Measured context-sensitive half-times of remifentanil and alfentanil

The pharmacokinetics of remifentanil in patients undergoing coronary artery bypass grafting with cardiopulmonary bypass

Remifentanil is a potent opioid with a short duration of action. It has the potential for large-dose opioid anesthesia without an obligatory prolonged period of mechanical ventilation. However, because of high clearance and rapid tissue distribution, cardiopulmonary bypass (CPB) may influence its pharmacokinetics and alter drug requirements. We administered remifentanil by continuous infusion to 68 patients having coronary artery bypass graft surgery during CPB with hypothermia to describe the effects of these interventions on its pharmacokinetics. Remifentanil concentrations were measured before, during, and after CPB. Disposition was best described by a two-compartment model. The volume of distribution increased by 86% with institution of CPB and remained increased after CPB. Elimination clearance decreased by 6.37% for each degree Celsius decrease from 37 degrees C.

IMPLICATIONS

Remifentanil concentrations decrease with the institution of cardiopulmonary bypass because of an increase in the volume of distribution. The decrease in elimination clearance with hypothermia results in increased total remifentanil concentrations during cardiopulmonary bypass if the infusion rate is not altered. More constant blood remifentanil levels may be obtained by reducing remifentanil infusion rate by 30% for each 5 degrees C decrease in temperature.

Hemodynamics and emergence profile of remifentanil versus fentanyl prospectively compared in a large population of surgical patients

STUDY OBJECTIVE

To compare the responses to, and hemodynamics associated with surgical stress, recovery profiles, and anesthesiologists' satisfaction following balanced general anesthesia using either remifentanil or fentanyl in a large-scale population.

DESIGN

Prospective, 1:1 single blind, randomized, controlled effectiveness study in which patients received either remifentanil or fentanyl in combination with a hypnotic-based anesthesia regimen of either isoflurane or propofol.

SETTING

Multicenter study including 156 hospitals and ambulatory surgery facilities.

PATIENTS

2,438 patients (1,496 outpatients and 942 inpatients), 18 years of age or older, scheduled for elective surgeries under general endotracheal anesthesia, with an expected duration of unconsciousness > or =30 minutes.

INTERVENTIONS

Patients were randomized to receive either intravenous (IV) remifentanil (0.5 microg/kg/min for induction and intubation, with the infusion rate decreased to 0.25 microg/kg/min after intubation) or IV fentanyl (administered according to anesthesiologists' usual practice) as the opioid during surgery. Concomitant hypnotic drugs were either propofol and/or isoflurane (with or without nitrous oxide) titrated according to protocol. Transition analgesia with either morphine or fentanyl was given to the remifentanil patients and, at the anesthesiologists' discretion, in the fentanyl patients.

MEASUREMENTS

Vital signs, adverse events, and emergence profiles were assessed and recorded. Recovery profile was assessed by recording time spent in the postanesthesia care unit and step-down recovery unit, number and timing of adverse events, timing and dosage of rescue medications, and time to eligibility for discharge (to home or to hospital room). Anesthesiologists' satisfaction with the anesthetic regimen was assessed at the end of surgery.

MAIN RESULTS

Remifentanil-treated patients exhibited lower systolic and diastolic blood pressures (by 10-15 mmHg) and lower heart rates (by 10-15 bpm) intraoperatively compared to the fentanyl-treated patients. This difference promptly disappeared on emergence. Remifentanil-treated patients responded to verbal command, left the operating room, and (for outpatients) were discharged home sooner than fentanyl-treated patients. Anesthesiologists rated the predictability of response to intraoperative titration, assessment of hemodynamic profiles, and the quality of anesthesia higher in the remifentanil-treated patients.

CONCLUSIONS

This study confirms previous observations on the hemodynamic properties associated with remifentanil and extends these to a wider context than previously reported. These characteristics provide clinicians with an alternative in opioid-based anesthesia.

Effects of combined methohexitone-remifentanil anaesthesia in electroconvulsive therapy

BACKGROUND

Methohexitone is widely used to provide anaesthesia for patients undergoing electroconvulstive therapy (ECT). Short seizure duration, high blood pressures (BP) and heart rates (HR) are usual in elderly patients. In this study, elderly patients undergoing ECT received low dose methohexitone with remifentanil or methohexitone alone and motor seizure duration, haemodynamic response and recovery time were compared.

METHODS

Ten patients, of mean age 74.3 years, were enrolled in this double-blind, randomised crossover trial, receiving a total of 38 ECTs. Each patient was given the following two i.v. regimens in random order: A) methohexitone 0.5 mg kg(-1) combined with remifentanil 1.0 microg kg(-1) and B) methohexitone 0.75 mg kg(-1). Additional methohexitone was given, if needed, until loss of consciousness, and then suxamethonium 1.0 mg kg(-1) for muscular paralysis.

RESULTS

Mean motor seizure duration was significantly longer with methohexitone-remifentanil (37.6 s (SD 12.0)) than with methohexitone alone (27.1 (SD11.5)) (P=0.0009). Recovery time, time to spontaneous breathing, peak postictal changes in BP and HR were similar with both regimens.

CONCLUSION

A reduced dose of methohexitone combined with remifentanil allows prolonged duration of motor seizures in ECT. We conclude that low dose methohexitone combined with a short-acting opioid is a reasonable alternative for elderly patients undergoing ECT, and for other patients with short seizure duration.

Pain management in the critically ill child

Children frequently received no treatment, or inadequate treatment, for pain and for painful procedures. The newborn and critically ill children are especially vulnerable to no treatment or under-treatment. Nerve pathways essential for the transmission and perception of pain are present and functioning by 24 weeks of gestation. The failure to provide analgesia for pain results in rewiring the nerve pathways responsible for pain transmission in the dorsal horn of the spinal cord and results in increased pain perception for future painful results. Many children would withdraw or deny their pain in an attempt to avoid yet another terrifying and painful experiences, such as the intramuscular injections. Societal fears of opioid addiction and lack of advocacy are also causal factors in the under-treatment of pediatric pain. False beliefs about addictions and proper use of acetaminophen and other analgesics resulted in the failure to provide analgesia to children. All children even the newborn and critically ill require analgesia for pain and painful procedures. Unbelieved pain interferes with sleep, leads to fatigue and a sense of helplessness, and may result in increased morbidity or mortality.

