Dose-dependent Regional Cerebral Blood Flow Changes during Remifentanil Infusion in Humans: A Positron Emission Tomography Study

The study was approved by the Ethics Committee of the Faculty of Medicine of the Technical University, Munich, and the radiation protection authorities. In accordance with the Declaration of Helsinki, all subjects gave written informed consent to participate in the study after the experimental procedure and radiation effects had been extensively explained.

Ten right-handed, healthy male volunteers were enrolled in the study. None had a history of neurologic or any other severe disease (American Society of Anesthesiologists physical status 1), and none had a history of drug abuse.

The volunteers had fasted for at least 6 h before the study. Electrocardiogram and arterial oxygen saturation (Sao²) were measured and continuously recorded (Capnomac Ultima; Datex, Helsinki, Finland). Noninvasive blood pressure measurements were performed at 5-min intervals (Dinamap 1846 SX; Criticon, Tampa, FL). End-tidal carbon dioxide concentrations were measured using a Capnomac Ultima monitor via  a catheter placed at the naso-pharyngeal border. Capillary carbon dioxide was measured immediately after every condition of drug administration by blood samples taken from a warm, nonheated finger tip. In a pilot study, remifentanil infusion was increased from 0.025 to 0.25 μg · kg⁻¹· min⁻¹. Significant respiratory depression (< 6 breaths/min) with increases in end-tidal carbon dioxide was observed with more than 0.20 μg · kg⁻¹· min⁻¹remifentanil. Therefore, the maximal dose in our study was limited to 0.15 μg · kg⁻¹· min⁻¹remifentanil; respiration was only slightly impaired and would be easily counteracted by verbal command. Each volunteer was instructed for respiration by verbal command twice per minute during every condition in our study.

A total of three different conditions of drug administration were investigated: control (saline), low-dose remifentanil (0.05 μg · kg⁻¹· min⁻¹), and moderate-dose remifentanil (0.15 μg · kg⁻¹· min⁻¹). A semi-randomized study protocol was used to overcome the problem of different dates of data acquisitions and possible residual remifentanil effects: one group of volunteers (n = 3) was first subjected to the control condition, whereas remifentanil measurements were performed 3 or more months after the control condition. In the second group, volunteers (n = 4) were first exposed to the remifentanil measurements and after 3 or more months to the control condition. Three additional volunteers received only the low and moderate doses of remifentanil and did not take part in further investigations. All the reported data are based on results from the seven volunteers (age range, 28–38 yr; mean, 32 years) who underwent all three conditions if not otherwise indicated (statistical results from the entire group are available on request). Statistical analysis was performed with data from the seven remaining subjects.

Each condition was repeated three times, resulting in a total of nine scans for each volunteer (n = 7). Because of its short half-life, remifentanil was delivered by an infusion pump (Combimat 2000; Döring, München, Germany) in a blinded, randomized order with a time interval of more than 30 min between subsequent remifentanil infusion rates. To establish steady state plasma concentrations, 17remifentanil was administered via  a separate intravenous line in a left antecubital vein to avoid bolus effects during ¹⁵O-water injections. All PET-scanning sessions were scheduled at similar times of the day in a quiet ambient environment. Subjects were instructed to remain in a supine position with their eyes closed and not to move or say anything until prompted for a subjective sedation rating (1–4: 1 = wide awake, 2 = awake, 3 = drowsy, 4 = sedated) after the scan.

Positron emission tomography measurements were performed using a Siemens 951 R/31 PET scanner (CTI, Knoxville, TN) in a three-dimensional mode with a total axial field of view of 10.5 cm and no interplane dead space. The patients’ heads were positioned parallel to the canthomeatal line with the primary sensorimotor cortex covered within the field of view. Attenuation was corrected using a transmission scan (two-dimensional) with an external ⁶⁸Ge/⁶⁸Ga ring source obtained before the tracer injection. A semibolus of 7 mCi ¹⁵O-water was administered intravenously via  a second intravenous line in a left antecubital vein over 35 s using an infusion pump (Harvard Apparatus SP22, South Natick, MA). The PET scan was initiated when the tracer bolus entered the brain, as indicated by an abrupt increase in the coincidence-counting rate of the tomograph. 18After correction for randoms, dead time, and scatter, images were three dimensionally reconstructed by filtered back-projection with a Hanning filter (cutoff frequency, 0.4 cycles per projection element), resulting in 31 slices with a 128 × 128 pixel matrix (pixel size, 2.0 mm) and interplane separation of 3.375 mm.

