Paroxysmal supraventricular tachycardia (PSVT or SVT) is a type of abnormal heart rhythm which affects around 2 million people worldwide.

It results in short episodes of very high heartbeat rate, typically 150 - 240 beats per minute. The episodes begin and end very abruptly, usually lasting a few minutes. It's not normally dangerous, but can be very distressing.

Although it has been found to result from faulty electrical feedback rhythms in the heart, the root cause (and what triggers the episodes) is unknown. Various factors such as caffeine, alcohol, infection, stress etc. etc. can make it more likely.

Further info : E-Medicine Health

Note : Many cases of PSVT are due to Wolff–Parkinson–White syndrome which itself has an unknown causative pathway.