A 28-year-old woman is admitted with suspected cyanide poisoning after ingestion of an unknown substance 45 minutes ago. She is drowsy, with respiratory rate 30/min, SpO₂ 98% on room air, and blood pressure 156/92 mmHg. Arterial blood gas: pH 7.19, PaCO₂ 28 mmHg, HCO₃⁻ 11 mEq/L, PaO₂ 105 mmHg. She has received sodium nitrite 300 mg IV and sodium thiosulfate 12.5 g IV. Fifteen minutes post-antidote, she remains drowsy with persistent tachypnea and metabolic acidosis (pH now 7.22, HCO₃⁻ 12 mEq/L). What is the most appropriate next step?