There are some significant warning signs. One is passing out or becoming dizzy during exercise. That can be a sign that your heart’s rhythm is out of whack enough to make you collapse but went back to normal fast enough to let you wake up.
Fainting as a result of a loud noise or being startled is a second red flag. The reasons for it aren’t really clear, but it’s a well-known occurrence in Long QT Syndrome.
Third is a family history of sudden cardiac death at a young age or of drowning. Many cardiac conditions can have you passing out during exercise, which may not be deadly in a marathon when you can stand back up after you recover. However, if you pass out in the water, you will drown.
See A Doctor
Even if none of these red flags are reason for pausing in your case, there still may be motives to see a doctor. If you have a family history of fainting, chest pain, cardiac irregularities, or heart disease, you should take a trip to see the doctor. That trip could result in getting your blood pressure checked or getting an electrocardiogram done which can detect some electrical issues with the heart.
Specialized tests are a heart ultrasound and an echocardiogram which can detect whether or not you have hypertrophic cardiomyopathy and can also pick up ARVC.
And if you’re older and want to run your first marathon, you may want to get a stress test. People over 40 are more probable to having coronary artery disease. You’ll ride a stationary bike or run on a treadmill to measure your blood pressure and heart rate while you do an increasingly difficult level of exercise. You may also take a nuclear stress test that looks at the blood flow to your heart.