A quantitative description of the pattern of breathing during steady-state CO2 inhalation in man, with special emphasis on expiration.

1. Quantitative data on the pattern of breathing in normal men and women (Gardner, 1977) have been used to derive expressions that are based on known physiological mechanisms.2. The relations between the applied chemical drive to breathing (expressed as DeltaP(A, CO2) in high O(2)) and the several components of the volume-time patterns described in the companion paper were examined. Neither mean tidal volume (V(T)), nor mean inspiratory nor mean expiratory times (T(I), T(E)) were uniquely related to the chemical drive across the breakpoint, which could be demonstrated in two and suspected in the third of these plots.3. Mean inspiratory flow (V(T)/T(I)) was linearly related to P(A, CO2) over the whole range and, like minute ventilation (V), showed no breakpoint. The mean relation was V(T)/T(I) = 0.11 (P(A, CO2) - 35.2). V(T)/T(I) was highly correlated with V; in individuals with healthy lungs and under relatively stable conditions of compliance and resistance it may be accepted as a wholly inspiratory alternative to V as an index, on the efferent side, of the total prevailing chemical drive.4. The description of the relation between T(I) and V(T) was essentially the same as that of Clark & Euler (1972): in range 1, T(I) = either 1.29 - 0.07 V(T)or the constant 1.24 sec, and in range 2, T(I) = 0.65/(V(T) - 0.88) + 0.59.5. Expiration was described by an equation based on the inverse linkage between T(E) and chemical drive and the direct link between both mean and breath-by-breath values of T(I) and T(E): T(E) = pT(I) + q/(drive - r) in which p was 0.64 +/- 0.09, q was 11.1 +/- 2.64 sec. (torr CO(2))(-1) and r was -2.73 +/- 1.09 torr CO(2). All three parameters were necessary for an adequate description.6. It is argued that the first term of the T(E) equation represents influences related to lung volume exerted through the vagus, and that the second represents the effects of over-all chemical stimulation exerted through other pathways.