For observer-independent determination of changes in rCBF, images were analyzed by statistical parametric mapping (SPM96, Wellcome Department of Cognitive Neurology, Institute of Neurology, University College London, London, United Kingdom). The emission scans were intra-individually realigned before transformation into a standard stereotactic space. 19As a final preprocessing step, the images were smoothed using an isotropic gaussian kernel (12-mm full width at half-maximum). 20Analysis of covariance (subject-specific) global normalization was used to adjust for intersubject and interscan variability in tracer count. Categoric comparisons were performed between control versus  low and low versus  moderate doses. Further data analysis included a regression analysis (Pearson) of the dose-dependent regional effects of remifentanil on rCBF. Correlation coefficients were transformed to z  scores by a Fisher transformation. The resulting foci of significant differences were characterized in terms of peak height (μ, z  score) at voxel level and combined μ and extent (κ, number of voxels) at cluster level. Correction for multiple nonindependent comparisons was performed, and significance level was defined at P < 0.05. For predefined brain structures, the corrected threshold (peak height, three-dimensional Hammersmith) was calculated taking into account the volume of search pixels and the smoothness (13.9-mm full width at half-maximum). The predefined structures were selected from brain areas with high opioid receptor binding capability 10and with previously reported response to a single dose of fentanyl 11: anterior cingulate cortex, thalamus, anterior striatum, anterior insula, and the periventricular grey of the brainstem, including the periaqueductal grey (PVG). These regions comprised a volume of approximately 60 ml according to the grid system of Talairach, and correspond to a z  score of 3.2 (P = 0.0006). For all analyses, the cluster extent threshold was set at a minimum of 25 voxels (uniform voxel size of 2 mm), passing the adjusted threshold of P = 0.0006. Sedation ratings were analyzed using a two-tailed paired t  test (P < 0.05).

Systemic hemodynamic and respiratory values are presented in table 1. Systolic blood pressure, diastolic blood pressure, mean arterial blood pressure, and heart rate were unchanged within and across the groups (unpaired t  test, P > 0.05). Similarly, respiratory parameters including capillary carbon dioxide, end-tidal carbon dioxide, and oxygen saturation were not different between groups. Subjective sedation ratings indicated a significant increase in sedation from control to low dose (P = 0.007) and a further significant increase from the low to moderate condition (P = 0.01).

Results are summarized in table 2and figure 1. The uncorrected P  value is < 0.001 for all regions. Accentuated increases in relative rCBF were observed in the right lateral prefrontal cortex and were separable into three clusters at a more stringent threshold of P = 0.0001. Increases in the left frontal cortex, in inferior parietal cortices, and in supplementary motor area (SMA) were less pronounced. Marked relative decreases in rCBF were observed in the basal part of the mediofrontal cortex, the cerebellum (vermis and right lateral part), and the rostromedial part of the left superior temporal lobe. The right mesial temporal lobe showed a similar decrease in rCBF slightly below the significance threshold. Within the predefined volume, there was a significant decrease in rCBF in the rostral PVG (z coordinate =−8 to 0; y coordinate =−30 to −20).

Results are summarized in table 3and figure 2. The uncorrected P  value is < 0.001 for all regions. From low to moderate remifentanil doses, increases in relative rCBF were dominant in the mediofrontal and anterior cingulate cortices simultaneous to increases in an occipital area comprising the lingula and the transition zone between posterior cingulate cortex, precuneus, and cuneus. Furthermore, an increase in relative rCBF was identified in the caudal PVG (z coordinate =−16 to −8; y coordinate =−20 to −30). A reduction in relative rCBF was observed bilaterally in the inferior parietal lobes.

Including the data of the three volunteers who underwent only low and moderate conditions in the analysis provided consistent results. The mediofrontal and anterior cingulate cortices as well as the occipital transition area clearly passed the probability threshold. The cluster in the PVG extended into the thalamus (z score = 4.91, x, y, z coordinates = 10, −20, 6). Reductions in relative rCBF in parietal cortices were also observed.

