Monday, June 18, 2012

Beta-Blockers: What do They Block?

Beta-blockers may be considered as a treatment for some forms of dysautonomia. The most likely diagnosis is postural orthostatic tachycardia syndrome (POTS), since tachycardia is a primary symptom.  Other typical symptoms of POTS include syncope or fainting, light-headedness, and general malaise.

To understand what B-blockers do, one considers the leg veins, the heart itself, and the stress response together with the stress hormones that are released.

First, imagine that your legs have both arteries and veins running up and down them. On the one hand, the arteries flow downward, carrying oxygen to the tissues. Once the tissues are perfused, the veins are called upon to perform a unique ability. They squeeze themselves. They do so in order to push the blood 'upstream' back to the heart. Trouble with this venous system can lead to varicose veins, puffy feet, dizziness, and orthostatic hypotension (low blood pressure upon standing).

Bear in mind that the heart controls its own rate. It does so via the sinoatrial node. The node 'talks' to the heart tissue via nervous system input. Also, there are structures called receptors that are anchored into of the actual cardiac muscles. They stick out, reaching out like little arms. The receptors respond to input. Turn the receptor on for action. Block the receptor for no action. Pharmacologic drugs actually 'sit' on the receptor to inactivate it. More on this later, but step back and view the whole scene. First the two top chambers contract in unison, then the two ventricles contract. In this manner, blood is pushed to the lungs for oxygen, and to the body for perfusion, respectively. Over and over again, hundreds and thousands and...well... 86,400 times per day, 604,800 times per week, and 31,449,600 times per year (for a heart rate of 60 beats/min), the heart beats.

Additionally, the endocrine system's  'fight or flight' hormones epinephrine and/or norepinephrine are released from the adrenal glands, located just above each kidney. Every one has two adrenal glands. Epinephrine is a stress hormone that gets released, for example, when a man sees a lion or a when woman sees a bear. The stress response gets the body ready to do one of two things: fight or run.  So, imagine all the things needed when under stress: big pupils, fast heart rate, high blood pressure, increased blood flow to the leg muscles, and decreased blood flow to the stomach and intestines (the body does not want to digest food; it wants to run). These are the things that normally occur during the normal stress response, to get the person ready to survive an insult.

A person with dysautonomia undergoes the 'fight or flight' response every time (s)he stands up. Blood crashes to the feet with gravity, and the normal compensation of the veins in the legs is lost. This failure to squeeze the leg veins means that blood is pooling in the legs. This, in turn, means that the brain is not getting enough blood.  The signal 'No Blood Flow to the Brain' leashes a cascade of intrinsic mechanisms for protection of the individual. Faster and faster, the heart rate seeks to pound because the brain is sensing that it needs more blood flow. This is the tachycardia of POTS. It is unleashed for protection.

However, compensation is lost. Despite tachycardia and increased epinephrine, cerebral blood flow continues to decrease. The patient is still trying to stand by now, but things are looking 'gray'. Sweaty palms, and a diaphoretic forehead say that the endocrine system output of epinephrine is in full gear. Things are still looking 'gray' and one way or another, the patient goes down.

Needless to say, this scenario can repeat itself over and over again, leading to exhaustion. Many patients are simply bed-ridden because their bodies are overwhelmed by the effort of attempting to counteract gravity. 

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