Final answer:
Joe's elevated heart rate is likely due to the problem with his autonomic nervous system (ANS). The sympathetic division of the ANS, which normally increases heart rate, may be overactive or disrupted after the shock of the attack.
Step-by-step explanation:
Joe's elevated heart rate is likely due to the problem with his autonomic nervous system (ANS) as determined by his doctor. The autonomic nervous system is responsible for regulating various bodily functions, including heart rate.
In Joe's case, the sympathetic division of the ANS, which normally increases heart rate, may be overactive or disrupted after the shock of the attack. As a result, Joe's heart rate remains abnormally high, even when the threat has subsided.