In Vitro Model of Prepacked Carbon Dioxide Absorber Use: Development and Testing.

BACKGROUND: Carbon dioxide absorbers allow the use of fresh gas flow below minute ventilation (V˙E). Models are developed and tested in vitro to quantify their performance with variable carbon dioxide load (V˙CO2), fresh gas flow, V˙E, end-tidal carbon dioxide (ETco2) fraction, and the type of workstation used. METHODS: First principles are used to derive a linear relationship between fresh gas flow and fractional canister usage or FCU0.5 (the reciprocal of the time for the inspiratory carbon dioxide fraction to reach 0.5%). This forms the basis for two basic models in which V˙E was measured by spirometry or calculated. These models were extended by multiplying V˙E with an empirical workstation factor. To validate the four models, two hypotheses were tested. To test whether the FCU0.5 intercept varied proportionally with V˙CO2 and was independent of V˙E, FCU was measured for 10 canisters tested with a fixed 0.3 l/min fresh gas flow and a range of V˙CO2 while V˙E was either constant or adjusted to maintain ETco2 fraction. A t test was used to compare the two groups. To confirm whether a change in V˙CO2 accompanied by a change in V˙E to maintain ETco2 fraction would shift the linear fresh gas flow-FCU0.5 relationship in a parallel manner, 19 canisters were tested with different combinations of V˙CO2 and fresh gas flow. These measured FCU values were compared to those predicted by the four models using Varvel's performance criteria. RESULTS: With 0.3 l/min fresh gas flow, FCU0.5 was proportional with V˙CO2 and independent of whether V˙E was adjusted to maintain ETco2 fraction or not (P = 0.962). The hypothesized parallel shift of the fresh gas flow-FCU0.5 relationship was confirmed. Both extended models are good candidate models. CONCLUSIONS: The models predict prepacked canister performance in vitro over the range of V˙E, fresh gas flow, and V˙CO2 likely to be encountered in routine clinical practice. In vivo validation is still needed.

Arterial to end-tidal CO2 gradients during isocapnic hyperventilation.

Isocapnic hyperventilation (ICHV) is occasionally used to maintain the end-expired CO2 partial pressure (PETCO2) when the inspired CO2 (PICO2) rises. Whether maintaining PETCO2 with ICHV during an increase of the PICO2 also maintains arterial PCO2 (PaCO2) remains poorly documented. 12 ASA PS I-II subjects undergoing a robot-assisted radical prostatectomy (RARP) (n = 11) or cystectomy (n = 1) under general endotracheal anesthesia with sevoflurane in O2/air (40% inspired O2) were enrolled. PICO2 was sequentially increased from 0 to 0.5, 1.0, 1.5 and 2% by adding CO2 to the inspiratory limb of the circle system, while increasing ventilation to a target PETCO2 of 4.7-4.9% by adjusting respiratory rate during controlled mechanical ventilation. Pa-ETCO2 gradients were determined after a 15 min equilibration period at each PICO2 level and compared using ANOVA. Mean (standard deviation) age, height, and weight were 66 (6) years, 171 (6) cm, and 75 (8) kg, respectively. Capnograms were normal and hemodynamic parameters remained stable. PETCO2 could be maintained within 4.7-4.9% in all subjects at all times except in 1 subject with 1.5% PICO2 and 5 subjects with 2.0% PICO2; data from the one subject in whom both 1.5 and 2.0% PICO2 resulted in PETCO2 > 5.1% were excluded from analysis. Pa-ETCO2 gradients did not change when PICO2 increased. The effect of a modest rise of PICO2 up to 1.5% on PETCO2 during RARP can be readily overcome by increasing ventilation without altering the Pa-ETCO2 gradients. At higher PICO2, airway pressures may become a limiting factor, which requires further study.

Development and validation of a model to calculate anesthetic agent consumption from inspired and end-expired concentrations, minute ventilation, fresh gas flow and dead space ventilation.

