In 1870, Frick pointed out that the output of the heart per minute could be determined from the difference in either the carbon dioxide or the oxygen tension of the blood before and after it passed through the lungs by direct determination of these gases in the arterial and venous blood. Subsequently, many attempts to adapt the Frick principle to the determination of the circulation rate in man were made by various investigators. The first of these attempts were based on equilibration of either the oxygen or the carbon dioxide in the lungs with the oxygen or the carbon dioxide in the blood, thus avoiding the dangers inherent in direct cardiac puncture as used originally. Bornstein, in 1910, suggested the use of high concentrations of nitrogen for equilibration, instead of carbon dioxide or oxygen, because it is an inert gas which does not enter into the various metabolic processes.
BOOTHBY WM, RYNEARSON EH. INCREASE IN CIRCULATION RATE PRODUCED BY EXOPHTHALMIC GOITERCOMPARED WITH THAT PRODUCED IN NORMAL SUBJECTS BY WORK. Arch Intern Med (Chic). 1935;55(4):547–557. doi:10.1001/archinte.1935.00160220017002
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