Postural Orthostatic Tachycardia Syndrome (POTS)
When the body is in upright posture (sitting or standing), the circulating blood volume tends to shift to the blood vessels in the lower part of the body (pelvis and the legs). This is due to elasticity (stretchability) of blood vessel system and the effect of gravity and is called 'gravitational pooling' of blood. As a result, brain will receive reduced blood flow leading to dizziness or even loss of consciousness.
In the normal circumstances, human body as an adaptation mechanism to avoid this from happening. There are pressure sensors (carotid baroreceptors) the carotid arteries in the neck that carry blood to the brain. If the pressure of blood that go to the brain drops, these sensors can send a signal to the autonomic (involuntary) nervous system that controls vital functions of the body. This signal is received by the 'sympathetic' part of the autonomic nervous system. This part of the autonomic system is also called 'fight or flight response system' and is activated in stress situations. When it received the 'distress' signal from the carotid baroreceptors, that the brain is not receiving enough blood, it activates a reflex nerve signal. This signal is sent to the blood vessels in the lower part of the body.
Blood vessel wall consists of a thick layer of muscle ('smooth muscle' that cannot be voluntarily controlled). The signal from the sympathetic nervous system activated this layer of muscle to constrict (vaso-constriction). If vasoconstriction happens the right way as soon as the body takes the upright posture, gravitational pooling is minimal. The position change does not cause any symptoms.
As long as the person is in upright (sitting or standing) posture, the signals from the sympathetic nervous system needs to maintain vasoconstriction in lower body blood vessels. Then the person is able to be standing or seated upright for any length of time.
Sometimes, this reflex does not happen quickly. Then the person could feel dizzy immediately after sitting up or standing. As the vasoconstriction happens slowly, the symptoms may abate. This is called postural ‘hypotension’ (low blood pressure). This is due to slowness of vasoconstriction but it happens in full with time.
In other circumstances, the vasoconstriction may not happen to the full extent even with time. In such cases, gravitational pooling keeps happening and the carotid baroreceptor keep sending the distress signal to the autonomic nervous system continuously that the blood delivery to the brain is not adequate. In this situation, the sympathetic nervous system uses a second mechanism to increase the blood flow to the brain, increasing the cardiac output. Sympathetic nervous system has numerous nerve fibres reaching the heart, to send this signal. As a result of this signal, the heart increases the beating rate (tachycardia) and increases the squeezing power (contractility). While this is happening, the person may feel the heart racing or ‘thumping’ hard. If the reflex to vasoconstrict is not corrected and the person remains upright, he/she may continue to experience dizziness, light-headedness, weakness, blurred vision fatigue, palpitations, tremulousness, and anxiety. Gastrointestinal symptoms such as nausea, abdominal cramps, early satiety, bloating, constipation, and diarrhea may be particularly problematic in some.
In some situations, compensation is not adequate to maintain the blood flow to the brain. As the heart contracts vigorously and fast, it can activate another reflex through the ‘strain receptors’ (mechanoreceptors) within the heart and great vessels. This reflex is carried via nerve fibres to the parasympathetic or vagal part of autonomic nervous system.
Parasympathetic system or vagal system is another part of autonomic nervous system. Its function is to reverse the activity of the sympathetic nervous system usually when the stress situation has resolved. The result of activation of parasympathetic system is slowing of the heart and relaxation of blood vessels (vasodilatation). When this system is overactivated or inappropriately activated, gravitational pooling can occur unhindered and the heart can be excessively slow down leading to deprivation of blood flow to the brain causing loss of consciousness. This event is called vasovagal syncope. Once upright posture is corrected i.e. patient’s head is at or below the level of lower body, blood flow to the brain is restored. A prompt recovery of alertness is expected.