title: Effect of hyper- and hypocapnia on cerebral arterial compliance in normal subjects.
abstract: BACKGROUND
  Changes in partial pressure of carbon dioxide (PaCO2) are associated with a decrease in cerebral blood flow (CBF) during hypocapnia and an increase in CBF during hypercapnia. However, the effects of changes in PaCO2 on cerebral arterial compliance (Ca) are unknown.
  
  
  METHODS
  We assessed the changes in Ca in 20 normal subjects using monitoring of arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV). Cerebral arterial blood volume (CaBV) was extracted from CBFV. Ca was defined as the ratio between the pulse amplitudes of CaBV (AMPCaBV ) and ABP (AMPABP). All parameters were recorded during normo-, hyper-, and hypocapnia.
  
  
  RESULTS
  During hypocapnia, Ca was significantly lower than during normocapnia (.10±.04 vs. .17±.06; P<.001) secondary to a decrease in AMPCaBV (1.3±.4 vs. 1.9±.5; P<.001) and a concomitant increase in AMPABP (13.8±3.4 vs. 11.6±1.7 mmHg; P<.001). During hypercapnia, there was no change in Ca compared with normocapnia. Ca was inversely correlated with the cerebrovascular resistance during hypo- (R2=0.86; P<.001), and hypercapnia (R2=0.61; P<.001).
  
  
  CONCLUSIONS
  Using a new mathematical model, we have described a reduction of Ca during hypocapnia. Further studies are needed to determine whether Ca may be an independent predictor of outcome in pathological conditions.
authors.affiliation: Department of Clinical Neurosciences, Addenbrooke's Hospital, University of Cambridge, Cambridge, United Kingdom. emmanuel.carrera@chuv.ch
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Carrera
authors.forename: Dong-Joo
authors.initials: DJ
authors.lastname: Kim
authors.forename: Gianluca
authors.initials: G
authors.lastname: Castellani
authors.forename: Christian
authors.initials: C
authors.lastname: Zweifel
authors.forename: Peter
authors.initials: P
authors.lastname: Smielewski
authors.forename: John D
authors.initials: JD
authors.lastname: Pickard
authors.forename: Marek
authors.initials: M
authors.lastname: Czosnyka
mesh_terms: D000328:Adult; D002536:Cerebral Arteries / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D005260:Female; D006801:Humans; D006935:Hypercapnia / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D016857:Hypocapnia / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D008297:Male; D008433:Mathematics; D011187:Posture; D012016:Reference Values; D012189:Retrospective Studies; D018709:Statistics, Nonparametric; D017585:Ultrasonography, Doppler, Transcranial; D014655:Vascular Resistance