Effect-site modelling of propofol using auditory evoked potentials

Critical view of the effect site modelling of propofol

Target controlled infusion (TCI) of Propofol has been the subject of discussion during its 20 years of use, including the validity of the models that represent the course of the effect, such as: Are the different EEG indexes representative of the effect? Is the reactivity of the EEG index used to build models comparable to each other? What is the real reacting time of each monitor? Is the ke0 influenced by the infusion speed? Is the ke0 or the time to peak effect affected by age? How valid are the current Emax models? Are the induction and wakening simple mirror phenomenon as they are represented in the E max models? This review discusses issues related to the complexity and difficulty in obtaining a representation of the effect, and the lack of agreed definitions to be able to construct representative models of the temporary installation of the effect of Propofol for its use in TCI.

Comparison of the potency of different propofol formulations: a randomized, double-blind trial using closed-loop administration

BACKGROUND

Several commercial formulations of propofol are available. The primary outcome of this study was the required dose of propofol alone or combined with lidocaine to achieve induction of general anesthesia.

METHODS

This multicenter, double-blinded trial randomized patients (American Society of Anesthesiologists physical status I-III) just before elective surgery with the use of a computer-generated list. Three different propofol 1% formulations-Diprivan (Astra-Zeneca, Cheshire, United Kingdom), Propoven (Fresenius-Kabi AG, Bad Homburg, Germany), and Lipuro (B-Braun, Melshungen AG, Germany)-were compared with either placebo (saline solution) or lidocaine 1% mixed to the propofol solution. Depth of anesthesia was automatically guided by bispectral index and by a computerized closed-loop system for induction, thus avoiding dosing bias. The authors recorded the total dose of propofol and duration of induction and the patient's discomfort through a behavioral scale (facial expression, verbal response, and arm withdrawal) ranging from 0 to 6. The authors further evaluated postoperative recall of pain using a Visual Analog Scale.

RESULTS

Of the 227 patients enrolled, 217 were available for analysis. Demographic characteristics were similar in each group. Propoven required a higher dose for induction (2.2 ± 0.1 mg/kg) than Diprivan (1.8 ± 0.1 mg/kg) or Lipuro (1.7 ± 0.1 mg/kg; P = 0.02). However, induction doses were similar when propofol formulations were mixed with lidocaine. Patient discomfort during injection was significantly reduced with lidocaine for every formulation: Diprivan (0.5 ± 0.3 vs. 2.3 ± 0.3), Propoven (0.4 ± 0.3 vs. 2.4 ± 0.3), and Lipuro (1.1 ± 0.3 vs. 1.4 ± 0.3), all differences significant, with P < 0.0001. No adverse effect was reported.

CONCLUSION

Plain propofol formulations are not equipotent, but comparable doses were required when lidocaine was concomitantly administered.

Reaction time-monitored patient-maintained propofol sedation: a pilot study in oral surgery patients

Previous volunteer studies of an effect-site controlled patient-maintained sedation system using propofol have demonstrated a risk of oversedation. We have incorporated a reaction time monitor into the handset to add an individualised patient-feedback mechanism. This pilot study assessed if the reaction time-feedback modification would prove safe and effective in 20 healthy patients receiving sedation while undergoing oral surgery. All patients successfully sedated themselves without reaching any unsafe endpoints. All 20 maintained verbal contact throughout. The mean (SD) lowest peripheral blood oxygen saturation was 98.0 (2.1)% breathing room air. No patient required supplementary oxygen. The mean (SD) maximum effect-site propofol concentration reached was 1.6 (0.5) μg.ml(-1). The present system was found to be safe and effective, allowing oral surgery treatment under conscious sedation, but preventing oversedation.

Bispectral index-guided induction of general anaesthesia in patients undergoing major abdominal surgery using propofol or etomidate: a double-blind, randomized, clinical trial

BACKGROUND

In a double-blind, randomized trial, we compared the haemodynamic effects of a bispectral index (BIS)-guided etomidate and propofol infusion for anaesthesia induction in patients undergoing major abdominal surgery.

METHODS

Forty-six patients were randomly assigned to two groups based on the induction of anaesthesia, performed with a BIS value of 60 titrated infusion of etomidate (E group) or propofol (P group). Mean arterial pressure (MAP), cardiac index (CI), heart rate, and systemic vascular resistance index (SVRI) measurements were taken 1 min before induction and recorded at 1-min intervals for 20 min. P<0.05 was considered statistically significant.

