Effective concentration 50 for propofol with and without 67% nitrous oxide

Inhibition of mitochondrial respiration by general anesthetic drugs

General anesthetic drugs have been associated with various unwanted effects including an interference with mitochondrial function. We had previously observed increases of lactate formation in the mouse brain during anesthesia with volatile anesthetic agents. In the present work, we used mitochondria that were freshly isolated from mouse brain to test mitochondrial respiration and ATP synthesis in the presence of six common anesthetic drugs. The volatile anesthetics isoflurane, halothane, and (to a lesser extent) sevoflurane caused an inhibition of complex I of the electron transport chain in a dose-dependent manner. Significant effects were seen at concentrations that are reached under clinical conditions (< 0.5 mM). Pentobarbital and propofol also inhibited complex I but at concentrations that were two-fold higher than clinical EC50 values. Only propofol caused an inhibition of complex II. Complex IV respiration was not affected by either agent. Ketamine did not affect mitochondrial respiration. Similarly, all anesthetic agents except ketamine suppressed ATP production at high concentrations. Only halothane increased cytochrome c release indicating damage of the mitochondrial membrane. In summary, volatile general anesthetic agents as well as pentobarbital and propofol dose-dependently inhibit mitochondrial respiration. This action may contribute to depressive actions of the drugs in the brain.

Current status of nitrous oxide use in pediatric patients

Nitrous oxide is one of the most commonly used inhalational anesthetic agents used in practice. It is a cost-effective, pleasant, safe, and versatile anesthetic agent with many desirable properties like good quality analgesia, decreased awareness, accelerated induction and recovery from anesthesia, and reduced utilization of other expensive inhalational agents with potential cost savings. The use of nitrous oxide has been questioned by a lot of studies and case reports perceiving its adverse systemic, hematological, immune, and neurologic adverse effects. However, the literature in the recent past has tried to resolve the controversies related to its use. The concerns over an increase in cardiovascular complications and mortality following nitrous oxide use have been negated by recent data. However, its use in certain vulnerable populations like children with cobalamin and folate deficiency or defects in their metabolic pathways remains a cause of concern for its toxic effects. In this narrative review, we aim to discuss the pharmacological properties of nitrous oxide, the potential advantages and drawbacks of the use of nitrous oxide in children, address the neurodevelopmental and other systemic effects, and throw light on the evidence regarding the safety of nitrous oxide use and its current role in pediatric procedural sedation and anesthesia practice. The literature related to its use in the pediatric population for painful procedures and surgeries has been summarized.

Influence of Depth of Hypnosis on Pupillary Reactivity to a Standardized Tetanic Stimulus in Patients Under Propofol-Remifentanil Target-Controlled Infusion: A Crossover Randomized Pilot Study

BACKGROUND

Pupillometry allows the measurement of pupillary diameter variations in response to nociceptive stimuli. This technique has been used to monitor the balance between analgesia and nociception. Under general anesthesia, the amplitude of pupillary dilation is related to the amount of administered opioids. The objective of this study was to determine whether at a constant infusion rate of opioids, the pupillary response was influenced by depth of hypnosis assessed by the bispectral index (BIS).

METHODS

Twelve patients (14-20 years) anesthetized for orthopedic surgery were included. Under propofol-remifentanil target-controlled infusion, remifentanil effect site target concentration was fixed at 1 ng/mL. Two measures of pupillary reflex dilation were performed on each patient in a randomized order: one at BIS 55 and one at BIS 25. These levels of BIS were obtained by adjusting propofol target concentration and maintained for 10 minutes before each measure. For each measure, we applied a standardized tetanic stimulation on the patient's forearm (60 mA, 100 Hz, 5 seconds). All measures were performed before the beginning of surgery.

RESULTS

Pupillary dilation was significantly greater at BIS 55 than at BIS 25: 32.1% ± 5.3% vs 10.4% ± 2.5% (mean difference estimate [95% confidence interval]: 21.8% [12.9-30.6], P < .001), without carryover effect (P = .30) nor period effect (P = .52). Hemodynamic parameters and BIS were not modified by the stimulation.

CONCLUSIONS

In patients receiving a constant infusion of remifentanil at a target concentration of 1 ng/mL, pupillary dilation after a standardized tetanic stimulation was influenced by depth of hypnosis assessed by the BIS.

Effects of Inhaled Nitrous Oxide on the Induction Dose and Time Requirements of Propofol: A Prospective, Randomized, Double-blind Study

Context:

Propofol is a commonly used induction agent during general anesthesia. As a sole agent, it does not provide any strong analgesic effect. The nitrous oxide (N₂O) used along with propofol for induction of anesthesia augments the induction characteristics and reduces the dose of propofol.

Aims:

To study the effects of inhaled N₂O on the induction dose and time of propofol during general anesthesia and also its hemodynamic response and adverse effects.

Settings and Design:

The present research is a prospective, randomized, double-blind comparative study.

Subjects and Methods:

The study population consisted of eighty patients aged 18–60 years from either sex, American Society of Anesthesiologists physical status 1 and 2 which were scheduled for various elective surgical procedures under general anesthesia. The patients were randomly allocated into two groups comprising forty patients in each group. All patients were premedicated with glycopyrrolate 0.2 mg, ondansetron 4 mg, and fentanyl 1 μg/kg intravenously. Group FN received breathing mixture of gases (67% N₂O @ 4 L/min and 33% O₂ @ 2 L/min), and propofol and Group FO received 100% O₂ @ 6 L/min and propofol. The different hemodynamic parameters (heart rate, mean arterial pressure, systolic blood pressure, diastolic blood pressure, and SpO₂) were measured.

Statistical Analysis:

All observations were analyzed using Chi-square test, Student's t-test, and analysis of variance.

Results:

The mean induction time and dose were significantly less in Group FN as compared to Group FO (P < 0.05). The mean induction time was 172 ± 32 s in Group FN as compared to 242 ± 43 s in Group FO (P < 001), whereas the mean induction dose was 56.10 ± 13.92 mg in Group FN as compared to 81.67 ± 17.64 mg in Group FO (P < 0.05). The hemodynamic parameters remained stable with no complications.

Conclusion:

The coadministration of N₂O during induction of anesthesia with propofol not only reduced the induction dose of propofol but also reduced induction time significantly. Furthermore, it provided stable hemodynamics without any complications.

