Automated noninvasive determination of mixed venous pCO2.

The determination of mixed venous pCO2 is desirable for assessing the metabolic and respiratory status of a patient. A totally automated, laboratory computer-controlled noninvasive system has been developed to determine mixed venous pCO2 by an equilibrium rebreathing method or by an exponential compartmental analysis for cases in which equilibrium is not achieved. A gas mixture is charged to a 2-liter anesthesia bag contained in a thermostatically controlled chamber used to maintain the temperature at 37 degrees C. This feature improves upon past rebreathing methods and eliminates water vapor as a variable in gas composition measurement. This bag is connected to a rebreathing circuit controlled by a minicomputer. The subject breathes from a mouthpiece attached to a two-way valve and rebreathes the gas mixture for a period of 30 seconds. Inspirate and expirate hoses are placed in the rebreathing bag to ensure a more uniform gas distribution than is generally found in rebreathing systems. Exchange of CO2 takes place between lungs and rebreathing bag, and the concentration of CO2 is continuously monitored by a mass spectrometer. After a period of time, the concentration of CO2 in the rebreathing bag, the alveoli, and the mixed venous blood come into equilibrium, demonstrated by a plateau on the record of CO2 concentration vs. time. Compartmental analysis predicts the mixed venous pCO2 even if an equilibrium is not established. This feature is a significant benefit of this new method, eliminating problems associated with establishing an equilibrium, such as gas mixture volume adjustment, recirculation, and poor ventilation. The predicted value agrees with the equilibrium valve for cases in which equilibrium is reached.(ABSTRACT TRUNCATED AT 250 WORDS)