RESULTS

Before intubation, no significant differences between the two groups regarding the haemodynamics were noticed. At intubation and up to 7 min after intubation MAP (P=0.019) was significantly higher in the E group. CI was significantly higher in the E group with respect to the P group 2, 6, and 7 min after intubation. Twenty-three patients developed complications. The incidence of hypotension was higher in the P group than that in the E group (8 vs 3; P=0.08), and the incidence of hypertension was significantly higher in the E group than that in the P group (10 vs 2; P=0.007).

CONCLUSIONS

Our study showed that the use of propofol resulted in less hypertension and tachycardia at and after intubation than etomidate. But even with the reduced doses given with the BIS-guided protocol, it often caused significant hypotension.

Patient-maintained propofol sedation using reaction time monitoring: a volunteer safety study

Previous volunteer studies of an effect-site controlled, patient-maintained sedation system using propofol have demonstrated a risk of over-sedation. We have incorporated a reaction-time monitor into the handset of the patient-maintained sedation system to add an individualised patient-feedback mechanism. This study assessed if such reaction-time feedback modification would reduce the risk of over-sedation in 20 healthy volunteers deliberately attempting to over-administer themselves propofol. All the volunteers successfully sedated themselves without reaching any unsafe endpoints. All volunteers maintained verbal contact throughout, in accordance with the definition of conscious sedation. The mean (SD) lowest S(p) O(2) was 97 (1.7) % when breathing room air and no volunteer required supplementary oxygen. The mean (SD) maximum effect-site propofol concentration reached was 1.7 (0.4) μg.ml(-1) . The present system was found to be safer than its predecessors, allowing conscious sedation, but preventing over-sedation.

Optimizing intravenous drug administration by applying pharmacokinetic/pharmacodynamic concepts

This review discusses the ways in which anaesthetists can optimize anaesthetic-analgesic drug administration by utilizing pharmacokinetic and pharmacodynamic information. We therefore focus on the dose-response relationship and the interactions between i.v. hypnotics and opioids. For i.v. hypnotics and opioids, models that accurately predict the time course of drug disposition and effect can be applied. Various commercial or experimental drug effect measures have been developed and can be implemented to further fine-tune individual patient-drug titration. The development of advisory and closed-loop feedback systems, which combine and integrate all sources of pharmacological and effect monitoring, has taken the existing kinetic-based administration technology forwards closer to total coverage of the dose-response relationship.

Episodic Waveforms in the Electroencephalogram during General Anaesthesia: A Study of Patterns of Response to Noxious Stimuli

Previous studies of the electroencephalogram (EEG) during anaesthesia have identified two distinct patterns of change in response to a noxious stimulus, a classical arousal pattern and a paradoxical arousal pattern. We developed methods of EEG analysis to quantify episodic EEG patterns – namely sleep spindle-like (‘10 Hz-score’) and burst-suppression-like fluctuations in high frequencies (‘high frequency variation index’) – and used traditional power spectral quantification of non-episodic delta waves. We studied 30 healthy adult patients undergoing elective surgery under general anaesthesia with propofol, fentanyl (1.0, 2.5 or 4.0 μg/kg, n=10 for each group), muscle relaxant and sevoflurane. Prefrontal EEG data were recorded during the operation and analysed for changes in episodic patterns before and after noxious stimuli (intubation and incision). Before noxious stimuli, the EEG patterns varied markedly between patients and were not strongly correlated to calculated effect-site concentrations of fentanyl, propofol or sevoflurane. Noxious stimuli reduced the 10Hz-score from 0.25 to 0.20 (P=0.01) after intubation and from 0.33 to 0.27 (P=0.01) after incision; and high frequency variation index from 2.8 to 2.0 (P=0.02) after incision – the classical arousal pattern. The nociception-induced reduction in spindles was greater in the low-dose fentanyl group (P=0.01). There was less tachycardia in the high-dose fentanyl group (P=0.002). It is possible to quantify such episodic EEG patterns during general anaesthesia and in this study noxious stimulation tended to reduce the prevalence of these patterns.

