Patience and love. If your parents are having health issues and you're concerned, first be sure they see a doctor. Aging can cause simple memory lapses but if there are blood pressure issues there may be other underlying factors. Be sure they seek medical advice.
Once you know what you might be dealing with, then you'll be better able to understand them. If their health is still relatively good and mild memory lapses cause you stress, I would suggest talking to them honestly. Understand that it's normal to have some kind of memory loss as we age. Practicing patience and loving them for who they are will eventually improve the relationship.