Time course of inhaled anaesthetic drug delivery using a new multifunctional closed-circuit anaesthesia ventilator. In vitro comparison with a classical anaesthesia machine †

Comparison of automated and manual control methods in minimal flow anesthesia

PURPOSE

New-generation anesthesia machines administer inhalation anesthetics and automatically control the fresh gas flow (FGF) rate. This study compared the administration of minimal flow anesthesia (MFA) using the automatically controlled anesthesia (ACA) module of the Mindray A9 (Shenzhen, China) anesthesia machine versus manual control by an anesthesiologist.

METHODS

We randomly divided 76 patients undergoing gynecological surgery into an ACA group (Group ACA) and a manually controlled anesthesia group (Group MCA). In Group MCA, induction was performed with a mixture of 40-60% O2 and air with a 4 L/min FGF until the minimum alveolar concentration (MAC) reached 1. Next, MFA was initiated with 0.5 L/min FGF. The target fraction of inspired oxygen (FiO2) value was 35-40%. In Group ACA, the MAC was defined as 1, and the FiO2 was adjusted to 35%. Depth of anesthesia, anesthetic agent (AA) consumption, time to achieve target end-tidal AA concentration, awakening times, and number of ventilator adjustments were analyzed.

RESULTS

The two groups showed no statistically significant differences in depth of anesthesia or AA consumption (Group ACA: 19.1 ± 4.9 ml; Group MCA: 17.2 ± 4.5; p-value = 0.076). The ACA mode achieved the MAC target of 1 significantly faster (Group ACA: 218 ± 51 s; Group MCA: 314 ± 169 s). The number of vaporizer adjustments was 15 in the ACA group and 217 in the MCA group.

CONCLUSION

The ACA mode was more advantageous than the MCA mode, reaching target AA concentrations faster and requiring fewer adjustments to achieve a constant depth of anesthesia.

Minimizing sevoflurane wastage by sensible use of automated gas control technology in the flow-i workstation: an economic and ecological assessment

Both ecological and economic considerations dictate minimising wastage of volatile anaesthetics. To reconcile apparent opposing stakes between ecological/economical concerns and stability of anaesthetic delivery, new workstations feature automated software that continually optimizes the FGF to reliably obtain the requested gas mixture with minimal volatile anaesthetic waste. The aim of this study is to analyse the kinetics and consumption pattern of different approaches of sevoflurane delivery with the same 2% end-tidal goal in all patients. The consumption patterns of sevoflurane of a Flow-i were retrospectively studied in cases with a target end-tidal sevoflurane concentration (Et_sevo) of 2%. For each setting, 25 cases were included in the analysis. In Automatic Gas Control (AGC) regulation with software version V4.04, a speed setting 6 was observed; in AGC software version V4.07, speed settings 2, 4, 6 and 8 were observed, as well as a group where a minimal FGF was manually pursued and a group with a fixed 2 L/min FGF. In 45 min, an average of 14.5 mL was consumed in the 2L-FGF group, 5.0 mL in the minimal-manual group, 7.1 mL in the AGC4.04 group and 6.3 mL in the AGC4.07 group. Faster speed AGC-settings resulted in higher consumption, from 6.0 mL in speed 2 to 7.3 mL in speed 8. The Et_sevo target was acquired fastest in the 2L-FGF group and the Et_sevo was more stable in the AGC groups and the 2L-FGF groups. In all AGC groups, the consumption in the first 8 min was significantly higher than in the minimal flow group, but then decreased to a comparable rate. The more recent AGC4.07 algorithm was more efficient than the older AGC4.04 algorithm. This study indicates that the AGC technology permits very significant economic and ecological benefits, combined with excellent stability and convenience, over conventional FGF settings and should be favoured. While manually regulated minimal flow is still slightly more economical compared to the automated algorithm, this comes with a cost of lower precision of the Et_sevo. Further optimization of the AGC algorithms, particularly in the early wash-in period seems feasible. In AGC mode, lower speed settings result in significantly lower consumption of sevoflurane. Routine clinical practice using what historically is called "low flow anaesthesia" (e.g. 2 L/min FGF) should be abandoned, and all anaesthesia machines should be upgraded as soon as possible with automatic delivery technology to minimize atmospheric pollution with volatile anaesthetics.

Advances in vaporisation: A narrative review

The output of inhalational agents from modern vaporisers are both electronically and pneumatically controlled. They are designed to deliver set agent concentrations accurately with low fresh gas flows and possess enhanced safety features. The purpose of this review article is to give an overview of three modern vaporisers, namely, the Aladin cassette vaporiser, injection vaporisers and AnaConDa™. The Aladin cassette is integrated with Datex Ohmeda S/5 ADU and GE Aisys anaesthesia machines. The electronic vapour control unit is incorporated within the anaesthesia machine. The agent specific cassettes act as a detachable vaporising chamber. The system can work as a variable bypass and measured flow vaporiser but requires a power supply to function. Injection vaporisers can achieve the set end-tidal agent concentration very rapidly with even metabolic flow rates. Hence, anaesthetic depth can be rapidly altered with minimal wastage and theatre pollution. The two types of injection vaporisers, namely, Maquet and DIVA™ are customised to function with Maquet FLOW-i and the Drager Zeus anaesthesia machine, respectively. AnaConDa™ is a combination of vaporiser and humidity and moisture exchange filter which can be fitted in the ventilatory circuit. It is primarily designed for use in intensive care for sedation and out of operating room use.

