Assessing Cerebral Autoregulation via Oscillatory Lower Body Negative Pressure and Projection Pursuit Regression

Abstract

The process by which cerebral perfusion is maintained constant over a wide range of systemic pressures is known as "cerebral autoregulation." Effective dampening of flow against pressure changes occurs over periods as short as ~15 sec and becomes progressively greater over longer time periods. Thus, slower changes in blood pressure are effectively blunted and faster changes or fluctuations pass through to cerebral blood flow relatively unaffected. The primary difficulty in characterizing the frequency dependence of cerebral autoregulation is the lack of prominent spontaneous fluctuations in arterial pressure around the frequencies of interest (less than ~0.07 Hz or ~15 sec). Oscillatory lower body negative pressure (OLBNP) can be employed to generate oscillations in central venous return that result in arterial pressure fluctuations at the frequency of OLBNP. Moreover, Projection Pursuit Regression (PPR) provides a nonparametric method to characterize nonlinear relations inherent in the system without a priori assumptions and reveals the characteristic non-linearity of cerebral autoregulation. OLBNP generates larger fluctuations in arterial pressure as the frequency of negative pressure oscillations become slower; however, fluctuations in cerebral blood flow become progressively lesser. Hence, the PPR shows an increasingly more prominent autoregulatory region at OLBNP frequencies of 0.05 Hz and below (20 sec cycles). The goal of this approach it to allow laboratory-based determination of the characteristic nonlinear relationship between pressure and cerebral flow and could provide unique insight to integrated cerebrovascular control as well as to physiological alterations underlying impaired cerebral autoregulation (e.g., after traumatic brain injury, stroke, etc.).

Identification of human sympathetic neurovascular control using multivariate wavelet decomposition analysis

“…The primary goal of this study was to establish the character of human sympathetic neurovascular control. Although studies have attempted to address this question in humans (14,28,44), precisely whether sympathetic modulation occurs remains controversial with studies reporting discrepant hemodynamic responses to sympathetic blockade (1). The discrepancies between studies have been partly attributed to the use of analytical methods like linear transfer function analysis that assume data linearity and stationarity (14,28,44).…”

Slow sinusoidal tilt movements demonstrate the contribution to orthostatic tolerance of cerebrospinal fluid movement to and from the spinal dural space

“…By the use of oscillatory LBNP, Hamner et al (2004) and Taylor et al (2014) found that the coherence between induced blood pressure oscillations and observed CBFv oscillations was decreasing at frequencies below 0.07 Hz. This phenomenon remained unexplained.…”

“…The dynamics of the fast component of autoregulation is generally defined by measurement of the transfer function (TF) from blood pressure to cerebral flow changes. For this measurement, the subject is kept in one position in relation to gravity, for example, lower body negative pressure (LBNP) (Birch et al 2002;Brown et al 2004;Hamner et al 2004;Taylor et al 2014;Smirl et al 2015), standing up from sitting (van Beek et al 2008;Oudegeest-Sander et al 2014), or squat-stand maneuvers (Birch et al 1995;Claassen et al 2009; Barnes et al 2017). No in-depth studies have analyzed the TF when the gravity vector itself is being manipulated.…”

Cerebral Autoregulation

“…A second recent review of autonomic neural control of the cerebral vasculature divided up this control into sympathetic and cholinergic control mechanisms, Tan and Taylor (2014). It was concluded that both played a role in autoregulation, although the evidence base is slim, particularly for the former.…”

“…Projection pursuit regression (PPR) was used by Taylor et al (2014) in conjunction with oscillatory LBNP testing to quantify the non-linear relationship between ABP and CBFV. Projection pursuit regression (PPR) was used by Taylor et al (2014) in conjunction with oscillatory LBNP testing to quantify the non-linear relationship between ABP and CBFV.…”