The additive contribution of nitrous oxide to isoflurane MAC in infants and children

Effects of midazolam and nitrous oxide on the minimum anesthetic concentration of isoflurane in the ball python (Python regius)

OBJECTIVE

To evaluate the effects of midazolam and nitrous oxide (N₂O) on the minimum anesthetic concentration of isoflurane (MAC_ISO) in ball pythons.

STUDY DESIGN

Prospective, crossover, randomized, semi-blinded study.

ANIMALS

A total of nine healthy adult female ball pythons (Python regius) weighing 2.76 ± 0.73 kg.

METHODS

In each snake, three protocols were evaluated with 2 week washouts: treatment MID-O₂, midazolam (1 mg kg^-1) administered intramuscularly (IM) and anesthesia induced with isoflurane-oxygen; treatment SAL-O₂, saline (0.2 mL kg^-1) IM and anesthesia with isoflurane-oxygen; and treatment SAL-N₂O, saline IM and anesthesia with isoflurane and 50% nitrous oxide (N₂O):50% oxygen. In each treatment, isoflurane was administered by face mask immediately after premedication. Snakes were endotracheally intubated and inspired and end-tidal isoflurane concentrations were monitored. The study design followed a standard bracketing technique, and the MAC_ISO was determined using logistic regression. Electrical stimulation using a Grass stimulator connected to the base of the tail (50 V, 50 Hz, 6.5 ms pulse^-1) was used as the supramaximal stimulus. Blood-gas analysis was performed on cardiac blood collected immediately following intubation and after the last stimulation. Blood-gas variables were compared over time and between treatments using linear mixed models.

RESULTS

MAC_ISO at a body temperature of 30.1 ± 0.4 °C was 1.11% (95% confidence interval, 0.94-1.28%) in SAL-O₂ and was significantly decreased to 0.48% (0.29-0.67%) in MID-O₂ (p < 0.001) and to 0.92% (0.74-1.09%) in SAL-N₂O (p = 0.016). PO₂ was significantly lower in MID-O₂ and SAL-N₂O than in SAL-O₂.

CONCLUSIONS AND CLINICAL RELEVANCE

Midazolam significantly decreased the MAC_ISO by 57% in ball pythons, whereas addition of N₂O resulted in a modest, although significant, decrease (17%). MAC_ISO in ball pythons was lower than those previously reported in reptiles.

Determination of the minimal alveolar concentration of sevoflurane associated with isoelectric electroencephalogram in children: A prospective, randomized, double-blind study

OBJECTIVE

We investigated the minimal alveolar concentration (MAC) of sevoflurane associated with the occurrence of isoelectric electroencephalogram in 50% of children under steady-state conditions (MAC IE). The MAC IE was determined in 100% oxygen and with the addition of 50% nitrous oxide or after the injection of fentanyl.

METHODS

Seventy-two children (ASA I or II, age 3-8 years) undergoing elective surgery were allocated to one of three groups: in 100% oxygen (group O₂ ), in 50% oxygen and 50% nitrous oxide (group N₂ O), or in 100% oxygen with a bolus of 3 μg/kg fentanyl (group FENTANYL). The state of isoelectric EEG was considered as significant when a burst suppression ratio of 100% lasted for >1 minute. The MAC IE was determined by the Dixon's up-and-down method after a 15-minute period with a stable endtidal concentration of sevoflurane. The concentration of sevoflurane was determined by the electroencephalogram of the previous patient: increase of 0.2% if isoelectric EEG were absent or decrease of 0.2% if isoelectric EEG were present.

RESULTS

The MAC IE in group O₂ (MAC IE_O2 ) was 5.30 (5.12-5.48)% (median [95% CI]). The MAC IE in group N₂ O (MAC IE_N2O ) was 5.83 (5.67-5.99)%. The MAC IE in group FENTANYL (MAC IE_FENTANYL ) was 5.37 (5.21-5.53)%, which was close to MAC IE_O2 .

CONCLUSION

The MAC IE of sevoflurane calculated in 100% O₂ was 5.30% in children. Addition of 50% N₂ O modestly increased MAC IE of sevoflurane, while 3 μg/kg fentanyl had no effect on MAC IE of sevoflurane.

