Oral clonidine premedication reduces minimum alveolar concentration of sevoflurane for laryngeal mask airway insertion in children

BACKGROUNDS: Sevoflurane is widely employed for inhalational induction in children. Clonidine deepens volatile anesthetics and reduces several types of MAC of sevoflurane. Laryngeal mask airway is a useful device for pediatric anesthesia. The aim of the current study was to determine whether oral clonidine premedication can reduce MAC of sevoflurane for an LMA insertion in children. METHODS: Fifty-six ASA physical status I patients (3-11 years) scheduled for general anesthesia were randomly divided into two groups of 28 patients each. One group (clonidine group) received clonidine 4 microg x kg(-1) approximately 100 min before anesthesia, and the other (control) group did not. Anesthesia was induced with sevoflurane. Each concentration of sevoflurane, at which an LMA insertion was attempted, was predetermined according to the modification of Dixon's up-and-down method with 0.25% as a step size and held constant for at least 20 min before the trial. All responses ('movement' or 'no movement') to an LMA insertion were assessed. RESULTS: Minimum alveolar concentration values of sevoflurane for an LMA insertion were lower in the clonidine group (1.31% +/- 0.18% [mean +/- sd]) than in the control group (2.00% +/- 0.16%). Logistic regression analysis revealed that sevoflurane EC95 values were 1.79% and 2.49% in the clonidine and control groups, respectively. CONCLUSIONS: Oral clonidine premedication reduced the MAC (EC50) and EC95 values of sevoflurane for LMA insertion by 38% and 28%, respectively.     

Additive contribution of nitrous oxide to sevoflurane minimum alveolar concentration for tracheal intubation in children.

BACKGROUND: To study the interaction between nitrous oxide and sevoflurane during trachea intubation, the authors determined the minimum alveolar concentration of sevoflurane for tracheal intubation (MAC(TI)) with and without nitrous oxide in children. METHODS: Seventy-two children aged 1-7 yr were assigned randomly to receive one of three end-tidal concentrations of nitrous oxide and one of four end-tidal concentrations of sevoflurane: 0% nitrous oxide with 2.0, 2.5, 3.0, or 3.5% sevoflurane: 33% nitrous oxide with 1.5, 2.0, 2.5, or 3.0% sevoflurane; or 66% nitrous oxide with 1.0, 1.5, 2.0, or 2.5% sevoflurane. After steady state end-tidal anesthetic concentrations were maintained for at least 10 min, laryngoscopy and intubation were attempted using a straight-blade laryngoscope and an uncuffed tracheal tube. The interaction between nitrous oxide and sevoflurane was investigated using logistic regression analysis of the responses to intubation. RESULTS: Logistic regression curves of the probability of no movement in response to intubation in the presence of sevoflurane and 0, 33, and 66% nitrous oxide were parallel. The interaction coefficient between nitrous oxide and sevoflurane did not differ significantly from zero (P = 0.89) and was removed from the logistic model. The MAC(TI) (+/- SE) of sevoflurane was 2.66+/-0.16%, and the concentration of sevoflurane required to prevent movement in 95% of children was 3.54+/-0.25%. Thirty-three percent and 66% nitrous oxide decreased the MAC(TI) of sevoflurane by 18% and 40% (P<0.001), respectively. CONCLUSIONS: We conclude that nitrous oxide and sevoflurane suppress the responses to tracheal intubation in a linear and additive fashion in children.     

The minimum alveolar concentration of enflurane for laryngeal mask airway extubation in deeply anesthetized children

