RELATIONSHIP OF THERMODILUTION CARDIAC-OUTPUT TO METABOLIC MEASUREMENTS AND MIXED VENOUS OXYGEN-SATURATION

To determine the individual contributions of variables in the Fick equ ation to cardiac output, we simultaneously measured oxygen uptake (VO2 ), carbon dioxide production (VCO2), venous oxygen saturation (SvO2BAR ) and thermodilution cardiac output (Qth) in 28 medical and surgical I CU patients. Patients were intubated and ventilated with the intermitt ent mandatory ventilation mode. VO2 and VCO2 (averaged over 3 min) wer e obtained from a metabolic cart. SvO2BAR was measured with fiberoptic reflectance oximetry (and COoximetry). Thirty-nine studies (average d uration, 4.3 h) with 151 Qth measurements were performed. The relation ships between Qth and VO2, Qth and VCO2, Qth and SvO2BAR and I/Qth and SvO2BAR, as well as between the sequential changes in these variables were analyzed by least squares linear regression. The ability of chan ges in the variables VO2, VCO2 and SvO2BAR to predict changes in Qth w ere analyzed by receiver operating characteristic (ROC) curves. Qth wa s weakly related to VO2 (r = 0.45), VCO2 (r = 0.45), or SvO2BAR (r = 0 .36). Changes in Qth were weakly related to changes in VCO2 (r = 0.40) , and even less to changes in VO2 (r = 0. 18) and SvO2BAR (r = 0. 13). The areas under the ROC curves for increases in Qth >10 percent were as follows: 0.66 for VCO2, 0.50 for VO2, and 0.55 for SvO2BAR. The are as for decreases in Qth <10 percent were as follows: 0.78 for VCO2 0.6 5 for VO2 and 0.49 for SvO2BAR. None of the above oximetry relationshi ps were substantially altered by use Of COoximetry venous oxygen satur ations. We conclude that Qth cannot be predicted well solely from VO2, VCO2, or SvO2BAR nor can changes in Qth be predicted well solely from changes in VO2, VCO2, or SvO2BAR. Of the metabolic variables, changes in VCO2 best predicted changes in Qth.