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.
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.
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.
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.
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KNOWLEDGE about the effects of different anesthetics and anesthetic regimens on cerebral circulation and brain metabolism is of vital clinical importance. Improper use of anesthetic techniques may have deleterious effects on the compromised brain.
The current understanding of the effects of general anesthetics on cerebral blood flow (CBF) and metabolism is largely based on laboratory data using a variety of different animal species and methods. In spite of partly contradictory results, intravenous anesthetics are commonly considered to reduce metabolism and flow to a comparable extent, whereas volatile anesthetics may perturb vascular reactivity, thereby challenging the classic “neuronal activity ≈ metabolism ≈ blood flow” paradigm. 1Even more variable results, depending on the experimental setting, have been reported for nitrous oxide (N2O). High concentrations of N2O have generally increased CBF and brain metabolism 2when given alone or with volatile anesthetic. 3–5Similarly, adding 70% N2O to deep propofol anesthesia increased cerebral blood flow velocity by 20% in humans. 6
Positron emission tomography (PET) allows accurate in vivo quantification with good regional resolution. Our main aim was to quantitatively assess the effects of a volatile anesthetic (sevoflurane) and an intravenous anesthetic (propofol) as sole agents and in combination with N2O on regional CBF (rCBF), regional cerebral metabolic rate of oxygen (rCMRO2), and blood volume (rCBV) in the living human brain using repetitive administration of 15O tracers and PET. From this data, the oxygen extraction fraction (OEF) was also calculated for each of the studied regions. Based on our previous study, 7we hypothesized that propofol and also sevoflurane would decrease both metabolism and flow compared to the awake state. Adding N2O was assumed to increase both metabolism and blood flow but more during sevoflurane than during propofol anesthesia. We also expected to see some extent of reduction in OEF and vasodilation (increase in rCBV) during sevoflurane and N2O.
Our secondary aim was to locate the peak effects of the drugs in greater detail and to identify the corresponding functional structures using voxel-based statistic parametric mapping (SPM) of the relative changes.
Materials and Methods
Subjects and Study Design
The study protocol was approved by Turku University and the Turku University Hospital Ethics Committee (Turku, Finland). After giving written informed consent, eight healthy (American Society of Anesthesiologists physical status class I), right-handed, nonsmoking male volunteers aged 23 (range, 20–26) years and with body mass index of 24.6 (range, 19.8–28.7) were enrolled in this open, nonrandomized, five-period study. All subjects underwent a detailed prestudy examination, including laboratory data collection and 12-lead electrocardiography. All subjects confirmed having no history of drug allergies or ongoing medications. Subjects restrained from using alcohol or any medication for 48 h and fasted at least 8 h before induction of anesthesia.
15O-labeled water, oxygen, and carbon monoxide were used as PET tracers to determine rCBF, rCMRO2, and rCBV, respectively, during the awake state (baseline) and 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 administered individually to keep a constant depth of anesthesia (titrated to Bispectral Index [BIS] value of 40) throughout the anesthesia. This value was based on our previous PET study characterizing rCBF changes during surgical levels of anesthesia. 7
Anesthesia and Monitoring
No premedication was given. The left radial artery and two large veins in the right forearm were cannulated for arterial sampling and for administration of anesthetic drugs, H215O-tracer boluses and 2.5% glucose plus 0.45% NaCl infusion (100 ml/h).
Throughout the study session, the electrocardiogram, noninvasive blood pressure, peripheral oxygen saturation, breathing gases (oxygen, carbon dioxide, sevoflurane, nitrous oxide), and nasopharyngeal temperature (during anesthesia only) were monitored using Datex AS3 equipment (Datex-Ohmeda Division, Instrumentarium Corp, Helsinki, Finland). Arterial blood gas analysis and acid–base balance was determined at baseline and at the end of each of the four anesthetic regimens. BIS® monitor A-1050 with software version 1.22 containing BI revision 3.3 (Aspect Medical Systems, Natick, MA) was used to measure the depth of anesthesia with two active electrodes placed on the temples and one reference electrode in the midline of the forehead. 8
Anesthesia was induced after the awake PET scans via mask with 8% sevoflurane (Sevorane; Abbot Oy, Espoo, Finland) in 100% oxygen. After disappearance of the eyelid reflex, 0.6 mg/kg rocuronium (10 mg/ml Esmeron; Organon, Helsinki, Finland) was administered intravenously to produce muscle relaxation. Subjects were intubated and connected to the Servo 900C ventilator (Siemens-Elema Ab, Solna, Sweden). First, an oxygen–air (30/70) mixture was used with the respiration frequency set to 15/min and ventilation set to 100 ml · kg−1· min−1. The ventilation was adjusted so that the individual end-tidal carbon dioxide measured before the induction was maintained throughout the anesthesia. Muscle relaxation was monitored using train-of-four and maintained at one or two twitches with bolus doses of rocuronium (5–10 mg). Arterial blood gas analysis and acid–base balance was determined at the end of each of the four anesthetic regimens.
