Question

A $9$-year-old boy is brought to the emergency department by his parents due to lethargy and recurrent vomiting for the past $24\text{ hours}$. Over the preceding two weeks, he has had increased thirst, frequent urination, and a $3\text{ kg}$ ($6.6\text{ lbs}$) weight loss. He has no significant past medical history.

On initial presentation, he is lethargic but arousable to verbal commands. Temperature is $37.0^\circ\text{C}$ ($98.6^\circ\text{F}$), blood pressure is $90/55\text{ mmHg}$, pulse is $135/\text{min}$, and respiratory rate is $32/\text{min}$ with deep, rapid respirations. Mucous membranes are dry, and capillary refill is $4\text{ seconds}$.

Initial laboratory studies:
- Serum sodium: $132\text{ mEq/L}$
- Serum potassium: $4.8\text{ mEq/L}$
- Serum chloride: $98\text{ mEq/L}$
- Serum bicarbonate: $8\text{ mEq/L}$
- Serum glucose: $580\text{ mg/dL}$
- Arterial pH: $7.15$
- Arterial $pCO_2$: $22\text{ mmHg}$
- TSH: $0.4\text{ mIU/L}$ (Normal: $0.5-5.0\text{ mIU/L}$)
- Free T4: $0.8\text{ ng/dL}$ (Normal: $0.9-1.7\text{ ng/dL}$)

The patient receives a $20\text{ mL/kg}$ bolus of $0.9\%$ normal saline over $1\text{ hour}$. Subsequently, a continuous intravenous infusion of regular insulin is started at $0.1\text{ units/kg/hr}$ along with appropriate maintenance fluids containing potassium.

Four hours later, the nurse urgently calls the physician because the patient has become increasingly obtunded and is complaining of a severe, diffuse headache.

Current vital signs:
- Blood pressure: $140/90\text{ mmHg}$
- Pulse: $58/\text{min}$
- Respiratory rate: $16/\text{min}$

Which of the following is the most appropriate next step in the management of this patient?