Effect of a fluid challenge on the Surgical Pleth Index during stable propofol-remifentanil anaesthesia.

BACKGROUND: The Surgical Pleth Index (SPI), derived from pulse amplitude and heartbeat interval, is proposed to monitor anti-nociception during anaesthesia. Its response to noxious stimulation can be affected by the intravascular volume status. This study investigated the effect of a fluid challenge (FC) on SPI during steady-state conditions. METHODS: After Institutional Review Board approval, 33 consenting patients undergoing neurosurgery received a 4 ml/kg starch FC over less than 5 min under stable surgical stimulation conditions and stable propofol (Ce(PPF) ) and remifentanil (Ce(REMI) ) effect-site concentrations as estimated by target-controlled infusion systems. Intravascular volume status was assessed using the Delta Down (DD). We looked at the SPI response to FC according to DD, Ce(PPF) , and Ce(REMI) . RESULTS: Following FC, SPI did not change in 16, increased in 12, and decreased in 3 patients. Ce(REMI) poorly affected the SPI response to FC. In normovolaemic patients, the probability of an SPI change after FC was low under common Ce(PPF) (0.9 to 3.9 µg/ml). A decrease in SPI was more probable with worsening hypovolaemia and lowering Ce(PPF) , while an increase in SPI was more probable with increasing Ce(PPF) . SPI changes were only attributable to modifications in pulse wave amplitude and not in heart rate. CONCLUSIONS: During stable anaesthesia and surgery, SPI may change in response to FC. The effect of FC on SPI is influenced by volaemia and Ce(PPF) through pulse wave amplitude modifications. These situations may confound the interpretation of SPI as a surrogate measure of the nociception-anti-nociception balance.

Comparison of the surgical Pleth Index™ with haemodynamic variables to assess nociception-anti-nociception balance during general anaesthesia.

BACKGROUND: The Surgical Pleth Index (SPI) is proposed as a means to assess the balance between noxious stimulation and the anti-nociceptive effects of anaesthesia. In this study, we compared SPI, mean arterial pressure (MAP), and heart rate (HR) as a means of assessing this balance. METHODS: We studied a standard stimulus [head-holder insertion (HHI)] and varying remifentanil concentrations (CeREMI) in a group of patients undergoing neurosurgery. Patients receiving target-controlled infusions were randomly assigned to one of the three CeREMI (2, 4, or 6 ng m⁻¹), whereas propofol target was fixed at 3 µg ml⁻¹. Steady state for both targets was achieved before HHI. Intravascular volume status (IVS) was evaluated using respiratory variations in arterial pressure. Prediction probability (Pk) and ordinal regression were used to assess SPI, MAP, and HR performance at indicating CeREMI, and the influence of IVS and chronic treatment for high arterial pressure, as possible confounding factors. RESULTS: The maximum SPI, MAP, or HR observed after HHI correctly indicated CeREMI in one of the two patients [accurate prediction rate (APR)=0.5]. When IVS and chronic treatment for high arterial pressure were taken into account, the APR was 0.6 for each individual variable and 0.8 when all of them predicted the same CeREMI. That increase in APR paralleled an increase in Pk from 0.63 to 0.89. CONCLUSIONS: SPI, HR, and MAP are of comparable value at gauging noxious stimulation-CeREMI balance. Their interpretation is improved by taking account of IVS, treatment for chronic high arterial pressure, and concordance between their predictions.

Responsiveness of the frontal EMG for monitoring the sedation state of critically ill patients.

