Postural tachycardia syndrome (POTS) may become debilitating condition due to inability to stand for even short times and even sitting may become a problem. Many patients which have been diagnosed with POTS cannot live a normal life since they get dizzy easily, faint, lack of stamina, cannot concentrate on mental tasks or feel vertigo and nausea.
Traditionally, POTS had been regarded a dysfunction of the autonomic nervous system. Thus, treatment attempts include medications to stabilize the sympathetic nervous system, to fill up circulating blood volume by the administration of mineralocorticoids, oral and intravenous supplementation of fluid and salt and administration of beta-blockers.
Unfortunately, diagnostic criteria of POTS have been imprecise and subject to change in the decades since the in the inauguration of the term. It is no surprise that with lacking understanding of the causal conditions the treatment attempts remain at best at guessing around on the basis of trial and error.
With functional colour Doppler sonographic ultrasound however it is possible to give a sound pathophysiological explanation for POTS.
Patients with vascular compression syndromes regularly develop POTS. Many of them are afflicted by connective-tissue disorders as hypermobile Ehlers-Danlos syndrome.
In these patients POTS is the answer of the circulatory system to the overwhelming stress of gravitation while standing upright.
But human beings are meant to be upright without clinical symptoms, aren’t they? If you are a healthy individual, the answer is yes.
But if your circulatory system cannot react properly onto the varying effect of gravitation then the answer is POTS.
Gravitation is pulling down the blood to the lowest part of your body. While lying horizontally a vertical movement of blood volumes is limited by the little height of the lying body (usually 20-30 cm at maximum).
When you sit or even stand the distance from the highest to the lowest point in your body is suddenly increasing tremendously. This means, that large blood volumes run down to your feet and calves and the pelvis as soon as your trunk is upright. Standing adds the length of the upper thighs to the maximum distance the blood can run down compared to a sitting posture.
So, depending on your height, large blood volumes move down by 80-100 cm in a body height of 160 to 190 cm.
To maintain a normal function of your brain a stable blood supply to the brain is an indispensable prerequisite. If suddenly large volumes are running away from the brain as soon as you sit up or stand up it is necessary to counteract bringing the blood back to the brain. This means, the blood which is running down into the lower body hemisphere needs to return to the heart without delay to be pumped back to the brain continuously, without interruption and at a constant volume. Minor reductions of the cerebral perfusion are well tolerated but if larger volumes cease to participate in the feeding of the brain, then it becomes vital for the body to act. If the supply is too low a brain dysfunction ensues. These are gradual changes starting from tiredness, sleepiness, inability to concentrate on mental tasks, to irritability, dizziness, near fainting and blackouts or even convulsions and loss of consciousness.
The shift of larger blood volumes in the lower body hemisphere while being in a posture with upright trunk (sitting) or standing – orthostasis – causes a pooling of about 500 – 800 ml in the lower hemisphere. The cardiac output can be measured by quantitative color Doppler ultrasound. Normal values for the cardiac output, the blood flow volume which is pumped into the entire body, range between 1.9 L/min/m² BSA and 4.3 L/min/m² BSA (body surface area). A typical adult female patient with 160 cm and 55 kg has a BSA of 1,56 m² for instance. Normally the heart then should pump 2,97 to 6,7 L/min.
Brain perfusion is age dependent. Between 20 to 39 years 727±102 mL/min and between 40 to 59 years 656±121 mL/min are normally expected.
Thus, brain perfusion may be affected by pooling of 800 ml/min in orthostasis. Orthostatic pooling under normal conditions in healthy subjects roughly equals normal brain perfusion volumes.
To prevent brain dysfunction mechanisms are activated to keep the pooling volume at bay. These are muscle contraction of the calves (voluntarily), constriction of the veins in the pool regions, increased heart rate.
Increasing heart rate (HR) is easily measured and can thus be easily used as a parameter of the orthostatic response.
Abnormally high HR alone however, the diagnostic hallmark of POTS, does tell little about the efficacy let alone causes of such a pathologic reaction with the above-mentioned symptoms.
With functional colour Doppler sonographic ultrasound however it is possible to measure immediately the volume shift in the central aortic circulation.
The comparison of the abdominal aortic flow volume while lying horizontally and while standing unveils the reasons for POTS. High pooling volumes are reflected by a sharp decline of this flow volume in orthostasis. A severe reduction of circulation volume would ensue brain dysfunction. Thus, the heart starts pumping faster to deliver a fairly constant perfusion volume to the brain.
Since the reduced circulating volume causes a reduced filling of the heart chambers (ventricles), the stroke volume (SV) of a single heart contraction will reduce in proportion to the pooled volume in the lower hemisphere. The more blood is trapped in legs, pelvis and abdomen the less the ventricle is filled. If the HR would remain constant, the brain perfusion would drop accordingly. Large pooled volumes beyond a critical limit would thus endanger brain function. The only chance to maintain a sufficient brain perfusion is thus an increasing HR since the perfusion volume (PV) is related to SV and HR as follows:
PV = SV * HR
If the counter mechanisms to hold the pooled volumes low fail, then the HR increases to uncomfortable heights and then these mechanisms may even fail to maintain the brain function if still not enough volume can be pumped. THIS IS POTS!
Now, which reasons may cause large pool volumes?
Unfortunately – reason 1 and 2 usually occur together.
The measurement of the pooled volume in orthostasis and its effect on kidney and pelvic organs with the PixelFlux-technique but opens a door for the cure of POTS since it allows a functional diagnostic of the circulatory issues in all parts of the circulation: the heart, the aorta, the abdominal, pelvic and peripheral arteries and veins. The effect of vascular compressions can be localized and quantified. A treatment plan can be established that goes to the root of POTS.
The physical, medical and surgical treatment of the compression syndromes significantly reduces and often completely obviates POTS symptoms.
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