After the induction of anesthesia, the end-tidal concentration of sevoflurane was adjusted so that the depth of anesthesia measured with the BIS® monitor was as close as possible to 40. After approximately 50 min, 70% N2O was added to the anesthetic regimen, and the depth of anesthesia was maintained by adjusting the sevoflurane vaporizer. After approximately 50 min, administration of sevoflurane and N2O was terminated, oxygen–air was restarted, and target-controlled infusion of propofol (20 mg/ml Diprivan; AstraZeneca Oy, Masala, Finland) was initiated with a target plasma concentration of 4 μg/ml. Infusion was accomplished with a Harvard 22 syringe pump (Harvard Apparatus, South Natick, MA) connected to a portable computer running Stanpump software with pharmacokinetic parameters by Marsh et al. 9
Finally, again after approximately 50 min, N2O was added to propofol. During both propofol regimens, the target concentration was adjusted at 0.1–0.2 μg/ml steps to maintain BIS at 40. At the end of both propofol regimens, 5-ml arterial blood samples were collected for the determination of plasma propofol concentrations. Plasma was immediately separated and kept frozen at −70°C until analyzed with high-performance liquid chromatography at Yhtyneet laboratoriot (Helsinki, Finland). 10The intraassay coefficient of variation of the assay was 7%.
After the PET scans, residual neuromuscular block was reversed with a neostigmine–glycopyrrolate combination, after which the subjects were extubated. They were then monitored until the vital functions had been stable at least for an hour.
We used 15O-labeled water to assess rCBF, 15O-labeled oxygen gas to assess rCMRO2, and 15O-labeled carbon monoxide gas to assess rCBV. All PET studies were performed in a dimly lit room with no sudden loud noises. A plastic head holder was used to minimize head movement. PET assessments were performed with the patients awake and during each anesthetic regimen after approximately 15 min stabilization. Each series of scans (H215O, 15O2, C15O) lasted for approximately 35 min. Thus, altogether, 15 (5 × 3) scans were performed in each subject. For anatomic reference, individual magnetic resonance images were acquired with 1.5-T scanner (Siemens Magnetom SP63, Erlangen, Germany) on a separate occasion.
15O was produced with a low-energy deuteron accelerator, Cyclone 3 (Ion Beam Applications Inc., Louvain-la-Neuve, Belgium) in Turku University Hospital. 11The 15O2gas (with radiochemical purity of 97%) was used as such or processed either into C15O in a charcoal oven 12or into H215O by diffusion membrane technique in a continuously working water module. 13The purity of the 15O2and C15O was defined with gas chromatography before each inhalation.
Scanner and Image Reconstruction.
A GE Advance PET scanner (General Electric Medical Systems, Milwaukee, WI) was used to assess tracer tissue concentration in 35 slices parallel to the canthomeatal line, spanning over a distance of 150 mm below cranial vertex. The three-dimensional (septa retracted) transaxial spatial resolution, full width at half maximum, at a radius of 10 cm in midplanes (used in the analysis) was 6 mm in the radial and 5 mm in the tangential direction. Axial resolution was 6.5 mm, full width at half maximum. 14All scan data were corrected for detector dead time, tracer decay, and measured tissue photon attenuation (assessed with the patient awake and after induction of anesthesia with transmission scans using two extracorporeal 68Ge rod sources to obtain 10 × 106counts/slice). Data were reconstructed into a 128 × 128 matrix using a three-dimensional transaxial Hann filter with a 4.6-mm cutoff and an axial Ramp filter with an 8.5-mm cutoff. The field of view was set to 30 cm, resulting in a pixel size of 300 mm/128 = 2.34 mm.
Tracer Delivery and Arterial Samples.
