The effects on breathing of alternate breaths of air and a carbon dioxide rich gas mixture in anaesthetized cats.

1. An in vivo pH electrode was used to assess the effect in anaesthetized cats of the administration of 5% CO(2) (21% O(2), balance N(2)) and air as alternate inspirates upon the time course of the carotid arterial pH, and by inference, the P(CO2).2. This method of administration of CO(2) and air resulted in a lowering of the recorded pH with the production of oscillations of twice the duration seen on air alone. These larger oscillations had a period of two respiratory cycles and their amplitude was approximately twice that of normal oscillations.3. The respiratory response consisted in all cases of an increase in mean tidal volume and ventilation, and in 50-60% of recorded runs, of a highly specific sequence of tidal volume changes. The specific sequence was composed of alternately larger and smaller breaths which persisted while the larger oscillations continued.4. The use of the in vivo pH electrode made it possible to determine whether or not the tidal volume alternation was a result of sensitivity to carotid arterial P(CO2) oscillations. By diverting the carotid arterial blood through a mixing chamber the amplitude of the double (respiratory) period oscillations was, in some cases, reduced but not eliminated. In these cases the specific pattern of tidal volume changes was still present in 60% of trials. In the cases where the pH fluctuations were completely eliminated the specific respiratory pattern was never present.5. Average breath to breath differences in tidal volume seen during control runs with large oscillations present ranged in size from 3.6 to 8.6% of the mean tidal volume. When the oscillations were completely eliminated by means of the mixing chamber the breath by breath differences only ranged from 0.2 to 2.7% of mean tidal volume. The change was highly significant.6. Mean tidal volume and ventilation were not altered by eliminating the large (double period) oscillations.7. It has been shown that the tidal volume is made to change from breath to breath in a persistent manner when there are recurrent changes in carotid arterial blood chemistry. The effects on respiration involve vascular receptors above the mid carotid arterial region. It is argued that the effects involve the sensitivity of the carotid bodies to rapid (within breath) changes in P(a, CO2). There appears to be no significant effect on mean ventilation of the dynamic component of the arterial P(CO2) changes produced in these experiments.