Note: Descriptions are shown in the official language in which they were submitted.
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ANESTHETIC AGENT IDE~TIFICATION ~N~LXZER
AND CONTA~ATION DETECTOR
Field of the Invention
This invention relates generally to infrared
gas analyzers and more particularly to an infrared gas -
analyzer which identifies and quantifies anesthetic
agents and detects contaminants. ---
,
Backaround of the Invention
Anesthetization is an inherently hazardous
undertaking. Any mistakes in the procedure, while not
15 common, can have catastrophic consequences both to the -;~
patient and to the hospital. It is thus extremely ~-- -
important for physicians to Xnow what anesthetizing
:
substanc~s are being administered to patients, the -~
concentrations of those substances, and whether there
is contamination. The use of an incorrect
anesthetizing agent, because of mislabelling or -- --~i
mistake, may seriously affect a patient's well-being
even to the point of causing death. Agents used in
incorrect concentrations are likewise dangerous, -
25 particularly when the physician must consider the ~-~
varying needs of different patients. For example, a
child or a particularly weak patient may require lower
concentrations of anesthetizing agents than an average
patient. Contamination of an anesthetizing gas is
similarly dangerous to the patient's health and to the
hospital in terms of liability. In spite of this, many
operating theaters have no capability for identifying
or measuring anesthetizing agents, or detecting ~-
contamination in the stream of gas flowing to the --
35 patient. ~;
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While there are instruments for identi~ying
and measuring concentrations o~ gases, if they are
sufficiently accurate ~or hospltal use, they are
typically very expensive and bulky. Ir they are
smaller and less expensive, they are generally not
sufficiently accurate or reliable.
To promote general safety through widespread
use, a device used for purposes of identification,
quantification and detection thus should be
conveniently portable and relatively inexpensive.
Also, because of the serious consequences of mistakes,
identification and measurements must be reliable and
accurate.
lS
Identification of substances is often
accomplished by mass spectrometers. These are
instruments which ionize the particles of the substance
(thereby giving it a positive charge) and then, by
means of electric and magnetic fields, selectively
deflect the particles onto a detector according to the
particles' mass, thereby identifying them from their -
mass value. Mass spectrometer measurements are very
accurate and have come into widespread use. However, a
mass spectrometer of high quality is relatively
expensive and typically not very portable, thus making
it unsuitable for most operating theater environments.
Another method of identification is to
scatter a beam of radiation off of the particles (so-
called Raman scattering), and by analyzing the
scattered radiation, identify the substances present.
This method, however, requires relatively high power ~-
for operation (for example, a typical instrument used
for the identification of anesthetizing agents uses 3
KW power supplies). Raman scattering devices also are
~,..~-
~, "~
r ~
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relatively expensive and likely produce radio frequency
interference problems. For these reasons, these
devices are not particularly suitable for operating
theater use.
Another type of gas analyzer uses the
radiation absorption characteristics of gases in the
infrared region of the electromagnetic spectrum. Many
different kinds of such infrared gas analyzers are
known in the art. They typically utilize an infrared
source and one or more filters to produce and direct
infrared radiation through an unknown gas mixture
contained in a sample cell. The absorption effect of
the gases on the radiation is detected and electrical --
lS signals are produced and analyzed to determine the
identities and/or concentrations of the gases in the ---~
gas mixture.
Because the absorption spectra of different -
20 gases may overlap and because some gases absorb more -
strongly than others, it is often necessary to limit
detected wavelength intervals by means of narrow --
bandwidth filters. For example, the absorption band of
water vapor is very wide and that of carbon dioxide is
very strongly absorbing. In order to detect other
gases in the wavelength intervals in which these gases -~
absorb, filters designed or chosen for the detection of - ~-
particular gases must be used. Often, these filters
are placed on a filter wheel placed between the
radiation source and the detector. A gas analyzer
exemplifying these principles is described in U.S.
Patent No. 4,692,621 to Passaro et al., and assigned to
the assignee of the present invention. Passaro et al.
teaches an improved infrared gas analyzer capable of
35 high accuracy and fast response at a relatively low -~ --
cost.
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Another prior art infrared device for
identifying and determining concentrations of
anesthetizing agents includes a black-body infrared
radiation source, four opto-electronic channels (one
channel for each of four predetermined agents), four
filters, and a protocol for identifying specific
agents. The protocol consists of taking ratios of the
outputs of the four detection channels and, with
knowledge of expected concentrations of agentis,
determining the identity of an agent by comparing the
various ratios. Concentrations are determined by
utilizing a normalized channel output vs. concentration
graph. Such a device is manufactured by Teledyne
Analytical Instruments in California.
