Loss of volition and pain response during induction of anaesthesia with propofol or sevoflurane

We compared the time to reach two anaesthetic end-points duringinduction of anaesthesia with a potent inhalation agent (sevoflurane)and an i.v. agent (propofol). We used a method to ensure steadybreathing during inhalation induction, and measured loss oftone in the outstretched arm and loss of response to a painfulstimulus. Thirty-eight female patients (age 39 (9) yr, weight65 (11) kg, and height 165 (8) cm) (mean (SD)) were randomlyallocated to receive either propofol or sevoflurane. The predictedinduction dose of propofol, estimated from age and weight foreach patient, was given at a rate of 1% of the induction doseper second, to a possible maximum of 2.5 times the predictedinduction dose. Sevoflurane was given with an inhaled concentrationof 8%, which was anticipated to cause loss of arm tone within90–120 s. After loss of consciousness, we applied a painfulelectrical stimulus to a finger at 15-s intervals and measuredthe time to loss of motor response. The median times and interquartilevalues for loss of arm tone were 105 (88–121) s for sevofluraneand 65 (58–80) s for propofol. This was equivalent to0.65 of the ED₅₀ of propofol. The time to loss of response topain was 226 (169–300) s for sevoflurane. The variancesof these three measurements were not significantly different,indicating that these dose–response relationships weresimilar. In contrast, only 11 of the patients given propofollost the response to pain after 2.5xED₅₀ had been given. Theseresults support previous evidence of substantial differencesbetween anaesthetic end-points, and show that this evidencecan be obtained using a simple and rapid method. Br J Anaesth 2001; 87: 283–6     

Influence of the baricity of a local anaesthetic agent on sedation with propofol during spinal anaesthesia

Background: This study examined the effect of different levels of spinalanaesthesia, induced by solutions of different baricity butcontaining the same amount of local anaesthetic agent, on therequirement for sedation with propofol. Methods: Thirty-six patients undergoing varicose vein surgery under spinalanaesthesia were randomly allocated to receive tetracaine 15mg in 3 ml of either glucose 5% (hyperbaric) or CSF (isobaric).I.V. propofol was started 5 min after the intrathecal injectionand was titrated to maintain a bispectral index (BIS) scoreof 65–75. The propofol requirements to maintain this rangein the two groups were compared every 5 min. Results: The propofol requirement was always lower in the hyperbaricgroup, with the differences becoming statistically significant20 min after the intrathecal injection. Total consumption ofpropofol over the 55 min of the study was also less in the hyperbaricgroup. Conclusion: The known difference in level of spinal anaesthetic block inducedby solutions of different baricity, but the same dose of localanaesthetic, was associated with different requirements forpropofol sedation as determined by BIS assessment.     

Early recovery after remifentanil-pronounced compared with propofol-pronounced total intravenous anaesthesia for short painful procedures

Background. We compared recovery from high-dose propofol/low-doseremifentanil (‘propofol-pronounced’) compared withhigh-dose remifentanil/low-dose propofol (‘remifentanil-pronounced’)anaesthesia. Methods. Adult patients having panendoscopy, microlaryngoscopy,or tonsillectomy were randomly assigned to receive either propofol-pronounced(propofol 100 µg kg^–1 min^–1; remifentanil0.15 µg kg^–1 min^–1) or remifentanil-pronounced(propofol 50 µg kg^–1 min^–1; remifentanil 0.45µg kg^–1 min^–1) anaesthesia. In both groups,the procedure was started with remifentanil 0.4 µg kg^–1,propofol 2 mg kg^–1, and mivacurium 0.2 mg kg^–1.Cardiovascular measurements and EEG bispectral index (BIS) wererecorded. To maintain comparable anaesthetic depth, additionalpropofol (0.5 mg kg^–1) was given if BIS values were greaterthan 55 and remifentanil (0.4 µg kg^–1) if heartrate or arterial pressure was greater than 110% of pre-anaestheticvalues. Results. Patient and surgical characteristics, cardiovascularmeasurements, and BIS values were similar in both groups. Therewere no differences in recovery times between the groups (timeto extubation: 12.7 (4.5) vs 12.0 (3.6) min, readiness for transferto the recovery ward: 14.4 (4.4) vs. 13.7 (3.6) min, mean (SD)). Conclusions. In patients having short painful surgery, lesspropofol does not give faster recovery as long as the same anaestheticlevel (as indicated by BIS and clinical signs) is maintainedby more remifentanil. However, recovery times were less variablefollowing remifentanil-pronounced anaesthesia suggesting a morepredictable recovery. Br J Anaesth 2003; 91: 580–2     