Evaluation of a new method of assessing depth of sedation using two-choice visual reaction time testing on a mobile phone

The utility of two-choice visual reaction time testing using a specially programmed mobile telephone as a measure of sedation level was investigated in 20 healthy patients sedated with target controlled infusions of propofol. At gradually increasing target concentrations visual reaction time was compared with patient-assessed visual analogue scale sedation scores and an observer-rated scale. Propofol sedation caused dose-dependent increases in visual reaction time and visual analogue scale scores that were statistically significant when the calculated effect-site concentration reached 0.9 microg.ml(-1) (p < 0.05) and 0.5 microg.ml(-1) (p < 0.01) respectively. While visual analogue scale scores were more sensitive at lower levels of sedation than visual reaction time, the latter demonstrated marked increase in values at higher levels of sedation. Visual reaction time may be useful for identifying impending over-sedation.

Sedation and regional anaesthesia in the adult patient

This review discusses sedation for regional anaesthesia in the adult population. The first section deals with general aspects of sedation and shows that the majority of patients receiving sedation for regional anaesthesia are satisfied and would choose it again. Methods of assessing the level of sedation are discussed with emphasis on clinical measures. The pharmacology of the drugs involved in sedation is discussed, with propofol and remifentanil appearing to be the combination of choice for sedation in regional anaesthesia. The techniques for administering sedation are discussed and replacement of the traditional repeated boluses or continuous infusion with pharmacokinetic and patient-controlled systems is supported. Patient satisfaction studies suggest that patient-controlled systems are preferred.

A comparison of pharmacokinetic/pharmacodynamic versus mass-balance measurement of brain concentrations of intravenous anesthetics in sheep

BACKGROUND

There are two recognized methods of estimating the brain concentrations of IV anesthetic drugs: (i) use of pharmacokinetic/pharmacodynamic (PK/PD) modeling of drug effect, from arterial concentrations and electroencephalogram changes, and (ii) direct measurement of the uptake of drug in the brain, by simultaneously measuring arterial and jugular concentrations and cerebral blood flow (mass-balance method). These two methods have not been directly compared. Because an accurate estimate of the time taken for transfer of anesthetic drug from arterial blood to its effect-compartment in the brain is critical for accurate effect-compartment dosing in IV anesthesia, we compared the PK/PD and mass-balance methods for propofol, methohexital, and ketamine in a sheep model.

METHODS

After instrumentation with arterial and sagittal-sinus cannulae, electrocorticogram, and sagittal sinus Doppler flow measurement seven adult sheep were given a random sequence of short anesthetic infusions with methohexital, ketamine, and propofol. Multiple blood samples were taken for measurement of the time course of the drug concentrations, and the electrocorticogram processed (approximate entropy, for propofol and methohexital and percentage high frequency time, for ketamine) to numerically quantify drug effect.

RESULTS

Using the PK/PD method the t(1/2)K(eo) was 2.0 +/- 0.4 min for ketamine, 2.7 +/- 1.1 min for propofol, and was significantly shorter (0.3 +/- 0.1 min) for methohexital. PK/PD and the mass-balance methods did not differ in the times to peak effect.

CONCLUSIONS

Both methods of calculating the delay in transfer of drug from arterial blood to brain give similar values. Methohexital crosses into the brain much faster than either propofol or ketamine.

Spectral entropy assessment with auditory evoked potential in neuroanesthesia

OBJECTIVE

The assessment of the level of anesthesia is a very hard task, since no gold standard has stood out in the past three decades. Middle Latency Auditory Evoked Potential (MLAEP) is one of the most popular neurophysiological tools for anesthesia monitoring. Recently, Spectral Entropy (SpEn) has been introduced: it provides two different parameters, State Entropy (SE) and Response Entropy (RE). The aim of this prospective study is to check SpEn end-point, comparing it to MLAEPs in neurosurgical anesthesia.

METHODS

Twenty patients submitted to elective supratentorial neurosurgery for removal of a temporal-parietal meningioma were included in the study. SpEn and MLAEPs were simultaneously monitored using the M-entropy module S/5 (GE Health Care, Helsinki, Finland) and Alaris Medical System AEP-ARX index monitor (AAI) (Kidemosevej, Denmark), respectively.

