The effect of a heat and moisture exchanger on humidity in a low-flow anaesthesia system

The heat and humidity in a low-flow breathing system was measured in order to study the inherent humidifying properties of the system at low fresh gas flows (< 1 and 2 l.min⁻¹) and whether a heat and moisture exchanger could compensate for the loss of heat and humidification occurring at higher fresh gas flows (5 l.min⁻¹) in these systems. Sixty patients were randomly divided into three groups (< 1, 2 and 5 l.min⁻¹ fresh gas flows) with a heat and moisture exchanger and three groups without a heat and moisture exchanger in the breathing system. Thirty minutes after the start of anaesthesia a control measurement was performed, after which a heat and moisture exchanger was inserted into the breathing system of the three groups randomly allocated to have one. Three more measurements were performed at 10, 30 and 60 min after control. At low fresh gas flows the humidifying properties of the low-flow breathing system are adequate (i.e. provide an absolute humidity > 20 mg.l⁻¹) but at a fresh gas flow of 5 l.min⁻¹ there is a need for a heat and moisture exchanger for adequate humidification of the inspired gas.     

Minimal-flow anaesthesia with controlled ventilation: comparison between laryngeal mask airway and endotracheal tube.

BACKGROUND AND OBJECTIVE: Minimal- and low-flow anaesthesia (fresh gas flow below 1 L min(-1)) provide many advantages, including reduced cost, conservation of body heat and airway humidity. An airtight seal is essential between the airway device and the airway of the patient. Therefore, we investigated whether the airtight seal created by a laryngeal mask airway allows controlled ventilation of the lungs when the fresh gas flow is reduced to 0.5 L min(-1) and compared this with an endotracheal tube. METHODS: In a prospective clinical study, 207 patients were managed using a laryngeal mask or an endotracheal tube. After intravenous induction of anaesthesia and 15 min of high fresh gas flow, the flow was reduced to 0.5 L min(-1). The breathing system was monitored for airway leaks, and the patients were assessed for complications after airway removal and postoperative discomfort. RESULTS: Both the laryngeal mask and endotracheal tube allowed fresh gas flow reduction to 0.5 L min(-1) in 84.7% and 98.3% of cases respectively (small leaks: 12% laryngeal mask, 1.7% endotracheal tube). Three patients with the laryngeal mask (3.3%) had airway leaks that were too large to permit any reduction in the fresh gas flow. CONCLUSIONS: The use of the laryngeal mask airway was more likely to be associated with a gas leak than use of an endotracheal tube; however, if modern anaesthesia machines and monitors are used, in 96.7% of the patients managed with a laryngeal mask a reduction in the fresh gas flow to 0.5 L min(-1) was possible. The incidence of coughing and postoperative complaints (sore throat, swallowing problems) was higher after use of an endotracheal tube.     

Economics of low-flow anaesthesia in children

We have measured the consumption of isoflurane and fresh gas flows in 77 infants and children during 20 all-day operating sessions using either the enclosed Mapleson A or the circle absorber mode of the Garden'Ventmasta'ventilator. The average consumption (SD) of isoflurane in 37 patients anaesthetised using the A mode of the Garden system with a mean fresh gas flow of 2.6l.min⁻¹ was 11.1 (4.2)g.h⁻¹, while that in 40 patients anaesthetised using the circle absorber mode with a mean fresh gas flow of 1.21.min⁻¹ was 4.7 (1.0)g.h⁻¹. These figures represent an overall saving of 58% in the use of isoflurane (p < 0.0001) and a mean reduction in fresh gas flow of 54% (p < 0.0001) as a result of using low-flow anaesthesia. With the addition of small bore breathing hoses the adult circle absorber system was practical to use in both infants and children. These findings should stimulate interest in the use of low-flow techniques in children.     

Spontaneous breathing with the use of a laryngeal mask airway in children: comparison of sevoflurane and isoflurane

