Effects of Xenon Anesthesia on Cerebral Blood Flow in Humans

Background

Animal studies have demonstrated a strong neuroprotective property of xenon. Its usefulness in patients with cerebral pathology could be compromised by deleterious effects on regional cerebral blood flow (rCBF).

Methods

15O-labeled water was used to determine rCBF in nine healthy male subjects at baseline and during 1 minimum alveolar concentration (MAC) of xenon (63%). Anesthesia was based solely on xenon. Absolute changes in rCBF were quantified using region-of-interest analysis and voxel-based analysis.

Results

Mean arterial blood pressure and arterial partial pressure for carbon dioxide remained unchanged. The mean (+/-SD) xenon concentration during anesthesia was 65.2+/-2.3%. Xenon anesthesia decreased absolute rCBF by 34.7+/-9.8% in the cerebellum (P<0.001), by 22.8+/-10.4% in the thalamus (P=0.001), and by 16.2+/-6.2% in the parietal cortex (P<0.001). On average, xenon anesthesia decreased absolute rCBF by 11.2+/-8.6% in the gray matter (P=0.008). A 22.1+/-13.6% increase in rCBF was detected in the white matter (P=0.001). Whole-brain voxel-based analysis revealed widespread cortical reductions and increases in rCBF in the precentral and postcentral gyri.

Conclusions

One MAC of xenon decreased rCBF in several areas studied. The greatest decreases were detected in the cerebellum, the thalamus and the cortical areas. Increases in rCBF were observed in the white matter and in the pre- and postcentral gyri. These results are in clear contradiction with ketamine, another N-methyl-D-aspartate antagonist and neuroprotectant, which induces a general increase in cerebral blood flow at anesthetic concentrations.

Increase in high frequency EEG activity explains the poor performance of EEG spectral entropy monitor during S-ketamine anesthesia

OBJECTIVE

To study the effects of S-ketamine on the EEG and to investigate whether spectral entropy of the EEG can be used to assess the depth of hypnosis during S-ketamine anesthesia.

METHODS

The effects of sub-anesthetic (159 (21); mean (SD) ng/ml) and anesthetic (1,959 (442) ng/ml) serum concentrations of S-ketamine on state entropy (SE), response entropy (RE) and classical EEG spectral power variables (recorded using the Entropy Module, GE Healthcare, Helsinki, Finland) were studied in 8 healthy males. These EEG data were compared with EEG recordings from 6 matching subjects anesthetized with propofol.

RESULTS

The entropy values decreased from the baseline SE 85 (3) and RE 96 (3) to SE 55 (18) and RE 72 (17) during S-ketamine anesthesia but both inter- and intra-individual variation of entropy indices was wide and their specificity to indicate unconsciousness was poor. Propofol induced more pronounced increase in delta power (P<0.02) than S-ketamine, whereas anesthetic S-ketamine induced more high frequency EEG activity in the gamma band (P<0.001). Relative power of 20-70 Hz EEG activity was associated with high SE (P=0.02) and RE (P=0.03) values during S-ketamine anesthesia.

CONCLUSIONS

These differences in low and high frequency EEG power bands probably explain why entropy monitor, while adequate for propofol, is not suitable for assessing the depth of S-ketamine anesthesia.

SIGNIFICANCE

The entropy monitor is not adequate for monitoring S-ketamine-induced hypnosis.

Subanesthetic ketamine does not affect 11C-flumazenil binding in humans

Positron emission tomography (PET) studies suggest that propofol and inhaled anesthetics increase (11)C-flumazenil binding in the living human brain, thus supporting the involvement of gamma-aminobutyric acid type A (GABA(A)) receptors in the mechanism of action of these drugs. Ketamine produces its anesthetic effects primarily by N-methyl-d-aspartate receptor antagonism, but it may also have GABA(A) receptor agonistic properties. By using PET, we studied the cerebral (11)C-flumazenil binding in 10 healthy subjects before and during a subanesthetic racemic ketamine infusion reaching a serum concentration of 350 +/- 42 ng/mL. Ketamine did not affect (11)C-flumazenil binding to GABA(A) receptor in the brain, indicating that this mechanism is of minor importance in the actions of subanesthetic ketamine.

