Method and apparatus for the removal of gases physically dissolved by dialysis in the blood

The disclosure herein describes a method and apparatus for the removal, by dialysis, of gases physically dissolved in the blood of a person, more particularly a diver or an aviator; the blood is brought into contact, in an exchange cell, with a membrane permeable to gases only; a liquid having high solubility for gases is circulated on the side of the membrane so that the gases dissolved in the blood are made to follow their chemical potential gradient across the membrane into the liquid.

FIELD OF THE INVENTION 
The present invention relates to a system for the removal of gases 
physically dissolved by dialysis in the blood of a person, more 
particularly a diver or an aviator. 
BACKGROUND OF THE INVENTION 
A diver has to breathe compressed air or a mixture of gases containing the 
appropriate amount of oxygen at a pressure equal to the hydrostatic 
pressure compressing the thorax, in order to be able to inflate the lungs. 
As the gas pressure is increased with the depth to which he descends, the 
quantity of gases physically dissolved in the blood plasma and the body 
tissues increases proportionally. While oxygen is metabolized, all other 
gases in the mixture will come out of solution again as soon as the 
hydrostatic pressure on the diver is reduced during his ascent to the 
surface. This decompression has to proceed at a very slow rate in order to 
prevent the occurrence of the phenomena seen when a bottle of a sparkling 
liquid is opened and its contents are abruptly decompressed: the dissolved 
gases are immediately released and coalesce to form bubbles. The gases 
that are not metabolized by the organism must be prevented from coming out 
of solution in this way since even small bubbles could block peripheral 
capillaries and thereby increase the load on the heart. In more serious 
cases, the gases form larger bubbles which can produce gas embolisms in 
the blood vessels of vital organs, infarctions and necroses of the 
surrounding tissues and will ultimately lead to death. 
The clinical signs of decompression sickness following a too rapid ascent 
to the surface correspond to the accumulation of gas bubbles in the 
joints. The person so afflicted minimizes the load on his/her 
articulations and maintains a distorted or bent posture, thus `the bends`. 
The later stages of decompression sickness are characterized by 
increasingly severe neurological disorders which are explained with 
functional losses in the central nervous system due to air embolisms. 
Decompression sickness is prevented by requiring divers to follow a strict 
ascent schedule which takes into account the depth reached and the 
duration of stay at depth. Ascent tables have been established 
semi-empirically and are constantly revised as the knowledge of the 
various physical and physiological parameters determining the distribution 
of gases in blood and the tissues as a function of muscular activity 
increases. Replacing nitrogen in the respired air with helium, a gas that 
has a much lower solubility in the tissues, has somewhat reduced the 
incidence of decompression sickness but an accident at work might require 
an ascent too rapid to prevent the bends even under such a "Heliox" 
(trademark) atmosphere. 
Divers brought to the surface too rapidly will always suffer from the bends 
to individually varying degrees. They will, in most cases, recuperate 
completely, i.e. without any apparent permanent lesions, if brought 
promptly into a recompression chamber where barometric pressure 
corresponding to the hydrostatic pressure at depth can be established and 
in which the slow ascent to the surface is simulated. Recompression 
chambers are very costly and are mainly found on land; only a few research 
vessels for deep-sea diving are so equipped. They are thus invariably 
located at considerable distances from the scene of a dive and the 
consequent delay in treatment increases the risk to the diver suffering 
from the symptoms of decompression sickness. Several symptomatic treatment 
schemes are available but they are only supplementary to recompression. 
The deep-sea exploration for petroleum and the mining of minerals bring 
divers even further away from the few centers where a prompt and complete 
treatment of decompression sickness can be effected. Emergency 
recompression chambers found on off-shore drilling rigs, for example, are 
small mobile units in which a diver can be evacuated but in which he 
cannot be treated. Some accidents in underwater work require immediate 
medical attention and do not leave time for the elaborate ascent schedule 
that can last several hours or even several days. Incidents have been 
reported in which recompression treatment was delayed by hours with, 
subsequently, the diver's complete recovery, but the individual variations 
are great and these cases cannot be used to establish a therapeutic 
strategy. Decompression by bringing the diver back into the water is not 
meant to replace treatment in a chamber. At present, divers are all too 
often confronted with the alternative to succumb to an injury at depth 
subsequent to only a minor working accident, or, to suffer the consequence 
of a rapid decompression, with a real possibility to die from it. 