Early Developmental Exposure to General Anesthetic Agents in Primary Neuron Culture Disrupts Synapse Formation via Actions on the mTOR Pathway

Human epidemiologic studies and laboratory investigations in animal models suggest that exposure to general anesthetic agents (GAs) have harmful effects on brain development. The mechanism underlying this putative iatrogenic condition is not clear and there are currently no accepted strategies for prophylaxis or treatment. Recent evidence suggests that anesthetics might cause persistent deficits in synaptogenesis by disrupting key events in neurodevelopment. Using an in vitro model consisting of dissociated primary cultured mouse neurons, we demonstrate abnormal pre- and post-synaptic marker expression after a clinically-relevant isoflurane anesthesia exposure is conducted during neuron development. We find that pharmacologic inhibition of the mechanistic target of rapamycin (mTOR) pathway can reverse the observed changes. Isoflurane exposure increases expression of phospho-S6, a marker of mTOR pathway activity, in a concentration-dependent fashion and this effect occurs throughout neuronal development. The mTOR 1 complex (mTORC1) and the mTOR 2 complex (mTORC2) branches of the pathway are both activated by isoflurane exposure and this is reversible with branch-specific inhibitors. Upregulation of mTOR is also seen with sevoflurane and propofol exposure, suggesting that this mechanism of developmental anesthetic neurotoxicity may occur with all the commonly used GAs in pediatric practice. We conclude that GAs disrupt the development of neurons during development by activating a well-defined neurodevelopmental disease pathway and that this phenotype can be reversed by pharmacologic inhibition.

Sevoflurane-induced isoelectric EEG and burst suppression: differential and antagonistic effect of added nitrous oxide

The objective of this study was to investigate whether nitrous oxide influenced the ED50 of sevoflurane for induction of isoelectric electroencephalogram (ED50_isoelectric ) differently from its influence on the ED50 of sevoflurane for electroencephalogram burst suppression (ED50_burst ). In a prospective, randomised, double-blind, parallel group, up-down sequential allocation study, 77 ASA physical status 1 and 2 patients received sevoflurane induction and, after tracheal intubation, were randomly allocated to receive sevoflurane with either 40% oxygen in air (control group) or 60% nitrous oxide in oxygen mixture (nitrous group). The ED50_isoelectric in the two groups was determined using Dixon's up and down method, starting at 2.5% with 0.2% step size of end-tidal sevoflurane. The electroencephalogram was considered as isoelectric when a burst suppression ratio of 100% lasted > 1 min. The subsequent concentrations of sevoflurane administered were determined by the presence or absence of isoelectric electroencephalogram in the previous patient in the same group. The ED50_isoelectric in the nitrous group 4.08 (95%CI, 3.95-4.38)% was significantly higher than that in the control group 3.68 (95%CI, 3.50-3.78)% (p < 0.0001). The values for ED50_burst were 3.05 (95%CI, 2.66-3.90)% and 3.02 (95%CI, 3.00-3.05)% in nitrous group and control group, respectively (p = 0.52). The addition of 60% nitrous oxide increases ED50_isoelectric , but not the ED50_burst of sevoflurane. Neither result indicates an additive effect of anaesthetic agents, as might be expected, and possible reasons for this are discussed.

Anesthetic Drugs in Congenital Heart Disease

The structural defects associated with the various forms of congenital heart disease lead to pathological and functional changes that place patients at risk for adverse events, and in fact the perioperative incidence of morbidity and mortality has been documented to be increased in children with congenital heart disease. Patients with congenital heart disease can present to the anesthesiologist in a relatively precarious state of balance of several hemodynamic factors, including preload, ventricular contractility, systemic vascular resistance, pulmonary vascular resistance, heart rate, and cardiac rhythm. Anesthetic drugs can affect each of these, and an ideal anesthetic drug for such patients does not exist. The purpose of this article is to review the hemodynamic effects of anesthetic drugs and how they may contribute to the occurrence of adverse events in children with congenital heart disease.

Effects of anesthetics, sedatives, and opioids on ventilatory control

This article provides a comprehensive, up to date summary of the effects of volatile, gaseous, and intravenous anesthetics and opioid agonists on ventilatory control. Emphasis is placed on data from human studies. Further mechanistic insights are provided by in vivo and in vitro data from other mammalian species. The focus is on the effects of clinically relevant agonist concentrations and studies using pharmacological, that is, supraclinical agonist concentrations are de-emphasized or excluded.

Effects of general anesthetics on substance P release and c-Fos expression in the spinal dorsal horn

BACKGROUND

The authors examined in vivo the effects of general anesthetics on evoked substance P release (primary afferent excitability) and c-Fos expression (neuronal activation) in superficial dorsal horn.