Sevoflurane and propofol increase 11C-flumazenil binding to gamma-aminobutyric acidA receptors in humans

Based on in vitro studies and animal data, most anesthetics are supposed to act via gamma-aminobutyric acid type A (GABA(A)) receptors. However, this fundamental characteristic has not been extensively investigated in humans. We studied (11)C-flumazenil binding to GABA(A) receptors during sevoflurane and propofol anesthesia in the living human brain using positron emission tomography (PET). Fourteen healthy male subjects underwent 2 60-min dynamic PET studies with (11)C-labeled flumazenil, awake and during anesthesia. Anesthesia was maintained with 2% end-tidal sevoflurane (n = 7) or propofol at a target plasma concentration of 9.0 +/- 3.0 (mean +/- sd) microg/mL (n = 7). The depth of anesthesia was measured with bispectral index (BIS). Values of regional distribution volumes (DV) of (11)C-flumazenil were calculated in several brain areas using metabolite-corrected arterial plasma curves and a two-compartment model. Separate voxel-based statistical analysis using parametric DV images was performed for detailed visualization. The average BIS index was 35 +/- 6 in the sevoflurane group and 28 +/- 8 in the propofol group (P = 0.02). Sevoflurane increased the DV of (11)C-flumazenil significantly (P < 0.05) in all brain areas studied except the pons and the white matter. In the propofol group the increases were significant (P < 0.05) in the caudatus, putamen, cerebellum, thalamus and the frontal, temporal, and parietal cortices. Furthermore, the DV increases in the frontal, occipital, parietal, and temporal cortical areas and in the putamen were statistically significantly larger in the sevoflurane than in the propofol group. Our findings support the involvement of GABA(A) receptors in the mechanism of action of both anesthetics in humans.

Ventilator add-on for delivering PET tracer gases

OBJECTIVES

The increasing use of PET for assessing cerebral blood flow, oxygen metabolism, and blood volume in critically ill patients has created a need for reliable technical solutions for delivering (15)O-tracer gases to mechanically ventilated subjects. Our objective was to create such a solution.

METHODS

We designed a ventilator add-on unit that enables complex functional brain studies using labeled oxygen and carbon monoxide gases as tracers. The unit manages both steady-state and bolus inhalations, and the latter can be manually initiated using a remote trigger. All parts conducting breathing gases can be sterilized. The unit can be operated during both spontaneous pressure support breathing and volume-controlled ventilation. It supports the standard safety features and alarms of the ventilator and includes an overflow valve in the bolus reservoir.

RESULTS

The count rate curves obtained using the new unit were similar to those from the standard bag-inhalation method.

CONCLUSION

The unit we describe offers an economical and easily operated solution for providing uninterrupted ventilator treatment while performing PET brain studies, and the provided treatment meets intensive care criteria.

Effects of subanesthetic ketamine on regional cerebral glucose metabolism in humans

BACKGROUND

The authors have recently shown with positron emission tomography that subanesthetic doses of racemic ketamine increase cerebral blood flow but do not affect oxygen consumption significantly. In this study, the authors wanted to assess the effects of racemic ketamine on regional glucose metabolic rate (rGMR) in similar conditions to establish whether ketamine truly induces disturbed coupling between cerebral blood flow and metabolism.

METHODS

18F-labeled fluorodeoxyglucose was used as a positron emission tomography tracer to quantify rGMR on 12 brain regions of interest of nine healthy male volunteers at baseline and during a 300-ng/ml ketamine target concentration level. In addition, voxel-based analysis was performed for the relative changes in rGMR using statistical parametric mapping.

