I would see a cardiologist before I started to medically manage your dog. An irregular heartbeat, depending on where the irregularity is being generated from, can be normal for a dog. A "regular irregularity" is common in dogs and no need for concern unless it becomes symptomatic.
"Dogs don't have a regular heartbeat as humans do..This is a helpful site>
http://www.petsdoc.com/html/askthevet/as...
Irregular heart beats are common, and usually not a problem unless associated with other cardiac related clinical signs. The most common origin is sinus arrhythmia, an irregularity in the pacemaker of the heart. If your dog is otherwise fine, you do not need to worry about the irregular heart beat."
As for heartworm meds, in Texas if your dog is outside for a season without heartworm preventative, chances are excellent the pet will have heartworms. Hopefully, at your pets age, she has been on heartworm preventative.....the vet will check her before starting anything for heartworms.