METHODS

Rats received saline, propofol (100 mg/kg), pentobarbital (50 mg/kg), isoflurane (2 minimum alveolar concentration), nitrous oxide (66%), or fentanyl (30 μg/kg). During anesthesia, rats received intraplantar 5% formalin (50 μl) to left hind paw. Ten minutes later, rats underwent transcardial perfusion with 4% paraformaldehyde. Substance P release from small primary afferents was assessed by incidence of neurokinin 1 receptor internalization in the superficial dorsal horn. In separate studies, rats were sacrificed after 2 h and c-Fos expression measured.

RESULTS

Intraplantar formalin-induced robust neurokinin 1 receptor internalization in ipsilateral dorsal horn (ipsilateral: 54 ± 6% [mean ± SEM], contralateral: 12 ± 2%; P < 0.05; n = 4). Fentanyl, but not propofol, pentobarbital, isoflurane, nor nitrous oxide alone inhibited neurokinin 1 receptor internalization. However, 2 minimum alveolar concentration isoflurane + nitrous oxide reduced neurokinin 1 receptor internalization (27 ± 3%; P < 0.05; n = 5). All agents reduced c-Fos expression (control: 34 ± 4, fentanyl: 8 ± 2, isoflurane: 12 ± 3, nitrous oxide: 11 ± 2, isoflurane + nitrous oxide: 12 ± 1, pentobarbital: 11 ± 2, propofol: 13 ± 3; P < 0.05; n = 3).

CONCLUSION

General anesthetics at anesthetic concentrations block spinal neuron activation through a mechanism that is independent of an effect on small primary afferent peptide release. The effect of fentanyl alone and the synergistic effect of isoflurane and nitrous oxide on substance P release suggest a correlative rationale for the therapeutic use of these anesthetic protocols by blocking nociceptive afferent transmitter release and preventing the initiation of cascade, which is immediately postsynaptic to the primary afferent.

Anesthetics interfere with the polarization of developing cortical neurons

Numerous studies from the clinical and preclinical literature indicate that general anesthetic agents have toxic effects on the developing brain, but the mechanism of this toxicity is still unknown. Previous studies have focused on the effects of anesthetics on cell survival, dendrite elaboration, and synapse formation, but little attention has been paid to possible effects of anesthetics on the developing axon. Using dissociated mouse cortical neurons in culture, we found that isoflurane delays the acquisition of neuronal polarity by interfering with axon specification. The magnitude of this effect is dependent on isoflurane concentration and exposure time over clinically relevant ranges, and it is neither a precursor to nor the result of neuronal cell death. Propofol also seems to interfere with the acquisition of neuronal polarity, but the mechanism does not require activity at GABAA receptors. Rather, the delay in axon specification likely results from a slowing of the extension of prepolarized neurites. The effect is not unique to isoflurane as propofol also seems to interfere with the acquisition of neuronal polarity. These findings demonstrate that anesthetics may interfere with brain development through effects on axon growth and specification, thus introducing a new potential target in the search for mechanisms of pediatric anesthetic neurotoxicity.

[General anaesthesia in children: a French survey of practices]

INTRODUCTION

The practice of pediatric anesthesia requires a regular update of scientific knowledge and technical skills. To provide the most adequate Continuing Medical Education programs, it is necessary to assess the practices of pediatric anesthesiologists. Thus, the objective of this survey was to draw a picture of the current clinical practices of general anesthesia in children, in France.

MATERIAL AND METHODS

One thousand one hundred and fifty questionnaires were given to anesthesiologists involved in pediatric cases. These questionnaires collected information on various aspects of clinical practice relative to induction, maintenance, recovery from general anaesthesia and also classical debated points such as children with Upper Respiratory Infection (URI), emergence agitation, epileptoid signs or anaesthetic management of adenoidectomy. Differences in practices between CHG (general hospital), CHU (teaching hospital), LIBERAL (private) and PSPH (semi-private) hospitals were investigated.

