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Department of Medicine, Charing Cross Hospital Medical School, London, W.6

Department of Anaesthesia, Charing Cross Hospital Medical School, London, W.6

Department of Medicine, Middlesex Hospital Medical School, London, W.1

Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH, U.S.A.

1. The effect of electrically induced `exercise' on the respiratory oscillation of arterial pH was studied in chloralose-anaesthetized dogs with spinal cord transection at T8/9 (dermatome level T6/7).

2. Respiratory oscillations of arterial pH (presumed to be due to oscillations of arterial P_CO2) were sensed with a fast-responding electrode in one carotid artery. Breath-by-breath estimates of the maximum rate of change of pH of the downstroke of the pH oscillation (dpH/dtmax) were obtained by differentiating the pH signal.

3. Consistent with the findings of the previous paper (Cross et al. 1982), the ventilatory response to exercise could not be explained on the basis of sensitivity to CO₂; the _I/P_{a, CO2} was significantly greater for `exercise' than for CO₂ inhalation.

4. On average, the amplitude of the pH oscillations decreased during `exercise'. The change in the phase relationship () between respiratory and pH cycles, although significant from the second breath onwards, was not thought to be responsible for the increased ventilation _I; the direction of the change was opposite to that previously found to increase _I.

5. Inspiratory duration (t_i), expiratory duration (t_e), _I and the dpH/dtmax changed significantly by the third breath of `exercise'. A significantly linear relationship was obtained between t<sub>e</sub> and dpH/dtmax during the on-transient (first ten breaths) of `exercise'. This relationship was maintained throughout `exercise'. <sub>I</sub> and dpH/dtmax were also linearly related during the on-transient, although the same relationship did not hold true throughout `exercise'.

6. The dpH/dtmax was related to CO₂ production (_CO2) lending support to the prediction that the slope of the downstroke of the pH oscillation is a function of _CO2.

7. It was concluded that the dpH/dtmax (dpCO₂/dtmax) is a potential humoral signal in `exercise' and could account totally for the shortening of t<sub>e</sub>. Since there was a late rise in <sub>I</sub> (due to an increase in tidal volume V<sub>T</sub>) in the absence of a change in dpH/dtmax, it was considered unlikely that the dpH/dtmax was the only humoral signal present during `exercise'.

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