Optimizing target control of the vessel rich group with volatile anesthetics

The ability to monitor the inspired and expired concentrations of volatile anesthetic gases in real time makes these drugs implicitly targetable. However, the end-tidal concentration only represents the concentration within the brain and the vessel rich group (VRG) at steady state, and very poorly approximates the VRG concentration during common dynamic situations such as initial uptake and emergence. How should the vaporization of anesthetic gases be controlled in order to optimally target VRG concentration in clinical practice? Using a generally accepted pharmacokinetic model of uptake and redistribution, a transfer function from the vaporizer setting to the VRG is established and transformed to the time domain. Targeted actuation of the vaporizer in a time-optimal manner is produced by a variable structure, sliding mode controller. Direct mathematical application of the controller produces rapid cycling at the limits of the vaporizer, further prolonged by low fresh gas flows. This phenomenon, known as "chattering", is unsuitable for operating real equipment. Using a simple and clinically intuitive modification to the targeting algorithm, a variable low-pass boundary layer is applied to the actuation, smoothing discontinuities in the control law and practically eliminating chatter without prolonging the time taken to reach the VRG target concentration by any clinically significant degree. A model is derived for optimum VRG-targeted control of anesthetic vaporizers. An alternate and further application is described, in which deliberate perturbation of the vaporization permits non-invasive estimation of parameters such as cardiac output that are otherwise difficult to measure intra-operatively.

High volatile anaesthetic conservation with a digital in-line vaporizer and a reflector

BACKGROUND

A volatile anaesthetic (VA) reflector can reduce VA consumption (VAC) at the cost of fine control of its delivery and CO₂ accumulation. A digital in-line vaporizer and a second CO₂ absorber circumvent both of these limitations. We hypothesized that the combination of a VA reflector with an in-line vaporizer would yield substantial VA conservation, independent of fresh gas flow (FGF) in a circle circuit, and provide fine control of inspired VA concentrations.

METHOD

Prospective observational study on six Yorkshire pigs. A secondary anaesthetic circuit consisting of a Y-piece with 2 one-way valves, an in-line vaporizer and a CO₂ absorber in the inspiratory limb was connected to the patient's side of the VA reflector. The other side was connected to the Y-piece of a circle anaesthetic circuit. In six pigs, an inspired concentration of sevoflurane of 2.5% was maintained by the in-line vaporizer. We measured VAC at FGF of 1, 4 and 10 l/min.

RESULTS

With the secondary circuit, VAC was 55% less than with the circle system alone at FGF 1 l/min, and independent of FGF over the range of 1-10 l/min. Insertion of a CO₂ absorber in the secondary circuit reduced P_et CO₂ by 1.3-2.0 kpa (10-15 mmHg).

CONCLUSION

A secondary circuit with reflector and in-line vaporizer provides highly efficient anaesthetic delivery, independent of FGF. A second CO₂ absorber was necessary to scavenge the CO₂ reflected by the anaesthetic reflector. This secondary circuit may turn any open circuit ventilator into an anaesthetic delivery unit.

End-tidal control vs. manually controlled minimal-flow anesthesia: a prospective comparative trial

BACKGROUND

To ensure safe general anesthesia, manually controlled anesthesia requires constant monitoring and numerous manual adjustments of the gas dosage, especially for low- and minimal-flow anesthesia. Oxygen flow-rate and administration of volatile anesthetics can also be controlled automatically by anesthesia machines using the end-tidal control technique, which ensures constant end-tidal concentrations of oxygen and anesthetic gas via feedback and continuous adjustment mechanisms. We investigated the hypothesis that end-tidal control is superior to manually controlled minimal-flow anesthesia (0.5 l/min).

METHODS

In this prospective trial, we included 64 patients undergoing elective surgery under general anesthesia. We analyzed the precision of maintenance of the sevoflurane concentration (1.2-1.4%) and expiratory oxygen (35-40%) and the number of necessary adjustments.