Results are summarized in table 4and figure 3. Significant positive correlations between remifentanil doses and relative rCBF were found in the left medial prefrontal gyrus, including the rostral part of the perigeniculate part of the anterior cingulate gyrus, the right lateral prefrontal cortex, the cuneus–cingulate transition, the lingula, and the right medial temporal gyrus. The only significant negative relation indicating a reduction in rCBF across the three dosage levels of remifentanil was observed in the fusiforme gyrus.

The current study investigated dose-dependent changes in relative rCBF in different brain areas after administration of the μ-selective opioid agonist remifentanil in healthy volunteers. Main findings were alterations of relative rCBF within structures involved in pain processing and dose-dependent changes in structures previously described to participate in modulation of vigilance and alertness.

The rCBF responses to remifentanil are within the widespread network associated with pain processing, 5demonstrating predominant effects in the opioid receptor–dense medial pain system. 9These findings are in agreement with the common clinical observation of a reduced intensity and emotional response to pain (medial pain system) but unaffected localization of a painful stimuli (lateral pain system) after synthetic opioid administration. In the current study, the anterior cingulate cortex was involved significantly at moderate doses (fig. 2). A positive response after increasing doses was confirmed in the correlation analysis (fig. 3). The anterior cingulate cortex is a main target of opioid receptor–binding substances and is the region most frequently reported to be activated in pain studies. 5It has extensive connections with the prefrontal cortex, medial thalamic nuclei, amygdala, and periventricular grey, and part of it projects to autonomic brainstem motor nuclei. The anterior cingulate cortex regulates autonomic and endocrine functions and is involved in emotional learning, emotional assessment of internal and external stimuli, 21and encoding of the degree of pain unpleasantness and pain threshold. 4The lateral prefrontal cortex was already activated at lower doses and did not show any further significant response when remifentanil infusion was increased. This brain area is involved in the cognitive evaluation of somatosensory stimuli and attention 6and has been related to pain coping strategies. 7,8The PVG was recruited with increasing opioidergic stimulation. Descending input from the limbic forebrain and diencephalon with ascending nociceptive afferents 22are integrated in the PVG, which also controls nociceptive transmission at the level of the spinal cord. 23Electrical stimulation of the PVG has been shown to exert analgesia related to the release of endogenous opioids. 24This emphasizes the relevance of the present rCBF responses in the PVG and its participation in opioidergic analgesia. The positive correlation between rCBF and remifentanil doses in the temporal cortex is in agreement with earlier studies, indicating an involvement of the temporal lobe in endogenous opioid neurotransmission by changes in opioid receptor binding 25and increased glucose metabolism in the temporal lobe in humans and monkeys during fentanyl and remifentanil administration. 26However, exogenous auditory stimuli, such as the verbal command for respiration, may have induced the observed increases in rCBF close to the primary auditory cortex. 27Based on the fundamental relation of stimulus frequency and activation level, it is improbable that a significant auditory activation measurable by PET can be induced by a single short verbal command during the 50-s acquisition period. 28However, it cannot be completely ruled out that attention may be modified by verbal commands.

A marked increase in relative rCBF during opioid administration was observed in the transition zone of the medial part of the occipital lobe, which has not been reported previously. Rainville et al.  29identified a specific pattern of cerebral activation associated with an experimental induced hypnotic state. The change from restful awake to a state with general relaxation, automatic responding, and slight disorientation in time corresponded to increases in occipital rCBF and an increase in occipital delta activity in the electroencephalogram. Likewise, conditions with altered states of consciousness, such as meditation 30and sleep, 31are associated with an rCBF increase in the same brain area and may be related to the sedation and decreased arousal common to these states.

The sites of action of opioids in the human brain were first described by Jones et al.  13in a case report using morphine for a patient suffering chronic pain from a carcinoma. Firestone et al.  11later examined the rCBF responses to a single dose of fentanyl in healthy subjects with similar results to our study using remifentanil. Adler et al.  12investigated the interaction of repeated boli of fentanyl and painful stimuli on rCBF. These results were only partly consistent with their previous report 11and our data. The differences probably reflect the residual effects of warm and painful stimuli applied simultaneously with fentanyl. Repeated boli of fentanyl are unlikely to provide a steady state opioid plasma level as we could expect in our study. Adler et al.  12used a very low significance level (P < 0.01, z > 2.5), whereas we used stricter thresholds with correction for multiple comparisons, which explains some of the different results (see Statistical Analysis, lowest threshold for a priori  defined areas: k ≥ 25 voxels, P < 0.0006, z > 3.2).

