Auditory event-related potentials, bispectral index, and entropy for the discrimination of different levels of sedation in intensive care unit patients

Cerebral monitoring in surgical ICU patients

PURPOSE OF REVIEW

To give an overview of cerebral monitoring techniques for surgical ICU patients.

RECENT FINDINGS

As the burden of postsurgical neurological and neurocognitive complications becomes increasingly recognized, cerebral monitoring in the surgical ICU might gain a relevant role in detecting and possibly preventing adverse outcomes. However, identifying neurological alterations in surgical ICU patients, who are often sedated and mechanically ventilated, can be challenging. Various noninvasive and invasive techniques are available for cerebral monitoring, providing an assessment of cortical electrical activity, cerebral oxygenation, blood flow autoregulation, intracranial pressure, and cerebral metabolism. These techniques can be used for the diagnosis of subclinical seizures, the assessment of sedation depth and delirium, the detection of an impaired cerebral blood flow, and the diagnosis of neurosurgical complications.

SUMMARY

Cerebral monitoring can be a valuable tool in the early detection of adverse outcomes in surgical ICU patients, but the evidence is limited, and clear clinical indications are still lacking.

Isoflurane effects on the N1 and other long-latency auditory evoked potentials in Wistar rats

Long-latency auditory evoked potentials (LLAEPs) may help further advances in understanding consciousness under general anesthesia and promote more objective means of assessing sedation depth than conventional clinical signs. Among the LLAEP components, the auditory N1 shows promise as a measure of sedation depth and a marker of consciousness, but findings are so far inconclusive. Research with animals can help elucidate the effects of various anesthetics on the N1 and other LLAEPs, but investigations of LLAEPs under anesthesia in animals is lacking. To address this deficit, we examined the P1, N1, P2, and N2, along with their corresponding peak-to-peak complexes, in 10 Wistar rats anesthetized with 1.5-2 % isoflurane in pure oxygen and again after recovery. While under anesthesia, subdermal needle electrodes were inserted and secured for electroencephalographic (EEG) recordings. LLAEPs were assessed during a 20-min, passive, two-tone (500 ms inter-tone interval) paradigm with randomized short (1 s) and long (5 s) inter-pair intervals (IPIs). Overall, while the LLAEP peaks under isoflurane were less defined, they were not eliminated. The peak-to-peak amplitudes, particularly the P1-N1, were significantly smaller under isoflurane than during post-recovery. Our preliminary findings indicate that isoflurane produces global suppression across LLAEP components, presumably reflecting impaired integration of top-down and bottom-up attention and sensory systems under profound sedation with isoflurane.

Time delay of the qCON monitor and its performance during state transitions

We investigated the performance of the qCON index regarding its time delay for sudden changes in the anesthetic level as well as to separate responsiveness from unresponsiveness during loss and return of responsiveness (LOR and ROR). For evaluation of the time delay, we replayed relevant EEG episodes to the qCON to simulate sudden changes between the states (i) awake/sedation, (ii) adequate anesthesia, or (iii) suppression. We also replayed EEG from 40 patients during LOR and ROR to evaluate the qCON’s ability to separate responsiveness from unresponsiveness. The time delays depended on the type of transition. The delays for the important transition between awake/sedation and adequate anesthesia were 21(5) s from awake/sedation to adequate anesthesia and 26(5) s in the other direction. The performance of the qCON to separate responsiveness from unresponsiveness depended on signal quality, the investigation window, i.e. ± 30 s or ± 60 s around LOR/ROR, and the specific transition being tested. AUC was 0.63–0.90 for LOR and 0.61–0.79 for ROR. Time delay and performance during state transitions of the qCON were similar to other monitoring systems such as bispectral index. The better performance of qCON during LOR than ROR probably reflects the sudden change in EEG activity during LOR and the more heterogeneous EEG during ROR.

