Studies on reflex control of breathing in pigs and baboons.

In 8 pigs and 4 baboons, spontaneously breathing, anaesthetized with halothane, Hering-Breuer reflex was tested by means of a total obstruction of the airway preventing either inspiration or expiration. Subsequently animals were paralysed and maintained on phrenic nerve driven servo-respirator. The response of phrenic motoneurone output to various degree of lung inflation, introduced for one breath only, was then carefully studied. This was achieved by varying the gain of servorespirator. Additionally in baboons, identical series of gain manoeuvres was performed against a background of different levels of the initial gain setting. Changes in both inspiratory time and peak amplitude of phrenic signal were monoexponentially dependent on gain of servorespirator and linearly dependent on tidal volume (all negatively correlated). The relationship between inspiratory time T(1) and subsequent expiratory duration T(E )existed only within a range of growing T(1). Vagal positive feedback phenomenon was apparent in pigs and negligible in baboons. It is postulated that inspiratory cut-off mechanism terminates inspiration when excitatory function are outbalanced by their integral.