Our heart has this ability to regulate its rate depending on the blood volume and pressure. Our heart has to maintain adequate brain perfusion. Due to any reason if brain blood flow decreases, heart increases its rate and force of contraction in order to counter-increase the blood flow to the brain. 

Some people complain of an increase in heart rate when they stand up from a sitting or lying position. 
The reason of this problem is that when you stand up from a lying or sitting position your blood pools in the veins of legs. The venous blood return to the heart decreases, which in turn decreases the cardiac output, blood pressure and brain blood flow. So in order to compensate for that the heart increases its rate and force of contraction and thus try to maintain the adequate brain blood flow. The affected person feels the sensation of palpitations, dizziness, headache, nausea or even he may faint. The increased heart rate or tachycardia continues until the venous return to the heart increases either by contraction of veins of the legs or by contraction of the muscles of the legs which pump the blood back to the heart. When venous return to heart increases, heart rate normalizes. 

When a normal person stands up from a sitting or lying position, the heart rate increases for a short time because his nervous system is working fine which immediately contracts the veins of the legs and prevents pooling of blood in there. This transient increase in heart rate is not usually felt by the normal individual. 

If the postural increase in heart rate in severe then this condition is called Postural Tachycardia Syndrome (PoTS). The exact cause of this PoST is not know. But it is thought that some viral or chronic illness might be responsible for that. 

Benign or mild postural increase in heart rate doesn't need any treatment and patient is advised 
1. to take 2-3 liters of fluid daily.
2. Increase salt intake, but not more than 10 grams per day.
3. Start exercising and target the core muscles of the legs so that they can effectively pump the blood back to the heart.
4. High body temperature may worsen the condition so keep your body temperature low.
5. Wear compression stockings up to the hips. The pressure should be about 30 mm of Hg.
6. Rise slowly from a sitting or lying position. Or if you try to stand up from a sitting or lying position then start by clinching you feet and hands. Contract your  muscles of arms, legs, thighs and buttocks while standing up. This will help pumping the blood back to the heart. 
7. If you feel early signs of PoST then immediately lie down.
If you have any health related question feel free to ask.
 
 
 
 
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