CEREBRAL VASCULAR AUTOREGULATION AND CO2 REACTIVITY FOLLOWING ONSET OF THE DELAYED POSTISCHEMIC HYPOPERFUSION STATE IN DOGS

A small number of animal studies have suggested that during the delayed postischemic hypoperfusion state, CO2 reactivity of the cerebral vasculature is lost whereas autoregulation is retained. These findings, however, are inconsistent with the bulk of experimental evidence which demonstrates that CO2 reactivity is more robust and may be retained in pathologic circumstances which abolish autoregulation. These opposing viewpoints were therefore further evaluated in 18 dogs in which complete global ischemia was induced by cerebrospinal fluid (CSF) compression for periods of 12 (n = 12) and 18 (n = 6) min. Following 45 min of reperfusion and with onset of the delayed postischemic hypoperfusion state, autoregulation and CO2 reactivity were evaluated using a continuous measurement of CBF (by sagittal sinus outflow). CO2 reactivity was tested over a PaCO2 range of 20 to 60 mm Hg; autoregulation was tested over a blood pressure range of 60 to 140 mm Hg. Results demonstrated that after both 12 and 18 min of complete global ischemia, autoregulation and CO2 reactivity of the cerebral vasculature were both present, but attenuated. In the case of CO2 reactivity, the slope of the CBF response was decreased approximately 75%. In the case of autoregulation, the response in some dogs was incomplete as compared with their preischemic response.