Remifentanil

Remifentanil is an opioid with a unique pharmacokinetic profile. Its metabolism by nonspecific esterases results in rapid and uniform clearance leading to highly predictable onset and offset of action. This review will describe the features that set remifentanil apart from other opioids and outline its potential usefulness in a critical care setting. Most studies on remifentanil have been in postoperative neurosurgical and cardiothoracic ICU settings. In the former, the agent has proved especially useful because rapid predictable emergence from sedation allows regular clinical evaluation. Remifentanil also prevents procedure-associated rise in intracranial pressure. In the cardiac setting, the drug provides excellent intraoperative hemodynamic control while allowing return of spontaneous ventilation within minutes of discontinuation of the infusion. One study suggests that the use of remifentanil will reduce the need for postoperative intensive care, especially after hepatic transplantation.

Recovery from propofol anaesthesia supplemented with remifentanil

We have examined the effects on recovery end-points of supplementation of a propofol-based anaesthetic with remifentanil. After induction of anaesthesia with propofol and remifentanil 1.0 microg kg(-1), 15 patients each were randomly allocated to target plasma propofol concentrations of 2, 3, 4 or 5 microg ml(-1) for maintenance of anaesthesia. Remifentanil was administered by infusion for supplementation in doses required for maintenance of adequate anaesthesia. All patients received 50% nitrous oxide in oxygen and ventilation was controlled. The total amount of drugs used and times to different recovery end-points were recorded. Cognitive function was also assessed using a Mini-Mental State questionnaire. The median dose of remifentanil for maintenance of adequate anaesthesia (excluding the initial bolus dose) in the four groups was 0.21, 0.15, 0.11 and 0.13 microg kg(-1) min(-1) respectively (P=0.0026). The median times to eye opening and orientation were shortest in the 2 microg ml(-1) group [6.0 and 6.5 min, 8.5 and 10.8 min, 13.4 and 15.8 min, and 14.2 and 19.5 min respectively in the propofol 2, 3, 4, and 5 microg ml(-1) groups respectively (P<0.001)]. The times to discharge from the recovery ward and the Mini-Mental State scores were not significantly different.

Sevoflurane--nitrous oxide anaesthesia supplemented with remifentanil: effect on recovery and cognitive function

The aim of this study was to compare recovery and psychomotor performance after maintenance of anaesthesia with sevoflurane or sevoflurane supplemented with remifentanil. Sixty-six per cent nitrous oxide was used in all patients. Twenty patients each were randomly allocated to maintenance of anaesthesia with sevoflurane only in concentrations necessary to maintain adequate anaesthesia or with 1.5, 1.0 or 0.5 MAC (end-tidal) of sevoflurane supplemented with remifentanil. The median dosage of remifentanil required in the last three groups was 0.21, 0.25 and 0.34 microg x kg(-1) x min(-1), respectively (p < 0.05). The median times to eye opening were 10.3, 12.7, 11.0 and 6.5 min in the four groups (p < 0.05 between the 0.5 MAC and the other groups) and for orientation 12.1, 14.9, 12.3 and 8.3 min, respectively (p < 0.05 between 0.5 and 1.5 MAC groups). There was no significant difference in the mini-mental state assessment scores or the actual discharge times from the recovery ward among the groups. Significantly greater numbers of patients could perform the critical flicker fusion test at 15 min in the group receiving the lowest concentration of sevoflurane and the highest dosage of remifentanil (p < 0.05). Patients in this group also showed the highest incidence of chest wall rigidity (p < 0.003). We conclude that, while the use of remifentanil with lower concentrations of sevoflurane facilitates early recovery, it does not influence discharge time from recovery ward and may be associated with side-effects such as chest wall rigidity.

Remifentanil versus meperidine for monitored anesthesia care: a comparison study in older patients undergoing ambulatory colonoscopy

Colonoscopy is one of the most frequently performed outpatient procedures in the United States. This study was designed to test the hypothesis that a remifentanil infusion would be superior to boluses of meperidine in older patients undergoing ambulatory colonoscopy. One hundred ASA physical status I-IV patients undergoing colonoscopy were randomized in this double-blinded study to receive either remifentanil infusions (n = 49) or titrated boluses of meperidine (n = 51). Patient tolerance was assessed using physiologic variables and side effects associated with opioid analgesia. Verbal pain/anxiety and patient/operator satisfaction were also assessed. As a group, the physiologic characteristics demonstrated no significant differences in the response to the colonoscopy procedure. Although the patient and operator satisfaction surveys were similar between groups, the incidences of tachycardia, hypotension, and nausea were less and the adjusted verbal pain and anxiety scores were more in the Remifentanil group compared with the Meperidine group. This study demonstrates that remifentanil and meperidine were equally well tolerated in older patients undergoing ambulatory colonoscopy when administered by an anesthesia provider. The differences in the pharmakinetics of remifentanil and meperidine most likely account for the differences noted between the two treatment groups.

IMPLICATIONS

Remifentanil infusions and meperidine boluses are equally well tolerated in older patients undergoing ambulatory colonoscopy when administered by an anesthesia provider.

All Half-Lives Are Wrong, But Some Half-Lives Are Useful

The half-life of a drug, which expresses a change in concentration in units of time, is perhaps the most easily understood pharmacokinetic parameter and provides a succinct description of many concentration-time profiles. The calculation of a half-life implies a linear, first-order, time-invariant process. No drug perfectly obeys such assumptions, although in practise this is often a valid approximation and provides invaluable quantitative information. Nevertheless, the physiological processes underlying half-life should not be forgotten. The concept of clearance facilitates the interpretation of factors affecting drug elimination, such as enzyme inhibition or renal impairment. Relating clearance to the observed concentration-time profile is not as naturally intuitive as is the case with half-life. As such, these 2 approaches to parameterising a linear pharmacokinetic model should be viewed as complementary rather than alternatives. The interpretation of pharmacokinetic parameters when there are multiple disposition phases is more challenging. Indeed, in any pharmacokinetic model, the half-lives are only one component of the parameters required to specify the concentration-time profile. Furthermore, pharmacokinetic parameters are of little use without a dose history. Other factors influencing the relevance of each disposition phase to clinical end-points must also be considered. In summarising the pharmacokinetics of a drug, statistical aspects of the estimation of a half-life are often overlooked. Half-lives are rarely reported with confidence intervals or measures of variability in the population, and some approaches to this problem are suggested. Half-life is an important summary statistic in pharmacokinetics, but care must be taken to employ it appropriately in the context of dose history and clinically relevant pharmacodynamic end-points.

