Medical Engineering & Physics 36 (2014) 592–600
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Nonstationary multivariate modeling of cerebral autoregulation during hypercapnia Kyriaki Kostoglou a , Chantel T. Debert b , Marc J. Poulin c,d,e,f,g , Georgios D. Mitsis a,∗ a
Department of Electrical and Computer Engineering, University of Cyprus, Nicosia, Cyprus Department of Physical Medicine and Rehabilitation, Faculty of Medicine, University of Calgary, AB, Canada Department of Physiology & Pharmacology, Faculty of Medicine, University of Calgary, AB, Canada d Department of Clinical Neurosciences, Faculty of Medicine, University of Calgary, AB, Canada e Faculty of Kinesiology, University of Calgary, AB, Canada f Hotchkiss Brain Institute, Faculty of Medicine, University of Calgary, AB, Canada g Libin Cardiovascular Institute of Alberta, Faculty of Medicine, University of Calgary, AB, Canada b c
a r t i c l e
i n f o
Article history: Received 5 March 2013 Received in revised form 7 September 2013 Accepted 13 October 2013 Keywords: Cerebral hemodynamics Time varying systems CO2 reactivity Laguerre functions Recursive Least Squares Multiple forgetting factors
a b s t r a c t We examined the time-varying characteristics of cerebral autoregulation and hemodynamics during a step hypercapnic stimulus by using recursively estimated multivariate (two-input) models which quantify the dynamic effects of mean arterial blood pressure (ABP) and end-tidal CO2 tension (PETCO2 ) on middle cerebral artery blood flow velocity (CBFV). Beat-to-beat values of ABP and CBFV, as well as breath-tobreath values of PETCO2 during baseline and sustained euoxic hypercapnia were obtained in 8 female subjects. The multiple-input, single-output models used were based on the Laguerre expansion technique, and their parameters were updated using recursive least squares with multiple forgetting factors. The results reveal the presence of nonstationarities that confirm previously reported effects of hypercapnia on autoregulation, i.e. a decrease in the MABP phase lead, and suggest that the incorporation of PETCO2 as an additional model input yields less time-varying estimates of dynamic pressure autoregulation obtained from single-input (ABP–CBFV) models. © 2013 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction Cerebral autoregulation collectively refers to the ability of the cerebrovascular bed to maintain a relatively constant cerebral blood flow (CBF) in response to variations in several physiological variables. The most important of these variables is arterial blood pressure (ABP); therefore, the relation between ABP and CBF is typically used to characterize autoregulation. Accurate quantitative assessment of cerebral autoregulation and, more generally, hemodynamics, is important in the context of cerebrovascular disease diagnosis and monitoring [1,2]. Following the advance of transcranial Doppler ultrasound (TCD), which yields accurate measurements of CBF velocity (CBFV) [3], it is now well established that autoregulation is a dynamic, frequency-dependent phenomenon [4–6]. Dynamic autoregulation may be assessed from the CBFV response to step-like, externally induced ABP stimuli or from spontaneous physiological variability, as the latter exhibits sufficiently broadband characteristics. The majority of the studies that have assessed dynamic cerebral autoregulation from spontaneous
∗ Corresponding author. E-mail address:
[email protected] (G.D. Mitsis).
variability have utilized univariate, linear techniques such as transfer function analysis [4]. However, in this latter study and other studies, low coherence values (
