Active and passive respiratory mechanics and control of breathing in kittens.

In five spontaneously breathing kittens (12-13 days old), anesthetized with pentobarbital sodium, we measured the passive and active elastances and resistances of the respiratory system and the decay of inspiratory muscle pressure (PmusI) during expiration. When normalized for body weight (BW), passive resistance (Rrs . BW) was smaller in kittens than in adult cats, whereas passive elastance (Ers . BW) did not differ significantly. As a result, passive time constant (tau rs = Rrs/Ers) was shorter in kittens (mean +/- SE: 0.073 +/- 0.011 s) than in cats (0.121 +/- 0.008 s). This, associated with a faster decay in PmusI in kittens, results in 2-3 times higher flows per kilogram body weight during spontaneous tidal expirations in kittens than in cats. As in the adult cats, the average values of active elastance and resistance were higher than the passive, the average percentage increase amounting to 59 and 49%, respectively. The greater active impedance reflects force-length and force-velocity properties of inspiratory muscles. Its price is higher work of breathing; its advantage is greater intrinsic load compensation.