100% oxygen. One hundred
percent oxygen may prevent existing bubbles from expanding, or
even
shrink them enough to provide symptomatic relief. The main reason
to have supplemental oxygen available at the site of any scuba dive
is
to treat DCS or AGE, two of the most serious dive-related problems.
For either problem hyperbaric oxygen therapy is optimal, but until
transport can be arranged to a hyperbaric chamber, the victim
should
receive 100% oxygen at atmospheric pressure, via a face mask.
The secondary reason to have supplemental oxygen available is to
treat hypoxia (low oxygen level), such as is found in victims of
near-drowning accidents. Oxygen can be very helpful for any
hypoxic
victim, but that is not the primary reason for having oxygen
available
when diving. Some might argue that this is quibbling over reasons,
but nonetheless there are two very different indications for
supplemental oxygen in dive accidents. It is primarily as treatment
for DCS/AGE that oxygen is recommended at every dive site.
REASONS TO HAVE OXYGEN AT EVERY DIVE SITE
Primary: To treat DCS/AGE
Secondary: To treat hypoxia (e.g., near-drowning, shock)
No matter what the indications, when oxygen is used as part of first
aid for a diving victim it is always given at atmospheric pressure
(i.e.,
not under hyperbaric conditions).
HOW IS OXYGEN USED AT ATMOSPHERIC PRESSURE?
On land, outside of a hyperbaric chamber, oxygen is always delivered
at the ambient (atmospheric) pressure. The oxygen concentration
administered can vary from just over 21% up to the maximum,
100%.
At sea level the ambient pressure is 760 mm Hg, also known as "one
atmosphere" (1 atm.; 14.7 psi). At an altitude of 18,000 feet the
ambient pressure is half that at sea level, and so equals 0.5 atm
(7.35
psi).
It should be apparent that the most oxygen a patient can receive on
land, outside of a hyperbaric chamber, is 100% given at sea level
pressure; this is one atmosphere of oxygen and is abbreviated "1 atm.
O2". (You could get a slightly higher oxygen pressure if you
administer 100% oxygen on land below sea level. The few habitable
areas below sea level are so close to sea level that their ambient
pressure can be considered 1 atm.)
In Denver, where the average atmospheric pressure is about 85% of
sea
level, 100% inspired oxygen equals .85 atmosphere of oxygen (.85
atm O2). One hundred per cent oxygen delivered at 18,000 feet,
where the ambient pressure is 0.5 atmosphere, is 0.5 atm. O2, etc.
WHAT ARE ATMOSPHERES OF OXYGEN?
Atmospheres of oxygen are based on the O2 concentration in the
inspired gas mixture and the ambient pressure. One atm. O2 is 100%
oxygen delivered at one atmosphere of pressure; other combinations
of concentration and pressure can also equal one atm. O2. Study the
following relationships for one, two and three atm. O2 to solidify
the concept of atm. O2 (fsw = feet sea water). Then study Figure 1,
which shows the rise in atmospheres of oxygen with depth, while
breathing compressed air.
HOW IS OXYGEN USED AT THE DIVE SITE?
Standard resuscitation and treatment protocols, including CPR when
necessary, should be followed in any life-threatening situation. The
information provided herein is intended only as a general discussion
of the rationale and use of oxygen therapy for suspected bubble
disease. Medical and first aid texts, several of which are listed in the
bibliography, can be consulted for more detailed information on
how
to provide oxygen, transport the patient, etc.
Oxygen toxicity should not be a concern when giving oxygen as part
of first aid. If possible, always give 100% oxygen via a positive
pressure (tight fitting) mask; this concentration of O2 is part of
first
aid for any scuba diving accident. At the same time, arrange to
transport the victim to an appropriate medical facility. Do not
worry
about oxygen toxicity unless 100% oxygen is administered
continuously for more than two hours. At that point, if an
appropriate medical facility has not been reached and supplemental
oxygen is still available, an air break of 15-30 minutes should begin,
followed by continuation of 100% oxygen.
If the diver's symptoms abate during treatment, 100% O2 is still
recommended if either DCS or AGE is suspected; bubbles could
reform and symptoms could recur. However, don't insist on a tight
fitting mask if the diver finds it very uncomfortable or is vomiting;
you can alternate with a loose fitting oxygen mask. In first aid for a
diving accident, it is always more important to give some oxygen
than none at all.
Since 1991 DAN has sponsored half-day courses on oxygen therapy
for certified divers. The DAN Oxygen First Aid in Dive Accidents
Course, usually held at a local dive shop, uses both lecture and
hands
on experience to teach three important aspects of first-aid oxygen
therapy: 1) how to make oxygen available at any dive site; 2) how to
use the recommended equipment; 3) the advantages of oxygen
therapy for dive accident victims.
1) OXYGEN AVAILABILITY. For field use oxygen comes in small
green tanks (typical capacity about 40 cu. ft. at 1500 psi) that look
like small scuba tanks, but contain 100% oxygen instead of
compressed air (Figure 2). Tanks of oxygen are always painted green,
either the entire tank (if made of steel) or the top portion (if
aluminum).
DAN recommends, and good practice mandates, that all dive boats
and dive facilities carry a readily accessible tank of oxygen, an
oxygen
regulator and appropriate face masks for first aid use. In fact, it is
recommended that one should avoid diving from a boat that does
not
have oxygen on board. Each year more and more dive boats are
carrying oxygen.
2) HOW TO USE. A principal focus of the DAN Oxygen Course is
on
how to use the equipment. It does no good for the distressed diver if
the boat contains a tank of oxygen that no one knows how to use.
(Note: This section is for informational purposes only. Formal
training in both CPR and use of oxygen equipment should be
obtained from qualified instructors in a hands-on situation.)
As with any tank of compressed air, a regulator is required to lower
the pressure so it can be safely delivered to the diver. Scuba air tank
regulators cannot be used for this purpose. You need a special
O2-tank regulator. A green plastic tube delivers oxygen from the
regulator to the mask, which fits over the diver's mouth and nose.
(Advantages of oxygen therapy are discussed later in this chapter.)
WHAT ARE THE TYPES OF OXYGEN MASKS AVAILABLE FOR
DIVE ACCIDENT VICTIMS?
Three types of oxygen delivery masks are available in the DAN
oxygen
kit: a demand-valve-with-mask, a pocket mask, and a
non-rebreathing mask (Table 2, Figures 3 and 4). The first two
masks
can deliver close to 100% oxygen to the diver; the last one can
deliver
about 70-80% oxygen.
The demand-valve-with-mask is similar to the demand valve found
in
all second stage scuba regulators; with each inhalation, it delivers
compressed gas (100% oxygen) to the diver's mouth and nose under
a
slight positive pressure. As with a second stage regulator, the diver
must first initiate a breath for the gas to be delivered; thus the
demand-valve-with-mask cannot be used on the non-breathing
victim. To assure close to 100% oxygen, the mask must make a tight
seal with the diver's face (i.e., no air leaks around the mask).
The positive pressure given by the demand-valve-with-mask is only
slightly higher than atmospheric. If atmospheric pressure is 760 mm
Hg, the pressure inside this mask on inhalation may be 770 mm Hg,
or only about .01% higher. This is obviously not hyperbaric therapy,
which is two or more times atmospheric pressure (i.e., 1520 mm Hg
or greater). The extra pressure inside the mask helps facilitate entry
of pure oxygen into the diver's lungs.
Three types of face masks recommend by DAN for oxygen delivery at
the site of a diving accident.
