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.     

Isoflurane concentrations using uncompensated vaporisers within circle systems

In a series of studies designed to investigate the need for a temperature-compensated vaporiser for use in a circle system, we first studied temperature changes within and isoflurane concentration delivered by a Komesaroff vaporiser during bench tests using different gas flows. Agent temperature and vapour concentration decreased as predicted by the Antoine equation. Using the vaporiser within a circle system during clinical anaesthesia, we then studied 20 patients breathing spontaneously and a further 10 patients receiving controlled ventilation, measuring the temperature of the agent within the vaporiser and the concentration of agent inspired by the patient. In clinical use with the fresh gas flows of 1–3 l.min⁻¹, the inspired agent concentration did not decrease despite the decrease in temperature of the liquid isoflurane in the vaporiser.     

Gas leakage and the laryngeal mask airway: A comparison with the tracheal tube and facemask during spontaneous ventilation using a circle breathing system

The ability of the laryngeal mask airway, tracheal tube and facemask to provide a leak free seal in a clinical setting was assessed by measuring the minimal fresh gas flows needed in a closed circle system during spontaneous ventilation on 60 subjects. The fresh gas flow was reduced until no spillage occurred from the pop-off valve. This fresh gas flow was taken to represent the sum of gas uptake by the subject and gas leakage from the circuit. The median fresh gas flow after 20 minutes was 350 ml. min⁻¹ in the laryngeal mask airway group, 350 ml. min⁻¹ in the tracheal tube group and 450 ml. min⁻¹ in the facemask group. The fresh gas flow required for the facemask group was significantly higher than that for the laryngeal mask airway or tracheal tube groups (p < 0.01). There was no significant difference between the fresh gas flows required for the tracheal tube and laryngeal mask airway groups. We conclude that the laryngeal mask airway provides as good a gas tight seal as a tracheal tube in this context and would be of benefit in reducing anaesthetic gas pollution.     

The estimation of inspired desflurane concentration in a low-flow system

We have examined the predictability of inspired desflurane concentration during low-flow anaesthesia using a to-and-fro breathing system. Twenty-two adult patients requiring mechanical ventilation of the lungs during surgery took part in the study. Using a fresh gas flow of 1 l.min¹, the ratio of inspired desflurane concentration to desflurane vaporizer setting was found to be approximately 0.75 after 9 min of anaesthesia and at 2 l.min¹ fresh gas flow the ratio was approximately 0.9 after 2 min of anaesthesia. These ratios were maintained throughout the procedure, except for a few minutes following each change in vaporizer setting.     

Continuous versus intermittent thermodilution cardiac output measurement during orthotopic liver transplantation

We evaluated intermittent and continuous thermodilution cardiac output data in 12 patients undergoing orthotopic liver transplantation. Measurements were performed at 16 predefined time points between induction of anaesthesia and 3 h after reperfusion of the liver graft. Cardiac output measurements yielded 192 data pairs (intermittent cardiac output range: 1.8–18.9 l.min⁻¹, continuous cardiac output range: 3.3–20.0 l.min⁻¹). During most of the procedure the correlation between intermittent and continuous cardiac output measurements was significant ( r  = 0.87, p < 0.0001), accompanied with a bias of −0.240 l.min⁻¹ and a degree of precision of 1.789 l.min⁻¹ (< 10.0 l.min⁻¹: 1.137 l.min⁻¹, ≥10.0 l.min⁻¹: 2.220 l.min⁻¹). However, in the early phases after caval clamping and after reperfusion, accuracy was not acceptable. Only during these phases did the difference between the mean values of pulmonary artery blood temperature and rectal temperature increase (after caval clamping) or decrease (after reperfusion). In conclusion, despite acceptable levels of accuracy and precision between intermittent and continuous cardiac output measurement under stable conditions, both methods showed markedly decreased accuracy and precision in the early phases after caval clamping and after reperfusion, possibly owing to increased thermal noise.     

