Oscillations of medullary extracellular fluid pH caused by breathing

The purpose of this study was to determine if respiratory oscillations of arterial PCO2 are transmitted to the medullary ECF. Anesthetized, paralyzed cats whose vagi and carotid sinus nerves had been cut were studied. A flat surfaced pH electrode (2 mm in diameter) and a specially built differential amplifier were used to measure pH on the ventral surface of the medulla. Elimination of noise and high sensitivity were obtained by integrating the output from the pH amplifier with a digital voltmeter . We measured ECF pH oscillations that had the same period as the ventilator. The amplitude of the oscillations was about 0.006 unit when the ventilator was slow (less than 10/min). The amplitude decreased progressively as the rate was increased and oscillations were usually undetectable at rates above 25/min. Experiments were performed that ruled out mechanical artifact and ventilator-related fluctuations in arterial pressure as being causal. We conclude that oscillations of alveolar CO2 are transmitted to the brain by the circulation and result in oscillations of ECF pH.