RESULTS

The mean +/- SD measured ketamine serum concentration was 326.4+/-86.3 ng/ml. The mean arterial pressure was slightly increased (maximally by 16.4%) during ketamine infusion (P < 0.001). Ketamine increased absolute rGMR significantly in most regions of interest studied. The greatest increases were detected in the thalamus (14.6+/-15.9%; P = 0.029) and in the frontal (13.6+/-13.1%; P = 0.011) and parietal cortices (13.1+/-11.2%; P = 0.007). Absolute rGMR was not decreased anywhere in the brain. The voxel-based analysis revealed relative rGMR increases in the frontal, temporal, and parietal cortices.

CONCLUSIONS

Global increases in rGMR seem to parallel ketamine-induced increases in cerebral blood flow detected in the authors' earlier study. Therefore, ketamine-induced disturbance of coupling between cerebral blood flow and metabolism is highly unlikely. The previously observed decrease in oxygen extraction fraction may be due to nonoxidative glucose metabolism during ketamine-induced increase in glutamate release.

Effects of sevoflurane, propofol, and adjunct nitrous oxide on regional cerebral blood flow, oxygen consumption, and blood volume in humans

BACKGROUND

Anesthetic agents, especially volatile anesthetics and nitrous oxide (N2O), are suspected to perturb cerebral homeostasis and vascular reactivity. The authors quantified the effects of sevoflurane and propofol as sole anesthetics and in combination with N2O on regional cerebral blood flow (rCBF), metabolic rate of oxygen (rCMRO2), and blood volume (rCBV) in the living human brain using positron emission tomography.

METHODS

15O-labeled water, oxygen, and carbon monoxide were used as positron emission tomography tracers to determine rCBF, rCMRO2 and rCBV, respectively, in eight healthy male subjects during the awake state (baseline) and at four different anesthetic regimens: (1) sevoflurane alone, (2) sevoflurane plus 70% N2O (S+N), (3) propofol alone, and (4) propofol plus 70% N2O (P+N). Sevoflurane and propofol were titrated to keep a constant hypnotic depth (Bispectral Index 40) throughout anesthesia. End-tidal carbon dioxide was strictly kept at preinduction level.

RESULTS

The mean +/- SD end-tidal concentration of sevoflurane was 1.5 +/- 0.3% during sevoflurane alone and 1.2 +/- 0.3% during S+N (P < 0.001). The measured propofol concentration was 3.7 +/- 0.7 microg/ml during propofol alone and 3.5 +/- 0.7 microg/ml during P+N (not significant). Sevoflurane alone decreased rCBF in some (to 73-80% of baseline, P < 0.01), and propofol in all brain structures (to 53-70%, P < 0.001). Only propofol reduced also rCBV (in the cortex and cerebellum to 83-86% of baseline, P < 0.05). Both sevoflurane and propofol similarly reduced rCMRO2 in all brain areas to 56-70% and 50-68% of baseline, respectively (P < 0.05). The adjunct N2O counteracted some of the rCMRO2 and rCBF reductions caused by drugs alone, and especially during S+N, a widespread reduction (P < 0.05 for all cortex and cerebellum vs. awake) in the oxygen extraction fraction was seen. Adding of N2O did not alter the rCBV effects of sevoflurane and propofol alone.

CONCLUSIONS

Propofol reduced rCBF and rCMRO2 comparably. Sevoflurane reduced rCBF less than propofol but rCMRO2 to an extent similar to propofol. These reductions in flow and metabolism were partly attenuated by adjunct N2O. S+N especially reduced the oxygen extraction fraction, suggesting disturbed flow-activity coupling in humans at a moderate depth of anesthesia.

Effects of subanesthetic doses of ketamine on regional cerebral blood flow, oxygen consumption, and blood volume in humans

BACKGROUND

Animal experiments have demonstrated neuroprotection by ketamine. However, because of its propensity to increase cerebral blood flow, metabolism, and intracranial pressure, its use in neurosurgery or trauma patients has been questioned.

