Frequency dependence of capnography in anesthetized rabbits

Aspirative capnography may be of help to diagnose early childhood asthma, but clinical usefulness in young children is limited by the relatively high respiratory rate. This study aimed to characterize the FCO2 time course during airway constriction in 8 anesthetized rabbits, artificially ventilated at 30, 60 and 80 breaths/min. Methacholine was inhaled to double the respiratory resistance measured at 8 Hz by the forced oscillation technique. The capnogram shape changed in response to both methacholine and ventilatory frequency. Slope of phase II, the peak of first-order time derivative and trough of the second-order time derivative of the FCO2 signal, were significantly attenuated after methacholine compared with baseline at all breathing rates (p < 0.02). Moreover, significant correlations between respiratory reactance and resistance were observed with the phase III slope and the angle described by phase II and phase III (p < 0.01). It is concluded that capnography may be useful to identify acute airway changes related to bronchoconstriction, even at high breathing frequencies.