For rCBF assessments, a 300-MBq dose of H215O was administered as a 15-s bolus using an automated infusion system. The 90-s static tissue activity scan was performed in three-dimensional mode. The arterial activity was measured using a peristaltic roller pump and a two-channel co-incidence detection system (GEMS, Uppsala, Sweden) cross-calibrated with the scanner.
During rCMRO2and rCBV assessments, a modified Servo 900C ventilator was used to enable bolus inhalation of 15O2and continuous inhalation of C15O in conscious as well as in anesthetized subjects. For rCMRO2assessment, a 350-MBq bolus dose of 15O2gas was collected and administered as a single deep inhalation, followed by a 20-s inspiratory pause. Simultaneously, a 5-min dynamic (16 frames) tissue activity scan was performed in three-dimensional mode. Arterial activity was measured as described above. For measuring arterial oxygen content (milliliters of oxygen per liter of arterial blood), manual samples were drawn before administering 15O2.
For rCBV assessments, the subjects inhaled in 2 min a 700-MBq dose of 7% C15O. Two minutes after completing the inhalation, a 4-min static tissue activity scan was performed in three-dimensional mode. Three manual samples were obtained at 2-min intervals during the scan and were measured for arterial (whole blood) activity using an automatic γ counter (Wizard 1480; Wallac, Turku, Finland) To ensure that the activity resided in the red blood cells, a 2-ml manual sample was taken after completing each CBV scan. After centrifugation, the activity of plasma and cells were measured with well type γ counter (Bicron MW 3″× 3″; Bicron Corp., Oakridge, TN).
The tissue tracer activity images were first computed into quantitative images of rCBF, rCMRO2, and rCBV by using arterial tracer activity data and the appropriate tracer kinetic models (see Appendix).
Quantitative ROI Analysis.
Each subject's anatomic magnetic resonance images and functional PET images were aligned and resliced using either the surface-fit 15or the amir-fit method 16to achieve matching image planes. The effects of subject movement were minimized by realigning other parametric images with the awake images for each individual using SPM99 software (see Voxel-based SPM Analyses).
Individual bilateral ROIs were drawn to outline frontal (on nine image planes), parietal (six planes), temporal (seven planes), and occipital gray matter (four planes), as well as the thalamus (three planes), caudate (three planes), putamen (three planes), and cerebellum (three planes). ROIs were transferred to the parametric PET images to obtain values for each structure.
The regional oxygen extraction fraction (rOEF) was calculated for each ROI as the ratio of consumption (rCMRO2) and delivery (rCBF × arterial oxygen content).
Statistical Analysis of ROI and Monitoring Data.
Quantitative ROI and monitoring data were analyzed with repeated-measures analysis of variance with the five different stages (baseline, sevoflurane alone, S+N, propofol alone, and P+N) as the within factor. A two-sided P value of less than 0.05 was considered statistically significant. The effect of increasing N2O was studied by forming linear contrasts and analyzing with paired t tests. Tukey-Kramer correction to P values in pairwise comparisons between the stages was applied to maintain the type I error rate at 0.05. Statistical analyses were conducted with SAS (version 8.02; SAS Institute Inc., Cary, NC). Data are presented as mean ± SD if not otherwise stated. Results of the ROI analysis are given as the mean of the left and right hemispheres.
Voxel-based SPM Analyses.
Statistical Parametric Mapping software version 99 (SPM99, the Wellcome Department of Cognitive Neurology, University College London, United Kingdom) 17running on Matlab 5.3 (The MathWorks Inc., Natick, MA) was used to offer visualization of areas of greatest changes in relative 7rCBF and rCMRO2and absolute rCBV.
Parametric (i.e. , quantified) rCBF, rCMRO2, and rCBV images were used as the source data. SPM99 preprocessing was performed as previously described, 7including the realignment of PET scans, conversion to common stereotactic space, and smoothing (14-mm kernel). For flow and metabolism data, proportional scaling was necessary to eliminate the effect of large global changes, whereas blood volume data were analyzed in absolute values without global normalization.