Anesthetic agent consumption is often calculated as the product of fresh gas flow (FGF) and vaporizer dial setting (FVAP). Because FVAP of conventional vaporizers is not registered in automated anesthesia records, retrospective agent consumption studies are hampered. The current study examines how FVAP can be retrospectively calculated from the agent's inspired (FIN) and end-expired concentration (FET), FGF, and minute ventilation (MV). Theoretical analysis of agent mass balances in the circle breathing reveals FVAP = [FIN - (dead space fraction * FIN + (1 - dead space fraction) * FET) * (1 - FGF/MV)]/(1-(1 - FGF/MV)). FIN, FET, FGF and MV are routinely monitored, but dead space fraction is unknown. Dead space fraction for sevoflurane, desflurane, and isoflurane was therefore determined empirically from an unpublished data set of 161 patient containing FVAP, FIN, FET, MV and FGF ranging from 0.25 to 8 L/min delivered via an ADU® (GE, Madison, WI, USA). Dead space fraction for each agent was determined empirically by having Excel's solver function calculate the value of dead space fraction that minimized the sum of the squared differences between dialed FVAP and predicted FVAP. With dead space fraction known, the model was then prospectively tested for sevoflurane in O2/air using data collected over the course of two weeks with one FLOW-i (Getinge, Solna, Sweden) and one Zeus workstation (Dräger, Lübeck, Germany). Because both workstations use an electronically controlled vaporizer/injector, the dialed FVAP were available to allow the calculation of median performance error (MDPE) and median absolute performance error (MDAPE). MDPE and MDAP are reported as median and interquartiles. The empirical dead space fraction for isoflurane, sevoflurane, and desflurane were 0.59, 0.49, and 0.66, respectively. For prospective testing, a total of 149.4 h of useful data were collected from 78 patient with the Zeus and Flow-i combined, with FGF ranging from 0.18 to 8 L/min. The model predicted dialed FVAP well, with a MDPE of -1 (-11, 6) % and MDAPE of 8 (4, 17) %. FVAP can be retrospectively calculated from FIN, FET, FGF, and MV plus an agent specific dead space fraction factor with a degree of error that we believe suffices for retrospective sevoflurane consumption analyses. Performance with other agents and N2O awaits further validation.

What is the optimal anesthetic monitoring regarding immediate and short-term outcomes after liver transplantation?-A systematic review of the literature and expert panel recommendations.

BACKGROUND: Liver transplant centers vary in approach to intraoperative vascular accesses, monitoring of cardiac function and temperature management. Evidence is limited regarding impact of selected modalities on postoperative outcomes. OBJECTIVES: To review the literature and provide expert panel recommendations on optimal intraoperative arterial blood pressure (BP), central venous pressure (CVP), and vascular accesses, monitoring of cardiac function and intraoperative temperature management regarding immediate and short-term outcomes after orthotopic liver transplant (OLT). METHODS: Systematic review following PRISMA guidelines and recommendations using the GRADE approach derived from an international expert panel. Recommendations made for: (1) Vascular accesses, arterial BP and CVP monitoring, (2) cardiac function monitoring, and (3) Intraoperative temperature management (CRD42021239908). RESULTS: Of 2619 articles screened 16 were included. Studies were small, retrospective, and observational. Vascular access studies demonstrated low rates of insertion complications. TEE studies demonstrated low rates of esophageal hemorrhage. One study found lower hospital-LOS and 30-day mortality in patients monitored with both PAC and TEE. Other monitoring studies were heterogenous in design and outcomes. Temperature studies showed increased blood transfusion and ventilation times in hypothermic groups. CONCLUSIONS: Recommendations were made for; routine arterial and CVP monitoring as a minimum standard of practice, consideration of discrepancy between peripheral and central arterial BP in patients with hemodynamic instability and high vasopressor requirements, and routine use of high flow cannulae while monitoring for extravasation and hematoma formation. Availability and expertise in PAC and/or TEE monitoring is strongly recommended particularly in hemodynamic instability, portopulmonary HT and/or cardiac dysfunction. TEE use is recommended as an acceptable risk in patients with treated esophageal varices and is an effective diagnostic tool for emergency cardiovascular collapse. Maintenance of intraoperative normothermia is strongly recommended.

