Patent ID: 7913690

Claim:
A method of determining the flux of a gas “X” in a subject that is ventilated or breathing spontaneously, comprising the steps of: a. providing to the subject, via a Conditional Breathing Circuit (CBC), a source gas and a second gas that has substantially the same concentration of gas “X” as in the alveoli of the lung, wherein the source gas for a given breath is provided at a flow rate (SGF) that results in the source gas entering the CBC being equal to or less than the subject's alveolar ventilation, any balance of the gas provided for the same breath being the second gas; b. determining the flux of gas “X” by: (i) determining the source gas flow (SGF) into the CBC; (ii) determining the concentration, F SX , of gas “X” in the source gas flow; (iii) determining the concentration, F EX , of gas “X” in the end expired gas; and (iv) processing data utilizing the relationship: Flux of gas “ X”=SGF ( F SX −F EX ); or Flux of gas “ X”=SGF ( F EX −F SX ); wherein: SGF=the rate of source gas flow into the CBC in liters/minute; F SX =Fractional concentration of gas “X” in the source gas; F EX =Fractional concentration of gas “X” in the end expired gas.