RESULTS

Target-concentrations of sevoflurane and oxygen were maintained at more stable levels with the use of end-tidal control (during the first 15 min 28% vs. 51% and from 15 to 60 min 1% vs. 19% deviation from sevoflurane target, P < 0.0001; 45% vs. 86% and 5% vs. 15% deviation from O₂ target, P < 0.01, respectively), while manual controlled minimal-flow anesthesia required more interventions to maintain the defined target ranges of sevoflurane (8, IQR 6-12) and end-tidal oxygen (5, IQR 3-6). The target-concentrations were reached earlier with the use of end-tidal compared with manual controlled minimal-flow anesthesia but required slightly greater use of anesthetic agents (6.9 vs. 6.0 ml/h).

CONCLUSIONS

End-tidal control is a superior technique for setting and maintaining oxygen and anesthetic gas concentrations in a stable and rapid manner compared with manual control. Consequently, end-tidal control can effectively support the anesthetist.

Wash-in and wash-out of sevoflurane in a test-lung model: A comparison between Aisys and FLOW-i

Background: Modern anaesthesia workstations are reassuringly tight and are equipped with effective gas monitoring, thus providing good opportunities for low/minimal flow anaesthesia. A prerequisite for effective low flow anaesthesia is the possibility to rapidly increase and decrease gas concentrations in the circle system, thereby controlling the depth of anaesthesia.  Methods: We studied the wash-in and wash-out of sevoflurane in the circle system with fixed fresh gas flow and vaporizer setting. We compared two modern anaesthesia work stations, the Aisys (GE, Madison, WI, USA) and FLOW-i (Maquet, Solna, Sweden) in a test lung model.  Results: We found fresh-gas flow to have, as expected, a major influence on wash-in, as well as wash-out of sevoflurane. The wash-in time to reach a stable circle 1 MAC (2.1%) decreased from an average of 547 ± 83 seconds with a constant fresh gas flow of 300 ml/min and vaporizer setting of 8%, to a mean of 38 ± 6 seconds at a fresh gas flow of 4 L/min. There were only minor differences between the two works-stations tested; the Aisys was slightly faster at both 300 and 4 L/min flow. Time to further increase circle end-tidal concentration from 1-1.5 MAC showed likewise significant associations to fresh gas and decreased from 330 ± 24 seconds at 300 ml/min. to less than a minute at constant 4 L/min (17 ± 11 seconds), without anaesthetic machine difference. Wash-out was also fresh gas flow dependent and plateaued at 7.5 L/min.  Conclusions: Circle system wash-in and wash-out show clear fresh gas dependency and varies somewhat between the Aisys and Flow-i. The circle saturation, reaching 1 MAC end-tidal or increasing from 1-1.5 MAC can be achieved with both work-stations within 1.5 minutes at a constant fresh gas flow of 2 and 4 L/min. Wash-out plateaued at 7.5 L/min.

An ADARPEF survey on respiratory management in pediatric anesthesia

BACKGROUND

There have been recent changes with regard to tools and concepts for respiratory management of children undergoing general anesthesia.

OBJECTIVES

To determine the practice of pediatric anesthetists concerning: preoxygenation, breathing systems, ventilation modes, anesthetic agent and airway device, strategies for a general anaesthetic of less than 30 min using spontaneous respiration, and opinion about technical aspects of ventilation.

METHODS

Online questionnaire sent by e-mail to all the anesthetists registered on the mailing list of the French-speaking Pediatric Anesthetists and Intensivists Association (ADARPEF).

RESULTS

232 questionnaires (46%) were returned. More than 25% of anesthetists surveyed declared that they do not perform preoxygenation before induction for children <15 years old, apart from neonates and clinical specific situations. When performed, <65% chose a FiO2 higher than 80%. Inhalational induction with sevoflurane is the preferred mode of induction set at 6% or 8%, respectively, 69% [62-75] vs 25% [18-31]. For induction, the circle system was the most popular circuit used in all ages. The accessory breathing system-Mapleson B type-was predominantly used for neonates (44% [37-54]). For maintenance of an anesthesia lasting <30 min in spontaneous breathing, the use of laryngeal mask increased with age, and the endotracheal tube was reserved for neonates (40% [33-48]). Pressure support ventilation was rarely used from the beginning of induction but was widely used for maintenance, whatever the age-group. Results differed according to the type of institution.

CONCLUSION

Ventilation management depends on the age and institutions in terms of circuit, airway device or ventilation mode, and specific differences exist for neonates.

Agent consumption with the Zeus® in the automated closed circuit anesthesia mode with O2/air mixtures

BACKGROUND

Earlier software versions of the Zeus® (Lübeck, Dräger, Germany) failed to provide true closed circuit anesthesia (CCA) conditions. We examined whether the latest software (SW 4.03 MK 04672-00) achieves this goal.

METHODS

In 8 ASA I-III patients, the CCA mode of the Zeus® was used to maintain the inspired O2 (FIO2) and end-expired sevoflurane % (FAsevo) at 50 and 1.8%, respectively. The fresh gas flow (FGF) of O2 and air and the sevoflurane injection rate (=Vinjsevo, mL liquid sevo/h) were videotaped from the control screen and entered offline into a spreadsheet. Cumulative sevoflurane usage during early wash-in (=0-1 min, CDsevo0-1), late wash-in (=1-5 min, CDsevo1-5), and maintenance (=5-60 min, CDsevo5-60) was calculated, and Vinjsevo between 1 and 60 min was compared with published uptake data.

