Cerebral autoregulation following head injury

Object. The goal of this study was to examine the relationship between cere bral autoregulation, intracranial pressure (ICP), arterial blood pressure ( ABP), and cerebral perfusion pressure (CPP) after head injury by using tran scranial Doppler (TCD) ultrasonography. Methods. Using ICP monitoring and TCD ultrasonography, the authors previous ly investigated whether the response of flow velocity (FV) in the middle ce rebral artery to spontaneous variations in ABP or CPP provides reliable inf ormation about cerebral autoregulatory reserve. In the present study, this method was validated in 187 head-injured patients who were sedated and rece iving mechanical ventilation. Waveforms of ICP, ABP, and FV were recorded o ver intervals lasting 20 to 120 minutes. Time-averaged mean FV and CPP were determined. The correlation coefficient index between FV and CPP (the mean index of autoregulation [Mx]) was calculated over 4-minute epochs and aver aged for each investigation. The distribution of averaged mean FV values converged with the shape of the autoregulatory curve, indicating lower (CPP < 55 min Hg) and upper (CPP > 105 min Hg) thresholds of autoregulation. The relationship between the Mx. and either the CPP or ABP was depicted as a U-shaped curve. Autoregulation was disturbed in the presence of intracranial hypertension (ICP greater tha n or equal to 25 mm Hg) and when mean ABP was too low (ABP < 75 mm Hg) or t oo high (ABP > 125 min Hg). Disturbed autoregulation (p < 0.005) and higher ICP (p < 0.005) occurred more often in patients with unfavorable outcomes than in those with favorable outcomes. Conclusions. Autoregulation not only is impaired when associated with a hig h ICP or low ABP, but it can also be disturbed by too high a CPP. The Mx ca n be used to guide intensive care therapy when CPP-oriented protocols are u sed.