A 52-year-old woman with bipolar disorder has been on lithium 900 mg daily for 3 years with good mood stabilization. She presents to the emergency department with severe nausea, vomiting, coarse tremor, confusion, and ataxia that started 6 hours ago. Her serum lithium level is 2.8 mEq/L (normal therapeutic range 0.6–1.2 mEq/L). On examination, she is disoriented to time and place, has hyperreflexia, and clonus is elicited. Her renal function is normal (creatinine 0.9 mg/dL), and she denies recent NSAIDs or diuretic use. What is the most appropriate immediate management?