If the step down you are going to is worth its salt they will give you classes and a strong orientation.
Things to Review:
EKG rhythms and strips, basic, anomalies, fatal rhythms, and ACLS protocols.
Make sure your BLS is up to date, which I assume it has to be and find out if they will provide ACLS or if you need to get it before.
Review cardiac medications. Here is a great table from the AHA that highlights many of them in oral form.
Here is a list of common cardiac drips and infusion rates, however your facilities protocols will lead this. However at least you see a list of common infusions, though these are for critical care, so you might not deal with all of them, however you never know!
IV Critical Care Infusion Drip Chart
Review your other common medications such as diabetic meds, and be aware of interactions with anti-depressants, sedatives (e.g. xanax and ativan).
Take time to prime and run a few pumps if it has been awhile since doing those tasks.
Brush up on chest tubes, catheter care, pacer wires, and wound care.
I don't bring up the nursing school stuff to be offensive to your expertise, however I am not sure what "office nurse" has entailed for you
I would also ask if you will expect to be dealing with LVADs and if there is a class you need to take.
Also brushing up on CHF, clotting disorders, common vascular surgeries, and new heart surgery techniques (robotic etc) might make you feel more confident.
Medscape has had a lot of articles lately on CHF, clotting medication management, and even some heparin/Lovenox information. Might be worth a browse since that is free to use.
Just for the record I am not a step-down nurse, however my experience is in complex cardiac.
Best of luck!!