Effect of Hyper- and Hypocapnia on Cerebral Arterial Compliance in Normal Subjects

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 (AMP(CaBV)) and ABP (AMP(ABP)). 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 AMP(CaBV) (1.3 +/- .4 vs. 1.9 +/- .5; P < .001) and a concomitant increase in AMP(ABP) (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). CONCLUSION 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.