The observed pattern of rCBF changes most likely reflects the agonist actions of remifentanil on neuron-located presynaptic or postsynaptic opioid receptors, 32resulting in an increased synaptic energy demand independent of the excitatory or inhibitory function of the neuron. 33Opioid receptor stimulation is generally believed to suppress brain activity, 34which results in a reduced rCBF. However, net changes in rCBF reflect local and remote synaptic activity as well as interaction with other cell assemblies. A direct action of remifentanil on opioid receptors on cells of the cerebrovascular bed is another possible explanation for the observed alterations in rCBF. Activation of different opioid receptor subtypes by morphine may result in changes of cerebral vascular resistance. 35A variety of factors (e.g. , adenosine, nitric oxide, cardiorespiratory parameters) are known to profoundly influence CBF. 36In the current study, no significant systemic cardiovascular changes occurred during low and moderate remifentanil infusion (table 2). Thus, these factors are unlikely to be responsible for the observed relative rCBF changes. Probably the most important reason for stable systemic hemodynamics was the constant infusion rate of remifentanil instead of a bolus administration. 37Because changes in arterial carbon dioxide partial pressure (Paco²) can substantially alter rCBF, volunteers’ normocapnia (table 2) was maintained by identical verbal command for respiration during every condition investigated. Corfield et al.  38detected limbic system activation during carbon dioxide breathing similar to the activation observed in our study. Their results likely reflect motor-related influences on breathing or uncomfortable sensations associated with carbon dioxide values of up to 50 mmHg. We relied on real-time capnography in addition to capillary blood gas monitoring to ensure Paco²concentrations within the physiologic range. On the premises of a relatively constant tidal volume and a constant alveolar dead space, there is a consistent correlation between end-tidal carbon dioxide and Paco². 39Factors that can alter the end-tidal carbon dioxide–Paco²relation (e.g. , chest wall rigidity, artificial ventilatory support) were not present in our study. From this we conclude that alterations in Paco²were not a reason for potential changes in global CBF and were unlikely responsible for the observed alterations in relative rCBF. Kofke et al.  26demonstrated an absolute increase in brain activity in the temporal lobe after opioid administration, supporting our interpretation that the relative increases in rCBF reflect an absolute increase in rCBF. Finally, even if CBF was globally reduced, brain regions with a relative increase would still be at least “relatively resistant” to the global effects and vice versa .

According to a pilot study, 0.15 μg · kg⁻¹· min⁻¹remifentanil was used, which provides moderate intraoperative analgesia in a variety of clinical settings. 40,41Pharmacodynamic opioid effects observed in the present study (table 1) correspond with mood effects and side effects obtained in other studies of healthy volunteers using full and partial μ-agonist opioids, e.g. , fentanyl and morphine. 42,43The semi-randomized study protocol (see Methods) may affect our results but avoids carryover effects and was necessary because psychomotor effects still may be apparent 60 min after termination of remifentanil infusion. 44The dose–response curve of remifentanil beyond the given doses is of interest because high doses of opioids may not actually produce more analgesia but can produce epileptiform activity and neuropathologic lesions. 45Defining this therapeutic window may result in an efficient use of opioids and indicate the necessity of administration of other anesthetics or vasoactive drugs to reduce risks of overdosage and delayed recovery.

In conclusion, this study identifies regional specific changes of relative rCBF after intravenous administration of remifentanil. The study provides the first human data on the rCBF response pattern to different doses of an opioid. The rCBF responses to remifentanil share a common network with pain-processing brain areas. Further investigation is needed to verify if increasing doses of other μ-agonists act in the same way.

The authors thank the staff at the RDS 112 cyclotron unit for reliable supply of ¹⁵O-water, the technical PET staff, Ngoc Ngyuen (Nuklearmedizinische Klinik und Poliklinik) for careful review of the manuscript, and Doris Droese (Klinik für Anaesthesiologie) for excellent technical assistance (all Technische Universität München, Klinikum rechts der Isar, München, Germany). We are grateful to the volunteers whose participation made this study possible.