RANSCRANIAL Doppler (TCD) monitoring is becom- ing a routinely used method for the evaluation of intracranial hemodynamics in patients with acute brain lesions, both traumatic and hemorrhagic. Early assessment of changes in cerebrovascular hemodynamics is of the greatest clinical importance in the prevention of cerebral perfusion derangement. Such a disturbance plays a pivotal role in the development of cerebral isch- emia or hyperemia, which leads to secondary brain dam- age. 1,2,7,8,14,15 Acute brain lesions are often associated with alterations in intracranial parameters, which may have a dramatic and sometimes unpredictable impact on cerebral hemodynam- ics and cerebral blood flow (CBF). The main alterations involve impairment in cerebrospinal fluid (CSF) outflow, reduced intracranial compliance, intracranial pressure (ICP) and cerebral blood volume increases, and damage to cerebral autoregulatory mechanisms. 7,14,15,18,19 In current practice several quantities are extrapolated from the TCD waveform in an effort to assess the status of cerebral hemodynamics: systolic and diastolic velocity, mean velocity, pulsatility index (PI), and peak-to-peak velocity amplitude. However, the way these parameters are related and affected by changes in cerebral hemody- namics and ICP are still not sufficiently understood and are the subject of many recent experimental and clinical J. Neurosurg. / Volume 89 / August, 1998 J Neurosurg 89:255–266, 1998 Relationships among cerebral perfusion pressure, autoregulation, and transcranial Doppler waveform: a modeling study MAURO URSINO, PH.D., MARCO GIULIONI, M.D., AND CARLO ALBERTO LODI, M.S. Department of Electronics, Computer Science and Systems, University of Bologna, Bologna, Italy; and Department of Neurosurgery, Bellaria Hospital, Bologna, Italy Object. The aim of this study was to analyze how the main values extrapolated from the transcranial Doppler (TCD) waveform (systolic, mean, and diastolic velocity; velocity peak-to-peak amplitude; and pulsatility index [PI]) are affected by changes in intracranial pressure (ICP), systemic arterial pressure (SAP), autoregulation, and intracranial compliance. Methods. The analysis was performed using a mathematical model of the intracranial dynamics. This model includes a passive middle cerebral artery, the biomechanics of large and small pial arteries subjected to autoregula- tory mechanisms, a collapsing venous cerebrovascular bed, the cerebrospinal fluid circulation, and the ICP–volume relationship. The results indicate that there are approximately three distinct zones characterized by different relationships between cerebral perfusion pressure (CPP) and velocity parameters in patients with preserved autoregulation. In the central autoregulatory zone (CPP . 70 mm Hg) the mean velocity does not change with decreasing CPP, whereas the PI and velocity peak-to-peak amplitude increase moderately. In a second zone (CPP between 40–45 and 70 mm Hg), in which vasodilation of small pial arteries becomes maximal, the mean velocity starts to decrease, whereas the PI and velocity amplitude continue to increase. In the third zone, in which autoregulation is completely exhausted (CPP , 40 mm Hg), arterioles behave passively, mean velocity and velocity amplitude decline abruptly, and the PI exhibits a dis- proportionate rise. Moreover, this rise is quite independent of whether CPP is reduced by increasing ICP or reducing mean SAP. In contrast, in patients with defective autoregulation, the mean velocity and velocity amplitude decrease linearly with decreasing CPP, but the PI still increases in a way similar to that observed in patients with preserved autoregula- tion. Conclusions. The information contained in the TCD waveform is affected by many factors, including ICP, SAP, autoregulation, and intracranial compliance. Model results indicate that only a comparative analysis of the con- comitant changes in ultrasonographic quantities during multimodality monitoring may permit the assessment of sev- eral aspects of intracranial dynamics (cerebral blood flow changes, vascular pulsatility, ICP changes, intracranial compliance, CPP, and autoregulation). KEY WORDS • transcranial Doppler ultrasound • cerebral autoregulation • cerebral perfusion pressure • intracranial pressure • pulsatility index • mathematical modeling T 255