Correlations among critical closing pressure, pulsatility index and cerebrovascular resistance.
ABSTRACT We attempted to explore the relationships among critical closing pressure (CrCP), resistance-area product (RAP) and traditional resistance indices of cerebral hemodynamics. Twenty healthy volunteers were studied. Blood pressure was obtained with servo-controlled plethysmography. Cerebral blood flow velocity (CBFV) was monitored by transcranial Doppler. Hemodynamic changes were induced by hyperventilation and by 5% CO(2) inhalation. Beat-to-beat CrCP and RAP values were extracted by linear regression analysis of instantaneous arterial blood pressure (ABP) and CBFV tracings. Gosling's pulsatility index (PI) and cerebrovascular resistance (CVR) were calculated. RAP correlated well with CVR at rest and during provocative tests (p = 0.006 approximately <0.001). There was no correlation among CrCP, CVR and PI. The changes in CVR correlated with those in RAP (p = 0.008 for the 5% CO(2) test and p = 0.014 for the hyperventilation test). The changes in PI and CrCP showed significant correlation (p = 0.004 for the 5% CO(2) test and p = 0.003 for the hyperventilation test). RAP reliably reflected cerebrovascular resistance. The changes in CrCP were valuable in assessing cerebrovascular regulation. Estimating changes in CrCP and RAP provided better understanding of the nature of cerebrovascular regulation.
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ABSTRACT: During the 1990s, Central Asia emerged as an idiom for the uncertainty pervading the post-Cold War climate of global life. This paper therefore queries the intertwining of the region in world politics and the ways in which the dynamics of international affairs affect Central Asia. In this respect, the investigation explores the scope and connotations of the “new Central Asia” label. Its framing provides a context for the conceptual engagement with the Central Asian agency of international actors. This assessment details the perception of a regional power vacuum and the emergence of awkward statehood as key contributing factors to the construction of Central Asia as a permissive environment for external agency. Consequently, the confrontation with the proliferation of “actorness” in Central Asia accounts for the dynamics of the “new great game” and the patterns of “hegemonic fragmegration” in the region.Journal of Eurasian Studies 01/2011; 2(1):21-29.
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ABSTRACT: Clinical transcranial Doppler assessment of cerebral vasomotor reactivity (CVMR) uses linear regression of cerebral blood flow velocity (CBFV) vs. end-tidal CO(2) (Pet(CO(2))) under steady-state conditions. However, the cerebral blood flow (CBF)-Pet(CO(2)) relationship is nonlinear, even for moderate changes in CO(2). Moreover, CBF is increased by increases in arterial blood pressure (ABP) during hypercapnia. We used a modified rebreathing protocol to estimate CVMR during transient breath-by-breath changes in CBFV and Pet(CO(2)). Ten healthy subjects (6 men) performed 15 s of hyperventilation followed by 5 min of rebreathing, with supplemental O(2) to maintain arterial oxygen saturation constant. To minimize effects of changes in ABP on CVMR estimation, cerebrovascular conductance index (CVCi) was calculated. CBFV-Pet(CO(2)) and CVCi-Pet(CO(2)) relationships were quantified by both linear and nonlinear logistic regression. In three subjects, muscle sympathetic nerve activity was recorded. From hyperventilation to rebreathing, robust changes occurred in Pet(CO(2)) (20-61 Torr), CBFV (-44 to +104% of baseline), CVCi (-39 to +64%), and ABP (-19 to +23%) (all P < 0.01). Muscle sympathetic nerve activity increased by 446% during hypercapnia. The linear regression slope of CVCi vs. Pet(CO(2)) was less steep than that of CBFV (3 vs. 5%/Torr; P = 0.01). Logistic regression of CBF-Pet(CO(2)) (r(2) = 0.97) and CVCi-Pet(CO(2)) (r(2) = 0.93) was superior to linear regression (r(2) = 0.91, r(2) = 0.85; P = 0.01). CVMR was maximal (6-8%/Torr) for Pet(CO(2)) of 40-50 Torr. In conclusion, CBFV and CVCi responses to transient changes in Pet(CO(2)) can be described by a nonlinear logistic function, indicating that CVMR estimation varies within the range from hypocapnia to hypercapnia. Furthermore, quantification of the CVCi-Pet(CO(2)) relationship may minimize the effects of changes in ABP on the estimation of CVMR. The method developed provides insight into CVMR under transient breath-by-breath changes in CO(2).Journal of Applied Physiology 03/2007; 102(3):870-7. · 3.48 Impact Factor
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ABSTRACT: The passive relationship between arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV) has been expressed by a single parameter [cerebrovascular resistance (CVR)] or, alternatively, by a two-parameter model, comprising a resistance element [resistance-area product (RAP)] and a critical closing pressure (CrCP). We tested the hypothesis that the RAP+CrCP model can provide a more consistent interpretation to CBFV responses induced by mental activation tasks than the CVR model. Continuous recordings of CBFV [bilateral, middle cerebral artery (MCA)], ABP, ECG, and end-tidal CO(2) (EtCO(2)) were performed in 13 right-handed healthy subjects (aged 21-43 yr), in the seated position, at rest and during 10 repeated presentations of a word generation and a constructional puzzle paradigm that are known to induce differential cortical activation. Due to its small relative change, the CBFV response can be broken down into standardized subcomponents describing the relative contributions of ABP, CVR, RAP, and CrCP. At rest and during activation, the RAP+CrCP model suggested that RAP might reflect myogenic activity in response to the ABP transient, whereas CrCP was more indicative of metabolic control. These different influences were not reflected by the CVR model, which indicated a predominantly metabolic response. Repeated-measures multi-way ANOVA showed that CrCP (P = 0.025), RAP (P = 0.046), and CVR (P = 0.002) changed significantly during activation. CrCP also had a significant effect of paradigm (P = 0.045) but not hemispheric dominance. Both RAP (P = 0.039) and CVR (P = 0.0008) had significant effects of hemispheric dominance but were not sensitive to the different paradigms. Subcomponent analysis can help with the interpretation of CBFV responses to mental activation, which were found to be dependent on the underlying model of the passive ABP-CBFV relationship.Journal of Applied Physiology 01/2006; 99(6):2352-62. · 3.48 Impact Factor