What types of cardiac side effects are associated with diphenhydramine HCI overdose?

Answer 1

the main side effect is tachycardia which is rapid heartbeat. Go to http://www.nlm.nih.gov/medlineplus/ency/article/002636.htm#Symptoms for advice if you have such a patient, and as soon as possible. That website is a national N.I.H. (U.S.A.), poison information site.
You should answers for the following questions before the contact.
the patient's age, weight, and condition
the name of the product (ingredients and strengths if known)
the time it was swallowed
the amount swallowed
Also contact the local poison control center.
Good luck

Answer 2

Diphenhydramine is one of many drugs which cause anticholingergic side effects. Some cardiac effects that may occur include palpitation, tachycardia (rapid heart beat), and hypotension (low blood pressure). Severe anticholingergic side effects can be reversed by using physostigmine given IV.