Fresh Gas Flow Research Articles

Spontaneous shutoff of desflurane vaporizer

To the Editor: We would like to report the intraoperative failure of a desflurane vaporizer. The incident occurred as a 57-yearold man underwent parathyroid surgery. After induction and intubation, anesthesia was maintained with 5 % inhaled desflurane, using a Tec 6 Plus vaporizer (GE Healthcare Japan Co., Tokyo, Japan), via a semiclosed circuit with a 3 l/min fresh gas flow. Approximately 7 min into the case, the ‘‘no output’’ vaporizer alarm sounded and the desflurane concentration in the anesthesia circuit began to fall. We immediately turned off the vaporizer. After several seconds, the vaporizer automatically reset but within several minutes, the ‘‘no output’’ error occurred again. The surgery was then completed using sevoflurane, without assessing the depth of anesthesia. The patient emerged from anesthesia uneventfully, with no recollection of the intraoperative events. We purchased the vaporizer 2 months prior to the incident; it was used almost every day for 2 months. Although clinical engineers and anesthesiologists maintain anesthetic machines and circuits, vaporizers are not inspected usually in our operating rooms. According to the manufacturer’s technical department, 10 incidences of initial failure have been confirmed in 431 products since the vaporizer’s 2011 release in Japan. The vaporizer was exchanged at no charge following this initial failure. Afterward, the manufacturer reported to us that the failure of an electronic pressure transducer caused this incident. They recommended the immediate use of alternative anesthetics if the ‘‘no output’’ error recurred. Reports of spontaneous shutoff during surgery of a correctly mounted desflurane vaporizer have not previously been published [1–3]. Unlike conventional mechanical vaporizers, the desflurane vaporizer has sophisticated electronic components. As a fail-safe, it completely halts agent output if an electronic failure occurs. Since the washout for desflurane is more rapid than that for other inhalation agents, patients emerge from anesthesia soon after shutoff of the vaporizer. The incidence of initial failure reported by the manufacturer seems relatively high. This case serves to emphasize the importance of fully understanding the features of equipment that is in frequent use [4].

Reducing Wastage of Inhalation Anesthetics Using Real-time Decision Support to Notify of Excessive Fresh Gas Flow

Reduced consumption of inhalation anesthetics can be safely achieved by reducing excess fresh gas flow (FGF). In this study the authors describe the use of a real-time decision support tool to reduce excess FGF to lower, less wasteful levels. The authors applied a decision support tool called the Smart Anesthesia Manager™ (University of Washington, Seattle, WA) that analyzes real-time data from an Anesthesia Information Management System to notify the anesthesia team if FGF exceeds 1 l/min. If sevoflurane consumption reached 2 minimum alveolar concentration-hour under low flow anesthesia (FGF < 2 l/min), a second message was generated to increase FGF to 2 l/min, to comply with Food and Drug Administration guidelines. To evaluate the tool, mean FGF between surgical incision and the end of procedure was compared in four phases: (1) a baseline period before instituting decision rules, (2) Intervention-1 when decision support to reduce FGF was applied, (3) Intervention-2 when the decision rule to reduce flow was deliberately inactivated, and (4) Intervention-3 when decision rules were reactivated. The mean ± SD FGF reduced from 2.10 ± 1.12 l/min (n = 1,714) during baseline to 1.60 ± 1.01 l/min (n = 2,232) when decision rules were instituted (P < 0.001). When the decision rule to reduce flow was inactivated, mean FGF increased to 1.87 ± 1.15 l/min (n = 1,732) (P < 0.001), with an increasing trend in FGF of 0.1 l/min/month (P = 0.02). On reactivating the decision rules, the mean FGF came down to 1.59 ± 1.02 l/min (n = 1,845). Through the Smart Anesthesia Messenger™ system, the authors saved 9.5 l of sevoflurane, 6.0 l of desflurane, and 0.8 l isoflurane per month, translating to an annual savings of $104,916. Real-time notification is an effective way to reduce inhalation agent usage through decreased excess FGFs.