BACKGROUND: Excessive sedation is associated with adverse patient outcomes during critical illness, and a validated monitoring technology could improve care. We developed a novel method, the responsiveness index (RI) of the frontal EMG. We compared RI data with Ramsay clinical sedation assessments in general and cardiac intensive care unit (ICU) patients. METHODS: We developed the algorithm by iterative analysis of detailed observational data in 30 medical-surgical ICU patients and described its performance in this cohort and 15 patients recovering from scheduled cardiac surgery. Continuous EMG data were collected via frontal electrodes and RI data compared with modified Ramsay sedation state assessments recorded regularly by a blinded trained observer. RI performance was compared with Entropy™ across Ramsay categories to assess validity. RESULTS: RI correlated well with the Ramsay category, especially for the cardiac surgery cohort (general ICU patients ρ=0.55; cardiac surgery patients ρ=0.85, both P<0.0001). Discrimination across all Ramsay categories was reasonable in the general ICU patient cohort [P(K)=0.74 (sem 0.02)] and excellent in the cardiac surgery cohort [P(K)=0.92 (0.02)]. Discrimination between 'lighter' vs 'deeper' (Ramsay 1-3 vs 4-6) was good for general ICU patients [P(K)=0.80 (0.02)] and excellent for cardiac surgery patients [P(K)=0.96 (0.02)]. Performance was significantly better than Entropy™. Examination of individual cases suggested good face validity. CONCLUSIONS: RI of the frontal EMG has promise as a continuous sedation state monitor in critically ill patients. Further investigation to determine its utility in ICU decision-making is warranted.

Autonomic nervous system state: the effect of general anaesthesia and bilateral tonsillectomy after unilateral infiltration of lidocaine.

BACKGROUND: Autonomic nervous system (ANS) sensitively responds to intraoperative stress. Several indices characterizing the state and responses of autonomic signs to nociceptive stimuli have been introduced. This study evaluated the behaviour of ANS descriptors after induction, before and during tracheal intubation, and during bilateral tonsillectomies after random and blinded unilateral infiltration of lidocaine 1% until emergence from anaesthesia. METHODS: Twelve patients undergoing bilateral tonsillectomy were anaesthetized with fentanyl and propofol (induction) and sevoflurane (maintenance). All patients were monitored throughout anaesthesia for middle finger temperature, non-invasive arterial pressure, heart rate (HR) and pulse rate (PR), state entropy (SE) and response entropy (RE), and surgical pleth index (SPI). New parameters complementing the above and characterizing the ANS state (ANSS) and responses are pulse-to-pulse interval (PPI), pulse plethysmographic amplitude (PPGA), ANSS, and an index based on maximal ANSS for the subject (ANSSI). Serial data were stored as 10 s averages into a laptop computer. RESULTS: Anaesthesia induction was associated with an increase in finger temperature to >30 degrees C within 10 min, whereas PPGA increased to their maximum levels within 5 min. Laryngoscopy and intubation were associated with transient autonomic responses in most patients. All autonomic signs indicated statistically significant sympathetic activation during saline-infiltrated tonsillectomies when compared with lidocaine-infiltrated sides (P<0001). Hypnotic measures (SE and RE) and finger temperatures did not differ between the sides. CONCLUSIONS: HR, PPI, PPGA, ANSS, ANSSI, SPI, and RE-SE detect autonomic responses to nociceptive stimuli and differentiate between tonsillectomies on locally anaesthetized tonsils from controls.

Audiovisual integration of letters in the human brain.

Letters of the alphabet have auditory (phonemic) and visual (graphemic) qualities. To investigate the neural representations of such audiovisual objects, we recorded neuromagnetic cortical responses to auditorily, visually, and audiovisually presented single letters. The auditory and visual brain activations first converged around 225 ms after stimulus onset and then interacted predominantly in the right temporo-occipito-parietal junction (280345 ms) and the left (380-540 ms) and right (450-535 ms) superior temporal sulci. These multisensory brain areas, playing a role in audiovisual integration of phonemes and graphemes, participate in the neural network supporting the supramodal concept of a "letter." The dynamics of these functions bring new insight into the interplay between sensory and association cortices during object recognition.

Measurement of surgical stress in anaesthetized children.