RESULTS

Four thousand and sixty four data points of SE, RE and AAI were recorded and ROC curves comparing AAI to RE and SE showed a highly significant (p<0.0001) area under the curve. The RE and SE cut-off values (showing maximal sensitivity with maximal specificity) to discriminate anesthesia from awake or consciousness sedation were 61 and 58, respectively. However, in a group of data points, low AAI was associated to high SpEn (577 data points for RE and 770 for SE) and vice versa (31 data points for RE and 43 for SE). The prediction probability for SE was 0.977 and for RE was 0.968.

CONCLUSIONS

Our results suggest that SpEn is as effective as AAI.

SIGNIFICANCE

Our results show that SpEn is able to discriminate between the levels of wakefulness and surgical anesthesia. However, the meaning of data showing a discrepancy between AAI and SpEn is not yet clear and calls for further study.

Does Cerebral Monitoring Improve Ophthalmic Surgical Operating Conditions During Propofol-Induced Sedation?

Sudden movements from over-sedation during ophthalmic surgery can be detrimental to the eye. Bispectral index (BIS) and middle-latency auditory-evoked potentials (Alaris AEP index, AAI) were reported to be accurate indicators for the level of sedation and loss of consciousness. We assessed these monitors during sedation with special emphasis on preventing over-sedation. One-hundred patients scheduled for elective eye surgery were sedated with target-controlled propofol infusion and randomly allocated to BIS-guided, AAI-guided, BIS/AAI-guided, or clinically guided groups (n = 25 each). The initial target concentration was 0.5 microg x mL(-1) in patients >70 yr and 1.0 microg x mL(-1) in all other patients. The concentration was increased every 3 min by 0.1 or 0.2 microg x mL(-1), respectively until the patient had reached a BIS value of 75 (range 70-90) or an AAI of 40 (range 35-60). The surgeon who was blinded to group allocation assessed treatment quality after the procedure. Sedation was converted into general anesthesia in four patients because of excessive head movements. BIS was out of range 7% of the time vs 58% for AAI. No significant differences in treatment quality were observed among the four groups. We conclude that propofol sedation, guided by BIS or AAI monitoring, did not enhance ophthalmic surgical operating conditions over sedation guided by clinical observation only.

Evaluation of a new effect-site controlled, patient-maintained sedation system in dental patients

We have designed a new effect-site controlled, patient-maintained sedation system for delivering propofol. In the previous systems we developed, the patients retained the use of the handset throughout the procedure and were able to increase the level of sedation. However, it was found that this could potentially lead to oversedation. In the present system, the patients were able to increase their level of sedation until a level was reached that was judged by the patients as being adequate to allow them to tolerate the injection of dental local anaesthetic. The handset was then taken from the patients and the effect site concentration of propofol was maintained at that level for the remainder of the procedure. To assess its safety and efficacy, the system was used to sedate 40 patients presenting for dental procedures under sedation. The system was used successfully and treatment was completed in 39 patients. The system was found to be safe. Both surgeon and patient approval scores were high. Although this study demonstrates the efficacy of effect-site controlled, patient-maintained propofol sedation in this group of patients, further work is required to confirm its safety.

Wirkortäquilibration, Anschlagzeit, „time to peak effect“

Contrary to the situation in "classical" clinical pharmacology, non-steady state phenomena play a fundamental role for clinical pharmacology in anesthesia. Their understanding is of tantamount importance for the safe and efficient application of drugs relevant to anesthesia. Concepts like optimised target-controlled infusion (TCI), effect compartment targeting and the small margin of error tolerable during maintained spontaneous ventilation, force the anesthesiologist to acquire a firm understanding of the difference between the concentration time course at the effect side vs. time course of the plasma concentration. The underlying concepts, their application for the rational use of muscle relaxants, propofol with TCI systems, volatile anaesthetics and opioids will be discussed.