METHODS

15O-labeled water, oxygen, and carbon monoxide were used as positron emission tomography tracers to determine quantitative regional cerebral blood flow (rCBF), metabolic rate of oxygen (rCMRO2), and blood volume (rCBV), respectively, on selected regions of interest of nine healthy male volunteers at baseline and during three escalating concentrations of ketamine (targeted to 30, 100, and 300 ng/ml). In addition, voxel-based analysis for relative changes in rCBF and rCMRO2 was performed using statistical parametric mapping.

RESULTS

The mean +/- SD measured ketamine serum concentrations were 37 +/- 8, 132 +/- 19, and 411 +/- 71 ng/ml. Mean arterial pressure was slightly elevated (maximally by 15.3%, P < 0.001) during ketamine infusion. Ketamine increased rCBF in a concentration-dependent manner. In the region-of-interest analysis, the greatest absolute changes were detected at the highest ketamine concentration level in the anterior cingulate (38.2% increase from baseline, P < 0.001), thalamus (28.5%, P < 0.001), putamen (26.8%, P < 0.001), and frontal cortex (25.4%, P < 0.001). Voxel-based analysis revealed marked relative rCBF increases in the anterior cingulate, frontal cortex, and insula. Although absolute rCMRO2 was not changed in the region-of-interest analysis, subtle relative increases in the frontal, parietal, and occipital cortices and decreases predominantly in the cerebellum were detected in the voxel-based analysis. rCBV increased only in the frontal cortex (4%, P = 0.022).

CONCLUSIONS

Subanesthetic doses of ketamine induced a global increase in rCBF but no changes in rCMRO2. Consequently, the regional oxygen extraction fraction was decreased. Disturbed coupling of cerebral blood flow and metabolism is, however, considered unlikely because ketamine has been previously shown to increase cerebral glucose metabolism. Only a minor increase in rCBV was detected. Interestingly, the most profound changes in rCBF were observed in structures related to pain processing.

Effects of surgical levels of propofol and sevoflurane anesthesia on cerebral blood flow in healthy subjects studied with positron emission tomography

BACKGROUND

The authors report a positron emission tomography (PET) study on humans with parallel exploration of the dose-dependent effects of an intravenous (propofol) and a volatile (sevoflurane) anesthetic agent on regional cerebral blood flow (rCBF) using quantitative and relative (Statistical Parametric Mapping [SPM]) analysis.

METHODS

Using H(2)(15)O, rCBF was assessed in 16 healthy (American Society of Anesthesiologists [ASA] physical status I) volunteers awake and at three escalating drug concentrations: 1, 1.5, and 2 MAC/EC(50), or specifically, at either 2, 3, and 4% end-tidal sevoflurane (n = 8), or 6, 9, and 12 microg/ml plasma concentration of propofol (n = 8). Rocuronium was used for muscle relaxation.

RESULTS

Both drugs decreased the bispectral index and blood pressure dose-dependently. Comparison between adjacent levels showed that sevoflurane initially (0 vs. 1 MAC) reduced absolute rCBF by 36-53% in all areas, then (1 vs. 1.5 MAC) increased rCBF in the frontal cortex, thalamus, and cerebellum (7-16%), and finally (1.5 vs. 2 MAC) caused a dual effect with a 23% frontal reduction and a 38% cerebellar increase. In the propofol group, flow was also initially reduced by 62-70%, with minor further effects. In the SPM analysis of the "awake to 1 MAC/EC(50)" step, both anesthetic agents reduced relative rCBF in the cuneus, precuneus, posterior limbic system, and the thalamus or midbrain; additionally, propofol reduced relative rCBF in the parietal and frontal cortices.

CONCLUSIONS

Both anesthetic agents caused a global reduction of rCBF (propofol > sevoflurane) at the 1 MAC/EC(50) level. The effect was maintained at higher propofol concentrations, whereas 2 MAC sevoflurane caused noticeable flow redistribution. Despite the marked global changes, SPM analysis enabled detailed localization of regions with the greatest relative decreases.