Automated control of end-tidal inhalation anaesthetic concentration using the GE Aisys Carestation™

Automated control of end-tidal inhalation anaesthetic concentration is now possible. The EtControl™ module of an Aisys Carestation Anaesthetic machine digitally adjusts fresh gas flow and plenum vaporizer output to achieve a target end-tidal concentration. We evaluated EtControl in clinical practice by measuring volatile agent consumption and the need for user input. We compared these values with contemporaneous controls using manual control of fresh gas flow rates. A total of 321 patients were anaesthetized with EtControl and 168 with manual control of fresh gas flow. The mean [95% confidence interval (CI)] sevoflurane usage for cases of 20-40 min duration was 14 (13-16) ml h(-1) with EtControl and 30 (26-35) ml h(-1) with manual control. For cases of the same duration, the mean (95% CI) desflurane consumption was 27 (21-33) ml h(-1) with EtControl and 45 (29-62) ml h(-1) with manual control. The average number of keypresses per case was 6.5 with EtControl and 13.6 during manual control of fresh gas flow. Automatic implementation of low-flow anaesthesia using EtControl allows the user to set and maintain a desired end-tidal volatile concentration while using less volatile agent.

Comparison of postoperative complication between Laryngeal Mask Airway and endotracheal tube during low-flow anesthesia with controlled ventilation

To compare the postoperative complications between Laryngeal Mask Airway (LMA) and endotracheal tube (ETT) during low-flow anesthesia with controlled ventilation. Eighty adult Patients with ASA class I or II were randomly allocated into two forty-patient groups (ETT or LMA). Cuff pressure was monitored during anesthesia. After high uptake period, fresh gas flow (FGF) was decreased to 1 lit/min and isoflurane set to 1%. Monitoring during anesthesia included non-invasive blood pressure, ECG, ETCO2 and pulse oximetry. System leakage (>100 ml/min), rebreathing and any attempt to increase FGF to overcome the leak were monitored during anesthesia. Later, patients were extubated and transferred to Post Anesthesia Care Unit (PACU). In PACU, the incidence of sore throat, cough, difficulty in swallowing and shivering was monitored for all patients. Leakage was observed in two and three cases in ETT and LMA groups respectively (P>0.05). Postoperative cough, sore throat and difficulty in swallowing were significantly less in LMA than ETT group. No significant difference was observed regarding ETCo2 values between 2 groups. If careful measures regarding insertion techniques, correct LMA position and routine monitoring of LMA cuff pressure are taken, LMA can be used as a safe alternative with lower incidence of post operation complication compared with ETT during low-flow controlled anesthesia with modern anesthetic machines.

Expiratory Limb Ventilation during Unique Failure of the Anesthesia Machine Breathing Circuit

Expiratory Limb Ventilation during Unique Failure of the Anesthesia Machine Breathing Circuit

Cuffed Endotracheal Tubes in Infants and Children

Background. Over the past 5 to 10 years, there has been a change in the clinical practice of pediatric anesthesiology with a transition to the use of cuffed instead of uncuffed endotracheal tubes (ETTs) in infants and children. When such tubes are used in clinical practice, it is imperative to ensure that the intracuff pressure is ≤30 cm H2O. To date, there are limited data regarding techniques to ensure this practice. Methods. Following endotracheal intubation with a cuffed ETT, a stethoscope was placed in the sternal notch and continuous positive airway pressure of 20 to 25 cm H2O held. The fresh gas flow was increased as needed to achieve a gradual rise of the airway pressure. Using a syringe, air was added to the cuff until no air escape or leak was heard. The intracuff pressure was checked using a handheld manometer (Posey Cufflator Endotracheal Tube Inflator and Manometer, JT Posey Company, Arcadia, CA). Results. The cohort for the study included 200 patients ranging in age from 6 months to 18 years. In 5 patients (2.5%), there was no audible air leak noted following endotracheal inflation at a continuous positive airway pressure of 20 to 25 cm H2O. In these patients, the ETT was removed and the trachea was intubated with a 0.5-mm size smaller ETT. In the entire cohort of 200 patients, the intracuff pressure was 21 ± 4 cm H2O. The intracuff pressure was ≥30 cm H2O in 1 of 200 patients (0.5%). Conclusions. The current study demonstrates a simple, bedside maneuver that requires no additional equipment and is effective at ensuring a safe intracuff pressure in virtually all patients.

