D. Hadzidiakos

Suppression of the human spinal H-reflex by propofol: a quantitative analysis

Background:  The spinal cord is an important site of anaesthetic action because it mediates surgical immobility. During anaesthesia with volatile anaesthetics, it has been shown that the suppression of the spinal H‐reflex correlates with surgical immobility. To evaluate whether the H‐reflex could also be a possible candidate for monitoring immobility during propofol anaesthesia, this study assessed the concentration‐dependent suppression of the H‐reflex by propofol. To discriminate different effect sites, the individual concentration response‐curves and the t1/2ke0 of the H‐reflex have been compared with those of two EEG parameters.

Analysis of Memory Formation during General Anesthesia (Propofol/Remifentanil) for Elective Surgery Using the Process-dissociation Procedure

Background:There have been reports of memory formation during general anesthesia. The process-dissociation procedure has been used to determine if these are controlled (explicit/conscious) or automatic (implict/unconscious) memories. This study used the process-dissociation procedure with the original measurement model and one which corrected for guessing to determine if more accurate results were obtained in this setting. Methods:A total of 160 patients scheduled for elective surgery were enrolled. Memory for words presented during propofol and remifentanil general anesthesia was tested postoperatively by using a word-stem completion task in a process-dissociation procedure. To assign possible memory effects to different levels of anesthetic depth, the authors measured depth of anesthesia using the BIS® XP monitor (Aspect Medical Systems, Norwood, MA). Results:Word-stem completion performance showed no evidence of memory for intraoperatively presented words. Nevertheless, an evaluation of these data using the original measurement model for process-dissociation data suggested an evidence of controlled (C = 0.05; 95% confidence interval [CI] 0.02–0.08) and automatic (A = 0.11; 95% CI 0.09–0.12) memory processes (P < 0.01). However, when the data were evaluated with an extended measurement model taking base rates into account adequately, no evidence for controlled (C = 0.00; 95% CI –0.04 to 0.04) or automatic (A = 0.00; 95% CI –0.02 to 0.02) memory processes was obtained. The authors report and discuss parallel findings for published data sets that were generated by using the process-dissociation procedure. Conclusion:Patients had no memories for auditory information presented during propofol/remifentanil anesthesia after midazolam premedication. The use of the process-dissociation procedure with the original measurement model erroneously detected memories, whereas the extended model, corrected for guessing, correctly revealed no memory.

Variability Comparison of the Composite Auditory Evoked Potential Index and the Bispectral Index During Propofol-fentanyl Anesthesia

BACKGROUND: Monitors of hypnotic depth help anesthesiologists to guide the anesthetic. The performance of different monitors depends on several factors, index variability at a steady state of hypnotic depth being one. We compared the recently introduced AAI1.6 with the established bispectral index (BIS), regarding index variability during stable values of propofol effect-site concentration. METHODS: After ethics committee approval and written informed consent, anesthesia was performed in 40 patients with propofol as the target controlled infusion and fentanyl. Variability of BIS and AAI1.6 was calculated during periods of constant predicted propofol effect compartment concentration and constant levels of surgical stimulation as the median absolute deviation (MAD) from the median value. A variability index was calculated as 1.48*MAD/(threshold − median value), with threshold being the division line between awake and asleep. Threshold crossing time was used to evaluate the performance in predicting return of consciousness. RESULTS: Variability index, however, was significantly larger for the AAI1.6, despite similar absolute variability measured as MAD. Lightening of anesthesia before recovery could be noticed earlier using the BIS than the AAI1.6, although consciousness was detected with a significantly higher Pk-value by the AAI1.6. CONCLUSION: Variability in relation to the difference between the median index value during anesthesia and the threshold necessary to detect consciousness with high sensitivity is higher for the AAI1.6 than for the BIS. This, as well as the steeper concentration–response function found for AAI1.6, impairs the performance of the AAI1.6 in predicting imminent return of consciousness during decreasing propofol concentrations. However, it makes AAI1.6 well suited to detect consciousness when it has occurred.

Comparison of a new composite index based on midlatency auditory evoked potentials and electroencephalographic parameters with bispectral index (BIS) during moderate propofol sedation

Background and objective: Derived parameters of the electroencephalogram and auditory evoked potentials can be used to determine depth of anaesthesia and sedation. However, it is not known whether any parameter can identify the occurrence of awareness in individual patients. We have compared the performance of bispectral index and a new composite index derived from auditory evoked potentials and the electroencephalogram (AAI 1.61) in predicting consciousness, explicit and implicit memory during moderate sedation with propofol. Methods: Twenty‐one patients with spinal anaesthesia received intraoperatively propofol at the age‐corrected C50 for loss of consciousness and were presented test words via headphones. Bispectral index and AAI 1.61 (auditory evoked potentials, AEP‐Monitor2) were recorded in parallel as well as the Observers Assessment of Alertness/Sedation‐score. Postoperatively, testing for explicit and implicit memory formation was performed. Results: Bispectral index and AAI 1.61 correlated well with loss of consciousness defined by an Observers Assessment of Alertness/Sedation‐score of 2 (identical PK of 0.87), but did not allow a prediction of postoperative explicit or implicit recall. Conclusions: Both bispectral index and AAI may be indices of depth of sedation rather than indicators of memory formation, which persists during propofol sedation even after loss of consciousness.