Is there a learning curve associated with the use of remifentanil?

This study prospectively determined whether there was a learning curve with the use of remifentanil, as indicated by decreased hemodynamic variability, improved recovery profile, and decreased incidence of opioid-related adverse events with increasing experience. Patients undergoing diverse surgical procedures (outpatient [n = 1340] and inpatient [n = 560]) were enrolled by investigators (n = 190) who had no previous experience with remifentanil use. Each investigator enrolled 10 patients. A standardized protocol for administration of remifentanil was used. Data were analyzed to determine differences between the first three patients and the last three patients enrolled for each anesthesiologist in the study. There were no differences in hemodynamic variables between the first triad and the last triad in either outpatients or inpatients. Requirements for hypnotic drugs and the doses of remifentanil used were also similar between groups. Analgesic medications administered at the end of surgery and in the postanesthesia care unit (PACU) were similar between groups, except that the last triad in the outpatient group received smaller doses of fentanyl compared with the first triad. Times to response to verbal command, tracheal extubation, and operating room discharge did not differ between groups. However, patients in the last triad undergoing outpatient surgery had shorter times to eligibility for PACU discharge, but times to eligibility for discharge home did not differ. The overall incidence of all adverse events (i.e., hypotension, hypertension, muscle rigidity, respiratory depression, apnea, nausea, and vomiting) was less in the last triad as compared with the first triad. When analyzed separately, only the incidence of vomiting (in the outpatient group) was decreased in the last triad as compared with the first triad. This study suggests that there is a learning curve that aids reduction of minor adverse effects associated with the use of analgesic medications administered at the end of surgery in outpatients, which might have reduced the incidence of postoperative vomiting and the duration of PACU stay.

IMPLICATIONS

This study demonstrated that anesthesiologists rapidly acquire the ability to use remifentanil with limited experience. However, there is a learning curve that aids reduction of minor adverse effects associated with the use of analgesic medications administered at the end of surgery in outpatients, which might have reduced the incidence of postoperative vomiting and the duration of postanesthesia care unit stay.

Comparison of intubating conditions following propofol and succinylcholine with propofol and remifentanil 2 micrograms kg-1 or 4 micrograms kg-1

We evaluated the intubating conditions, haemodynamic responses and duration of apnoea in 60 healthy adult patients after propofol 2 mg kg-1 combined with either a bolus of remifentanil 2 micrograms kg-1 or 4 micrograms kg-1, or succinylcholine 1 mg kg-1. Patients intubated following remifentanil showed dose-dependent intubating conditions, similar at 4 micrograms kg-1 to the conditions produced with succinylcholine. Post-induction mean arterial pressure decreased from baseline values by 21% (P < 0.0001), 28% (P < 0.0001) and 8% (P > 0.05) in the remifentanil 2 micrograms kg-1, remifentanil 4 micrograms kg-1 and succinylcholine 1 mg kg-1 groups, respectively. The mean (SD) duration of apnoea following induction was 9.3 (2.6) min and 12.8 (2.9) min in the remifentanil 2 micrograms kg-1 and 4 micrograms kg-1 groups, and 6.0 (0.9) min in the succinylcholine group (P < 0.001 between groups).

Comparison of remifentanil versus regional anaesthesia in children anaesthetised with isoflurane/nitrous oxide. International Remifentanil Paediatric Anaesthesia Study group

We compared the efficacy and safety of a remifentanil (0.25 microg x kg(-1) x min(-1)-based balanced anaesthetic technique with a bupivacaine-based regional anaesthetic technique in an open label, multicentre study in 271 ASA physical status 1 or 2 children aged 1-12 years. Subjects requiring major intra-abdominal, urological or orthopaedic surgery were randomly allocated to receive either intravenous remifentanil (group R; n = 185) or epidural bupivacaine (group B; n = 86) with isoflurane/nitrous oxide for their anaesthesia. The majority of children in both groups (85% in group R, 78% in group B) showed no defined response to skin incision, and although the mean increase in systolic blood pressure (+11 mm Hg) was significantly greater in group R than in group B, this change did not represent a serious haemodynamic disturbance. More children in group R (31%) required interventions to treat hypotension and/or bradycardia than those in group B (12%), but these were easily managed by administration of fluids or anticholinergic drugs. Adverse events, mainly nausea and/or vomiting, occurred in 45% of group R and 42% of group B (NS). The adverse event profile of remifentanil in this study was typical of a potent mu-opioid receptor agonist. Remifentanil was as effective as epidural or caudal block in providing analgesia and suppressing physiological responses to surgical stimuli in children aged between 1 and 12 years undergoing major abdominal, urological, or orthopaedic surgery under isoflurane/nitrous oxide anaesthesia.

Tracheal intubation after induction of anesthesia in children with propofol--remifentanil or propofol-rocuronium

PURPOSE

To compare the intubating conditions after remifentanil-propofol with those after propofol-rocuronium combination with the aim of determining the optimal dose of remifentanil.

METHODS

In a randomized, double-blind study 80 healthy children aged three to nine years were assigned to one of four groups (n=20): 2 or 4 microgxkg(-1) remifentanil (Re2 or Re4); 2 microgxkg(-1) remifentanil and 0.2 mgxkg(-1) rocuronium (Re2-Ro0.2); 0.4 mgxkg(-1) rocuronium (Ro0.4). After atropine, remifentanil was injected over 30 sec followed by 3.5 mgxkg(-1) propofol and rocuronium. After 60 sec, laryngoscopy and intubation were attempted. Intubating conditions were assessed as excellent, good or poor based on ease of ventilation, jaw relaxation, position of the vocal cords, and coughing to intubation.