Regional drug-induced changes were explored using subtraction analysis with multisubject, multiple-condition models (with eight subjects and five scans per subject) for rCBF, rCMRO2, and rCBV data separately. The changes between drug conditions were tested for statistical significance in each voxel using t statistics. From these t statistic maps, areas of greatest changes were located without an a priori hypothesis regarding the spatial distribution of the effect, as anesthetics at the studied concentrations may induce both subtle widespread effects, but also greater localized effects. To visualize both, two levels of inference were defined as follows. 18Peak effects were revealed by areas where the parallel significant change occurred in every voxel, each with P < 0.05 (corrected for multiple comparisons). Conversely, the more subtle changes (having lower height threshold) are significant only if they occur in a sufficiently large clusters (defined by the minimum cluster size). Using height threshold T = 3.41 for rCBF data, the cluster level significance P < 0.05 (corrected) was achieved by adjusting the minimum cluster size to 100 (sevoflurane vs. awake), 50 (propofol vs. awake), 105 (S+N vs. sevoflurane), or 30 (P+N vs. propofol). Using height threshold T = 3.47 for rCMRO2data, the required minimum cluster sizes were 150 (sevoflurane vs. awake), 100 (propofol vs. awake), or 200 (P+N vs. propofol).
All PET assessments were performed successfully, except the awake rCMRO2measurement in the first subject (oxygen content was not analyzed from the arterial samples). BIS was kept near 40 during anesthesia as planned (fig. 1). With sevoflurane alone, the end-tidal concentration range for maintaining BIS at 40 was 1.1–1.9%. After adding N2O, the sevoflurane concentration could be reduced on average by 22% (P < 0.001) while maintaining a steady BIS level. The range of measured plasma concentrations for propofol alone was 2.6–4.6 μg/ml. The introduction of N2O did not allow reducing the plasma propofol concentrations (table 1).
All anesthetic regimens reduced mean arterial pressure on average by 18% compared to awake values (P < 0.05 for all). Heart rate remained close to awake values, although differences between the anesthetic regimens caused a significant overall effect in the statistical analysis (P = 0.0034). Nasopharyngeal temperatures showed a slight decrease during anesthesia, with 0.24°C average reduction from awake values during the last drug combination (P < 0.001 overall). The end-tidal carbon dioxide concentrations were well maintained near the awake values (range, 4.6–5.7%) throughout anesthesia and matched concomitant arterial values (fig. 1) very well. Numeric values for hemodynamic and ventilation monitoring parameters and drug concentrations during the PET assessments are presented in table 1.
rCBF, rCMRO2, rOEF, and rCBV
During the awake state, rCBF was 37–55 ml · 100 g−1· min−1in the studied regions. Propofol reduced absolute rCBF in all studied areas to 53–70% of baseline (P < 0.001), whereas the reduction by sevoflurane was significant only in the occipital cortex, cerebellum, caudate, and thalamus (73–80% of baseline, P < 0.01) (fig. 2and table 2). More detailed regional visualization of areas with largest flow reduction can be seen in figure 3, showing SPM analysis of the relative rCBF changes. Information on stereotactic coordinates of the SPM findings can be found on the Anesthesiology Web site. Although adding N2O to either drug increased rCBF significantly compared to drug alone in most regions (fig. 2), during P+N, rCBF was maintained below the awake levels in almost all regions (62–78% of baseline, P < 0.05 for all except the occipital cortex), whereas S+N caused a global return to awake rCBF values (83–112% of baseline, not significant). In addition, the absolute flow increases were significantly greater (vs. drugs alone) in the caudate, putamen, and cerebellum (P < 0.05) during S+N (range, 4.9–12.9 ml · 100 g−1· min−1) than during P+N (range 4.4–8.5 ml · 100 g−1· min−1). The areas of greatest relative flow increase caused by adding N2O to either drug are shown in figure 4.
During the awake state, rCMRO2was 4.2–5.6 ml · 100 g−1· min−1in the studied regions. Both sevoflurane and propofol markedly reduced rCMRO2in all brain areas to 56–74% and 50–68% of baseline, respectively (P < 0.05;fig. 5and table 2). Areas of greatest relative metabolic reduction are shown in figure 3. Although N2O increased rCMRO2in parietooccipital regions and the cerebellum during S+N and in all cortical areas during P+N, metabolism was maintained at lower than in the awake state (62–82% and 59–76% of baseline, respectively, P < 0.01) during all regimens.