Use of a target-controlled infusion system for propofol does not improve subjective assessment of anaesthetic depth by inexperienced anaesthesiologists

Background and objectives: Target‐controlled infusion, via the calculated effect compartment concentrations, may help anaesthesiologists to titrate anaesthetic depth and to shorten recovery from anaesthesia. Methods: In this prospective, randomized clinical study, we compared the performance of six inexperienced anaesthesiologists with <1 yr of training when using target‐ or manually controlled infusion of propofol, combined with manual dosing of fentanyl. Ninety‐two premedicated ASA I‐III patients undergoing minor elective urological or gynaecological surgery were assigned to the manual‐ or target‐controlled infusion group. Bispectral index was recorded in a blinded manner. Subjective assessment of anaesthetic depth on a 10 point numerical scale (1 = very deep anaesthesia, 10 = awake) was asked at regular intervals and the correlation with the blinded bispectral index was analysed using the prediction probability, PK. The propofol concentration profile was calculated post hoc. Results: Propofol administration was similar in both groups with no significant difference for the administered amount and concentrations of propofol. Recovery times were also not different. In both groups, a large percentage of the bispectral index data points recorded during surgery showed bispectral index values below the recommended value of 40, but in the target‐controlled infusion group there were significantly less bispectral index values above the recommended upper limit of 60 (2.5% vs. 5.1%). Conclusions: A target‐controlled infusion system does not help inexperienced anaesthesiologists to assess anaesthetic depth or to shorten recovery times, but may reduce episodes of overly light anaesthesia and thus help to prevent awareness.

subjective assessment of depth of anaesthesia by experienced and inexperienced anaesthetists

Background and objective: To measure ‘depth of anaesthesia’, anaesthesiologists use a combination of observable end‐points such as immobility and autonomic stability. Unconsciousness and amnesia are not reliably observable end‐points, but correlate with parameters derived from the electroencephalogram. We investigated the association of subjective assessment and electroencephalographic measures of anaesthetic depth in a group of experienced (>4 yr of experience) and a group of inexperienced (<2 yr of experience) anaesthesiologists. Methods: One hundred ASA I or II patients were assigned to either group. Anaesthesiologists assessed ‘anaesthetic depth’ using an 11‐point numeric and a 5‐point verbal scale. Bispectral index and spectral entropy were recorded as electroencephalogram parameters. The association between the subjective assessment and the electroencephalogram parameters was calculated using the prediction probability, PK. Results: Association between subjective assessment and electroencephalographic parameters showed a tendency to a better prediction probability in the experienced group. The difference was significant (P < 0.05) for the bispectral index (PK 0.76 ± 0.01 for experienced and 0.71 ± 0.01 for inexperienced anaesthesiologists). In both groups, a large percentage of the data points recorded during surgery showed bispectral index values above the recommended value of 60 (13.2% in the experienced and 34.3% in the inexperienced group) despite a subjective assessment of ‘deep’ or ‘very deep’ anaesthetic depth. Conclusion: The study demonstrates that the association between subjectively assigned values of anaesthetic depth and electroencephalographic parameters of anaesthetic depth is better for anaesthesiologists with more clinical experience. However, in the ‘inexperienced’ as well as ‘experienced’ group a high percentage of bispectral index and entropy values above 60 occurred despite a subjective assessment of adequate anaesthetic depth. Although there was no evidence for explicit memory, this may indicate a risk for memory formation.

Pre- and postoperative management of risk factors for postoperative delirium: who is in charge and what is its essence?

Postoperative delirium (POD) is an adverse clinical outcome characterized by cognitive, affective and behavioral symptoms with typically an acute onset and a fluctuating course. POD is attributed to certain patients’ predisposing factors as well as to treatment-related precipitating factors. While there are several single-component interventions for the prevention of POD, evolving evidence suggests the importance of a system approach in the prevention of POD. This involves strategies by multidisciplinary teams with additional geriatric consultation services to identify risk factors for POD and to modify their impact on the perioperative course. Some patients may profit from postponing an elective surgery and undergoing a prehabilitation program to optimize his/her resilience for the surgical and anesthesiologic stressors.