Noninvasive determination of cardiac output in patients with severe airflow limitation

The noninvasive measurement of cardiac output (over dotQ) by the Indirect Fick CO2-rebreathing technique requires mixed venous PCO2(P (V) over bar(CO2)) to be determined by the rebreathing maneuver, and Pa-CO2 to be estimated from end-tidal P-CO2 (PET(CO2)). Previous work has suggested that although P (V) over bar(CO2) can be determined, Pa-CO2 cannot be accurately estimated in patients with significant airflow limitation. Nineteen patients with cystic fibrosis who had severe airflow limitation (%FEV(1), 29.3 +/- 7.12 SD) were studied during steady-state exercise at 50% of their measured maximal work capacity. Estimated Pa-CO2 was slightly lower than Pa-CO2 measured from blood samples obtained from an indwelling arterial catheter (measured: 45.2 +/- 4.92; estimate: 42.7 +/- 5.68 mm Hg). To calculate arterial blood content, the values derived from Pa-CO2, pH, hemoglobin (Hb), and O-2 saturation were compared with those derived from PET(CO2) and O-2 saturation, where (1) pH was assumed to be 7.40 and Hb was measured, and (2) pH was assumed to be 7.40 and Hb was assumed to be 15 g/dl (measured mean pH, 7.34; Hb, 14.4 g/dl). No difference in arterial CO2 content was seen between the three methods (measured: 47.53 +/- 5.17; estimate 1:49.57 +/- 6.58; estimate 2: 49.12 +/- 6.61 ml/100 mi). As pH and Hb can also affect mixed venous CO2 content, the effect on over dotQ was also assessed. Both estimates fit closely with measured over dotQ (r(2) = 0.77 and 0.76), with intercepts not different from zero and slopes not different from 1, and coefficients of variation of 13.5 and 14.6%. When viewed with regard to the confidence intervals for over dotQ as a function of O-2 consumption, over dotQ was altered to a minor extent. We conclude that the use of PET(CO2) to estimate Pa-CO2 can give reasonable values for over dotQ determined noninvasively in patients with severe airflow limitation.