This Web page is designed for the general education of patients and their families.
Important correction made 6/13/97 (in bold, in the text).
Disclaimer: The answers provided are the opinion of Dr.
Tom Chelimsky. No patient should make any changes in their care without
first consulting their physician.
Question: I am a 22 year old female and have been diagnosed with neurally mediated syncope (vasodepressor). During my tilt table test, I had a severe reaction to the infusion of isoproterenol, within four minutes my heart rate jumped to 170 and my blood pressure dropped to 70/30. Could you explain what causes this?
I also faint when I am not exercising, often when I am standing still. Is the same mechanism causing this? I am currently talking atenolol and am on a high salt diet. It is too soon to tell if it is working. Any information you could give me would be appreciated.
Is there anything I should do on my own to help myself get better? Is exercising dangerous?
Dr. Chelimsky: You are describing a fairly classical syndrome called postural orthostatic tachycardia syndrome or POTS. There has been a fair amount written about this disorder, and it is one that our laboratory has been studying for some time.
Isoproterenol is a drug which mimics epinephrine, the body's fight or flight hormone. It binds to a type of receptor called the "beta-adrenergic" receptor. These receptors are located in the heart, where they cause the heart to pump faster, and in the veins where they cause dilation. They are also located in numerous other organs, includeing the lungs, where they cause the airways to open up (hence the use of similar drugs for asthma), the kidneys, where they alter fluid handling, the pupils, where they cause dilation, and in the central nervous system.
There are several THEORIES as to why a person with POTS might faint when given this drug: (1) the heart pumps faster, and your illness (POTS) causes it to be "sensitive", so that the heart sends a signal back to the brain complaining of overload, with the result that the brain tells the circulatory system to quit (blood pressure drops, heart slows). (2) POTS might cause the veins to become sensitive, so that a little isoproterenol dilates the veins a lot, and causes similar findings. (3) POTS is disorder of the central nervous system, and involves a beta-adrenergic receptor in the brain which is malfunctioning, causing fatigue, low blood pressure and sensitivity of the heart to any change in setting (such as sitting to standing). I prefer the last hypothesis as it makes the most sense to explain all of the findings, and our results in the laboratory which suggest a central nervous system, not a peripheral nervous system problem.
It should be kept in mind that the isoproterenol tilt can cause fainting in normal people as well. When a beta-blocker such as atenelol works, it does so immediately. If it is not working now, it probably will not. I would not give up on beta-blockers, however. They are a very good treatment for this disorder. The problem with atenolol is that it binds only the beta-1 subtype, and does not cross into the brain. An alternative, if your physician thinks this is appropriate, would be nadolol, which binds both beta-1 and beta-2 and does cross into the brain a little better, but not so much as to cause fatigue.
People can help themselves by taking a multivitamin B complex tablet with no more than 50 mg of B6 in it, and 800 units of vitamin E per day. Heavy exercise can be dangerous, if the heart rate goes up too high (>150-160). You can exercise in the water (water jogging), and the pressure of the water on your lower body will cause blood to return to your heart more efficiently, hence this is generally recommended. These recommendations are general only, and I would check everything with your own physician before doing any of these.
Here are some references for your further reading:
1) Bou-Holaigah et al.: JAMA 1995 Vol 274 p 961
2) Schondorf and Low: Neurology 1993 Vol 43 p 132 Wishing you the best. Dr. Chelimsky.
If you have a question, please submit it to Dr. Chelimsky.