The science behind banning desflurane: A narrative review.

Potent inhaled anaesthetics are halogenated hydrocarbons with a large global warming effect. The use of fluorinated hydrocarbons (most are not anaesthetics) are being restricted but volatile anaesthetics have been exempted from legislation, until now: the EU has formulated a proposal to ban or at least severely restrict the use of desflurane starting January 2026. This narrative review addresses the implications of a politics-driven decision - without prior consultation with major stakeholders, such as the European Society of Anaesthesiology and Intensive Care (ESAIC) - on daily anaesthesia practice and reviews the potential scientific arguments that would support stopping the routine use of desflurane in anaesthetic practice. Of note, banning or severely restricting the use of one anaesthetic agent should not distract the user from sensible interventions like reducing fresh gas flows and developing technology to capture and recycle or destroy the wasted potent inhaled anaesthetics that we will continue to use. We call to join efforts to minimise our professional environmental footprint.

Prospective validation of gas man simulations of sevoflurane in O2/air over a wide fresh gas flow range.

The use of inhaled anesthetics has come under increased scrutiny because of their environmental effects. This has led to a shift where sevoflurane in O2/air has become the predominant gas mixture to maintain anesthesia. To further reduce environmental impact, lower fresh gas flows (FGF) should be used. An accurate model of sevoflurane consumption allows us to assess and quantify the impact of the effects of lowering FGFs. This study therefore tested the accuracy of the Gas Man® model by determining its ability to predict end-expired sevoflurane concentrations (FETsevo) in patients using a protocol spanning a wide range of FGF and vaporizer settings. After IRB approval, 28 ASA I-II patients undergoing a gynecologic or urologic procedure under general endotracheal anesthesia were enrolled. Anesthesia was maintained with sevoflurane in O2/air, delivered via a Zeus or FLOW-i workstation (14 patients each). Every fifteen min, FGF was changed to randomly selected values ranging from 0.2 to 6 L/min while the sevoflurane vaporizer setting was left at the discretion of the anesthesiologist. The FETsevo was collected every min for 1 h. For each patient, a Gas Man® simulation was run using patient weight and the same FGF, vaporizer and minute ventilation settings used during the procedure. For cardiac output, the Gas Man default setting was used (= Brody formula). Gas Man®'s performance was assessed by comparing measured with Gas Man® predicted FETsevo using linear regression and Varvel's criteria [median performance error (MDPE), median absolute performance error (MDAPE), and divergence]. Additional analysis included separating performance for the wash-in (0-15 min) and maintenance phase (15-60 min). For the FLOW-i, MDPE, MDAPE and divergence were 1% [- 6, 8], 7% [3, 15] and - 0.96%/h [- 1.14, - 0.88], respectively. During the first 15 min, MDPE and MDAPE were 18% [1, 51] and 21% [8, 51], respectively, and during the last 45 min 0% [- 7, 5] and 6% [2, 10], respectively. For the Zeus, MDPE, MDAPE and divergence were 0% [- 5, 8], 6% [3, 12] and - 0.57%/h [- 0.85, - 0.16], respectively. During the first 15 min, MDPE and MDAPE were 7% [- 6, 28] and 13% [6, 32], respectively, and during the last 45 min - 1% [- 5, 5] and 5% [2, 9], respectively. In conclusion, Gas Man® predicts FETsevo in O2/air in adults over a wide range of FGF and vaporizer settings using different workstations with both MDPE and MDAPE < 10% during the first hour of anesthesia, with better relative performance for simulating maintenance than wash-in. In the authors' opinion, this degree of performance suffices for Gas Man® to be used to quantify the environmental impact of FGF reduction in real life practice of the wash-in and maintenance period combined.

Memsorb™, a novel CO2 removal device part I: in vitro performance with the Zeus IE®.

