[PhysioFlex: a target-controlled self-regulating closed-circuit inhalation anesthesia regulator]

End-Tidal Control Versus Manual Control of Inhalational Anesthesia Delivery: A Randomized Controlled Noninferiority Trial

BACKGROUND

Precise anesthesia delivery helps ensure amnesia, analgesia, and immobility. Conventionally, the end-tidal anesthetic concentration is maintained through manual adjustment of the fresh gas flow and anesthetic vaporizer output. Some anesthesia delivery systems can deliver and maintain clinician-selected end-tidal anesthetic agent (EtAA) concentration using a modified closed-loop system. We evaluated the performance of an End-tidal Control (EtC) system on the Aisys CS 2 anesthesia machine (GE HealthCare). We hypothesized EtC anesthetic delivery would be noninferior to manually controlled anesthetic delivery.

METHODS

The Multi-site Anesthesia randomized controlled STudy of End-tidal control compared to conventional Results (MASTER) Trial evaluated anesthetic delivery in 210 adult patients receiving inhaled anesthesia. Patients were randomized to either EtC or manual control (MC) anesthetic delivery. The primary objective was to determine whether, compared to conventional anesthesia practice, EtC achieves and maintains clinician-specified EtAA and end-tidal oxygen (Et o2 ) concentrations within defined noninferiority limits. Noninferiority was concluded if the lower limit of the 95% confidence interval (CI) of the difference between the percent duration within the acceptable range (5% of steady state or a margin of ~10% of each agent's minimum alveolar concentration) for EtC and MC was ≥ -5% for both EtAA and Et o2 . Secondary objectives included performance measures: response time: time required to attain 90% of the first desired EtAA, overshoot: amount the controller (or vaporizer delivery) exceeded the desired EtAA, and accuracy: average deviation from the desired EtAA.

RESULTS

EtC achieved and sustained targeted EtAA and Et o2 concentrations within the noninferiority threshold. The EtAA was within 5% of the desired value 98% ± 2.05% of the time with EtC compared to 45.7% ± 31.7% of the time with MC (difference 52.3% [95% CI, 45.9%-58.6%], P < .0001). For Et o2 , EtC was within the noninferiority limit 86.3% ± 22.8% of the time compared with MC at 41% ± 33.3% ( P < .0001, difference 45.3% [95% CI, 36.1%-54.5%]). The median response time for achieving 90% of the initial EtAA desired value was 75 seconds with EtC and 158 seconds with MC ( P = .0013). EtC exhibited a median overshoot of 6.64% of the selected EtAA concentration, whereas MC often failed to reach the clinician's desired value. The difference in median percent deviation from desired EtAA value was 15.7% ([95% CI, 13.5%-19.0%], P < 0001).

CONCLUSIONS

EtC achieves and maintains the EtAA and Et o2 concentration in a manner that is noninferior to manually controlled anesthesia delivery.

Xenon consumption during general surgery: a retrospective observational study

BACKGROUND

High costs still limits the widespread use of xenon in the clinical practice. Therefore, we evaluated xenon consumption of different delivery modes during general surgery.

METHODS

A total of 48 patients that underwent general surgery with balanced xenon anaesthesia were retrospectively analysed according to the mode of xenon delivery during maintenance phase (ECO mode, AUTO mode or MANUAL mode).

RESULTS

Xenon consumption was highest during the wash-in phase (9.4 ± 2.1l) and further decreased throughout maintenance of anaesthesia. Comparison of different xenon delivery modes revealed significant reduced xenon consumption during ECO mode (18.5 ± 3.7L (ECO) vs. 24.7 ± 11.5L (AUTO) vs. 29.6 ± 14.3L (MANUAL); p = 0.033). No differences could be detected with regard to anaesthetic depth, oxygenation or performance of anaesthesia.

CONCLUSION

The closed-circuit respirator Felix Dual offers effective reduction of xenon consumption during general surgery when ECO mode is used.