The end-tidal anesthetic gas concentration required to prevent the anesthetized patient from coughing or moving during or immediately after the laryngeal mask airway (LMA) extubation is not known. We sought to determine the minimum alveolar concentration of enflurane required for the removal of the LMA in children. We studied 21 nonpremedicated children between 4 and 11 yr of age, ASA physical status I, undergoing procedures below the umbilicus. General anesthesia was induced with a mask by using sevoflurane, nitrous oxide, and oxygen, and the LMA was inserted. Anesthesia was maintained with enflurane, nitrous oxide, and oxygen. At the end of surgery, a predetermined end-tidal enflurane concentration was achieved, and the LMA was removed. Each concentration at which the LMA extubation was attempted was predetermined by the up-and-down method (with 0.1% as a step size). When LMA removal was accomplished without coughing, clenching teeth, or gross purposeful muscular movements during or within 1 min after removal, it was considered a successful LMA removal. Removal was considered to be unsuccessful in patients who developed breath holding or laryngospasm during or immediately after LMA removal. The minimum alveolar concentration of enflurane at which 50% of children had a successful LMA removal was found to be 1.02% (95% CL, 0.95%-1.11%), and the 95% effective dose for successful extubation was 1.14% (95% CL, 1.07%-1.66%). In conclusion, the LMA removal may be accomplished without coughing or moving at 1.02% end-tidal enflurane concentration in 50% of anesthetized children aged 4-11 yr. Implications: There may be fewer problems associated with the laryngeal mask airway extubation when patients are deeply anesthetized. The purpose of this study was to determine the minimum concentration of enflurane for successful removal of the laryngeal mask in children.     

右美托咪定降低小儿Ambu AuraOnce喉罩置入所需七氟醚呼气末半数有效浓度

目的 观察右美托咪定(dexmedetomidine,Dex)1μg/kg降低小儿Ambu AuraOnce喉罩(Ambu喉罩)置入所需七氟醚呼气末半数有效浓度(median effective concentration,EC50)作用. 方法 4～10岁全身麻醉下行择期整形外科手术患儿50例,ASA分级Ⅰ级,计算机随机抽样表格法分为Dex组(D组,26例)和对照组(C组,24例).患儿入室后均采用8％七氟醚和50％氧化亚氮(nitrous oxide,N2O)吸入行麻醉诱导,建立静脉通路后停止N2O吸入,调节七氟醚吸入浓度使七氟醚呼气末EC50达到预定浓度.同时D组开始静脉输注Dex 1 μg/kg,C组注射生理盐水,10 min注射完毕后置入一次性Ambu喉罩.两组首例患儿的七氟醚呼气末EC50均设定为1.8％,采用改良Dixon up-down法确定下一例喉罩置入的七氟醚呼气末EC50,相邻七氟醚的浓度梯度值为0.2％. 结果 小儿置入Ambu喉罩时所需七氟醚呼气末EC50值(95％CI)D组和C组分别为1.44％(1.17％,1.60％)和1.77％(1.52％,2.01％). 结论 Dex 1 μg/kg可有效降低小儿置入Ambu喉罩所需七氟醚呼气末EC50.     

Propofol or sevoflurane for laryngeal mask airway insertion

PURPOSE: Sevoflurane is a volatile anesthetic agent, which combines rapid, smooth inhalational induction of anesthesia with rapid recovery, making it particularly suitable for day case anesthesia. The laryngeal mask airway is often also used in ambulatory anesthesia, with intravenous propofol being the agent of choice for its insertion. Our objective was to compare the conditions for laryngeal mask airway (LMA) insertion obtained by modified vital capacity breath sevoflurane inhalational induction of anesthesia with propofol intravenous induction. METHODS: Eighty-eight patients, aged 18-65 yr, ASA I-II, undergoing general anesthesia for elective surgery were randomized into two groups in a prospective, single-blind study. Patients in Group P (n=44) received 2.5 mg x kg(-1) propofol i.v. and in Group S (n=44) received sevoflurane 8% in nitrous oxide 50% and oxygen. Ventilation was not assisted. Laryngeal mask airway insertion was attempted at one minute intervals from loss of both verbal response and eyelash reflex, by an anesthesiologist unaware of the induction technique. Complications, such as coughing and head movement, were also noted at each attempt. RESULTS: Mean time to successful LMA insertion was 1.3 (1-3) min in P and 2.2 (1-3) min in S, P < 0.05. Eleven patients in Group P, (25%) required additional propofol compared with four (9%) in S, P < 0.05. Incidence of complications was similar in both groups and by 3 min, LMA was successfully inserted in all patients. CONCLUSION: Modified vital capacity breath inhalational induction with sevoflurane 8% is efficient for LMA insertion in most cases, but takes slightly longer than propofol.     

