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Q. Why do I have rapid heart rate and dizziness after doing strenuous activity?

Answered by
Dr. Nishant Kumar
and medically reviewed by iCliniq medical review team.
This is a premium question & answer published on Sep 17, 2018

Hello doctor,

About a year ago, I had a long break from doing any type of fitness. About five months ago, I started doing more things again, mainly little things like going out to hang out with my horses, and going to a therapist for a bit. Then around two or three months ago, I decided to go to a fitness trainer to get myself even more fit for my horse riding. I have been held back quite a bit by an issue, what tends to happen is normally when I do something strenuous (but it also happens when temperature switches or the air just becomes more humid) I will start feeling my heart rate become quite rapid and heavy feeling, then I feel dizzy (my head feels really weird) and I will quite quickly feel sick and have to throw up or at least dry-retch. The doctor thought it might be something to do with my thyroid glands so took a full blood test, everything looked normal, my trainer thought it might be anxiety but it has happened at times where I am at my calmest, and my parents thought it might be because I am unfit, but after doing training and my fitness is pretty much back up to where it was before my break, it is still happening and has not improved at all. I have always done sports for most of my life before the break, the main two being athletics and horse ridings (though I only do horse riding now as well as fitness training).

Dr. Nishant Kumar

Family Physician General Practitioner


Welcome to

As per your symptoms, you feel dizzy when you do strenuous work or when the temperature changes (I guess when it becomes hotter) and when it is humid. As you feel nauseated. And you said that all blood reports with thyroid reports are also normal. The chances are this is due to vasodilation, that is the blood vessels dilate so the adequate amount is not reaching heart which in turn reduces blood supply to organs.

The Probable causes:


Differential diagnosis:

1.Vasovagal syncope.
3.Postural orthostatic tachycardia syndrome.

Probable diagnosis:

Postural orthostatic tachycardia syndrome.

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