As its name implies, sinus tachycardia is a condition in which the sinus node fires too rapidly. It is usually due to some other condition, such as anemia, fever, an overactive thyroid gland, or as a side effect of certain medications. Also, the sinus node is very sensitive to physical and emotional cues (mainly through the effects of adrenaline), so sinus tachycardia can often be due to anxiety. Sinus tachycardia may cause symptoms of palpitations, lightheadedness, or fatigue. It may also cause no symptoms at all. Your physician will usually try to find some hidden cause for sinus tachycardia. Generally, treatment is aimed at the underlying cause, but occasionally no other cause is found. In these cases, a diagnosis of "Inappropriate Sinus Tachycardia" (a disorder of impulse generation) is made. In symptomatic patients, medications are available to slow the sinus node and relieve symptoms. If sinus tachycardia is due to another condition, the arrhythmia should resolve after that condition is treated.
Over the last 10-15 years, a condition known as "Postural Tachycardia Syndrome" or POTS has been recognized more and more. This is a disturbance in the part of the nervous system known as the Autonomic Nervous System, which controls heart rate and blood pressure. POTS primarily affects young women, but it can occur in men also. It causes symptoms of palpitations, chronic fatigue, lightheadedness, among others, but most of the symptoms only occur with standing up. There is a thorough discussion of POTS in the section on Dysautonomia on this website.