Pulmonary scintigraphy for diagnosis of aspiration during intravenous propofol anaesthesia for colonoscopy

A specific technique for detection of pulmonary aspiration duringthe perioperative period is lacking. In this study, we developeda scintigraphic method for its diagnosis. Technetium 99m sulphurcolloid was given orally 2 h before an i.v. infusion of propofolin patients undergoing elective colonoscopy. During the procedure,patients were spontaneously breathing 100% oxygen via a facemask. After recovery from anaesthesia, patients had a chestscinti-scan. As a control group, 10 healthy men were studied.The lung scan was considered positive if any tracer activitygreater than background level was detected in the lung field.Among 96 patients studied, three patients had a positive chestscinti-scan. One of the three patients developed pneumonia whilethe other two remained asymptomatic. In none of the controlasymptomatic group was tracer detected in the chest. We suggestthat this technique is specific and can be used as a tool toassess the risk of pulmonary aspiration during different anaestheticprocedures. Br J Anaesth 2001; 87: 204–6     

Recent advances in intravenous anaesthesia

Efforts to develop new hypnotic compounds continue, althoughseveral have recently failed in development. Propofol has beenreformulated in various presentations with and without preservatives.Pharmacokinetic and pharmacodynamic differences exist betweensome of these preparations, and it is currently unclear whetherany have substantial advantages over the original presentation.The use of target-controlled infusion (TCI) has been extendedto include paediatric anaesthesia and sedation. Applicationof TCI to remifentanil is now licensed. Linking of electroencephalogram(EEG) monitoring to TCI for closed-loop anaesthesia remainsa research tool, although commercial development may follow.The availability of stereoisomer ketamine and improved understandingof its pharmacology have increased non-anaesthetic use of ketamineas an adjunct analgesic. It may be useful in subhypnotic dosesfor postsurgical patients with pain refractory to morphine administration.      

Comparison of isoflurane and propofol-fentanyl anaesthesia in a swine model of asphyxia

Background. There have been few studies comparing the responseto asphyxia and the effectiveness of typical cardiopulmonaryresuscitation (CPR) using exogenous epinephrine administrationand manual closed-chest compression between total intravenousanaesthesia (TIVA) and inhalational anaesthesia. Methods. Twenty pigs were randomly assigned to two study groupsanaesthetized using either 2% end-tidal isoflurane (n=10) orpropofol (12 mg kg^–1 h^–1)–fentanyl (50 µgkg^–1) (n=10). Asphyxia was induced by clamping the trachealtube until the mean arterial pressure (MAP) decreased to 40%of the baseline value (40% MAP time). The tracheal tube wasdeclamped at that point, and CPR was performed. Haemodynamicparameters and blood samples were obtained before the inductionof asphyxia, at 1-min intervals during asphyxia, and 1, 2, 3,5, 10, 30 and 60 min after asphyxia. Results. TIVA maintained the MAP against hypoxia–hypercapniastress significantly longer than isoflurane anaesthesia (mean(SD) 40% MAP time 498 (95) and 378 (104) s respectively). Inall animals in the isoflurane group, spontaneous circulationreturned within 1 min of the start of CPR. In six of the TIVAanimals, spontaneous circulation returned for 220 (121) s; spontaneouscirculation did not return within 5 min in the remaining fouranimals. Conclusions. Although TIVA is less prone than isoflurane anaesthesiato primary cardiovascular depression leading to asphyxia, TIVAis associated with reduced effectiveness of CPR in which resuscitationbecause of asphyxic haemodynamic depression occurs. Br J Anaesth 2003; 91: 871–7     

Absence of memory for intra-operative information during surgery with total intravenous anaesthesia

While using the isolated forearm technique, we wished to determinewhether patients who did not respond to commands during generalanaesthesia with a total intravenous technique (propofol andalfentanil with atracurium) had any evidence of post-operativeexplicit or implicit memory. Forty women undergoing major gynaecologicalsurgery were randomized, in a double-blind design, to hear twodifferent tapes during surgery. Psychological tests of explicitand implicit memory were conducted within 2 h of surgery. Therewas no evidence of implicit or explicit memory, nor any recall,in the seven women who responded to commands during surgery.We conclude that during total intravenous anaesthesia with propofoland alfentanil, there is no evidence that learning takes placewhen anaesthesia is adequate. Furthermore, with this anaesthetictechnique, it would seem that—provided any period of patientresponsiveness is short and that unconsciousness is inducedrapidly again—there is no evidence of implicit or explicitmemory. Br J Anaesth 2001; 86: 196–202     