Close to 100% Victim is breathing, not vomiting; victim initiates
each breathe, which is delivered under positive pressure Tight
mask-face seal mandatory pocket mask Close to 100% Victim is not
breathing; rescuer provides all breaths Tight mask-free seal
mandatory non-rebreathing mask 70%-80% Victim is breathing
Loose fitting mask; uses large amount of oxygen
For the victim who is not breathing (apneic), the pocket mask can
be
used; it is so-called because the mask easily folds and fits into one's
pocket (Figure 4). The pocket mask is designed to fit snugly over
the
victim's mouth and nose. As long as the mask-face seal is airtight,
the victim can be ventilated through a port on the mask by the
rescuer using artificial respiration but without mouth-to-mouth
contact
The pocket mask technique of resuscitation is a variation of
standard
mouth-to-mouth breathing taught in basic CPR. (Obviously, if the
victim is also pulseless cardiac compressions must be delivered at the
same time.) A plastic tube connects the oxygen regulator to the
mask, so that each breath delivered by the rescuer should provide a
high concentration of oxygen. A one-way valve diverts the victim's
exhaled air away from the rescuer.
The third type of mask is called a non-rebreather, because it does
not
allow the victim to rebreathe any of his own exhaled air; the flow of
oxygen-enriched air through the mask is great enough to quickly
wash away exhaled air. It is an excellent backup mask to the first two
types, and can deliver somewhere around 70-80% oxygen. The
non-rebreather is useful for the victim who can't tolerate the
positive pressure mask, or who is vomiting. Also, if there are two
dive accident victims, the non-rebreather can be used for one while
the demand-valve-with-mask is used for the other (both masks are
fed from a single tank of oxygen and single regulator). One
disadvantage of the non-rebreather is that it delivers oxygen
continuously instead of only on demand, and so is relatively
wasteful
of the gas. DAN markets all the necessary oxygen therapy equipment
(regulator, masks) in one convenient kit, available to oxygen course
instructors and dive shop/boat operators. DAN will also supply the
oxygen tank and a training mannequin, as well as a special carrying
case for boats.
WHAT ARE THE ADVANTAGES OF SUPPLEMENTAL OXYGEN
THERAPY FOR DIVE ACCIDENT VICTIMS?
The overall goals of supplemental oxygen therapy are to hasten
recovery, preserve organ function, save a life. Behind these goals are
two basic reasons for using 100% oxygen when bubble disease (AGE
or DCS) is suspected:
If the victim is hypoxic (low in oxygen), the extra oxygen might
raise
the blood oxygen level enough to provide more (and sufficient)
oxygen for the brain, heart and other vital organs.
The extra oxygen will help shrink existing nitrogen or air bubbles
and
prevent others from forming, and so provide important first aid
while
the victim is transported to a hyperbaric chamber. The victim may
experience symptomatic relief with just 100% oxygen delivered at
atmospheric pressure.
WHAT IS THE PHYSIOLOGIC BASIS FOR OXYGEN THERAPY?
The two stated reasons for oxygen therapy can be better understood
by reference to blood oxygen pressures. Oxygen pressure in the
blood
is called the PaO2: partial pressure (P) of oxygen (O2) in the
arterial
blood (a). At sea level and breathing air (21% O2), normal PaO2 is
about 80 to 100 mm Hg. This means that the oxygen pressure in
the
blood, by itself, will support a column of mercury 80 to 100 mm
high. Because air pressure falls with altitude, the normal PaO2 also
falls with altitude. In Denver, for example, the normal PaO2 is only
65 to 85 mm Hg. On the summit of Mt. Everest PaO2 has been
estimated in air-breathing climbers to be only about 28 mm Hg!
(Any lower and the climbers would not have lived to report the
ascent.)
If a healthy person inhales 100% oxygen at sea level, nitrogen in the
lungs and tissues is replaced by the pure oxygen. This "washout" of
nitrogen by 100% oxygen is reflected in a much higher PaO2, about
600 mm Hg at sea level. (During the washout with 100% O2 at sea
level, the blood nitrogen pressure actually falls from about 573 mm
Hg to almost zero.)
While PaO2 goes up markedly with 100% oxygen, the actual number
of oxygen molecules in the blood goes up only slightly. A PaO2 of
600 mm Hg puts only about 7% more oxygen molecules into the
blood than a PaO2 of 100 mm Hg, even though the oxygen pressure
is six times higher.
WHY DOES A PaO2 SIX TIMES NORMAL PROVIDE ONLY 7%
MORE
OXYGEN IN THE BLOOD?
The point made in the last paragraph may be confusing and it bears
some elaboration. The reason a given percentage increase in PaO2
does not translate into the same percentage increase in oxygen
molecules (oxygen content) is because the blood hemoglobin is
almost fully saturated with oxygen at a PaO2 of 100 mm Hg.
In the normal situation, 98% of all oxygen in the blood is carried by
hemoglobin; the other 2% is dissolved in the plasma. Oxygen
pressures above 100 mm Hg add only dissolved oxygen to the blood;
at an ambient pressure of one atmosphere (sea level), this extra
dissolved oxygen is a small amount when compared to the
hemoglobin-bound oxygen, and not enough to make a big
difference
to patients with lung or heart disease.
There is a major distinction between oxygen pressure and oxygen
content in the blood. Oxygen pressure directly reflects the pressure
of inhaled oxygen, and is due solely to the unbound (dissolved)
fraction of oxygen in the blood; it is the same value regardless of
the
hemoglobin content. Oxygen content, on the other hand, reflects
the actual number of oxygen molecules in the blood, both bound to
hemoglobin and unbound (dissolved). A low hemoglobin value will
not affect a diver's oxygen pressure, but will have a direct effect in
reducing oxygen content. Table 3 shows these differences.
Inhaling 100% oxygen at one atmosphere of pressure (sea level) adds
greatly to oxygen pressure, but very little to the oxygen content.
That is why, when the goal is to improve a low PaO2 (the situation
in virtually all non-diving-accident patients who receive oxygen
therapy), there is no reason to exceed the normal PaO2 range of
80-100 mm Hg, especially as it may add some risk of oxygen
toxicity.
The goal is different for victims of AGE or DCS, where the principal
rationale for oxygen therapy is to shrink gas bubbles in the tissues
and circulation. The higher the oxygen pressure in the blood, the
faster nitrogen is kicked out of the bubbles and the quicker they will
shrink. The limiting factor in maintaining high oxygen pressures in
victims of DCS/AGE is oxygen toxicity.
WHY DOES REPLACING NITROGEN WITH OXYGEN
SHRINKBUBBLES?
Nitrogen and oxygen are handled very differently in the body.
Nitrogen, is inert; it is not metabolized by the tissues. It is just
there. Under ordinary (non-diving) conditions, the pressure of
nitrogen in our blood and tissues is the same as in our lungs and in
the atmosphere (Figure 5).
Diving upsets this equilibrium because it subjects you to changing
ambient pressures. As you descend breathing compressed air, the
pressure of inhaled air (and its individual components) changes with
surrounding water pressure. The pressure of inhaled nitrogen and
oxygen increase, and so more nitrogen and oxygen molecules enter
the blood and tissues (according to Henry's Law). Figure 5 also
shows that on descent there is a nitrogen pressure gradient from
inhaled air and lungs (highest) to blood (intermediate) to tissues
(lowest).
Nitrogen, being inert, accumulates in the body with every dive.
Oxygen, by contrast, is metabolized and doesn't accumulate in the
tissues to any great extent. (Oxygen toxicity comes from high
inspired oxygen pressures, not from any accumulation of oxygen
molecules in the body).
