Retinal vascular autoregulation and CO₂ reactivity in man

Abstract

The purpose of the present study was to investigate the possibility to estimate CBF autoregulation and cerebrovascular CO₂ reactivity by the retinal vascular responsiveness to changes in perfusion pressure and in arterial carbon dioxide tension (Paco₂).
The retinal arterial diameter and its response to changes in blood pressure and in Paco₂ were measured by means of a fundus camera in 6 control subjects without intracranial lesion, 15 patients with cerebrovascular disease (CVD), 2 with Shy-Drager syndrome and 3 with spinocerebellar degeneration.
Reduction of effective mean arterial blood pressure (effective MABP) was induced by the postural change from a recumbent to an erect position. The increase of Paco2 was induced by 5% CO₂ inhalation for 3 to 5 minutes in sitting position.
Changes in diameter of the retinal vessels were quantitatively analysed by the use of Retinal Vascular Index (RVI) (-a change in diameter (%) / a change in effective MABP (mmHg)) and Retinal Vascular CO₂ Reactivity Index (CO₂-I) (a change in diameter (%) / a change in Paco₂ (mmHg))
The results obtained were summarized as follows :
1) Both RVI and CO₂-I in patients with CVD were significantly lower when compared with those in control subjects (p<0.001 and p<0.05, respectively).
RVI in cases with acute CVD (<a month from stroke) and chronic CVD (>a month from stroke) was both significantly lower than that in control subjects (p<0.001 and p<0.05, respectively). RVI in acute CVd was lower than that in chronic CVD, but the difference was not significant. CO₂-I in cases with acute CVD was significantly lower than that in control subjects (p<0.001), but there was no significant difierence in CO₂-I between chronic CVD and control subjects.
In cases with unilateral CVD, RVI in both affected and non-affected side of the lesion was significantly lower when compared with that in control subjects (p<0.001 and p<0.02, respectively), but there was no significant difference in RVI between affected and non-affected side. CO₂-I was slightly lower in non-affected side and moderately lower in affected side whem compared with that in control subjects, but these difference was not significant among three groups.
2) In patients with Shy-Drager syndrome with severe impairment of the autonomic nervous system, RVI was significantly lower than that in control subjects (p<0.01). CO₂-I, however, did not show any significant difference between Shy-Drager syndrome and control subjects.
In patients with spino-cerebellar degeneration without impairment of the autonomic nervous system, RVI and CO₂-I were not significantly different from those in control subjects.
The above data indicate that the autonomic nervous system plays a role in the mechanism of autoregulation of retinal blood flow and that the mechanism is independent of chemical control of retinal vessels mediated by carbon dioxide.
Present results of the retinal vessels are compatible with the previous data of autoregulation and CO₂ reactivity obtained from the cerebral vessels. It is concluded that the observation of the retinal blood flow autoregulation and retinal vascular CO₂ reactivity are a useful method to estimate the CBF autoregulation and cerebrovascular CO₂ reactivity.