The Collaborative International Dictionary of English v.0.48:

Arrhythmic \Ar*rhyth"mic\, Arrhythmous \Ar*rhyth"mous\, a. [Gr.
   ?; 'a priv. + ? rhythm.] (Med.)
   Being without rhythm or regularity, as the pulse.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arrhythmic
    adj 1: lacking a steady rhythm; "an arrhythmic heartbeat" [syn:
           arrhythmic, jerking, jerky]
    2: without regard for rhythm [syn: arrhythmic, arrhythmical]