Techniques of preoxygenation in patients with ineffective face mask seal

Background:Ineffective face mask seal is the most common cause for suboptimal pre-oxygenation. Room air entrainment can be more with vital capacity (VC) breaths when the mask is not a tight fit.Aims:This study was designed to compare 5 min tidal volume (TV) breathing and eight VC breaths in patients with ineffective face mask seal.Methods:Twenty eight ASA I adults with ineffective face mask seal were randomized to breathe 100% oxygen at normal TV for 5 min (Group TV) and eight VC breaths (Group VC) in a cross over manner through circle system at 10 L/min. End tidal oxygen concentration (EtO2) and arterial blood gas analysis was performed to evaluate oxygenation with each technique.Statistical and Analysis:Data were analysed using SPSS statistical software, version 16. Friedman's two-way analysis of variance by ranks was used for non-parametric data.Results:Significant increase in EtO2 (median 90) and PaO2 (228.85) was seen in group TV when compared to group VC (EtO2 median 85, PaO2 147.65), P<0.05. Mean total ventilation volume in 1 min in group VC was 9.4±3.3 L/min and more than fresh gas flow (10 L/min) in seven patients. In group TV, the fresh gas flow (50 L/5 min) was sufficient at normal TV (mean total ventilation in 5 min 36.7±6.3 L/min).Conclusions:TV breathing for 5 min provides better pre-oxygenation in patients with ineffective mask seal with fresh gas flow of 10 L/min delivered through a circle system.

The closed circuit and the low flow systems

A breathing system is defined as an assembly of components, which delivers gases from the anesthesia machine to the patients’ airways. When the components are arranged as a circle, it is termed a circle system. The flow of exhaled gases is unidirectional in the system. The system contains a component (absorber), which absorbs exhaled carbon dioxide and it is not necessary to give high fresh gas flows as in Mapleson systems. When the adjustable pressure limiting (APL) valve is closed and all the exhaled gases without carbon dioxide are returned to the patient, the system becomes a totally closed one. Such a circle system can be used with flows as low as 250 to 500 mL and clinically can be termed as low-flow systems. The components of the circle system can be arranged in different ways with adherence to basic rules: (1) Unidirectional valve must be present between the reservoir bag and the patient on both inspiratory and expiratory sides; (2) fresh gas must not enter the system between the expiratory unidirectional valve and the patient; and (3) the APL valve must not be placed between the patient and the inspiratory unidirectional valve. The functional analysis is explained in detail. During the function, the arrangement of components is significant only at higher fresh gas flows. With the introduction of low resistance valves, improved soda lime canisters and low dead space connectors, the use of less complicated pediatric circle systems is gaining popularity to anesthetize children. There are bidirectional flow systems with carbon dioxide absorption. The Waters to and fro system, a classic example of bidirectional flow systems with a canister to absorb carbon dioxide, is valveless and portable. It was widely used in the past and now is only of historical importance.

Inhalational anaesthesia with low fresh gas flow

During the inhalation of anaesthesia use of low fresh gas flow (0.35-1 L/min) has some important advantages. There are three areas of benefit: pulmonary - anaesthesia with low fresh gas flow improves the dynamics of inhaled anaesthesia gas, increases mucociliary clearance, maintains body temperature and reduces water loss. Economic - reduction of anaesthesia gas consumption resulting in significant savings of > 75% and Ecological - reduction in nitrous oxide consumption, which is an important ozone-depleting and heat-trapping greenhouse gas that is emitted. Nevertheless, anaesthesia with high fresh gas flows of 2-6 L/min is still performed, a technique in which rebreathing is practically negligible. This special article describes the clinical use of conventional plenum vaporizers, connected to the fresh gas supply to easily perform low (1 L/min), minimal (0.5 L/min) or metabolic flow anaesthesia (0.35 L/min) with conventional Primus Draeger® anaesthesia machines in routine clinical practice.

