Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
1999-05-18
2001-08-21
Lacyk, John P. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C250S345000
Reexamination Certificate
active
06277081
ABSTRACT:
TECHNICAL FIELD
This invention relates to an apparatus for detecting and measuring the concentration of various unknown gases. In particular, this system may be used to detect and measure the concentrations of CO
2
, N
2
O, and five anesthetic agents in a respiratory gas stream.
BACKGROUND
In order to safely administer anesthetic gases to a patient under anesthesia, anesthesiologists need to be able to monitor the concentration of certain gases which patients inhale and exhale to prevent the risk of supplying too much or too little gas. Carbon dioxide (CO
2
) is not administered, but it and the administered anesthetic gases nitrous oxide (N
2
O), halothane, enflurane, isoflurane, sevoflurane, and desflurane are monitored. These anesthetic gases may be administered as a single agent gas, or as a mixture of agent gases.
Because expired CO
2
is a reliable indicator of the carbon dioxide concentration in the arterial blood, the concentration of expired CO
2
is often the most critical of the gases to observe. This supervision helps to prevent excess CO
2
from being delivered to the patient, by preventing malfunctions in the anesthetic breathing apparatus.
With the help of modern technology, gas analyzers which utilize infrared radiation have been developed to measure the concentration of CO
2
and the anesthetic gases. These gas analyzers take advantage of the known infrared absorption characteristics of CO
2
and the anesthetic gases to determine which gases are present. In other words, in a typical gas analyzer, an infrared light would be emitted through a respiratory gas stream in the main (or side) airway, and onto an infrared sensor/detector device which contain certain infrared bandpass filters and a set of thermopiles to ultimately determine the concentration of carbon dioxide or any of the anesthetic gases. For convenience purposes, each combination of an infrared bandpass filter and its respective thermopile will be referred to as a “channel”. In this respect, a “channel” thus does not define an actual physical pathway, but rather, is an imaginary pathway arbitrarily defined as in the previous sentence in order to easier describe the invention herein.
One example of such analyzers includes that disclosed in U.S. Pat. No. 5,081,998 entitled OPTICALLY STABILIZED INFRARED ENERGY DETECTOR (the disclosure of which is incorporated herein by reference). As shown in
FIG. 1
, this analyzer uses pairs-of thermopiles connected in series opposed relation, with the first and second thermopiles of each pair located next to each other, whereby each thermopile is preceded by an optical bandpass or a neutral density filter to permit different infrared radiation wavelengths to reach each thermopile in the pair. This resulting difference in output is then used to eliminate the effects of background thermal noise. However, difficulties were encountered due to the space limitations in trying to arrange six or more independent analytical channels to detect additional anesthetic gases over a restricted mounting area.
This space arrangement problem was apparently improved upon in the apparatus disclosed in U.S. Pat. No. 5,296,706, entitled SHUTTERLESS MAINSTREAM DISCRIMINATING ANESTHETIC AGENT ANALYZER (the disclosure of which is incorporated herein by reference). As shown in
FIG. 2
, this apparatus uses a first and second thermopile connected in a “parallel opposed” fashion for each independent detector channel. In other words, there are two thermopiles for every detector channel: the first thermopile located in the path of the incident light and the second thermopile shielded from all incident light and located directly behind the first. The purpose of the second thermopile is to produce reference output representative of ambient temperature transients. Upon detection of carbon dioxide and the anesthetic gases, the concentrations of the gases are then calculated using a second order polynomial equation having cross product terms.
The present inventors have found that it was difficult to construct a sensor with seven to ten channels, each with a reference detector connected in a parallel opposed manner directly behind its corresponding infrared detector. The present inventors also found that the use of large order polynomials to calibrate and compensate the measurement was difficult to accomplish.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an improved anesthetic gas detection apparatus suitable for measuring carbon dioxide and more than five anesthetic gases in a respiratory gas stream.
It is another object of this invention to provide an improved anesthetic gas detection apparatus in which only one blocked detector reference detector is used as a reference point for ambient temperature transients.
It is still another object of this invention to provide an improved anesthetic gas detection apparatus which uses a novel calibration process to compensate for errors such as thermal drifts and cross coupling when detecting carbon dioxide and the anesthetic gases.
It is yet another object of this invention to provide an improved anesthetic gas detection apparatus which uses a novel processing means to determine the concentration of carbon dioxide and the anesthetic gases.
Still a further object of the present invention is to provide a durable, reliable, and economical anesthetic gas detection apparatus for use with patients under anesthesia.
Thus, the present invention overcomes the above mentioned problems in the art by using the output of a single blocked detector as a reference point for any ambient temperature transients. This novel aspect of the invention eliminates the need to have a corresponding reference infrared detector for each independent infrared detector.
In the preferred embodiment, this apparatus comprises:
a source of infrared radiation for illuminating the gas conduit;
a plurality of independent infrared detectors, each with an infrared bandpass filter positioned before the detector to permit only infrared radiation of a particular wavelength to reach each respective infrared detector;
one or more reference infrared detectors, each with an infrared bandpass filter positioned before the detector to permit only infrared radiation of a particular wavelength to reach each respective reference infrared detector;
a blocked detector infrared detector, with an opaque filter to prevent substantially all infrared radiation from reaching the blocked detector infrared detector; and
processing means for thermally correcting each independent infrared detector output by algebraic removal of a scaled version of a channel's output and the blocked detector infrared detector from the output of each independent infrared detector.
Since each gas will absorb different wavelengths of the infrared radiation, each infrared filter preceding a detector element is designed to only allow selected wavelengths of the infrared radiation to pass through the filter.
Another novel aspect of this invention is the process by which the sensor is calibrated and the gas concentrations are determined. Such a method, in accordance with a preferred embodiment of the invention, may comprise the steps of:
measuring the voltage across each individual thermopile,
measuring and compensating for the effects of certain factors such as offset voltage, wideband and thermal changes, and cross coupling, which may distort the accuracy of the thermopile measurements;
determining the concentration of the selected gases based on the results of the calibration process; and
zeroing the system at predetermined intervals.
By using the novel calibration and concentration determination process of this invention, accurate results can be obtained when determining the concentration of the various gases in this system since the factors which would distort such measurements are accounted for. Such method will be further described below.
REFERENCES:
patent: 4069420 (1978-01-01), Ross
patent: 4180734 (1979-12-01), Gedeon
patent: 4549080 (1985-10-01), Baskins
patent: 4692621 (1987-09-01), Passaro
patent:
Hefele David
Moore John
Susi Roger
Weeks Arthur R.
Hopgood, Calimafde, Judlowe & Monodolino LLP
Invivo Research Inc.
Lacyk John P.
Szmal Brian
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