The extra nitrogen that accumulates on descent begins to leave on
ascent (Figure 5). As the ambient pressure decreases the nitrogen
pressure gradient reverses, so that nitrogen pressure is highest in the
tissues, intermediate in the blood and lowest in the lungs. Without
this reverse gradient divers would never remove the excess nitrogen
accumulated on any dive. DCS occurs when the nitrogen that
accumulated in the tissues on descent comes out too fast on ascent;
instead of dissolving harmlessly in the blood, from where it can be
exhaled, it forms nitrogen bubbles large enough to inflict pain or
cause blockage of blood flow.
The origin and composition of bubbles differ between AGE and DCS,
but the principle of treatment is the same for both: exchanging
nitrogen in the bubbles for oxygen. Both types of gas bubbles
contain
a high nitrogen content. In AGE bubbles it is 78% (same as ordinary
air), and in DCS bubbles it is near 100%.
Since nitrogen concentration in AGE bubbles is the same as in the
blood and air (78%), there is no gradient for nitrogen to leave the
bubbles. Eventually some oxygen from inspired air finds its way into
the bubbles and they do shrink, but that is a slow process, "too
little,
too late" for the victim. Treating AGE (and DCS) requires hastening
shrinkage of bubbles, and there are only two ways to accomplish
this:
compress the bubbles by increasing the ambient pressure, or speed
up
nitrogen's exit from the bubbles.
Increasing the ambient pressure requires a hyperbaric chamber. (In
theory, sending the diver back down in the water would also work,
but this is a tricky form of therapy and impossibly dangerous for
any
seriously ill diver. Although in-water recompression is a routine
practice in some remote commercial dive operations, it is never
recommended for the recreational diver. Without a full face mask
for
the diver, and professional support personnel both below the water
and on the surface, sending an ill recreational diver back down is an
invitation for disaster.) In lieu of a hyperbaric chamber,
supplemental oxygen is used to hasten nitrogen's exit from the
bubbles. If effect, 100% inspired oxygen is used to de-nitrogenate
the
blood and shrink any gas bubbles.
As the victim inhales 100% oxygen, nitrogen in the blood is "kicked
out" by the oxygen and is exhaled by the lungs. Initially, the
bubbles
still contain a high nitrogen pressure. However, as nitrogen leaves
the blood (replaced by oxygen), a large nitrogen gradient forms
between the inside of the bubble and the surrounding blood; this
fosters nitrogen's exit from the bubble (Figure 6). At the same time
that nitrogen exits, some oxygen enters the bubble. However,
because this O2 is given up to the tissues for metabolism, there
remains a net loss of gas molecules from the bubble and the bubble
shrinks.
In summary, the purpose in giving 100% O2 to anyone with
suspected bubble disease is to rid the blood of nitrogen, so that a
large nitrogen gradient forms between inside and outside the bubble.
In this manner nitrogen flows out of the bubbles and into the
blood,
from where it can be excreted by the lungs. This principle of therapy
requires only a high inspired oxygen concentration; it does not
require a hyperbaric chamber. Pure oxygen is the preferred washout
gas because it contains no nitrogen, and the extra oxygen does not
accumulate in the body. The risk of oxygen toxicity can be reduced
by decreasing the concentration of inhaled oxygen (e.g., a mixture
of
50% O2 + 50% N2), but this reduces the gradient for nitrogen
diffusion nitrogen egress.
Fortunately, it is not difficult to get oxygen into the bubbles.
Under
normal circumstances there is always a positive pressure gradient for
oxygen between air sacs in the lungs and the blood bathing them;
otherwise, no oxygen would enter the blood and be delivered to the
tissues. Likewise, there is a positive pressure gradient between
oxygen
in the blood and inside the bubbles; and a positive gradient between
oxygen in the bubbles and the tissues. Thus, when supplemental O2
is given, oxygen will flow:
from the lungs into the blood, then from blood into any bubbles in
the blood, and from the bubbles into the surrounding tissues, where
it is metabolized A bubble containing air will shrink slowly, as it
contains 78% non-metabolized nitrogen (which moves nowhere) and
only 21% oxygen. A bubble containing pure nitrogen (as found in
DCS) will shrink even more slowly; that it shrinks at all is due to
the
fact that some oxygen eventually finds its way into the bubble,
replacing nitrogen.
HOW IS OXYGEN THERAPY USED AT HYPERBARIC PRESSURES?
Unless trained in hyperbaric medicine, you will never be called upon
to administer this type of therapy. Even so, all divers should be
aware
of the role of hyperbaric chambers in treating bubble disease.
Hyperbaric chambers are heavy, rigid structures that can hold one or
more people in an environment of high ambient pressure. Every
hyperbaric chamber treatment is like a compressed air dive, only
there is no water and the limits of compression are rigorously
controlled by the hyperbaric operator.
In the U.S. there are about 300 operating hyperbaric chambers;
about 68% of them are the monoplace type Monoplace means that
only one person can fit inside the chamber at a time. Multiplace
chambers can accommodate two or more people, depending on their
size. In addition to treating more than one patient at a time,
multiplace chambers offer a great deal more flexibility than
monoplace chambers:
A hyperbaric tender (nurse or other medical assistant) can
accompany the patient inside the multiplace chamber and administer
to his or her needs. Tenders can rotate in and out of the chamber
while the patient continues to receive treatment.
Any ancillary equipment needed can be easily entered into the
chamber, including a mechanical ventilator.
The patient can receive 100% oxygen through a head tent or face
mask, while the tender is breathing compressed air.
Hyperbaric therapy can be administered with any percentage of
oxygen from 21% (ordinary air) to 100%. Either way, hyperbaric
therapy will increase the oxygen pressure in the patient's blood.
(Two atm. O2 can be achieved at 33 fsw breathing 100% O2, or at
282 fsw breathing compressed air). Two fundamental principles of
hyperbaric therapy hold regardless of the type of chamber used, the
concentration of oxygen inhaled, the ambient pressure achieved, or
the duration of therapy:
1) The higher the ambient pressure, the greater the shrinkage of
bubbles. Whereas 100% oxygen at atmospheric pressure can only
denitrogenate the blood and hasten nitrogen out of the bubbles,
hyperbaric pressures will actually compress the bubbles. The higher
ambient pressure inside the chamber increases the gas pressure in
the
blood, which in turn compresses the bubble according to Boyle's
law.
Even with minimal compression the victim may find immediate pain
relief. When the hyperbaric pressure is removed (equivalent to an
ascent) the bubble could re-expand; however, it does not re-expand
to the same pre-compression size, because its nitrogen content is
also reduced during compression. This is where oxygen plays a vital
role in hyperbaric therapy.
2) The higher the blood oxygen pressure, the faster the gas bubbles
will shrink. Hyperbaric oxygen therapy greatly increases oxygen
pressure in the blood, driving oxygen into the bubbles and
hastening
nitrogen's exit. This is the same mechanism as when 100% O2 is
given under atmospheric pressure, but greatly accelerated. Thus
there
are two reasons for bubble shrinkage from 100% O2 inhaled under
pressure:
denitrogenation of the blood creates a favorable nitrogen gradient
(N2 out); the high blood oxygen pressure creates a favorable oxygen
gradient (O2 in). If oxygen was not metabolized, merely replacing
nitrogen with oxygen would not decrease the bubble's size; however
oxygen, once inside the bubble, then enters the tissues, where it is
metabolized. Thus the bubble shrinks.
Hyperbaric oxygen therapy (HBO) goes one giant step further than
is
possible with 100% oxygen at atmospheric pressure. Whereas one
atm. O2 is achievable without a hyperbaric chamber, with a chamber
the patient can receive two, three or any number of O2
atmospheres.
