A 28-year-old woman is admitted to the ICU with suspected cyanide poisoning from ingestion of cassava root (containing cyanogenic glycosides). She is drowsy, has a respiratory rate of 32/min, and blood pressure is 88/54 mmHg. Arterial blood gas shows severe metabolic acidosis (pH 7.15, HCO₃⁻ 10 mEq/L) and lactate 10.5 mmol/L. She has already received sodium nitrite 10 mL IV and sodium thiosulfate 50 mL IV 15 minutes ago but remains critically ill. What is the most appropriate next step?