A non-rebreathing coaxial anaesthesia system: dependence of end-tidal gas concentrations on fresh gas flow and tidal volume

A non-rebreathing adaptation of the Bain coaxial anaesthesia circuit was developed in Nepal as a simple and economical anaesthetic system for underdeveloped countries. It was made by inserting a coaxial (Bain) tubing between an Ambu-E valve and an Ambu self-inflating bag. The present study examined the dependence of end-tidal gas concentrations on fresh gas flow and tidal volume during halothane/oxygen/air inhalation anaesthesia. Four levels of fresh gas flow with normocapnia (0.2–3 l.min⁻¹) and three levels of tidal volume at a constant respiratory rate of 15 breath.min⁻¹ (to achieve end-tidal carbon dioxide values of 4 ± 0.5%, 5 ± 0.5% and 6 ± 0.5%) were introduced in random order. Twelve ASA class 1 and 2 adult patients having intra-abdominal or pelvic surgery were studied. With increasing fresh gas flow rates, there were proportionate increases in the end-tidal concentrations of oxygen and halothane; with decreasing tidal volume and therefore less air dilution, there were proportionate increases in the end-tidal concentrations of carbon dioxide, oxygen and halothane. Both effects were statistically and clinically significant. Thus, when this system is used as described, the end-tidal concentrations of oxygen and halothane are highly dependent upon both the fresh gas flow and the tidal volume.     

Techniques for emergency ventilation through a needle cricothyroidotomy

We examined the pressures produced by a construction intended for emergency ventilation through a needle cricothyroidotomy. This construction consisted of a standard hospital wall oxygen supply, flowmeter, oxygen tubing and a three-way tap. We measured the flow achieved through a transtracheal catheter and compared the construction to a Manujet jet ventilator and to a Sanders injector. The construction performed similarly to the Manujet set at low pressures (0–100 kPa). To achieve similar pressures and flow to the Manujet set at pressures higher than 100 kPa required opening of the flowmeter beyond its calibrated range. The flow through the transtracheal catheter was almost three times higher when the flowmeters were fully opened than when they were opened to the 15 l.min⁻¹ mark (44.5 vs 15.8 l.min⁻¹, respectively; p < 0.0001). When the flowmeters were fully opened the pressure measured before the catheter was over four times higher than when they were only opened to the 15 l.min⁻¹ mark (285.3 vs 66.4 kPa, respectively; p < 0.0001). This system of ventilation is inferior to a Manujet in terms of robustness and calibration throughout its range of pressures and flows, but seems appropriate for emergency use in the absence of a purpose-made jet ventilator.     

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.     

Humidification during low-flow anesthesia in children

Purpose The aim of this study was to compare the effect of low-flow anesthesia with or without a heat and moisture exchanger with high-flow anesthesia on airway gas humidification in children. Methods One hundred twenty children were randomly assigned to one of three groups: low-flow anesthesia with 0.5l·min⁻¹ of total gas flow (LFA,n=40), low-flow anesthesia with 0.5l·min⁻¹ using a heat and moisture exchanger (HME,n=40), and high-flow anesthesia with 6l·min⁻¹ (HFA,n=40). The temperature and relative humidity of the inspired gas were measured throughout anesthesia. Results The relative humidity of the inspired gas in the HME group was increased compared with that of the LFA and HFA groups 20 min after induction (p<0.05). The airway humidification in the LFA group was higher than that in the HFA group 10 min after induction (p<0.05). The temperature of the inspired gas in the HME group was increased compared with that in the LFA and HFA groups after 70 min (P<0.05). Conclusion Low-flow anesthesia is less effective in providing adequate humidification of inspired gas than low-flow anesthesia with a heat and moisture exchanger, but significantly better than high-flow anesthesia in children.     

