Asymmetric Dynamic Cerebral Autoregulatory Response to Cyclic Stimuli

Abstract

Background and Purpose-Dynamic cerebral autoregulation has been shown to be fast and effective, but it is not well known if the mechanism is symmetric, that is to say, it acts with equal compensatory action to upward as compared with downward abrupt changes in arterial blood pressure (ABP). Methods-Fourteen patients with head injuries and 10 normal subjects had bilateral transcranial Doppler and continuous ABP recording. Cyclic ABP stimuli were generated by large thigh cuffs, which were rapidly inflated above systolic pressure for 15 seconds alternating with 15 seconds of deflation. At least 8 such cycles were ensemble-averaged and the dynamic autoregulatory gain (AG up and AG dn ) was estimated separately for upward and downward changes in ABP. The results were compared with the autoregulation index using conventional leg cuff releases. Results-In normal subjects, AG dn was 0.74Ϯ0.18 and AG up was 0.77Ϯ0.17 (meanϮSD); the difference was insignificant.The correlation between AG dn and AG up , however, was weak (rϭ0.24). In the patients with head injury, AG dn was 0.30Ϯ0.21 and AG up was 1.27Ϯ0.76, the difference being highly significant (PϽ0.001). There was a negative relationship between AG dn and AG up (rϭϪ0.33). Autoregulation index correlated well with AG dn (rϭ0.79) and weakly negatively with AG up (rϭϪ0.47). Conclusions-A strongly asymmetric dynamic response of the cerebral autoregulation was seen the majority of patients with head injury. It might also have been present, albeit to a lesser degree, in the normal subjects. The findings suggest that nonlinear effects may be present in the operation of the cerebral autoregulation mechanism.

Adaptive feedback analysis and control of programmable stimuli for assessment of cerebrovascular function

“…In 2007 Aaslid et al [7] reported the combined use of three Hokanson 1 units (i.e., a controller, an air source unit and a timer) to drive bilateral thigh cuffs on and off at a constant frequency of 0.05 Hz. Their system did not allow changes in frequency or other controls, and also did not incorporate the ability to assess reactivity to CO 2 changes.…”

Detection of Impaired Cerebral Autoregulation Improves by Increasing Arterial Blood Pressure Variability

“…Compared with the classical single inflation/release of thigh cuffs 3 or the fixed frequency square wave inflation/deflation approach, 20 PRBS-controlled inflation/deflation of thigh cuffs present several important advantages. First of all, the method allows the time during which the cuffs remain inflated to be much reduced compared with the conventional thigh cuff test, 16 thus reducing patient discomfort and minimizing the possibility of increased sympathetic activity, as indicated by the stable HR values in Table 2.…”

Cerebrovascular Regulation During Transient Hypotension and Hypertension in Humans

“…Although it is assumed that this reflex vasomotion is equally effective in attenuating both cerebral hypoperfusion and hyperperfusion, 7 dCA may be more adept at compensating for transient hypertension than for hypotension, as induced by cyclic inflation and deflation of thigh cuffs. 8 However, extrapolation of these data to healthy individuals needs to be done with caution, because asymmetry was only apparent in head-trauma patients under conditions of artificial ventilation and hypocapnia. Furthermore, thigh cuffs only elicited minor changes in blood pressure (Ϸ8 mm Hg).…”