Elevated heart rate captured on a Holter monitor

Dear Ask The Doctor: Hi. I am 19 years old girl-normal weight going to thin. Non smoker,non drinker,never used drugs. My problem is the fast heart rate. I get tired so easily. Mine heart rate in rest is ussually normal min 70 max 90. But problem is when i am standing up,it start to jump and raise up the tahycardia to 150. Then i feel fatique and dizziness,weakness in legs and arms. Even i can't walk anymore,it beats so fast so i lost my breathe and i need to sit. After sit it gets calm for 5 min. I did all blood analizys and they all came back normal. I checked heart with ecg,holter monitor and echo,and doctors here said that everything is ok with heart. Another thing is my bp which is 90-60 generally. Isn't it wired to have low bp and high hr? I am in big worry and i ask for help as soon as it is possible.

Dear Anita:A tachycardia is defined as a heart rate >100 beats per minute. If the heart rate rapidly increases to 150, this may be normal sinus rhythm of the heart, known as sinus tachycardia, or it may be a manifestation of an underlying heart rhythm abnormality. The only way to diagnose a heart rhythm disturbance is to record a tracing of the heart (ECG), during these episodes. It is reassuring to note that all the investigations to date have been normal including the 24 hour Holter monitor and echocardiogram. A blood pressure of 90/60mmHg is low, but may be normal in some patients. I advise attending your family doctor for a full history and clinical examination and a repeat ECG. Sometimes a 24 hour recorder may not capture when you experience symptoms, and a 48 hour holter or patient activated recorder may be appropriate in your case, to reach a diagnosis.