RESULTS

There were 1025 questionnaires completed. Fifty-five percent of responders worked in public hospitals (CHG and CHU); 77% had a practice that was 25% or less of pediatric cases. In children from 3 to 10 years: 72% of respondents used always premedication and two thirds performed inhalation induction in more than 50% of cases. For induction, 53% used sevoflurane (SEVO) at 7 or 8%. Respondents from LIBERAL used higher SEVO concentrations. Tracheal intubation was performed with SEVO alone (37%), SEVO and propofol (55%) and SEVO with myorelaxant (8%), 93% of respondents used a bolus of opioid. For maintenance, the majority of respondents used SEVO associated with sufentanil; desflurane and remifentanil were more frequently used in CHU. Two thirds of respondents used N(2)O. Depth of anesthesia was commonly assessed by hemodynamic changes (52%), end tidal concentration of halogenated (38%) or automated devices based on EEG (7%). In children with URI, 98% of respondents used SEVO for anesthesia. To control the airway 42% used a tracheal tube, 30% a laryngeal mask and 20% a facial mask. Emergence agitation was an important concern for two thirds of respondents, while epileptoid signs were considered as important by only 20%. Eighty-nine percent of respondents practiced anesthesia for adenoidectomy. Anesthesia was induced by inhalation of SEVO 7-8% (41%), 6% (39%) or 4% (12%), 66% put an intravenous line (less frequently in LIBERAL). 67% of the responders managed adenoidectomy without any device to control the airway (more frequently in LIBERAL), 32% administrated a bolus of opioid (less frequently in LIBERAL).

DISCUSSION

This survey demonstrated that the practices regarding general anesthesia in children are relatively homogenous. Most of the differences appeared between LIBERAL and the others structures; the anaesthetic management for adenoidectomy illustrates these findings.

Airway responses to desflurane during maintenance of anesthesia and recovery in children with laryngeal mask airways

BACKGROUND

We sought to characterize the airway responses to desflurane during maintenance of and emergence from anesthesia in children whose airways were supported with laryngeal mask airways (LMAs).

METHODS/MATERIALS

Four hundred healthy children were randomized in a 3 : 1 ratio to either desflurane or isoflurane (reference group) during anesthetic maintenance. After induction of anesthesia, anesthesia was maintained with the designated anesthetic. The investigator chose the airway (LMA and facemask), ventilation strategy and when to remove the LMA. The incidence of airway events during maintenance, emergence and recovery was recorded.

RESULTS

Ninety percent of children received LMAs. The frequency of major airway events after desflurane (9%) was similar to that after isoflurane (4%) (number needed to harm [NNH] 20), although the frequency of major events after the LMA was removed during deep desflurane anesthesia (15%) was greater than during awake removal (5%) (NNH 10) (P < 0.006) and during deep isoflurane removal (2%) (NNH 8) (P < 0.03). The frequency of airway events of any severity after desflurane was greater than that after isoflurane (39% vs 27%) (P < 0.05). The frequencies of laryngospasm and coughing of any severity after desflurane were greater than those after isoflurane (13% vs 5% and 26% vs 14%, respectively) (P < 0.05).

CONCLUSIONS

When an LMA is used during desflurane anesthesia in children, fewer airway events occur when it is removed when the child is awake. Although the time to discharge from recovery was not delayed and no child required overnight admission, caution should be exercised when using an LMA in children who are anesthetized with desflurane.

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.

The effect of nitrous oxide on halothane, isoflurane and sevoflurane requirements in ventilated dogs undergoing ovariohysterectomy

OBJECTIVE

To examine the effect of 64% nitrous oxide (N2O) on halothane (HAL), isoflurane (ISO) or sevoflurane (SEV) requirements in dogs undergoing ovariohysterectomy.

STUDY DESIGN

Prospective, randomized, clinical trial.

ANIMALS

Ninety, healthy dogs of (mean +/- SD) body weight 21.2 +/- 10.0 kg and age 17.8 +/- 22.8 months.

MATERIALS AND METHODS

After premedication with acepromazine, hydromorphone and glycopyrrolate, anesthesia was induced with thiopental administered to effect. Dogs received one of six inhalant protocols (n = 15 group): HAL; HAL/N2O; ISO; ISO/N2O; SEV; or SEV/N2O. End-tidal CO2 was maintained at 40 +/- 2 mmHg with intermittent positive pressure ventilation (IPPV). Body temperature, heart rate, indirect systemic arterial blood pressures, inspired and end-tidal CO2, volatile agent, N2O and O2 were recorded every 5 minutes. The vaporizer setting was decreased in 0.25-0.5% decrements to elicit a palpebral reflex, and this level maintained. Statistical analysis included two-way anova for repeated measures with Bonferroni's correction factor and statistical significance assumed when p < 0.05. Percentage reduction in end-tidal volatile agent was calculated at 60 minutes after starting study.

