Note: Descriptions are shown in the official language in which they were submitted.
90BllOUK/PG/ALF
1- ~Q~g~5
IMPROVEHENTS IN ANAEST~ETIC VAPORISERS
The present invention relates to anaesthetic vaporisers.
UK Patent No 1 224 478, describes an anaesthetic vaporiser of the
by-pass type in vhich a carrier gas such as oxygen, air or nitrous
oxide is initially divided on entry to the vaporiser between a first
stream which is directed towards the sump or vaporising chamber of the
vaporiser to entrain vapour from a volatile liquid anaesthetic
contained therein; and a second by-pass stream, the first and second
streams subsequently recombining prior to leaving the vaporiser for
delivery to a patient.
This known vaporiser has been used successfully over a number of years
for delivering anaesthetic agents such as halothane, trichloroethlene
and ether derivatives including enflurane, fluoroxene, methoxyflurane
and isoflurane. All the aforementioned anaesthetic agents have a
boiling point at atmospheric pressure well above 40C.
However, a new anaesthetic agent has been developed namely
2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane which has a boiling
point at atmospheric pressure of between 20 and 25C. This physical
characteristic of 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane
renders existing anaesthetic vaporisers unsuitable for delivering said
agent to a patient.
Conventional vaporisers of the by-pass type are unsuitable for this
new anaesthetic agent in that its boiling point is approximately in
the middle of a conventional vaporisers operating ambient temperature
range of between 15C and 35C. ~hen the ambient temperature and
hence the vaporiser temperature is above 25C heat is transferred to
the anaesthetic agent and causes an amount of vapour to boil off such
that heat lost by the latent heat of vaporisation is equal to the heat
transferred to it.
90BllOUK/PG/ALF
_ 2 - 2~ ~9 ~5
It is an aim of the present invention to provide an anaesthetic
vaporiser which is capable of deliving a predetermined concentration
of an anaesthetic agent having a boiling point at normal atmospheric
pressure of less than 30C to a patient.
According to the present invention, an anaesthetic vaporiser comprises
inlet for carrier gas and an outlet for carrier gas and anaesthetic
agent for delivery to a patient, a passageway from said inlet to a
vaporising chamber and a passage from said vaporising chamber to the
outlet, said passageway having located therein a laminar restrictor,
and a constant level tank containing liquid anaesthetic agent, a
second passage extending between said tank and the vaporising chamber,
a laminar control valve being located in said second passage and means
for transmitting the pressure in said passageway to the liquid in the
liquid level tank to drive the liquid anaesthetic agent along the
second passage for injection into the vaporising chamber.
An embodiment of the invention will now be described, by way of
example, reference being made to the Figures of the accompanying
drawings in which:-
Figure 1 is a diagrammatic sketch of an anaesthetic vaporiseraccording to the present invention; and
Figure 2 is a diagrammatic sketch of a modification of the anaestheitc
vaporiser as illustrated in Figure 1.
As shown in Figure 1, an anaesthetic vaporiser 1 has an inlet 2 for
carrier gas and an outlet 4 for carrier gas and gaseous anaesthetic
agent. Extending between the inlet 2 and the outlet 4 is a passage 6
in which is located a fixed laminar flow restrictor 8. The restrictor
8 exhibits laminar flow characteristics over its operating range.
90BllOUK/PGtALF
- 3 - ~ ~ ~9 Q75
Exending from the passage 6 from a location 9 upstream of the
restrictor 8 ls a second passage 10 vhich communications ~ith a first
chamber 14 of a constant level tank 12. The tank 12 includes a second
chamber 16 ~hich is separated from the first chamber 14 by a diaphragm
18.
The second chamber 16 is connected via a passage 20 vith a liquid
anaesthetic reservoir 22. A further passage 24 extends from the
second chamber 16 to a vaporising chamber 26. In the passage 24
bet~een the tank 12 and the vaporising chamber 26 there is located a
laminar control valve 28.
A passage 30 extends between the passage 10 and the vaporising chamber
26 and located in the passage 30 is a further fixed laminar restrictor
32. As will be explained, the passages 10, 30 form a passage~ay for a
first stream of carrier gas to flo~ to~ards the vaporising chamber 26.
Extending from the vaporising chamber 26 to join the passage 6
adjacent the oulet 4 is a passage 34.
