Critical Thresholds for Cerebrovascular Reactivity: Facts, No Fiction!

Excerpt

We are grateful for the letter [1], which gives us an opportunity to reiterate some important points regarding continuous monitoring of cerebrovascular reactivity. Nordstrom and Nielsen are correct about the definition of autoregulation. However, pressure-reactivity index (PRx) is not an index of cerebral autoregulation, as noted before [2]. There is a link between vascular reactivity and autoregulation, but the overall correlation coefficient between PRx and autoregulation derived from Transcranial Doppler (TCD) or other CBF monitors (direct or indirect) is far from +1 (average values range from 0.4 to 0.7, depending on computational details, pathology studied, and so forth [2, 3]). Another point is that the limits of vascular reactivity extend beyond the limits of autoregulation. Obviously, non-reactive vessels cannot autoregulate, but in reactive arterioles, too weak response of cerebrovascular resistance to a change in cerebral perfusion pressure may still make CBF pressure passive. …