BACKGROUND: The surgical stress index (SSI), derived from a combination of heart rate (HR) and photoplethysmographic amplitude (PPGA) time series, is a novel method for continuous monitoring of intraoperative stress and has been validated in adults. The applicability of SSI and its constituents to monitoring children has not been previously evaluated. METHODS: In this controlled trial, 22 anaesthetized patients, aged 4-17 yr, undergoing strabismus surgery were randomized into two groups, Group LL and Group BSS. Patients in Group LL received topical conjunctival anaesthesia with a 1:1 mixture of lidocaine 2% and levobupivacaine 0.75%, and patients in Group BSS received balanced salt solution. RESULTS: Endotracheal intubation (n=22) increased median (range) SSI from 39.2 (22.6-55.6) to 53.6 (35.8-63.3) (P<0.001), decreased PPGA from 5.62 (2.79-9.69) to 5.27 (2.59-7.54)% (P=0.001), and increased the difference of response entropy (RE) and state entropy (SE) of frontal biopotentials (RE-SE) from 3.1 (0.06-9.1) to 5.7 (0.6-9.4) (P=0.01). Conventional haemodynamic variables also increased, median (range) HR from 72.9 (56.7-113.8) to 84.2 (60.4-124.8) beats min(-1) (P<0.001), and systolic non-invasive arterial pressure (S-NIBP) from 87 (78-143) to 103 (79-125) (P=0.007). When 3 min baseline before surgery was compared with 12 min of surgery, median (range) SSI increased from 43.3 (31.2-58.0) to 49.9 (39.3-57.2) (P=0.042) vs from 46.6 (26.8-57.8) to 52.1 (31.7-60.1) (P=0.024) and PPGA decreased from 6.60 (3.10-8.24) to 5.80 (3.03-7.65)% (P<0.001) vs from 5.51 (3.25-9.84) to 5.06 (3.08-8.99)% (P=0.042), in Groups LL and BSS, respectively, but SSI or other indicators did not differ significantly between the groups. CONCLUSIONS: SSI, PPGA, HR, NIBP, RE, and RE-SE detect autonomic responses to nociceptive stimuli in anaesthetized children undergoing strabismus surgery.

Surgical stress index as a measure of nociception/antinociception balance during general anesthesia.

BACKGROUND: No validated monitoring method is available for evaluating the nociception/antinociception balance. We assessed the surgical stress index (SSI), computed from finger photoplethysmographic waveform amplitudes and pulse-to-pulse intervals, in patients undergoing shoulder surgery under general anesthesia (GA) and interscalene plexus block and in patients with GA only. METHODS: In this prospective, randomized study in 26 patients, increased blood pressure (BP) or heart rate, movement, and coughing were considered to be signs of intraoperative nociception and were treated with alfentanil. GA was maintained with desflurane aiming at a State Entropy level of 50. Photoplethysmographic waveforms were collected from the contra-lateral arm to the surgery and SSI values from 0 (no surgical stress) to 100 (maximal surgical stress) were calculated off-line. RESULTS: Two minutes after skin incision, SSI had not increased in the plexus group and was lower in the plexus group (38 +/- 13) compared with the controls (58 +/- 13, P<0.005). Among the controls, 1 min before alfentanil administration, the SSI value was higher than during periods of adequate antinociception, 59 +/- 11 vs. 39 +/- 12 (P<0.01). The total cumulative need for alfentanil was higher in controls (2.7 +/- 1.2 mg) compared with the plexus group (1.6 +/- 0.5 mg; P=0.008). Tetanic stimulation to the ulnar region of the hand increased SSI significantly only among the patients with plexus block not covering the site of the stimulation. CONCLUSION: SSI values were lower in patients with plexus block covering the sites of nociceptive stimuli. In detecting nociceptive stimuli, SSI had better performance than heart rate, BP, or response entropy.

Non-impaired auditory phase locking in dyslexic adults.

Dyslexic adults have profound difficulties in discriminating rapidly presented sound sequences. To test whether these deficits might be caused by impaired neuronal phase locking to the envelopes of the sound stimuli, 20 normal-reading and 13 dyslexic adults discriminated pitches of pure tones at approximately 1 kHz (producing spectral pitch due to place coding in the cochlea) and of approximately 80 Hz amplitude modulations of white noise (producing periodicity pitch based on temporal information only). We proposed that a specific deficit in phase locking would result in a worse ability to discriminate periodicity than spectral pitch. The dyslexics were significantly less accurate than the control subjects in discriminating both spectral and periodicity pitch stimuli but their performance was not disproportionally worse in the periodicity pitch task. Thus it seems that impaired neuronal phase-locking cannot explain the problems dyslexics face in processing of rapid sound sequences.

