Breath-by-Breath Measurement of Gas Volume and Concentration with Airtight Facemask in Anesthetized Rats

Abstract

To measure ventilatory variables and analyze endtidal and expired gasses of anesthetized rats, we devised a respiratory circuit composed of polyvinyl facemask (dead space 0.3 ml), small spirometer (125 ml) and massspectrometer. The face-mask has the inlet, outlet and sampling tube of the gas analyzer. For the airtight seal, we plastered the animal with non-smelling paste and fastened the mask by hand to make the closed circuit for ventilation. On measurement, we introduced the inspired air into the mask from the glass cylinder under constant flow rate (178 ml/min) with infusion pump, and collected expired air into the spirometer. The endtidal gas was sampled from the small sampling tube placed into the rat's nostril. On the other hand, the tidal volume and respiratory frequency were measured from recorded traces of electrical gain of the potentiometer attached to the spirometer. Minute ventilation was calculated by the tidal volume times respiratory frequency.
Our present respiratory circuit using tight mask has benefits as follows; firstly, small dead space without using non-rebreathing valve, secondly breth-by-breath analyses of endtidal gas concentration, and thirdly no trache ostmy. Then we can measure outputs of other indicator gas and ventilatory variables in steady state.