RESULTS

In all children intubation was successful. Overall intubating conditions were better (P < 0.01), and the frequency of excellent conditions, 85%, was higher (P < 0.01) in the Re4 group than in the Ro0.4 group. No child manifested signs of muscular rigidity. In the remifentanil groups, arterial pressure decreased 11-13% and heart rate 6-9% after anesthetic induction, and remained at that level throughout the study.

CONCLUSION

The best intubating conditions were produced by the combination of 4 microgxkg(-1) remifentanil and 3.5 mgxkg(-1) propofol. It provided excellent or good intubating conditions in all children without causing undue cardiovascular depression.

A preliminary investigation of remifentanil as a labor analgesic

IMPLICATIONS

In this preliminary investigation, we evaluated the safety and analgesic efficacy of IV remifentanil for labor pain. Four women were studied, and then the trial was terminated because administration of this novel synthetic opioid produced significant maternal side effects in the absence of effective pain control.

A comparison of anaesthesia using remifentanil combined with either isoflurane, enflurane or propofol in patients undergoing gynaecological laparoscopy, varicose vein or arthroscopic surgery

BACKGROUND

Anaesthesia comprising remifentanil plus isoflurane, enflurane or propofol was randomly evaluated in 285, 285 and 284 patients, respectively, undergoing short-procedure surgery.

METHODS

Anaesthesia was induced with propofol (0.5 mg x kg(-1) and 10 mg x 10 s(-1)), and a remifentanil bolus (1 microg x kg(-1)) and infusion at 0.5 microg x g(-1) x min(-1). Five minutes after intubation, remifentanil infusion was halved and 0.5 MAC of isoflurane or enflurane, or propofol at 100 microg x kg(-1) x min(-1) were started and titrated for maintenance.

RESULTS

Patient demography and anaesthesia duration were similar between the groups. Surgery was performed as daycases (52%) or inpatients (48%). The median times (5-7 min) to extubation and postoperative recovery were similar between the groups. Responses to tracheal intubation (15% vs 8%) and skin incision (13% vs 7%) were significantly greater in the total intravenous anaesthesia (TIVA) group (P<0.05). Fewer patients given remifentanil and isoflurane (21%) or enflurane (19%) experienced > or =1 intraoperative stress response compared to the TIVA group (28%) (P<0.05). Median times to qualification for and actual recovery room discharge were 0.5-0.6 h and 1.1-1.2 h, respectively. The most common remifentanil-related symptoms were muscle rigidity (6-7%) at induction, hypotension (3-5%) and bradycardia (1-4%) intraoperatively and, shivering (6-7%), nausea and vomiting postoperatively. Nausea (7%) and vomiting (3%) were significantly lower with TIVA compared with inhaled anaesthetic groups (14-15% and 6-8%, respectively; P<0.05).

CONCLUSION

Anaesthesia combining remifentanil with volatile hypnotics or TIVA with propofol was effective and well tolerated. Times of extubation, postanaesthesia recovery and recovery room discharge were rapid, consistent and similar for all three regimens.

The use of remifentanil for Cesarean section in a parturient with recurrent aortic coarctation

PURPOSE

To illustrate the clinical utility of a short acting opioid (remifentanil) based general anesthetic for Cesarean section in a parturient with compromised cardiac function.

CLINICAL FEATURES

A 23-yr-old primigravida, complicated by a recurrent aortic coarctation with an approximate 50% narrowing of the aortic arch, presented for elective Cesarean section at 37 wk gestational age. Initially asymptomatic, her clinical condition had deteriorated as the pregnancy progressed, with worsening episodes of mild chest pain and shortness of breath. A semi-elective Cesarean section under general anesthesia was planned at 37 wk to minimize the potential for aortic complications associated with the hemodynamic stress of labour. Remifentanil was infused at 0.05 to 0.1 microg x kg(-1) x min(-1) with good sedation and analgesia for the placement of invasive monitors. The infusion was increased to 0.2 microg x kg(-1) x min(-1) for induction, and combined with isoflurane 0.4 to 0.6% for maintenance of anesthesia. The patient maintained stable hemodynamics throughout and her trachea was extubated without difficulty at the end of the procedure. The newborn did not require tracheal intubation, mask ventilation or naloxone and was in excellent condition upon transfer to the well baby nursery.

CONCLUSION

Remifentanil, when used as part of an opioid-based general anesthetic for Cesarean section, can provide maternal hemodynamic stability with minimal neonatal respiratory depression and should allow for immediate postoperative tracheal extubation of the mother.

Tracheal intubation without the use of muscle relaxants: remifentanil or alfentanil in combination with propofol

BACKGROUND

Alfentanil-propofol combination provides adequate conditions for tracheal intubation without neuromuscular blocking drugs in most patients. Providing an option for intense opioid effect without compromising recovery after short operations, remifentanil might offer benefits over alfentanil, especially in ambulatory surgery. In this study intubating conditions after remifentanil-propofol were compared to those after alfentanil-propofol.

METHODS

In a randomized, double-blind study 60 healthy patients were assigned to one of three groups (n=20). After intravenous atropine, remifentanil 3 or 4 microg kg(-1) (Rem3 or Rem4) or alfentanil 30 microg kg(-1) (Alf30) was injected over 30 s followed by propofol 2.5 mg kg(-1). Sixty seconds after the administration of propofol, laryngoscopy and intubation were attempted. Intubating conditions were assessed as excellent, good or poor on the basis of ease of ventilation, jaw relaxation, position of the vocal cords, and patient response to intubation and slow inflation of the endotracheal tube cuff.

RESULTS

Overall conditions at intubation were significantly (P<0.05) better, and the frequency of excellent conditions was significantly (P<0.05) higher in the Rem4 group compared with the Alf30 group. Intubation was judged to be impossible in 20%, 25% or 5% of the patients in the Alf30, Rem3 or Rem4 groups, respectively. No patient manifested signs of opioid-induced muscular rigidity. In terms of arterial pressures or heart rate, there were no differences between the groups.

CONCLUSION

The best method was the combination of remifentanil 4 microg kg(-1) and propofol 2.5 mg kg(-1). This provided satisfactory intubating conditions in 93%, and prevented cardiovascular intubation response.

