An 83-year-old patient is unresponsive and lying on the floor.

Comprehensive and Detailed Explanation From Exact Extract:

Given fall with head trauma and absent respirations, the most concerning cause is spinal injury, particularly a cervical spine fracture. A high cervical injury (C1–C4) can paralyze the diaphragm, leading to apnea despite a beating heart.

Brain herniation can also depress respirations but often presents with unequal pupils, posturing, and Cushing’s triad (not described here). Commotio cordis is sudden cardiac arrest from blunt chest trauma (not head). Open pneumothorax affects chest mechanics, not directly linked here.