Evaluation of tissue saturation as a noninvasive measure of mixed venous saturation in children

Background: Mixed venous saturation (SV̄O₂) is an important measurement that helps guide the care of critically ill patients. Invasive SV̄O₂ assessment in infants and children is often avoided because of the inherent risks. A noninvasive tissue saturation (STO₂) monitor has recently been developed that uses near-infrared spectroscopy to measure oxyhemoglobin saturation in muscle. In adult and animal studies, STO₂ correlated with oxygen delivery and SV̄O2. There has been no evaluation in pediatric patients. Objective: To evaluate tissue saturation as a noninvasive measure of mixed venous saturation in children. Design: A prospective observational study. Setting: Catheterization laboratory in a tertiary care children's medical center. Patients: We studied 98 children (49 without intracardiac mixing and 49 with intracardiac mixing) ≤12 yrs of age who underwent cardiac catheterizatlon. Under general anesthesia, we compared STO₂ measured over the deltoid muscle with superior vena cava saturation in both groups and STO₂ with pulmonary artery saturation in patients without intracardiac mixing. Paired measurements were analyzed for bias, precision, and correlation via Bland-Altman plot and linear regression. Resulte: No meaningful correlation was found between STO₂ and superior vena cava saturation or pulmonary artery saturation. Bland-Altman analyses of STO₂ with superior vena cava saturation yielded bias values of -6.67 ± 37.33% in patients with intracardiac mixing and -0.82 ± 41.31% in patients without mixing. Bland-Altman analysis of STO ₂ with pulmonary artery saturation yielded a bias of 3.61 ± 41.32% in patients without mixing. Differences between noninvasive and invasive measurements were greatest in smaller children. Conclusion: Noninvasive tissue saturation over the deltoid does not correlate with SV̄O₂ in children. It is possible that more precise probe spacing, coupled with optimal muscle-mass location, could result in more accurate measures in future investigations.