The Physiopathology of Tachycardia  

Before going over the physiopathology of tachycardia, it’s necessary to have a look at how the circulation system functions in a properly working heart.

Normally, the impulse that causes a contraction in the heart comes out from the focus called ‘the sinus knot’.  This focus, compared with the other automatic focuses creates higher amounts of impulse, therefore, it is more dominant and it also has a control over the heart palpitation.

These impulses first go to the arteriovenous (A.V) knot via internodal routes. Stopping here for a while, they move on to the ventricles via the A.V bundle and finally spread out to the ventricles with the lymph of purkinje, thus provide the necessary support for the heart palpitation. So, the heart beats  60 to 100 times, in average 72 times per minute.

Obviously, there are systems that are responsible for the control of this circulatory system. The unbalanced condition between the two parts of this autonomic nervous system namely; the parasympathetic and the sympathetic nervous systems which are functioning involuntarily, causes the state of tachycardia, one of the variations of a disorder. In the case of tachycardia, the balance has been spoiled in favor of the sympathetic nervous system. The stimulating factors, such as stress, worry, excitement, fear, other stimulating substances, the intake of drugs,  increase both the number of contractions in the sinus knot and the circulation speed of this contraction into the heart, meanwhile the adrenaline and the nonadrenaline hormones provide the formation of a hundred or sometimes more impulses.

The original phase of this happening in a molecular base is also explained as follows:

It is related with the increase of the permeability of  Na +1 and Ca +2 ions  in the cell membrane with the help of the impulse mentioned above.

Such a stimulation of the heart has a control over ‘’the heart rate control  system’’. This system  is based on the principle of the heart slowing down as a consequence of the increase in the permeability of the potassium ion (k + a) in the cell membrane and with the acetylcholine,a neurotransmitter secreted by the vagus nerves (the nerves of the parasympathetic system that stimulates the heart), thus  the speed of the heart is decreased. The cause of this mechanism of dominance is based on the principle of the stimulation of the adrenaline hormone.

Although the parasympathetic system is considered to be the main effective system in stimulating the heart, the sympathetic system dominates it. Besides,  the thyroxine, one of the two thyroid hormones gives also rise to the speed of the heart by sending direct impulses.

  Another main mechanism that has an effect on the speed of the heart is the depressor and the impulsive reflex that is created by the stimulation of the centers placed on the various parts of the circulatiory system, eg. heart, the aortic arch, depending on the level of oxygen and carbon dioxide in the blood. The depressors decrease the speed of the heart, whereas the impulsive reflex increases it. This happens via the vagus nerves of the parasympathetic system.  

Dr. Mehmet Ozdemir