I suspect that your doctor would deal with the significance of ST Segment Depression if he/she felt it is of concern. If you have concerns, best to speak with him/her.
That said, I wondered about ST Segment Depression in the context of PSVT. From a site that provides instruction to professionals (of which I am not) concerning interpretations of EKGs, I did find the following:
"Supraventricular tachycardia (e.g. AVNRT) typically causes widespread horizontal ST depression, most prominent in the left precordial leads (V4-6). This rate-related ST depression does not necessarily indicate the presence of myocardial ischaemia.........".
The list of causes of ST Segment depression is not limited to the more serious things you have been reading about. I read other things as well that can cause it, anything from poor EKG lead contact (I believe referred to as Pseudo ST Segment Depression), MVP, and more. Only your doctor would be able to put it into context.
If he/she (both your GP and Cardiologists) were unconcerned, there is no reason for you to be concerned.
Hope this helps.