Target-controlled infusion (Propofol) versus inhaled anaesthetic (Sevoflurane) in patients undergoing shoulder arthroscopic surgery

BACKGROUND

One of the challenges of anaesthesia for shoulder arthroscopic procedures is the need for controlled hypotension to lessen intra-articular haemorrhage and thereby provide adequate visualisation to the surgeon. Achievement of optimal conditions necessitates several interventions and manipulations by the anaesthesiologist and the surgeon, most of which directly or indirectly involve maintaining intra-operative blood pressure (BP) control.

AIM

This study aimed to compare the efficacy and convenience of target controlled infusion (TCI) of propofol and inhalational agent sevoflurane in patients undergoing shoulder arthroscopic surgery after preliminary inter-scalene blockade.

METHODS

Of thirty four patients studied, seventeen received TCI propofol (target plasma concentration of 3 μg/ml) and an equal number, sevoflurane (1.2-1.5 Minimum Alveolar Concentration). N2O was used in both groups. Systolic, diastolic, mean blood pressures and heart rate were recorded regularly throughout the procedure. All interventions to control BP by the anaesthesiologist and pump manipulation requested by the surgeon were recorded. The volume of saline irrigant used and the haemoglobin (Hb) content of the return fluid were measured.

RESULTS

TCI propofol could achieve lower systolic, mean BP levels and the number of interventions required was also lower as compared to the sevoflurane group. The number of patients with measurable Hb was lower in the TCI propofol group and this translated into better visualisation of the joint space. A higher volume of saline irrigant was required in the sevoflurane group. No immediate peri-operative anaesthetic complications were noted in either category.

CONCLUSION

TCI propofol appears to be superior to and more convenient than sevoflurane anaesthesia in inter-scalene blocked patients undergoing shoulder arthroscopy.

Predicted effect-site concentration of propofol and sufentanil for gynecological laparoscopic surgery

BACKGROUND

this study was to estimate the predicted effect-site concentration of propofol administered by a target-controlled infusion (TCI) for maintenance of anesthesia based on the bispectral (BIS) index as a measure of hypnosis in laparoscopic surgery.

METHOD

one-hundred and sixty unpremedicated patients undergoing gynecologic laparoscopy were assigned randomly to receive one of the target effect-site concentrations of propofol 2.0, 2.5, 3.0, 3.5 and 4.0 microg/ml during TCI with propofol and sufentanil. The dose-response relationship of propofol for the maintenance of adequate anesthesia based on BIS, movement and hemodynamic response was investigated using a fixed effect-site concentration of sufentanil (0.2 ng/ml). The BIS values, hemodynamic variables, time course during emergence and intraoperative awareness were also assessed.

RESULTS

the predicted effect-site propofol concentrations for adequate anesthesia at the skin incision in 50% (EC(50) ) and 95% (EC(95) ) of patients undergoing gynecologic laparoscopy were 2.2 and 3.7 microg/ml, respectively. The predicted propofol EC(50) and EC(95) to maintain adequate anesthesia in these patients were 2.6 microg/ml (95% CI 2.3-2.7 microg/ml) and 3.6 microg/ml (95% CI 3.3-4.0 microg/ml), respectively. The BIS values, effect-site concentration of propofol, hemodynamic data and time course during emergence and post-operative adverse events were comparable in each group. There were no reports of intraoperative awareness in the post-anesthetic care unit.

CONCLUSION

based on the anesthetic depth assessed by the clinical signs and BIS monitoring, the predicted effect-site propofol concentrations for the maintenance of anesthesia in patients undergoing gynecologic laparoscopy were similar in those administered adequate anesthesia at the skin incision during TCI.

A comparison of parametric and non-parametric approaches to target-controlled infusion of propofol

Nineteen adult patients of either gender received intravenous infusions of propofol, scaled to estimated lean body mass (LBM), for 150 minutes as part of a balanced anaesthetic. Arterial blood was assayed for whole blood propofol. The first subject received propofol at a fixed rate of 0.058 mg x min(-1) x kg(LBM)(-1). Subsequent groups received variable rate infusions based on the ratio of the infusion rate to the propofol concentration at each sampling point in the previous group, multiplied by the target concentration. After groups of one, two, five and 11 subjects, the median weighted residual was 0.040 and median absolute weighted residual was 0.153. Population pharmacokinetic analysis of the final group of six females and five males, aged 29 to 70 years and of 16.5 to 44.2% body fat, resulted in a two compartment pharmacokinetic model with coefficients and standard errors of V = 0.102 (0.0155) l/kg(LBM), V2 = 0.257 (0.079) l/kg(LBM), k10 = 0.423 (0.069)/min, k12 = 0.222 (0.051)/min, k21 = 0.084 (0.02)/min and clearance = 0.0418 (0.0023) L x min(-1) x kg(LBM)(-1). The only significant covariate was LBM. Within infusion data improved prediction when compared with data derived in previous studies from random observations.

Pharmacokinetic models for propofol--defining and illuminating the devil in the detail

The recently introduced open-target-controlled infusion (TCI) systems can be programmed with any pharmacokinetic model, and allow either plasma- or effect-site targeting. With effect-site targeting the goal is to achieve a user-defined target effect-site concentration as rapidly as possible, by manipulating the plasma concentration around the target. Currently systems are pre-programmed with the Marsh and Schnider pharmacokinetic models for propofol. The former is an adapted version of the Gepts model, in which the rate constants are fixed, whereas compartment volumes and clearances are weight proportional. The Schnider model was developed during combined pharmacokinetic-pharmacodynamic modelling studies. It has fixed values for V1, V3, k(13), and k(31), adjusts V2, k(12), and k(21) for age, and adjusts k(10) according to total weight, lean body mass (LBM), and height. In plasma targeting mode, the small, fixed V1 results in very small initial doses on starting the system or on increasing the target concentration in comparison with the Marsh model. The Schnider model should thus always be used in effect-site targeting mode, in which larger initial doses are administered, albeit still smaller than for the Marsh model. Users of the Schnider model should be aware that in the morbidly obese the LBM equation can generate paradoxical values resulting in excessive increases in maintenance infusion rates. Finally, the two currently available open TCI systems implement different methods of effect-site targeting for the Schnider model, and in a small subset of patients the induction doses generated by the two methods can differ significantly.

