The critical closing pressure contribution to dynamic cerebral autoregulation in humans: influence of arterial partial pressure of CO₂

Abstract

Dynamic cerebral autoregulation (CA) is often expressed by the mean arterial blood pressure (MAP)-cerebral blood flow (CBF) relationship, with little attention given to the dynamic relationship between MAP and cerebrovascular resistance (CVR). r In CBF velocity (CBFV) recordings with transcranial Doppler, evidence demonstrates that CVR should be replaced by a combination of a resistance-area product (RAP) with a critical closing pressure (CrCP) parameter, the blood pressure value where CBFV reaches zero due to vessels collapsing. r Transfer function analysis of the MAP-CBFV relationship can be extended to the MAP-RAP and MAP-CrCP relationships, to assess their contribution to the dynamic CA response. r During normocapnia, both RAP and CrCP make a significant contribution to explaining the MAP-CBFV relationship. r Hypercapnia, a surrogate state of depressed CA, leads to marked changes in dynamic CA, that are entirely explained by the CrCP response, without further contribution from RAP in comparison with normocapnia.

Cerebral critical closing pressure and resistance-area product: the influence of dynamic cerebral autoregulation, age and sex

“…31,54,57 In a recent study, SRV CrCP , but not SRV RAP , explained the corresponding changes in SRV CBFV in response to hypercapnia. 26 In this study, estimates of SRV RAP (Figures 3(b) and 5(b)) were in broad agreement with previous estimates of RAP, or CVR response to a step change in MAP, 14,15,54,58 remembering that in our case the SRV RAP curves are inverted, to represent the corresponding reduction in V CBFV , following a rise in RAP. What has not been described previously though, is how the SRV RAP temporal pattern varies with the efficiency of dynamic CA (Figure 5(b)) and the linear association of the SRV RAP with ARI at T2 (Table 2), and both ARI and age at T3 (Table 2, Figure 5(b)).…”

“…31,53 Although hypercapnia induces significant increases in CBFV and depression of CA, the contribution of RAP to explain these changes has been minimal or entirely absent. 23,26,54 Moreover, time-varying changes in RAP have been closely associated with a response to preceding changes in MAP, and not with EtCO 2 25,.25,31,55,56 On the other hand, different authors have reported a strong association between CrCP and PaCO 2 16,23 and dynamic analyses have also shown that CrCP could explain better the changes in CBFV resulting from neural stimulation than RAP. 31,54,57 In a recent study, SRV CrCP , but not SRV RAP , explained the corresponding changes in SRV CBFV in response to hypercapnia.…”

“…Based on the linear properties of the FFT, a similar relationship between the responses to a step change in MAP can be written as the summation in equation (1). 26 By changing the order of the parcels, it is possible to express the step change in MAP as:…”

“…On the other hand, SRV CrCP has only been reported by our group, first in a NVC study, 54 and more recently in the comparison between hypercapnia and normocapnia. 26 Interpretation of its temporal pattern (Figures 3(c) and 5(c)) is much more challenging than for SRV RAP . To start with, why has SRV CrCP shown its largest changes for the lowest range of ARI (0-4) in Figure 5 (c)?…”

“…Hypercapnia is often used as a surrogate of dynamic CA impairment, [21][22][23] but it is not clear to what extent it can mimic impairment of the myogenic response, or if its effect is mediated by impacting mainly on metabolic pathways. 24,25 We have previously described the effects of hypercapnia on the CrCP and RAP step responses, 26 but in this study we aimed to understand how these responses are affected by dynamic CA, as reflected by ARI. ARI has been validated as a sensitive index of dynamic CA, with extensive literature describing its use in physiological and clinical studies.…”