Effect-site controlled patient maintained propofol sedation: a volunteer safety study

Effect-site concentration is a mathematical term related to the clinical effect of a drug. We have designed a patient-maintained sedation system for delivering propofol to the predicted effect-site concentration rather than plasma concentration. To assess its efficacy and safety, 20 healthy volunteers deliberately attempted to over-sedate themselves using the system. The new effect-site concentration driven system delivered sedation successfully, and more rapidly than its predecessor. Fifteen volunteers ended the study when the system automatically reduced the effect-site concentration after 6 min of no button presses despite verbal contact and maintenance of arterial oxygen saturation (at effect-site concentration 1.8-3.8 microg x ml(-1)). Four out of 20 volunteers ended with minor arterial desaturation (lowest 88% at effect-site concentration 2.6-3.4 microg x ml(-1)). One volunteer ended after loss of verbal contact at effect-site concentration 3.4 microg x ml(-1). Further modification of the system's design is necessary before it can be recommended for routine practice.

[Clinical applications of intravenous anaesthetics pharmacology: the example of hypnotics and opioids]

OBJECTIVE

To review the general principles of pharmacokinetics and pharmacodynamics models by focusing on intravenous anaesthetics (hypnotics and opioids).

DATA SOURCES

Medline references, lectures from the French congress of anaesthesiology and intensive care medicine, abstracts.

DATA SYNTHESIS

Pharmacokinetic and pharmacodynamic modelling allows simple estimation of becoming of anaesthetic drugs in the body, instead of classical pharmacologic approach. However, pharmacokinetic as well as pharmacodynamic parameters are often considered as resulting from complex mathematic approaches and remain then poorly used in practice by physicians. The aim of this article is to simply expose concepts underlying PK-PD models building and to explain significance of the main PK-PD parameters (first-order rate constants, k(e0), T(1/2)k(e0), T(peak), context-sensitive half-time, context-sensitive decrement times). Clinical consequences for using intravenous anaesthetic drugs (hypnotics and opioids) are exposed either during bolus injection or continuous infusion, when injected alone or co-administered.

The A-line ARX index may be a more sensitive detector of arousal than the bispectral index during propofol-fentanyl-nitrous oxide anesthesia: A preliminary investigation

PURPOSE

To compare changes in the A-line ARX index (AAI) by the Alaris AEP monitor(TM) with those of the bispectral index (BIS) during propofol-fentanyl-nitrous oxide anesthesia.

METHODS

Eighty female patients undergoing partial mastectomy were randomly allocated to AAI or BIS (40 per group). Anesthesia was induced with propofol 2 mg x kg(-1) and fentanyl 3 micro g x kg(-1) during the inhalation of oxygen. A laryngeal mask airway (LMA) #3 was inserted. Anesthesia was maintained with propofol 4 mg x kg(-1) x hr(-1), fentanyl 1 micro g x kg(-1) given at the start of surgery, and nitrous oxide 4 L x min(-1) in oxygen 2 L x min(-1). Blood pressure, heart rate, and AAI or BIS were monitored, including recovery time of the index after disturbance by electrocautery.

RESULTS

The AAI but not the BIS increased significantly with LMA insertion and skin incision, while blood pressure and heart rate did not change. The BIS decreased from 87 +/- 7 to 30-60 while the AAI decreased from 75 +/- 8 to 10-25 during anesthesia. The increase of the AAI was larger than that of the BIS at recovery from anesthesia. The variation of the index was smaller in the AAI than in the BIS. Recovery time of the index after electrocautery was significantly longer in the BIS group (21 +/- 9 sec) than that in the AAI group (5 +/- 3 sec).

CONCLUSIONS

During propofol-fentanyl-nitrous oxide anesthesia, the AAI responded to LMA insertion or surgical incision, but not the BIS, and the AAI had smaller variations. The AAI recovered faster from the disturbance by electrocautery than the BIS. Thus, the AAI may be a more sensitive and useful detector of arousal than the BIS.

A Comparison of the Clinical Usefulness of Three Different Electroencephalogram Monitors: Bispectral Index, Processed Electroencephalogram, and Alaris Auditory Evoked Potentials

We compared the usefulness of the Bispectral Index (BIS), Processed electroencephalogram (pEEG), and Alaris auditory evoked potentials (A-AEP). Ninety females scheduled for mastectomy were divided into three groups. Anesthesia was induced with propofol and fentanyl to insert a laryngeal mask airway (LMA) and was maintained by adding nitrous oxide. EEG was monitored by either BIS, spectral edge frequency by pEEG, or A-AEP index by A-AEP. We recorded the number of patients with impedance low enough to extract good EEG signals at the first electrodes application (success rate), the number with an index outside of the range considered appropriate for general anesthesia (inappropriateness rate), changes of the index by LMA insertion or surgical incision (responsiveness), and time to return to good EEG signals after signal disturbance by electric cautery (recovery time). The success rate was larger in BIS > or = A-AEP > pEEG. The inappropriateness rate was smaller in A-AEP < or = BIS , or = pEEG. The A-AEP group showed the largest responsiveness. The recovery time was shorter in pEEG < A-AEP < BIS. In summary, the BIS had the largest success rate, the A-AEP had the least inappropriateness rate and the largest responsiveness, and the pEEG had the fastest recovery time.