Mid-Latency Auditory Evoked Potentials Differentially Predict Sedation and Drug Level Under Opioid and Hypnotic Agents

Background: Auditory-evoked brain potentials (AEPs) are widely used to assess depth of the sedative component of general anesthesia. Depth of sedation as induced by hypnotic drugs (e.g., propofol) is characterized by a gradual decline of mid-latency cortical AEPs (10–50 ms). Using the decline of mid-latency AEPs as a reliable index for sedation requires its robustness against confounding pharmaceutical influences, e.g., analgesic opioids such as remifentanil. Critically, in this context the following two questions remained unresolved so far: First, it is unclear whether opioids directly affect mid-latency AEPs. Second, high doses of opioids decrease arousal, but it is unknown whether opioid-induced sedation is reflected by the diminution of mid-latency AEPs. We hypothesized that opioids affect mid-latency AEPs and that these effects rely on different mechanisms compared to hypnotic agents.

Methods: To address both questions, we performed a series of experiments under the participation of healthy human volunteers. We measured AEPs and quantified participants’ sedation state by a standardized rating scale during stepwise increase of different pharmaceutical agents (remifentanil, propofol or placebo).

Results: Our results revealed a decline of mid-latency AEPs during remifentanil medication. This decrease was predicted by drug dose, rather than sedation level. In contrast, attenuation of the mid-latency AEPs during propofol was predicted by sedation level and was not related to hypnotic drug dose. We did not find any drug-induced changes of brainstem AEPs (1–10 ms).

Conclusion: As remifentanil reduced mid-latency AEPs without inducing strong sedation levels, a decrease of this evoked brain component does not constitute an unequivocal index for the depth of sedation. These results challenge the use of mid-latency AEPs as a reliable marker of depth of the sedative component of anesthesia if hypnotic drugs are combined with opioids.

Feasibility of continuous sedation monitoring in critically ill intensive care unit patients using the NeuroSENSE WAVCNS index

Sedation in the intensive care unit (ICU) is challenging, as both over- and under-sedation are detrimental. Current methods of assessment, such as the Richmond Agitation Sedation Scale (RASS), are measured intermittently and rely on patients' behavioral response to stimulation, which may interrupt sleep/rest. A non-stimulating method for continuous sedation monitoring may be beneficial and allow more frequent assessment. Processed electroencephalography (EEG) monitors have not been routinely adopted in the ICU. The aim of this observational study was to assess the feasibility of using the NeuroSENSE™ monitor for EEG-based continuous sedation assessment. With ethical approval, ICU patients on continuous propofol sedation were recruited. Depth-of-hypnosis index (WAV_CNS) values were obtained from the NeuroSENSE. Bedside nurses, blinded to the NeuroSENSE, performed regular RASS assessments and maintained the sedation regimen as per standard of care. Participants were monitored throughout the duration of their propofol infusion, up to 24 h. Fifteen patients, with median [interquartile range] age of 57 [52-62.5] years were each monitored for a duration of 9.0 [5.7-20.1] h. Valid WAV_CNS values were obtained for 89% [66-99] of monitoring time and were widely distributed within and between individuals, with 6% [1-31] spent < 40 (very deep), and 3% [1-15] spent > 90 (awake). Significant EEG suppression was detected in 3/15 (20%) participants. Observed RASS matched RASS goals in 36/89 (40%) assessments. The WAV_CNS variability, and incidence of EEG suppression, highlight the limitations of using RASS as a standalone sedation measure, and suggests potential benefit of adjunct continuous brain monitoring.

Neurally adjusted ventilatory assist feasibility during anaesthesia: A randomised crossover study of two anaesthetics in a large animal model

BACKGROUND

Spontaneous breathing during mechanical ventilation improves gas exchange by redistribution of ventilation to dependent lung regions. Neurally adjusted ventilatory assist (NAVA) supports spontaneous breathing in proportion to the electrical activity of the diaphragm (EAdi). NAVA has never been used in the operating room and no studies have systematically addressed the influence of different anaesthetic drugs on EAdi.

OBJECTIVES

The aim of this study was to test the feasibility of NAVA under sedation and anaesthesia with two commonly used anaesthetics, sevoflurane and propofol, with and without remifentanil, and to study their effects on EAdi and breathing mechanics.

DESIGN

A crossover study with factorial design of NAVA during sedation and anaesthesia in pigs.

SETTING

University basic science laboratory in Uppsala, Sweden, from March 2009 to February 2011.

ANIMALS

Nine juvenile pigs were used for the experiment.

INTERVENTIONS

The lungs were ventilated using NAVA while the animals were sedated and anaesthetised with continuous low-dose ketamine combined with sevoflurane and propofol, with and without remifentanil.