A xenon recirculating ventilator for the newborn piglet: developing clinical applications of xenon for neonates

CONTEXT

The clinical applications of xenon for the neonate include both anaesthesia and neuroprotection. However, due to the limited natural availability of xenon, special equipment is required to administer and recapture the gas to develop xenon as a therapeutic agent.

OBJECTIVE

In order to test the xenon recirculating ventilator for the application of neuroprotection in a preclinical trial, our primary objective was to test the efficiency, reliability and safety of administering 50% xenon for 24 h in hypoxic ischaemic piglets.

DESIGN

A prospective observational study.

SETTING

Institute for Women's Health, University College London, January 2008 to March 2008.

ANIMALS

Four anaesthetised male piglets, less than 24 h old, underwent a global hypoxic ischaemic insult for approximately 25 min prior to switching to the xenon recirculating ventilator.

INTERVENTION

Between 2 and 26 h after hypoxic ischaemia, anaesthetised piglets were administered a mixture of 50% xenon, air, oxygen and isoflurane.

MAIN OUTCOME MEASURES

The primary outcome measure was blood gas PaCO2 (kPa) and secondary outcome measure was xenon gas use (l h), over the 24-h duration of xenon administration.

RESULTS

The xenon recirculating ventilator provided effective ventilation, automated control of xenon/air gas mixtures, and stable blood gas PaCO2 (4.5 to 6.3 kPa) for 24 h of ventilation with the xenon recirculating ventilator. Total xenon use was minimal at approximately 0.6 l h at a cost of approximately &OV0556;8 h. Additional features included an isoflurane scavenger and bellows height alarm.

CONCLUSION

Stable gas delivery to a piglet with minimal xenon loss and analogue circuitry made the xenon recirculating ventilator easy to use and it could be modified for other large animals and noble gas mixtures. The technologies, safety and efficiency of xenon delivery in this preclinical system have been taken forward in the development of neonatal ventilators for clinical use in phase II clinical trials for xenon-augmented hypothermia and for xenon anaesthesia.

A closed-circuit neonatal xenon delivery system: a technical and practical neuroprotection feasibility study in newborn pigs

BACKGROUND

Asphyxia accounts for 23% of the 4 million annual global neonatal deaths. In developed countries, the incidence of death or severe disability after hypoxic-ischemic (HI) encephalopathy is 1-2/1000 infants born at term. Hypothermia (HT) benefits newborns post-HI and is rapidly entering clinical use. Xenon (Xe), a scarce and expensive anesthetic, combined with HT markedly increases neuroprotection in small animal HI models. The low-Xe uptake of the patient favors the use of closed-circuit breathing system for efficiency and economy. We developed a system for delivering Xe to mechanically ventilated neonates, then investigated its technical and practical feasibility in a previously described neonatal pig model approximating the clinical scenario of global HI injury, prolonged Xe delivery with and without HT as a potential therapy, subsequent neonatal intensive care unit management, and tracheal extubation.

METHODS

Sixteen newborn pigs underwent a global 45 min HI insult (4%-6% inspired oxygen reducing the electroencephalogram amplitude to <7 microV), then received 16 h 50% inspired Xe during normothermia (39.0 degrees C) or HT (33.5 degrees C). A conventional neonatal ventilator provided breaths of oxygen to a lower chamber compressing a hanging bag within. This bag communicated with the upper closed part of the breathing system containing soda lime, unidirectional valves, Xe/oxygen analyzers, and a tracheal tube connection. At each end-inspiration, this bag emptied fully and a bolus of oxygen, the driving gas, crossed from the lower to upper chamber via an additional valve. This mechanically substituted the gas uptake from the circle during the previous breath cycle (oxygen + small volume of Xe) with an equivalent volume of oxygen creating a slow-rising inspired oxygen concentration. This was offset by manual injection of Xe boluses, infrequently at steady state, due to the low-Xe uptake of the patient.