We compared respiratory parameters during anaesthesia with sevoflurane and isoflurane through a laryngeal mask airway (LMA). Children were anaesthetized with O₂ and air with 2.3% (1MAC) sevoflurane ( n =20) or 1.5% (1MAC) isoflurane ( n =20). After insertion of LMA, patients were allowed to breathe spontaneously and respiratory rate (RR) and P ECO2 were measured (presurgery state). After the measurement, anaesthetic concentration was increased to 1.3 MAC (3.0% sevoflurane or 2.0% isoflurane) and surgical stimulation was added. Fifteen min after incision, the measurements were again performed (during surgery). In the sevoflurane group, mean RR and P ECO2 were 32 breaths.min⁻¹, and 6.0 kPa (45 mmHg) respectively, before surgery, and 35 breaths.min⁻¹ and 7.0 kPa (52 mmHg) during surgery. In the isoflurane group, mean RR and P ECO2 were 32 breaths.min⁻¹ and 6.1 kPa (46 mmHg) respectively, before surgery, and 37 breaths.min⁻¹ and 6.7 kPa (52 mmHg) during surgery. There were no statistical differences between the two anaesthetic groups. Clinical respiratory and cardiovascular parameters during spontaneous breathing with LMA in children are similar during sevoflurane and isoflurane anaesthesia.     

Effect of the size of a tracheal tube and the efficacy of the use of the laryngeal mask for fibrescope-aided tracheal intubation

We randomly allocated 60 patients with normal airways into three groups to compare the ease of fibrescope-aided tracheal intubation using 8.0-mm internal diameter (group F₈) and 6.0-mm (group F₆) reinforced tracheal tubes and to evaluate the efficacy of the laryngeal mask as an aid for fibreoptic tracheal intubation (group L). In group F₈ tracheal intubation was optimal in 2 of 20 patients and in two patients in whom intubation over the fibrescope was difficult the attempts resulted in inadvertent oesophageal intubation. In group F₆ intubation was always successful and significantly easier than in group F₈ (p < 0.005; 95% confidence interval for the difference in the proportion of the optimal intubation grade: 20–70%). In group L tracheal intubation was optimal in 18 of 20 patients and easier than in group F₆ (p = 0.014; 95% confidence interval for difference: 10–60%). In both groups F₆ and L tracheal intubation was completed within less than about 1 min. We conclude that conventional fibrescope-aided tracheal intubation with a 6.0-mm tracheal tube is easier than with an 8.0-mm tube and that the laryngeal mask facilitates fibrescope-aided tracheal intubation.     

The effect of introducing fibreoptic bronchoscopes on gas flow in laryngeal masks and tracheal tubes

The laryngeal mask airway, reinforced laryngeal mask airway and tracheal tube were studied to determine (1) flow resistance during simulated inspiration and (2) the maximum size of fibreoptic scope which can he passed down the lumen at clinically useful ventilatory settings. In addition, the flow resistance imposed by the mask aperature bars was quantified. The laryngeal mask airway can accommodate a larger fibrescope than the corresponding sizes of reinforced laryngeal mask airway or tracheal tube. Mean (range)flow resistance was 2.3 (1.7-35) times higher with the reinforced laryngeal mask airway compared to the laryngeal mask airway, 2.1 (1.2-4.2) times higher with the tracheal tube compared with the laryngeal mask airway and 1.2 (0.7-1.8) times lower with the tracheal compared with the reinforced laryngeal mask airway. Removal of the mask aperture bars resulted in a mean decrease in flow resistance of 3.6%. Our data have shown that the laryngeal mask airway can accommodate a larger fibrescope than either the reinforced laryngeal mask airway or tracheal tube at clinically useful ventilatory settings and that the current recommendations for maximum size of fibrescope should be revised.     

Intubation in a patient with lingual tonsil hypertrophy using an intubating laryngeal mask airway in combination with fiberoptic intubation

A 66-yr-old man was scheduled for colon resection under general anesthesia. There were no findings suggesting difficulty of airway management. After induction of anesthesia, manual ventilation via a facemask was suboptimal, but increased fresh gas flow improved it. At direct laryngoscopy after achieving muscular relaxation, the arytenoids and epiglottis could not be seen even by an expert anesthesiologist. Intubating laryngeal mask airway (ILMA) was inserted to patient's larynx and ventilation could be continued. Tracheal intubation through ILMA was impossible because of hard resistance for inserting the endotracheal tube. Fiberoptic bronchoscopy revealed that the hypertrophied lingual tonsil obstructed the aperture of ILMA. Several attempts were made for intubation using fiberoptic tracheal intubation technique through ILMA and finally the patient's trachea was intubated without any bleeding or swelling of laryngeal tissues. The effectiveness of ILMA for the patient with lingual tonsil hypertrophy is still unknown, but the insertion of ILMA might be considered for safe airway management in combination with a fiberscope.     