The Teledyne device, however, is accurate
only for exPected concentrations of agent.
Concentrations are not actually calculated and the
Teledyne device does not detect contamination.
Some common anesthetizing agents and their
trade names are enflurane (ethrane), isoflurane
(forane), and halothane. An anesthesized patient's
inhaled and exhaled breath will likely contain these
gases and also carbon dioxide, water vapor, nitrous
oxide, and oxygen. ~ ~-
The absorption bands of the anesthetizing
agents forane, ethrane, and halothane strongly overlap
one another and have similarly shaped absorption
curves. Halothane also is very weakly absorbing and
thus difficult to measure. Further, the concentrations
of these agents in typical anesthetizing dosages is ~;
very low (5% for forane and ethrane and .8% for
halothane), making them even more difficult to measure.
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ThQse facts, together with the presence o~ carbon
dioxide and water vapor absorption bands in the same
wavelength region, make identification and measurement
extremely difficult.
This difficulty is compounded by a source of
error common to gas analyzers called "zero-drift."
Zero drift may cause erroneous infrared radiation
transmission values which produce incorrect
identification or concentration results. Zero drift
can be produced by contamination in the measuring
system, shifts in the output of detectors (which are
inherent in many types of detectors), and temperature
changes in the measuring system. Some gas analyzers,
such as U.S. Patent No. 4,692,621 to Passaro et al.,
compensate for zero-drift by using reference filters to
provide a reference channel against which the measured
signals may be compared. Since the reference channel
utilizes the same optical path (except for the filter),
the effects of zero-drift may be compensated for to
some extent. Zero-drift, however, is still a potential
source of serious error because of differences among
the various filters and electro-optical channels in the
measuring instrument. These errors must be considered
if reliable and accurate measurements are to be made.
Accordingly, it is an object of the present
invention to provide a conveniently portable, low-cost
anesthetizing agent identification analyzer and
30 contamination detector. ;~ -
It is a further object of the present
invention to accurately identify one of the
anesthetizing agents ethrane, forane, and halothane in -~
the inhaled and exhaled breath of patients undergoing
anesthesia.
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It is another ob~ect Or the present lnvention
to detect and report the presence of contamination of
the identified anesthetizing agent.
It is yet a further object of the present
invention to measure and continuously report in real
time the concentrations of the identified anesthetizing
agent.
It is still a further ob;ect of the present
invention to identify, determine, and continuously
report the concentrations of all of the anesthetizing
agents ethrane, forane, and halothane.
Summary of the Invention
The present invention is a gas analyzer
apparatus for measuring infrared transmission through a
mixture of gases, determining the concentrations of
those gases, identifying one of the gases, reporting
the concentration of the identified gas, and detecting ` -~.
contamination of the gas. The gas analyser comprises a
sample cell for containing the gases, a source of
infrared radiation, a set of specifically chosen
filters on a filter holder, a signal processor, and a
microprocessor that computes the concentrations of the
gases and implements decision logic for identifying one
gas and detecting contamination of that gas. In one
embodiment, a filter wheel holds the filters between
the source and the sample cell and there is a single
detector placed downstream from the sample cell. In a
second embodiment, a chopper produces an AC signal from
the infrared radiation source and there are three
filtsrs, one in front of each of three detectors. An
alternate embodiment measures, calculates, and reports
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the concentrations of three anesthetizing agents.
Brief Description of the D~inq~
Figure 1 is block diagram of an infrared gas
analyzer according to the present invention.
Figure 2 illustrates the transmission curves
versus wavenumber of ethrane, forane, halothane and
three filters utilized in an embodiment of the present
invention.
