Fast 12 hrs before tx and 6 hours after. Give Diatomaceous Earth supplement daily in food between treatments. And give Garlic to food daily along with diatomaceous earth. Or one caps. There is a lot of controversy about heartworm preventative.
Any symptoms can become worse.
Over the years at conferences, various veterinarians have reported problems with any of the preventatives. Most feel has the fewest problems is the once a day, DEC, but many dogs do fine on the monthly ones as well. Observing your dog will give you clues that you need to try one of the other preventatives or use none at all. The fewer drugs the better, so use ones just for heartworms, not other worms. When giving the preventative, daily or monthly, give it less frequently that recommended.