MAIN OUTCOME MEASURES

During the last 5 min of each study period (total eight steps) EAdi, breathing pattern, blood gas analysis, neuromechanical efficiency (NME) and neuroventilatory efficiency (NVE) during NAVA were determined.

RESULTS

EAdi was preserved and normoventilation was reached with both sevoflurane and propofol during sedation as well as anaesthesia. Tidal volume (Vt) was significantly lower with sevoflurane anaesthesia than with propofol. NME was significantly higher with sevoflurane than with propofol during anaesthesia with and without remifentanil. NVE was significantly higher with sevoflurane than with propofol during sedation and anaesthesia.

CONCLUSION

NAVA is feasible during ketamine-propofol and ketamine-sevoflurane anaesthesia in pigs. Sevoflurane promotes lower Vt, and affects NME and NVE less than propofol. Our data warrant studies of NAVA in humans undergoing anaesthesia.

Entropy correlates with Richmond Agitation Sedation Scale in mechanically ventilated critically ill patients

Sedation is routinely used in intensive care units. However due to absence of objective scoring systems like Bispectral Index and entropy our ability to regulate the degree of sedation is limited. This deficiency is further highlighted by the fact that agitation scores used in intensive care units (ICU) have no role in paralyzed patients. The present study compares entropy as a sedation scoring modality with Richmond Agitation Sedation Scale (RASS) in mechanically ventilated, critically ill patients in an ICU. Twenty-seven, mechanically ventilated, critically ill patients of either sex, 16-65 years of age, were studied over a period of 24 h. They received a standard sedation regimen consisting of a bolus dose of propofol 0.5 mg/kg and fentanyl 1 lg/kg followed by infusions of propofol and fentanyl ranging from 1.5 to 5 mg/kg/h and 0.5 to 2.0 lg/kg/h, respectively. Clinically relevant values of RASS for optimal ICU sedation (between 0 and -3) in non-paralyzed patients were compared to corresponding entropy values, to find if any significant correlation exists between the two. These entropy measurements were obtained using the Datex-Ohmeda-M-EntropyTM module. This module is presently not approved by Food and Drug Administration (FDA) for monitoring sedation in ICU. A total of 527 readings were obtained. There was a statistically significant correlation between the state entropy (SE) and RASS [Spearman's rho/rs = 0.334, p\0.0001]; response entropy (RE) and RASS [Spearman's rho/rs = 0.341, p\0.0001]). For adequate sedation as judged by a RASS value of 0 to -3, the mean SE was 57.86 ± 16.50 and RE was 67.75 ± 15.65. The present study illustrates that entropy correlates with RASS (between scores 0 and -3) when assessing the level of sedation in mechanically ventilated critically ill patients.

Perioperative use of bispectral (BIS) monitor for a pressure ulcer patient with locked-in syndrome (LIS)

The bispectral (BIS) monitor uses brain electroencephalographic data to measure the depth of sedation and pharmacological response during anaesthetic procedures. In this case, the BIS monitor was used for another purpose, to demonstrate postoperatively to the nursing staff that a patient with history of locked-in syndrome (LIS), who underwent pressure ulcer debridement, had periods of wakefulness and apparent sensation, even with his eyes closed. Furthermore, as patients with LIS can feel pain, despite being unable to move, local block or general anaesthesia should be provided for sharp surgical debridement and other painful procedures. This use of the BIS has shown that as a general rule, the staff should treat the patient as though he might be awake and sensate even if he does not open his eyes or move his limbs. The goal of this study was to continuously monitor pain level and communicate these findings to the entire wound team, i.e. anaesthesiologists, surgeons and nurses.

Propofol versus flunitrazepam for inducing and maintaining sleep in postoperative ICU patients

Context:

Sleep deprivation is a common problem on intensive care units (ICUs) influencing not only cognition, but also cellular functions. An appropriate sleep-wake cycle should therefore be maintained to improve patients’ outcome. Multiple disruptive factors on ICUs necessitate the administration of sedating and sleep-promoting drugs for patients who are not analgo-sedated.

Aims:

The objective of the present study was to evaluate sleep quantity and sleep quality in ICU patients receiving either propofol or flunitrazepam.

Settings and Design:

Monocentric, randomized, double-blinded trial.