~etailed Description of the Invention
The present invention is a conveniently
portable, inexpensive infrared gas analyzer system
utilizing a combination of filters chosen for their
discriminability, sensitivity, and availability at -~
relatively low cost. These filters are part of an
infrared gas analyzer system which is capable of
accurately measuring infrared transmission through
gases, and includes a microprocessor-embedded
mathematical algorithm for calculating agent
concentrations, identifying anesthetizing agents, and ~-~
25 detecting the presence of certain contaminating gases. --
A simplified block diagram of the system of
the present invention is shown in Figure 1. A gas
analyzer 10 comprises a sample cell 21, an infrared `--
source 11 which produces and transmits radation through
sample cell 21 via a filter wheel 17 having at least ---~
one filter thereon. Filter wheel 17 rotates to
successively interpose filters between source 11 and
sample cell 21. A motor 19 and a belt drive 20 operate
to rotate filter wheel 17 under the control of a signal
processor 24. Infrared radiation passing through - -
200(1305
sample cell 21 is detected by detector 15 and an
electrical signal i8 produced which i8 representative
of the intensity of the in~rared radiation by signal
processor 24. Signal processor 24 is described in
detail in U.S. Patent No. 4,692,621 to Passaro et al.
which is hereby incorporated by reference. Sample cell
21 has an inlet connection to a tube 23 which is
connected to a valve 51 which regulates the intake
between the ambient air passing through a scrubber 53
and a patient's airway. Valve 51 is controlled by
signal processor 24. Sample cell 21 has an outlet
connection to an exhaust tube 18 which is connected to
a pump 16 which is itself connected by tubing to an
oxygen (2) sensor 49 and is controlled by an
electrical connection to signal processor 24. Gases
inhaled by the patient take the "to patient airway"
path and gases exhaled by the patient take the ~-
"exhaust" path. Also electrically connected to signal
processor 24 is an ambient temperature sensor 47.
Electrically connected to signal processor 24 is a
communications board 55 for communicating between
signal processor 24 and an outboard computer 57 and
host computer 59. Host computer 59 reports the data
regarding agent identification, agent concentration,
and contamination detection.
In a first embodiment of the present
invention, filter wheel 17 has three interference type
filters which are selected to pass narrow bands of
infrared radiation, each having different band centers
at predetermined wavelengths, to provide three
measuring signals, plus a fourth filter which is added
to provide a reference signal. A fifth segment of
filter holder 17 may be used to block radiation from
sample cell 21 so that the associated signal may be
used to measure background noise from extraneous
y~
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g
radiation, electronics, detector null, and any other
optical or electronic noise. A more detailed
description of a sim~lar filter system is given in the
above-referenced U.S. Patent No. 4,692,621 to Passaro
et al.
In a second embodiment of the present
invention (not shown in Figure 1), there is a chopper
for producing a square wave AC signal between source 11
and sample cell 21. Instead of a single detector 15,
there are three detectors, one for each gas of
interest. Disposed between sample cell 21 and the
three detectors is a holder for the three filters.
Each of the three detectors has a filter in front of
its receiving end.
The difficulty of isolating the absorption
bands of the anesthetizing agents forane, ethrane, and
halothane is indicated in Figure 2 showing the ~ -~
transmission curves of these agents superimposed with
the transmission curves of three exemplary filters. ~- `
These curves are measures of the amount of infrared
radiation from infrared source 11 tFigure 1) that is -
transmitted through the agent gases contained in sample
cell 21 and detected by detector 15 as a function of -
wavenumber (the reciprocal of wavelength). The greater
the transmission, the less the absorption by the
particular gas. Curve 201 is the transmission curve
for forane, curve 202 is for ethrane, and curve 203 is
for halothane. It can be seen that these curves are
strongly overlapping, that forane and ethrane have very ~
similar transmission curve shapes, and that halothane ~-
is very weakly absorbing. The presence of carbon
.,....-.
dioxide and water vapor absorption bands in the same -
wavelength region make identification and measurement
even more difficult. Filters 210, 220, and 230 are
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chosen for discriminability sufficient to distinguish
among the gases, for sensitivity to allow measurement
of concentrations of the gases to a precision
sufficient for identification, and because they are
relatively inexpensive. Such filters are available
from, among others, Barr Associates of Nassachusetts.
The filters utilized in one embodiment of the present
invention have the following specifications (in
wavenumber units) at the operating temperatures o~ the
filters in the analyzer:
,,
Filter Center and Tolerance Bandwidth
210 3038 + 5 33 + 5
220 3012 + 8 46 + 5
230 2998 + 5 3s ~ 5
In another embodiment of the present ~.
invention, thè filters have these specifications (in
wavenumber units) again at the operating temperatures
of the filters in the analyzer:
Filter Center and Tolerance Bandwidth
210 3047 + 5 33 + 5
220 300g + 8 46 + 5
230 3017 + 5 35 ~ 5
Various other combinations of filters will be
apparent to those skilled in the art and are utilizable
without departing from the scope of the present
invention.
