The Collaborative International Dictionary of English v.0.48:

Tricrotism \Tri"cro*tism\, n. (Physiol.)
   That condition of the arterial pulse in which there is a
   triple beat. The pulse curve obtained in the sphygmographic
   tracing characteristic of tricrotism shows two secondary
   crests in addition to the primary.
   [1913 Webster]
   [1913 Webster]