Materials and Methods:

A total of 66 ICU patients were enrolled in the study (flunitrazepam n = 32, propofol n = 34). Propofol was injected continuously (2 mg/kg/h), flunitrazepam as a bolus dose (0.015 mg/kg). Differences between groups were evaluated using a standardized sleep diary and the bispectral index (BIS).

Statistical Analysis Used:

Group comparisons were performed by Mann-Whitney U-Test. P < 0.05 was considered to be statistically significant.

Results:

Sleep quality and the frequency of awakenings were significantly better in the propofol group (Pg). In the same group lower BIS values were recorded (median BIS propofol 74.05, flunitrazepam 78.7 [P = 0.016]). BIS values had to be classified predominantly to slow-wave sleep under propofol and light sleep after administration of flunitrazepam. Sleep quality improved in the Pg with decreasing frequency of awakenings and in the flunitrazepam group with increasing sleep duration.

Conclusions:

Continuous low-dose injection of propofol for promoting and maintaining night sleep in ICU patients who are not analgo-sedated was superior to flunitrazepam regarding sleep quality and sleep structure.

An evaluation of the validity and potential utility of facial electromyelogram Responsiveness Index for sedation monitoring in critically ill patients

PURPOSE

The purpose of this study is to explore the validity of a novel sedation monitoring technology based on facial electromyelography (EMG) in sedated critically ill patients.

MATERIALS AND METHODS

The Responsiveness Index (RI) integrates the preceding 60 minutes of facial EMG data. An existing data set was used to derive traffic light cut-offs for low (red), intermediate (amber), and higher (green) states of patient arousal. The validity of these was prospectively evaluated in 30 sedated critically ill patients against hourly Richmond Agitation Sedation Scale (RASS) assessments with concealment of RI data from clinical staff.

RESULTS

With derivation data, an RI less than or equal to 35 had best discrimination for a Ramsay score of 5/6 (sensitivity, 90%; specificity, 79%). For traffic lights, we chose RI less than or equal to 20 as red, 20 to 40 as amber, and more than 40 as green. In the prospective study, RI values were red/amber for 76% of RASS -5/-4 assessments, but RI varied dynamically over time in many patients, and discordance with RASS may have resulted from the use of 1 hour of data for RI calculations. We also noted that red/amber values resulted from sleep, encephalopathy, and low levels of stimulation.

CONCLUSIONS

Responsiveness Index is not directly comparable with clinical sedation scores but is a potential continuous alert to possible deep sedation in critically ill patients.

Effect of sedation level on the prevalence of delirium when assessed with CAM-ICU and ICDSC

PURPOSE

We hypothesized that reduced arousability (Richmond Agitation Sedation Scale, RASS, scores -2 to -3) for any reason during delirium assessment increases the apparent prevalence of delirium in intensive care patients. To test this hypothesis, we assessed delirium using the Confusion Assessment Method for the Intensive Care Unit (CAM-ICU) and Intensive Care Delirium Screening Checklist (ICDSC) in intensive care patients during sedation stops, and related the findings to the level of sedation, as assessed with RASS score.

METHODS

We assessed delirium in 80 patients with ICU stay longer than 48 h using CAM-ICU and ICDSC during daily sedation stops. Sedation was assessed using RASS. The effect of including patients with a RASS of -2 and -3 during sedation stop ("light to moderate sedation", eye contact less than 10 s or not at all, respectively) on prevalence of delirium was analyzed.

RESULTS

A total of 467 patient days were assessed. The proportion of CAM-ICU-positive evaluations decreased from 53 to 31 % (p < 0.001) if assessments from patients at RASS -2/-3 (22 % of all assessments) were excluded. Similarly, the number of positive ICDSC results decreased from 51 to 29 % (p < 0.001).

CONCLUSIONS

Sedation per se can result in positive items of both CAM-ICU and ICDSC, and therefore in a diagnosis of delirium. Consequently, apparent prevalence of delirium is dependent on how a depressed level of consciousness after sedation stop is interpreted (delirium vs persisting sedation). We suggest that any reports on delirium using these assessment tools should be stratified for a sedation score during the assessment.

Neurophysiological testing in neurocritical care

PURPOSE OF REVIEW

To summarize a consensus of European authorities about the applications of clinical neurophysiology in the ICU and, particularly, for a clinically useful management of individual patients.

