Domperidone Rx for Gastroparesis: Which Test First?

Domperidone is better tolerated by some patients than metoclopramide. Before a switch, however, this test must be performed.

A 42-year-old woman has gastroparesis which has responded well to metoclopromide. Given concerns over the side effects of metoclopramide, consideration is given to prescribing domperidone. 

Which of the following should be checked prior to initiation of domperidone?