Reproducibility of a semiautomated acetylene rebreathing technique for measuring cardiac output in humans at rest. 
The specific aims of the present study were to determine: (1) the day-to-day reproducibility of a semiautomated acetylene rebreathing technique for measuring cardiac output under resting conditions; (2) the reproducibility of this technique among subjects differing in gender and age; and (3) the number of trials within a session necessary to maximize the day-to-day reproducibility of the technique.  To address these aims, cardiac output was measured in 21 healthy men (n = 8) and women (n = 13) between the ages of 25 and 71 years in the supine posture on two separate days.  Mean levels of cardiac output at rest were similar on day 1 vs. day 2 in the overall group.  Cardiac output measured on day 1 was highly correlated (r = 0.98, P < 0.001) with cardiac output measured on day 2.  The day 1 to day 2 mean difference in cardiac output for the individual subjects was < 4%.  The mean levels of heart rate and stroke volume also were similar between day 1 and day 2.  The relation between cardiac output measured on day 1 vs. day 2 in the gender and age subgroups was similar to that observed in the overall group.  The mean absolute difference among the three rebreathing trials within a day was 360 ml min-1 in the overall group, with a coefficient of variation of 7%.  The variability between rebreathing trials measured on day 1 vs. day 2 in the gender and age subgroups was similar to that observed in the overall group.  The reliability of cardiac output measured on different days was excellent with a single rebreathing trial (r = 0.93) and improved significantly up to three trials (r = 0.98).  In conclusion, the findings of the present study indicate that the acetylene rebreathing technique can be a highly reproducible method for measuring cardiac output under resting conditions.  The reproducibility is consistently strong in healthy humans of varying age and in both genders, and is enhanced by the use of multiple trials.