The Diamedica Draw-Over Vaporizer: a comparison of a new vaporizer with the Oxford Miniature Vaporizer

The Diamedica Draw-Over Vaporizer (DDV) has been developed as an alternative to the Oxford Miniature Vaporizer (OMV). Both can function as draw-over or plenum vaporizers. The performances of these two vaporizers were compared under conditions simulating intermittent positive pressure ventilation (IPPV) and continuous flow (CF). Series 1 experiments were conducted with the vaporizers in water baths at 20, 25 and 30 °C. Vaporizers were tested at dial settings of 1–4% over a range of minute volumes (1.75–6 l.min⁻¹) and flow rates (3–8 l.min⁻¹). Series 2 experiments compared output of the vaporizers over time at ambient temperatures of 20, 25 and 30 °C. A minute volume of 6 l.min⁻¹ (IPPV) and a gas flow of 8 l.min⁻¹ (CF) were used with a vaporizer setting of 2%. Vapour concentrations were recorded at 5-min intervals. In series 1 IPPV experiments, the DDV vaporizer was more accurate, producing significantly fewer vapour concentrations 0.5% more than or less than setting (p = 0.013). The OMV tended to produce more favourable results under continuous flow (p = 0.42). In series 2 experiments, the accuracy of both vaporizers was similar but consistency of output over time was better for the DDV and consistency of output according to differences in ambient temperature was better for the DDV. The OMV produced more vapour concentrations that were markedly higher than dial setting, particularly at high ambient temperatures. The DDV is a suitable alternative to the OMV with some distinct advantages. These include a larger reservoir, tendency towards greater accuracy during IPPV and improved consistency of output.     

An assessment of the efficiency of the Glostavent® ventilator

In parts of the world where supplies of oxygen and electricity are erratic, ventilating patients' lungs can be problematic. Should the electricity supply fail, gas driven ventilators have an advantage as they can continue functioning. However, many are extravagant in their requirement for the driving gas. The Glostavent^® ventilator was designed to minimise these requirements. We measured the duration of ventilation achieved by the Glostavent ventilator using an E-size oxygen cylinder at a range of minute volumes, and the inspired oxygen concentration achieved by recycling the driving gas. The period of mechanical ventilation from a single E-size cylinder ranged from 11 h 8 min (SD 4 min) with a minute volume of 7 l.min⁻¹ to 18 h 15 min (SD 7 min) with a minute volume of 3 l.min⁻¹. The mean fractional inspired oxygen concentration achieved by recycling the driving gas without further inspired oxygen supplementation was 0.33. We conclude that the Glostavent ventilator performs as efficiently and cost effectively as predicted.     

Preservation of humidity and heat of respiratory gases during anaesthesia - a laboratory investigation

Humidification and heating of anaesthetic gases are desirable to prevent respiratory tract damage and a fall in body temperature during operative procedures. Numerous studies on the humidity and temperature of inspiratory gases in different breathing systems for anaesthesia have been carried out, but comparisons are difficult since different methods have been used. In this laboratory set-up we studied a non-rebreathing system with and without humidifiers and a circle absorber system with low (0.5 l/min) or medium (5 l/min) fresh gas flows regarding their ability to heat and humidify anaesthetic gases. The humidity of inspired gases was acceptable in the non-rebreathing system using either a Bennett Cascade humidifier or disposable humidifiers and in the circle absorber system using a fresh gas flow of 5 l/min or less. The temperature of the inspired gases was highest with the Bennett Cascade humidifier, followed by the low-flow circle system. The circle absorber system used with low fresh gas flow gave higher inspiratory gas temperature and humidity than the non-rebreathing system with a good disposable humidifier.     

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.     

Effect of nasal cannula oxygen administration on oxygen concentration at facial and adjacent landmarks

An oxygen-enriched atmosphere enhances the potential for operating-room fires. We thus determined oxygen concentrations at various facial landmarks during oxygen administration via nasal cannulae. Thirteen supine volunteers were draped similarly to patients undergoing a cervical-node biopsy. Oxygen was delivered in random order through nasal cannulae at rates of 2, 4, and 6 l.min⁻¹. Oxygen concentration was measured at pre-determined facial landmarks and also distal to the drape at non-facial sites. At a flow of 2 l.min⁻¹, oxygen concentrations exceeded 23% only within a few centimetres of the nasal cannula. Concentration increased as a function of flow, but rarely exceeded 26%. At all flow rates, concentrations distal to the drape were < 24%. To reduce combustion risk, ignition sources should be kept at least 10 cm from the oxygen outlet when using nasal cannula at a flow rate ≥ 4 l.min⁻¹.     

