In 1974, after I was in a minor accident in high school, a doctor who checked my vital signs told me that I had a “heart murmur” and shouldn’t do stressful things like sports. Since I was in my third year of cross-country and track, I was worried but also slightly skeptical. Luckily, soon afterward I read Dr. George Sheehan’s first book on running, in which he wrote that he’d been told the same thing and was denied insurance because of his “heart condition.” He was a cardiologist and also a masters’ mile record-holder at the time, so I decided to ignore the warning. It’s 43 years of races later now and I’ve never had the slightest bit of heart-related trouble.
About vital signs in my case: my blood pressure isn't low enough to get attention, but before getting out of bed in the morning after an easy few days of training, my pulse is around 34-36. The lowest it ever got (that I know about) was during a study that I volunteered for that looked at the effects of an antidepressant drug. I sat in an isolation chamber for about 45 minutes, responding as quickly as possible by clicking either the mouse under my left hand or the one under my right hand, depending on what kind of symbol was flashed on a monitor in front of me, while hooked up by a whole lot of electrodes on my head to machines that registered brain activity. I felt very awake and not very relaxed, but my pulse went down to 30 in there. (It had the usual alarming effect on the testers, who could see it on their equipment.) It happened during both tests—drugged and not)—so that wasn’t the reason.
Doctors might be more aware of athletes’ physiology now, but it also wasn’t long ago that I was told (in my 50s) that I shouldn’t be running marathons “at my age” by a primary-care physician (whose services I declined after that).