Objectives: The objectives were to compare three methods of estimating critical closing pressure (CrCP) in a scenario of a controlled increase in intracranial pressure (ICP) induced during an infusion test in patients with suspected normal pressure hydrocephalus (NPH).
Methods: We retrospectively analyzed data from 37 NPH patients who underwent infusion tests. Computer recordings of directly measured intracranial pressure (ICP), arterial blood pressure (ABP) and transcranial Doppler cerebral blood flow velocity (CBFV) were used. The CrCP was calculated using three methods: first harmonics ratio of the pulse waveforms of ABP and CBFV (CrCPA) and two methods based on a model of cerebrovascular impedance, as a function of cerebral perfusion pressure (CrCPinv), and as a function of ABP (CrCPninv).
Results: There is good agreement among the three methods of CrCP calculation, with correlation coefficients being greater than 0.8 (p < 0.0001). For the CrCPA method, negative values were found for about 20% of all results. Negative values of CrCP were not observed in estimators based on cerebrovascular impedance. During the controlled rise of ICP, all three estimators of CrCP increased significantly (p < 0.05). The strongest correlation between ICP and CrCP was found for CrCPinv (median R = 0.41).
Conclusion: Invasive CrCP is most sensitive to variations in ICP and can be used as an indicator of the status of the cerebrovascular system during infusion tests.
Keywords: Cerebral autoregulation; Critical closing pressure; Infusion test; Intracranial pressure; Wall tension.