Low-flow anaesthesia at a fixed flow rate

Aims and Objectives: This study attempts to assess the safety of low-flow anaesthesia (LFA) at fixed flow rates with particular reference to the incidence of a decline in FiO₂ below safe levels of 0.3 and to determine whether LFA can be used safely in the absence of an FiO₂ monitor.
Methods: A total of 100 patients undergoing procedures under general anaesthesia at fresh gas flows of 300 ml/min of O₂ and 300 ml/min of N₂O were monitored while maintaining the dial setting of isoflurane at 1.5% for 2 h. The changes in gas composition were analysed and even a single recording of FiO₂ of <0.3 was considered sufficient to render the technique unsafe in the absence of gas monitors.
Results: The lowest recorded value of FiO₂ was 31% (v/v%). There was no incidence of adverse events necessitating the conversion from low flows to conventional flows.
Conclusions: We conclude that low flows of 300 ml/min of N₂O and 300 ml/min of oxygen can be used safely for a period of 2 h without the use of monitors for gas analysis of oxygen and agent in adult patients weighing between 40 and 75 kgs.     

Oxygenation of patients undergoing ophthalmic surgery under local anaesthesia

The oxygenation of 30 patients undergoing elective ophthalmic surgery without sedation whilst breathing air was studied and was compared with two methods of oxygen supplementation. Arterial oxygen saturation, inspired and expired oxygen and carbon dioxide were analysed. The delivery of oxygen at 2 l.min⁻¹ via nasal cannulae was shown to be superior to a method which directed oxygen from under the surgical drapes.     

Neuromuscular interactions between mivacurium and esmolol in rabbits

We compared the dose–response relationship and the neuromuscular blocking effects of mivacurium during infusions of esmolol in 40 anaesthetised rabbits. Train-of-four stimuli were applied every 10 s to the common peroneal nerve and the force of contraction of the tibialis anterior muscle was measured. Plasma cholinesterase activity decreased by 13% after esmolol infusion. The ED₉₅ of mivacurium increased significantly from 29 (4.8) μgkg⁻¹ with placebo to 61 (9.8) μgkg⁻¹ during esmolol 100 μgkg⁻¹.min⁻¹, 49 (8.2) μgkg⁻¹ during esmolol 300 μgkg⁻¹.min⁻¹ and 54 (7.3) μgkg⁻¹ during esmolol 500 μgkg⁻¹.min⁻¹, respectively (p < 0.001). The duration of neuromuscular block with mivacurium 0.16 mgkg⁻¹ was prolonged by 30% with esmolol due to diminished plasma cholinesterase activity (p < 0.05). Heart rate and mean arterial blood pressure decreased by 15% with esmolol (p < 0.05). The results of this study show that, in rabbits, esmolol decreased plasma cholinesterase activity, antagonised the neuromuscular blocking potency of mivacurium and prolonged its neuromuscular blocking effect.     

Klimatisierung von Narkosegasen bei Einsatz unterschiedlicher Patientenschlauchsysteme

