Cardiac output by rebreathing in patients with cardiopulmonary diseases

Noninvasive estimates of cardiac output by rebreathing soluble gases (Qc) can be unreliable in patients with cardiopulmonary diseases because of uneven distribution of ventilation to lung gas volume and pulmonary blood flow. To evaluate this source of error, we compared rebreathing Qc with invasive measurements of cardiac output performed by indicator-dilution methods (COID) in 39 patients with cardiac or pulmonary diseases. In 16 patients with normal lung volumes and 1-s forced expiratory volumes (FEV1), Qc measured with acetylene [Qc(C2H2)] overestimated COID insignificantly by 2 +/- 9% (SD). In subjects with mild to moderate obstructive lung disease, Qc(C2H2) slightly overestimated COID by 6 +/- 15% (P = 0.11). In patients with restrictive disease or combined obstructive and restrictive disease, Qc(C2H2) underestimated COID significantly by 9 +/- 14% (P less than 0.04). The magnitude of the discrepancy between Qc and COID correlated with size of the volume rebreathed and an index of uneven ventilation calculated from helium mixing during rebreathing that determined a dead space to inspired volume ratio (VRD/VI). Rebreathing volumes less than 40% of the predicted FEV or VRD/VI of 0.4 or greater identified all subjects with a discrepancy between Qc(C2H2) and COID of 20% or greater.