NONINVASIVE DETERMINATION OF CARDIAC-OUTPUT IN PATIENTS WITH SEVERE AIR-FLOW LIMITATION

The noninvasive measurement of cardiac output (over dotQ) by the Indir ect Fick CO2-rebreathing technique requires mixed venous PCO2(P (V) ov er 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 sug gested that although P (V) over bar(CO2) can be determined, Pa-CO2 can not be accurately estimated in patients with significant airflow limit ation. Nineteen patients with cystic fibrosis who had severe airflow l imitation (%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 obtain ed from an indwelling arterial catheter (measured: 45.2 +/- 4.92; esti mate: 42.7 +/- 5.68 mm Hg). To calculate arterial blood content, the v alues derived from Pa-CO2, pH, hemoglobin (Hb), and O-2 saturation wer e 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 assu med to be 7.40 and Hb was assumed to be 15 g/dl (measured mean pH, 7.3 4; Hb, 14.4 g/dl). No difference in arterial CO2 content was seen betw een 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 aff ect mixed venous CO2 content, the effect on over dotQ was also assesse d. Both estimates fit closely with measured over dotQ (r(2) = 0.77 and 0.76), with intercepts not different from zero and slopes not differe nt from 1, and coefficients of variation of 13.5 and 14.6%. When viewe d 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 conclu de that the use of PET(CO2) to estimate Pa-CO2 can give reasonable val ues for over dotQ determined noninvasively in patients with severe air flow limitation.