Transient analysis of the canine cerebrovascular response to carbon dioxide

Cerebral venous outflow and carbon dioxide transients were studied during five different transitional states: (1) on and off 10% carbon dioxide breathing, (2) on and off hyperventilation, (3) on 7% carbon dioxide breathing, (4) on 10% carbon dioxide breathing initiated from 7% carbon dioxide breathing, and (5) on 10% carbon dioxide breathing initiated during intracarotid papaverine infusion, in pentobarbital anesthetized, paralyzed, mechanically ventilated dogs. Plots of the temporal relationships between these variables indicated that cerebral blood flow is closely related with cerebral venous carbon dioxide tension but not arterial carbon dioxide tension. The rate at which flow changed upon transition from one steady state to another was phase dependent, in that longer times were required to establish stable conditions in the on phase than in the off phase. The magnitude of the maximum rates of change in cerebral blood flow achieved during transition was influenced both by the size of the forcing function and the level of flow present at the time the response was initiated. Directional changes had no effect upon the maximum rate of the flow change as long as equivalent-sized forcing functions were employed and the initial blood flow levels were similar between responses. However, faster flow transients could be produced by increasing either of the latter two factors. These findings are consistent with the hypothesis that it is either tissue carbon dioxide tension or cerebral venous carbon dioxide tension that is the important variable regulated by cerebral blood flow. The rate-limiting factor in the response appears to be carbon dioxide delivery rate and not the rate of carbon dioxide diffusion.