Because hyperbaric therapy with air actually puts more nitrogen into
the patient's blood (albeit while shrinking the bubbles), most
hyperbaric physicians prefer using short periods of 100% hyperbaric
oxygen in treating bubble disease rather than just hyperbaric air.
WHAT ELSE IS HYPERBARIC THERAPY USED FOR?
Diving accidents are just one of several conditions handled by most
hospital hyperbaric centers. There is much controversy about
hyperbaric therapy in some diseases. The Undersea and Hyperbaric
Medical Society (10531 Metropolitan Ave., Kensington, MD 20895)
has developed a list of approved conditions that are generally
reimbursable by third party payers. This list includes:
Gas gangrene (infection with Clostridium perfringens bacteria)
Other severe, acute wound infections Chronic refractory
osteomyelitis (bone infection) Vascular insufficiency to an
extremity Severe carbon monoxide poisoning Life-threatening
anemia when the patient refuses blood transfusion or blood is
unavailable For non-diving conditions, hyperbaric therapy aims to
raise blood oxygen pressure much higher than achievable by 100%
O2
at one atmosphere in order to put more oxygen into the blood. For
DCS and AGE, on the other hand, the goal is to replace any
nitrogen
in the gas bubbles with oxygen, so the bubbles will shrink faster.
Note that with HBO therapy the extra dissolved oxygen in the blood
can appreciably increase the blood's total oxygen content. One
hundred percent oxygen at three atmospheres can add an extra 5
vol.% to the blood, about a quarter of the normal value. In
extremely
anemic patients this extra dissolved oxygen can be important
therapeutically, and in some cases sustain a life that might
otherwise
be lost because of low oxygen content. In bubble disease, however,
HBO is given primarily to shrink the bubbles, not to increase the
dissolved oxygen content.
WHAT IS A HYPERBARIC TREATMENT TABLE?
The U.S. Navy has developed treatment tables for DCS and AGE. An
example of the DCS table was shown in Figure 2, Section G. Note
that
these tables are always amenable to modification by the physician
treating the patient. However, all treatment tables alternate a high
concentration of oxygen (usually 100% ) with periods of breathing
air. The reason, of course, is to prevent oxygen toxicity, a potential
hazard of hyperbaric oxygen therapy.
The specific treatment table chosen, and the number of times it is
administered, will depend on the clinical course of the patient and
assessment of the hyperbaric physician. Following is a case history.
About 30 minutes after J.W. surfaced from his second dive of the
morning he felt numbness and tingling in his legs. This dive, 60
feet
for 40 minutes, followed a first dive to 110 feet for 10 minutes.
However, he admitted to diving "for just a few seconds" to 120 feet
on his first dive, to chase a large grouper.
On the boat J.W. received 100% O2 by positive pressure face mask.
The boat captain radioed ahead for an ambulance to meet at the
dock.
When the boat docked 30 minutes later, J.W. could not walk; he
had
near total paralysis of both legs but was mentally alert. He was
carried off the boat and into the waiting ambulance.
The closest hyperbaric facility was one hour away, in a local
hospital.
The ambulance attendant called ahead to have the chamber ready.
J.W. continued to receive 100% oxygen en route. In the emergency
room he was quickly examined by the hyperbaric physician on duty.
Diagnosis: Type II DCS, bubble damage to the spinal cord. A
catheter
was inserted into J.W.'s bladder so he could urinate, and an
intravenous line was started for fluids.
Approximately 15 minutes after arrival to the hospital J.W. entered
the multiplace chamber with a nurse tender; treatment was begun on
U.S. Navy Table 6. At the end of treatment he felt considerably
better
and could move his legs a little, but still felt some numbness.
He stayed in the hospital and over the next week received five more
hyperbaric treatments. By the end of the week he could walk
unassisted. He was discharged at that time, with only some residual
decrease in sensation, which improved over time.
J.W. believes he became "bent" solely because he exceeded the diving
tables. He plans to dive again and pay much more attention to his
depth and bottom time. His physician cautioned him to be very
conservative and to dive well within the dive tables.
J.W. was fortunate. The dive boat carried oxygen, a chamber was in
the area, he never developed cerebral symptoms (mental confusion,
stroke, etc.), and his paralysis cleared with hyperbaric therapy.
REFERENCES AND BIBLIOGRAPHY
SECTION H. Oxygen Therapy: At Atmospheric and Hyperbaric
Pressure
DAN Oxygen Provider Course Manual. Divers Alert Network,
Durham, NC. Updated frequently.
DAN Underwater Diving Accident Manual including Oxygen First
Aid Manual. Divers Alert Network, Durham, NC. Updated
frequently.
Donaldson, K. Oxygen and the Diver.
Hendrick W, Thompson B. Oxygen and The Scuba Diver. Best
Publishing Co., Flagstaff, AZ, 1993.
Lippmann J. Oxygen First Aid for Divers. Divers Alert Network,
1993.
Martin L. Pulmonary Physiology in Clinical Practice. C.V. Mosby
Co., St. Louis, 1987.
Wright SE, Jenkinson SG. Clinical use of hyperbaric oxygen. Clinical
Pulmonary Medicine 1994;1:237-249.
Pulmonary Medicine Home Page
Oxygen Therapy For Diving Accidents: At Atmospheric and
Hyperbaric Pressure WHAT IS OXYGEN THERAPY?
Oxygen, of course, is part of the air we breathe. Oxygen is also the
most widely prescribed "drug" in hospitals; about a quarter of all
patients entering an acute care hospital will receive inhaled oxygen
at
some point in their stay. Since air already contains 21% oxygen,
what
doctors prescribe is more accurately known as supplemental oxygen,
i.e., an inhaled oxygen concentration greater than the 21% in
surrounding air. Sometimes the gas mixture prescribed is called
"enriched" air, to distinguish it from "ordinary" air.
In hospitals, 100% oxygen is piped into each patient's room, ready
for delivery at whatever concentration needed. The actual percentage
of oxygen delivered is determined by the type of appliance used to
bring the pure oxygen from the wall source to the patient's face,
e.g.,
nasal prongs or various types of face mask. These appliances serve to
mix the 100% oxygen from the wall source with the 21% oxygen
from
ordinary air; depending on the appliance used, the percentage of
oxygen delivered to the patient can range between just above 21% to
over 90%.
The oxygen percentage a doctor orders depends on the clinical
condition of the patient. Generally, the lower the patient's oxygen
level, the higher the O2 concentration. Pure or undiluted (100%)
oxygen is only used rarely, and then only in an intensive care unit.
To minimize the risk of oxygen toxicity physicians try to keep the
oxygen concentration at 40% or lower.
WHAT IS OXYGEN TOXICITY?
Oxygen toxicity is damage to some part of the body from inhaling
too much oxygen. It is impossible to get oxygen toxicity by
inhaling
air (21% oxygen) at sea level or lower pressures. A significantly
higher percentage of oxygen than the normal 21%, or inhalation at a
significantly higher ambient pressure than sea level, can cause
oxygen toxicity. As used in hospitals, outside of a hyperbaric
chamber, the main risk is lung damage. Under water, inhaling too
much oxygen (usually a result of the higher ambient pressures), the
main risk is seizures.
WHAT IS THE DIFFERENCE BETWEEN OXYGEN THERAPY FOR
BUBBLE DISEASE AND OTHER MEDICAL CONDITIONS?
Supplemental oxygen is widely employed to improve a patient's low
oxygen level. Virtually any condition affecting the lungs can lead to
a low oxygen level: asthma, bronchitis, emphysema, pneumonia,
heart failure, etc. Apart from patients with "bubble disease" - DCS
or
AGE - if the oxygen level is not reduced there is usually no need to
prescribe oxygen therapy.
