Etymology
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Words related to cardio-

*kerd- 
Proto-Indo-European root meaning "heart."

It forms all or part of: accord; cardiac; cardio-; concord; core; cordial; courage; credence; credible; credit; credo; credulous; creed; discord; grant; heart; incroyable; megalocardia; miscreant; myocardium; pericarditis; pericardium; quarry (n.1) "what is hunted;" record; recreant; tachycardia.

It is the hypothetical source of/evidence for its existence is provided by: Greek kardia, Latin cor, Armenian sirt, Old Irish cride, Welsh craidd, Hittite kir, Lithuanian širdis, Russian serdce, Old English heorte, German Herz, Gothic hairto, "heart;" Breton kreiz "middle;" Old Church Slavonic sreda "middle."
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cardiogram (n.)
"a tracing of the beating of the heart made with a cardiograph," 1876, from cardio- + -gram.
cardiograph (n.)

"apparatus for recording by tracing the beating of the heart," 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]
cardiology (n.)
"the study of the heart," 1847, from cardio- + -logy. Cardiologist attested from 1885.
cardiopulmonary (adj.)
also cardio-pulmonary, "pertaining to both the heart and the lungs," 1879, from cardio- + pulmonary.
cardiovascular (adj.)
also cardio-vascular, "pertaining to both the heart and the blood vessels," 1870, from cardio- + vascular. Cardiovascular system is recorded by 1918.