七氟烷抑制全麻患儿喉罩通气道拔除反应的最低肺泡有效浓度

目的 确定七氟烷抑制全麻患儿喉罩通气道拔除反应的最低肺泡有效浓度(MAC).方法 择期全麻手术患儿25例,ASA Ⅰ或Ⅱ级,年龄3～8岁.吸人8%七氟烷麻醉诱导后置人喉罩,吸入3%七氟烷维持麻醉.术中均保留自主呼吸.术毕吸除口腔分泌物后维持预定七氟烷浓度10 min,采用序贯法调整七氟烷浓度,初始呼气末七氟烷浓度为1%,相邻浓度比值为1.2,喉罩通气道拔除反应阳性时,则下一例患儿升高1个浓度梯度;喉罩通气道拔除反应阴性时,则下一例患儿降低1个浓度梯度.拔除喉罩后1 min内患儿出现有目的性的肢体运动、屏气、喉痉挛和低氧血症(spO2<95%)为喉罩通气道拔除反应阳性,否则为阴性.将阳性反应到阴性反应的中点设为平衡点,计算所有平衡点七氟烷浓度的平均值,即为MAC.结果 七氟烷抑制喉罩通气道拔除反应的MAC为0.98%.结论 七氟烷抑制全麻患JL(3～8岁)喉罩通气道拔除反应的最低肺泡有效浓度为0.98%.     

Influence of sepsis on sevoflurane minimum alveolar concentration in a porcine model

Sevoflurane is widely used in anaesthetic protocols for patientsundergoing surgical procedures. However, there are no reportson the influence of sepsis on minimum alveolar concentrationof sevoflurane (MAC_SEV) in animals or in humans. The aim ofthis study was to test the hypothesis that sepsis could alterthe MAC_SEV in a normotensive septic pig model. Twenty young,healthy pigs were used. After they had received 10 mg kg^–1of ketamine i.m. for premedication, anaesthesia was establishedwith propofol 3 mg kg^–1 and the trachea wasintubated. Sevoflurane was used as the sole anaesthetic agent.Baseline haemodynamic recording included electrocardiography,carotid artery blood pressure and a pulmonary thermodilutioncatheter. Baseline MAC_SEV in each pig was evaluated by pinchingwith a haemostat applied for 1 min to a rear dewclaw. MAC_SEVwas determined using incremental changes in sevoflurane concentrationuntil purposeful movement appeared. Pigs were assigned randomlyto two groups: the saline group (n=10) received a 1-h i.v. infusionof sterile saline solution while the sepsis group (n=10) receiveda 1-h i.v. infusion of live Pseudomonas aeruginosa. Epinephrineand hydroxyethylstarch were used to maintain normotensive andnormovolemic haemodynamic status. In both groups, MAC_SEV wasevaluated 5 h after infusion. Significant increases inmean artery pulmonary pressure, filling, epinephrine and vascularpulmonary resistances occurred in the sepsis group. MAC_SEV forthe saline group was 2.4% [95% confidence interval (CI) 2.1–2.55%]and the MAC_SEV for the sepsis group was 1.35% (95% CI 1.2–1.45%,P<0.05). These data indicate that MAC_SEV is significantlydecreased in this normotensive septic pig model. Br J Anaesth 2001; 86: 832–6     

小儿食管引流型喉罩与经典喉罩插入时适宜呼气末七氟醚浓度的比较

目的比较小儿食管引流型喉罩(PLMA)与经典喉罩(CLMA)插入所需的七氟醚最低肺泡有效浓度(MAC)。方法择期全麻手术患儿54例,年龄3~9岁,ASAⅠ或Ⅱ级,随机均分为PLMA组(P组)和CLMA组(C组)。患儿吸入七氟醚行麻醉诱导,待呼气末七氟醚浓度(CETSev)达到预设值并稳定10 min后插入喉罩。七氟醚的预设值根据Dixon序贯法确定,P、C组首例患儿初始CETSev分别设为2.2%和2.0%,浓度梯度为0.2%。结果患儿插入PLMA和CLMA所需的七氟醚MAC分别为(2.21±0.11)%和(1.99±0.11)%(P<0.01),P、C组CETSev的EC95分别为2.40%(95%CI 2.28%~2.74%)和2.14%(95%CI 2.03%~2.44%)。结论 3~9岁小儿PLMA插入条件满意所需的七氟醚MAC较插入CLMA高。     

