Critical closing pressure in cerebrovascular circulation

Objective-Cerebral critical closing pressure (CCP) has been defined as an a rterial pressure threshold below which arterial vessels collapse. Hypotheti cally this is equal to intracranial pressure (ICP) plus the contribution fr om the active tone of cerebral arterial smooth muscle. The correlation of C CP with ICP, cerebral autoregulation, and other clinical and haemodynamic m odalities in patients with head injury was evaluated. Method-intracranial pressure, arterial blood pressure (ABP) and middle cere bral artery blood flow velocity were recorded daily in ventilated patients. Waveforms were processed to calculate CCP, the transcranial Doppler-derive d cerebral autoregulation index (Mx), mean arterial pressure (ABP), intracr anial pressure (ICP), and cerebral perfusion pressure (CPP). Results-Critical closing pressure reflected the time related changes in ICP during plateau and B waves. Overall correlation between CCP and ICP was mi ld but significant (R=0.41; p<0.0002). The mean difference between ABP and CCP correlated with CPP (R=0.57, 95% confidence interval (95% CI) for predi ction 25 mm Hg). The difference between CCP and ICP, described previously a s proportional to arterial wall tension, correlated with the index of cereb ral autoregulation Mx (p<0.0002) and CPP (p<0.0001). However, by contrast w ith the Mx index, CCP-ICP was not significantly correlated with outcome aft er head injury. Conclusion-Critical closing pressure, although sensitive to variations in I CP and CPP, cannot be used as an accurate estimator of these modalities wit h acceptable confidence intervals. The difference CCP-ICP significantly cor relates with cerebral autoregulation, but it lacks the power to predict out come after head injury.