The importance of flow and pressure release in emergency jet ventilation devices

Background:  Several self-assembled devices, consisting of a three-way stopcock connected to a high pressure oxygen source, have been proposed for transtracheal jet ventilation in an emergency situation. As a three-way stopcock acts as a 'flow splitter' it will, when connected to a continuous oxygen flow, never ensure total flow and pressure release through its side port. The aim of the present study was to measure the efficacy of flow and pressure release of three previously described self-assembled jet devices and one commercially available tool.
Methods:  In a laboratory setting simulating an obstructed upper airway the generated pressure at the cannula tip (PACT) during the expiration phase was measured in three self-assembled jet devices consisting of a three-way stopcock with an inner diameter of 2 mm (device A), 2.5 mm (device B), and 3 mm (device C), respectively, and in the Oxygen Flow Modulator (OFM) at oxygen flows of 6, 9, 12, and 15 l·min⁻¹.
Results:  The PACT of device A at on oxygen flow of 15 l·min⁻¹ was 71.1 (±0.08) cm H₂O. At a reduced flow of 9 l min⁻¹ the PACT of device A was still 25.8 (±0.08) cm H₂O. In device B and C the PACT was 35.6 (±0.04) and 17.6 (±0.04) cm H₂O, respectively, at an oxygen flow of 15 l·min⁻¹. In contrast, the PACT in the OFM (five side holes open) was 4.4 (±0.02) cm H₂O at the same flow.
Conclusion:  In case of complete upper airway obstruction the OFM provides sufficient flow and pressure release, whereas the self-assembled jet devices tested are inherently dangerous constructions.     

Resistance and additional inspiratory work imposed by the laryngeal mask airway

Laryngeal mask airways and tracheal tubes were studied to determine both their resistance to constant gas flows and additional inspiratory work during simulated inspiration. Laryngeal mask airways imposed less resistance and required lower additional inspiratory work compared with the corresponding sized tracheal tubes. If inspiratory loading during anaesthesia is an important consideration, then the laryngeal mask airway may be preferable to a tracheal tube.     

Intermittent positive ventilation through a laryngeal mask in children: does it cause gastric dilatation?

After obtaining Ethics Committee approval and informed consent, sixty children, ASA Grade 1 or 2 and aged six months to ten years, were randomly allocated to receive intermittent positive pressure ventilation through either a laryngeal mask or a tracheal tube. Inflation pressures were maintained below 20 cm H₂O, and gas aspirated from the stomach via an orogastric tube over a one h period. No large volumes were aspirated and no differences were detected between the groups. We conclude that healthy children over the age of six months can be safely ventilated through the laryngeal mask airway without gastric distension.     

Laryngeal mask airway and uncuffed tracheal tubes are equally effective for low flow or closed system anaesthesia in children

Low flow and closed system anaesthesia have considerable advantages in economy, limited atmospheric pollution, and maintenance of humidification and temperature. To benefit from these techniques leakage from the breathing system should be as low as possible. The sealing of the airway is crucial to ensure this. Therefore, we have investigated in 30 children, aged 2-6 yr, the effectiveness of the laryngeal mask airway (LMA) and the uncuffed tracheal tube (TT) for closed system paediatric anaesthesia, during positive pressure ventilation, in a prospective, randomized study. Ventilation was adequate in all cases with both devices. Loss of gas from the breathing system was less than 100 ml min-1 in 13 (87%) patients in the LMA and in 12 (80%) patients in the TT group, with a maximum of approximately 700 ml min-1 in the TT and approximately 350 ml min-1 in the LMA group. We conclude that the airway sealing with both devices was tight enough to perform low flow or closed system anaesthesia in paediatric patients aged 2-6 yr.      

A comparison of the laryngeal mask airway with the facemask and oropharyngeal airway for manual ventilation by first responders in children

In adults, first responders to a cardiopulmonary arrest provide better ventilation using a laryngeal mask airway than a facemask. It is unclear if the same is true in children. We investigated this by comparing the ability of 36 paediatric ward nurses to ventilate the lungs of 99 anaesthetised children (a model for cardiopulmonary arrest) using a laryngeal mask airway and using a facemask with an oropharyngeal airway. Anteroposterior chest wall displacement was measured using an ultrasonic detector. Nurses achieved successful ventilation in 74 (75%) of cases with the laryngeal mask airway and 76 (77%) with facemask and oropharyngeal airway (p = 0.89). Median (IQR [range]) time to first breath was longer for the laryngeal mask airway (48 (39–65 [8–149])) s than the facemask/airway (35 (25–53 [14–120]) s; p < 0.0001). In 10 cases (10%) the lungs were ventilated using the laryngeal mask airway but not using the facemask/oropharyngeal airway. We conclude that ventilation is achieved rapidly using a facemask and oropharyngeal airway, and that the laryngeal mask airway may represent a useful second line option for first responders.     

