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Object. The aim of this study was to investigate potential episodes of cerebral ischemia during surgery for large and complicated aneurysms, by examining the effects of arterial temporary clipping and the impact of confounding variables such as blood pressure and cerebrospinal fluid (CSF) drainage.

Methods. Brain tissue PO₂, PCO₂, and pH, as well as temperature and extracellular glucose, lactate, pyruvate, and glutamate were monitored in 46 patients by using multiparameter sensors and microdialysis. Baseline data showed that brain tissue PO₂ decreased significantly, below a mean arterial pressure (MAP) threshold of 70 mm Hg. Further evidence of its relationship with cerebral perfusion pressure was shown by an increase in mean brain tissue PO₂ after drainage of CSF from the basal cisterns (Wilcoxon test, p < 0.01). Temporary clipping was required in 31 patients, with a mean total duration of 14 minutes (range 3–52 minutes), causing brain tissue PO₂ to decrease and brain tissue PCO₂ to increase (Wilcoxon test, p < 0.01). In patients in whom no subsequent infarction developed in the monitored region, brain tissue PO₂ fell to 11 mm Hg (95% confidence interval 8–14 mm Hg). A brain tissue PO₂ level below 8 mm Hg for 30 minutes was associated with infarction in any region (p < 0.05 according to the Fisher exact test); other parameters were not predictive of infarction. Intermittent occlusions of less than 30 minutes in total had little effect on extracellular chemistry. Large glutamate increases were only seen in two patients, in both of whom brain tissue PO₂ during occlusion was continuously lower than 8 mm Hg for longer than 38 minutes.

Conclusions. The brain tissue PO₂ decreases with hypotension, and, when it is below 8 mm Hg for longer than 30 minutes during temporary clipping, it is associated with increasing extracellular glutamate levels and cerebral infarction.

Object. The benefits of measuring cerebral oxygenation in patients with brain injury are well accepted; however, jugular bulb oximetry, which is currently the most popular monitoring technique used has several shortcomings. The goal of this study was to validate the use of a new multiparameter sensor that measures brain tissue oxygenation and metabolism (Neurotrend) by comparing it with positron emission tomography (PET) scanning.

Methods. A Neurotrend sensor was inserted into the frontal region of the brain in 19 patients admitted to the neurointensive care unit. After a period of stabilization, the patients were transferred to the PET scanner suite where C¹⁵O, ¹⁵O₂, and H₂ ¹⁵O PET scans were obtained to facilitate calculation of regional cerebral blood volume, O₂ metabolism, blood flow, and O₂ extraction fraction (OEF). Patients were given hyperventilation therapy to decrease arterial CO₂ by approximately 1 kPa (7.5 mm Hg) and the same sequence of PET scans was repeated. For each scanning sequence, end-capillary O₂ tension (PvO₂) was calculated from the OEF and compared with the reading of brain tissue O₂ pressure (PbO₂) provided by the sensor.

In three patients the sensor was inserted into areas of contusion and these patients were eliminated from the analysis. In the subset of 16 patients in whom the sensor was placed in healthy brain, no correlation was found between the absolute values of PbO₂ and PvO₂ (r = 0.2, p = 0.29); however a significant correlation was obtained between the change in PbO₂ (ΔPbO₂) and the change in PvO₂ (ΔPvO₂) produced by hyperventilation in a 20-mm region of interest around the sensor (ρ = 0.78, p = 0.0035).

Conclusions. The lack of correlation between the absolute values of PbO₂ and PvO₂ indicates that PbO₂ cannot be used as a substitute for PvO₂. Nevertheless, the positive correlation between ΔPbO₂ and ΔPvO₂ when the sensor had been inserted into healthy brain suggests that tissue PO₂ monitoring may provide a useful tool to assess the effect of therapeutic interventions in brain injury.

Object. Clinical microdialysis enables monitoring of the cerebral extracellular chemistry of neurosurgical patients. Introduction of the technique into different hospitals' neurosurgical units has resulted in variations in the method of application. There are several variables to be considered, including length of the catheter membrane, type of perfusion fluid, flow rate of perfusion fluid, and on-line compared with delayed analysis of samples. The objects of this study were as follows: 1) to determine the effects of varying catheter characteristics on substance concentration; 2) to determine the relative recovery and true extracellular concentration by varying the flow rate and extrapolating to zero flow; and 3) to compare substance concentration obtained using a bedside enzyme analyzer with that of off-line high-performance liquid chromatography (HPLC).

Methods. A specially designed bolt was used to conduct two adjacent microdialysis catheters into the frontal cortex of patients with head injury or poor-grade subarachnoid hemorrhage who were receiving ventilation. One reference catheter (10-mm membrane, perfused with Ringer's solution at 0.3 µl/minute) was constant for all studies. The other catheter was varied in terms of membrane length (10 mm or 30 mm), perfusion fluid (Ringer's solution or normal saline), and flow rate (0.1–1.5 µl/minute). The effect of freezing the samples on substance concentration was established by on-line analysis and then repeated analysis after storage at −70°C for 3 months. Samples assayed with the bedside enzyme analyzer were reassessed using HPLC for the determination of glutamate concentrations.

Conclusions. Two adjacent microdialysis catheters that were identical in membrane length, perfusion fluid, and flow rate showed equivalent results. Variations in perfusion fluid and freezing and thawing of samples did not result in differences in substance concentration. Catheter length had a significant impact on substance recovery. Variations in flow rate enabled the relative recovery to be calculated using a modification of the extrapolation-to-zero-flow method. The recovery was approximately 70% at 0.3 µl/minute and 30% at 1 µl/minute (10-mm membrane) for all analytes. Glutamate results obtained with the enzyme analyzer showed good correlation with those from HPLC.