Differential effects of propofol, ketamine, and thiopental anaesthesia on the skeletal muscle microcirculation of normotensive and hypertensive rats in vivo

Background. This study utilized the dorsal microcirculatorychamber (DMC) model to determine differential effects of i.v.propofol, ketamine, and thiopental anaesthesia on the skeletalmuscle microcirculation (10–180 µm) of normotensive(Male Wistar Kyoto, WKY) and hypertensive (spontaneously hypertensiveHarlan, SHR) rats, importantly, comparing responses to a consciousbaseline. Methods. Three weeks following implantation of the DMC in WKY(n=8) and SHR (n=6) (130 g) 0.25 ml 100 g^–1 FITC–BSA(i.v.) was administered and the microcirculation viewed usingfluorescent in vivo microscopy for a 30 min baseline (t=0–30min). This was followed by either propofol, thiopental, ketamine,or saline (i.v. bolus induction over 5 min (t=30–35 min)),then maintenance step-up infusion for 60 min (t=45–105min), so that animals received all four agents 1 week apart(56 experiments). Results. Dilation of A3 arterioles (15–30 µm) andV3 venules (20–40 µm) with propofol was greaterin SHR (t=95 min, A3 36.7 (12)%, V3 15.5 (2.3)%) than WKY (t=95min, A3 19.4 (7.4)%, V3 8.0 (2.3)%) (P<0.05). Constrictionof A3 with ketamine was greater in SHR (t=95 min, A3 –29.1(6.4)%) than WKY (A3 –17.5 (8.8)%) (P<0.05). This wasaccompanied by hypotension with propofol in SHR (–32%decrease in systolic arterial pressure), but not WKY (–6%)and hypertension with ketamine in WKY (–15%) and SHR (–24%)(P<0.05). During thiopental anaesthesia there was dilationof A1 (80–180 µm), A3, and V3 in WKY (P<0.05).Conversely, in SHR dilation of venules (29.2 (8.7)%) was accompaniedby constriction of A1 and A3 (t=95 min, A1 –25.1 (5.9)%,A3 –45.2 (3.1)%) (P<0.05). Conclusion. Within the skeletal muscle microcirculation of hypertensiverats there is enhanced dilation with propofol and constrictionwith ketamine, associated with exaggerated changes in arterialpressure. Thus, dysfunctional control mechanisms at the levelof the microcirculation alter responses to anaesthesia duringhypertension.      

BIS guided sedation with propofol during spinal anaesthesia: influence of anaesthetic level on sedation requirement

Background. In this prospective, clinical study we tested thehypothesis whether two different doses of spinal administeredbupivacaine and accordingly, two different levels of spinalanaesthesia can affect the dose requirement of propofol duringBIS guided sedation. Methods. Fifty women undergoing vaginal hysterectomy (high spinalgroup, HS) or transvaginal tape (TVT) procedure for urinaryincontinence (low spinal group, LS) under spinal anaesthesiawere enrolled to the study. In group HS, 17.5 mg and in groupLS, 7.5 mg of hyperbaric bupivacaine were given intrathecally.After 15 min to obtain the appropriate level of spinal anaesthesia,propofol infusion was started at a rate of 100 µg kg^–1min^–1 to reach a BIS level of less than 75 (onset time),and titrated to maintain the BIS value between 65 and 75. Propofolinfusion was stopped 45 min after placing the spinal to measurethe time to reach a BIS level of 90 (recovery time). Results. Median anaesthetic level was T3 (T1–4) in theHS group and T10 (T9-11) in the LS group. In both the HS andthe LS groups, onset time was 226 (47) vs 273 (48) s (P=0.001),recovery time was 234 (47) vs 202 (56) s (P=0.03), total doseof propofol was 2.17 (0.43) vs 3.14 (0.56) mg kg^–1 (P<0.001),respectively. Conclusion. A high spinal block obtained with hyperbaric bupivacaine17.5 mg was associated with a faster onset, delayed recoveryand lower doses of propofol sedation compared with a low spinalblock with 7.5 mg of the same drug.     