During the awake state, rOEF was 0.46–0.55 in the studied regions. Sevoflurane alone caused a tendency of reduced rOEF (P < 0.05 in the frontal and occipital cortices and in the cerebellum;fig. 5and table 3). The sevoflurane–N2O combination, on the other hand, uniformly reduced OEF on average by 0.12 units in all cortical areas and the cerebellum (P < 0.05) compared to awake values. Propofol alone or with N2O did not affect rOEF compared to baseline in any of the studied regions.
During the awake state rCBV was 2.9–4.4% in the studied regions. Sevoflurane alone or with N2O did not affect rCBV in any of the studied areas (fig. 2and table 3). Propofol with or without N2O reduced quantitative rCBV on average by 0.66% units (P < 0.05) from baseline values in all cortex and the cerebellum. SPM was used for visualization of significant absolute rCBV changes, displayed over group mean magnetic resonance image planes (fig. 6) to reveal changes also outside our ROIs.
At concentrations producing a BIS value of 40, propofol reduced rCBF and rCMRO2comparably, to approximately 60% of baseline. The intense reduction in cerebral perfusion was seen also as a reduction in tissue blood volume in the cortex and cerebellum. At the same BIS level, sevoflurane reduced rCBF less than propofol but rCMRO2to an extent similar to propofol, thereby causing a tendency of reduced oxygen extraction. Adjunct N2O increased both flow and metabolism with either drug. Metabolism was still maintained below the awake values during all regimens, whereas the flow-reducing effect of sevoflurane, but not of propofol, was abolished. The combination of sevoflurane and N2O especially decreased rOEF.
Quantitative Effects of Sevoflurane and Propofol on rCMRO2and rCBF
Previous information on propofol is quite coherent, and anticipated reductions were seen in both rCMRO221,22and rCBF. 23,24The similar extent of these changes ensured that rOEF was maintained at normal i.e. , awake level, suggesting that this drug leaves the coupling of neuronal activity and blood flow intact despite surgical levels of anesthesia.
Also for sevoflurane, the reduction in rCMRO2was as anticipated, 25,26but the reduction was surprisingly similar to that of propofol, which could suggest that these drugs are actually equally potent suppressors of neuronal metabolism. Regarding the flow effects of sevoflurane, we have previously shown that 1 minimal alveolar concentration (MAC) sevoflurane reduces rCBF by 36–53% from awake values in similar ROI regions, but the effect was partly due to concomitant reduction in arterial carbon dioxide tension. In the current study, end-tidal carbon dioxide was maintained at baseline level, and we were able to confirm that 1.5% sevoflurane as a sole anesthetic does not increase perfusion in any region in intact human brain when compared to a proper awake baseline, but rather causes a global declining tendency in rCBF.
Quantitative Effects of Adjunct Nitrous Oxide on rCMRO2, rCBF, and rOEF
At sedative doses (20–50%), N2O alone has been shown to increase cerebral blood flow in humans 27–29while maintaining coupling, 30and at high doses (70%), it has been shown to increase flow and metabolism in goats. 2Adjunct N2O has been shown to alleviate or abolish the rCBF (or mean middle cerebral artery flow velocity)–reducing effect of sevoflurane in pigs 31and humans 32and that of isoflurane 33in humans. Similarly, adding N2O to propofol has been shown to increase middle cerebral artery flow velocity (vs. propofol alone) in humans. 6Our rCBF findings agree well with these studies: N2O increased metabolism and flow with both anesthetics. The increases in rCMRO2and rCBF (vs. drug alone) were more pronounced during S+N than during P+N. This may be partly explained by the fact that the sevoflurane concentration, but not that of propofol, was reduced during adjunct N2O, because the BIS® monitor did not detect N2O-related deepening of anesthesia during P+N the same way as previously reported for remifentanil–propofol, 34fentanyl–midazolam, 35and isoflurane. 36So, although the different conduct makes comparison between S+N and P+N a bit problematic, we believe that our goal to maintain equihypnotic depth was more or less achieved during S+N, and the increases in rCBF and rCMRO2versus sevoflurane alone are essentially valid. It should also be pointed out that sevoflurane concentration was reduced by merely 22%, whereas the rCBF-reducing effect of sevoflurane was entirely abolished. Furthermore, if MAC values are compared, the anesthesia was still deeper during S+N than during sevoflurane alone.
