Quantitative assessment of cerebral autoregulation from transcranial Doppler pulsatility: a computer simulation study

Transcranial Doppler (TCD) ultrasonography is largely used today to achieve non-invasive assessment of cerebral autoregulation and cerebrovascular reactivity in neurosurgical patients. Recent experimental and clinical studies suggest that not only the pattern of mean velocity, but also velocity pulse amplitude alterations during changes in cerebral perfusion pressure (CPP) contain information on autoregulation status. The aim of this work is to investigate the relationship between cerebral autoregulation and TCD pulsatility by means of a comprehensive mathematical model of intracranial dynamics and cerebrovascular regulation. Simulation results, performed using different values of the most important clinical parameters of the model (autoregulation strength, cerebrospinal fluid (CSF) outflow resistance and intracranial elastance coefficient) show that velocity pulse amplitude increases with a reduction in CPP in patients with intact autoregulation, whereas changes in velocity pulsatility are modest in patients with weak autoregulation. Finally, velocity pulse amplitude decreases during a CPP reduction in patients with impaired autoregulation. Moreover, the relationship between the velocity pulse amplitude changes and autoregulation strength is almost linear in a wide range of CPP values, and is scarcely affected by changes in CSF circulation and intracranial elasticity. Starting from these results, we suggest a new quantitative index to assess autoregulation strength, i.e. G(aut)% = (s-b)/a, where G(aut)% is autoregulation strength (100% means intact autoregulation, 0% means impaired autoregulation), a approximately -0.03; b approximately 1.5 and s is the slope of the relationship ' percentage changes of velocity pulse amplitude to arterial pressure pulse amplitude vs. CPP changes'.