1. The CO2-rebreathing indirect Fick method of cardiac-output measurement was compared with the standard invasive direct Fick method for oxygen during rest and treadmill exercise in 15 patients with pulmonary disease characterized by the common feature of expiratory-airflow limitation.

2. The correlation coefficient for all 37 determinations was 0.94 (P < 0.01); in patients who had a Pa-ETco2 (arterial end-tidal Pco2 gradient) of greater than 5 mmHg, the coefficient was 0.69 (P < 0.05), and in the remainder, with a normal Pa-ETco2, 0.98 (P < 0.01).

3. The difference in correlation between these two latter groups is statistically significant.

4. It is concluded that the indirect method is valid in most patients with pulmonary disease, but that the correlation with the direct Fick method is significantly worse when the Pa-ETco2 exceeds 5 mmHg.

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