Spanish mortgages usually work on the 12 month Euribor rate, which at the moment is very very low and will go up and down with the ECB rate.
If the apartments rent will cover the mortgage on a ECB variable but not a Euribor variable, then the mortgage would not be covered with even a .75% increase in ECB.
Dont buy property because interest rates are low, they wont be low forever and from personal experience (i'm just back for spain today) i wouldn't touch spanish property with a barge poll for atleast a few years, theres better returns in Ireland than spain.