The effects of surgical levels of sevoflurane and propofol anaesthesia on heart rate variability

BACKGROUND AND OBJECTIVE

The laryngeal mask has become a widely accepted alternative to endotracheal intubation and mask ventilation. The laryngeal tube is a relatively new supraglottic airway device for airway management. We compared the new version of the laryngeal tube with the laryngeal mask.

METHODS

In a randomized design, either a laryngeal tube (n = 66) or a laryngeal mask (n = 66) were inserted. Ease of insertion, oxygenation and ventilation, spirometry data and postoperative airway morbidity were determined.

RESULTS

After successful insertion, it was possible to maintain oxygenation and ventilation in all the patients. Insertion success rates after the first, second and third attempts were 84.8% (n = 56), 12.1% (n = 8) and 3% (n = 2) for the laryngeal tube compared with 56.1% (n = 37), 25.8% (n = 17) and 18.2% (n = 12) for the laryngeal mask (P = 0.001). There was no significant difference in peak airway pressure, and dynamic compliance between the groups (P > 0.05). Blood on the cuff after removal of the device was noted in one patient with the laryngeal tube and in 10 patients with the laryngeal mask. Six patients in the laryngeal mask group complained of hoarseness (P = 0.012).

CONCLUSION

With respect to clinical function, the new version of the laryngeal tube and the laryngeal mask are similar and either device can be used to establish a safe and effective airway in paralysed patients.

Nitrous Oxide Does Not Aggravate Postoperative Emesis After Orthognathic Surgery in Female and Nonsmoking Patients

PURPOSE

The purpose of this study was to evaluate the effect of supplemental nitrous oxide on postoperative nausea and vomiting (PONV) after propofol anesthesia for orthognathic surgery in female and nonsmoking patients.

PATIENTS AND METHODS

We compared PONV in 28 ASA-I female nonsmoking patients undergoing orthognathic surgery. Anesthesia was induced with propofol combined with fentanyl, and tracheal intubation was facilitated with vecuronium. Anesthesia was maintained with propofol with or without nitrous oxide. No patient received neostigmine. PONV was assessed as score 0 (no PONV), score 1 (nausea), and score 2 (vomiting) during the 24-hour recovery period.

RESULTS

There were no differences in the patients' characteristics, operation, anesthesia and emergence time, fluid transfusion, blood loss, urine output, and total propofol and fentanyl doses between the 2 groups. There was also no difference in PONV score in 2 groups. Only 1 patient in each group vomited.

CONCLUSIONS

It is suggested that supplemental nitrous oxide does not aggravate PONV after propofol anesthesia for orthognathic surgery in female nonsmoking patients.

Pharmacokinetic/pharmacodynamic modeling of anesthetics in children: therapeutic implications

Modeling the pharmacokinetics and pharmacodynamics of anesthetics in children is performed as a response to the clinical need for safe and efficacious administration of drugs with a low therapeutic index. Rates and concentrations of these drugs, which are the primary parameters used by anesthesiologists, depend on physiologic parameters that are markedly affected by development. Volatile anesthetics have been used for >50 years in pediatric patients. The pharmacokinetics of inhalation agents are context sensitive, but little difference between age groups has been described. These agents are not only eliminated unchanged by the lung but they are also metabolized by the liver. Halothane has Michaelis-Menten kinetics, with up to 40% of the administered dose metabolized by the liver. For volatile anesthetics, the effect measured is the minimum alveolar concentration (MAC) that leads to movement of the limb in response to skin incision in 50% of the patients studied. The MAC is higher in infants than in children and adults. Infants aged 6 months have a MAC 1.5-1.8 times the MAC observed in adults aged 40 years. Children have a greater clearance and volume of distribution of propofol than adults. In order to achieve similar plasma concentrations, children require three times the initial dose used in adults. In adults, an increased sensitivity to propofol has been demonstrated with aging, but nothing is known about the effects in children. However, it is clear that equipotent doses of propofol induce marked deleterious hemodynamic effects in infants compared with children. Regional anesthesia is used in pediatrics, both in combination with general anesthesia during surgery or alone for postoperative analgesia. A marked decrease in protein binding has been described in infants. In the postoperative period, a rapid increase in binding because of inflammation decreases the free fraction, but the free drug concentration remains constant because of the resulting decrease in total clearance. A low clearance because of liver function immaturity has been observed during the first year(s) of life for bupivacaine and ropivacaine. Pharmacodynamic interactions between general anesthesia and regional anesthesia need to be modeled. This is one of the future tasks for pharmacokineticists. Methods such as the Dixon up-and-down allocation and the isobolographic technique are promising in this field.

Nitrous oxide can enhance the hypnotic effect, but not the suppression of spinal motor neuron excitability by propofol in humans

PURPOSE

We investigated whether nitrous oxide can enhance the suppressive effect of propofol on spinal motor neuron excitability in humans.

METHODS

Sixteen adult patients were prospectively randomly assigned to be given either propofol alone (group P; n = 8) or a supplement of 66% nitrous oxide with propofol (group PN; n = 8) for intraoperative sedation. Propofol was administered by a target-controlled infusion system to maintain sequentially increasing plasma propofol concentrations (Cpt) of 0.5, 0.8, 1.0, 1.3, 1.5 and 1.8 microg x ml(-1) in all patients. Assessment of the patient's level of sedation in both groups was performed with the Wilson Sedation Scale (WSS). F-wave analysis on the left abductor pollicis brevis muscle was carried out for the assessment of spinal motor neuron excitability at each plasma propofol concentration.

RESULTS

Significant differences in the WSS scores between group P and group PN were observed at 0.8, 1.0, 1.3, and 1.5 microg x ml(-1) of Cpt (group P < group PN; P < 0.01). Cpt greater than 1.0 microg x ml(-1) significantly reduced F-wave persistence in a concentration-dependent manner, and the ICpt 50 and ICpt 95 values for plasma propofol concentration (plasma propofol concentrations that produced 50% and 95% inhibition of the baseline, respectively) were 1.05 and 1.95 microg x ml(-1) in group P, and 1.07 and 2.14 microg x ml(-1) in group PN, respectively.

CONCLUSION

These results suggest that nitrous oxide can enhance the hypnotic effect, but not the suppression of spinal motoneuron excitability by propofol in humans at clinical levels of Cpt.

