A 5-year-old girl with known generalized tonic-clonic epilepsy on sodium valproate presents with status epilepticus. She received lorazepam 4 mg IV 10 minutes ago, but seizures have not stopped. She is now intubated and on mechanical ventilation. Vital signs: HR 140/min, BP 95/60 mmHg, temperature 39.2°C, SpO₂ 98% on FiO₂ 0.6. What is the most appropriate next step in management?