Mapleson′s breathing systems

Mapleson breathing systems are used for delivering oxygen and anaesthetic agents and to eliminate carbon dioxide during anaesthesia. They consist of different components: Fresh gas flow, reservoir bag, breathing tubes, expiratory valve, and patient connection. There are five basic types of Mapleson system: A, B, C, D and E depending upon the different arrangements of these components. Mapleson F was added later. For adults, Mapleson A is the circuit of choice for spontaneous respiration where as Mapleson D and its Bains modifications are best available circuits for controlled ventilation. For neonates and paediatric patients Mapleson E and F (Jackson Rees modification) are the best circuits. In this review article, we will discuss the structure of the circuits and functional analysis of various types of Mapleson systems and their advantages and disadvantages.

Hyperventilation increases the rate of the rise in arterial blood desflurane concentration during induction - a Gas Man® simulation

Lu et al. described that hyperventilation accelerated the desflurane uptake from the alveolar to the arterial blood (1). It is a very intriguing study of a direct determination of the desflurane concentration in the arterial blood (Ades). In comparison to the end-tidal volatile anesthetic concentration, the arterial concentration of a volatile anesthetic is more likely to reflect its concentration in the brain (1-3). A pharmacokinetic model for volatile anesthetics has been developed and used for over 25 years (4). Therefore, the effect of ventilation on Ades can be simulated with this model. Gas Man® (Version 4.1, Med Man Simulations, Inc., Chestnut Hill, MA, USA) is a computer simulation program which contains a five-compartment system built statistically which simulates the pharmacokinetics for volatile anesthetic uptake and distribution (4) with embedded parameters (5). This program helps users and readers understand anesthetic uptake and distribution with graphs. If the embedded parameters are adjusted prior to a simulation, simulated results can be seen in graphs individually. For the simulations, the necessary parameters and patient data were taken from the report of Lu et al. (1): an anesthesia breathing circuit volume of 6 L; a fresh gas flow of 6 L/min; a functional residual capacity of 2.4 L; 61.8, 62.0, and 58.0 kg patient (hyperventilation, normal ventilation, and hypoventilation, mean values from each group); an alveolar minute ventilation of 3.33, 2.38, and 1.80 L/min, mean values from each group; cardiac output which was derived from the cardiac index of the report of Lu et al. with Du Bois method (5.38, 5.56, and 5.36 L/min, mean values from each group). Other parameters were not manipulated. Therefore, the values for compartment volumes (vessel rich group, muscle, fat, and venous group) and the values for regional blood flow percentages (vessel rich group, muscle, and fat) showed some differences depending on the entered weight. At 0, 1, 3, 5, 10, 20, 30, and 40 minutes, the simulated Ades were recorded. In these simulations, despite having some limitations on the data used (e.g. using mean values of weight and alveolar ventilation) in the report of Lu et al, Gas Man® shows similar trends with the simulated Ades just as those of Lu et al (1). Hyperventilation seems to produce a higher Ades-over-time curve than normal ventilation and hypoventilation. Simulated Ades in the hyperventilation group also seems to increase more rapidly than those in the normal ventilation and the hypoventilation groups during the first 5 minutes (Figure 1. Figure 1 Simulated arterial concentration of desflurane (%) time curves under three different ventilatory settings, including hyperventilation, normal ventilation, and hypoventilation in Gas Man® (Version 4.1, Med Man Simulations, Inc., Chestnut Hill, ...

Modern anaesthesia vapourisers

Inhalational anaesthetic agents are usually liquids at room temperature and barometric pressure and need to be converted to vapour before being used and this conversion is effected using a vapouriser. Vapourisers have evolved from very basic devices to more complicated ones. Anaesthetists should understand the basic principles of anaesthetic vapouriser, including the principles that affect vapouriser output and how they influence vapouriser design. Most of the modern vapourisers in use are designed to be used between the flow meter and the common gas outlet on the anaesthesia machine. Modern vapourisers are flow and temperature compensated, concentration calibrated, direct reading, dial controlled and are unaffected by positive-pressure ventilation. Safety features include an anti-spill and a select-a-tec mechanism and a specific vapouriser filling device. Desflurane has unique physical properties requiring the use of a specific desflurane vapouriser. The most recently designed vapourisers are controlled by a central processing unit in the anaesthetic machine. The concentration of vapour is continuously monitored and adjusted by altering fresh gas flow through the vapouriser. This article looks at the basic design and functioning of the modern vapourisers.