IMPLICATIONS

We compared the usefulness of three electroencephalogram monitors. The Bispectral Index was the easiest for obtaining low impedance, the auditory evoked potential index had the least inappropriateness rate for general anesthesia and had the largest responsiveness, and the spectral edge frequency was the fastest in stabilizing measurement after electric cautery.

Relationship between bispectral index, auditory evoked potential index and effect-site EC50 for propofol at two clinical end-points

BACKGROUND

Many anaesthetists are deterred from using total i.v. anaesthesia because of uncertainty over the concentration of propofol required to prevent awareness. We predicted blood and effect-site concentrations of propofol at two clinical end-points: loss of consciousness and no response to a painful stimulus.

METHODS

Forty unpremedicated Caucasian patients were anaesthetized with i.v. propofol delivered by a Diprifusor target-controlled infusion (TCI). Bispectral index (BIS) and auditory evoked potential index (AEPex) were measured and blood and effect-site propofol concentrations were predicted. Logistic regression was used to estimate population values for predicted blood and effect-site propofol concentrations at the clinical end-points and to correlate these with BIS and AEPex.

RESULTS

The effect-site EC(50) at loss of consciousness was 2.8 micro m ml(-1) with an EC(05) and an EC(95) of 1.5 and 4.1 micro m ml(-1), respectively. The predicted EC(50) when there was no response to a tetanic stimulus was 5.2 micro m ml(-1) with an EC(05) and an EC(95) of 3.1 and 7.2 micro m ml(-1), respectively.

CONCLUSIONS

Unconsciousness and lack of response to a painful stimulus occur within a defined range of effect-site concentrations, predicted by Diprifusor TCI software.

Tracheal intubating conditions using propofol and remifentanil target-controlled infusions

Using target-controlled infusions (TCI) we aimed to determine the most appropriate dose of remifentanil required for intubation, using a steady effect-site concentration of propofol and without the use of neuromuscular blocking drugs. Sixty ASA III patients presenting for elective surgery were randomly allocated to one of three groups. Anaesthesia was induced in all patients using a target-controlled infusion of propofol 6.5 microg x ml(-1). This was reduced to 3 microg x ml(-1) after 1 min. Each group received a different TCI of remifentanil, 19, 15 or 11 ng x ml(-1), which was reduced to 10, 8 or 6 ng x ml(-1), respectively, after 1 min. Laryngoscopy and intubation were attempted at 4 min. Laryngoscopy and ease of intubation were assessed using a standard scoring system. Intubation was considered satisfactory in 75% of patients in groups 1 and 2 and 35% of patients in group 3. Intubation was successful in 20/20, 19/20 and 15/20 patients in groups 1, 2 and 3, respectively. Pulse oximetry, heart rate and noninvasive arterial pressure were measured pre-induction, and at intervals until after laryngoscopy and intubation. Mean arterial pressure (MAP) and heart rate decreased following induction of anaesthesia in all groups, which was statistically significant. Following laryngoscopy, MAP and heart rate increased, but were significantly less than the corresponding baseline values.

Patient-maintained propofol sedation: a follow up safety study using a modified system in volunteers

Patient-maintained sedation is a mode of patient-controlled sedation during which propofol is administered using a target-controlled infusion, with patient demand increasing the target concentration. A system tested previously for safety in our institution resulted in oversedation. Aiming to improve safety, we modified the system by increasing the lockout period to 4 min,reducing the starting concentration to 0.5 microg x ml(-1) and the increments on demand to 0.1 microg x ml(-1). As in the previous study, healthy volunteers attempted to render themselves unconscious by frequently pressing the demand button. To assess effects on memory, volunteers were given keywords to remember every 15 min. The maximum target concentration reached varied between 1.0 and 2.5 microg x ml(-1). No volunteers lost consciousness, however, one volunteer had a brief period of apnoea and oxygen desaturation. The Cp50 for loss of memory for words was 1.26 microg x ml(-1). Although this version represents an improvement, we conclude that the system is not yet completely suitable for use without anaesthetic supervision.