The minimum alveolar concentration of sevoflurane in cats

Eight adult cats of either sex were studied. The minimal alveolar concentration (MAC) for sevoflurane in the cats was found to be 2.58 ± 0.30% (mean ± SD). The ratios of MAC values between sevoflurane and halothane, enflurane and isoflurane in cats were very similar to those ratios found in humans and dogs. This observation suggests that the results of this study are correct and allows us to estimate unknown MAC values for sevoflurane in other species using known MAC values for other anesthetic agents.(Doi M, Yunoki H, Ikeda K: The minimum alveolar concentration of sevoflurane in cats. J Anesth 2: 113–114, 1988)     

Caudal anesthesia reduces the minimum alveolar concentration of enflurane for laryngeal mask airway removal in boys

PURPOSE: To investigate the effects of caudal analgesia on the minimal alveolar concentration of enflurane for laryngeal mask airway (LMA) smooth extubation (MACex). METHODS: We studied 50 nonpremedicated children, aged three to ten years, ASA physical status I, undergoing surgery for hypospadias repair. After a sevoflurane inhalation induction, children were randomized to receive LMA insertion with or without ropivacaine caudal analgesia. At the end of surgery, a predetermined end-tidal enflurane concentration was achieved, and the LMA was removed by an anesthesiologist blinded to group allocation. Each concentration at which LMA extubation was attempted was predetermined by the up-and-down method (with 0.1% as the step size). When LMA removal was accomplished without coughing, clenching teeth or gross purposeful muscular movements during or within one minute after removal, it was considered successful. RESULTS: MACex of enflurane for LMA removal in the group without caudal anesthesia was 1.04% (95% confidence interval, 1.00-1.10) and the LMA MACex of enflurane in the group with caudal anesthesia was 0.74% (95% confidence interval, 0.63-0.81). Caudal analgesia significantly reduced enflurane requirements by 29% (95% confidence interval, 22-36%). CONCLUSION: In conclusion, caudal analgesia significantly reduced the LMA MACex of enflurane by approximately 29%. Possible mechanisms may be related to the analgesic effect of caudal blockade or to the sedative properties of neuraxial anesthesia.     

The effect of nitrous oxide diffusion on laryngeal mask airway cuff inflation in children

We studied the effect of nitrous oxide (N₂O) diffusion on size 2 LMA cuff inflation in 52 children, 38±21 months of age. LMA cuffs were inflated with air (Group A) or 65% N₂O, oxygen and halothane (Group B). Cuffs were inflated in a stepwise manner, achieving a cuff leak pressure of 17±4 cm H₂O for all patients. Cuff volume and intracuff pressure increased in Group A (8.5±1.0 ml to 10.8±1.4 ml and 101±36 mmHg to 152±42 mmHg, respectively) and decreased in Group B (8.9±1.0 ml to 6.6±1.5 ml and 90±30 mmHg to 53±37 mmHg, respectively). Cuff leak pressure did not change significantly in either group. We conclude that cuff inflation with a mixture of N₂O/oxygen prevents N₂O diffusion into the cuff, avoiding cuff overinflation without compromising LMA function.     

Comparison of sevoflurane with propofol for laryngeal mask airway insertion in adults

We performed a prospective, randomized, controlled trial to compare the quality and ease of laryngeal mask airway (LMA) insertion after either rapid inhaled sevoflurane or i.v. propofol induction of anesthesia. Seventy-six unpremedicated ASA physical status I or II patients were anesthetized with either a single vital capacity breath of sevoflurane 8% or i.v. propofol 3 mg/kg, which produced equally rapid loss of consciousness (40.5 +/- 13.9 vs 37.7 +/- 9.9 s; P > 0.05). The LMA was inserted more rapidly in patients in the propofol group (74 +/- 29 vs 127 +/- 35 s; P < 0.01) and required fewer attempts (1.2 vs 1.6; P < 0.05) than the sevoflurane group. There was a greater incidence of initially impossible mouth opening in the sevoflurane group (45% vs 21%; P < 0.05). Once mouth opening was possible, the degree of attenuation of laryngeal reflexes was similar. The overall incidence of complications related to LMA insertion, especially apnea (32% vs 0%; P < 0.01), was more frequent in the propofol group (82% vs 26%; P < 0.01). There were four failures of LMA insertion in the propofol group and none in the sevoflurane group. Both groups had stable hemodynamic profiles and good patient satisfaction. We conclude that sevoflurane vital capacity breath induction compares favorably with i.v. propofol induction for LMA insertion in adults. However, prolonged jaw tightness after the sevoflurane induction of anesthesia may delay LMA insertion. Implications: In this randomized, controlled trial, we compared the ease of insertion of the laryngeal mask airway in adults after induction of anesthesia with either a sevoflurane vital capacity breath technique or propofol i.v.. We conclude that sevoflurane compares favorably with propofol, although prolonged jaw tightness may delay laryngeal mask airway insertion.     