Vapourisers: Physical principles and classification

Vapourisers have evolved from rudimentary inhalers to the microprocessor controlled, temperature compensated and flow sensing devices, which are universal today. The improvements in the design was influenced by the development of potent inhalational anaesthetics, unique properties of some agents, a deeper understanding of their mechanism of action, inherent flaws in the older vapourisers, mechanical problems due to thymol deposition, factors influencing their output such as temperature and pressure variations. It is important to review the principles governing the design of the vapouriser to gain insight into their working. It is fascinating to know how some of the older vapourisers, popularly used in the past, functioned. The descendant of Oxford Miniature Vapourizer, the Triservice vapouriser is still a part of the military anaesthesia draw over equipment meant for field use whereas the Copper Kettle the first precision device is the fore-runner of the Tec 6 and Aladdin cassette vapouriser. Anaesthesia trainees if exposed to draw over techniques get a deeper understanding of equipment and improved skills for disaster situations. In the recent advanced versions of the vapouriser a central processing unit in the anaesthetic machine controls the operation by continuously monitoring and adjusting fresh gas flow through the vapouriser to maintain desired concentration of the vapour.

Labor analgesia and anesthetic management during emergency cesarean section of parturient with spinal cord injury (SCI)

Labor analgesia and anesthetic management during emergency cesarean section of parturient with spinal cord injury (SCI)

Theoretical effect of hyperventilation on speed of recovery and risk of rehypnotization following recovery - a GasMan®simulation

BackgroundHyperventilation may be used to hasten recovery from general anesthesia with potent inhaled anesthetics. However, its effect may be less pronounced with the newer, less soluble agents, and it may result in rehypnotization if subsequent hypoventilation occurs because more residual anesthetic will be available in the body for redistribution to the central nervous system. We used GasMan® simulations to examine these issues.MethodsOne MAC of isoflurane, sevoflurane, or desflurane was administered to a fictitious 70 kg patient for 8 h with normoventilation (alveolar minute ventilation [VA] 5 L.min-1), resulting in full saturation of the vessel rich group (VRG) and >95% saturation of the muscle group. After 8 h, agent administration was stopped, and fresh gas flow was increased to 10 L.min-1 to avoid rebreathing. At that same time, we continued with one simulation where normoventilation was maintained, while in a second simulation hyperventilation was instituted (10 L.min-1). We determined the time needed for the partial pressure in the VRG (FVRG; representing the central nervous system) to reach 0.3 MAC (MACawake). After reaching MACawake in the VRG, several degrees of hypoventilation were instituted (VA of 2.5, 1.5, 1, and 0.5 L.min-1) to determine whether FVRG would increase above 0.3 MAC(= rehypnotization).ResultsTime to reach 0.3 MAC in the VRG with normoventilation was 14 min 42 s with isoflurane, 9 min 12 s with sevoflurane, and 6 min 12 s with desflurane. Hyperventilation reduced these recovery times by 30, 18, and 13% for isoflurane, sevoflurane, and desflurane, respectively. Rehypnotization was observed with VA of 0.5 L.min-1 with desflurane, 0.5 and 1 L.min-1 with sevoflurane, and 0.5, 1, 1.5, and 2.5 L.min-1 with isoflurane. Only with isoflurane did initial hyperventilation slightly increase the risk of rehypnotization.ConclusionsThese GasMan® simulations confirm that the use of hyperventilation to hasten recovery is marginally beneficial with the newer, less soluble agents. In addition, subsequent hypoventilation results in rehypnotization only with more soluble agents, unless hypoventilation is severe. Also, initial hyperventilation does not increase the risk of rehypnotization with less soluble agents when subsequent hypoventilation occurs. Well-controlled clinical studies are required to validate these simulations.

