What is the X in X-mas?
1867, from cardio- + -graph "something written."
Although the work does not treat of the recent means of diagnosis--the thermometer, laryngoscope, cardiograph, etc.,--still it is complete as far as it goes. [book review in "Medical Investigator," May 1867, p.94]
cardiograph car·di·o·graph (kär'dē-ə-grāf')n.
An instrument used to record the mechanical movements of the heart.