EEG-controlled closed-loop dosing of propofol in rats

Background. Based on previous pharmacokinetic and pharmacodynamicstudies, we have developed an EEG-controlled closed-loop systemfor the i.v. hypnotic agent propofol in rats. Methods. Seven adult male Sprague–Dawley rats (weight423–584 g) were included in the study. EEG was recordedwith occipito–occipital needle electrodes and the EEGpower spectrum was estimated. The median frequency (MEF) wasextracted from the power spectrum and was modified MEF (mMEF)to account for the occurrence of spikes and burst suppressionpatterns in the EEG. Propofol infusion was controlled by a model-basedadaptive control algorithm to maintain a set point of mMEF=3.0(SD 0.5) Hz. The performance of the feedback system was characterizedby the median performance error MDPE=median{(mMEF–setpoint)/set point} and the median absolute performance error(MDAPE). The effective therapeutic infusion (ETI) to maintainthe set point was determined from the resulting infusion rates. Results. In all rats a feedback period of 90 min could be performed.Mean MDPE was 1.2 (SE 0.4)% and MDAPE was 13.9 (0.3)%. The ETIwas 0.73 (SD 0.20) mg kg^–1 min^–1. Mean arterialpressure before propofol infusion was 148 (14) mm Hg, with thelowest value during closed-loop infusion being 110 (20) mm Hg. Conclusions. The feedback system presented here may be a usefultool not only for automatic drug control to maintain a definedhypnotic effect but may also be a powerful device in pharmacologicalstudies such as the determination of dose requirements or theassessment of drug–drug interactions. Br J Anaesth 2004; 92: 564–9     

Plasma propofol concentration and EEG burst suppression ratio during normothermic cardiopulmonary bypass

Background. During cardiopulmonary bypass (CPB), several factorsaffect drug disposition and action. This topic has not beenstudied extensively during normothermic CPB. In this study,we related propofol dose to plasma propofol concentration andburst suppression of the EEG during normothermic bypass. Methods. After institutional approval and informed consent,45 patients having cardiac surgery were assigned randomly toreceive propofol infusions at 4 (Group A), 5 (Group B) and 6(Group C) mg kg^–1 h^–1 during normothermic CPB. Inall patients, small to moderate doses of fentanyl were alsoadministered. Plasma propofol concentration and burst suppressionratio (BSR) were measured at the following times: (1) 10 minbefore CPB, (2) 10 min after the start of CPB, (3) 30 min afterthe start of the CPB, (4) just after aortic declamping, and(5) 60 min after CPB. Results. At baseline, plasma propofol concentrations were similaramong the three groups. After the start of CPB, the concentrationsof propofol decreased significantly by 41, 35, and 30% of controlvalues in Groups A, B, and C, respectively. In Group A, theconcentration of propofol during CPB remained unchanged at lessthan the concentration before bypass. In Groups B and C, plasmapropofol concentrations gradually increased during CPB to thepre-bypass concentrations. In Group A, BSR values did not changesignificantly during CPB. In Groups B and C, BSR values graduallyincreased and became significantly greater than baseline values.No patient reported intraoperative awareness. Conclusion. The pharmacokinetics and pharmacodynamics of propofolchange during normothermic CPB. During normothermic CPB, theefficacy of propofol may be enhanced compared with before CPB. Br J Anaesth 2003; 90: 122–6     

Pressure support ventilation during fibreoptic intubation under propofol anaesthesia

Goal of the study. To assess the benefit of pressure supportventilation during fibreoptic intubation performed under propofolanaesthesia in patients having an anticipated difficult intubation. Procedures. Thirty-two patients with ENT cancer, and havingat least two criteria for anticipated difficult intubation wereprospectively included. All patients received topical lidocaine2% and propofol by plasma target control infusion (initial targetconcentration 3 µg ml^–1, then adjusted to maintainloss of consciousness without apnoea). They were randomly assignedbetween two groups: spontaneous breathing (SB) or pressure supportventilation (with a support level set at 10 cm H₂O) both usingFI_O2=1. Conditions for fibreoptic intubation, respiratory parameters(pulse oxymetry, ventilatory frequency, tidal volume and after intubation) and haemodynamicparameters were recorded. Results. Patient characteristic data and intubation conditionswere similar between both groups. All patients had a successfulfibreoptic intubation and none needed a rescue procedure becauseof desaturation. In spite of a longer duration of intubation, after intubation was lower and tidal volume during intubation was higher with pressuresupport ventilation than in SB patients [38.1 (4.2) vs 42.3(4.7) mm Hg and 371 (139) vs 165 (98) ml, respectively]. Desaturationepisodes were observed in two SB patients conversely to no episodeduring pressure support ventilation, probably because of thehigher minute ventilation. Conclusion. Pressure support represents a useful method to improveventilation during fibreoptic intubation under propofol anaesthesiain patients with an anticipated difficult intubation. Presented in abstract at the ASA meeting 2003.     

