Am. J. Respir. Crit. Care Med., Vol 153, No. 3, 03 1996, 981-984.
Noninvasive determination of cardiac output in patients with severe airflow limitation
LC Lands, G Canny, F Xu and AL Coates
Montreal Children's Hospital-McGill University, Quebec, Canada.
The noninvasive measurement of cardiac output (Q) by the Indirect Fick
CO2-rebreathing technique requires mixed venous P CO2 (P 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 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 (%FEV1, 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 O2 saturation were
compared with those derived from PET CO2 and O2 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 ml). As pH and Hb can also affect mixed venous CO2
content, the effect on Q was also assessed. Both estimates fit closely with
measured Q (r2=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 Q as a
function of O2 consumption, Q was altered to a minor extent. We conclude
that the use of PET CO2 to estimate Pa CO2 can give reasonable values for Q
determined noninvasively in patients with severe airflow limitation.