Pollution with nitrous oxide using laryngeal mask or face mask

BACKGROUND: As environmental pollution by nitrous oxide may influence the health of the personnel working in operating theatres, the incidence and magnitude of nitrous oxide (N2O) leakage, when using a face mask or a laryngeal mask airway (LMA) for controlled ventilation, were studied in 34 patients scheduled for elective cystoscopy. METHODS: A semi-closed gas delivery ventilation system with active scavenging was used. The N2O concentrations were measured every 8 s at a position 30 cm above the patient's mouth with a N2O gas monitor (GD 200, Simrad Optronics). RESULTS: When using a face mask, the leakage of N2O resulted in a N2O concentration of 157 (85-332) p.p.m. (parts per million) (median concentration and 25% and 75% percentiles). With the LMA, a lower median concentration of N2O of 60 (28-126) p.p.m. was found (P=0.04). With the face mask, a concentration above 100 p.p.m. was found during 51% of the exposure time compared to 24% of the time in the LMA group. CONCLUSION: Environmental pollution was less with the LMA than the face mask, but under the conditions of the study both modes of airway management were associated with levels of N2O peak concentrations in the breathing zone of anaesthetists that are deemed to be excessively high by the Danish National Institute for Occupational Safety.     

Predicted values of propofol EC50 and sevoflurane concentration for insertion of laryngeal mask Classic and ProSeal

Background. A new laryngeal mask airway, the ProSeal^TM (PLMA),is said to be more difficult to insert than the laryngeal maskairway Classic^TM (CLMA) using propofol anaesthesia. Therefore,we expected a greater dose of propofol and sevoflurane to berequired to insert the PLMA compared with the CLMA. We determinedthe effective concentration 50% (EC₅₀) of propofol and end-tidalsevoflurane to allow insertion of the PLMA and the CLMA. Methods. Seventy-six elective female patients (aged 20–60yr and ASA I–II) were randomly assigned to one of fourgroups. Either a PLMA or a CLMA was inserted using either propofoltarget controlled infusion or sevoflurane. Both propofol andsevoflurane targets were determined with a modified Dixon’sup-and-down method. After equilibration between the predeterminedblood and effect site concentrations, which had been held steadyfor more than 10 min, LMA insertion was attempted without neuromuscularblock. Results. The predicted EC_50CLMA and EC_50PLMA for propofol were3.14 (0.33) and 4.32 (0.67) µg ml^–1. E'_CLMAand E'_PLMA of sevoflurane (mean (SD)) were 2.36 (0.22) and 2.82(0.45)% (P<0.01 and 0.05, respectively). Conclusions. The estimated concentration of propofol and thesevoflurane concentration needed to allow insertion of the ProSeal^TMare respectively 38 and 20% greater than those needed for insertionof the Classic LMA. Br J Anaesth 2004; 92: 242–5     

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.     

The effect of nitrous oxide on laryngeal mask cuff pressure

We have studied the effect of nitrous oxide on the cuff pressure of a laryngeal mask both in vitro and in vivo. In laboratory tests, we showed that nitrous oxide and carbon dioxide diffuse across the cuff wall much more rapidly than nitrogen and oxygen. Differing partial pressures of these gases across the cuff wall therefore give rise to changes in volume and pressure within the cuff. We then studied 18 patients undergoing general anaesthesia with nitrous oxide, and found a consistent and linear increase in cuff pressure in all patients. After 30 min, the mean pressure had increased by 30 mmHg, and there was approximately 10% nitrous oxide in the cuff. It is difficult to relate these findings to pressure on pharyngeal structures, but methods of limiting the rise in intracuff pressure are discussed.