A 10-year-old girl (33 kg) undergoes left upper-arm fracture fixation under general endotracheal anesthesia. Sixty minutes after placement of a supraclavicular block, she develops acute hypotension (BP 65/40 mm Hg), tachycardia (HR 145 bpm), frequent Premature ventricular contractions (PVCs), and desaturation (SpO₂ 82%). At this time, reduced movement of the left hemithorax is visible. What is the most critical immediate next step?