Pressure Autoregulation Measurement Techniques in Adult TBI, Part II: A Scoping Review of Continuous Methods

To perform systematically a scoping review of the literature on commonly described continuous autoregulation measurement techniques in adult TBI. The goal was to provide an overview of methodology and comprehensive reference library of the available literature for each technique. Five separate small systematic reviews were conducted for each of the continuous techniques: pressure reactivity index (PRx), laser Doppler flowmetry (LDF), near infrared spectroscopy (NIRS) techniques, brain tissue oxygen tension (PbtO2), and thermal diffusion (TD) techniques. Articles from MEDLINE, BIOSIS, EMBASE, Global Health, Scopus, Cochrane Library (inception to December 2016) and reference lists of relevant articles were searched. A two-tier filter of references was conducted. The literature base identified from the individual searches was limited, except for PRx. The total number of articles utilizing each of the 5 searched techniques for continuous autoregulation in adult TBI were: PRx (28), LDF (4), NIRS (9), PbtO2 (10), and TD (8). All continuous techniques described in adult TBI are based on moving correlation coefficients. The premise behind the calculation of these moving correlation coefficients focuses on the impact of slow fluctuations in either MAP or CPP on some indirect measure of CBF, such as: intracranial pressure (ICP), LDF, NIRS signals, PbtO2 or TD CBF. The thought is the correlation between a hemodynamic driving factor, such as MAP or CPP, and a surrogate for CBF or cerebral perfusion sheds insight on the state of cerebral autoregulation. Both PRx and NIRS indices were validated experimentally against ‘golden standard’ static autoregulatory curve (Lassen curve) at least around lower threshold of autoregulation. PRx has the largest literature base supporting the association with patient outcome. Various methods of continuous autoregulation assessment are described within the adult TBI literature. Many studies exist on these various indices, suggesting an association between their values and patient morbidity/mortality.