By contrast, oxygen is used in DCS and AGE to shrink bubbles that
have formed in the blood and tissues. It is usually not given to
improve a low oxygen level. A low blood oxygen level is not the
problem in DCS or AGE (unless the lungs are clogged with bubbles,
or there is some other direct effect on the lungs, e.g., aspiration
from near drowning). In any case, the initial blood oxygen level
doesn't matter. For all cases of DCS or AGE the first aid goal is to
administer 100% inhaled oxygen.
Once extra oxygen is inhaled the gas does not accumulate for later
use; it is metabolized right away. The fact that oxygen is not stored
by the body is why people die quickly when air is cut off; all
available
oxygen at that point is completely exhausted in about four minutes.
This is also the reason why, if a patient with pneumonia is low on
oxygen, it does no good to prescribe "30 minutes of oxygen twice a
day." Intermittent dosing may be appropriate for most other drugs,
because they stay in the body for some time, but not for
supplemental oxygen. To be effective for patients whose blood is low
in oxygen, supplemental oxygen has to be supplied and inhaled
continuously, until the underlying problem is corrected. (Football
players who come off the field for a few whiffs of oxygen on the
sidelines are not really benefited in any physiologic sense; by the
time they get back in the game the supplemental oxygen they
inhaled
has been completely used up.)
WHY IS 100% OXYGEN RECOMMENDED FOR ALL VICTIMS OF A
SCUBA DIVING ACCIDENT?
Scuba accident victims may be low on oxygen, especially if they have
aspirated water or developed some other lung complication, but the
main reason for 100% oxygen therapy is to shrink any gas bubbles
formed during ascent. Treatment of gas bubbles requires 100%
oxygen regardless of the blood oxygen level, and for as long as is
practical; just how long is governed by the risk of oxygen toxicity.
(Note. Examination of the patient is not a reliable guide to the
blood oxygen level. A patient can be in distress with a normal
oxygen
level, or appear calm and peaceful while succumbing from hypoxia.
Neither respiratory rate and nor the skin color are reliable guides to
hypoxia. Hospitals can measure the patient's oxygen level in various
ways. It is unlikely that any test for blood oxygen will be available
at
the site of a diving accident but, for reasons stated, none is really
needed.)
Unlike the typical patient with a low oxygen level (from pneumonia,
heart failure, asthma, etc.), the victim of bubbles from diving
should
benefit from even a short period of 100% oxygen. One hundred
percent oxygen may prevent existing bubbles from expanding, or
even
shrink them enough to provide symptomatic relief. The main reason
to have supplemental oxygen available at the site of any scuba dive
is
to treat DCS or AGE, two of the most serious dive-related problems.
For either problem hyperbaric oxygen therapy is optimal, but until
transport can be arranged to a hyperbaric chamber, the victim
should
receive 100% oxygen at atmospheric pressure, via a face mask.
The secondary reason to have supplemental oxygen available is to
treat hypoxia (low oxygen level), such as is found in victims of
near-drowning accidents. Oxygen can be very helpful for any
hypoxic
victim, but that is not the primary reason for having oxygen
available
when diving. Some might argue that this is quibbling over reasons,
but nonetheless there are two very different indications for
supplemental oxygen in dive accidents. It is primarily as treatment
for DCS/AGE that oxygen is recommended at every dive site.
REASONS TO HAVE OXYGEN AT EVERY DIVE SITE
Primary: To treat DCS/AGE
Secondary: To treat hypoxia (e.g., near-drowning, shock)
No matter what the indications, when oxygen is used as part of first
aid for a diving victim it is always given at atmospheric pressure
(i.e.,
not under hyperbaric conditions).
HOW IS OXYGEN USED AT ATMOSPHERIC PRESSURE?
On land, outside of a hyperbaric chamber, oxygen is always delivered
at the ambient (atmospheric) pressure. The oxygen concentration
administered can vary from just over 21% up to the maximum,
100%.
At sea level the ambient pressure is 760 mm Hg, also known as "one
atmosphere" (1 atm.; 14.7 psi). At an altitude of 18,000 feet the
ambient pressure is half that at sea level, and so equals 0.5 atm
(7.35
psi).
It should be apparent that the most oxygen a patient can receive on
land, outside of a hyperbaric chamber, is 100% given at sea level
pressure; this is one atmosphere of oxygen and is abbreviated "1 atm.
O2". (You could get a slightly higher oxygen pressure if you
administer 100% oxygen on land below sea level. The few habitable
areas below sea level are so close to sea level that their ambient
pressure can be considered 1 atm.)
In Denver, where the average atmospheric pressure is about 85% of
sea
level, 100% inspired oxygen equals .85 atmosphere of oxygen (.85
atm O2). One hundred per cent oxygen delivered at 18,000 feet,
where the ambient pressure is 0.5 atmosphere, is 0.5 atm. O2, etc.
WHAT ARE ATMOSPHERES OF OXYGEN?
Atmospheres of oxygen are based on the O2 concentration in the
inspired gas mixture and the ambient pressure. One atm. O2 is 100%
oxygen delivered at one atmosphere of pressure; other combinations
of concentration and pressure can also equal one atm. O2. Study the
following relationships for one, two and three atm. O2 to solidify
the concept of atm. O2 (fsw = feet sea water). Then study Figure 1,
which shows the rise in atmospheres of oxygen with depth, while
breathing compressed air.
HOW IS OXYGEN USED AT THE DIVE SITE?
Standard resuscitation and treatment protocols, including CPR when
necessary, should be followed in any life-threatening situation. The
information provided herein is intended only as a general discussion
of the rationale and use of oxygen therapy for suspected bubble
disease. Medical and first aid texts, several of which are listed in the
bibliography, can be consulted for more detailed information on
how
to provide oxygen, transport the patient, etc.
Oxygen toxicity should not be a concern when giving oxygen as part
of first aid. If possible, always give 100% oxygen via a positive
pressure (tight fitting) mask; this concentration of O2 is part of
first
aid for any scuba diving accident. At the same time, arrange to
transport the victim to an appropriate medical facility. Do not
worry
about oxygen toxicity unless 100% oxygen is administered
continuously for more than two hours. At that point, if an
appropriate medical facility has not been reached and supplemental
oxygen is still available, an air break of 15-30 minutes should begin,
followed by continuation of 100% oxygen.
If the diver's symptoms abate during treatment, 100% O2 is still
recommended if either DCS or AGE is suspected; bubbles could
reform and symptoms could recur. However, don't insist on a tight
fitting mask if the diver finds it very uncomfortable or is vomiting;
you can alternate with a loose fitting oxygen mask. In first aid for a
diving accident, it is always more important to give some oxygen
than none at all.
Since 1991 DAN has sponsored half-day courses on oxygen therapy
for certified divers. The DAN Oxygen First Aid in Dive Accidents
Course, usually held at a local dive shop, uses both lecture and
hands
on experience to teach three important aspects of first-aid oxygen
therapy: 1) how to make oxygen available at any dive site; 2) how to
use the recommended equipment; 3) the advantages of oxygen
therapy for dive accident victims.
1) OXYGEN AVAILABILITY. For field use oxygen comes in small
green tanks (typical capacity about 40 cu. ft. at 1500 psi) that look
like small scuba tanks, but contain 100% oxygen instead of
compressed air (Figure 2). Tanks of oxygen are always painted green,
either the entire tank (if made of steel) or the top portion (if
aluminum).
