Respiratory gas analyzer

An apparatus for determining the concentration of a selected gas in a mixture of gases containing the selected gas and providing an electrical signal representative thereof, comprising:

• a housing;
• a sample cell supported in said housing for containing the gas mixture, said sample cell having first and second windows positioned transversely to said beam;
• source means mounted in said housing for emitting infrared radiation;
• optical means mounted in said housing for directing said radiation into a beam traveling down a beam path through said sample cell;
• measuring detector means for detecting infrared radiation passing through said sample cell at the characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;
• reference detector means for detecting infrared radiation passing through said sample cell at a wavelength other than that of said characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;
• holder means formed in said housing for manually releasably holding said sample cell in said beam path, said holder means having first and second windows corresponding to said first and second windows of said sample cell and positioned transversely to said beam, said first holder means window being positioned proximate and upstream of said first sample cell window, said second holder means window being positioned proximate and downstream of said second sample cell window;
• window temperature regulation means, for inhibiting condensate build-up on said first and second sample cell windows, comprising window heating means for applying heat to said first and second sample cell windows and window temperature sensor means for monitoring the temperature of said sample cell, said window temperature sensor means providing an output signal indicative of said temperature to a temperature control means capable of increasing or decreasing the amount of energy provided to said window heating means so as to inhibit condensate build-up on said sample cell windows, wherein said window heating means comprises resistive elements attached to said first and second windows of said holder means and means for supplying a current to said resistive elements, and further wherein said window temperature sensor means comprises a thermistor mounted on said second window of said holder means;
• and
• detector temperature regulation means, for minimizing variations in the ambient temperature surrounding said measuring detector means and said reference detector means, comprising a thermally-conductive block in which said measuring detector means and said reference detector means are seated, block heating means for applying heat to said thermally-conductive block, comprising a heater strip of resistive material attached to the outside of a wall of said thermally-conductive detector block, and block temperature sensor means for monitoring the temperature of said block and for providing an output signal indicative of said block temperature to said temperature control means, said temperature control means being capable of increasing or decreasing the amount of energy provided to said block.

For use with a signal processing system, an apparatus for determining the concentration of a selected gas in a mixture of gases containing the selected gas and providing an electrical signal representative thereof, comprising:

• a housing;
• a sample cell supported in said housing for containing the gas mixture, said sample cell having first and second windows positioned transversely to said beam;
• source means mounted in said housing for emitting infrared radiation;
• optical means mounted in said housing for directing said radiation into a beam travelling down a beam path through said sample cell;
• measuring detector means for detecting infrared radiation passing through said sample cell at the characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;
• reference detector means for detecting infrared radiation passing through said sample cell at a wavelength other than that of said characteristic absorption wavelength of the selected gas and providing an output signal representative thereof;
• holder means formed in said housing for holding said sample cell in said beam path between said source means and said receiver means;
• calibration means mounted in said housing for storing characterization information characterizing the performance of said source means, said measuring detector means, and said reference detector means, wherein said calibration means provides electrical signals representative of said information to the processing system;
• and
• span check filter means, for determining whether said characterization information accurately represents the performance of said apparatus, comprising hardware means for simulating the radiation absorption characteristics of a selected gas concentration and software means for running a span check algorithm, wherein said hardware means comprises a holder with a first window, a filter, and a second window, and means for securing said first window, said filter and said second window within said holder, whereby said hardware means is supported in said housing in lieu of said sample cell, and further wherein said software means verifies whether the value of the detector output signals representative of said simulated selected gas concentration lies within a range of prescribed values representing said selected gas concentration.

A method for measuring the concentration of carbon dioxide in a stream of respiratory gases and processing the measurement so as to provide the user with information indicative of an individual's respiratory status, comprising the steps of:

• emitting radiation;
• directing said radiation into a beam;
• exposing said stream of gases to said beam of radiation;
• splitting said beam into first and second component beams having preselected wavelengths;
• filtering said first and second component beams;
• collecting said first and second component beams;
• detecting said first and second component beams;
• measuring the intensity of radiation in said first and second component beams;
• ratioing the measured values of said radiation in said first and second component beams;
• providing an electrical signal indicative of the ratioed value to a processing apparatus, said ratioed value indicative of the concentration of carbon dioxide in said stream of respiratory gases;
• and
• processing said signal for providing the individual's respiratory rate and respiratory carbon dioxide concentration continuously at end-tidal peak and during the inspired phase to a host unit.