The sudden failure of a pilot's pressurization equipment at altitude 
(explosive decompression) leads to the same phenomena and produces the 
same results. 
OBJECTS AND STATEMENT OF THE INVENTION 
It is an object of the present invention to provide a method and apparatus 
to eliminate gases physically dissolved in blood by dialysis in the 
treatment of decompression sickness of divers and aviators. As a result, 
the ascent time for divers can be reduced dramatically. 
Blood from a cannulated vein is brought into contact with a membrane 
permeable to gases only and not to the blood plasma, to the formed 
elements of the blood, nor to any ions or molecules dissolved in it. The 
gases dissolved in the plasma are made to follow their chemical potential 
gradient across the membrane or film selectively permeable to them, into a 
liquid that has a very high solubility for them and extracts them from the 
blood by virtue of its affinity. A liquid characterized by its very high 
solubility for gases, typically a perfluorocarbon or a silicone, 
circulates on the side of the membrane opposite the blood. 
The families of liquids prescribed for the present application have the 
highest uptake capacities for gases known. They are chemically inert and 
do not corrode or attack in any way the gas-permeable membrane, nor do 
they dissolve into it. They are not toxic and are, in fact, used in the 
formulation of `artificial blood` to carry oxygen to the tissues. In the 
most efficient hemodialysis arrangement, blood and the dialyzate are 
pumped through the exchange cell in a countercurrent fashion. 
Several polymers have been developed that permit the selective passage of 
gases. They are either of the solution-diffusion type, i.e., only those 
molecules soluble in the membrane itself can diffuse through it (examples: 
Silastic brand sheeting, Dow Corning, Mississauga, Ont.; dimethyl 
silicone, various types, General Electric, Schenectady, N.Y.), or, they 
permit the transfer of gases by the process of ultrafiltration, i.e., 
through exclusive pores. Vycor brand glass (Corning Glass Works, Elmira, 
N.Y.) is selectively permeable to gases by virtue of the fact that it 
contains pores with an average size of 40 .ANG. and a very narrow size 
distribution. The materials mentioned are typical and have nitrogen 
permeabilities of, respectively, 110 (Silastic), 330 (Vycor) and 400 
(silicone) ml per (min) (m.sup.2) (atm). Vycor glass has the advantage 
that it could eventually be drawn into fine capillary tubing for the 
construction of `hollow fiber` dialysis cartridges to present a very high 
exchange area. Perfluorocarbon liquids with a high and reversible uptake 
capacity for gases are available in a wide variety; typically, 
perfluorodecalin and perfluorotributylamine dissolve 28 ml nitrogen gas 
per 100 ml at temperatures between 25.degree. and 35.degree. C. 
The apparatus intended to perform gas removal by dialysis according to the 
present invention consists of a gas exchange cell, also called a dialyzer, 
and ancillary equipment whose function is to maintain the relative fluid 
flows, to monitor the conditions pertaining to the gas exchange and, 
through feedback circuits, to keep the conditions within the range set. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given hereinafter. It 
should be understood, however, that this detailed description, while 
indicating preferred embodiments of the invention, is given by way of 
illustration only since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1, blood is drawn from a cannulated vein 10 of a diver or 
aviator's arm 12 through a venous puncture. This is possible by the use, 
for example, of a commercially available single-access dual-flow catheter 
with a 18-gauge needle, as is securely used in clinical hemodialysis. Of 
course, conventional double-needle cannulas could also be used. The 
collected blood is transported through a conduit 14 to a gas exchange cell 
16. The cell may consist of a cylindrically-shaped container which is 
sealingly closed at opposite ends by means of caps or manifolds 18 and 20, 
respectively provided with an inlet port 22 and an outlet port 24. Blood 
received in conduit 14 passes by an afferent flow monitor 25 to enter 
inlet port 22 and then passes through a plurality of hollow fibers 26, 
each fiber having a gas permeable membrane into a liquid having high 
uptake capacities for gases. The blood then exits at the outlet port 24 
and is returned to the vein through conduit 28. Blood flow is supported by 
a pump 30. In the blood return conduit, an electronic air/foam detector 32 
with a warning light is provided in order to stop pump 30 if a passing 
bubble changes the optical density in the conduit. An afferent flow 
monitor 34 is also provided in the return conduit as well as a further 
monitor 36 which is used to indicate the pressure of the blood being 
returned. 
