Disorders In Depth
Postural Tachycardia Syndrome (POTS)
When a patient’s heart rate speeds up 30 beats per minute or more without much change in blood pressure on standing, the patient may have orthostatic intolerance (OI). This disorder is heterogeneous and has had many different names, including neurasthenia, soldier’s heart, mitral valve prolapse syndrome, vasoregulatory asthenia, neurocirculatory asthenia, and and effort syndrome. Because upright heart rate is usually greatly increased, the more common term used now is Postural Tachycardia Syndrome (POTS). The increase in heart rate may be a sign that the cardiovascular system is working hard to maintain blood pressure and blood flow to the brain. Orthostatic intolerance affects an estimated 500,000 Americans and causes a wide range of disabilities. It is most frequently seen in young women, often less than 35 years of age.
- Lightheadedness, palpitations and tremulousness during standing
- Other upright posture symptoms may include:
- Visual changes
- Discomfort in the head and neck
- Throbbing of the head
- Poor concentration
- Occasionally fainting
- Chest discomfort
- Shortness of breath
The etiology of POTS is unknown. The onset of POTS is sometimes predated by a recent viral infection. Patients can undergo extensive clinical evaluation without identification of a specific abnormality, and therefore most patients remain undiagnosed. These difficulties are compounded by the heterogeneity of disease states in patients with orthostatic symptoms, spontaneous fluctuations in disease severity, and no uniformity in nomenclature of disease classification. Another problem in the diagnosis of OI is its overlap with other conditions such as Chronic Fatigue Syndrome (CFS), Neurally Mediated Syncope (NMS), physical deconditioning, etc. Improving characterization of the underlying circulatory responses may lead to a clarification of some of those issues, and will facilitate the discovery of the pathophysiologies of POTS.
The Role of Hypovolemia in POTS
Hypovolemia is an abnormal decrease in blood volume, or more specifically an abnormal decrease in the volume of blood plasma. This sometimes occurs in POTS patients. The patients may have a reduced blood volume throughout their body, or the hypovolemia may occur due to blood pooling in the abdomen and legs. Reduced plasma renin activity often accompanies the low blood volume. Reduced levels of renin release consequently result in reduced secretion of aldosterone. This would be expected to impair renal sodium conservation thereby contributing to hypovolemia. Findings suggest that the impaired renin release may possibly result from sympathetic denervation. Abnormalities in the kidney are also suspected of causing the reduced renin and aldosterone levels. Physicians believe hypovolemia and inappropriately low levels of plasma renin activity may be important pathophysiological components of orthostatic intolerance. The findings in hypovolemic POTS patients have been dubbed the "renin-aldosterone paradox" and are explained as follows:
Under normal circumstances, low plasma volume is sensed in the kidney (and in the heart and aorta) and stimulates an increase in plasma renin activity (renin), and aldosterone (ALDO). The increase in plasma renin activity and aldosterone promotes salt and water retention, which leads to an increase in extracellular fluid volume and plasma volume. In POTS, there is a failure to sense and appropriately respond to low plasma volume. There is no appropriate increase in plasma renin activity and aldosterone given the hypovolemia. Because plasma renin activity and aldosterone are not increased, salt and water retention is not increased, and plasma volume is not increased.
Long-Term Outlook for Patients with POTS
The majority of patients with POTS have a relatively mild disorder which improves over succeeding weeks and months. Most patients will eventually be free of symptoms. However, in some patients, the symptoms are more severe, the duration of the illness may be longer, and the expected recovery may not occur.
In individual patients, different therapeutic regimens may result in improvement of symptoms. These include:
- Orthostatic “exercise”
- Low dose beta blockade
- Salt and /or fludrocortisones
- Low dose alpha 2 agonist (clonidine)
- Low dose alpha-1 agonist (midodrine)
- Alpha-Methyldopa (Aldomet)