Irrespective of the depth of anesthesia, the clear reduction in rOEF (vs. awake) during sevoflurane but especially S+N is important because it suggests that blood flow is no longer tightly regulated by oxygen requirements. On the other hand, regionally, the association between flow and metabolism is not completely abolished because regions of highest rCMRO2still had highest the rCBF, and vice versa , during all anesthetic regimens (fig. 7). Especially in a compromised brain, however, a reduced rOEF is most probably an undesirable phenomenon, irrespective of the actual mechanism. Thus, our findings support the current clinical practice that in such circumstances, volatile anesthetics and N2O should be used very cautiously.
Quantitative Effects of Sevoflurane, Propofol, and Adjunct Nitrous Oxide on rCBV
The effects of propofol on rCBV are straightforward. The marked reduction in rCBF was accompanied with a reduction in rCBV, suggesting vasoconstriction to suit the reduced requirements of diminished neuronal activity. This should be a reassuring finding for neuroanesthetists, although our study does not prove that exactly the same occurs in the compromised brain.
Based on previous studies showing that CBV does not reflect CBF 37or intracranial pressure 38,per se but could correlate inversely with regional cerebral vascular resistance 39–41and is globally increased because of N2O, 42halothane, 43and isoflurane, 38,43we hoped that rCBV assessment would help us to estimate the vasodilatory effects of sevoflurane and N2O within the cortical tissue. Sevoflurane reduced rCBF by 6–27% and S+N maintained it at the awake level, whereas neither regimen affected rCBV. Taking into account the concomitant decrease in blood pressure as shown in figure 8, rCBV does not seem to reveal the probable vasodilation during S+N. To examine also, for example, the white matter, we looked for changes in absolute rCBV at voxel-level detail using SPM, showing that S+N increased blood volume (vs. awake) only in the transverse venous sinus and probably the anterior and medial cerebral arteries (fig. 6) but not within the brain tissue. Thus, our results suggest that rCBV is a fairly insensitive indicator of vasodilation. The most likely reasons for this insensitivity are that even a small change in vascular diameter alters CBF considerably, that the actual resistance vessels represent only a small fraction of the whole blood volume shown by CBV imaging, and/or that anesthesia may cause local changes in capillary hematocrit, 44which we assumed constant in our measurements.
Results of Voxel-based rCBF and rCMRO2Analyses
While quantitative ROI analysis is restricted to anatomic regions manually defined for each brain structure for every individual, SPM explores statistically significant changes for every voxel and enables the localization of the changes according to a common stereotactic atlas. At best, such changes in flow or metabolism may reveal regions with drug-induced changes in neuronal activity. However, in the presence of large global effects, it is important to understand that the effects of the drugs are not restricted to the areas of greatest changes. Furthermore, although regions of least changes can be visualized as well, these findings may not be functionally valid because they seem to contain structures such as white matter with very stable blood flow.
Although rCBF reflects neuronal activity well in normal circumstances and repeated measurements are widely used in neurofunctional studies, interpretation of results becomes difficult when studying drugs that may alter flow–activity coupling. ROI-quantified rCBF and rCMRO2provide some assistance because they can be used to verify intact flow–metabolism correlation. If, on the other hand, rOEF is found to be decreased, the reliability of rCBF findings is questionable because a disturbance in flow–activity coupling cannot be excluded. Theoretically, rCMRO2could substitute for rCBF in voxel-based analyses during disturbed coupling because it also can be quickly and repeatedly assessed. However, according to the present SPM results with propofol, rCMRO2seems to lack the intensity of rCBF changes despite normal rOEF. The reason why rCMRO2seems to be a fairly insensitive indicator of regional changes is unclear, but recent evidence suggests that the regional metabolic rate of glucose 45would actually correlate more directly with neuronal activity than that of oxygen 46during local activation, whereas flow–CMRO2correlation is sustained in the nonactivated regions. 47,48This would make regional metabolic rate of glucose more accurate in revealing neuronal activity, but, because certain anesthetics could theoretically alter the glucose–oxygen consumption ratio (see Långsjöet al. 49in this issue), further studies are needed to establish whether rCMRO2or regional metabolic rate of glucose provides a more reliable estimate of neuronal activity. The practical problem in regional metabolic rate of glucose assessments is that using 18F-fluorodeoxyglucose, a single assessment takes approximately 60 min, and performing repeated assessments in a single session is very difficult (the half-life of 18F is 110 min).
