A 25-year-old male injured his bilateral lower extremities after a high fall. There are bilateral mid-shaft femur fractures and a left tibial shaft fracture. He underwent an open reduction. Post-op day 1, he had decreased level of consciousness and SpO2 was 80%. ABG showed respiratory alkalosis. Intubation caused an improvement in his hypoxemia. An ECG done showed sinus tachycardia. What caused this post-surgery?