The Collaborative International Dictionary of English v.0.48:

cardiogram \car"di*o*gram`\, n. [Gr. kardi`a heart + -gram.]
   (Physiol.)
   Any instrumental record made of heart activity, especially
   the curve or tracing made by a cardiograph. Sometimes used
   synonymously with electrocardiogram.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

cardiogram
    n 1: a graphical recording of the cardiac cycle produced by an
         electrocardiograph [syn: electrocardiogram, cardiogram,
         EKG, ECG]