Based on the above discussion, the relative rCBF changes by propofol should reflect neuronal activity reasonably well. Because sevoflurane tended to reduce absolute rOEF, neurofunctional deduction should be especially cautious. The relative rCBF effects of sevoflurane and propofol were, however, surprisingly identical in the current study and similar to our own previous study on 1 MAC sevoflurane and 1 EC50propofol. 7The same areas have also shown greatest reductions in voxel-based analysis during 0.7% halothane (with reduced rGMR in the basal forebrain, thalamus, limbic system, cerebellum, and occipital cortex) 50and 2 μg/ml propofol (with reduced rCBF in the frontal and parietal cortex, posterior cingulate, and thalamus) 23and in ROI-based analysis during sedative concentrations of propofol (with reduction in rCBF in the thalamus, cuneus, precuneus, posterior cingulate, and orbitofrontal and right angular gyri). 24
Limitations of the Study Design
The nonbalanced design of the current study can be criticized. However, randomization was not considered possible because it would have lengthened the anesthesia considerably if propofol were given before sevoflurane. We also wanted to study the drugs alone and in combination with N2O consecutively. The study conditions were rigorously standardized, and we consider significant order effects extremely unlikely, although body temperature was slightly but statistically significantly reduced toward the end of anesthesia. It should also be pointed out that only trace amounts of sevoflurane were detected during the propofol and P+N periods (table 1), suggesting that the time interval between the PET assessments at different anesthetic regimens was sufficient.
Propofol reduced rCBF and rCMRO2comparably at a BIS value of 40. Sevoflurane reduced rCBF less than propofol but rCMRO2to an extent similar to propofol. Adjunct N2O increased flow and metabolism with both drugs. Sevoflurane, but especially the combination of sevoflurane and N2O, decreased rOEF, which may suggest disturbed flow–activity coupling in humans already at a moderate depth of anesthesia.
The authors thank Paul Manberg, Ph.D. (Vice President of Aspect Medical Systems, Newton, Massachusetts), for providing the Bispectral Index® monitor for this study. They also thank the personnel of Turku PET Center (University of Turku, Finland) for excellent technical assistance and Steven Shafer, M.D. (Department of Anesthesia, Stanford University, Stanford, California), for the free use of his STANPUMP computer program.
Appendix: Parametric Images—Tracer Kinetic Modeling
H215O, being a freely diffusing substance, effectively washes into brain tissue, thereby enabling the quantification of tissue perfusion. The tissue tracer activity images were computed into quantitative images by using corrected 51arterial tracer activity data and in vivo autoradiography 52as described in detail previously. 7
The inhaled labeled oxygen is effectively bound by hemoglobin, carried to the brain, extracted into the brain tissue, and metabolized into H215O. Using a two-compartment model, the regional tissue oxygen uptake rate can be calculated into a parametric rCMRO2image.
The original model for assessing rCMRO2with oxygen-bolus inhalation technique 53states that rCMRO2can be calculated when oxygen concentration in arterial blood (Cao2), cerebral blood flow (CBF), and oxygen extraction fraction (OEF) are measured (equation 1).
We applied a well-documented, more simplified modification of this model, the one-step method by Ohta et al. 54As described in equation 2, the model states that the product of flow and EOF can be estimated from unidirectional clearance of labeled oxygen from blood to brain (K1).
In detail, the measured tissue (Ci) and arterial (Ca) radioactivity concentrations can be used in equation 3, allowing the estimation of the wash-in (K1) rate of oxygen and the washout (k2) rate of metabolized water by multiple linear least-squares regression, while vascular volume (V0) is assumed constant (3%).
The parameters were estimated for each pixel to give a K1image, which was multiplied by arterial oxygen concentration and divided by average brain density 1.04 g/ml to produce the quantitative (ml O2· 100 g−1· min−1) rCMRO2image.
In a comparison 55between the traditional steady state technique 53and the “single-step” bolus method of Ohta et al. 54used in this study, rCMRO2values were comparable except for venous sinuses (overestimated by bolus technique, probably due to diffusion delay). These areas were therefore cautiously avoided with the help of the rCBV image when drawing ROIs.
Labeled carbon monoxide reveals the distribution of red blood cells as it is effectively and almost irreversibly bound by hemoglobin. rCBV images are calculated simply by dividing the tissue tracer activity value in each pixel (Ci) by arterial radioactivity concentration (Ca) and by the cerebral–to–large vessel hematocrit ratio (R = 0.85) 56,57according to equation 4. 58