RESULTS

End-tidal HAL, ISO and SEV decreased when N2O was administered. Percentage reduction: HAL (12.4%); ISO (37.1%) and SEV (21.4%). Diastolic, mean and systolic blood pressures increased in ISO/N2O compared with ISO. Heart rate increased in ISO/N2O and SEV/N2O compared with ISO and SEV, respectively. Systolic, mean and diastolic blood pressures increased in SEV compared with HAL and ISO. Systolic, mean, diastolic blood pressures and heart rate increased in SEV/N2O and ISO/N2O compared with HAL/N2O.

CONCLUSIONS

N2O reduces HAL, ISO and SEV requirements in dogs undergoing ovariohysterectomy. Cardiovascular stimulation occurred when N2O was used with ISO, less so with SEV and not with HAL

Anesthetic potency of sevoflurane with and without nitrous oxide in mechanically ventilated Dumeril monitors

OBJECTIVE

To determine the minimum alveolar concentration (MAC) of sevoflurane and assess the sevoflurane-sparing effect of coadministration of nitrous oxide in mechanically ventilated Dumeril monitors (Varanus dumerili).

DESIGN

Prospective crossover study.

ANIMALS

10 healthy adult Dumeril monitors.

PROCEDURE

Anesthesia was induced with sevoflurane in 100% oxygen or sevoflurane in 66% nitrous oxide (N2O) with 34% oxygen, delivered through a face mask. Monitors were endotracheally intubated, and end-tidal and inspired isoflurane concentrations were measured continuously; MAC was determined by use of a standard bracketing technique. An electrical stimulus (50 Hz, 50 V) was delivered to the ventral aspect of the tail as the supramaximal stimulus. A blood sample for blood gas analyses was collected from the ventral coccygeal vessels at the beginning and end of the anesthetic period. An interval of at least 7 days was allowed to elapse between treatments.

RESULTS

The MAC +/- SDs of sevoflurane in oxygen and with N2O were 2.51 +/- 0.46% and 1.83 +/- 0.33%, respectively. There was a significant difference between the 2 treatments, and the mean MAC-reducing effect of N2O was 26.4 +/- 11.4%. Assuming simple linear additivity of sevoflurane and N2O, the MAC for N2O was estimated to be 244%. No significant differences in blood gas values--with the predictable exception of oxygen pressure--were detected between the 2 groups.

CONCLUSIONS AND CLINICAL RELEVANCE

The MAC of sevoflurane in Dumeril monitors is similar to that reported for other species. The addition of N2O significantly decreased the MAC of sevoflurane in this species.

INHALATION ANESTHESIA IN DUMERIL'S MONITOR (VARANUS DUMERILI) WITH ISOFLURANE, SEVOFLURANE, AND NITROUS OXIDE: EFFECTS OF INSPIRED GASES ON INDUCTION AND RECOVERY

Induction and recovery from inhalation anesthesia of Dumeril's monitors (Varanus dumerili) using isoflurane, sevoflurane, and nitrous oxide (N2O) were characterized using a randomized crossover design. Mean times to induction for isoflurane in 100% oxygen (O2), sevoflurane in 100% O2, sevoflurane in 21% O2:79% nitrogen (N2; room air), and sevoflurane in 66% N2O:34% O2 were 13.00 +/- 4.55, 11.20 +/- 3.77, 10.40 +/- 2.50, and 9.40 +/- 2.80 min, respectively, at 26 degrees C (n = 10). Mask induction with sevoflurane was significantly faster than with isoflurane. There was no significant difference between the induction time for sevoflurane in O2 or in room air, but sevoflurane combined with N2O resulted in significantly faster inductions than were obtained with sevoflurane in 100% O2. All treatments resulted in a significantly higher respiratory rate than in undisturbed animals. There were no significant differences in respiratory rate among lizards receiving O2, isoflurane in 100% O2, sevoflurane in room air, and sevoflurane combined with N2O, but animals receiving sevoflurane in O2 had a lower respiratory rate than those receiving pure O2. The sequence of complete muscle relaxation during induction was consistent and not significantly different among the four treatments: front limbs lost tone first, followed by the neck and the hind limbs; then the righting reflex was lost and finally tail tone. There were no significant differences in recovery times between isoflurane and sevoflurane or between sevoflurane in 100% O2 and sevoflurane combined with N2O. Similar recovery times were observed in animals recovering in 100 and 21% O2.

