The Reliability of the Carbon Dioxide-Rebreathing, Indirect Fick Method of Cardiac Output Determination in Patients with Pulmonary Disease

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Abstract
The CO2-rebreathing indirect Fick method of cardiac-output measurement was compared with the standard invasive direct Fick method for O2 during rest and treadmill exercise in 15 patients with pulmonary disease characterized by the common feature of expiratory-airflow limitation. 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 > 5 mmHg, the coefficient was 0.69 (P < 0.05), and in the remainder, with a normal Pa-ETCO2, 0.98 (P < 0.01). The difference in correlation between these 2 latter groups is statistically significant. The indirect method apparently 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.