A 35-year-old female agricultural worker is found unconscious in a field 4 hours after suspected organophosphate pesticide ingestion (empty container of malathion nearby). On arrival at hospital: GCS 7/15, pinpoint pupils, severe bronchospasm with pink frothy sputum, muscle fasciculations visible, BP 76/48 mmHg, HR 42/min. Arterial blood gas: pH 7.18, PaCO₂ 68 mmHg, PaO₂ 52 mmHg on 100% O₂. After immediate airway management and atropine 5 mg IV, which of the following is the most critical next intervention to prevent irreversible enzyme damage and improve long-term neurological outcome?