RESULTS

FAsevo reached 1.8% within 101 (23) sec. CDsevo0-1 was between 1.24 (0.03) and 3.01(0.25) mL (a range is provided because no absolute Vinjsevo values were displayed once Vinjsevo was > 100 mL/h, which occurred between 15 ± 2 and 46 ± 6 sec). CDsevo1-5 was 0.81 (0.37) mL, and CDsevo5-60 was 4.63 (0.94) mL. The Vinjsevo pattern between 1 and 60 min matched previously published uptake data. Brief high FGF periods were used to maintain the target FIO2, and to refill the reservoir bag after external pressure had been applied to the abdomen; subsequent "spikes" wasted 0.08-0.19 mL and 0.14-0.49 mL sevoflurane (1-3% and 3-9% of total agent usage between 1 and 60 min, respectively).

CONCLUSION

Under the conditions specified, the Zeus® approaches CCA conditions so closely that further reductions in agent usage would have minimal economic significance.

Cost efficiency of target-controlled inhalational anesthesia

BACKGROUND

Cost and environmental pollution are two prime concerns with general anesthesia. We hypothesized that target-controlled (TC) anesthesia drug delivery system also called as end-tidal (ET) control is an effective and safe system that would reduce the cost and also environmental pollution.

MATERIALS AND METHODS

We studied 200 patients undergoing laparoscopic abdominal and pelvic surgeries and randomly distributed those in 2 groups of 100 each, TC and manual-controlled (MC) group. We reviewed the two groups in term of consumption of gases, time required to achieve the ET concentration of sevoflurane of 1.5%, maximum inspired concentration of sevoflurane achieved, and number of adjustments required to maintain the depth of anesthesia.

RESULTS

We found that the consumption of nitrous oxide and sevoflurane was significantly less in TC group than MC group (P < 0.05), oxygen consumption was also less in TC group but not statistically significant. The time required to achieve the desired levels, maximum inspired sevoflurane concentration achieved, and the number of drug delivery adjustments required were statistically significant in TC group (P < 0.05). As the consumption reduced in TC group, the cost of the inhalational anesthesia reduced by approximately Rs. 64/h ($1.12) and thus the environmental pollution.

CONCLUSION

We concluded from our study that ET control is a good system for conserving the consumption of gases and thus is efficient as it reduces both the cost and the environmental pollution.

[The choice of a pediatric anesthesia ventilator]

The technology of anesthesia ventilators has substantially progressed during last years. The choice of a pediatric anesthesia ventilator needs to be led by multiple parameters: requirement, technical (pneumatic performance, velocity of halogenated or oxygen delivery), cost (purchase, in operation, preventive and curative maintenance), reliability, ergonomy, upgradability, and compatibility. The demonstration of the interest of pressure support mode during maintenance of spontaneous ventilation anesthesia makes this mode essential in pediatrics. In contrast, the financial impact of target controlled inhalation of halogenated has not be studied in pediatrics. Paradoxically, complex and various available technologies had not been much prospectively studied. Anesthesia ventilators performances in pediatrics need to be clarified in further clinical and bench test studies.

Cost analysis of two anaesthetic machines: "Primus®" and "Zeus®"

Background

Two anaesthetic machines, the "Primus^®" and the "Zeus^®" (Draeger AG, Lübeck, Germany), were subjected to a cost analysis by evaluating the various expenses that go into using each machine.

Methods

These expenses included the acquisition, maintenance, training and device-specific accessory costs. In addition, oxygen, medical air and volatile anaesthetic consumption were determined for each machine.

Results

Anaesthesia duration was 278 ± 140 and 208 ± 112 minutes in the Primus^® and the Zeus^®, respectively. The purchase cost was €3.28 and €4.58 per hour of operation in the Primus^® and the Zeus^®, respectively. The maintenance cost was €0.90 and €1.20 per hour of operation in the Primus^® and the Zeus^®, respectively. We found that the O₂ cost was €0.015 ± 0.013 and €0.056 ± 0.121 per hour of operation in the Primus^® and the Zeus^®, respectively. The medical air cost was €0.005 ± 0.003 and €0.016 ± 0.027 per hour of operation in the Primus^® and the Zeus^®, respectively. The volatile anaesthetic cost was €2.40 ± 2.40 and €4.80 ± 4.80 per hour of operation in the Primus^® and the Zeus^®, respectively.

Conclusion

This study showed that the "Zeus^®" generates a higher cost per hour of operation compared to the "Primus®".