A multicenter comparison of isoflurane and propofol as adjuncts to remifentanil-based anesthesia

STUDY OBJECTIVE

To compare recovery, hemodynamics, and side effects of remifentanil-based anesthesia with hypnotic concentrations of isoflurane or propofol.

DESIGN

Multicenter, prospective, randomized, two-group study.

SETTING

15 university and 5 municipal hospitals.

PATIENTS

249 ASA physical status I, II, and III adult patients scheduled for elective gynecological laparoscopy, varicose vein, or arthroscopic surgery of at least 30 minutes' duration.

INTERVENTIONS

Anesthesia was induced in the same manner in both groups: remifentanil-bolus (1 microg/kg), start of remifentanil-infusion (0. 5 microg/kg/min), followed by propofol as needed for induction. Five minutes after intubation, remifentanil was reduced to 0.25 microg/kg/min, and it was combined with either a propofol-infusion (0.1 mg/kg/min) or with isoflurane (0.6 vol% end-tidal) in O(2)/air. Adverse hemodynamic responses of heart rate and systolic blood pressure were recorded and treated according to a predefined protocol. With termination of surgery, anesthetic delivery was discontinued simultaneously without tapering, and recovery times were recorded.

MEASUREMENTS AND MAIN RESULTS

No significant differences were observed between the remifentanil-isoflurane or remifentanil-propofol treatment regimens. Recovery times (means +/- SD) were similar for spontaneous ventilation (5.8 +/- 3.2 min vs. 6. 3 +/- 3.7 min), extubation (7.6 +/- 3.5 vs. 8.5 +/- 4.2 min), eye opening (6.8 +/- 3.2 vs. 7.5 +/- 3.8 min), and arrival to the postanesthesia care unit (16.5 +/- 7.0 vs.18.0 +/- 7.2 min). There were no significant differences in adverse hemodynamic responses, postoperative shivering, nausea, or vomiting between the groups.

CONCLUSIONS

Emergence after remifentanil-based anesthesia with 0.6 vol% of isoflurane is at least as rapid as with 0.1 mg/kg/min propofol. Both isoflurane and propofol are suitable adjuncts to remifentanil, and the applied dosages are clinically equivalent with respect to emergence and recovery. Therefore, both combinations should be appropriate, particularly in settings in which rapid recovery from anesthesia is desirable, such as fast tracking and/or ambulatory surgery.

Safety and feasibility of continuous infusion of remifentanil in the neurosurgical intensive care unit

OBJECTIVE

Remifentanil is a selective mu-opioid agonist with a context-sensitive half-time of 3 to 5 minutes, independent of dose or administration duration. Other desirable effects include decreased cerebral metabolism and intracranial pressure (ICP) with minimal cerebral perfusion pressure changes. We present six cases illustrating indications for the use of remifentanil in the neurosurgical intensive care unit.

METHODS

Patients received bolus doses of remifentanil of 0.05 to 1.0 microg/kg, followed by continuous infusions of 0.03 to 0.26 microg/kg/min, titrated to effect. When infusions were discontinued for neurological examinations, another bolus dose preceded infusion reinstitution. Indications for the use of remifentanil included mean arterial pressure and cerebral perfusion pressure decreases with the use of other agents (e.g., codeine or propofol) for ICP control, elevated ICP that was refractory to propofol/mannitol treatment, agitation that was unresponsive to standard therapies, and coughing that caused ICP increases after subarachnoid hemorrhage.

RESULTS

Three patients experienced spontaneous intracranial bleeding (two cases of subarachnoid hemorrhage and one case of intraventricular hemorrhage), and three patients exhibited severe traumatic subdural hemorrhage. All patients recovered from the effects of remifentanil within 3 minutes after discontinuation of infusion, which allowed frequent rapid neurological assessments. Procedures for pulmonary toilet (i.e., endotracheal suctioning, postural drainage, and bronchoscopy) were performed without deleterious ICP increases or mean arterial pressure or cerebral perfusion pressure decreases during remifentanil infusions.

CONCLUSION

The ultrashort duration of action of remifentanil allowed easy performance of frequent neurological examinations in the neurosurgical intensive care unit. No patient experienced deleterious hemodynamic or neurological effects as a result of remifentanil use.

The pharmacokinetics of anesthetic drugs and adjuvants during cardiopulmonary bypass

The institution of cardiopulmonary bypass during cardiac surgery has profound effects on the plasma concentration of drugs and thus their therapeutic effectiveness. These changes occur through acute hemodilution, altered plasma protein binding, hypotension, as well as the use of hypothermia and heparin administration. Isolation of the lungs from the circulation and the possible sequestration of drugs in the bypass circuit also affect drug plasma concentrations on bypass. The individual characteristics of the drug in question are also important in determining the final plasma concentration: Lipid soluble drugs with a high volume of distribution may be more readily taken up by bypass equipment, but the initial fall in concentration at the start of cardiopulmonary bypass may be more readily counteracted by back diffusion into plasma, if large tissue stores have accumulated. The extent of the drug's plasma protein binding is of importance as the effective free fraction in plasma for highly bound drugs will be sensitive to changes in plasma protein binding brought on by factors such as hemodilution, heparin administration as well as alpha, acid-glycoprotein binding. Clearly the fate of drugs administered before or on bypass is complex and can only be accurately determined by specific studies evaluating drug plasma concentrations. This review updates the available data on anesthetics and drugs used during cardiac surgery in order that anesthetists may predict better the likely effect of drugs administered before or during cardiopulmonary bypass.