Nitrous oxide induces paradoxical electroencephalographic changes after tracheal intubation during isoflurane and sevoflurane anesthesia

In this randomized, double-blind, controlled study, we tested the hypothesis that nitrous oxide (N2O) affects bispectral index (BIS) and 95% spectral edge frequency (SEF95) in response to tracheal intubation during anesthesia with isoflurane and sevoflurane. In protocol 1, we randomly allocated 90 ASA physical status I patients to 6 groups (n = 15 each). Anesthesia was induced with isoflurane or sevoflurane with 0%, 33%, or 66% N2O. The concentration of isoflurane and sevoflurane was gradually increased and end-tidal concentrations were maintained at 1.1% and 1.7%, respectively. Tracheal intubation was performed 12 min after induction of anesthesia. BIS was significantly increased 1 min after tracheal intubation compared before laryngoscopy in patients receiving only isoflurane or sevoflurane (P = 0.001 and 0.007, respectively). In patients receiving 66% N2O-isoflurane or 66% N2O-sevoflurane, both BIS and SEF95 were significantly decreased after tracheal intubation and significantly lower than in those patients receiving only isoflurane or sevoflurane, respectively (P < 0.01 for both). In protocol 2, 3 microg/kg of IV fentanyl completely abolished the decrease of BIS and SEF95 after tracheal intubation during anesthesia with 66% N2O-isoflurane and 66% N2O-sevoflurane (n = 10). We conclude that 66% N2O induced a paradoxical decrease of BIS in response to tracheal intubation during anesthesia with isoflurane and sevoflurane.

Nitrous Oxide and Anesthetic Requirement for Loss of Response to Command During Propofol Anesthesia

The blood concentration associated with loss of response (LOR) to command in 50% of subjects (CP50(LOR)) is an important measure of anesthetic potency. We therefore determined the CP50(LOR) in 40 healthy surgical patients, aged 18-60 yr old, receiving propofol alone or propofol with 67% nitrous oxide (N2O). Patients were randomized to receive 100% oxygen or 67% N2O in oxygen via facemask. Three minutes later, a target-controlled propofol infusion was commenced at a concentration determined by the response of the previous patient in the same group. Fifteen minutes later, response to command was assessed by a blinded observer. Arterial blood samples were taken for propofol assay, and the bispectral index (BIS) was monitored continuously. At testing for response to command, both the measured and target propofol concentrations were significantly larger and BIS values significantly smaller in the propofol-alone group compared with the propofol-N2O group. The CP50(LOR) of propofol in the propofol-alone group was 4.58 mug/mL (95% confidence interval [CI], 1.14-15.36) and 2.67 microg/mL (95% CI, 2.28-3.17) in the propofol-N2O group. The BIS value when 50% of patients responded to command was 60 (95% CI, 55-65) in the propofol-alone group and 75 (95% CI, 73-83) in the propofol-N2O group.

Predictive performance of 'Diprifusor' TCI system in patients during upper abdominal surgery under propofol/fentanyl anesthesia

OBJECTIVE

To evaluate the predictive performance of 'Diprifusor' TCI (target-controlled infusion) system for its better application in clinical anesthesia.

METHODS

The predictive performance of a 'Diprifusor' TCI system was investigated in 27 Chinese patients (16 males and 11 females) during upper abdominal surgery under total intravenous anesthesia (TIVA) with propofol/fentanyl. Measured arterial propofol concentrations were compared with the values predicted by the TCI infusion system. Performance was determined by the median performance error (MDPE), the median absolute performance error (MDAPE), the divergence (the percentage change of the absolute PE with time), and the wobble (the median absolute deviation of each PE from the MDPE).

RESULTS

The median (range) values of 14.9% (-21.6%-42.9%) for MDPE, 23.3% (6.9%-62.5%) for MDAPE, -1.9% h(-1) (-32.7%-23.0% h(-1)) for divergence, and 18.9% (4.2%-59.6%) for wobble were obtained from 227 samples from all patients. For the studied population, the PE did not increase with time but with increasing target propofol concentration, particularly following induction. conclusions: The control of depth of anaesthesia was good in all patients undergoing upper abdominal surgical operation and the predictive performance of the 'Diprifusor' target controlled infusion system was considered acceptable for clinical purposes. But the relatively bigger wobble showed that the pharmacokinetic model is not so suitable and requires improvement.

Individual effect-site concentrations of propofol are similar at loss of consciousness and at awakening

Reported effect-site concentrations of propofol at loss of consciousness and recovery of consciousness vary widely. Thus, no single concentration based on a population average will prove optimal for individual patients. We therefore tested the hypothesis that individual propofol effect-site concentrations at loss and return of consciousness are similar. Propofol effect-site concentrations at loss and recovery of consciousness were estimated with a target-control infusion system in 20 adults. Propofol effect-site concentrations were gradually increased until the volunteers lost consciousness (no response to verbal stimuli); unconsciousness was maintained for 15 min, and the volunteers were then awakened. This protocol was repeated three times in each volunteer. Our major outcomes were the concentration producing unconsciousness and the relationship between the estimated effect-site concentrations at loss and recovery of consciousness. The target effect-site propofol concentration was 2.0 +/- 0.9 at loss of consciousness and 1.8 +/- 0.7 at return of consciousness (P <0.001). The average difference between individual effect-site concentrations at return and loss of consciousness was only 0.17 +/- 0.32 microg/mL (95% confidence interval for the difference 0.09-0.25 microg/mL). Our results thus suggest that individual titration to loss of consciousness is an alternative to dosing propofol on the basis of average population requirements.