RECENT FINDINGS

Clinical neurophysiology is useful for diagnosis (epilepsy, brain death, and neuromuscular disorders), prognosis (anoxic ischemic encephalopathy, head trauma, and neurological disturbances of metabolic and toxic origin), and follow-up. The prognostic significance of each test varies as a function of coma etiology. A distinction should be made between tests whose abnormalities are indicative of a poor prognosis (bilateral absence of N20 in anoxic coma, abnormalities suggesting pontine involvement in head trauma) and those whose relative normalcy constitutes an argument for a good prognosis (integrity of brainstem conductions in head trauma, presence of cognitive evoked potentials - mismatch negativity, P300 - irrespective of coma etiology).The highlights of the recent literature mainly concern continuous neuromonitoring for early detection of nonconvulsive seizures, both in adult and neonatal ICU, brain entry into the ischemic penumbra zone, and neuronal functional consequences of intracranial hypertension.

SUMMARY

The domain of clinical neurophysiology is similar to that of clinical examination and complementary to that of imaging techniques. It substantially improves the individual management of ICU patients.

Influence of progressive hemorrhage and subsequent cardiopulmonary resuscitation on the bispectral index during isoflurane anesthesia in a swine model

BACKGROUND

The bispectral index for measurement of anesthetic depth may be modified by extreme hypotension during hemorrhagic shock. In this study, the influence of progressive hemorrhage and subsequent cardiopulmonary resuscitation on the bispectral index was investigated under controlled anesthetic depth.

METHODS

Fifteen swine were anesthetized through inhalation of isoflurane under bispectral index monitoring. Hemorrhagic shock was induced using a stepwise hemorrhage model in which 20%, 10%, and 10% of estimated blood volume were removed over three 30-minute periods and then 5% was removed every 30 minutes until the mean arterial pressure was less than 10 mm Hg. After reaching this criterion, chest compression with 0.2-mg/kg epinephrine and hydroxyethyl starch infusion was performed for 20 minutes or until the mean arterial pressure exceeded 50 mm Hg. The pharmacodynamics of the isoflurane effect was examined before hemorrhage, after 40% bleeding, and after resuscitation.

RESULTS

A mean (SD) volume of 836 (78) mL of blood was drained before resuscitation. The bispectral index suddenly decreased at a mean (SD) arterial pressure of 22 (3) mm Hg and showed isoelectric activity in most animals before resuscitation. Eight pigs were resuscitated, and the bispectral index recovered during a range of periods after recovery of the mean arterial pressure. The pharmacodynamic effect of isoflurane did not change after 40% bleeding but increased after resuscitation, with the alteration correlated with the time for resuscitation.

CONCLUSION

In hemorrhagic shock, the bispectral index merely reflects the anesthetic depth until development of lethal hypotension at which brain electrical activity cannot be sustained. After recovery from depression, the potency of isoflurane can increase depending on the cerebral hypoperfusion time. The increased bispectral index for anesthetics after resuscitation might reflect the degree of cerebral damage due to hypoperfusion.

The utility of bispectral index monitoring for sedated patients treated with low-dose remifentanil

The aim of the present study was to determine the effect of low-dose remifentanil on the monitoring quality of the Bispectral index for mechanically ventilated patients. Twelve patients who underwent elective surgery and required mechanical ventilation post-operatively were enrolled in this study with written informed consent. Eligible patients were divided into two groups. Patients in the remifentanil group received low-dose remifentanil (0.05-0.125 μg/kg/min) and propofol (1-3 mg/kg/h). Patients in the control group received propofol (1-3 mg/kg/h). Levels of sedation were evaluated by both the Richmond Agitation Sedation Scale (RASS) and BIS monitor (A2000-XP, version 4.0, Aspect Medical Systems, Newton, USA). Monitoring quality was assessed by a correlation between RASS and BIS values. These values were assessed by single regression analysis and a P value of <0.05 was considered significant. There was a significant correlation between RASS and BIS values (P = 3 × 10(-12), R (2) = 0.67) in the remifentanil group, but not in the control group (P = 0.50, R (2) = 0.057). The administration of low-dose remifentanil makes BIS a more precise tool for sedated patients under mechanical ventilation in the ICU.

