Cause
The cause of tachycardia may lie within the conduction system of the heart itself or outside of it. For instance, cells in the sinus node (in the right atrium) that normally act as pacemakers can sometimes initiate a faster rhythm. The cause of this is currently unknown. This phenomenon is called sinus tachycardia. If the conduction of electric impulses that normally runs from the sinus node through the atria and the AV node before entering the ventricles is somehow disrupted (e.g. by a so-called atrioventricular block), it is possible that other groups of cells begin to act as pacemakers. Depending on their location, the resulting tachycardia is labelled “supraventricular” or “ventricular”.
Exogenous causes for tachycardia include foods and beverages containing e.g. caffeine, illegal drugs such as amphetamines or cocaine, but also medications like beta-mimetics (which are contained in many asthma inhalers), theophylline, nitrates, or calcium antagonists. Some foods like nutmeg contain essential oils such as myristicin and elemicin that can also cause tachycardia if consumed in larger quantities.
Tachycardias can be a physiological reaction to sudden decreases in blood pressure. This is called reflex tachycardia. Finally, psychological states, such as fear or excitement, can cause tachycardias.