Sevoflurane and propofol increase 11C-flumazenil binding to gamma-aminobutyric acidA receptors in humans

Based on in vitro studies and animal data, most anesthetics are supposed to act via gamma-aminobutyric acid type A (GABA(A)) receptors. However, this fundamental characteristic has not been extensively investigated in humans. We studied (11)C-flumazenil binding to GABA(A) receptors during sevoflurane and propofol anesthesia in the living human brain using positron emission tomography (PET). Fourteen healthy male subjects underwent 2 60-min dynamic PET studies with (11)C-labeled flumazenil, awake and during anesthesia. Anesthesia was maintained with 2% end-tidal sevoflurane (n = 7) or propofol at a target plasma concentration of 9.0 +/- 3.0 (mean +/- sd) microg/mL (n = 7). The depth of anesthesia was measured with bispectral index (BIS). Values of regional distribution volumes (DV) of (11)C-flumazenil were calculated in several brain areas using metabolite-corrected arterial plasma curves and a two-compartment model. Separate voxel-based statistical analysis using parametric DV images was performed for detailed visualization. The average BIS index was 35 +/- 6 in the sevoflurane group and 28 +/- 8 in the propofol group (P = 0.02). Sevoflurane increased the DV of (11)C-flumazenil significantly (P < 0.05) in all brain areas studied except the pons and the white matter. In the propofol group the increases were significant (P < 0.05) in the caudatus, putamen, cerebellum, thalamus and the frontal, temporal, and parietal cortices. Furthermore, the DV increases in the frontal, occipital, parietal, and temporal cortical areas and in the putamen were statistically significantly larger in the sevoflurane than in the propofol group. Our findings support the involvement of GABA(A) receptors in the mechanism of action of both anesthetics in humans.

Effects of halothane, sevoflurane and propofol on left ventricular diastolic function in humans during spontaneous and mechanical ventilation

BACKGROUND

There is limited knowledge of the effects of anaesthetics on left ventricular (LV) diastolic function in humans. Our aim was to evaluate these effects in humans free from cardiovascular disease.

METHODS

Sixty patients (aged 18-47 yr) who had no history or signs of cardiovascular disease were randomized to receive general anaesthesia with halothane, sevoflurane or propofol. Echocardiography was performed at baseline and during spontaneous respiration at 1 minimum alveolar concentration (MAC) of the inhalational agents or propofol 4 microg ml(-1) (step 1), and repeated during positive-pressure ventilation with 1 and 1.5 MAC of the inhalational agents or with propofol 4 and 6 microg ml(-1) (steps 2a and 2b). Analysis of echocardiographic measurements focused on heart rate corrected isovolumic relaxation time (IVRT(c)) and early diastolic peak velocity of the lateral mitral annulus (E(a)).

RESULTS

IVRT(c) decreased from baseline to step 1 in the halothane group (82 [95% CI, 76-88] ms and 74 [95% CI, 68-80] ms respectively; P=0.02), remained stable in the sevoflurane group (78 [95% CI, 72-83] ms and 73 [95% CI, 67-81] ms; n.s.) and increased in the propofol group (80 [95% CI, 74-86] ms and 92 [95% CI, 84-102] ms; P=0.02). E(a) decreased in the propofol group only (18.8 [95% CI, 16.5-19.9] cm s(-1) and 16.0 [95% CI, 14.9-17.9] cm s(-1); P=0.003). From step 2a to step 2b, IVRT(c) increased further in the propofol group (109 [95% CI, 99-121] ms and 119 [95% CI, 99-135] ms; P=0.04) but remained stable in the other two groups. E(a) did not change from step 2a to step 2b.

CONCLUSIONS

Halothane and sevoflurane did not impair LV relaxation, whereas propofol caused a mild impairment. However, the impairment by propofol was of a magnitude that is unlikely to cause clinical diastolic dysfunction.

Manual versus target-controlled infusions of propofol

Target-controlled infusion systems have been shown to result in the administration of larger doses of propofol, which may result in delayed emergence and recovery from anaesthesia. The aim of this study was to investigate if this was due to a difference in the depth of hypnosis (using the bispectral index monitoring) between the manual and target controlled systems of administration. Fifty unpremedicated patients undergoing elective surgery were randomly allocated to have their anaesthesia maintained with manual or target-controlled propofol infusion schemes. In both groups, the rate of propofol administration was adjusted according to standard clinical criteria while bispectral index scores were recorded by an observer not involved in the delivery of anaesthesia. The total dose of propofol used was higher in the target controlled group (mean 9.9 [standard deviation 1.6] compared with 8.1 [1.0] mg.kg(-1).h(-1) in the manual group [p < 0.0001]). The times to emergence and recovery end-points were comparable between the two groups. The difference in the total dosage of propofol was mainly due to higher rate of propofol administration in the first 30 min in the target controlled infusion group. The bispectral index scores were lower in the target controlled group during this time, being significantly so over the first 15 min of anaesthesia. We conclude that propofol administration by a target controlled infusion system results in the administration of higher doses of propofol and lower bispectral index values mainly in the initial period of anaesthesia.

Ceasing routine use of nitrous oxide-a follow up

BACKGROUND

The role of nitrous oxide in modern anaesthesia is questioned. The routine use of nitrous oxide was almost completely stopped in our department after November 1, 2000, and we now report some consequences.

METHODS

Staff completed a questionnaire after 6 months, and we analysed the use of hypnotics and opioids after 12 months. The cost of drugs for the year after stopping nitrous oxide was compared with the cost 2 yr before.

RESULTS

Less than half of the 55 staff members who answered the questionnaire used nitrous oxide in the 6 months after the stop, and they did so on only a few occasions. Half of the staff members thought the benefit of nitrous oxide was small. Most supported the change. The use of opioids was stable during the study period, and there was an annual increase of 12-14% in the use of hypnotics during the 3 yr.

CONCLUSIONS

The staff questionnaire showed a strong acceptance of the new policy, and the use of other anaesthetic agents did not increase as expected. Has the value of nitrous oxide been overestimated?

Narcotrend, bispectral index, and classical electroencephalogram variables during emergence from propofol/remifentanil anesthesia

The aim of this study was to investigate modern and classical electroencephalographic (EEG) variables in response to remifentanil and propofol infusions. We hypothesized that modern EEG variables may indicate the effects of propofol but not of remifentanil. Twenty-five patients were included in the study after the end of elective spine surgery without any surgical stimulation. Baseline values were defined with remifentanil 0.3 microg. kg(-1). min(-1) and target-controlled infusion of propofol 3.0 microg/mL. EEG changes were evaluated 1, 3, 5, 7, and 9 min after the stop of remifentanil infusion, followed by a step-by-step reduction (0.2 microg/mL) every 3 min of target-controlled infusion propofol. Narcotrend (NT; classifying EEG stages from awake to deep anesthesia), bispectral index (BIS), EEG spectral frequency bands (%), 50% (Median) and 95% percentiles (spectral edge frequency), mean arterial blood pressure, heart rate, and oxygen saturation were detected at every time point. The end of remifentanil application resulted in significant increases in %alpha, spectral edge frequency, mean arterial blood pressure, and %theta and decreases in %delta (P < 0.05). NT, BIS, Median, heart rate, and oxygen saturation were unchanged. Decreases in propofol concentration were associated with statistically significant increases in NT and BIS (P < 0.05). Thus, the sedative-hypnotic component of propofol could be estimated by modern EEG variables (NT and BIS), whereas the analgesic component provided by remifentanil was not indicated. However, during conditions without surgical stimulation, neither NT nor BIS provided an adequate assessment of the depth of anesthesia when a remifentanil infusion was used.

