Hi, Tuliip.
Orthostatic problems, including OH (orthostatic hypotension) as you have reported, and POTS (postural orthostatic tachycardia syndrome) are common in ME/CFS.
In my view, there are three things that occur in CFS that are likely contributors to the orthostatic problems:
1. The total volume of blood in the body is often lower than normal. This likely results from a "mild" diabetes insipidus (not the same as diabetes mellitus) which is often present in CFS. This in turn is caused by lower than normal secretion of antidiuretic hormone (ADH) by the hypothalamus/pituitary. In my hypothesis, this is caused by glutathione depletion in these organs. The low ADH secretion causes the kidneys to dump too much water from the blood into the urine, and that lowers the total blood volume. People with diabetes insipidus have high 24-hour urine volumes (sometimes much higher than the normal average of 1.5 liters per day), and they have constant thirst. They drink a lot of fluids, but can't keep up with the loss of water into the urine, so they run at lower than normal total blood volume all the time. The osmolality (depends on concentrations of solutes, including the electrolytes) of the blood is higher than normal, and the osmolality (and specific gravity) of the urine is lower than normal.
The low total blood volume causes the venous return of blood to the heart to be lower than normal, and that lowers the stroke volume, since the heart can pump out only what it receives. The cardiac output (the product of the stroke volume and the heart rate) is particularly lowered when standing, because of the additional effect of gravity. Low cardiac output prompts the autonomic nervous system to speed up the heart rate in an effort to raise the cardiac output and deliver more blood when standing, and that constitutes POTS. The low cardiac output can also result in low blood pressure, which constitutes OH. In hot weather, or in taking a warm shower, more of the circulating blood is routed to the skin, in order to help cool the body, and that means that less flow is available for other organs, which exacerbates the problems.
2. Most people who have CFS have dysfunction of the HPA (hypothalamus-pituitary-adrenal) axis. This causes the cortisol output to be abnormal, usually too high in the early part of the illness, and then dropping down to be too low later in the illness. This interacts with the secretion of adrenaline (epinephrine), and noradrenaline (norepinephrine). Among other things, these hormones control the operation of the heart and the circulatory system. This dysfunction may also contribute to POTS and OH. In my hypothesis, this dysfunction is also due to glutathione depletion in the hypothalamus and pituitary. (I might mention, though, that Dr. de Meirleir attributes the hypothalamic dysfunction in ME/CFS to hydrogen sulfide, produced by dysbiotic bacteria in the gut. I think he plans to publish evidence for this mechanism.)
3. Many (or most) people with CFS have diastolic dysfunction of the heart, according to Dr. Paul Cheney, who runs detailed echocardiography exams on his patients. This means that the main pumping chamber of the heart (the left ventricle) does not fill with enough blood during the diastolic phase of the heart's cycle, and that also limits how much blood is pumped out (stroke volume). A low stroke volume again prompts the autonomic nervous system to speed up the heart rate in order to raise the cardiac output. It can also result in lowering the blood pressure. Diastolic dysfunction is caused by too low a rate of supply of ATP to the heart muscle cells by their mitochondria. In my hypothesis, this is caused by glutathione depletion in the heart.
The bottom line is that glutathione depletion can explain all of these aspects as well as many more aspects of CFS. In recent years, we have found that the glutathione depletion in CFS is linked to a dysfunction in the methylation cycle, and that lifting the partial block in the methylation cycle will restore glutathione and correct many of the aspects of CFS. If you want to read more about this, you can find information at
http://www.aboutmecfs.org/Trt/TrtGSHIntro.aspx
As has been posted by others, there are also some temporary remedies that can help until the fundamental causes are dealt with. These include consuming electrolyte drinks or additional salt and water and using compression stockings. Some drugs, such as Florinef combined with salt, have been used, also.
I do agree that it would be very advisable to see a cardiologist to have your heart tested to see if there are other problems as well. If the cardiologist could run a Doppler echocardiograph exam including evaluation of cardiac output and measurement of not only the ejection fraction, but also the IVRT (isovolumetric relaxation time) and the e/a ratio, that would tell you whether you have diastolic dysfunction. The ejection fraction is often actually higher than normal, rather than lower, in cases of diastolic dysfunction. Dr. Cheney has suggested that this is an effort to compensate for the lower amount of blood in the left ventricle. This can be misleading to cardiologists who are accustomed to looking for low ejection fraction as the indicator of other types of heart failure.
I hope this is helpful.
Best regards,
Rich