1. A study of exercise physiology in 117 children aged 6–16 is described including the measurement of cardiac output by the Indirect (CO2) Fick Principle.

2. Computer produced graphs enabled various alternative values for arterial Pco2 (Pa,co2) to be inserted simultaneously into the Fick equation for cardiac output and the Bohr equation for dead space.

3. Dead space could be estimated with reasonable accuracy using end-tidal Pco2.

4. Cardiac output determination was unreliable at rest due to the small venoarterial Pco2 difference but it was very good on exercise when this difference is much larger.

5. The Pa,co2 used for the calculation of cardiac output could be derived either from ear lobe Pco2 or end-tidal Pco2. However, the best estimate of cardiac output was that using the Pa,co2 implied by assuming a normal dead space.

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