IMPLICATIONS

We investigated modern and classical electroencephalographic (EEG) variables during emergence from propofol/remifentanil anesthesia. Modern EEG variables indicate changes of infusion in propofol, but not in remifentanil. Thus, modern EEG variables did not provide an adequate assessment of depth of anesthesia when remifentanil was used.

Cp50 of propofol for laryngeal mask airway insertion using predicted concentrations with and without nitrous oxide

This study sought to determine the predicted Cp50 of propofol required for laryngeal mask airway insertion (Cp50LMA) and to investigate whether nitrous oxide reduces these required concentrations. Using target-controlled infusion and incorporating the standard Diprifusor pharmacokinetic model, 46 unpremedicated patients were randomly assigned to one of two groups. The patients received either 40% oxygen in air (control group: n = 23), or 60% nitrous oxide in oxygen (nitrous oxide group: n = 23). The target concentration for each patient was determined using the up and down method. Following equilibration between the predetermined blood and effect site concentrations, had been established for > 10 min, laryngeal mask airway insertion was attempted without neuromuscular relaxants. The data were analysed using a probit analysis to obtain Cp50LMA levels. The values for Cp50LMA were 3.24 micro g.ml-1 in the control group and 1.93 microg.ml-1 in the nitrous oxide group.

Clinical comparison of 'single agent' anaesthesia with sevoflurane versus target controlled infusion of propofol

The introduction of total intravenous anaesthesia (TIVA) and the use of volatile induction/maintenance anaesthesia (VIMA) has led to the rediscovery of 'single agent' anaesthesia, eliminating the transition phase from induction to maintenance. We compared quality, patient acceptability and cost of TIVA using target control infusion (TCI) with propofol and VIMA with sevoflurane. Forty patients undergoing spinal surgery of 1-3 h were assigned to one of two groups. Group I received propofol-air-oxygen for induction followed by propofol-air-oxygen for maintenance. Group II received 8% sevoflurane-oxygen for induction and sevoflurane-oxygen-nitrous oxide for maintenance. Propofol had a significantly faster mean (SD) induction time (67 (20) s) than sevoflurane (97 (38) s) but was associated with double the incidence of involuntary movements. Although not significant, twice the number of interventions by the anaesthetist were required to maintain an adequate level of anaesthesia in the sevoflurane group. Emergence times, characteristics, postoperative nausea, vomiting and pain were unaffected by the anaesthetic technique. However, a more predictable emergence time was found following sevoflurane. Cardiovascular stability was good and comparable in both groups. The majority of patients found either technique acceptable and would choose the same anaesthetic again. Induction and maintenance was substantially cheaper with sevoflurane (28.06 Pounds) compared with propofol (41.43 Pounds).

Blood alcohol concentration and psychomotor effects

This study assessed the effect of intravenous alcohol infusions on psychomotor impairment and compared it with that of alcohol administered orally. Comparisons were made between three European drink-driving limits of blood alcohol concentration (BAC) (20, 50 and 80 mg 100 ml-1) and an oral dose of alcohol 0.75 mg kg-1. Twelve volunteers, aged 22-34 yr, were recruited. At targets of 20, 50 and and 80 mg 100 ml-1, the mean (SD) BAC was 22.1 (3.7), 51.5 (3.3) and 80.5 (4.2) mg 100 ml-1, respectively. The peak BAC following an oral dose of alcohol 0.75 mg kg-1 ranged from 19 to 68 mg 100 ml-1. In psychomotor testing, choice reaction time deteriorated with increasing BAC and showed significant differences between baseline and the 50 (P < 0.05) and 80 mg 100 ml-1 (P < 0.01) conditions. Dual-task secondary reaction time deteriorated with increasing BAC and showed a statistically significant difference between all groups and baseline (oral and 20 mg groups, P < 0.05; 50 and 80 mg groups, P < 0.01). Dual-task tracking in the 50 and 80 mg groups was significantly different from baseline (P < 0.05 and P < 0.01, respectively). Oral dosing resulted in widely variable BACs, making it difficult to assess psychomotor impairment reliably. An intravenous infusion enables the BAC to be maintained within a narrow range. This allows precision when investigating the effects of alcohol on psychomotor performance.

The effect of two low-dose propofol infusions on the relationship between six-pulse transcranial electrical stimulation and the evoked lower extremity muscle response

BACKGROUND

Transcranial stimulation of the motor cortex using high-voltage electrical stimuli given in train is a method of monitoring the integrity of the motor pathways during thoracoabdominal aortic aneurysm surgery. The purpose of this study was to assess the relationship between the stimulus intensity and the corresponding amplitude of the myogenic motor evoked potential (tcMEP) in response to six-pulse transcranial electrical stimulation during two levels of low-dose propofol infusion and stable fentanyl/nitrous oxide anaesthesia.

METHODS

Nine patients (37-78 yr) scheduled to undergo surgery on the thoracoabdominal aorta were studied. After achieving a stable anaesthetic state the output voltage was decreased with 50 V intervals from 350 V to 200 V during a target propofol infusion aimed at a plasma steady-state concentration of 0.7 microg x ml(-1) and increased with 50 V intervals from 200 V to 450 V during a target propofol infusion aimed at a plasma steady-state concentration of 1.4 microg x ml(-1). TcMEPs were recorded from the right tibialis anterior muscle.

RESULTS

Doubling the target propofol infusion to 1.4 microg x ml(-1) resulted in a 30-50% decrease in tcMEP amplitude. The largest tcMEP amplitude using the six-pulse paradigm was found during a propofol infusion aimed at a plasma concentration of 0.7 microg x ml(-1) and demanded a stimulus output of 350 V, corresponding to a charge density of 7.5 microC x cm(-2) per phase.