Apparent asynchrony between interictal electric and magnetic spikes.

We recorded simultaneous multichannel electroencephalogram (EEG) and magnetoencephalogram (MEG) in four children with partial epilepsy. Sources of averaged spikes were modelled with current dipoles. Of 10 spike averages obtained, three peaked simultaneously in MEG and EEG, and in seven averages, the MEG peak preceded the main EEG peak by 9-40 ms. A small positive early EEG signal coincided with the MEG peak in six asynchronous spikes. The simultaneous MEG and EEG spikes originated within 5-23 mm, while sources of asynchronous peaks were 12-67 mm apart. We conclude that non-identical neurone currents underlie the MEG and EEG signals, and emphasize the importance of modelling early phases of EEG spikes when localizing interictal epileptic zones.

Prolonged attentional dwell time in dyslexic adults.

Dyslexic adults have been shown to be slow in processing rapid sequences of stimuli in all sensory modalities. We now demonstrate, by means of an attentional blink task, that the attentional dwell time is prolonged by approximately 30% in dyslexic adults compared with normal readers. Thus a target captures attentional resources for considerably longer time in dyslexics than control subjects. The observed prolongation could significantly contribute to the sluggish temporal processing of dyslexic adults.

Global optimization in the localization of neuromagnetic sources.

The locations of active brain areas can be estimated from the magnetic field produced by the neural current sources. In many cases, the actual current distribution can be modeled with a set of stationary current dipoles with time-varying amplitudes. This work studies global optimization methods that find the minimum of the least-squares error function of the current dipole estimation problem. Three different global optimization methods were investigated: clustering method, simulated annealing, and genetic algorithms. In simulation studies, the genetic algorithm was the most effective method. The methods were also applied to analysis of actual measurement data.

Performance of Surgical Stress Index during Sevoflurane-Fentanyl and Isoflurane-Fentanyl Anesthesia.

The performance of recently introduced Surgical Stress Index (SSI), based on heart rate and photoplethysmography, was estimated during sevoflurane-fentanyl and isoflurane-fentanyl anesthesia during surgical procedures. Forty ASA I-III patients were enrolled. Anesthesia was induced with fentanyl 2 mug kg(-1) and thiopentone 3-5 mg kg(-1). Tracheal intubation was performed 5 minutes after fentanyl bolus. Patients were randomly allocated to receive sevoflurane (n = 20) or isoflurane (n = 20) in 30% oxygen/air. State entropy was kept at 40-60, target being 50. During surgery, fentanyl boluses 1.5 mug kg(-1) were given at 30-40-minute intervals. SSI increased significantly after intubation. During surgery, the decrease of SSI after fentanyl boluses was similar in sevoflurane and isoflurane groups but SSI values were higher in sevoflurane than in isoflurane group. Tracheal intubation, skin incision, and surgical stimuli increased SSI from baseline, indicating that nociceptive stimuli increase SSI. Fentanyl boluses during surgery decreased SSI, indicating that increasing analgesia decreases SSI.

Surgical stress index reflects surgical stress in gynaecological laparoscopic day-case surgery.