DAN recommends, and good practice mandates, that all dive boats
and dive facilities carry a readily accessible tank of oxygen, an
oxygen
regulator and appropriate face masks for first aid use. In fact, it is
recommended that one should avoid diving from a boat that does
not
have oxygen on board. Each year more and more dive boats are
carrying oxygen.
2) HOW TO USE. A principal focus of the DAN Oxygen Course is
on
how to use the equipment. It does no good for the distressed diver if
the boat contains a tank of oxygen that no one knows how to use.
(Note: This section is for informational purposes only. Formal
training in both CPR and use of oxygen equipment should be
obtained from qualified instructors in a hands-on situation.)
As with any tank of compressed air, a regulator is required to lower
the pressure so it can be safely delivered to the diver. Scuba air tank
regulators cannot be used for this purpose. You need a special
O2-tank regulator. A green plastic tube delivers oxygen from the
regulator to the mask, which fits over the diver's mouth and nose.
(Advantages of oxygen therapy are discussed later in this chapter.)
WHAT ARE THE TYPES OF OXYGEN MASKS AVAILABLE FOR
DIVE ACCIDENT VICTIMS?
Three types of oxygen delivery masks are available in the DAN
oxygen
kit: a demand-valve-with-mask, a pocket mask, and a
non-rebreathing mask (Table 2, Figures 3 and 4). The first two
masks
can deliver close to 100% oxygen to the diver; the last one can
deliver
about 70-80% oxygen.
The demand-valve-with-mask is similar to the demand valve found
in
all second stage scuba regulators; with each inhalation, it delivers
compressed gas (100% oxygen) to the diver's mouth and nose under
a
slight positive pressure. As with a second stage regulator, the diver
must first initiate a breath for the gas to be delivered; thus the
demand-valve-with-mask cannot be used on the non-breathing
victim. To assure close to 100% oxygen, the mask must make a tight
seal with the diver's face (i.e., no air leaks around the mask).
The positive pressure given by the demand-valve-with-mask is only
slightly higher than atmospheric. If atmospheric pressure is 760 mm
Hg, the pressure inside this mask on inhalation may be 770 mm Hg,
or only about .01% higher. This is obviously not hyperbaric therapy,
which is two or more times atmospheric pressure (i.e., 1520 mm Hg
or greater). The extra pressure inside the mask helps facilitate entry
of pure oxygen into the diver's lungs.
Three types of face masks recommend by DAN for oxygen delivery at
the site of a diving accident.
Close to 100% Victim is breathing, not vomiting; victim initiates
each breathe, which is delivered under positive pressure Tight
mask-face seal mandatory pocket mask Close to 100% Victim is not
breathing; rescuer provides all breaths Tight mask-free seal
mandatory non-rebreathing mask 70%-80% Victim is breathing
Loose fitting mask; uses large amount of oxygen
For the victim who is not breathing (apneic), the pocket mask can
be
used; it is so-called because the mask easily folds and fits into one's
pocket (Figure 4). The pocket mask is designed to fit snugly over
the
victim's mouth and nose. As long as the mask-face seal is airtight,
the victim can be ventilated through a port on the mask by the
rescuer using artificial respiration but without mouth-to-mouth
contact
The pocket mask technique of resuscitation is a variation of
standard
mouth-to-mouth breathing taught in basic CPR. (Obviously, if the
victim is also pulseless cardiac compressions must be delivered at the
same time.) A plastic tube connects the oxygen regulator to the
mask, so that each breath delivered by the rescuer should provide a
high concentration of oxygen. A one-way valve diverts the victim's
exhaled air away from the rescuer.
The third type of mask is called a non-rebreather, because it does
not
allow the victim to rebreathe any of his own exhaled air; the flow of
oxygen-enriched air through the mask is great enough to quickly
wash away exhaled air. It is an excellent backup mask to the first two
types, and can deliver somewhere around 70-80% oxygen. The
non-rebreather is useful for the victim who can't tolerate the
positive pressure mask, or who is vomiting. Also, if there are two
dive accident victims, the non-rebreather can be used for one while
the demand-valve-with-mask is used for the other (both masks are
fed from a single tank of oxygen and single regulator). One
disadvantage of the non-rebreather is that it delivers oxygen
continuously instead of only on demand, and so is relatively
wasteful
of the gas. DAN markets all the necessary oxygen therapy equipment
(regulator, masks) in one convenient kit, available to oxygen course
instructors and dive shop/boat operators. DAN will also supply the
oxygen tank and a training mannequin, as well as a special carrying
case for boats.
WHAT ARE THE ADVANTAGES OF SUPPLEMENTAL OXYGEN
THERAPY FOR DIVE ACCIDENT VICTIMS?
The overall goals of supplemental oxygen therapy are to hasten
recovery, preserve organ function, save a life. Behind these goals are
two basic reasons for using 100% oxygen when bubble disease (AGE
or DCS) is suspected:
If the victim is hypoxic (low in oxygen), the extra oxygen might
raise
the blood oxygen level enough to provide more (and sufficient)
oxygen for the brain, heart and other vital organs.
The extra oxygen will help shrink existing nitrogen or air bubbles
and
prevent others from forming, and so provide important first aid
while
the victim is transported to a hyperbaric chamber. The victim may
experience symptomatic relief with just 100% oxygen delivered at
atmospheric pressure.
WHAT IS THE PHYSIOLOGIC BASIS FOR OXYGEN THERAPY?
The two stated reasons for oxygen therapy can be better understood
by reference to blood oxygen pressures. Oxygen pressure in the
blood
is called the PaO2: partial pressure (P) of oxygen (O2) in the
arterial
blood (a). At sea level and breathing air (21% O2), normal PaO2 is
about 80 to 100 mm Hg. This means that the oxygen pressure in
the
blood, by itself, will support a column of mercury 80 to 100 mm
high. Because air pressure falls with altitude, the normal PaO2 also
falls with altitude. In Denver, for example, the normal PaO2 is only
65 to 85 mm Hg. On the summit of Mt. Everest PaO2 has been
estimated in air-breathing climbers to be only about 28 mm Hg!
(Any lower and the climbers would not have lived to report the
ascent.)
If a healthy person inhales 100% oxygen at sea level, nitrogen in the
lungs and tissues is replaced by the pure oxygen. This "washout" of
nitrogen by 100% oxygen is reflected in a much higher PaO2, about
600 mm Hg at sea level. (During the washout with 100% O2 at sea
level, the blood nitrogen pressure actually falls from about 573 mm
Hg to almost zero.)
While PaO2 goes up markedly with 100% oxygen, the actual number
of oxygen molecules in the blood goes up only slightly. A PaO2 of
600 mm Hg puts only about 7% more oxygen molecules into the
blood than a PaO2 of 100 mm Hg, even though the oxygen pressure
is six times higher.
WHY DOES A PaO2 SIX TIMES NORMAL PROVIDE ONLY 7%
MORE
OXYGEN IN THE BLOOD?
The point made in the last paragraph may be confusing and it bears
some elaboration. The reason a given percentage increase in PaO2
does not translate into the same percentage increase in oxygen
molecules (oxygen content) is because the blood hemoglobin is
almost fully saturated with oxygen at a PaO2 of 100 mm Hg.
In the normal situation, 98% of all oxygen in the blood is carried by
hemoglobin; the other 2% is dissolved in the plasma. Oxygen
pressures above 100 mm Hg add only dissolved oxygen to the blood;
at an ambient pressure of one atmosphere (sea level), this extra
dissolved oxygen is a small amount when compared to the
hemoglobin-bound oxygen, and not enough to make a big
difference
to patients with lung or heart disease.
