Pentobarbital-induced apneusis in intact, vagotomized, and pneumotaxic-lesioned cats

While recording several respiratory parameters, sodium pentobarbital (PB) was infused into the inferior vena cava of spontaneously breathing, PB anesthetized cats. Three cat groups were investigated: intact control (group A); vagotomized (group B); pneumotaxic center-lesioned (group C). With a few exceptions, all cats developed PB-induced inspiratory apneusis. Groups B and C exhibited 10-sec inspiratory hold patterns at significantly lower PB levels than group A cats. All groups developed apnea at different PB levels. Ventilation was consistently depressed, predominantly by breathing frequency attenuation. Tidal volume remained comparable to control, but decreased in vagotomized cats at high PB levels. These results are interpreted to signify that (1) inspiratory inhibitory inputs are more susceptible to depression by PB than inspiratory drive mechanisms; (2) the breathing pattern of apneusis results when summed inspiratory inhibition is reduced below a critical minimum level; (3) vagal and pneumotaxic center inhibitions on inspiration are equally weighted at apneusis, but not at apnea. These results are further discussed in terms of the inspiratory off-switch model. A possible model of Biot respiration is also introduced.