Control of the haemodynamic response to surgical stimuli in semi-closed circuit or closed circuit anaesthesia using a multifunctional anaesthesia system

This study compared the ability of the Zeus multifunctional anaesthesia system to control haemodynamic response to surgical stimulation in semi-closed (SCA) or closed circuit anaesthesia (CCA) modes. Fifty patients undergoing gynaecological surgery were randomly assigned to SCA or CCA. Anaesthesia was induced with 2 mg propofol and 0.9 mg/kg rocuronium, intravenously, and maintained using sevoflurane (minimum alveolar concentration [MAC], 1.0) using 2 l/min oxygen plus 2 l/min nitrous oxide (SCA 4 l/min group) or 50% oxygen plus 50% nitrous oxide (CCA group). An increase in mean arterial pressure (MAP) > 20% above baseline in response to surgical stimulation provoked a stepwise increase in sevoflurane (1.3 MAC and then 1.6 MAC), followed by fentanyl 1 pg/kg intravenously (rescue drug). The time required for MAP to return to within 10% of baseline was significantly shorter in the CCA group (6.4 +/- 3.6 min) compared with the SCA 4 l/min group (10.2 +/- 6.0 min). The percentage of patients requiring fentanyl was significantly greater in the SCA 4 l/min group than in the CCA group. In conclusion, CCA controlled acute haemodynamic responses to surgical stimuli more successfully and rapidly than SCA 4 l/min, using a multifunctional anaesthesia machine.

The effect of predictive display on the control of step changes in effect site sevoflurane levels

Graphical displays of past and future levels of drugs may be a useful adjunct to manual dosing. We have previously found that a display of predicted future values speeds step changes in end-tidal sevoflurane. In this study anaesthetists made step changes of 0.3% in effect site sevoflurane, with and without the display and as increases and decreases. We analysed 91 changes. When the predictive display was present, users made larger vaporiser dial changes of 3.9% vs 3.1% (95% CI for the difference -1.3% to -0.01%, p = 0.046) reflected in larger end-tidal changes (95% CI for the difference -0.009 vol% to -0.34 vol%, p = 0.06). There was no difference in the speed of change (220 vs 227 s (95% CI for the difference -51 to 32 s)), or in the accuracy of the change. In this study the predictive display influenced the magnitude of the step changes made by anaesthetists but did not affect the speed or overall accuracy of the change.

Effect of the mode of administration of inhaled anaesthetics on the interpretation of the F(A)/F(I) curve--a GasMan simulation

The effects of blood solubility, cardiac output and ventilation on the rise of the alveolar towards the inspired concentration, the F(A)/F(I) curve, of an inhaled anaesthetic are often thought to reflect how these factors affect wash-in of the central nervous system compartment and, therefore, speed of induction because F(A) is the partial pressure ultimately attained in the central nervous system (F(VRG)). These classical F(A)/F(I) curves assumed a constant F(I). We used GasMan to examine whether changes in solubility, cardiac output and ventilation affect the relationship between the F(A)/F(I) curve and F(VRG) differently while either F(I) or F(A) are kept constant. Using GasMan, we studied the effects of solubility (desflurane vs isoflurane), cardiac output (5 vs. 10 l x min(-1)) and minute ventilation (4 vs. 8 l x min(-1)) on F(A), F(I), F(A)/F(I) and F(VRG) with either F(I) kept constant or F(A) kept constant (at 1 minimum alveolar concentration). High fresh gas flows were used to avoid rebreathing, so that the delivered concentration matched F(I). Despite similar effects on the F(A)/F(I) curve, the effects on F(VRG) differed. With constant F(I), lower solubility or higher ventilation results in a higher F(VRG) and a higher cardiac output results in a lower F(VRG). With constant F(A), solubility has only a minimal effect on F(VRG); an increase in cardiac output hastens the rise of F(VRG) to the same plateau value; and a change in ventilation has minimal effect on F(VRG). Despite similar effects on the F(A)/F(I) curve, the effects of solubility, cardiac output and ventilation on the F(VRG) are different when either F(I) or F(A) are kept constant. With the F(I) kept constant, induction of anaesthesia is slower with a higher cardiac output, but with F(A) kept constant, induction of anaesthesia is faster with a higher cardiac output. The introduction of an end-expired closed-loop feedback administration of inhaled anaesthetics makes this distinction clinically relevant.

Automated anesthesia

PURPOSE OF REVIEW

Anesthesiologists are overloaded with information and multitasking necessities in an extremely complex work environment. The purpose of this review is to present recent developments toward automated anesthesia and present future technologies for everyday clinical practice.

RECENT FINDINGS

Decision support systems integrate different parameters, clinical scenarios and assessments by (non)-trained personnel into algorithms, which lead to diagnostic suggestions, triage evaluations or treatment options. Target-controlled anesthesia infusion systems reduce the anesthesiologist's workload; target-controlled analgesia systems have the potential to provide more stable hemodynamic control. Closed-loop delivery of anesthesia is feasible and provides anesthetic control as good as or better than human delivery. Teleanesthesia offers the possibility of distant preoperative assessment of the patient's fitness for anesthesia, aid of trained personnel to perform anesthetic tasks and the control of anesthesia delivery in a distant location.

