Possible causes of POTS

Not many people know that, even though the upright posture is a fundamental human activity, it requires several rapid and effective circulatory and neurological compensations in order to maintain blood pressure and consciousness.
Almost immediately after assuming the upright posture, gravity causes a redistribution of about 500 ml of blood to the lower extremities. One half of this amount is redistributed within seconds after standing up and up to 25% of the total blood volume may be involved in the process. This causes a decrease in venous return to heart and stroke volume may fall by 40%. In a normal subject, orthostatic stabilization after standing up is achieved in 1 min or less.
Anyway, our understanding of the pathophysiology of this condition remains incomplete. While the etiology of POTS is still unclear, it probably represents a heterogeneous group of disorders with similar clinical characteristics.

Heart problems

It is logical to assume that, because the heart is the central blood pump, a weak or irregular heart can cause orthostatic hypotension. The conditions that can affect the heart in such manner are:

    * arrhythmia,
    * heart failure, and
    * pregnancy

These are the examples where the heart may not be up to the task of providing an adequate blood pressure. Since the heart is not pumping the blood strong enough, the pressure drops significantly.
 

Hypovolemia

The other possible causes not associated with the heart are anemia, dehydration or dialysis which are causing the the blood volume in the bloodstream to become insufficient. The condition is called Hypovolemia.
 

Blood vessels conditions

The blood vessels may also be the problem. Their role isn't just passive conduction of the blood. The blood vessels in the body also can squeeze to raise the blood pressure, and if this action is prevented for some reason, the blood pressure may fall. The most common reason behind the loss of this function of the blood vessels are different medications. Numerous medications affect blood vessels including most of the medications used for blood pressure, and many of the medications used in psychiatry and for heart pain. Heat, such as a hot shower or from a fever can also dilate blood vessels and cause orthostasis.
 

Nervous system problems

The nervous system normally senses and responds to regulate blood pressure. There are several neurological mechanisms of controlling blood pressure which affect heart pumping, blood vessels, kidneys etc. If something is wrong with this control system, the blood pressure may fluctuate.

Ear vestibular problems

It is proven that vestibular disorders may interact with blood pressure and heart rate control. The vestibular system is a source of information about uprightness and the whole system is localized in inner ear. 
 

Medications

Medication related pressure drop could be caused by high doses of blood pressure or depression medications.

Drugs that can cause or worsen orthostatic intolerance:

    * A-Receptor blockers
    * Angiotensin-converting-enzyme inhibitors
    * B-Blockers
    * Bromocriptine
    * Calcium channel blockers
    * Diuretics
    * Ethanol
    * Ganglionic blocking agents
    * Hydralazine
    * Monoamine oxidase inhibitors
    * Nitrates
    * Opiates
    * Phenothiazines
    * Sildenafil citrate (Viagra)
    * Tricyclic antidepressants
 

Primary adrenal insufficiency

Persons with primary adrenal insufficiency usually have symptoms of glucocorticoid deficiency and low blood pressure. Their skin may be dark, serum potassium high, and there may be associated hypothyroidism, diabetes and vitiligo.
POTS can be caused by genetically inherited neurotransmitter disorders, including disorders of catecholamine production and release, such as Norepinephrine-Transporter Deficiency.