Reducing the demand for admission to intensive care after major abdominal surgery by a change in anaesthetic practice and the use of remifentanil

We have used a remifentanil-based anaesthetic for patients undergoing major abdominal surgery who would normally have gone to the intensive care unit in this hospital. Avoiding intensive care unit admission was considered an advantage as a shortage of intensive care unit beds was leading to the cancellation of operations. We first used remifentanil as part of a safety and efficacy study. The aim was to see if the rapid and complete awakening obtained when using this drug would allow us to avoid the need for admission to an intensive care unit and use a high dependency unit instead. This was shown to be practicable. In comparison with a group of patients matched retrospectively for the type of operation before remifentanil was used there was a reduction in the length of time (mean+/- SD) patients' lungs were ventilated (612+/-417 vs. 9.9+/-28.9 min P< 0.0001). There was no saving in cost ( pound808.71+/- pound187.06 vs. pound795.27+/- pound253.49). When remifentanil was used routinely (after the safety and efficacy study), there were significant reductions in the time to tracheal extubation (612+/-417 vs. 4+/-10 min P < 0.0001) and costs (808.71I vs. 392.10 I P < 0.0001) compared with other patients in whom it was not used. Patients waiting for a liver transplant were also being cancelled when a donor organ became available because of the shortage of intensive care unit beds. Based on our other experience with remifentanil, we used a similar anaesthetic technique for these patients. It proved possible to extubate the trachea in 12 of 15 patients at the end of the operation. No patient needed re-intubation. The need for intensive care and therefore cancellation of surgery was reduced. In contrast, only one patient's trachea was extubated immediately after surgery in the group of patients anaesthetized before the introduction of remifentanil. A remifentanil-based technique in combination with a change in organization has therefore enabled us to avoid admission to the intensive care unit for these patients.

Continuous infusion of remifentanil and target-controlled infusion of propofol for patients undergoing cardiac surgery: a new approach for scheduled early extubation

OBJECTIVE

To assess hemodynamic stability, postoperative pain management, and the control and timing of early extubation of a total intravenous anesthetic technique using propofol target-controlled infusion (TCI) and remifentanil in cardiac surgery.

DESIGN

Prospective study.

SETTING

University hospital.

PARTICIPANTS

Fifty patients scheduled for elective cardiac surgery.

INTERVENTIONS

Premedication consisted of oral midazolam, 0.1 mg/kg. Anesthesia was induced with propofol TCI at a target concentration of 1.5 to 2 microg/mL; remifentanil, 1 microg/kg; and rocuronium. Anesthesia was maintained with propofol at the same target concentration and remifentanil titrated between 0.25 and 1 microg/kg/min. Thirty minutes before the end of surgery, a 0.1-mg/kg bolus of morphine was administered intravenously. Postoperative sedation was achieved by maintaining the propofol infusion until the patient was deemed ready for extubation. Postoperative pain relief was evaluated using a visual analog scale. The intervals between arrival in the intensive care unit, spontaneous ventilation, and extubation were recorded.

MEASUREMENTS AND MAIN RESULTS

Included in this study were 36 men and 14 women (American Society of Anesthesiologist = III; New York Heart Association = II) scheduled for cardiac surgery. All patients remained hemodynamically stable throughout the perioperative period. Thirty-seven patients were successfully extubated during the first 4 postoperative hours. Spontaneous breathing was achieved at a mean interval of 15+/-5 minutes after propofol discontinuation. The mean interval to extubation was 163+/-45 minutes after arrival in the intensive care unit. Extubation was performed 48+/-12 minutes after patients were considered ready to awaken. During spontaneous ventilation, 36 patients received additional boluses of morphine (mean, 2.5+/-1 mg). Subsequently, all patients achieved a visual analog scale less than 40 mm.

CONCLUSION

The combination of remifentanil and propofol TCI resulted in hemodynamic stability and good postoperative analgesia. This technique allows physicians to schedule the time of extubation in patients undergoing cardiac anesthesia.

Propofol anesthesia for craniotomy: a double-blind comparison of remifentanil, alfentanil, and fentanyl

For patients undergoing craniotomy, it is desirable to have stable and easily controllable hemodynamics during intense surgical stimulation. However, rapid postoperative recovery is essential to assess neurologic function. Remifentanil, an ultra-short-acting mu-opioid receptor agonist, may be the ideal agent to confer the above characteristics. In this prospective randomized study, we compared the hemodynamic stability, recovery characteristics, and the dose of propofol required for maintaining anesthesia supplemented with an infusion of remifentanil, alfentanil, or fentanyl in 34 patients scheduled for supratentorial craniotomy. With routine monitors in place, anesthesia was induced with propofol (2-3 mg/kg), atracurium (0.5 mg/kg), and either remifentanil (1 microg/kg), alfentanil (10 microg/kg), or fentanyl (2 micro/kg). The lungs were ventilated with O2/air to mild hypocapnia. Anesthesia was maintained with infusions of propofol (50-100 microg/kg/min) and either remifentanil (0.2 microg/kg/min), alfentanil (20 microg/kg/h), or fentanyl (2 microg/kg/h). There were no significant differences among the groups in the dose of propofol maintenance required, heart rate, or mean arterial pressure. However, the time to eye opening (minutes) was significantly shorter in the remifentanil compared to the alfentanil group (6+/-3; 21+/-14; P = 0.0027) but not the fentanyl group (15+/-9). We conclude that remifentanil is an appropriate opioid to use in combination with propofol during anesthesia for supratentorial craniotomy.

Dosing study of remifentanil and propofol for tracheal intubation without the use of muscle relaxants

Sixty ASA I and II patients, premedicated with midazolam, were administered propofol 2 mg x kg-1 and remifentanil 3 microg x kg-1 (group R3), remifentanil 4 microg x kg-1 (group R4) and remifentanil 5 microg x kg-1 (group R5). Laryngoscopy and intubation were performed 1 min after the administration of the study drugs and the intubating conditions were assessed. Good to excellent conditions were observed in 12 patients in group R3 compared with 19 patients each in groups R4 and R5 (p = 0.004). Significant reductions in mean arterial pressure (MAP) and heart rate (HR) after administration of the study drug were observed in each group, p < 0.01. There was, however, no difference in mean MAP and HR between the three groups at all time points. We conclude that remifentanil 4-5 microg x kg-1 may reliably provide good to excellent conditions for tracheal intubation when administered after propofol 2 mg x kg-1.

Lack of rapid development of opioid tolerance during alfentanil and remifentanil infusions for postoperative pain

Studies in animals and volunteers have suggested the development of acute tolerance to opioid analgesics. In this article, we present data from patients who regulated their own target-controlled infusions of alfentanil and remifentanil to provide analgesia in the immediate postoperative period. Fifty-one patients received alfentanil for 24 h after cardiac surgery, and 30 patients received remifentanil for 6 h after orthopedic surgery. Satisfactory analgesia, defined as a rating of < or =3 on an 11-point visual analog scale, was obtained by patients after each type of surgery. The target concentrations of the opioids required to produce postoperative analgesia and the cumulative opioid doses administered over the course of the clinical observation suggest there was no tolerance to the analgesic effects of the opioids. The requirements for both analgesic drugs in individual patients had a large variation (>200%). We conclude that our results may indicate an absence of tolerance to opioids in postoperative analgesia. Nonetheless, our data show that the postoperative requirement for these rapidly acting drugs is qualitatively similar to that for other opioids in that dosage escalation does not occur.