Electroacupuncture reduces the dose of midazolam monitored by the bispectral index in critically ill patients with mechanical ventilation: an exploratory study

OBJECTIVE

Electroacupuncture, a modern variation on a traditional Chinese treatment, might be useful for sedation and analgesia. This study aims to investigate whether electroacupuncture can modify the dose of midazolam monitored by the bispectral index (BIS) in critically ill patients with mechanical ventilation.

METHODS

Orotracheally intubated patients undergoing mechanical ventilation were randomly assigned into three groups (groups A, B and C). All patients were given an intravenous infusion of midazolam. Patients in group A received no additional treatment. Patients in group B were given acupuncture without electrical stimulation at acupuncture points GV24 and EX-HN3 (Yintang) for 6 h simultaneously, and patients in group C were given electroacupuncture to the same points as in group B.

RESULTS

Maintaining the BIS between 60 and 80, the hourly mean one dose of midazolam within the first 6 h after sedation in group C was 0.05 (±0.02 mg/kg per hour), which was significantly lower than both group A (0.08 ± 0.03 mg/kg per hour, p<0.001) and group B (0.07 ± 0.01 mg/kg per hour, p<0.021). The doses in groups A and B showed no significant difference. Between-group comparison analysis of hepatic and renal function and severe adverse reactions all showed no significant difference between the three groups.

CONCLUSIONS

Electroacupuncture appears to reduce markedly the dose of sedative drug required in critically ill patients with mechanical ventilation monitored by BIS, without any obvious severe adverse action, and larger studies to confirm the effect are justified.

Bispectral index to monitor propofol sedation in trauma patients

BACKGROUND

This study tested the hypothesis that the bispectral index (BIS) is reliable relative to clinical judgment for estimating sedation level during daily propofol spontaneous awakening trials (SATs) in trauma patients.

METHODS

This was a prospective observational trial with waiver of consent conducted in the intensive care unit of Level I trauma center in 94 mechanically ventilated trauma patients sedated with propofol alone or in combination with midazolam. BIS, Richmond Agitation Sedation Scale (RASS), electromyography, and heart rate variability, as a test of autonomic function, were measured for 45 minutes during daily SATs. Data were evaluated with analysis of variance, linear regression, and nonparametric tests.

RESULTS

The BIS wave form coincided almost exactly with propofol on/off. Steady-state BIS correlated with RASS (p < 0.0001) and with propofol dose (p < 0.0001), but the strengths of association were relatively low (all r(2) < 0.5). BIS wave form was not altered by age, heart rate, or heart rate variability and was similar with propofol alone or propofol plus midazolam, but the presence of brain injury or the use of paralytics shifted the curve downward (both p < 0.001). The overall test characteristics for BIS versus RASS without neuromuscular blockade were sensitivity: 90% versus 77% (p = 0.034); specificity: 90% versus 75% (p = 0.021); positive predictive value: 90% versus 76% (p = 0.021), and negative predictive value: 90% versus 76% (p = 0.021).

CONCLUSIONS

In the first trial in trauma patients and largest trial in any surgical population, the (1) BIS was reliable and has advantages over RASS of being continuous and objective, at least during a propofol SAT; (2) BIS interpretation remains somewhat subjective in patients receiving paralytic agents or with traumatic brain injury.

Administration and monitoring of intravenous anesthetics

PURPOSE OF REVIEW

The importance of accuracy in controlling the dose-response relation for intravenous anesthetics is directly related to the importance of optimizing the efficacy and quality of anesthesia while minimizing adverse drug effects. Therefore, it is important to measure and control all steps of the pharmacokinetic and dynamic cascade influencing this dose-effect relationship.

RECENT FINDINGS

The ultimate goal when administering a particular dose of a drug is to obtain the desired clinical effect, taking into account interindividual pharmacokinetic and dynamic variability. Recent findings suggest that effect compartment-controlled target-controlled infusion systems and measurement of (surrogate) clinical drug effects might be helpful in an attempt to optimize the administration intravenous anesthetics and opioids. Additionally, recent findings suggest that the pharmacokinetic and dynamic interaction between anesthetics and opioids is important and such be taking into account when optimizing drug administration. Hereby, feedback control technology and advisory displays depicting these interactions have been studied.

SUMMARY

Anesthetic drug administration might be optimized by applying knowledge from clinical pharmacokinetics and dynamics.