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Fainting while Exercising

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A little background-- I'm 27 years old, normal weight. I was a D1 athlete for 4 years in undergrad, so not new to exercise. In 2011, I switched to a vegetarian diet (no meat, yes eggs and dairy) and have been vegetarian since. I was in grad school the past 2 years and have certainly been less active than during my time in undergrad, however I do occasionally run and do yoga and generally walk and hike a fair amount. I drink about 2.5-3 L of water per day.

To get to the real problem-- At the beginning of this past June I went to run on the treadmill at the gym after around 2 mostly sedentary months. After running a mile at a pretty easy pace, I started to see spots in my vision so I slowed the treadmill to a jog at which time I started to get tunnel vision and felt weak. I stopped the treadmill as I knew I should sit down, went to take a sip of water, and passed out. I was only out for a few seconds. I knew what happened immediately after waking up and was able to stand up and walk around within a few minutes of falling. Prior to June, I had never passed out or felt lightheaded with exercise. I have occasionally felt lightheaded when standing after lying down or sitting, but these symptoms generally pass within seconds and I have never actually fainted during these times.

I went to the ER where they did a number of tests and found I was not dehydrated and salt and glucose levels were normal. I had eaten a good sized meal around 1.5 hours before working out. Over the past two months they have done a number of cardiac tests (EKG, echocardiogram, cardiac CT scan with contrast), which have yielded all normal results excluding a treadmill stress test. During the stress test they found my heart rate and BP rose normally over the stages of the Bruce protocol and then dropped suddenly at around 15 minutes of exercise (heart rate still only around 165) at which point I felt lightheaded, had blurred vision, and became pale. They laid me down to prevent me passing out (I’m certain I would have, had I not been lying down) and my BP shot up extremely high (260/41) after I was lying down. As I said, all further tests indicate it's not a result of a heart abnormality, and my cardiologist basically told me this was an abnormal no response to exercise, but he didn’t have an explanation for it and told me I should drink more water and go back to my normal exercise routine but “not push myself as hard”.

Since the treadmill stress test I have gotten tunnel vision on two additional occasions, once while on a stationary bike (max heart rate 155) and once while hiking up a fairly steep incline (heart rate 144). On these occasions I have been able to sit on the ground and rest for a few minutes to avoid passing out. I know fainting with exercise is common when you stop exercise too quickly, but in these cases my blood pressure begins to drop and I get symptoms when I am still exercising.

I know iron and B12 deficiencies are common in vegetarians and could potentially explain fainting and feeling lightheaded, however I had blood tests done in mid August and I am neither anemic nor B12 deficient according to these tests.

I’ve talked to 2 cardiologists, 3 general practitioners, a sports medicine doctor, and a sports nutritionist and nobody can tell me what’s going on. And all of them seem to have different perspectives as to whether I should be working out or not, but I’m a little wary. At the suggestion of one of the cardiologists, I have an appointment with a kidney specialist soon. Anyone ever have symptoms like this or hear of someone fainting with exercise like this? Any insights would be greatly appreciated.

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