Large concentrations of nitrous oxide decrease the isoflurane minimum alveolar concentration sparing effect of morphine in the rat

Many adjuvant drugs have demonstrated anesthetic-sparing properties when combined with volatile anesthetics. Nitrous oxide is combined with volatile anesthetics to reduce the concentrations of volatile anesthetics required to produce anesthesia. Analgesic doses of opioids clearly reduce the requirement for inhaled anesthetics in both human patients and experimental animals. We performed this study to determine whether the combination of nitrous oxide and morphine decreased isoflurane minimum alveolar anesthetic concentration (MAC) even further in the rat. Fifty-eight female rats were used. The rats were divided into 8 groups: isoflurane in 4 possible nitrous oxide concentrations (0%, 30%, 50%, or 70%) with saline or morphine (1 mg/kg). Then the MAC of isoflurane (MAC(ISO))was determined from alveolar gas samples at the time of tail clamp. The MAC of isoflurane was significantly different at each nitrous oxide concentration, and increasing nitrous oxide concentrations reduced anesthetic requirements for isoflurane. The administration of morphine reduced the MAC(ISO) when used with 0% or 30% nitrous oxide. This MAC(ISO) by morphine reduction was less with 50% nitrous oxide and nonexistent at 70% nitrous oxide. However, with morphine present the MAC(ISO) was independent of the nitrous oxide concentration in the 30%-70% range.

Bispectral Index During Isoflurane Anesthesia in Pediatric Patients

Bispectral index (BIS) was developed to monitor anesthetic depth in adults, but has been investigated for use in children, using sevoflurane. We examined the concentration-response relationship between BIS and isoflurane. Thirty children undergoing cardiac catheterization received continuous intraoperative BIS monitoring and had BIS values recorded at 6 steady-state end-tidal isoflurane (Et(Iso)) concentrations between 1.5% and 0.5% and at first arousal. The mean (SD) values for BIS were as follows: 1.5%, 32.3 +/- 11.7; 1.3%, 34.7 +/- 12.5; 1.1%, 40.5 +/- 13.3; 0.9%, 48.0 +/- 13.7; 0.7%, 55.9 +/- 14.4; and 0.5%, 61.8 +/- 13.1. There was an inverse correlation between Et(Iso) and BIS (r = -0.634; P < 0.01). There were significant differences (P < 0.0001) in mean BIS values between adjacent Et(Iso) in all cases except 1.5% versus 1.3%. An inhibitory sigmoid E(max) model best described the BIS-isoflurane concentration relationship, with an 50% effective dose of 0.85% (95% confidence interval, 0.72%-0.98%). The mean value of BIS at first arousal was 78.5 +/- 12.3. The relationship between Et(Iso) and BIS is qualitatively and quantitatively similar to that described for isoflurane in adults and sevoflurane in children. These results add to the body of evidence that BIS is adequately calibrated for use in children older than 1 yr.

IMPLICATIONS

This observational study of children undergoing cardiac catheterization characterizes the concentration-response relationship between bispectral index and isoflurane and demonstrates that bispectral index seems adequately calibrated for monitoring the depth of isoflurane anesthesia in pediatric patients.

Effects of 0.5 and 1.0 MAC isoflurane, sevoflurane and desflurane on intracranial and cerebral perfusion pressures in children

BACKGROUND

Isoflurane has been a commonly used agent for neuroanesthesia, but newer agents, sevoflurane and desflurane, have a quicker onset and shorter emergence from anesthesia and are increasingly preferred for general pediatric anesthesia. But their effects on intracranial pressure (ICP) and cerebral perfusion pressure (CPP), especially in pediatric patients with already increased ICP, have not been well documented.

METHODS

We studied 36 children scheduled for elective implantation of an intraparenchymal pressure device for 24 h monitoring for suspected elevated ICP. After a standardized intravenous anesthesia, the patients were moderately hyperventilated with 60% nitrous oxide (N2O) in oxygen. The patients were then randomized to receive 0.5 and 1.0 MAC of isoflurane (Group I, n = 12), sevoflurane (Group S, n = 12) or desflurane (Group D, n = 12) in 60% N2O in oxygen. Respiratory and hemodynamic variables, ICP and CPP were recorded at baseline and after exposure to a target level of test drug for 10 min or until CPP fell below 30 mmHg (recommended lower ICP level is 25 mmHg in neonates, rising to 40 mmHg in toddlers).

