Effect of F_{L CO 2} Dynamics on T_i and T_tot in Spontaneously Breathing Birds

Summary

This study investigates whether the model of the respiratory pacemaker devised by Kunz and Miller (1974) is responsible for respiratory frequency in awake spontaneously breathing birds. This model hypothesizes that each inspiration causes an oscillation in fractional concentration of CO₂ in the lung (${\operatorname{F} _{{{\operatorname{LCO} }_2}}}$) and that the timing of an event in this ${\operatorname{F} _{{{\operatorname{LCO} }_2}}}$ cycle sets the time of the next inspiration. These experiments used awake chickens which were spontaneously breathing from a gas flow line in which the fractional concentration of CO₂ (${\operatorname{F} _{{{\operatorname{ICO} }_2}}}$) could be rapidly changed. During the first portion of the inspiratory period, ${\operatorname{F} _{{{\operatorname{ICO} }_2}}}$ was kept at a value near that of end expiratory ${\operatorname{F} _{{{\operatorname{CO} }_2}}}$ (0.05). Then τ seconds after the beginning of inspiration, the ${\operatorname{F} _{{{\operatorname{ICO} }_2}}}$ was decreased to 0.0 for 0.6 s and then returned to 0.05. Ventilatory movements were monitored by a whole body plethysmograph. When τ was varied within the range 0.1 to 0.6 s the inspiratory period, T_i, and total respiratory period, T_tot, were found to change proportionately. Sinusoidal and step forcing of τ showed an immediate response of T_i and T_tot with almost no hysteresis. These findings are believed to support the model.