Can invisible fences interfere with pacemakers?

No. It is very unlikely that an invisible fence would interfere with a pacemaker. Normally, and this is generally true of the pacemakers that have been implanted over the past 10 years or so, not much on the outside of the body can affect their function with the exception of large magnetic fields (like that of an mri). Even then, the interference is not permanent and not significantly detrimental.