In studying the attacks of tachycardia which have been known since the description of Cotton in 1867, and which Bouveret has designated as "essential" or "idiopathic" paroxysmal tachycardia, it is necessary to distinguish very carefully between "paroxysms of tachycardia" and idiopathic paroxysmal tachycardia. In the ordinary tachycardias of emotion, excitement or exercise, the pulse-rate gradually quickens and finally reaches its maximum, and when the cause is over, the rate subsides gradually. In the idiopathic paroxysmal tachycardia the rate rises and falls suddenly. The heart-beats just preceding the paroxysm have the usual rate. With the second beat of the paroxysm the heartbeat is already about double that of the preceding and has reached the maximal rate of the paroxysm. The rate then continues practically unchanged throughout the paroxysm, and without warning, the paroxysm subsides as suddenly as it has come.
This sudden onset and sudden subsidence is