Effect of total intravenous anaesthesia and balanced anaesthesia on the frequency of coughing during emergence from the anaesthesia

Background: The effects of total intravenous anaesthesia (TIVA) and balancedanaesthesia (BAL) on coughing during emergence from the generalanaesthesia have not yet been compared. Methods: Fifty patients, aged 18–60 yr, undergoing elective lumbardisk surgery were randomly allocated to undergo TIVA (propofol–remifentanil)or BAL (fentanyl–nitrous oxide–sevoflurane). Extubationwas performed in the knee–elbow position, documented onvideo, and subsequently evaluated by blinded examiners. Results: There was no difference between TIVA and BAL patients with respectto patient characteristics, proportion of smokers, surgicaltime, or time of emergence. The median number of coughs wassignificantly lower in the TIVA group (1, range 0–9) thanin the BAL group (4, range 0–20, P = 0.007). Mean maximalheart rate and mean maximal arterial pressure measured duringemergence were also significantly lower in the TIVA group (P= 0.009 and P = 0.006, respectively). Conclusions: During emergence from anaesthesia in the knee–elbow position,TIVA is associated with significantly less coughing and reducedhaemodynamic response when compared with BAL.     

Comparison of the effects of sevoflurane and propofol on cooling and rewarming during deliberate mild hypothermia for neurosurgery

Background. Because the time available for cooling and rewarmingduring deliberate mild hypothermia is limited, studies of therate of the cooling and rewarming are useful. The decrease incore hypothermia caused by heat redistribution depends on theanaesthetic agent used. We therefore investigated possible differencesbetween sevoflurane and propofol on the decrease and recoveryof core temperature during deliberate mild hypothermia for neurosurgery. Methods. After institutional approval and informed consent,26 patients were assigned randomly to maintenance of anaesthesiawith propofol or sevoflurane. Patients in the propofol group(n=13) received propofol induction followed by a continuousinfusion of propofol 3–5 mg kg^–1 h^–1.Patients in the sevoflurane group (n=13) received propofol inductionfollowed by sevoflurane 1–2%. Nitrous oxide and fentanylwere also used for anaesthetic maintenance. After inductionof anaesthesia, patients were cooled and tympanic membrane temperaturewas maintained at 34.5°C. After surgery, patients were activelyrewarmed. Results. There was no difference in the rate of decrease andrecovery of core temperature between the groups. There was alsono difference in skin surface temperature gradient (forearmto fingertip), heart rate and mean arterial blood pressure betweenthe groups. Conclusions. Sevoflurane-based anaesthesia did not affect coolingand rewarming for deliberate mild hypothermia compared withpropofol-based anaesthesia. Br J Anaesth 2003; 90: 32–8     

Measuring depth of anaesthesia using changes in directional connectivity: a comparison with auditory middle latency response and estimated bispectral index during propofol anaesthesia

General anaesthesia is associated with changes in connectivity between different regions of the brain, the assessment of which has the potential to provide a novel marker of anaesthetic effect. We propose an index that quantifies the strength and direction of information flow in electroencephalographic signals collected across the scalp, assess its performance in discriminating ‘wakefulness’ from ‘anaesthesia’, and compare it with estimated bispectral index and the auditory middle latency response. We used a step-wise slow induction of anaesthesia in 10 patients to assess graded changes in electroencephalographic directional connectivity at propofol effect-site concentrations of 2 μg.ml⁻¹, 3 μg.ml⁻¹ and 4 μg.ml⁻¹. For each stable effect-site concentration, connectivity was estimated from multichannel electroencephalograms using directed coherence, together with middle latency response and estimated bispectral index. We used a linear support vector machine classifier to compare the performance of the different electroencephalographic features in discriminating wakefulness from anaesthesia. We found a significant reduction in the strength of long-range connectivity (interelectrode distance > 10 cm) (p < 0.008), and a reversal of information flow from markedly postero-frontal to fronto-posterior (p < 0.006) between wakefulness and a propofol effect-site concentration of 2 μg.ml⁻¹. This then remained relatively constant as effect-site concentration increased, consistent with a step change in directed coherence with anaesthesia. This contrasted with the gradual change with increasing anaesthetic dose observed for estimated bispectral index and middle latency response. Directed coherence performed best in discriminating wakefulness from anaesthesia with an accuracy of 95%, indicating the potential of this new method (on its own or combined with others) for monitoring adequacy of anaesthesia.     

