[A study on shifts of cerebral autoregualtion following end-tidal CO2 by critical closing pressure].

OBJECTIVE: To investigate the shifts of cerebral autoregulation following end-tidal CO2, and set up a new clinical way to evaluate the lower limit of cerebral autoregulation.

METHODS: The cerebral blood flow spectrum of middle cerebral artery, radial blood pressure and end-tidal CO2 (ETco(2)) were simultaneously monitored among 70 healthy volunteers, 38 males and 41 females, aged 21-77. The Lower limit of cerebral autoregulation (LLCA) was determined by critical closing pressure (CCP). We observed the shifts of LLCA of healthy subjects respectively between normocapnia and hyper- and hypocapnia.

RESULTS: The LLCA of healthy subjects was 58 mm Hg +/- 10 mm Hg at normocapnia, increased at hypercapnia and decreased at hypocapnia significantly (69 mm Hg +/- 15 mm Hg and 44 mm Hg +/- 11 mm Hg, P < 0.05). The 95% CI of difference were 2.70 mmHg between hyper- and normocapnia and 2.18 mm Hg between hypo- and normocapnia. The shifting rates of LLCA correlated inversely to the rates of CCP at both hyper- and hypocapnia (r = -0.610 5, -0.555 1, both P < 0.05), and the relation between LLCA's and CCP's shifts displayed an "S" pattern curve at hypocapnia. The rate of mean velocity changing in middle cerebral artery was significantly correlative to the LLCA shifts' rate (r = 0.584 1, P < 0.05), and showing an "S" pattern curve.

CONCLUSION: The lower limit of cerebral autoregulation can be determined by CCP exactly. The cerebral autoregulation can shift up or down following ETco(2), and its physiological basis is closely correlated with the cerebrovascular tone.