1. Resting pulmonary blood flow (), using the uptake of the soluble inert gas Freon-22 and an indirect estimate of lung tissue volume, has been estimated during breath-holding (c) and compared with direct Fick cardiac output (f) in 16 patients with various cardiac disorders.
2. The effect of breath-hold time was investigated by comparing c estimated using 6 and 10 s of breath-holding in 17 patients. Repeatability was assessed by duplicate measurements of c in the patients and in six normal subjects.
3. c tended to overestimate f, the bias and error being 0.09 l/min and 0.59, respectively. The coefficient of repeatability for c in the patients was 0.75 l/min and in the normal subjects was 0.66 1/min. For f it was 0.72 l/min. There was no significant difference in c measured at the two breath-hold times.
4. The technique is simple to perform, and provides a rapid estimate of , monitoring acute and chronic changes in cardiac output in normal subjects and patients with cardiac disease.