RESULTS

When comparing baseline values with values at 1.0 MAC, mean arterial pressure (MAP) decreased (P < 0.001) in all groups, with no differences between the groups. ICP increased (P < 0.001) with all agents, mean +2, +5, and +6 mmHg in Group I, S and D, respectively, with no differences between the groups. Regression analyzes found no relationship between baseline ICP and the increases in ICP from baseline to 1.0 MAC for isoflurane or sevoflurane. However, increased baseline ICP tended to cause a higher ICP increase with 1.0 MAC desflurane; regression coefficient +0.759 (P = 0.077). The difference between regression coefficients for Group I and Group D were not significant (P = 0.055). CPP (MAP-ICP) decreased (P < 0.001) in all groups, mean -18, -14 and -17 mmHg in Group I, S and D, respectively, with no significant difference between the groups.

CONCLUSIONS

0.5 and 1.0 MAC isoflurane, sevoflurane and desflurane in N2O all increased ICP and reduced MAP and CPP in a dose-dependent and clinically similar manner. There were no baseline dependent increases in ICP from 0 to 1.0 MAC with isoflurane or sevoflurane, but ICP increased somewhat more, although statistically insignificant, with higher baseline values in patients given desflurane. The effect of MAP on CPP is 3-4 times higher than the effect of the increases in ICP on CPP and this makes MAP the most important factor in preserving CPP. In children with known increased ICP, intravenous anesthesia may be safer. However, maintaining MAP remains the most important determinant of a safe CPP.

Age, minimum alveolar anesthetic concentration, and minimum alveolar anesthetic concentration-awake

Two defining effects of inhaled anesthetics (immobility in the face of noxious stimulation, and absence of memory) correlate with the end-tidal concentrations of the anesthetics. Such defining effects are characterized as MAC (the concentration producing immobility in 50% of patients subjected to a noxious stimulus) and MAC-Awake (the concentration suppressing appropriate response to command in 50% of patients; memory is usually lost at MAC-Awake). If the concentrations are monitored and corrected for the effects of age and temperature, the concentrations may be displayed as multiples of MAC for a standard age, usually 40 yr. This article provides an algorithm that might be used to produce such a display, including provision of an estimate of the effect of nitrous oxide.

IMPLICATIONS

Two defining effects of inhaled anesthetics (immobility in the face of noxious stimulation, and absence of memory) correlate with the end-tidal concentrations of the anesthetics. Thus, these defining effects may be monitored and the results displayed if the concentrations are known and corrected for the effects of age and temperature.

Comparison of recovery after intermediate duration of anaesthesia with sevoflurane and isoflurane

BACKGROUND

The purpose of this study was to compare recovery from anaesthesia after sevoflurane and isoflurane were administered to children for more than 90 min.

METHODS

After parental informed consent and ethical committee approval, children aged between 2 months and 6 years, ASA I or II, were randomly allocated to sevoflurane (n=20) or isoflurane (n=20) groups. Halogenated agents were discontinued following skin closure and patients were ventilated mechanically with 100% oxygen until minimum alveolar concentration (MAC) values awake were obtained (endtidal concentrations 0.6 MAC for sevoflurane and 0.4 MAC for isoflurane). Effective perioperative analgesia was provided by a caudal block.

RESULTS

The mean (+/- SD) duration of anaesthesia was 132 +/- 38 min and 139 +/- 49 min for sevoflurane and isoflurane, respectively. Early recovery occurred sooner in the isoflurane group (time to extubation was 16 +/- 7 min and 11 +/- 5 min, P<0.01; Aldrete's score at 0 min was 5.5 +/- 1.5 and 7.4 +/- 1.8, P<0.001, respectively). But the time to be fit for discharge from recovery room was similar at 136 +/- 18 min and 140 +/- 20 min, respectively.

CONCLUSIONS

After intermediate duration of anaesthesia administered to children for up to 90 min, isoflurane and sevoflurane allow recovery after approximatively the same lapse of time.

The safety and efficacy of sevoflurane anesthesia in infants and children with congenital heart disease

We tested the hypothesis that sevoflurane is a safer and more effective anesthetic than halothane during the induction and maintenance of anesthesia for infants and children with congenital heart disease undergoing cardiac surgery. With a background of fentanyl (5 microg/kg bolus, then 5 microg. kg(-1). h(-1)), the two inhaled anesthetics were directly compared in a randomized, double-blinded, open-label study involving 180 infants and children. Primary outcome variables included severe hypotension, bradycardia, and oxygen desaturation, defined as a 30% decrease in the resting mean arterial blood pressure or heart rate, or a 20% decrease in the resting arterial oxygen saturation, for at least 30 s. There were no differences in the incidence of these variables; however, patients receiving halothane experienced twice as many episodes of severe hypotension as those who received sevoflurane (P = 0.03). These recurrences of hypotension occurred despite an increased incidence of vasopressor use in the halothane-treated patients than in the sevoflurane-treated patients. Multivariate stepwise logistic regression demonstrated that patients less than 1 yr old were at increased risk for hypotension compared with older children (P = 0.0004), and patients with preoperative cyanosis were at increased risk for developing severe desaturation (P = 0.049). Sevoflurane may have hemodynamic advantages over halothane in infants and children with congenital heart disease.

