Influence of PaO₂ on Cerebral Macro- and Microcirculation as Observed by Light Reflection: Time Course of Changes

Summary

A silicon intensified target camera was used to study cerebral cortical vessels of the cat through a skull window implant, and red cell content changes were measured by light reflectance. Red cell content changes were observed in cerebral arterioles, capillaries, and venules when the PaO₂ was decreased by lowering the PiO₂.

The time course of change in the diameter of the arterioles and venules was measured by selecting a cross section of the vessel plus some surrounding tissue. From the averaged cross-sectional reflectance signal, the change in vessel diameter was followed as a function of time following the PiO₂ change.

All vessels of greater than 10 microns were observed in focus. Substantial areas where no vessels could be discriminated would contain only capillaries, and changes in light reflectance from such areas would indicate changes in capillary red cell content. The time course of these changes following a step decrease in PiO₂ was recorded.

Results show that the sequence of red cell content increase in cerebral microcirculation during hypoxia is capillary before venule and arteriole. The times of initial red cell content increase are 37.9 ± 7 s, 59.7 ± 7.9 s, and 60.8 ± 9.1 s, respectively. These results suggest an increase in the capillary bed red cell content as the initial response to hypoxia, but venules and arterioles change only on longer exposure to hypoxia. The sequence of the increase in red cell content suggests the capillaries rather than the arterioles are the vessels which respond to the oxygen autoregulation signal.