CO2 responsivity in the mouse measured by rebreathing

We have modified the rebreathing method to study CO₂ responsivity in very small mammals. Tidal volume (V _T) and frequency (f) of pentobarbital-anesthetized mice were measured during rebreathing from a closed circuit, primed with 95% O₂, 5% CO₂, through which the gas was constantly circulated at 0.5 l·min^–1. The circuit consisted of T-tube from a plethysmograph, Tygon tubing with compliant element, CO₂ analyzer and pump, in series. CircuitPCO₂ (PctCO₂), which was recorded continuously during spontaneous breathing, rapidly equilibrated with end-tidalPCO₂. CO₂ response curves were constructed from extrapolated minute ventilation ( ),V _T,f and parameters of breath-to-breath timing, respectively, onPctCO₂. Analyses of slopes of the response curves, change from onset of rebreathing to peak response, andPctCO₂ at which the response peaked revealed that CO₂ stimulates by increasingf andV _T and that this is effected by facilitation of central inspiratory-expiratory phase switching and inspiratory drive mechanisms. However, the stimulatory effect of CO₂ on phase switching was not sustained, with maximal effect occurring before peak . The advantages and facility of the modified rebreathing method make it suitable for studies of other small mammals, including neonates.