Age-related variation in oxygen transport but stable mixed venous oxygen saturation in anaesthetised children

BACKGROUND: Interpretation of cardiovascular function during paediatric anaesthesia requires knowledge of normal ranges of cardiac output (CO) and oxygen transport. However, most available paediatric haemodynamic data originate from awake or critically ill populations, and flow-based variables are rarely available during routine anaesthesia. OBJECTIVES: To characterise age-related variation in CO and oxygen transport variables in anaesthetised children and to evaluate whether mixed venous oxygen saturation (SvO2) remains stable across paediatric age groups. DESIGN: Retrospective observational study. SETTING: Single tertiary paediatric university hospital. Study period: retrospective data from 2016 to 2024. PATIENTS: Fifty-eight anaesthetised children with structurally normal cardiac physiology undergoing elective surgery or catheter-based interventions. INTERVENTIONS: None. MAIN OUTCOME MEASURES: CO, stroke volume (SV), cardiac index (CI), oxygen delivery (DO2), and SvO2 measured using capnodynamic assessment of CO and SvO2 under stable baseline anaesthesia. RESULTS: CO, SV, and absolute DO2 increased significantly with age (all P < 0.0001). Indexed variables demonstrated age-dependent patterns: CI and DO2 index varied across age groups (P < 0.0001), with infants showing lower values compared with older children. In contrast, SvO2 did not differ between age groups (P = 0.19) and showed no linear association with age (P = 0.10). CONCLUSION: Absolute and indexed CO and DO2 variables demonstrated age-dependent variation in anaesthetised children, whereas SvO2 remained relatively stable across developmental stages. This stability suggests that SvO2 may represent a physiologically robust and age-independent indicator of adequate systemic DO2 during paediatric anaesthesia.