Paroxysmal tachycardia is defined by Lewis as "a condition in which from time to time the normal mechanism of the heart beat is interrupted by a series of rapid and regular beats varying in rate between 100 and 200 beats per minute, the attacks starting and ending quite abruptly."
The normal impulse to the heart beat originates in the sinus node, or pacemaker, situated at the junction of the superior vena cava and the right auricle. In paroxysmal tachycardia, a new center of impulse formation arises outside the sinus node, in either the auricle or the ventricle, and the rate is much greater than normal. In the ordinary case of paroxysmal tachycardia, the onset is sudden. The patient is perfectly comfortable and may be aware of the increase in the rate of the heart beat or of a fluttering sensation about the heart. The average pulse rate is about 160
MEYER J, LACKNER JE, SCHOCHET SS. PAROXYSMAL TACHYCARDIA IN PREGNANCY. JAMA. 1930;94(24):1901–1904. doi:10.1001/jama.1930.02710500019007