CO2-ventilatory response of the anesthetized rat by rebreathing technique

The ventilatory response to CO2 in rats under sodium pentobarbital anesthesia has been measured using the rebreathing technique. The animal rebreathed through a tracheal cannula for a period of 4 min from an apparatus of 200–400 ml capacity, containing 5–6% CO2 in O2.\(P_{{\text{CO}}_{\text{2}} } \) in the rebreathing apparatus (PappCO2), instantaneousVT,f, and\(\dot V_{\text{E}}\) were monitored before, during, and after rebreathing. During the rebreathing run,PappCO2 andPaCO2 rose linearly from 35–40 to 65–70 mm Hg; there was no significant difference betweenPappCO2 andPaCO2 at any time during rebeathing.VT and\(\dot V_{\text{E}}\) increased almost linearly with the rise inPappCO2, whilef increased to a maximum within 2 min of rebreathing. In the rat,VT regulation seemed to operate exclusively as a proportional control system in response to linearly increasing CO2 stimulus. The slopes ofPappCO2,VT or\(\dot V_{\text{E}}\) response curves varied considerably during the time course of the experiment, depending upon the level of anesthesia, even though there was no large change in\(\dot V_{\text{E}}\) in the control periods which were under hyperoxic conditions. However, a significant linear relationship was seen betweenf in the respective control period and the slope ofPappCO2-VT response at various levels of anesthesia. We concluded that the rebreathing technique can be applied in small experimental animals and that changes in the sensitivity of the respiratory control system to a CO2 stimulus by anesthesia can be easily monitored by repeating the rebreathing test.