AUTOMATED NONINVASIVE DETERMINATION OF MIXED VENOUS pCO//2.

A totally automated, laboratory computer-controlled noninvasive system has been developed to determine mixed venous pCO//2 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 degree C. 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. Exchange of CO//2 takes place between lungs and rebreathing bag, and the concentration of CO//2 is continuously monitored by a mass spectrometer. After a period of time, the concentration of CO//2 in the rebreathing bag, the alveoli, and the mixed venous blood come into equilibrium, demonstrated by a plateau on the record of CO//2 concentration vs. time. Compartmental analysis predicts the mixed venous pCO//2 even if an equilibrium is not established.