Source : Webster's Revised Unabridged Dictionary (1913)

Hemisystole \Hem`i*sys"to*le\, n. (Physiol.)
   Contraction of only one ventricle of the heart.

   Note: Hemisystole is noticed in rare cases of insufficiency
         of the mitral valve, in which both ventricles at times
         contract simultaneously, as in a normal heart, this
         condition alternating with contraction of the right
         ventricle alone; hence, intermittent hemisystole.