Initiation of each avian inspiration by a CO2 threshold mechanism

The avian respiratory oscillator has been investigated in a unidirectionally ventilated chicken by changing the dynamic pattern of inflow CO2 concentration (FCO2). Stimulation with periodic FCO2 results in a one-to-one synchronization of the respiratory movements that we have called pacing (Respir. Physiol. 22: 167--177, 1974). A two-parameter CO2 threshold model is proposed to explain this behavior. The model states that when FCO2 reaches a threshold level (L), it initiates the beginning of inspiration a constant time interval (LB) later. According to this model, when a triangular FCO2 concentration is used to synchronize the breathing pattern, the time from the minimum of the wave form to the beginning of inspiration (C-B interval) is dependent on the mean value and the rate of rise of FCO2 as determined by period and amplitude of the triangle. Particularly interesting is the prediction that the direction of the relationship (increasing or decreasing) between FCO2 amplitude and the C-B interval is dependent on whether the mean value of FCO2 is above or below the threshold level. Experimental data obtained during amplitude changes support the above prediction.