RESULTS

Total mean Xe usage was 0.18 (0.16-0.21) L/h with no differences between Xe-HT and Xe-NT groups, which had weights of 1767 (1657-1877) g and 1818 (1662-1974) g, respectively (95% CI). HT reduced heart rate in the cooled animals; 180 (165-195) vs 148 (142-155) bpm (P < 0.0001) with no differences in arterial blood pressure, oxygen saturation, arterial carbon dioxide tension, or weaning times between these groups.

CONCLUSION

We describe a closed-circuit Xe delivery system with automatic mechanical oxygen replenishment, which could be developed as a single use device. Gas exchange was maintained while Xe consumption was minimal (<$2/h at $10/L*). We have shown it is both feasible and cost-efficient to use this Xe delivery method in newborn pigs for up to 16 h with or without concurrent cooling after a severe HI insult.

A cryogenic machine for selective recovery of xenon from breathing system waste gases

BACKGROUND

Xenon has many characteristics that make it very attractive as an anesthetic and therapeutic drug. Unfortunately, the supply of xenon is fixed, and therefore reclamation and recovery from even the most efficient breathing circuits is desirable. We built and evaluated a cryogenic device to recover xenon from waste anesthetic gases.

METHODS

Xenon was selectively frozen to -139.2 degrees C from test gas mixtures at ambient pressure (STP). The machine ran on standard 240 V 13 A electrical current without refrigerants that required replenishing, e.g., liquid nitrogen. A wide range of xenon/oxygen mixtures were processed over a range of freezing chamber temperatures. Efflux gas and thawed reclaimed xenon were collected separately. Xenon purity and yield (fraction recovered) were measured and calculated on each occasion.

RESULTS

Gas was processed at 300 mL/min, and the operating temperature was -139.2 (0.096) degrees C [Mean (sd)]. Purity and yield were >90% and >70% for gas mixtures containing > or =20% xenon, increasing to >95% and >85%, respectively, with an input gas xenon fraction > or =40%. Efficiency improved linearly with reducing temperature.

CONCLUSIONS

Xenon of high purity (>90%) and yield (>70%) for such a machine was recovered from all gas mixtures containing > or =20% xenon. The operating temperature of the freezing chamber is a major influence on the efficiency of recovery.

Induction par inhalation de sévoflurane à objectif de concentration chez l'adulte avec le respirateur ZEUS®

OBJECTIVES

To evaluate if the new anaesthesia platform ZEUS (Dräger Medical) allows the induction of anaesthesia with target-controlled inhalation of sevoflurane.

STUDY DESIGN

Prospective clinical study.

PATIENTS

Adult ASA I or II patients scheduled for elective surgery under general anaesthesia.

METHODS

After preoxygenation during 3 min at 100% oxygen, patients were asked to breathe normally; the target end-tidal concentration of sevoflurane was fixed at 4% without priming of the circuit. Sufentanil (target concentration 0.5 ng/ml) was administered 40 s after.

RESULTS

Ten patients (48+/-22 yrs) were included. Sevoflurane was detected in the circuit after 36+/-5 s; the target end-tidal concentration of sevoflurane was obtained at 130+/-19 s. Loss of consciousness was observed after 119+/-7 s. The induction was achieved in all patients without any incident.

CONCLUSION

This new anaesthesia system make available the induction of anaesthesia with sevoflurane without priming of the circuit.