Comparison of propofol/remifentanil and sevoflurane/remifentanil for maintenance of anaesthesia for elective intracranial surgery

Background. Propofol and sevoflurane are suitable agents formaintenance of anaesthesia during neurosurgical procedures.We have prospectively compared these agents in combination withthe short-acting opioid, remifentanil. Methods. Fifty unpremedicated patients undergoing elective craniotomyreceived remifentanil 1 µg kg^–1 followed by an infusioncommencing at 0.5 µg kg^–1 min^–1 reducing to0.25 µg kg^–1 min^–1 after craniotomy. Anaesthesiawas induced with propofol, and maintained with either a target-controlledinfusion of propofol, minimum target 2 µg ml^–1 orsevoflurane, initial concentration 2%_ET. Episodes of mean arterialpressure (MAP) more than 100 mm Hg or less than 60 mm Hg formore than 1 min were defined as hypertensive or hypotensiveevents, respectively. A surgical assessment of operating conditionsand times to spontaneous respiration, extubation, obey commandsand eye opening were recorded. Drug acquisition costs were calculated. Results. Twenty-four and twenty-six patients were assigned topropofol (Group P) and sevoflurane anaesthesia (Group S), respectively.The number of hypertensive events was comparable, whilst morehypotensive events were observed in Group S than in Group P(P=0.053, chi-squared test). As rescue therapy, more labetolol[45 (33) vs 76 (58) mg, P=0.073] and ephedrine [4.80 (2.21)vs 9.78 (5.59) mg, P=0.020] were used in Group S. Between groupdifferences in recovery times were small and clinically unimportant.The combined hourly acquisition costs of hypnotic, analgesic,and vasoactive drugs appeared to be lower in patients maintainedwith sevoflurane than with propofol. Conclusion. Propofol/remifentanil and sevoflurane/remifentanilboth provided satisfactory anaesthesia for intracranial surgery.     

Propofol 1% versus propofol 2% in children undergoing minor ENT surgery

Background. The induction characteristics of propofol 1% and2% were compared in children undergoing ENT surgery, in a prospective,randomized, double-blind study. Methods. One hundred and eight children received propofol 1%(n=55) or 2% (n=53) for induction and maintenance of anaesthesia.For induction, propofol 4 mg kg^–1 was injectedat a constant rate (1200 ml h^–1), supplementedwith alfentanil. Intubating conditions without the use of aneuromuscular blocking agent were scored. Results. Pain on injection occurred in 9% and 21% of patientsafter propofol 1% and 2%, respectively (P=0.09). Loss of consciousnesswas more rapid with propofol 2% compared with propofol 1% (47 svs 54 s; P=0.02). Spontaneous movements during inductionoccurred in 22% and 34% (P=0.18), and intubating conditionswere satisfactory in 87% and 96% (P=0.19) of children receivingpropofol 1% or 2%, respectively. There were no differences betweenthe two groups in respect of haemodynamic changes or adverseevents. Conclusions. For the end-points tested, propofol 1% and propofol2% are similar for induction of anaesthesia in children undergoingminor ENT surgery. Br J Anaesth 2003: 90: 375–7     

Midazolam versus propofol for reducing contractility of fatigued canine diaphragm