Ambient oxygen concentrations during simulated cardiopulmonary resuscitation

Oxygen concentrations were measured at 12 points around a cardiopulmonary resuscitation practice mannequin following simulated ventilation with a self-inflating bag, a 'Waters' bag and a ventilator to determine whether increased oxygen concentrations may contribute to the risk of combustion from arcing defibrillator paddles. Ventilation was simulated using either a mask or via a tracheal tube fitted to the airway. The head of the mannequin rested upon a 10-cm-high pillow. Gas sampling took place after 5 min of ventilation with subsequent removal of the ventilatory device and placement on the pillow to the left of the mouth, with the tubing of the device removed to a point 1 m behind the mouth and with the device left connected to the tracheal tube. Gas was sampled after using all devices at oxygen flows of 10 lmin⁻¹ and 15 lmin⁻¹. Slightly increased oxygen concentrations were noted over the anterior chest after placement of all devices on the pillow at the higher flow. Concentrations of greater than 30% were measured in the left axilla after placement of all devices on the pillow at both flows. No increase in oxygen concentration was seen when the devices were either left connected to the tracheal tube or removed to a distance of 1 m. It would appear that leaving a patient connected to a ventilator poses no increase in risk of fire from ignition of combustible material in an oxygen-enriched atmosphere during defibrillation. Disconnecting any device which continues to discharge oxygen and leaving it on the pillow before defibrillation is dangerous.     

异丙酚抑制老年患者术毕喉罩替换气管导管时机体反应的药效学

目的 探讨异丙酚抑制老年患者术毕喉罩替换气管导管时机体反应的药效学.方法 择期腹腔镜下拟行胆囊切除术的老年患者20例,ASA Ⅰ或Ⅱ级,年龄65～70岁.术毕行喉罩替换气管导管时,根据是否诱发机体反应,采用序贯法调节异丙酚血浆靶浓度,采用概率单位法得出异丙酚抑制喉罩替换气管导管时机体反应的半数有效血浆靶浓度(EC_(50))和95%有效血浆靶浓度(EC_(95))及其95%可信区间(95%CI).结果 异丙酚抑制喉罩替换气管导管时机体反应的EC_(50)(95%CI)为2.79(2.44～3.04)μg/ml;EC_(95)(95%CI)为3.61(3.27～4.78)μg/ml.结论 老年患者术毕靶控输注异丙酚血浆靶浓度3.61 μg/ml可满足喉罩平稳替换气管导管.     

A comparison of the laryngeal mask airway with facemask and oropharyngeal airway for manual ventilation by critical care nurses in children

The laryngeal mask airway is included as a first line airway device during adult resuscitation by first responders. However, there is little evidence for its role in paediatric resuscitation. Using anaesthetised children as a model for paediatric cardiopulmonary arrest, we compared the ability of critical care nurses to manually ventilate the anaesthetised child via the laryngeal mask airway compared with the facemask and oropharyngeal airway. The airway devices were inserted in random order and chest expansion was measured using an ultrasound distance transducer. The critical care nurses were able to place the laryngeal mask airway and achieve successful ventilation in 82% of children compared to 70% using the facemask and oropharyngeal airway, although the difference was not statistically significant (p = 0.136). The median time to first successful breath using the laryngeal mask airway was 39 s compared to 25 s using the facemask (p < 0.001). In this group of nurses, we did not show a difference in ventilation via a laryngeal mask airway or facemask, although facemask ventilation was achieved more quickly.     

Complications following the use of the Combitube, tracheal tube and laryngeal mask airway

In a prospective, randomised trial, 75 patients scheduled for routine surgery were randomly allocated to one of three groups to evaluate trauma and postoperative complications after insertion of the Combitube, tracheal tube or laryngeal mask airway. Insertion of the Combitube was associated with a higher incidence of sore throat (48% vs. 16% vs. 12% [p < 0.01]) and dysphagia (68% vs. 12% vs. 8% [p < 0.01]) compared with tracheal intubation or insertion of the laryngeal mask airway, respectively. Hoarseness was significantly less common in both the Combitube and the laryngeal mask groups (both 12%) than in the tracheal tube group (44%; p < 0.01). Haematoma occurred in 36% of the Combitube group compared with 4% in both the laryngeal mask and the tracheal tube groups (p < 0.01). The higher incidence of complications should be considered when using the Combitube.     