IMPLICATIONS

In infants and children with congenital heart disease, anesthesia with sevoflurane may result in fewer episodes of severe hypotension and less emergent drug use than anesthesia with halothane.

Nitrous oxide lacks the antinociceptive effect on the tail flick test in newborn rats

Nitrous oxide (N(2)O) is commonly used for pediatric anesthesia under the assumption that it produces a similar analgesic response to that seen in adults. We examined the antinociceptive effect of 75% N(2)O on tail flick latency response in newborn rats at postnatal Day 1 (PD 1), PD 8, PD 15, PD 22, and PD 29. Up to PD 15, rats showed no analgesic effect to N(2)O. By PD 29, rats exhibited a comparable analgesic effect to that seen in adult animals. These data are consistent with the fact that the descending noradrenergic neurons, which are required for the analgesic action of N(2)O, are not anatomically or functionally developed at birth and take more than three weeks to fully develop in rats.

IMPLICATIONS

The present study indicates that rats below 3 wk old lack an antinociceptive effect to nitrous oxide by using the tail flick test. Because a 3-wk-old rat is comparable in neurological development with the toddler stage in humans, we may anticipate that patients below this age may not experience the usual analgesic effect of nitrous oxide.

The interaction between nitrous oxide and isoflurane on suppression of learning: a study using classical conditioning in rabbits

BACKGROUND

Nitrous oxide (N2O) is commonly combined with a volatile agent for administration of general anesthesia. We studied the effects of N2O and isoflurane on learning of the rabbit nictitating membrane responses (NMRs).

METHODS

Classical conditioning of the NMR was accomplished by presenting a 400 ms tone conditioned stimulus before the presentation of a 100 ms shock unconditioned stimulus over 6 daily training sessions. The percentages of conditioned responses (CRs) were calculated for animals treated with 0% (n = 10), 33% (n = 11), 67% (n = 11), and 75% (n = 7) N2O and for those treated with 0% (n = 8), 0.2% (n = 7), 0.4%, (n = 13) and 0.8% (n = 9) isoflurane separately. ED-50 for suppression of learning for each drug were calculated. Percentages of CRs were calculated for treatments with combinations of 0.2% isoflurane with either 32 or 48% N2O (n = 14, for each).

RESULTS

Isobolographic analysis demonstrated that the combination of the two drugs exerted no greater effect than that seen with either agent administered alone; for well-established CRs (mean of days 5 and 6), the estimated concentrations corresponding to a rate of 70% CRs were 0.31% isoflurane with no N2O, 65.3% N2O with no isoflurane, and 0.2% isoflurane combined with 32.4% N2O.

CONCLUSIONS

N2O and isoflurane interact additively on suppression of learning.

Halothane concentrations required to block the cardiovascular responses to incision (MAC CVR) in infants and children

The purpose of this study was to determine the halothane concentration in N2O required to block the cardiovascular responses to skin incision (MAC CVR) in infants and children. We studied 64 unpremedicated ASA 1 infants and children (one month to seven years). In each infant or child, anaesthesia was induced slowly with halothane and N2O, and an endotracheal tube was placed. The MAC CVR was assessed, after a steady state end-tidal halothane concentration had been established for ten minutes, by the "up and down technique" of Dixon. Positive responses were defined as an increase in MAP or HR > 10%. The MAC CVR50 values of halothane with 60% N2O were 1.16 +/- 0.23% at 1-6 mo, 1.17 +/- 0.18% at 7-12 mo, 0.95 +/- 0.26% at 1-3 yr, and 1.12 +/- 0.16% at 4-7 yr. The value at 1-3 years children was less than those in the other age groups (P < 0.05). The changes of MAP were correlated with changes of both HR and pupillary diameter. These results indicate that the values of MAC CVR50 of halothane in infants and children are higher than those required to block motor responses (MAC). The halothane requirement to block cardiovascular responses is lowest in the children aged one to three years.