Sevoflurane Requirement to Maintain Bispectral Index–Guided Steady-State Level of Anesthesia During the Rewarming Phase of Cardiopulmonary Bypass With Moderate Hypothermia

The authors aimed to quantify any increase and the extent of the increase in sevoflurane requirements for maintaining hypnosis during hypothermic cardiopulmonary bypass (CPB) with the bispectral index (BIS) maintained between 40 and 50. An observational prospective study. A single-center tertiary referral center at a university hospital. Fifty patients undergoing elective coronary artery bypass graft surgery with CPB. End-tidal oxygenator expiratory gas concentrations were used to quantify sevoflurane requirements while maintaining a BIS level between 40 and 50 during the rewarming phase. Sevoflurane requirements progressively increased as temperature increased. The difference in sevoflurane requirement at 35°C and 29°C was compared using analysis of variance for repeated measures, which was statistically significant. When relating temperature and sevoflurane requirement, the Pearson correlation coefficient was 0.67. Linear regression analysis using temperature as the independent variable and expiratory sevoflurane as the dependent variable showed a temperature β-coefficient of 0.11 and a constant of -2.34. Other parameters like fresh gas flows and pump flows were correlated to find out if they affected end-tidal sevoflurane levels on CPB. They were not significant in multivariate analysis. The sevoflurane requirement increases during the rewarming phase of hypothermic CPB. The percent increase in the requirement for sevoflurane is uniform and follows a particular pattern, which may be predicted.

Brief review: Theory and practice of minimal fresh gas flow anesthesia

The aim of this brief review is to provide an update on the theory regarding minimal fresh gas flow techniques for inhaled general anesthesia. The article also includes an update and discussion of the practical aspects associated with minimal-flow anesthesia, including the advantages, potential limitations, and safety considerations of this important anesthetic technique. Reducing the fresh gas flow to < 1 L·min(-1) during maintenance of anesthesia is associated with several benefits. Enhanced preservation of temperature and humidity, cost savings through more efficient utilization of inhaled anesthetics, and environmental considerations are three key reasons to implement minimal-flow and closed-circuit anesthesia, although potential risks are hypoxic gas mixtures and inadequate depth of anesthesia. The basic elements of the related pharmacology need to be considered, especially pharmacokinetics of the inhaled anesthetics. The third-generation inhaled anesthetics, sevoflurane and desflurane, have low blood and low tissue solubility, which facilitates rapid equilibration between the alveolar and effect site (brain) concentrations and makes them ideally suited for low-flow techniques. The use of modern anesthetic machines designed for minimal-flow techniques, leak-free circle systems, highly efficient CO(2) absorbers, and the common practice of utilizing on-line real-time multi-gas monitor, including essential alarm systems, allow for safe and cost-effective minimal-flow techniques during maintenance of anesthesia. The introduction of new anesthetic machines with built-in closed-loop algorithms for the automatic control of inspired oxygen and end-tidal anesthetic concentration will further enhance the feasibility of minimal-flow techniques. With our modern anesthesia machines, reducing the fresh gas flow of oxygen to 0.3-0.5 L·min(-1) and using third-generation inhaled anesthetics provide a reassuringly safe anesthetic technique. This environmentally friendly practice can easily be implemented for elective anesthesia; furthermore, it will facilitate cost savings and improve temperature homeostasis.

Improving the cost-effectiveness of volatile anaesthesia - agent choice or fresh gas flow?

Improving the cost-effectiveness of volatile anaesthesia - agent choice or fresh gas flow?

The effect of the introduction of automated gas flow control on fresh gas flow rates

The effect of the introduction of automated gas flow control on fresh gas flow rates

Inside Anesthesia Breathing Circuits

We measured the time it takes to reach the desired inspired anesthetic concentration using the Primus (Drägerwerk, AG, Lübeck, Germany) and the Avance (GE Datex-Ohmeda, Munich, Germany) anesthesia machines with toddler and newborn ventilation settings. The time to reach 95% of inspired target sevoflurane concentration was measured during wash-in from 0 to 6 vol% sevoflurane and during wash-out from 6 to 0 vol% with fresh gas flows equal to 1 and 2 times the minute ventilation. The Avance was faster than the Primus (65 seconds [95% confidence interval (CI): 55 to 78] vs 310 seconds [95% CI: 261 to 359]) at 1.5 L/min fresh gas flow, tidal volume of 50 mL, and 30 breaths/min. Times were shorter by the same magnitude at higher fresh gas flows and higher minute ventilation rates. The effect of doubling fresh gas flow was variable and less than expected. The Primus is slower during newborn than toddler ventilation, whereas the Avance's response time was the same for newborn and toddler ventilation. Our data confirm that the time to reach the target-inspired anesthetic concentration depends on breathing circuit volume, fresh gas flow, and minute ventilation.