CONCLUSION

Doubling the target propofol infusion to 1.4 microg x ml(-1) provides less robust, but still recordable tcMEPs in response to six-pulse electrical stimulation. Safety guidelines are discussed.

Nitrous oxide prevents movement during orotracheal intubation without affecting BIS value

We sought to determine whether the addition of nitrous oxide (N(2)O) to an anesthetic with propofol and remifentanil modifies the bispectral index (BIS) during the induction of anesthesia and orotracheal intubation. Thirty ASA physical status I or II patients were randomly allocated to receive either 50% air in oxygen (control group) or 60%-70% N(2)O in oxygen (N(2)O group) that was commenced via a mask simultaneously with the induction of anesthesia. Anesthesia was performed in all the patients with IV propofol at the target effect compartment site concentration of 4 microg/mL throughout the study. A target-controlled infusion (TCI) of remifentanil was initiated 3 min after the TCI of propofol and maintained at the effect-site concentration of 4 ng/mL until the end of the study. After loss of consciousness, and before the administration of vecuronium 0.1 mg/kg, a tourniquet was applied to one arm and inflated to a value more than the systolic blood pressure. An examiner, blinded to the presence of N(2)O, sought to detect any gross movement within the first minute after tracheal intubation, which was performed 10 min after remifentanil TCI began. Inspired and expired oxygen, N(2)O, and carbon dioxide were continuously monitored. A BIS value was generated every 10 s. Arterial blood pressure and heart rate (HR) were measured noninvasively every minute. Measures of mean arterial pressure (MAP), HR, and BIS were obtained before the induction, before the start of the remifentanil TCI, before laryngoscopy, and 5 min after intubation. No significant intergroup differences were seen in BIS, HR, and MAP throughout the study. Maximum changes in BIS, HR, and MAP with intubation were significant (P < 0.01) for both groups but comparable. Six patients in the control group and none in the N(2)O group moved after intubation (P < 0.05).

IMPLICATIONS

We demonstrated that 0.6 minimal alveolar concentration of nitrous oxide combined with a potent anesthetic and an opioid prevents movement after orotracheal intubation without affecting the bispectral index. This demonstrates that the bispectral index is not a useful neurophysiologic variable to monitor the level of anesthesia when nitrous oxide is added to a general anesthetic regimen using propofol and remifentanil.

Interaction of nitrous oxide and propofol to reduce hypertensive response to stimulation

PURPOSE

To evaluate the interaction between nitrous oxide and propofol for the suppression of hypertension following electrical stimulation of the mental nerve in the rabbit.

METHODS

Male Japan White rabbits were tracheostomized, cannulated and mechanically ventilated under isoflurane anesthesia. Square wave pulses (5 V, 0.5 msec, 50 Hz for 5 sec) were delivered to the left mental nerve. Animals received nitrous oxide 20, 40, 60 and 80% (Group 1); propofol 200, 400, 600 and 800 microg x kg(-1) min(-1) (Group 2); or combinations of nitrous oxide and propofol at 10 + 100, 20 + 200, 30 + 300 and 40 % + 400 microg x kg(-1) x min(-1) (Group 3). Systolic blood pressure was recorded from immediately before to maximal increase following nerve stimulation. Probit analysis was used to obtain ED(50) values for 50% suppression of blood pressure elevation. Isobolographic analysis was used to evaluate the interaction between nitrous oxide and propofol.

RESULTS

ED(50) values are 52.9% for nitrous oxide (Group 1), 464.1 microg x kg(-1) min(-1) for propofol (Group 2), 21.7 % + 217.1 microg x kg(-1) min(-1) for nitrous oxide and propofol combination (Group 3) and 24. 7 % + 247.1 microg x kg(-1) x min(-1) for the theoretically additive combination of nitrous oxide and propofol, respectively.

CONCLUSION

The interaction between nitrous oxide and propofol for the suppression of blood pressure elevation following electrical stimulation of the mental nerve is additive.

Inhaling nitrous oxide reduces the induction dose requirements of propofol

Inhaling nitrous oxide (N(2)O) before propofol induction appears to decrease propofol usage. To investigate the efficacy of N(2)O as a component of the drugs used to induce anesthesia, the effect of inhaling a N(2)O:oxygen (O(2)) mixture on the dose of propofol required to induce anesthesia was determined in a double-blinded manner. We randomized 117 unpremedicated patients scheduled for elective surgery into three groups. Group FN received 1 microg/kg fentanyl and breathed 4 L/min N(2)O + 2 L/min O(2). Group PN received placebo and breathed 4 L/min N(2)O + 2 L/min O(2). Group FO received 1 microg/kg fentanyl and breathed 6 L/min O(2). Propofol was infused at 20 mg/min after 1 min of gas mixture inhalation, and the infusion stopped when there was loss of response to verbal command. The mean (SD) propofol dose was 0.75 (0.30), 0.84 (0.26), and 1.33 (0.51) mg/kg, and the induction time 133 (57), 142 (47), and 226 (78) s for Groups FN, PN, and FO, respectively. We conclude that inhalation of 66% N(2)O in O(2) 1 min before the IV induction of anesthesia with propofol at 20 mg/min, reduces the induction dose of propofol by 44% and decreases the time required for the induction of anesthesia (P < 0.001).

Target-controlled remifentanil in combination with propofol for spontaneously breathing day-case patients

Remifentanil is a new potent opioid with a very short duration of action irrespective of duration of infusion. It may have a role in day-case anaesthesia as part of a balanced total intravenous anaesthetic technique with propofol. We examined the respiratory depressant effects of remifentanil in 20 patients undergoing day-case anaesthesia. The target plasma concentration of remifentanil was varied while maintaining a constant target-controlled infusion of 4.5 microg x ml-1 propofol. In only 12 patients was satisfactory spontaneous respiration maintained. In these patients the median remifentanil target concentration was 1.6 ng x ml-1 and was achieved with a median infusion rate of 0.05 microg x kg-1 x min-1. The range of target concentrations associated with satisfactory spontaneous respiration was wide and varied over a 4.7-fold range from 0.6 to 2.8 ng x ml-1.

Nitrous oxide: still useful in the year 2000?

