arrhytmias post pacemaker implantation

My mother had her pacemaker replaced in March 09  She went for a pacemaker check today.  She has had arrhytmias the past several months so she has an appt next week to see her doc.  What causes arrhytmias even when you have a pacemaker.  Is this normal?  Pacemakers are supposed to treat this, right?  

[Jerry_NJ]

There are many reasons for pacemakers.  One is to cut in when the HR gets too low.  I believe this is done in some applications and leaves the heart signal/timing functioning (otherwise one is completely Pacemaker dependent).  In this case any existing arrhythmia would just continue.  Perhaps someone who has a better understanding will also respond.