The effects of midazolam and propofol on the contractility offatigued canine diaphragm were examined. Diaphragmatic fatiguewas induced by intermittent supramaximal bilateral electrophrenicstimulation at a frequency of 20 Hz applied for 30 min.After fatigue had been induced, group I (n=10) receivedno study drug, group II (n=10) was given a propofol infusion(0.1 mg kg^–1 loading dose plus 1.5 mg kg^–1 h^–1maintenance dose) and group III (n=10) was given a midazolaminfusion (0.1 mg kg^–1 loading dose plus 0.1 mg kg^–1 h^–1maintenance dose). Diaphragmatic contractility was assessedby measuring transdiaphragmatic pressure (P_di). After the fatigue-inducingperiod in each group, P_di at low-frequency (20 Hz) stimulationwas lower than the baseline values (P<0.05), whereas no changein P_di at high-frequency (100 Hz) stimulation was observed.In group II, P_di at 20 Hz stimulation was lower than fatiguedvalues (P<0.05); P_di at 100 Hz stimulation did not change.In group III, P_di at both stimulation frequencies was lowerthan fatigued values (P<0.05). Compared with group I, P_diat 20 Hz stimulation was lower than fatigued values (P<0.05)during administration of the study drug in groups II and III.The decrease in P_di was greater in group III than in group II(P<0.05). In conclusion, midazolam compared with propofolis associated with an inhibitory effect on contractility inthe fatigued canine diaphragm. Br J Anaesth 2001; 86: 879–81     

Predictive performance of 'Servin's formula' during BIS-guided propofol-remifentanil target-controlled infusion in morbidly obese patients

Background. The aim of this study was to assess the predictiveperformance of ‘Servin's formula’ for bispectralindex (BIS)-guided propofol-remifentanil target-controlled infusion(TCI) in morbidly obese patients. Methods. Twenty patients (ASA physical status II–III,age 32–64 yr) undergoing bilio-intestinal bypass surgery,were recruited. Anaesthesia was induced by using a TCI of propofolwith an initial target plasma concentration of 6 µg ml^–1,then adapted to maintain stable BIS values ranging between 40and 50. A TCI of remifentanil was added to achieve pain controland haemodynamic stability. For propofol, weight was correctedas suggested by Servin and colleagues. With ideal body weight(IBW) corrected according to formula suggested by Lemmens andcolleagues. For remifentanil, weight was corrected accordingto IBW. Arterial blood samples for the determination of bloodpropofol concentrations were collected at different surgicaltimes. The predictive performance of propofol TCI was evaluatedby examining performance accuracy. Results. Median prediction error and median absolute predictionerror were –32.6% (range –53.4%; –2.5%) and33.1% (10.8%; 53.4%), respectively. Wobble median value was5.9% (2.5%; 25.2%) while divergence median value was –1.5%h^–1 (–7.7; 33.8% h^–1). Conclusion. Significant bias between predicted and measuredplasma propofol concentrations was found while the low wobblevalues suggest that propofol TCI system is able to maintainstable drug concentrations over time. As already suggested before,a computer simulation confirmed that the TCI system performancecould be significantly improved when total body weight is used.     

Remifentanil target-controlled infusion vs propofol target-controlled infusion for conscious sedation for awake fibreoptic intubation: a double-blinded randomized controlled trial

BACKGROUND: Awake fibreoptic intubation (AFOI) is a technique used in patients with difficult airways. This study compares the suitability of remifentanil target-controlled infusion (TCI) to propofol TCI for conscious sedation during AFOI in patients with bona fide difficult airways. METHODS: We recruited 24, ASA I-III patients, who were undergoing sedation for elective AFOI. Patients were randomized to one of the two groups, Group P (n=10) received propofol TCI and Group R (n=14) received remifentanil TCI. Primary outcome measures were conditions achieved at endoscopy, intubation, and post-intubation, which were graded using scoring systems. Other parameters measured were the endoscopy time, intubation time, and number of attempts at intubation. A postoperative interview was conducted to determine recall of events and level of patient satisfaction. RESULTS: Endoscopy scores (0-5) and intubation scores (0-5) were significantly different [Group P 3 (1-4) vs Group R 1 (0-3) P<0.0001, Group P 3 (2-4) vs Group R 1 (0-3) P<0.0001, respectively]; with much better conditions in Group R, endoscopy times and intubation times were also significantly different, being shorter in Group R (P<0.007 and P<0.023, respectively). Patient tolerance of the procedure, judged by the discomfort scores (P<0.004) and the post-intubation scores (P<0.08), was significantly better in Group R. The level of recall for events was higher in Group R. However, there were no significant differences in the patient satisfaction scores. CONCLUSIONS: Remifentanil TCI appears to provide better conditions for AFOI when compared with propofol TCI. The disadvantage of remifentanil in this setting may be a higher incidence of recall.