Propofol effective concentration 50 and its relationship to bispectral index

Sixty unpremedicated healthy adult patients were studied during induction of anaesthesia with intravenous propofol delivered by a 'Diprifusor' target-controlled infusion. Bispectral index (BIS) and spectral edge frequency (SEF95) were measured concurrently with the predicted blood and effect site propofol concentrations. Logistic regression was used to calculate the predicted propofol blood and effect site concentrations required to produce unconsciousness and no response to a noxious stimulus in 50% and 95% of patients and to correlate BIS with these end-points. The Diprifusor TCI software produces anaesthesia at consistent target concentrations. Bispectral index correlates well with clinical end-points and may be useful during propofol anaesthesia.

A comparison of the effects of propofol and midazolam on memory during two levels of sedation by using target-controlled infusion

We examined memory during sedation with target-controlled infusions of propofol and midazolam in a double-blinded five-way, cross-over study in 10 volunteers. Each active drug infusion was targeted to sedation level 1 (asleep) and level 4 (lethargic) as determined with the Observer Assessment of Alertness/Sedation scale. At the target level of sedation, drug concentration was clamped for 30 min, during which time neutral words were presented. After 2 h, explicit memory was assessed by recall, and implicit memory by using a wordstem completion test. Venous drug concentrations (mean +/- SD) were 1350 ng/mL (+/-332 ng/mL) for propofol and 208 ng/mL (+/-112 ng/mL) for midazolam during Observer Assessment of Alertness/Sedation scale level 4; and 1620 ng/mL (+/-357 ng/mL) and 249 ng/mL (+/-82 ng/mL) respectively during level 1. The wordstem completion test frequencies at low level sedation were significantly higher than spontaneous frequencies (8.7% + 2.4%; P: < 0.05 in all cases), and lower than during placebo (33.6% + 23%) (P: < 0.05 in all cases, except P: = 0.076 for propofol at level 4). Clinically distinct levels of sedation were accompanied by small differences in venous propofol or midazolam concentrations. This indicates steep concentration-effect relationships. Neutral information is still memorized during low-level sedation with both drugs. The memory effect of propofol and midazolam did not differ significantly.

IMPLICATIONS

Implicit memory can occur during different states of consciousness and might lead to psychological damage. In 10 volunteers, implicit memory was investigated during sedation with propofol and midazolam in a double-blinded, placebo-controlled study. To compare the effects of both drugs, they were titrated using a computer-controlled infusion system to produce similar high and low levels of sedation.

Safety of patient-maintained propofol sedation using a target-controlled system in healthy volunteers

We investigated the safety of a patient-maintained system that allows individuals to operate a target-controlled infusion of propofol to achieve sedation. Ten healthy volunteers were recruited and instructed to try to anaesthetize themselves with the system. A target-controlled infusion of propofol was set to deliver a target propofol concentration of 1 microgram ml-1, and the subjects allowed to increase the target in increments of 0.2 microgram ml-1 by pressing a control button twice in 1 s. There was a lockout time of 2 min and a maximum permitted target concentration of 3 micrograms ml-1. Heart rate and pulse oximetry oxygen saturation (SpO2) were monitored continuously, and non-invasive arterial pressure, ventilatory frequencies and sedation scores were measured every 5 min. Sedation was continued until the subject stopped pressing the button. A keyword was then read for the individual to remember and sedation discontinued. There were no instances of significant decrease of SpO2 or loss of airway control. Maximum target blood concentration of propofol recorded ranged from 1.4 to 3 micrograms ml-1. Two subjects became oversedated, one of whom was unrousable with loss of eyelash reflex. No subject could recall the keyword, although one recognized it from a list of 10 words. We conclude that the patient-maintained sedation system described could not be guaranteed to produce only conscious sedation in all patients, and that close clinical supervision by an anaesthetist would still be required for safe operation.