Final answer:
The first sign of hypotension after a regional block is typically bradycardia. This condition, known as orthostatic hypotension, occurs when the sympathetic nervous system fails to compensate for changes in posture, leading to a drop in blood pressure.
Step-by-step explanation:
One of the first signs of hypotension occurring immediately after administration of a regional block is bradycardia (a). Hypotension after regional anesthesia indicates that the sympathetic nervous system is not able to compensate by increasing cardiac output, which results in decreased blood pressure and, subsequently, reduced cerebral perfusion. This can present as orthostatic hypotension, a condition where blood pressure drops below the homeostatic set point when a person stands up, leading to symptoms like dizziness or lightheadedness due to a transient decrease in brain oxygenation.