SUMMARY

Decision support systems help to make reliable and standardized decisions in complex environments. Target-controlled infusion systems reduce the anesthetic workload. Closed-loop systems will automate anesthesia care in the near future. Teleanesthesia offers the opportunity to provide safe anesthetic care whenever trained personnel are not available or need support.

A Comparison of Consumption and Recovery Profiles According to Anaesthetic Circuit Mode using a New Multifunctional Closed-Circuit Anaesthesia System during Desflurane Anaesthesia: A Clinical Study

This clinical study compared induction time, consumed anaesthetic dose, and haemodynamic and recovery profiles when using a new type of multifunctional anaesthesia machine (Zeus®) in semi-closed or closed circuit modes. Sixty female patients undergoing gynaecological surgery were randomly assigned to three groups and received desflurane anaesthesia through a semi-closed circuit (SCC) at fresh gas flow rates of 4 l/min (SCC 4 l/min) or 2 l/min (SCC 2 l/min), or through a closed circuit (CC). Anaesthesia was maintained at the minimum alveolar concentration for blocking the adrenergic response to painful stimulus (MACBAR) (4.6% end-tidal desflurane) during each operation. The time required to reach MACBAR was significantly shorter and the dose of desflurane was significantly smaller in the CC group compared with the other groups. There were no differences in haemodynamic and recovery profiles between the groups. It is concluded that the CC mode allowed a faster and more reliable induction, lower anaesthetic consumption and stable haemodynamic and recovery profiles.

Assessing the clinical or pharmaco-economical benefit of target controlled desflurane delivery in surgical patients using the Zeus anaesthesia machine

The Zeus anaesthesia machine includes an auto-control mode which allows targeting of end-tidal volatile and inspired oxygen concentrations. We assessed the clinical benefits and economic impact of this target-controlled anaesthesia compared with conventional manually controlled anaesthesia. Eighty patients were randomly assigned to receive desflurane either with a fresh gas flow set by the anaesthetist or in auto-control mode. Drug delivery was adjusted to maintain bispectral index between 40-60 units and systolic arterial pressure under 15 mmHg above its pre-induction value (upper limit) and over 90 mmHg (lower limit). Blood pressure was maintained in the desired range for 89% and 91% of the maintenance period for auto-control and manual control respectively (p = 0.49). Bispectral index was in the desired range for 82% and 79% of the maintenance period, for auto-control and manual control respectively (p = 0.46). Oxygen consumption was more than halved by the use of auto-control mode, and mean (SD) desflurane consumption during surgery was 0.07 (0.04) vs 0.2 (0.07) ml.min(-1) in auto-control and manual control respectively (p < 0.0001). The number of drug delivery adjustments per hour was significantly lower in auto-control mode (mean (SD) 7 (2) vs 15 (12); p < 0.0001). Thus, the auto-control mode provided similar haemodynamic stability and bispectral control as did conventional manually controlled anaesthesia, but led to a reduction in gas and vapour consumption with a more clinically acceptable workload.

[Optimization of anesthesia for emergency abdominal surgery in the elderly]

OBJECTIVES

Peroperative haemodynamic profile comparison of two anaesthetic protocols for emergency abdominal surgery of old patients.

PATIENTS AND METHODS

Non-randomized monocentric study. Patients in the Optimization group were prospectively studied. Anaesthesia was induced by etomidate-succinylcholine and maintained with effect site and end-tidal target controlled administration of remifentanil and desflurane respectively to keep the BIS values between 45 and 55. These patients were matched with retrospectively studied patients constituting the Control group. The latter's were anaesthetized with etomidate-succinylcholine and anaesthesia was maintained by manually controlled administration of sufentanil and desflurane to keep systolic arterial pressure (SAP) within a range of more or less 30% of preoperative baseline SAP.

RESULTS

Twelve patients (86+/-5 yrs) were included in the Optimization group, 11 (86+/-4 yrs) in the Control group. The time spent at a SAP within more or less 30% of baseline values was 92+/-7% and 71+/-29% of total anesthesia time in the Optimization and Control groups respectively (p<0.05). That spent at a SAP less than 15 and 30% of baseline values was 23+/-11% et 3+/-5% of total anaesthesia time in the Optimization group, whereas in the MAN group it was 65+/-21% and 27+/-30% respectively (p<0.05). Desflurane and ephedrine consumption was less in the Optimization group as well as crystalloid or colloid volume loading.

CONCLUSION

Anaesthetic agents target controlled administration and/or neurophysiologic depth of anaesthesia monitoring improve the time course of the haemodynamic effects in elderly patients undergoing abdominal surgery in emergency.

