The other item that than easily throw a diagnostic ECG is poor lead placement. Ive had one chap in for an ECG and the medic brought me the printout. I looked at it, and asked him did he recheck the leads and reanalyse. He said no. The thing is, as this chap was flight crew and he was only in for a medical, I thought the diagnosis on the top of the ecg was a bit over the top, given he should have been showing some significant symptoms.
I found the operator had some of the leads placed incorrectly. Once the leads were replaced in their correct locations, the ECG said normal. The diagnostic facilty in the machine can only interpret what information it can gather from the leads. Incorrect lead placement only can give an errant diagnosis.