Hello: My Husband Had A Heart Valve Replacement 4 Years

Hello: my husband had a heart valve replacement 4 years ago and has been well since then. goes to the cardiologist once per year now. Doctor says he's well. In the past year he has had dizzy spells 5 time. tonight he almost fainted on the sidewalk. his heart rate was 82 then steadily dropped to 54 over a half hour. He is laying down now and feeling nausea. He didn't have much solid food today and may not have had enough water. probably drank to much coffee too. There is a stomach bug going around. he wouldn't go to the hospital. should I be worried?