IMPLICATIONS

The development of acute tolerance to opioid analgesics has been suggested based on experimental studies in animals and volunteers. Our report from patients who self-controlled their analgesic requirements by using target-controlled infusions of alfentanil and remifentanil for postoperative analgesia provides no evidence of tolerance to opioids.

Context-sensitive half-time and anesthesia: how does theory match reality?

The pharmacokinetic model, which describes context-sensitive decrement times, has received validation through direct measurement. Further validation has come from the ability of targeted infusion schemes to obtain reliable and stable concentrations of a variety of intravenous drugs. A complete understanding of the concept will enable the appropriate selection of drugs and an appreciation of the strengths and limitations of current drug delivery systems.

Pharmacokinetics of opioids in liver disease

The liver is the major site of biotransformation for most opioids. Thus, the disposition of these drugs may be affected in patients with liver insufficiency. The major metabolic pathway for most opioids is oxidation. The exceptions are morphine and buprenorphine, which primarily undergo glucuronidation, and remifentanil, which is cleared by ester hydrolysis. Oxidation of opioids is reduced in patients with hepatic cirrhosis, resulting in decreased drug clearance [for pethidine (meperidine), dextropropoxyphene, pentazocine, tramadol and alfentanil] and/or increased oral bioavailability caused by a reduced first-pass metabolism (for pethidine, dextropropoxyphene, pentazocine and dihydrocodeine). Although glucuronidation is thought to be less affected in liver cirrhosis, and clearance of morphine was found to be decreased and oral bioavailability increased. The consequence of reduced drug metabolism is the risk of accumulation in the body, especially with repeated administration. Lower doses or longer administration intervals should be used to remedy this risk. Special risks are known for pethidine, with the potential for the accumulation of norpethidine, a metabolite that can cause seizures, and for dextropropoxyphene, for which several cases of hepatotoxicity have been reported. On the other hand, the analgesic activity of codeine and tilidine depends on transformation into the active metabolites, morphine and nortilidine, respectively. If metabolism is decreased in patients with chronic liver disease, the analgesic action of these drugs may be compromised. Finally, the disposition of a few opioids, such as fentanyl, sufentanil and remifentanil, appears to be unaffected in liver disease.

General anesthesia with remifentanil for Cesarean section in a parturient with an acoustic neuroma

PURPOSE

To describe the anesthetic management of a parturient with a large acoustic neuroma undergoing general anesthesia with remifentanil for Cesarean section.

CLINICAL FEATURES

A near-term parturient presented with a large intracranial mass. Cesarean section under general anesthesia was elected one week prior to craniotomy for tumour resection. Remifentanil infusion, 0.2-1.0 microg x kg(-1) x min(-1), was used from induction to emergence of general anesthesia. The neonate was born seven minutes after the remifentanil infusion was started. She had normal umbilical cord pH and her Apgar scores were 7 and 8, at one and five minutes respectively. Although the neonate received supplemental oxygen, she did not require naloxone. Both mother and neonate made an uneventful recovery.

CONCLUSION

Remifentanil was effective in producing stable hemodynamic conditions, without severe neonatal respiratory depression, during induction and maintenance of general anesthesia for a Cesarean delivery in a parturient with a large intracranial tumour.

RETRACTED: Total intravenous anesthesia with remifentanil and propofol for implantation of cardioverter-defibrillators in patients with severely reduced left ventricular function

OBJECTIVE

To determine the cardiocirculatory effects of total intravenous anesthesia (TIVA) using remifentanil and propofol in high-risk cardiac surgical patients.

DESIGN

Prospective study of 20 patients undergoing first-time implantation of a cardioverter-defibrillator (ICD).

SETTING

Major, community, university-affiliated hospital.

PARTICIPANTS AND INTERVENTIONS

In 20 patients with severely reduced left ventricular function (left ventricular ejection fraction <30%) undergoing first-time implantation of an ICD, TIVA using remifentanil and propofol was performed.

MEASUREMENTS AND MAIN RESULTS

Extensive hemodynamic monitoring using a pulmonary artery catheter was performed: (T1) before induction of anesthesia, (T2) after intubation, (T3) after skin incision, (T4) after first defibrillation, and (T5) 10 minutes after extubation. Propofol, 3.0 +/- 0.6 mg/kg/h (range, 1.9 to 4.4 mg/kg/h), and remifentanil, 0.30 +/- 0.05 microg/kg/min (range, 0.21 to 0.40 microg/kg/min), were used. Total costs added up to US $44.60 per patient. Patients could be extubated within 12.5 +/- 4.2 minutes after stopping anesthesia. There were significant decreases in heart rate (HR; from 77 +/- 12 to 57 +/- 10 beats/min [T3]), mean arterial blood pressure (MAP; from 98 +/- 14 to 70 +/- 12 mmHg [T2]), and systemic vascular resistance (from 1,551 +/- 309 to 1,233 +/- 274 dyne x s x cm(-5) [T2]). Cardiac index (CI) slightly decreased only at T3 (from 2.46 +/- 0.42 to 1.92 +/- 0.29 L/min/m2; p = 0.04). The decrease in MAP could easily be treated by volume infusion in most patients (17 patients). Sixty-five percent of the patients needed dobutamine to increase CI to greater than 2.0 L/min/m2 (mean dose, 2.2 +/- 1.8 microg/kg/min). Dobutamine could be stopped before extubation in all patients. No patient needed sustained inotropic or ventilatory support and intensive care therapy could be avoided.

CONCLUSION

TIVA using remifentanil and propofol in patients with severely reduced left ventricular function is safe, well-controllable, and allows early extubation after implantation of an ICD. Because patients without complications did not need a postoperative intensive care stay, costs may be considerably reduced.