Target-controlled inhalation induction with sevoflurane in children: a prospective pilot study

BACKGROUND

Target-controlled inhalation induction (TCII) with sevoflurane is becoming possible with new anesthesia platforms. Although TCII has already been performed in adults, it remains to be evaluated in children.

METHODS

In a prospective study, we compared TCII using the Felix AInOC anesthetic station (Taema, Anthony, France) to our standard protocol inhalation induction in children scheduled for elective surgery under general anesthesia. After preoxygenation, sevoflurane induction was performed in both groups without priming of the circuit. Sufentanil was administered after venous line placement.

RESULTS

In the TCII group, no overdosage or underdosage was observed except in two children where TCII failed owing to high agitation, and the number of adjustments was lower compared with our standard protocol inhalation induction (1(1-2.5[0-5]) vs 6(5-6[4-10]) respectively). Moreover, the delay to obtain target end-tidal sevoflurane concentration was shorter in the TCII group (2(1.6-2.7[1.3-4]) min vs 3.4(2.5-3.8[2.3-6.5]) min respectively). No significant difference in the delay of loss of consciousness or in the conditions for intubation or laryngeal mask placement was observed between the groups.

CONCLUSION

The Felix AInOC allows TCII to be performed satisfactorily in children. Manual inhalation induction induced a higher number of adjustments and overdosages.

Mechanical ventilator for delivery of ¹⁷O₂ in brief pulses

The ¹⁷O nucleus has been used recently by several groups for magnetic resonance (MR) imaging of cerebral metabolism. Inhalational delivery of ¹⁷O₂ in very brief pulses could, in theory, have significant advantages for determination of the cerebral metabolic rate for oxygen (CMRO₂) with MR imaging. Mechanical ventilators, however, are not typically capable of creating step changes in gas concentration at the airway. We designed a ventilator for large animal and human studies that provides mechanical ventilation to a subject inside an MR scanner through 25 feet of small-bore connecting tubing, and tested its capabilities using helium as a surrogate for ¹⁷O₂. After switching the source gas from oxygen to helium, the 0-90% response time for helium concentration changes at the airway was 2.4 seconds. The capability for creating rapid step changes in gas concentration at the airway in large animal and human studies should facilitate the experimental testing of the delivery ¹⁷O₂ in brief pulses, and its potential use in imaging CMRO₂.

Desflurane Consumption During Automated Closed-circuit Delivery is Higher Than When a Conventional Anesthesia Machine is Used With a Simple Vaporizer-O₂-N₂O Fresh Gas Flow Sequence

Background

The Zeus^® (Dräger, Lübeck, Germany), an automated closed-circuit anesthesia machine, uses high fresh gas flows (FGF) to wash-in the circuit and the lungs, and intermittently flushes the system to remove unwanted N₂. We hypothesized this could increase desflurane consumption to such an extent that agent consumption might become higher than with a conventional anesthesia machine (Anesthesia Delivery Unit [ADU^®], GE, Helsinki, Finland) used with a previously derived desflurane-O₂-N₂O administration schedule that allows early FGF reduction.

Methods

Thirty-four ASA PS I or II patients undergoing plastic, urologic, or gynecologic surgery received desflurane in O₂/N₂O. In the ADU group (n = 24), an initial 3 min high FGF of O₂ and N₂O (2 and 4 L.min^-1, respectively) was used, followed by 0.3 L.min^-1 O₂ + 0.4 L.min^-1 N₂O. The desflurane vaporizer setting (F_D) was 6.5% for the first 15 min, and 5.5% during the next 25 min. In the Zeus group (n = 10), the Zeus^® was used in automated closed circuit anesthesia mode with a selected end-expired (F_A) desflurane target of 4.6%, and O₂/N₂O as the carrier gases with a target inspired O₂% of 30%. Desflurane F_A and consumption during the first 40 min were compared using repeated measures one-way ANOVA.

Results

Age and weight did not differ between the groups (P > 0.05), but patients in the Zeus group were taller (P = 0.04). In the Zeus group, the desflurane F_A was lower during the first 3 min (P < 0.05), identical at 4 min (P > 0.05), and slightly higher after 4 min (P < 0.05). Desflurane consumption was higher in the Zeus group at all times, a difference that persisted after correcting for the small difference in F_A between the two groups.

Conclusion

Agent consumption with an automated closed-circuit anesthesia machine is higher than with a conventional anesthesia machine when the latter is used with a specific vaporizer-FGF sequence. Agent consumption during automated delivery might be further reduced by optimizing the algorithm(s) that manages the initial FGF or by tolerating some N₂ in the circuit to minimize the need for intermittent flushing.