Oxygen Consumption Measurement: Agreement Between the Closed-Circuit PhysioFlex Anesthesia Machine and the Deltatrac II Indirect Calorimeter

We designed this study to ascertain whether, for the purpose of clinical interpretation, the direct measurement of O(2) consumption with the PhysioFlex closed-circuit anesthesia machine and with the Deltatrac II indirect calorimeter are interchangeable. Oxygen consumption was measured using the two instruments successively in critically-ill, mechanically-ventilated patients. Measurements were recorded as the mean of 10 consecutive, minute-by-minute, stable readings. The degree of agreement between the measurements obtained with the two systems was estimated using Bland-Altman analysis and the intraclass correlation coefficient. Forty-four pairs of measurements made in 21 patients were analyzed, yielding a mean bias of 6.32 mL/min and limits of agreement of 40.28 and -27.63 mL/min. The intraclass correlation coefficient was 0.95, and the 95% confidence interval ranged from 0.91 to 0.97. The measurement of O(2) consumption obtained with the PhysioFlex anesthesia machine is interchangeable with that obtained by indirect calorimetry.

IMPLICATIONS

The PhysioFlex anesthesia machine (Dräger Inc., Lübeck, Germany) is a closed circuit anesthesia delivery device. The oxygen delivered by this device to maintain a steady-state inspired oxygen concentration is therefore a measure of the patient's oxygen consumption. This study was designed to evaluate the accuracy of the PhysioFlex for measuring oxygen consumption by comparing it with an established technology (Deltatrac II Calorimeter) for making this measurement.

[Xenon anesthesia: from myth to reality]

OBJECTIVE

To analyze the current knowledge concerning xenon anaesthesia.

DATA SOURCES

References were obtained from computerized bibliographic research (Medline), recent review articles, the library of the service and personal files.

STUDY SELECTION

All categories of articles on this topic have been selected.

DATA EXTRACTION

Articles have been analysed for history, biophysics, pharmacology, toxicity and environmental effects and using prospect.

DATA SYNTHESIS

The noble gas xenon has anaesthetic properties that have been recognized 50 years ago. Xenon is receiving renewed interest because it has many characteristics of an ideal anaesthetic. In addition to its lack of effects on cardiovascular system, xenon has a low solubility enabling faster induction of and emergence from anaesthesia than with other inhalational agents. Nevertheless, at present, the cost and arety of xenon limit its widespread use in clinical practice. The developement of closed rebreathing system that allowed recycling of xenon and therefore reducing its waste has led to a recent interest in this gas. Reducing its cost will help xenon to find its place among anaesthetic agents. An European multicentric clinical trial under submission will contribute to the discussion of the opportunity for xenon introduction in anaesthesia.

[Performance studies of 6 new anesthesia ventilators: bench tests]

OBJECTIVE

To assess the pneumatic performances of six new anaesthesia ventilators.

STUDY DESIGN

Bench test study.

MATERIAL

The study included one ventilator operated by an electric motor: ABT 5300 (Kontron); four ventilators of "bellows-in-bottle" category: ADU version 97 (Datex-Ohmeda); Aestiva 3000 (Datex-Ohmeda), Kion (Siemens), the two versions of Julian (Dräger); and an original ventilator devised for quantitative, or self-regulating target controlled inhalation anaesthesia, with a totally closed circuit, made of four ventilating chambers: PhysioFlex (Dräger).

METHODS

The bench test included a passive lung model with adjustable compliance and resistances, and flow and pressure gauges. The accuracy of volume and pressure measurements was tested in various conditions of resistance and compliance.

RESULTS

Pneumatic performance and accuracy were satisfactory, even in severe ventilatory conditions. All the ventilators, except ABT 5300 and Julian 1, have a compliance compensation system permitting to deliver and to maintain a constant tidal volume under various conditions of downstream load, particularly under maximal load condition. Variations of tidal volume with the increase of the fresh gas flow are negligible.

CONCLUSION

Recent technological progress has improved pneumatic performance of anaesthesia ventilators and the marketed models are more homogeneous at present. Ergonomics and training for the use of the machine are becoming major criteria for the global assessment and the choice of a ventilator.

[Anesthetic equipment: fresh gas delivery systems. II. Electronic systems]

OBJECTIVES

To analyse the design, functioning, benefits and drawbacks of electronic fresh gas delivery units (FGDUs) included in anaesthetic machines marketed in France in 1999.

