Final answer:
The likely cause of the patient's heart racing after receiving local anesthesia prior to a dental procedure is anxiety or adrenaline release.
Step-by-step explanation:
The likely cause of the patient's heart racing after receiving local anesthesia prior to a dental procedure is anxiety or adrenaline release. Adrenaline, also known as epinephrine, is a hormone that is released during moments of stress or excitement. It has a stimulatory effect on heart rate and can cause the heart to race. This is a normal response to stress or anxiety and should subside once the patient is calm.