CARDIAC-OUTPUT DETERMINATION BY SIMPLE ONE-STEP REBREATHING TECHNIQUE

A rebreathing technique was developed for measuring cardiac output in resting or exercising subjects. The data needed are the persons CO2 dissociation curve, the initial volume and CO2 fraction of the rebreathing bag and a record of CO2 at the mouth during the maneuver. From these all the values required to solve the Fick equation can be obtained. The combined error due to inaccuracy in reading the tracings and to the simplifying assumptions was found to be small (mean = 0.5%, SD = 2.5%). Cardiac output values determined with this technique in normal subjects were on the average 2% higher than those obtained simultaneously with an acetylene rebreathing method (n = 49, SD = 11%). Among the advantages of the technique are that it requires analysis of a single gas, takes less than 30 s per determination, allows repeated measurements to be obtained at rapid intervals, is not affected by the ability of lung tissue to store CO2, and eliminates many of the assumptions usually made in non-invasive measurements of cardiac output.