Measuring and testing – Gas analysis
Reexamination Certificate
2000-04-07
2001-10-23
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
C073S024010, C073S061610, C073S061790, C073S054060, C073S053010, C073S03200R, C422S082000
Reexamination Certificate
active
06305212
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a universal method and apparatus for determining, in real time, the individual concentrations of fluid constituents of any mixture of a predetermined number of fluids using, in the preferred embodiment, fluidic sensors. Further, the invention relates to a method and apparatus for determining or verifying the identity and/or purity of a single gas or an unknown gas in a mixture of gasses.
2. Description of the Prior Art
The determination of the relative concentrations of gasses in a mixture has been the subject of numerous inventions and intensive research over the years. Particularly, when noxious, poisonous or otherwise hazardous gasses are present, knowledge of the amount of such gasses is important to alert personnel in the area of any potential danger. In medical and clinical settings, awareness of the concentrations of respired gasses is important in the determination of patient metabolic conditions, especially the relative and absolute amounts of oxygen and carbon dioxide which provide information on the metabolization of oxygen as well as respiratory functioning. Under operating room conditions, anesthesiologists must be careful in administering anesthesia gasses and do so as a function of metabolic rate, and also must be aware of the absolute amount of anesthetic being provided in order to prevent overdosing or underdosing which would cause a patient to be aware during an operation. Also, when several different potent anesthetics must be administered during a procedure, the net amounts of the anesthetics need to be monitored to prevent overdosing.
Multiple medical gas monitors (MMGMs) continuously sample and measure inspired and exhaled (end-tidal) concentrations of respiratory gasses, including anesthetic gasses during and immediately following administration of anesthesia. These monitors are required since an overdose of anesthetic agent, and/or too little oxygen, can lead to brain damage and death, whereas too little agent results in insufficient anesthesia and subsequent awareness. The current development of these monitoring devices is described in the extensive anesthesia and biomedical engineering literature. Complete and specific information about the principles and applications of these devices is well reviewed in several recent texts (see, e.g., Lake,
Clinical Monitoring
, WB Saunders Co., pp. 479-498 (ch. 8),1990, incorporated herein by reference in its entirety), manufacturer's and trade publications (see, e.g., ECRI, “Multiple Medical Gas Monitors, Respired/Anesthetic”, August 1983, incorporated herein by reference in its entirety), and in extensive anesthesia literature describing this equipment and its principles, methods and techniques of operation.
Medical gas monitoring provides the clinician with information about the patient's physiologic status, verifies that the appropriate concentrations of delivered gases are administered, and warns of equipment failure or abnormalities in the gas delivery system. These monitors display inspired and exhaled gas concentrations and may sound alarms to alert clinical personnel when the concentration of oxygen (O
2
), carbon dioxide (CO
2
), nitrous oxide (N
2
O), or anesthetic agent falls outside the desired set limits.
Most MMGMs utilize side-stream monitoring wherein gas samples are aspirated from the breathing circuit through long, narrow-diameter tubing lines. A water trap, desiccant and/or filter may be used to remove water vapor and condensation from the sample before the gas sample reaches the analysis chamber. Gas samples are aspirated into the monitor at either an adjustable or a fixed flow rate, typically from 50 to 250 ml/min. Lower rates minimize the amount of gas removed from the breathing circuit and, therefore, from the patient's tidal volume; however, lower sampling flow rates increase the response time and typically reduce the accuracy of conventional measurements. These gas monitors eliminate the exhaust gas through a scavenging system or return certain gas constituents to the patient's breathing circuit.
There are several methods and techniques of anesthetic gas monitoring that are currently used. These methods and techniques are briefly reviewed below to distill their intrinsic advantages and disadvantages. A brief comparison is provided that includes both stand-alone and multi-operating room gas monitors that can determine concentrations of anesthetic and respiratory gases in the patient breathing circuit during anesthesia. Much of the research and development of these monitors have followed the long use of similar detector principles from analytical chemistry.
Because of the chemically diverse substances that they measure, MMGMs commonly combine more than one analytical method. Most MMGMs measure concentrations of halogenated anesthetic agents, CO
2
, and N
2
O using nondispersive infrared (IR) absorption technology; however, there are others that use photoacoustic spectroscopy, based on the sound produced when an enclosed gas is exposed to pulsed optical energy. Other MMGMs use a piezoelectric method to measure anesthetic agent concentration. Electrochemical (e.g., galvanic)fuel cells and/or paramagnetic sensors are typically used to measure oxygen concentration, primarily because of their performance characteristics. Some MMGMs also have built-in or modular pulse oximeters to monitor tissue oxygen perfusion, although there is a major problem with the ambiguity between the presence of oxygen and carbon monoxide because hemoglobin bonds with both oxygen and carbon monoxide and conventional single wavelength pulse oximeters cannot distinguish between the two.
Infrared analyzers have been used for many years to identify and assay compounds for research applications. More recently, they have been adapted for respiratory monitoring of CO
2
, N
2
O and halogenated agents. Dual-chamber nondispersive IR spectrometers pass IR energy from an incandescent filament through the sample chamber and an identical geometry but air-filled reference chamber. Each gas absorbs light at several wavelengths, but only a single absorption wavelength is selected for each gas to determine the gas concentration. The light is filtered after it passes through the chambers, and only that wavelength selected for each gas is transmitted to a detector. The light absorption in the analysis chamber is proportional to the partial pressure (e.g., concentration) of the gas. To detect halothane, enflurane, isoflurane, and other related potent anesthetics, most manufacturers use a wavelength range around 3.3 &mgr;m, the peak wavelength at which the hydrogen-carbon bond absorbs light. In one monitor that identifies and quantifies halogenated agents, the analyzer is a single-channel, four-wavelength IR filter photometer. In this monitor, each of four filters (i.e., one for each anesthetic agent and one to provide a baseline for comparison) transmits a specific wavelength of IR energy, and each gas absorbs differently in the selected wavelength bands. In another monitor, the potent anesthetic agent is assayed by determining its absorption at three different wavelengths of light. The (Vickers Medical) Datex Capnomac, a multi-gas anesthetic agent analyzer, is based on the absorption of infrared radiation. This unit accurately analyzes breath-to-breath changes in concentrations of CO
2
, NO
2
, and N
2
O and anesthetic vapors (See, McPeak et al., “Evaluation of a multigas anaesthetic monitor: the Datex Capnomac”, Anaesthesia, Vol. 43, pp.1035-1041, 1988, incorporated herein by reference in its entirety). It is accurate with CO
2
for up to 60 breaths/min, and 30 breaths/min for O
2
, but N
2
O and anesthetic vapors show a decrease in accuracy at frequencies higher than 20 breaths/min. The use of narrow wave-band filters to increase specificity for CO
2
and N
2
O makes the identification of the anesthetic vapors which are measured in the same wave band more difficult. The lnov3100 near-infrared spectroscopy monitor has been offered as a moni
metaSENSORS, Inc.
Wiggins David J.
Williams Hezron
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