The adverse effects listed in MIMS does not include any cardiac symptoms. However as I am avoiding writing my thesis today I did a literature search and came up with:
Increased HR in 4.3 % of patients presenting with a Lamotrigine overdose.
Increased HR in a patient presenting with Venlafaxine and Lamotrigine overdose, attributed to the Venlafaxine.
Increased HR associated with an abrupt cessation of Lamotrigine in one patient.
A patient on Lithium and Lamotrigine with a pre-existing condition exhibited increased HR. However this was attributed to the lithium.
So it's not particularly convincing...

I am so procrastinating to avoid writing my thesis!!!!!