microtonometer

A small tonometer invented by Krogh, originally intended for animals but later adapted to humans, for determining the tensions of oxygen and carbon dioxide in arterial blood; it provides the means of bringing a small bubble of air into gaseous equilibrium with a sample of blood obtained by arterial puncture. [micro- + G. tonos, tone, + metron, measure]

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mi·cro·to·nom·e·ter (mi″kro-to-nomґə-tər) a small tonometer for measuring the oxygen and carbon dioxide tension in arterial blood.