DATA SOURCES

Articles were obtained from a Medline review (1980-1999; search terms: anaesthetic machine, flowmeter, vaporizer), textbooks and personal files; specific data were also provided by manufacturers.

STUDY SELECTION AND DATA EXTRACTION

The articles were analysed in considering the differences between mechanical and electronic FGDUs.

DATA SYNTHESIS

Four anaesthetic machines out of 11 are equipped with electronic FGDUs. In comparison to mechanical units, they include the following benefits: wide range of accurate gas flow, especially in the low flow range, with analog and digital display; intermittent delivery of each gas, which is essential for automated gas delivery and quantitative anaesthesia or target controlled (FET vapor) inhalational anaesthesia; facilitated oxygen ratio control; possibility to print the gas and vapor flows on the automated anaesthetic record; interruption of gas flow when the main switch of the machine is set to off; and absence of risk for retropollution. Three FGDUs out of four are adapted for gas delivery to an accessory (ancillary) anaesthetic circuit. However, only one of them delivers a gas flow up to 30 L.min-1. The specificity of the ADU AS/3 (Datex-Ohmeda) is the bypass vaporizer included in the FGDU with an exchangeable vaporizing chamber (cassette). The specificity of the Julian (Dräger) is the delivery of fresh gas limited to the expiratory phase during mechanical ventilation. The specificity of the the FGDU from Kion (Siemens) is the possibility to act as an open circuit ventilator, similar to a Servoventilator. The specificity of the PhysioFlex is an FGDU included in the circle circuit, each gas and liquid anaesthetic being directly injected into the circuit. Data from the French medical device surveillance commission indicate that the main failures occur in the power supply device and the microprocessor.

Feedback control of anaesthesia

Feedback control of anaesthesia can improve the quality of patient care while reducing the administration and cost of anaesthetic drugs. Systems have been available for several years to control blood pressure and neuromuscular blockade. Control of anaesthetic depth has been reported using the median frequency of the electroencephalogram. Recently, the monitoring of anaesthetic depth using the bispectral index or auditory evoked potentials has greatly improved feedback control of the depth of anaesthesia.

[Anesthesia ventilators]

OBJECTIVE

To review anaesthesia ventilators in current use in France by categories of ventilators.

DATA SOURCES

References were obtained from computerized bibliographic search. (Medline), recent review articles, the library of the service and personal files.

DATA SYNTHESIS

Anaesthesia ventilators can be allocated into three groups, depending on whether they readminister expired gases or not or allow both modalities. Contemporary ventilators provide either constant volume ventilation, or constant pressure ventilation, with or without a pressure plateau. Ventilators readministering expired gases after CO2 absorption, or closed circuit ventilators, are either of a double- or a single-circuit design. Double-circuit ventilators, or pneumatical bag or bellows squeezers, or bag-in-bottle or bellows-in-bottle (or box) ventilators, consist of a primary, or driving circuit (bottle or box) and a secondary or patient circuit (including a bag or a bellows or membrane chambers). Bellows-in-bottle ventilators have either standing bellows ascending at expiration, or hanging bellows, descending at expiration. Ascending bellows require a positive pressure of about 2 cmH2O throughout exhalation to allow the bellows to refill. The expired gas volume is a valuable indicator for leak and disconnection. Descending bellows generate a slight negative pressure during exhalation. In case of leak or disconnection they aspirate ambient air and cannot act therefore as an indicator for integrity of the circuit and the patient connection. Closed circuit ventilators with a single-circuit (patient circuit) include a insufflating device consisting either in a bellows or a cylinder with a piston, operated by a electric or pneumatic motor. As the hanging bellows of the double circuit ventilators, they generate a slight negative pressure during exhalation and aspirate ambient air in case of leak or disconnection. Ventilators not designed for the readministration of expired gases, or open circuit ventilators, are generally stand-alone mechanical ventilators modified to allow the administration of inhalational anaesthetic agents.