The risk-benefit ratio of using nitrous oxide has been debated for many years. In this article the adverse effects of nitrous oxide on patient well-being, including its role in postoperative nausea and vomiting, its toxic effects and adverse physiological changes are reviewed. Guidelines for the rational use of the drug are suggested.

The distribution of the maximum likelihood estimator in up-and-down experiments for quantal dose-response data

Standard maximum likelihood logistic or probit regression has been used in biopharmaceutical practice for inference about tolerance threshold distributions in situations where subjects (patients) have been allocated doses according to an up-and-down design. For example, a steeper dose-response curve than expected was reported in one such study. This article demonstrates that the maximum likelihood estimator systematically and considerably exaggerates the regression parameter with moderately large sample sizes. Thus a probable explanation for finding a steeper curve than expected is the method used to analyze the experiment, that is, the bias in the maximum likelihood estimator. An additional consequence of this bias is that the mean/median/ED50 are estimated with a misleading precision. In particular, confidence intervals are much too narrow. As a conclusion, we warn against conventional logistic or probit regression in combination with up-and-down designs.

Effects of nitrous oxide on haemodynamic and electroencephalographic responses induced by tetanic electrical stimulation during propofol anaesthesia

We studied the effect of nitrous oxide on haemodynamic and electroencephalographic responses caused by noxious stimulation during propofol anaesthesia. Thirty-four patients (ASA I-II) were anaesthetised with propofol 3 mg x kg(-1) and were randomly allocated to receive either 60% nitrous oxide in oxygen or 40% oxygen in air. Anaesthesia was maintained using propofol infusion of 10 mg x kg(-1) x h(-1) for the first 10 min, 8 mg x kg(-1) x h(-1) for the next 10 min and 6 mg x kg(-1) x h(-1) thereafter. Thirty minutes after the induction of anaesthesia, tetanic electrical stimulation (80 mA, 100 Hz) was applied to the ulnar nerve. Tetanic stimulation significantly increased blood pressure and heart rate in both groups (p < 0.005 or less), but did not induce any arousal pattern on the electroencephalograph. Nitrous oxide significantly attenuated the tetanic stimulation-induced increase in blood pressure (p < 0.05 or less), but not the heart rate.

[Target-controlled intravenous anesthesia]

Target-controlled infusion (TCI) is a new delivery system for i.v. anaesthetic agents with which the anaesthetist targets a plasma drug concentration to achieve a predetermined effect. With this system, the tedious task of calculating the amount of administered drug required to achieve the target concentration is left in charge of a microprocessor which commands the infusion device. TCI has long been used only by a few research teams, but this year a much wider field opens to this delivery system through marketing of Diprifusor, a TCI system specifically designed for administration of propofol in everyday practice. This article describes the rationale for administering i.v. agents through TCI delivery systems, the pharmacokinetic basis of TCI, the regulations and a broad overview of clinical applications, both recent and yet to come.

Induction of anaesthesia with sevoflurane, preprogrammed propofol infusion or combined sevoflurane/propofol for laryngeal mask insertion: cardiovascular, movement and EEG bispectral index responses

Inhalation induction with sevoflurane was compared with propofol or sevoflurane/propofol in 60 unpremedicated adults. Target concentrations for the three groups (with 60% nitrous oxide) were 3% end-tidal sevoflurane, 12 mg/l propofol and 1.5% sevoflurane/6 mg/l propofol respectively, prior to insertion of a laryngeal mask airway (LMA) at 10 minutes. Induction of anaesthesia was satisfactory in each group, but movement response to LMA insertion was observed in 20 patients (least in the sevoflurane group). Cardiovascular responses were similar except for a lower heart rate in the sevoflurane group. EEG bispectral index suggested a greater depth of anaesthesia in the inhalation induction group. A bispectral index of 60 separated patients responding to LMA insertion from nonresponders (P = 0.006), and had a sensitivity of 68% and specificity 70%. Movement response was not predicted by cardiovascular changes.

Comparison of spontaneous frontal EMG, EEG power spectrum and bispectral index to monitor propofol drug effect and emergence

BACKGROUND

The aim of this study was to investigate the accuracy of frontal spontaneous electromyography (SEMG) and EEG spectral edge frequency (SEF 95%), median frequency (MF), relative delta power (RDELTA) and bispectral index (BIS) in monitoring loss of and return of consciousness and hypnotic drug effect during propofol administration at different calculated plasma target concentrations.

METHODS

Propofol was administered by using a target-controlled infusion at different propofol steady-state concentrations. All variables were measured simultaneously at specific calculated concentrations and endpoints.

RESULTS

Loss of consciousness was accurately monitored by BIS, SEMG and SEF 95%, and propofol drug effect by BIS only. Return of consciousness was predicted by BIS, MF and SEF 95%. Due to the biphasic EEG pattern of propofol and the lack of reproducible data at specific propofol concentrations, the clinical usefulness of SEF 95%, MF and RDELTA was very limited. SEMG was useful to detect loss and return of consciousness, but without predictive value.

CONCLUSIONS

The BIS might be an accurate measure to monitor depth of anaesthesia and hypnotic drug effect. Other neurophysiologic measures have limited value to monitor depth of anaesthesia and hypnotic drug effect.

Influence of analgesic supplementation on the target propofol concentrations for anaesthesia with 'Diprifusor' TCI

Forty healthy patients undergoing orthopaedic surgery were randomly allocated to receive an initial blood propofol target concentration of either 4 micrograms.ml-1 or 6 micrograms.ml-1 for induction of anaesthesia with a 'Diprifusor' target controlled infusion system for propofol, and analgesic supplementation with either nitrous oxide 67% in oxygen or alfentanil 15-20 micrograms.kg-1.h-1. Anaesthesia was induced within 3 min in 80% and 95% of patients with propofol target concentrations of 4 micrograms.ml-1 and 6 micrograms.ml-1, respectively. The frequency of discomfort on infusion was similar for both target concentrations. During maintenance, supplementary doses of alfentanil were required to provide adequate surgical conditions in approximately half of the patients receiving nitrous oxide. There was no statistically significant difference between the target concentration [mean (SD)] of propofol for total intravenous anaesthesia [5.1 (2.0) micrograms.ml-1] compared with a technique using nitrous oxide [4.6 (1.2) micrograms.ml-1] supplemented as needed with small doses of alfentanil.