1. Field of the Invention
The present invention relates generally to a gas monitoring system, and, in particular, to an integrated sample cell and filter for use in a sidestream gas monitoring system, such filter for separating undesired liquid components from respiratory gases to be monitored in the sample cell.
2. Description of the Related Art
During medical treatment, it is often desirable to monitor and analyze a patient's exhalations to determine the gaseous composition of the exhalate. For instance, monitoring the carbon dioxide (CO2) content of a patient's exhalations is often desirable. Typically, the carbon dioxide (or other gaseous) content of a patient's exhalation is monitored by transferring a portion, or sample, of the patient's expired gases to a suitable sensing mechanism and monitoring system.
Monitoring of exhaled gases may be accomplished utilizing either mainstream or sidestream monitoring systems. In a mainstream monitoring system, the gaseous content of a patient's exhalations is measured in-situ in the patient circuit or conduit coupled to the patient's airway. In a sidestream monitoring system, on the other hand, the gas sample is transported from the patient circuit through a gas sampling line to a sensing mechanism located some distance from the main patient circuit for monitoring. As a patient's expired gases are typically fully saturated with water vapor at about 35° C., a natural consequence of the gas transport is condensation of the moisture present in the warm, moist, expired gases.
Accurate analysis of the gaseous composition of a patient's exhalation is dependent upon a number of factors including collection of a gaseous sample that is substantially free of liquid condensate, which might distort the results of the analysis. As an expired gas sample cools during transport through the gas sampling line to the sensing mechanism in a sidestream monitoring system, the water vapor contained in the sample may condense into liquid or condensate. The liquid or condensate, if permitted to reach the sensing mechanism, can have a detrimental effect on the functioning thereof and may lead to inaccurate monitoring results. Condensed liquid in the gas sampling line may also contaminate subsequent expired gas samples by being re-entrained into such subsequent samples.
In addition to the condensate, it is not uncommon to have other undesirable liquids, such as blood, mucus, medications, and the like, contained in the expired gas sample. Each of these liquids, if present in the gas sample to be monitored, may render analytical results that do not accurately reflect the patient's medical status.
There are numerous ways in which to separate undesired liquids from the patient's expired gas stream to protect the sensing mechanism. For instance, it is known place a moisture trap between the patient and the sensing mechanism to separate moisture from the exhalation gas before it enters the sensing mechanism. The challenge, however, is to achieve the separation without affecting the characteristics of the parameters being measured, e.g., the waveform of the gas to be monitored.
By way of example, carbon dioxide (CO2) is effectively present only in the patient's expired gases. Therefore, the CO2 in an exhaled gas sample, transported through a gas sampling line to the sensing mechanism, fluctuates according to the CO2 present at the point at which the sample is taken. Of course the CO2 level also varies with the patient respiratory cycle. Disturbance to this fluctuation, i.e., decreases in the fidelity of the CO2 waveform, are undesirable, because such disturbances can affect the accuracy of the CO2 measurement and the graphical display of the waveform. For this reason, removal of liquids from the exhaled gas sample is desirably accomplished in such a way that it does not substantially degrade the fidelity of the CO2 waveform. Unfortunately, conventional moisture traps often disturb the waveform to a substantial degree.
Various other techniques have been employed to filter the expired gas stream of the undesired condensate while attempting to permit the waveform to be transported undisturbed. Such techniques include absorbents, centrifugal filters, desiccants, hydrophobic membranes and hydrophilic membranes. One well-established application which has shown some success is the use of hydrophobic hollow fibers as a filter for fluid separation. However, this application often-times still results in degradation of the waveform to some degree due to the physical requirements of the interface between the hollow fibers and the sensing mechanism.
Furthermore, a prominent existing application of hydrophobic hollow fibers as a filter provides a disposable gas sample collection unit that connects to a reusable sensing mechanism. It this conventional filter arrangement, the gas sample collection unit, i.e., the sample cell, is physically located some distance from the filter. The gas passing through the filter is transported to the sample cell via a relatively long tube.
The present inventor recognized that the conventional technique of gathering and transporting the filtered gas sample to a remote sensing mechanism degrades the CO2 waveform measured at the sample cell. More specifically, the present inventor recognized that by locating the filter element some distance from the sample cell and causing the filtered gas to pass through a relatively long conduit to get from the filter to the sample cell, this arrangement effectively dampens the fidelity of the CO2 waveform, for example, by dulling the rising and falling edges of the waveform. Accordingly, a gas sampling assembly that effectively and efficiently separates moisture from a gas sample and that does not substantially degrade the waveform of expired gases measured at the sample would be advantageous.