In operation, returning to Figure 1, inhaled
and exhaled gases from a patient are supplied to sample
cell 21 through tubes 23 and 18 respectively. Source
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--11--
11 emits infrared radiation in the wavelength region o~
interest, which radiation passes through the ~ilters in
filter wheel 17 which i8 rotated to successively
interpose the desired filter in the radiation beam by
means of control signals from signal processor 24. The
transmitted radiation is detected by detector lS which
converts the measured transmission into electrical
signals for processing by signal processor 24. Various
operating conditions sensed by appropriate sensing
devices, only some of which are illustrated, are
applied to signal processor 24. For example, ambient
temperature and oxygen are sensed by ambient
temperature sensor 47 and 2 sensor 49 respectively and
fed into signal processor 24 for inclusion in the data
stream if desired.
The operation of the second embodiment is
similar to that described above except that the chopper
chops the radiation from source ll and the AC signals
pass through sample cell 21, and the three filters in
front of the three detectors.
- - .::, .' . ,,
The transmission values and other data are
then fed by signal processor 24 to communications board ~ ~-
25 55 which controls the data to be sent to either -- -
outboard computer 57 or host computer 59. Outboard
computer 57 monitors the optical transmission values.
When transmission is detected below (or absorption is --~
above) a certain threshold level, outboard computer 57
30 initiates an agent identification algorithm (to be -
described in detail below). Agent concentrations are
calculated (using a method to be described in detail
below) and compared with threshold values to determine
which of the following is true: (1) an agent is not
present at levels above the threshold level, (2) an
agent is present at a level above the threshold level
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and that agent is identified, or (3) there is
contamination by another agent. once ldentlfled, the
appropriate one of the set o~ three filters i8
interposed by filter holder 17 and the concentration of
that agent is determined by a microprocessor-based
table look-up procedure of concentration versus
transmission. This is done by outboard computer 57
which then continuously reports the concentration of
that identified agent in approximately 14 msec
intervals. This information is then transmitted along
with the identity of the agent in the gas and the
calculated concentration of that agent to host computer~.
59 which reports the data for display.
In an alternate embodiment, for those cases
where all three agents forane, ethrane, and halothane
may be present, the present invention can calculate the
concentration of each agent and report those
concentrations. This embodiment requires a co-
processor as part of outboard computer 57 to speed up
the measuring, calculationf and reporting functions. ~-
The essential procedure is identical to that described
below.
Before operation, gas analyzer lO is
calibrated at the factory to determine the absorption
coefficients for the particular gases of interest. In
the calibration procedure, three binary gases each
consisting of one of the agent gases (forane, ethrane,
or halothane) and a carrier gas (usually nitrogen) are
passed through sample cell 21. Each filter in filter
holder 17 is interposed successively and the absorption
(transmission) of each of the agent gas/filter
combinations is measured. To calculate the absorption
coefficients, the following known procedure is utilized
with the understanding that other numbers of agents and
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filters could be used without departing from the scope
of this invention.
The absorption (A) and transmission (T) is
given by
T = ( 1 - A) = exp(-kc) (1)
where k is the absorption coefficient and c is the
concentration of the agent. For each combination of
agent and filter, the transmission ~i~ i8 given by
Ti; = eXP(-kijCj) (2)
where i = 1, 2, 3 designates each filter and ; = 1, 2,
3 designates each agent. Now, Cj is known because a -~
known concentration of each agent gas is successively
run through sample cell 21, ~ij is measured by gas
analyzer 10 in the manner described above, and equation
20 (2) is used to calculate kij, the absorption ~ -
coefficents for each combination of agent gas and
filter. The absorption coefficients are stored in - ~-
outboard computer 57 for use in the subsequent
concentration calculations. - ^-
^-
In operation of an embodiment utilizing three
filters only (no reference filter), when a gas
containing two or more agents is to be measured, the
radiation transmitted by each filter can be
approximated by
ri exp( ki1Cl) * eXp(-ki2c2) * exp(-k
.
