 We plan a randomized double blind trial comparing intravenous Mg with placebo in children admitted to the pediatric intensive care unit with clinical HFMD, ANS dysregulation, and Stage 1 hypertension. Patients will be eligible for enrolment if the arterial blood pressure exceeds the 95th centile for age, gender and lengthheight for at least 30 minutes while the child is not distressed, they exhibit at least one other criterion for ANS dysregulation, and a parentguardian gives written informed consent. Specific renal and cardio-respiratory exclusion criteria will apply. 