The effect of hypercapnia on the directional sensitivity of dynamic cerebral autoregulation and the influence of age and sex.

The cerebral circulation responds differently to increases in mean arterial pressure (MAP), compared to reductions in MAP. We tested the hypothesis that this directional sensitivity is reduced by hypercapnia. Retrospective analysis of 104 healthy subjects (46 male (44%), age range 19-74 years), with five minute recordings of middle cerebral blood velocity (MCAv, transcranial Doppler), non-invasive MAP (Finometer) and end-tidal CO (capnography) at rest, during both poikilocapnia and hypercapnia (5% CO breathing in air) produced MCAv step responses allowing estimation of the classical Autoregulation Index (ARI), and corresponding values for both positive (ARI) and negative (ARI) changes in MAP. Hypercapnia led to marked reductions in ARI, ARI and ARI (p < 0.0001, all cases). Females had a lower value of ARI compared to males (p = 0.030) at poikilocapnia (4.44 ± 1.74 vs 4.74 ± 1.48) and hypercapnia (2.44 ± 1.93 vs 3.33 ± 1.61). The strength of directional sensitivity (ARI-ARI) was not influenced by hypercapnia (p = 0.46), sex (p = 0.76) or age (p = 0.61). During poikilocapnia, ARI decreased with age in females (p = 0.027), but not in males. Directional sensitivity was not affected by hypercapnia, suggesting that its origins are more likely to be inherent to the mechanics of vascular smooth muscle than to myogenic pathways.