BACKGROUND: Monitoring of analgesia remains a challenge during general anaesthesia. The surgical stress index (SSI) is derived from the photoplethysmographic waveform amplitude and the heart beat-to-beat interval. We evaluated the ability of SSI to measure surgical stress in patients undergoing gynaecological laparoscopy. Our hypothesis was that while keeping State Entropy (SE) at a predetermined level, SSI would be higher in patients receiving a beta-blocking agent (esmolol) than in those receiving an opioid (remifentanil) during laparoscopy. METHODS: Thirty women undergoing gynaecological laparoscopy were assigned randomly to receive esmolol (n = 15) or remifentanil (n = 15). Anaesthesia was induced with propofol and fentanyl and maintained with desflurane and nitrous oxide 50% in oxygen to keep SE at 50(5). The infusion of esmolol or remifentanil was started before laparoscopy and adjusted to keep the systolic blood pressure at -20 to +10% from the preoperative value. RESULTS: During the fentanyl phase, before surgery, both groups behaved similarly, with an increase in SSI after intubation. In the patients receiving esmolol, the SSI reacted to the initial incision (P < 0.05), and remained high after trocar insertion (P < 0.05). In patients receiving remifentanil, it did not react to the initial incision, but increased after trocar insertion (P < 0.05), and it remained lower both after incision (P < 0.05) and after trocar insertion (P < 0.05). CONCLUSION: SSI was higher in patients receiving esmolol. The index seems to reflect the level of surgical stress and may help guide the use of opioids during general anaesthesia.

Changes in a surgical stress index in response to standardized pain stimuli during propofol-remifentanil infusion.

BACKGROUND: The surgical stress index (SSI) is based on a sum of the normalized pulse beat interval (PBI) and the pulse wave amplitude (PPGA) time series of the photoplethysmography. As a measure of the nociception-anti-nociception balance in response to a standardized pain stimulus, SSI was compared with EEG changes in state and response entropy (SE and RE), PPGA, and heart rate (HR) during various targeted pseudo-steady-state concentrations of propofol and remifentanil. METHODS: Forty ASA I patients were allocated to one of the four groups to receive a remifentanil step-up/-down effect-compartment target-controlled infusion (Ce(remi)) of 0, 2, 6, 2, 0 ng ml(-1), or 6, 2, 0, 2, 6 ng ml(-1), and an effect-compartment target-controlled propofol infusion (Ce(prop)) to keep the SE between 30 and 50 or 15 and 30, respectively. At each steady-state Ce(remi), maximum change in SSI, SE, RE, PPGA, and HR after a noxious stimulus was compared with the baseline value. A correlation and prediction probability (P(K)) with Ce(prop) and Ce(remi) was measured. RESULTS: Static and dynamic values of SSI correlated to Ce(remi) better than SE, RE, HR, and PPGA. SSI was independent of Ce(prop), in contrast to SE and RE. The P(K) for Ce(remi) both before and during a noxious stimulus was better with SSI. CONCLUSIONS: SSI appeared to be a better measure of nociception-anti-nociception balance than SE, RE, HR, or PPGA.

Assessment of surgical stress during general anaesthesia.

BACKGROUND: Inadequate analgesia during general anaesthesia may present as undesirable haemodynamic responses. No objective measures of the adequacy of analgesia exist. We aimed at developing a simple numerical measure of the level of surgical stress in an anaesthetized patient. METHODS: Sixty and 12 female patients were included in the development and validation data sets, respectively. All patients had elective surgery with propofol-remifentanil target controlled anaesthesia. Finger photoplethysmography and electrocardiography waveforms were recorded throughout anaesthesia and various waveform parameters were extracted off-line. Total surgical stress (TSS) for a patient was estimated based on stimulus intensity and remifentanil concentration. The surgical stress index (SSI) was developed to correlate with the TSS estimate in the development data set. The performance of SSI was validated within the validation data set during and before surgery, especially at skin incision and during changes of the predicted remifentanil effect-site concentration. RESULTS: SSI was computed as a combination of normalized heart beat interval (HBI(norm)) and plethysmographic pulse wave amplitude (PPGA(norm)): SSI = 100-(0.7*PPGA(norm)+0.3*HBI(norm)). SSI increased at skin incision and stayed higher during surgery than before surgery; SSI responded to remifentanil concentration changes and was higher at the lower concentrations of remifentanil. CONCLUSIONS: SSI reacts to surgical nociceptive stimuli and analgesic drug concentration changes during propofol-remifentanil anaesthesia. Further validation studies of SSI are needed to elucidate its usefulness during other anaesthetic and surgical conditions.

Foveal attention modulates responses to peripheral stimuli.