Background. During general anaesthesia gas climate significantly is improved by performance of low flow techniques. Gas climatisation, however, markedly also will be influenced by the temperature loss at, and corresponding water condensation within the hoses, factors which are related to the technical design and material of the patient hose system. The objective of this prospective study was to investigate 1. how anaesthetic gas climatisation during minimal flow anaesthesia is influenced by the technical design of different breathing hose systems in clinical practice. 2. to investigate, whether a sufficient gas climatisation also can be gained with higher fresh gas flows if that hose system is used, proven beforehand to optimally warming and humidifying the anaesthetic gases. Methods. Three different systems, a conventional two-limb hosing consisting of smooth silicone hoses, a coaxial hosing, and a hosing consisting of actively heated breathing hoses, attached to a Dräger Cicero EM anaesthesia machine, were used during minimal flow anaesthesia with a fresh gas flow of 0.5 l/min. Gas temperature and absolute humidity were measured at the tapered connection between the inspiratory limb and the breathing system as well as at its connection to the endotracheal tube. The best gas climatisation was observed if heated breathing hoses were used. Thus, using this hosing, additionally gas temperature and humidity in the inspiratory limb were taken at fresh gas flow rates of 1.0, 2.0 and 4.4 l/min respectively. Measurements were performed in all groups at all general anaesthesias lasting at least 45 minutes during the lists of eight different days each. Results. In minimal flow anaesthesia, with all hose systems likewise, generally an absolute humidity between 17 to 30 mgH₂O/l is reached at the endotracheal tube's connector during the course of the list. Only in the first cases of the day there was a short delay of 15 to 30 minutes before reaching a humidity of at least 17 mgH₂O/l. Only with heated hoses, however, humidity frequently even exceeded 30 mgH₂O/l. If conventional or coaxial hosings were used, during minimal flow anaesthesia gas temperatures in an acceptable range between 23 to 30 °C were measured at the tube connector. With heated hoses, however, warming of the gases was excellent with gas temperatures betwen 28 to 32 °C. In minimal flow anaesthesia climatisation of the anaesthetic gases proved to be best if heated hoses were used. Thus, using heated hose systems another three trials with increasing fresh gas flow rates of 1.0, 2.0 and 4.4 l/min respectively were performed. Whereas climatisation of the anaesthetic gases still was found to be optimal with a fresh gas flow of 1.0 l/min, the humidity dropped drastically to values lower than 17 mgH₂O/l at 2.0 l/min and even down to 10 mgH₂O/l at a flow rate of 4.4 l/min. Gas temperatures, however, turned out to be independent of the flow and remained at 28–32°C, even at a flow as high as 4.4 l/min. Conclusions. Using conventional hose systems and coaxial hosings acceptable, but not optimal climatisation of the anaesthetic gases can be gained if minimal flow anaesthesia is performed. The use of a coaxial hose system seems to lead to improved climatisation in long lasting procedures only. In routine clinical practice, however, conventional and coaxial hose systems are similar in respect to the climatisation of breathing gases. Heated breathing hoses performed markedly better in terms of climatisation of the breathing gas than the coaxial and the conventional hose system. With this hosing not only sufficient but optimal moisture and temperature values are realized. Optimal climatisation, however, only can be gained if low flow anesthetic techniques with fresh gas flows equal or less than 1 l/min are performed. With higher fresh gas flow rates the humidity decreases markedly while high gas temperatures are maintained. It seems justified to assume, that ventilation with warm but dry gases may result in increasingly drying out of the respiratory epithelium of the lower air ways. Heated hoses only should be used if low flow anaesthetic techniques are performed. While moisture content of the breathing gases mainly is influenced by the fresh gas flow rate, temperature mainly is depending on the convectional loss of heat at the inspiratory limb of the hosing.     

A new, minimally invasive technique for measuring cardiac index: clinical comparison of continuous cardiac dynamic monitoring and pulmonary artery catheter methods

To assess the utility of a relatively simple bedside method of estimating cardiac index during major surgery or in the intensive care unit, we conducted a prospective study in patients undergoing elective cardiac bypass surgery where a pulmonary artery catheter was inserted as part of routine monitoring. The cardiac index was estimated using standard techniques and compared with estimates from continuous cardiac dynamic monitoring using heartsmart ^® software. Two hundred and seventy sets of measurements were suitable for comparison. The mean bias (95% limits of agreement), for the pre-bypass cardiac index was −0.09 (−1.26 to 1.08) l.min⁻¹.m⁻², and post-bypass was 0.12 l.min⁻¹.m⁻² (−1.32 to 1.56). These results suggest that continuous cardiac dynamic monitoring using heartsmart ^® is sufficiently accurate for assessment of haemodynamic variables in critically ill patients, facilitating goal-directed therapies.     

Controlled ventilation using isoflurane with an in-circle vaporiser

We studied 19 patients anaesthetised for routine surgery using isoflurane delivered from a Komesaroff vaporiser mounted within a circle system. Their lungs were ventilated using a Penlon Nuffield ventilator attached to the circle system by a trunk of tubing. Fresh gas flow rates of 1, 2 or 3 l.min⁻¹ were used. The inspired agent concentration was measured using a Datex Ultima multigas analyser and was found to be stable and easily controlled.