There is a major distinction between oxygen pressure and oxygen
content in the blood. Oxygen pressure directly reflects the pressure
of inhaled oxygen, and is due solely to the unbound (dissolved)
fraction of oxygen in the blood; it is the same value regardless of
the
hemoglobin content. Oxygen content, on the other hand, reflects
the actual number of oxygen molecules in the blood, both bound to
hemoglobin and unbound (dissolved). A low hemoglobin value will
not affect a diver's oxygen pressure, but will have a direct effect in
reducing oxygen content. Table 3 shows these differences.
Inhaling 100% oxygen at one atmosphere of pressure (sea level) adds
greatly to oxygen pressure, but very little to the oxygen content.
That is why, when the goal is to improve a low PaO2 (the situation
in virtually all non-diving-accident patients who receive oxygen
therapy), there is no reason to exceed the normal PaO2 range of
80-100 mm Hg, especially as it may add some risk of oxygen
toxicity.
The goal is different for victims of AGE or DCS, where the principal
rationale for oxygen therapy is to shrink gas bubbles in the tissues
and circulation. The higher the oxygen pressure in the blood, the
faster nitrogen is kicked out of the bubbles and the quicker they will
shrink. The limiting factor in maintaining high oxygen pressures in
victims of DCS/AGE is oxygen toxicity.
WHY DOES REPLACING NITROGEN WITH OXYGEN
SHRINKBUBBLES?
Nitrogen and oxygen are handled very differently in the body.
Nitrogen, is inert; it is not metabolized by the tissues. It is just
there. Under ordinary (non-diving) conditions, the pressure of
nitrogen in our blood and tissues is the same as in our lungs and in
the atmosphere (Figure 5).
Diving upsets this equilibrium because it subjects you to changing
ambient pressures. As you descend breathing compressed air, the
pressure of inhaled air (and its individual components) changes with
surrounding water pressure. The pressure of inhaled nitrogen and
oxygen increase, and so more nitrogen and oxygen molecules enter
the blood and tissues (according to Henry's Law). Figure 5 also
shows that on descent there is a nitrogen pressure gradient from
inhaled air and lungs (highest) to blood (intermediate) to tissues
(lowest).
Nitrogen, being inert, accumulates in the body with every dive.
Oxygen, by contrast, is metabolized and doesn't accumulate in the
tissues to any great extent. (Oxygen toxicity comes from high
inspired oxygen pressures, not from any accumulation of oxygen
molecules in the body).
The extra nitrogen that accumulates on descent begins to leave on
ascent (Figure 5). As the ambient pressure decreases the nitrogen
pressure gradient reverses, so that nitrogen pressure is highest in the
tissues, intermediate in the blood and lowest in the lungs. Without
this reverse gradient divers would never remove the excess nitrogen
accumulated on any dive. DCS occurs when the nitrogen that
accumulated in the tissues on descent comes out too fast on ascent;
instead of dissolving harmlessly in the blood, from where it can be
exhaled, it forms nitrogen bubbles large enough to inflict pain or
cause blockage of blood flow.
The origin and composition of bubbles differ between AGE and DCS,
but the principle of treatment is the same for both: exchanging
nitrogen in the bubbles for oxygen. Both types of gas bubbles
contain
a high nitrogen content. In AGE bubbles it is 78% (same as ordinary
air), and in DCS bubbles it is near 100%.
Since nitrogen concentration in AGE bubbles is the same as in the
blood and air (78%), there is no gradient for nitrogen to leave the
bubbles. Eventually some oxygen from inspired air finds its way into
the bubbles and they do shrink, but that is a slow process, "too
little,
too late" for the victim. Treating AGE (and DCS) requires hastening
shrinkage of bubbles, and there are only two ways to accomplish
this:
compress the bubbles by increasing the ambient pressure, or speed
up
nitrogen's exit from the bubbles.
Increasing the ambient pressure requires a hyperbaric chamber. (In
theory, sending the diver back down in the water would also work,
but this is a tricky form of therapy and impossibly dangerous for
any
seriously ill diver. Although in-water recompression is a routine
practice in some remote commercial dive operations, it is never
recommended for the recreational diver. Without a full face mask
for
the diver, and professional support personnel both below the water
and on the surface, sending an ill recreational diver back down is an
invitation for disaster.) In lieu of a hyperbaric chamber,
supplemental oxygen is used to hasten nitrogen's exit from the
bubbles. If effect, 100% inspired oxygen is used to de-nitrogenate
the
blood and shrink any gas bubbles.
As the victim inhales 100% oxygen, nitrogen in the blood is "kicked
out" by the oxygen and is exhaled by the lungs. Initially, the
bubbles
still contain a high nitrogen pressure. However, as nitrogen leaves
the blood (replaced by oxygen), a large nitrogen gradient forms
between the inside of the bubble and the surrounding blood; this
fosters nitrogen's exit from the bubble (Figure 6). At the same time
that nitrogen exits, some oxygen enters the bubble. However,
because this O2 is given up to the tissues for metabolism, there
remains a net loss of gas molecules from the bubble and the bubble
shrinks.
In summary, the purpose in giving 100% O2 to anyone with
suspected bubble disease is to rid the blood of nitrogen, so that a
large nitrogen gradient forms between inside and outside the bubble.
In this manner nitrogen flows out of the bubbles and into the
blood,
from where it can be excreted by the lungs. This principle of therapy
requires only a high inspired oxygen concentration; it does not
require a hyperbaric chamber. Pure oxygen is the preferred washout
gas because it contains no nitrogen, and the extra oxygen does not
accumulate in the body. The risk of oxygen toxicity can be reduced
by decreasing the concentration of inhaled oxygen (e.g., a mixture
of
50% O2 + 50% N2), but this reduces the gradient for nitrogen
diffusion nitrogen egress.
Fortunately, it is not difficult to get oxygen into the bubbles.
Under
normal circumstances there is always a positive pressure gradient for
oxygen between air sacs in the lungs and the blood bathing them;
otherwise, no oxygen would enter the blood and be delivered to the
tissues. Likewise, there is a positive pressure gradient between
oxygen
in the blood and inside the bubbles; and a positive gradient between
oxygen in the bubbles and the tissues. Thus, when supplemental O2
is given, oxygen will flow:
from the lungs into the blood, then from blood into any bubbles in
the blood, and from the bubbles into the surrounding tissues, where
it is metabolized A bubble containing air will shrink slowly, as it
contains 78% non-metabolized nitrogen (which moves nowhere) and
only 21% oxygen. A bubble containing pure nitrogen (as found in
DCS) will shrink even more slowly; that it shrinks at all is due to
the
fact that some oxygen eventually finds its way into the bubble,
replacing nitrogen.
HOW IS OXYGEN THERAPY USED AT HYPERBARIC PRESSURES?
Unless trained in hyperbaric medicine, you will never be called upon
to administer this type of therapy. Even so, all divers should be
aware
of the role of hyperbaric chambers in treating bubble disease.
Hyperbaric chambers are heavy, rigid structures that can hold one or
more people in an environment of high ambient pressure. Every
hyperbaric chamber treatment is like a compressed air dive, only
there is no water and the limits of compression are rigorously
controlled by the hyperbaric operator.
In the U.S. there are about 300 operating hyperbaric chambers;
about 68% of them are the monoplace type Monoplace means that
only one person can fit inside the chamber at a time. Multiplace
chambers can accommodate two or more people, depending on their
size. In addition to treating more than one patient at a time,
multiplace chambers offer a great deal more flexibility than
monoplace chambers:
A hyperbaric tender (nurse or other medical assistant) can
accompany the patient inside the multiplace chamber and administer
to his or her needs. Tenders can rotate in and out of the chamber
while the patient continues to receive treatment.
