Intravenous magnesium sulphate is routinely administered in paediatric cardiac ICUs for the correction of hypomagnesemia. Its physiologic effects in critically ill children have not been clearly characterised. We aimed to describe circulatory and oximetric responses following intravenous magnesium sulphate administration.

A single-centre retrospective, descriptive cohort study of paediatric cardiac ICU patients aged 1–12 years who received intravenous magnesium sulphate. Temporal responses in physiologic indices were characterised using locally estimated scatterplot smoothing and restricted cubic spline regression adjusted for patient characteristics.

57 magnesium administrations were analysed. Blood pressure demonstrated early and minor relative declines followed by a progressive increase above baseline. Modelled venous resistance reached a nadir of 7.2% below baseline (95% bootstrap confidence interval −24.77% to 25.19%) and modelled mean circulatory filling pressure reached a nadir of 2.2% below baseline (95% bootstrap confidence interval −10.85% to 8.55%) at 124 minutes. Regional oxygen saturation remained above baseline throughout most of the observation period, reaching a maximum increase of 2.6% (95% bootstrap confidence interval −0.38% to 7.55%) at 480 minutes among 30 administrations with regional oxygen saturation data, while modelled oxygen extraction showed a sustained reduction below baseline.

Findings suggest modest, coordinated changes in venous and arterial vascular physiology, with preserved regional oxygenation. Wide confidence intervals indicate uncertainty and heterogeneity in physiologic response patterns. These findings provide novel insight into the temporal circulatory response to magnesium sulphate in critically ill children and support prospective studies to further characterise these responses and establish causal relationships.