Are Slow Waves of Intracranial Pressure Suppressed by General Anaesthesia?

OBJECTIVES: Slow waves of intracranial pressure (ICP) are spontaneous oscillations with a frequency of 0.3-4 cycles/min. They are often associated with pathological conditions, following vasomotor activity in the cranial enclosure. This study quantifies the effects of general anaesthesia (GA) on the magnitude of B-waves compared with natural sleep and the conscious state. MATERIALS AND METHODS: Four groups of 30 patients each were formed to assess the magnitude of slow waves. Group A and group B consisted of normal pressure hydrocephalus (NPH) patients, each undergoing cerebrospinal fluid (CSF) infusion studies, conscious and under GA respectively. Group C comprised conscious, naturally asleep hydrocephalic patients undergoing overnight ICP monitoring; group D, which included deeply sedated head injury patients monitored in the intensive care unit (ICU), was compared with group C. RESULTS: The average amplitude for group A patients was higher (0.23 ± 0.10 mmHg) than that of group B (0.15 ± 0.10 mmHg; p = 0.01). Overnight magnitude of slow waves was higher in group C (0.20 ± 0.13 mmHg) than in group D (0.11 ± 0.09 mmHg; p = 0.002). CONCLUSION: Slow waves of ICP are suppressed by GA and deep sedation. When using slow waves in clinical decision-making, it is important to consider the patients' level of consciousness to avoid incorrect therapeutic and management decisions.

Critical Closing Pressure During a Controlled Increase in Intracranial Pressure.

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.

Is There a Link Between ICP-Derived Infusion Test Parameters and Outcome After Shunting in Normal Pressure Hydrocephalus?

OBJECTIVE: The term "hydrocephalus" encompasses a range of disorders characterised by clinical symptoms, abnormal brain imaging and derangement of cerebrospinal fluid (CSF) dynamics. The ability to elucidate which patients would benefit from CSF diversion (a shunt or third ventriculostomy) is often unclear. Similar difficulties are encountered in shunted patients to predict the scope for improvement by shunt re-adjustment or revision. MATERIALS AND METHODS: We compared retrospective pre-shunting infusion test results performed in 310 adult patients diagnosed with normal pressure hydrocephalus (NPH) and their improvement after shunting. RESULTS: Resistance to CSF outflow correlated significantly with improvement (p < 0.05). Other markers known from the literature, such as amplitude in CSF pulse pressure, the slope of the amplitude-pressure regression line, or elasticity did not show any correlation with outcome. CONCLUSION: Outcome following shunting in adult NPH is associated with resistance to CSF outflow; however, the latter cannot be taken as an absolute predictor of shunt response.