Anaesthesia and analgesia: contribution to surgery, present and future

Anaesthetists provide comprehensive perioperative medical care to patients undergoing surgical and diagnostic procedures, including postoperative intensive care when needed. They are involved in the management of perioperative acute pain as well as chronic pain. This manuscript considers some of the recent advances in modern anaesthesia and their contribution to surgery, from the basic mechanisms of action, to the delivery systems for general and regional anaesthesia, to the use of new drugs and new methods of monitoring. It assesses the resulting progress in acute and chronic pain services and looks at patient safety and risk management. It speculates on directions that may shape its future contributions to the management of the patient undergoing surgery.

Changing patterns in anesthetic fresh gas flow rates over 5 years in a teaching hospital

BACKGROUND

Reducing anesthetic fresh gas flows can reduce volatile anesthetic consumption without affecting drug delivery to the patient. Delivery systems with electronic flow transducers permit the simple and accurate collection of fresh gas flow information. In a 2001 audit of fresh gas flow, we found little response to interventions designed to foster more efficient use of fresh gas. We compared current practice with our earlier results.

METHODS

Flow data were collected in areas with a mix of general and acute surgery in March and November 2001, and again during 2006, by recording directly from the Datex ADU to a computer every 10 s. We extracted the distribution of flow rates when a volatile anesthetic was being administered. Data collection in March 2001 and 2006 was not advertised.

RESULTS

In 2001, the mean flow rates were 1.95 and 2.1 L/min with a median flow of 1.5 L/min. In 2006, the mean was 1.27 and the median in the range 0.5-1.0 L/min. Isoflurane use decreased from 47% in 2001 to 4% in 2006.

CONCLUSIONS

Fresh gas flows used in our department have decreased by 35% over 4 years. Although the absolute change in flow rate is not large, this represents potential annual savings of more than $US130,000. This occurred without specific initiatives, suggesting an evolution in practice towards lower fresh gas flow. Improvements in equipment and monitoring, including a locally developed system, which displays forward predictions of end-tidal and effect-site vapor concentrations, may be factors in this change.

Closed system anaesthesia – historical aspects and recent developments

Closed circuit anaesthesia was described decades ago but did not achieve wide popularity among anaesthesiologists mainly because reliable control of inspiratory gas concentrations was not possible. Recent innovations including fast gas analysers, electronically controlled dosage systems and algorithms for feedback control have made possible the development of sophisticated closed circuit ventilators designed for routine clinical practice. The main advantages comprise economic use of medical gases and volatile anaesthetics, reduction of anaesthetic gas loss into the atmosphere, improved airway acclimatization as well as estimations of oxygen consumption. This article reviews historical aspects, recent developments as well as advantages and limitations of closed system anaesthesia.

Depth of anaesthesia monitoring: what's available, what's validated and what's next?

Depth of anaesthesia monitors might help to individualize anaesthesia by permitting accurate drug administration against the measured state of arousal of the patient. In addition, the avoidance of awareness or excessive anaesthetic depth might result in improved patient outcomes. Various depth of anaesthesia monitors based on processed analysis of the EEG or mid-latency auditory-evoked potentials are commercially available as surrogate measures of anaesthetic drug effect. However, not all of them are validated to the same extent.

Closed loops in anaesthesia

Closed-loop systems are able to make their own decisions and to try to reach and maintain a preset target. As a result, they might help the anaesthetist to optimise the titration of drug administration without any overshoot, controlling physiological functions and guiding monitoring variables. Thanks to the development of fast computer technology and more reliable pharmacological effect measures, the study of automation in anaesthesia has regained popularity. This short review focuses on the most recently developed and tested feedback systems in anaesthesia. Various new approaches for controlling the administration of intravenous and inhaled hypnotic-anaesthetic drugs have recently been published. For analgesics, a framework for further research has been presented in the literature. For other drugs, such as muscle relaxants and haemodynamic agents, only short reviews can be found. Until now, most of these systems have had to be under development. The challenge is now fully to establish the safety, efficacy, reliability and utility of closed-loop anaesthesia so that it can be adopted in the clinical setting. Besides, their role in optimising the controlled variables and control models, these systems have to be tested in extreme circumstances in order to test their robustness.

An innovative anaesthesia machine: the closed system

PURPOSE OF REVIEW

Closed system anaesthesia allows economic use of medical gases and volatile anaesthetics, maintenance of airway climatization and reduction of anaesthetic gas loss into the environment. In this context we reviewed papers addressing recent technical and clinical advances of closed system anaesthesia.

RECENT FINDINGS

New anaesthesia ventilators with computer-aided control of inspiratory oxygen, nitrous oxide and expiratory volatile anaesthetic concentrations have been developed. When compared with low flow or minimal flow techniques applied to classical anaesthesia ventilators with out-of-circle vaporizers these new ventilators provide fast and reliable achievement of the targeted volatile agent concentrations. The profile of desired end-tidal concentrations are stable with minimal or no operator intervention and the times to washout volatile anaesthetics are short.

SUMMARY

Modern feedback controlled ventilators allow the application of closed system anaesthesia as a safe and economic technique for routine clinical practice.