The exchange cell further includes an inlet port 38 and an outlet port 40 
to allow the dialysis liquid to pass along membranes 26. Conduits 42 and 
44 connect the inlet and outlet ports to a reservoir 46 containing the 
perfluorocarbon or silicone liquid. Pump 30 also serves to circulate the 
liquid between the reservoir 46 and the exchange cell 16. The liquid is 
brought to the body temperature by means of a heating element 48, 
controlled by a thermostat 50. On return to the reservoir, the optical 
absorbance of the liquid is monitored at 52, at the exit manifold, in 
order to detect possible blood leaks through the exchange surface. 
The exchange surface in the hollow fiber cartridge should be as large as 
possible. Areas of up to 1.5 m.sup.2 are typical for cartridges used in 
conventional hemodialysis. Cartridges about 40 cm long can hold enough 
fibers with a diameter of 200 micrometers to give an exchange area of more 
than 3 m.sup.2. Several valves (not shown) permit to interrupt the flow 
safely; ports (not shown) are provided to inject heparin or other 
anticoagulants continuously. 
The maximum volume of metabolically inert gas that dissolves in the body 
tissues hyperbaric conditions is estimated to be about 4.5 liters, a value 
extrapolated from measurements performed normobarically. Accordingly, the 
dialyzate reservoir should contain about 20 liters of perfluorocarbon 
liquid in order to maintain the appropriate partial pressure gradient with 
the blood at all times. In reality, the fluid volume can be much smaller 
than that since the release of nitrogen from the tissues is 
diffusion-limited and the 1 to 2 liters of circulating blood are rapidly 
cleared of gas. The apparatus illustrated schematically in FIG. 1 will 
easily fit into a portable canister 58 appropriately armoured to withstand 
the water pressure. 
The equipment to be used for the cannulation of a blood vessel under water 
is shown in FIG. 2. A perspex cup 60 is attached to the bend of the arm by 
two straps 62 and 64 fixing it in an extended position. A piston 66 draws 
water out of the cup in order to establish a negative pressure with 
respect to the outside. This seals the cup with its cushioned rim 68 
against the arm and makes the blood vessels under it stand out. The 
catheter and needle 70 enter the perspex cup at a shallow angle through a 
port 72, closed by a rubber membrane 74, to facilitate cannulation. The 
tubings, 14 and 28 from the cup to the canister containing the dialysis 
equipment is wrapped in a flexible armoured conduit 74. 
Only ab out 5 vol % of the oxygen bound to hemoglobin inside the red blood 
cells is normally used up by the body so that the hemoglobin on the venous 
side still holds about 16 of the 21 vol % to which the blood is charged in 
the lungs. The small fraction of oxygen dissolved physically in the plasma 
is in equilibrium with that bound to hemoglobin in the erythrocytes and 
decompression dialysis will thus also reduce the amount of oxygen carried 
by the red blood cells. It is therefore recommended that persons 
undergoing decompression dialysis respire an atmosphere enhanced in 
oxygen. Before use, the apparatus for decompression dialysis is to be 
primed, on the blood side, with a degassed physiological saline solution 
and, on the side of the dialyzate, with the degassed perfluorocarbon 
liquid. This prevents that bubbles form in the tubing or that the 
procedure would be started with bubbles adhering to the surfaces. 
Decompression dialysis as described will remove gases dissolved in the 
blood and the tissues of a diver much faster than they can be passed 
through the lungs during the slow ascent in stages prescribed by 
International Diving Tables. As a consequence, divers can be brought to 
the surface rapidly in an emergency, without suffering the effects of 
decompression sickness, or, suffering them to a much lesser extent. 
Conceivably, many lives may be saved which are otherwise lost due to the 
fact that recompression facilities are not immediately at hand. The 
principles involved in decompression dialysis are simple and the apparatus 
required is far from complicated so that the divers themselves could be 
instructed in its use at the surface or even under water as a first-aid 
measure. The process of hemodialysis started at depth can then continue 
above the surface. 
Decompression dialysis could also be used as a preventive measure to extent 
the diving times which are, at present, limited by the long ascent periods 
more than by the degree of physical exertion at depth. This application 
would require, of course, that divers submit to the inconvenience of 
having a vein cannulated before the dive, to permit prompt access to the 
dialysis equipment.