Comparison of times to achieve tracheal intubation with three techniques using the laryngeal or intubating laryngeal mask airway

We compared the times to intubate the trachea using three techniques in 60 healthy patients with normal airways: (i) fibreoptic intubation with a 6.0-mm reinforced tracheal tube through a standard laryngeal mask airway (laryngeal mask-fibreoptic group); (ii) fibreoptic intubation with a dedicated 7.0-mm silicone tracheal tube through the intubating laryngeal mask airway (intubating laryngeal mask-fibreoptic group); (iii) blind intubation with the dedicated 7.0-mm silicone tracheal tube through the intubating laryngeal mask airway (intubating laryngeal mask-blind group). Mean (SD) total intubation times were significantly shorter in the intubating laryngeal mask-blind group (49 (20) s) than in either of the other two groups (intubating laryngeal mask-fibreoptic 74 (21) s; laryngeal mask-fibreoptic group 75 (36) s; p < 0.001). However, intubation at the first attempt was less successful with the intubating laryngeal mask-blind technique (15/20 (75%)) than in the other two groups (intubating laryngeal mask-fibreoptic 19/20 (95%) and laryngeal mask-fibreoptic 16/20 (80%)) although these differences were not statistically significant. We conclude that in this patient group, all three techniques yield acceptable results. If there is a choice of techniques available, the intubating laryngeal mask-blind technique would result in the shortest intubation time.     

The effect of flow on the filtration performance of paediatric breathing system filters

The effect of flow on the filtration performance of six different types of filter intended for use in paediatric anaesthesia was measured by challenging the filters with sodium chloride particles at five different flows: 6, 10, 15, 20 and 30 l.min⁻¹. Twenty-five unused samples of each filter type were evaluated. The pressure drop across each filter was measured at the same flows as those used to measure penetration. The pressure drop varied both between and within the types of filter. Mean pressure drop varied between 89 and 262 Pa at a flow of 15 l.min⁻¹ for the six different filters. Penetration of sodium chloride particles varied from 1.9 to 18% at 15 l.min⁻¹ for the six filters. Allowing for the variation in pressure drop, the penetration of particles increased fairly linearly as flow increased for all six filter types. The increase in penetration per unit increase in flow varied from 0.11 to 0.82% per litre per minute. Over the range of flows studied, the increase in penetration with flow is fairly predictable.     

Intra-ocular pressure changes following laryngeal mask airway insertion: a comparative study

We compared the effects of the Brain laryngeal mask airway with a tracheal tube on intra-ocular pressure. Propofol was used as induction agent and atracurium as relaxant. Twenty-six patients with normal intra-ocular pressure undergoing cataract surgery were randomly allocated to two groups. Group A ( n  = 13) had a laryngeal mask airway inserted and Group B ( n  = 13) had a tracheal tube inserted. Intra-ocular pressure was measured just before insertion of the airway, 20 s after insertion and at 2 min. In the laryngeal mask airway group there were no significant changes in mean intra-ocular pressure. In the tracheal tube group there was a significant rise in mean intra-ocular pressure at 20 s (p = 0.0056) which returned to pre-insertion levels at 2 min. We conclude that the laryngeal mask airway continues to have advantages over the tracheal tube for ophthalmic surgery despite the use of propofol and atracurium as anaesthetic agents.     

Effect-site concentration of remifentanil for laryngeal mask airway insertion during target-controlled infusion of propofol

The purpose of this study was to determine the effect-site concentration of remifentanil that would provide optimal conditions for successful laryngeal mask airway insertion during a target-controlled infusion (TCI) of propofol at 3.5 μg.ml⁻¹ without the use of neuromuscular blockade. Five minutes after propofol infusion, remifentanil was infused at a dose determined by a modified Dixon's up-and-down method. Five minutes after remifentanil infusion, the laryngeal mask was inserted. The effect-site concentration of remifentanil for successful laryngeal mask insertion in 50% of adults (EC₅₀) was 3.04 (SD 0.49) ng.ml⁻¹ during a TCI of 3.5 μg.ml⁻¹ propofol without neuromuscular blockade. From the probit analysis, the EC₅₀ and EC₉₅ of remifentanil were 2.84 ng.ml⁻¹ (95% CI 2.09–3.57 ng.ml⁻¹) and 3.79 ng.ml⁻¹ (95% CI 3.26–9.25 ng.ml⁻¹), respectively.