In use, fresh carrier gas is fed to the inlet 2 of the vaporiser 1
from a conventional flow metering bank delivering typically 0.2 to 15
litres per minute of air, oxygen or nitrous oxide in various
proportions.
The carrier gas enters the vaporiser 1 and at location 9 is divided
into a first stream ~hich passes initially along passage 10 and a
second by-pass stream which passes along passage 6 through restrictor
8 towards the outlet 4. The proportion of the gas which enters the
passages 6 and 10 is determined by the value of the restrictors 8, 32
~hich have a fixed and linear (laminar) flow characteristic. The
carrier gas in the first stream passes from passage 10 into passage 30
through restrictor 32 and into vaporising chamber 26. Vithin the
vaporising chamber 26 volative liquid anaesthetic agent is injected
into the vaporising chamber and evaporated into the carrier gas
90BllOUK/PG/ALF
_ 4 _ 2 0 3 q ~ 7 ~
stream. Carrier gas and anaesthetic agent vill then leave the
vaporising chamber 26 along passage 34 and exit the vaporisor 1 at
outlet 4.
It will be evident, that the pressure of the first stream in passages
10, 30 is reflected in the pressure existing in the first chamber 14
and this pressure is transferred across the diaphragm 18 in the
constant level tank 12 to liquid contained vithin the second chamber
16. The temperature of the liquid in the tank 12 is kept sufficiently
constant so that variations in temperature hence vapour pressure of
the volatile anaesthetic agent are small compared vith the pressure of
the first stream in passages 10, 30. The liquid level in the second
chamber 16 is kept constant and at the same height as the point of
injection in the vaporising chamber 26 so that no liquid head effects
are superimposed on the pressure available to drive the liquid agent
into the chamber 26. The liquid flow from the tank 12 along passage
24 is determined by the pressure in the first stream that is the
pressure in passages 10, 30, and the setting of the laminar control
valve 28 which is connected to a dial calibrated in percentage volume
of drug in the delivered mixture. By use of the laminar
characteristics of the fixed restrictors 8, 32 and the laminar control
valve 28 the concentration of drug is independent of the carrier gas
flow into the vaporiser 1.
~ith the embodiment illustrated in Figure 1, as previously stated, it
is necessary to maintain the temperature of the liquid anaesthetic
agent in the constant level tank 12 constant so that variations in
vapour pressure are small compared vith the pressure in passages 10,
30.
In practice this is difficult to achieve particularly ~hen heating or
cooling is possible vith the flov of liquid anaesthetic agent from the
reservoir 22 to the tank 12. An alternative approach is illustrated
in Figure 2 in vhich the passage 10 is interrupted by a balanced
venting system 50.
90BllOUK/PG/ALF
- 5 - ~03~7~
If, for example, the pressure in chamber 14 increases above the
pressure in passage 10 due to the heating of the liquid anaesthetic
agent, the pressure in chamber a increases to the same value as in
chamber 14. This will cause diaphragm c to move upwards (as shown)
against the lower pressure in chamber d which is equal to the pressure
in passage 10. Movement of the diaphragm c opens a valve b releasing
pressure to atmosphere until the pressure imbalance ceases.
Conversely, if the pressure in chamber 14 falls relative to the
pressure in passage lO a valve _ opens due to the imbalance in
pressure between chambers f and h allowing gas to flow from the
passage 10 into the chamber ~ until the imbalance is eliminated.
Means (not shown) is connected between the constant level tank 12 and
the reservoir 22 for sensing the level of liquid anaesthetic in the
chamber 16 and automatically feeding liquid anaesthetic from the
reservoir to the chamber 16 when the liquid level drops below a
predetermined level.
Anaesthetic agents such as halothane, enflurane and isoflurane are
normally delivered in concentrations of approximately 5Z of the total
flow of gas administered to a patient. However, the relatively new
anaesthetic agent 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane has a
much higher vapour delivery concentration up to, for example, 18Z of
the total flow of gas administered to a patient. It is an advantage
therefore that the laminar control valve 28 be associated with the
liquid flow passage 24 rather than the usual line passing gaseous
mixture of carrier gas and anaesthetic agent.
In a modification that portion of the passage 6 between the junction 9
and the point where the passage 34 joins the passage 6 can be deleted
so that in effect all the carrier gas can be made to pass along the
passageway 10, 30 into the vaporising chamber 26.