or
Ti = eXp-(kilcl + ki2C2 + ki3C3) (4) ~-^
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which can be written as
- ln ri ~ ki1Cl + ki2C2 + kl3C3
The set of equations represented by equation (5) can be
expressed using matr~x algebra as
- ln ~ = K * c (6)
where T and Q are the column vectors (ri) and ~c~ and
K is the 3 x 3 matrix ~kij} which is just
kll kl2 kl3
` K = . k21 k22 k23 (7)
k31 k32 k33
Equation (6) can be solved using the inverse matrix K 15 to yield the concentrations c, namely
c = - ln(K~l * ~) (8)
The procedure described above is a first
order approximation. A better approximation is
produced using a set of three filters and a reference
filter which produces negligible interference. The
transmissivities of the four filters may be represented
using Beer's Law and mathematical curve-fitting
techniques for experimental concentration/transmission
data, all of which techniques are well known. Using
-
: 200Q;~05
-15-
this approach, those skilled in the art will appreciate
that a set of four non-linear equations may be
produced; These may be solved using a suitable
algorithm incorporated in outboard computer 57 of the
present invention for the gas concentrations Cf, ce,
and Ch, and the non-interfered reference transmission
Tref. The algorithm may incorporate Newton's method of
numerical solution, as is known in the art.
In this way, the concentrations of each of
the anesthetizing agents, forane, ethrane, and
halothane, can be calculated. In one embodiment of the
present invention, these concentrations may be reported
directly to a display by a co-processor as part of
outboard computer 57.
The procedures described previously generate
three gas concentrations, one each for forane, ethrane,
and halothane~ If, however, the measured filter
transmisRions ti are inaccurate (due to zero-drift)
there will be a corresponding error in the calculated
agent concentrations. In instruments of this kind,
there are also the well-known span errors (when there ~ -
are actual measurements taken with unknown gases in
sample cell 21), and errors from noise which is
inherent in detector 15. The cumulative error has been
determined empirically and has been used to establish
the threshold for identification and detection.
Cf = Cpf + dcpf -~
Ce = Cpe + dCpe
Ch = Cph + dCph
where cp; = the calculated concentration of gas j
~. ,~.. . .
200(~05
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dcp~ - the maximum error o~ the calculated
concentration of gas ~.
The errors represented by dcp~ form the detection
thresholds for each gas. The errors are different for
each analyzer, depending on many different factors
contributing to variations in analyzer performance.
The identification software of the present invention
computes (cp; - dcpj) for each gas and then implements
the following decision logic:
(a) If (cp; - dcpj) S O for each gas, then UNDETERMINED
(b) If (cp; - dcpj) > O for exactly one gas, then
SINGLE GAS
~c) If (cp; - dcpj~ ~ 0 for more than one gas, then
CONTAMINATED
~ecision (a) means that measured and then
calculated concentrations which are less than the
maximum errors represented by dcpj are not sufficiently
large for a determination.
Decision (b) indicates the presence of a
single, significantly different from zero,
concentration which serves to identify the agent.
The first time the analyzer identifies the
presence of an agent, its status is changed ~rom "no
agent identified" to "agent identified". The analyzer
then reports out the agent's concentration after
calculations, as described above, are done
automatically.
Contamination decision (c) operates as
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follows: If only one anesthetizing agent i5 being u~ed
and the derived concentrations o~ two or more agents
are larger than the assigned measurement uncertainty,
this indicates that there is contamination. If the
agent is uncontaminated, then two of the three
concentrations derived will be close to zero. Because
of the relatively low concentrations of anesthetizing
agents typically used (5% for forane and ethrane)
8% for halothane, any agent contaminant will likely
be at very low levels.
Any other substances having absorption bands
in the wavelength region covered by the present
invention may be detected as a contaminant providing
the concentration is above the detection threshold.
This would include many hydrocarbon-based substances
which have absorption bands in the region and includes
alcohol and acetone which may be present in operating
theaters. A contaminant in the wavelength region of
the anesthetizing agent in use will also be detected in
the form of greater than expected concentrations of -
that agent.
The above description of the present -
25 invention has been made with reference to an infrared `
gas analyzer for the identification and quantification `
of anesthetic agents and the detection of
contamination. It will be apparent to those skilled in ~ -
the art that the present invention is applicable to a
much larger class of gas analyzers.
Accordingly, there has been described herein
an accurate, fast-response time, conveniently portable
infrared gas analyzer suitable for operating theater
35 use. Various modifications to the present invention ` -
will become apparent to those skilled in the art from
o ~ :
200(~ 05
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the foregoing description and accompanying drawing~ and
the present invention 1~ to be limited ~olely by the
scope of the following claims.
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