Any ancillary equipment needed can be easily entered into the
chamber, including a mechanical ventilator.
The patient can receive 100% oxygen through a head tent or face
mask, while the tender is breathing compressed air.
Hyperbaric therapy can be administered with any percentage of
oxygen from 21% (ordinary air) to 100%. Either way, hyperbaric
therapy will increase the oxygen pressure in the patient's blood.
(Two atm. O2 can be achieved at 33 fsw breathing 100% O2, or at
282 fsw breathing compressed air). Two fundamental principles of
hyperbaric therapy hold regardless of the type of chamber used, the
concentration of oxygen inhaled, the ambient pressure achieved, or
the duration of therapy:
1) The higher the ambient pressure, the greater the shrinkage of
bubbles. Whereas 100% oxygen at atmospheric pressure can only
denitrogenate the blood and hasten nitrogen out of the bubbles,
hyperbaric pressures will actually compress the bubbles. The higher
ambient pressure inside the chamber increases the gas pressure in
the
blood, which in turn compresses the bubble according to Boyle's
law.
Even with minimal compression the victim may find immediate pain
relief. When the hyperbaric pressure is removed (equivalent to an
ascent) the bubble could re-expand; however, it does not re-expand
to the same pre-compression size, because its nitrogen content is
also reduced during compression. This is where oxygen plays a vital
role in hyperbaric therapy.
2) The higher the blood oxygen pressure, the faster the gas bubbles
will shrink. Hyperbaric oxygen therapy greatly increases oxygen
pressure in the blood, driving oxygen into the bubbles and
hastening
nitrogen's exit. This is the same mechanism as when 100% O2 is
given under atmospheric pressure, but greatly accelerated. Thus
there
are two reasons for bubble shrinkage from 100% O2 inhaled under
pressure:
denitrogenation of the blood creates a favorable nitrogen gradient
(N2 out); the high blood oxygen pressure creates a favorable oxygen
gradient (O2 in). If oxygen was not metabolized, merely replacing
nitrogen with oxygen would not decrease the bubble's size; however
oxygen, once inside the bubble, then enters the tissues, where it is
metabolized. Thus the bubble shrinks.
Hyperbaric oxygen therapy (HBO) goes one giant step further than
is
possible with 100% oxygen at atmospheric pressure. Whereas one
atm. O2 is achievable without a hyperbaric chamber, with a chamber
the patient can receive two, three or any number of O2
atmospheres.
Because hyperbaric therapy with air actually puts more nitrogen into
the patient's blood (albeit while shrinking the bubbles), most
hyperbaric physicians prefer using short periods of 100% hyperbaric
oxygen in treating bubble disease rather than just hyperbaric air.
WHAT ELSE IS HYPERBARIC THERAPY USED FOR?
Diving accidents are just one of several conditions handled by most
hospital hyperbaric centers. There is much controversy about
hyperbaric therapy in some diseases. The Undersea and Hyperbaric
Medical Society (10531 Metropolitan Ave., Kensington, MD 20895)
has developed a list of approved conditions that are generally
reimbursable by third party payers. This list includes:
Gas gangrene (infection with Clostridium perfringens bacteria)
Other severe, acute wound infections Chronic refractory
osteomyelitis (bone infection) Vascular insufficiency to an
extremity Severe carbon monoxide poisoning Life-threatening
anemia when the patient refuses blood transfusion or blood is
unavailable For non-diving conditions, hyperbaric therapy aims to
raise blood oxygen pressure much higher than achievable by 100%
O2
at one atmosphere in order to put more oxygen into the blood. For
DCS and AGE, on the other hand, the goal is to replace any
nitrogen
in the gas bubbles with oxygen, so the bubbles will shrink faster.
Note that with HBO therapy the extra dissolved oxygen in the blood
can appreciably increase the blood's total oxygen content. One
hundred percent oxygen at three atmospheres can add an extra 5
vol.% to the blood, about a quarter of the normal value. In
extremely
anemic patients this extra dissolved oxygen can be important
therapeutically, and in some cases sustain a life that might
otherwise
be lost because of low oxygen content. In bubble disease, however,
HBO is given primarily to shrink the bubbles, not to increase the
dissolved oxygen content.
WHAT IS A HYPERBARIC TREATMENT TABLE?
The U.S. Navy has developed treatment tables for DCS and AGE. An
example of the DCS table was shown in Figure 2, Section G. Note
that
these tables are always amenable to modification by the physician
treating the patient. However, all treatment tables alternate a high
concentration of oxygen (usually 100% ) with periods of breathing
air. The reason, of course, is to prevent oxygen toxicity, a potential
hazard of hyperbaric oxygen therapy.
The specific treatment table chosen, and the number of times it is
administered, will depend on the clinical course of the patient and
assessment of the hyperbaric physician. Following is a case history.
About 30 minutes after J.W. surfaced from his second dive of the
morning he felt numbness and tingling in his legs. This dive, 60
feet
for 40 minutes, followed a first dive to 110 feet for 10 minutes.
However, he admitted to diving "for just a few seconds" to 120 feet
on his first dive, to chase a large grouper.
On the boat J.W. received 100% O2 by positive pressure face mask.
The boat captain radioed ahead for an ambulance to meet at the
dock.
When the boat docked 30 minutes later, J.W. could not walk; he
had
near total paralysis of both legs but was mentally alert. He was
carried off the boat and into the waiting ambulance.
The closest hyperbaric facility was one hour away, in a local
hospital.
The ambulance attendant called ahead to have the chamber ready.
J.W. continued to receive 100% oxygen en route. In the emergency
room he was quickly examined by the hyperbaric physician on duty.
Diagnosis: Type II DCS, bubble damage to the spinal cord. A
catheter
was inserted into J.W.'s bladder so he could urinate, and an
intravenous line was started for fluids.
Approximately 15 minutes after arrival to the hospital J.W. entered
the multiplace chamber with a nurse tender; treatment was begun on
U.S. Navy Table 6. At the end of treatment he felt considerably
better
and could move his legs a little, but still felt some numbness.
He stayed in the hospital and over the next week received five more
hyperbaric treatments. By the end of the week he could walk
unassisted. He was discharged at that time, with only some residual
decrease in sensation, which improved over time.
J.W. believes he became "bent" solely because he exceeded the diving
tables. He plans to dive again and pay much more attention to his
depth and bottom time. His physician cautioned him to be very
conservative and to dive well within the dive tables.
J.W. was fortunate. The dive boat carried oxygen, a chamber was in
the area, he never developed cerebral symptoms (mental confusion,
stroke, etc.), and his paralysis cleared with hyperbaric therapy.
REFERENCES AND BIBLIOGRAPHY
SECTION H. Oxygen Therapy: At Atmospheric and Hyperbaric
Pressure
DAN Oxygen Provider Course Manual. Divers Alert Network,
Durham, NC. Updated frequently.
DAN Underwater Diving Accident Manual including Oxygen First
Aid Manual. Divers Alert Network, Durham, NC. Updated
frequently.
Donaldson, K. Oxygen and the Diver.
Hendrick W, Thompson B. Oxygen and The Scuba Diver. Best
Publishing Co., Flagstaff, AZ, 1993.
Lippmann J. Oxygen First Aid for Divers. Divers Alert Network,
1993.
Martin L. Pulmonary Physiology in Clinical Practice. C.V. Mosby
Co., St. Louis, 1987.
Wright SE, Jenkinson SG. Clinical use of hyperbaric oxygen. Clinical
Pulmonary Medicine 1994;1:237-249.
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