The safety and effectiveness of remifentanil as an adjunct sedative for regional anesthesia

We assessed the sedative potential of continuous infusions of remifentanil with a validated composite alertness scale in 160 patients (ASA physical status I or II) undergoing hip replacement surgery with spinal block (n = 61) or hand surgery using brachial plexus block (n = 93). They were randomized to receive one of the following initial dose regimens in double-blinded fashion: placebo or 0.04, 0.07, or 0.1 microg x kg(-1) x min(-1) remifentanil subsequently titrated to effect. Additional midazolam IV was allowed for adequate sedation as required. The combined analysis of both surgery groups revealed a dose-related increase in achievement of sedation level > or =2 within 15 min of the start of the study drug infusion; all remifentanil dose comparisons with placebo reached significance (P < 0.001). The remifentanil 50% effective dose for a composite sedation level > or =2 within 15 min of the start of drug infusion was estimated as 0.043 microg x kg(-1) x min(-1) (95% confidence interval 0.01, 0.059). The requirement for midazolam decreased with increasing remifentanil dose compared with placebo (P < 0.001). The median time to return to alertness after the end of infusion was 10-12 min in the remifentanil groups and 5 min in the placebo group. Significant incidences of nausea, pruritus, sweating, and respiratory depression were reported during remifentanil infusions compared with placebo. The data suggest that remifentanil may be useful for supplementation of regional anesthesia, provided that ventilation is carefully monitored.

IMPLICATIONS

In this dose-finding, placebo-controlled study, remifentanil infusions were used to provide sedation during spinal and brachial plexus regional anesthesia. The 50% effective dose for achievement of sedation was 0.043 microg x kg(-1) x min(-1). Return to alertness occurred after 10-12 min (median time). Remifentanil infusions can be used to supplement regional anesthesia, but this requires careful monitoring of ventilation.

What is new in monitored anesthesia care?

The use of monitored anesthesia care (MAC) techniques is increasing in popularity because recovery profiles seem to be improved compared with general and regional anesthesia. This article describes the conceptual basis for MAC and reviews the current MAC practices.

Use of remifentanil in infants

We describe the use of remifentanil in three infants with complex medical issues (hepatic failure, cyanotic heart disease and renal compromise). The short duration of opioid effect even after a long period of drug infusion (18 h) suggests this drug may be useful in some infants. Continued study is warranted.

Promising non-narcotic analgesic techniques for labour

Epidural analgesia and spinal analgesia are the most effective techniques for relieving labour pain. Basically, local anaesthetics (i.e. bupivacaine) and opioids (i.e. fentanyl or sufentanil), especially when combined, produce excellent analgesia with minimal motor blockade. However, none of these agents is devoid of side-effects and analgesia remains sometimes imperfect, suggesting that new drugs would be welcome. Adrenalin and clonidine act on a2-adrenoceptors in the spinal cord and both have been found to improve analgesia. These two drugs have already been used in many patients and studies because the absence of neurotoxicity has been well documented. Clonidine looks more attractive, although sedation and hypotension limit its use. Other analgesic drugs are promising alternatives but are still at an experimental or very early clinical stage. Neostigmine and ketamine (without preservative) are not neurotoxic while midazolam neurotoxicity is still controversial. Intravenous remifentanil might prove useful when neuraxial analgesia is contraindicated.

Rapid development of tolerance to analgesia during remifentanil infusion in humans

Studies in experimental animals have demonstrated a rapidly developing acute tolerance to the analgesic effect of opioids administered by continuous i.v. infusion. The aim of the present study was to determine whether acute tolerance plays an important role in the analgesic effect of remifentanil provided by i.v. infusion to humans. The analgesic effect of remifentanil, infused at a constant rate of 0.1 microg x kg(-1) x min(-1) for 4 h, was evaluated by measuring pain tolerance with thermal (2 degrees C water) and mechanical (pressure) noxious stimulations in 13 paid volunteers. The constant-rate infusion of remifentanil resulted in a threefold increase in pain tolerance with both tests. After reaching its maximum in 60-90 min, the analgesic effect of remifentanil began to decline despite the constant-rate infusion, and after 3 h of infusion, it was only one fourth of the peak value. A comparative rate in the development of acute tolerance measured in terms of time to 50% recovery during infusion was 129 +/- 27 min (mean +/- SD) with the cold water test and 138 +/- 39 min with the pressure test. We conclude that the development of tolerance should be included in the calculations for target-controlled infusions.

IMPLICATIONS

Our study shows that tolerance to analgesia during remifentanil infusion is profound and develops very rapidly. The administration of opioids during anesthesia based on target-controlled infusions should include corrections for the development of tolerance.

Recovery from opioid anesthesia: the clinical implication of context-sensitive half-times

The context-sensitive half-time, the time required for a 50% decrease in drug concentration, has been proposed to predict the speed of recovery after infusions of i.v. anesthetics. We studied 40 patients to compare the clinical recovery characteristics of alfentanil and sufentanil. Patients were randomly allocated to receive either sufentanil/propofol (Group 1) or alfentanil/propofol (Group 2) total i.v. anesthesia by target-controlled infusions (TCI), assuming an equipotency ratio of 500:1. After discontinuation, times to tracheal extubation and to discharge from the postanesthesia care unit were measured, as were drug concentrations up to 24 h. The TCI bias was -17.1% for sufentanil and -16.9% for alfentanil. We found no difference in mean extubation times between the groups (48.7 min in Group 1 versus 46.4 min in Group 2), whereas discharge criteria were fulfilled significantly (P = 0.039) earlier after alfentanil (99.5 min) compared with sufentanil (131.3 min). The relative decrement values to tracheal extubation were 62.1% for sufentanil and 48.0% for alfentanil, compared with 75.7% and 65.0% for discharge, respectively. Based on a difference in propofol requirements, we suggest an actual sufentanil to alfentanil equipotency ratio of 1:300. We conclude that the decay in pharmacodynamic effect is not only the result of pharmacokinetics.

IMPLICATIONS

Computer simulations may help to anticipate the clinical behavior of anesthetic drugs. In a clinical setting, we tested whether the recovery characteristics after i.v. anesthesia could be explained by a pharmaco-kinetic value, which describes the decline of drug concentrations in the body. This was not fully achieved.