A method for monitoring end-tidal CO2 during neurophysiological experiments on small laboratory animals

The Hewlett Packard Model 47210A Capnometer, intended for monitoring expired CO2 in human patients, can be modified for use in acute neurophysiological studies on small animals. The airway adaptor which would normally be attached to an endotracheal tube is modified and its internal volume is reduced to 0.2 ml, and is incorporated into a guinea pig head-holder. We have verified that the modification does not alter the Capnometer's accuracy. There is no significant increase in resistance in the airway when the adaptor is in place, and the waveform of instantaneous CO2 shows rapid changes on inspiration and expiration with well-resolved peaks and troughs. Measurement of arterial pCO2 suggests that the Capnometer and modified airway adaptor form an accurate system for obtaining a continuous record of end-tidal CO2 in small guinea pigs.