Recirculation times in exercising children.

Recirculation times were measured with indicator-dilution methods in 19 subjects, aged 5--14 yr, during near-maximal supine bicycle exercise. Recirculation times were as short as 4.5 s and peak recirculation occurred within 10 s. When rebreathing methods are used to measure exercise cardiac output in children, their very rapid recirculation needs to be considered.