When attending to a visual object, peripheral stimuli must be monitored for appropriate redirection of attention and gaze. Earlier work has revealed precentral and posterior parietal activation when attention has been directed to peripheral vision. We wanted to find out whether similar cortical areas are active when stimuli are presented in nonattended regions of the visual field. The timing and distribution of neuromagnetic responses to a peripheral luminance stimulus were studied in human subjects with and without attention to fixation. Cortical current distribution was analyzed with a minimum L1-norm estimate. Attention enhanced responses 100-160 ms after the stimulus onset in the right precentral cortex, close to the known location of the right frontal eye field. In subjects whose right precentral region was not distinctly active before 160 ms, focused attention commonly enhanced right inferior parietal responses between 180 and 240 ms, whereas in the subjects with clear earlier precentral response no parietal enhancement was detected. In control studies both attended and nonattended stimuli in the peripheral visual field evoked the right precentral response, whereas during auditory attention the visual stimuli failed to evoke such response. These results show that during focused visual attention the right precentral cortex is sensitive to stimuli in all parts of the visual field. A rapid response suggests bypassing of elaborate analysis of stimulus features, possibly to encode target location for a saccade or redirection of attention. In addition, load for frontal and parietal nodi of the attentional network seem to vary between individuals.

Detecting and correcting for head movements in neuromagnetic measurements.

Head movements during neuromagnetic measurements may cause a significant error in the estimated locations of active brain areas. In this study we present a fast method for measuring the head position while neuromagnetic data are acquired. We then compare two methods for removing the effect of the movements in neuromagnetic source estimation and magnetic field alignment: minimum-norm estimate alignment and forward calculation correction. Simulations and a test measurement show that the proposed head position measurement method works with millimeter precision and that incorporating the effect of the head movements into the magnetic field forward calculations is an efficient and a sufficiently accurate method for correcting for the head movements.

Auditory stream segregation in dyslexic adults.

Developmental dyslexia is often associated with problems in phonological processing based on, or accompanied by, deficits in the perception of rapid auditory changes. Thirteen dyslexic adults and 18 control subjects were tested on sequences of alternating tones of high (1000 Hz) and low (400 Hz) pitch, which at short stimulus onset asynchronies (SOAs) led to perceptual separation of the sound sequence into high- and low-pitched streams. The control subjects perceived the tone sequence as connected down to SOAs of 130 ms, with segregation of the streams at shorter SOAs; in dyslexic subjects the segregation occurred already at 210 ms. Auditory stream segregation has previously been shown to impair the detection of phoneme order in segments of speech sounds. The observed aberrant segregation of sound streams in dyslexic subjects might thus contribute to their difficulties in achieving awareness of phonemes or phoneme order and in the acquisition of literacy.

Visualization of magnetoencephalographic data using minimum current estimates.

The locations of active brain areas can be estimated from the magnetic field the neural current sources produce. In this work we study a visualization method of magnetoencephalographic data that is based on minimum[symbol: see text] (1)-norm estimates. The method can represent several local or distributed sources and does not need explicit a priori information. We evaluated the performance of the method using simulation studies. In a situation resembling typical magnetoencephalographic measurement, the mean estimated source strength exceeded baseline level up to 2 cm from the simulated point-like source. The method can also visualize several sources, activated simultaneously or in a sequence, which we demonstrated by analyzing magnetic responses associated with sensory stimulation and a picture naming task.

Cortical representation of sign language: comparison of deaf signers and hearing non-signers.

Numerous studies have demonstrated activation of the classical left-hemisphere language areas when native signers process sign language. More recently, specific sign language-related processing has been suggested to occur in homologous areas of the right hemisphere as well. We now show that these cortical areas are also activated in hearing non-signers during passive viewing of signs that for them are linguistically meaningless. Neuromagnetic activity was stronger in deaf signers than in hearing non-signers in the region of the right superior temporal sulcus and the left dorsal premotor cortex, probably reflecting familiarity and linguistic meaningfulness of the observed movement sequences. In contrast, the right superior parietal lobule, the mesial parieto-occipital region, and the mesial paracentral lobule were more strongly activated in hearing non-signers, apparently reflecting active visuomotor encoding of complex unfamiliar movement sequences.