Introduction: Pragmatic and reproducible method to measure or trend cardiac output (CO) remain a challenge in ICUs. We previously showed, the functional CO (fCO), a multiplication product of alveolar functional fraction (AFF, ratio of EtCO2 to PaCO2) with minute ventilation (MV), correlated moderately with measured CO in pig model of shock. Human data validating the fCO are lacking, so we hypothesized that fCO would at least moderately correlate with measured CO in children. Methods: We performed a retrospective observational study of children ≤18-yrs who had cardiac catheterization (CATH) since 2009. Children without intracardiac shunts who had ventilation data with EtCO2 & an arterial blood gas with PaCO2 during CATH were included. Data were excluded if pH was 7.45 indicating ventilation-perfusion uncoupling from anesthesia. The fCO was calculated as AFF X MV from values during the CATH. The reference was Thermodilution Cardiac Output (TdCO) measured by the CATH. Repeated paired measures for each CATH were included if a patient had >1 CATH with TdCO. The paired fCO and TdCO were compared using unadjusted Pearson’s correlation, repeated measures correlation (for repeated measures per patient), Bland-Altman plot and an equivalence test (both accounting for repeated measures) with margines of 0.8 to 1.2 for acceptability of 20% difference between two COs. Results: Out of 133 CATHs from 22 children, 24 were excluded for missing values and 29 for low or high pH. The remaining 80 CATHs with fCO & TdCO pairs were analyzed. Median age at CATH was 6(IQR 1-11) yrs. Eighteen(18) patients had >1 CATH with a mean interval of 0.9 ± 2.2 yrs between CATHs. The unadjusted Peason’s correlation coefficient was 0.64 (95% CI:0.49, 0.75, p< 0.001) and repeated measures correlation was 0.40 (95%CI:0.16, 0.60, p=0.002). The Bland-Altman plot showed bias of 0.17 L/m (95% CI: -1.2, 1.6). The equivalence test showed mean ratio of 0.97 (95% CI: 0.89, 1.06), statistically different from upper (t = -7.2, p < 0.001) and lower (t = 5.4, p< 0.001) equivalence bounds. Conclusions: The fCO showed a strong clinical correlation, agreement and equivalence with TdCO in children undergoing cardiac CATH. Additional studies are warranted to test validity of fCO as a point estimate and/or trend of measured CO in critically ill children.
Chauhan et al. (Sun,) studied this question.