The first priority in assessing an unconscious patient is ensuring a patent airway because without airway patency, oxygen cannot reach the lungs, leading to rapid hypoxia and brain injury. Even if circulation and breathing are intact initially, an obstructed airway can quickly cause life-threatening respiratory arrest. Securing the airway allows effective ventilation and oxygenation, which is critical before proceeding to other assessments such as pulse or neurological status. This approach follows the Airway-Breathing-Circulation (ABC) principle in emergency care.

Reference: Tintinalli's Emergency Medicine: A Comprehensive Study Guide, Judith E. Tintinalli, 9th Edition.