Soda lime-based CO2 absorbents are safe, but not ideal for reasons of ecology, economy, and dust formation. The Memsorb™ is a novel CO2 removal device that uses cardiopulmonary bypass oxygenator technology instead: a sweep gas passes through semipermeable hollow fibers, adding or removing gas from the circle breathing system. We studied the in vitro performance of a prototype Memsorb™ used with a Zeus IE® anesthesia machine when administering sevoflurane and desflurane in O2/air mixtures. The Zeus IE® equipped with Memsorb™ ventilated a 2L breathing bag with a CO2 inflow port in its tip. CO2 kinetics were studied by using different combinations of CO2 inflow (VCO2), Memsorb™ sweep gas flow, and Zeus IE® fresh gas flow (FGF) and ventilator settings. More specifically, it was determined under what circumstances the inspired CO2 concentration (FICO2) could be kept < 0.5%. O2 kinetics were studied by measuring the inspired O2 concentration (FIO2) resulting from different combinations of Memsorb™ sweep gas flow and O2 concentrations, and Zeus IE® FGFs and O2 concentrations. Memsorb™'s sevoflurane and desflurane waste was determined by measuring their injection rates during target-controlled closed-circuit anesthesia (TCCCA), and were compared to historical controls when using a soda lime absorbent (Draegersorb 800+) under identical conditions. With 160 mL/min VCO2 and 5 L/min minute ventilation (MV), lowering the sweep gas flow at any fixed Zeus IE® FGF increased FICO2 in a non-linear manner. Sweep gas flow adjustments kept FICO2 < 0.5% over the entire Zeus IE® FGF range tested with VCO2 up to 280 mL/min; tidal volume and respiratory rate affected the required sweep gas flow. At 10 L/min MV and low FGF (< 1.5 L/min), even a maximum sweep flow of 43 L/min was unable to keep FICO2 ≤ 0.5%. When the O2 concentration in the Zeus IE® FGF and the Memsorb™ sweep gas flow differed, FIO2 drifted towards the sweep gas O2 concentration, and more so as FGF was lowered; this effect was absent once FGF > minute ventilation. During sevoflurane and desflurane TCCCA, the Zeus IE® FGF remained zero while agent usage per % end-expired agent increased with increasing end-expired target agent concentrations and with a higher target FIO2. Agent waste during target-controlled delivery was higher with Memsorb™ than with the soda lime product, with the difference remaining almost constant over the FGF range studied. With a 5 L/min MV, Memsorb™ successfully removes CO2 with inflow rates up to 240 mL/min if an FICO2 of 0.5% is accepted, but at 10 L/min MV and low FGF (< 1.5 L/min), even a maximum sweep flow of 43 L/min was unable to keep FICO2 ≤ 0.5%. To avoid FIO2 deviating substantially from the O2 concentration in the fresh gas, the O2 concentration in the fresh gas and sweep gas should match. Compared to the use of Ca(OH)2 based CO2 absorbent, inhaled agent waste is increased. The device is most likely to find its use integrated in closed loop systems.

Memsorb™, a novel CO2 removal device part II: in vivo performance with the Zeus IE®.

Memsorb™ (DMF Medical, Halifax, Canada) is a novel device based upon membrane oxygenator technology designed to eliminate CO2 from exhaled gas when using a circle anesthesia circuit. Exhaled gases pass through semipermeable hollow fibers and sweep gas flowing through these fibers creates a diffusion gradient for CO2 removal. In vivo Memsorb™ performance was tested during target-controlled closed-circuit anesthesia (TCCCA) with desflurane in O2/air using a Zeus IE® anesthesia workstation (Dräger, Lübeck, Germany). Clinical care protocols for using this novel device were guided by in vitro performance results from a prior study (submitted simultaneously). After IRB approval, written informed consent was obtained from 10 ASA PS I-III patients undergoing robot-assisted radical prostatectomy. TCCCA targets were 39% inspired O2 concentration (FIO2) and 5.0% end-expired desflurane concentration (FETdes). Minute ventilation (MV) was adjusted to maintain 4.5-6.0% FETCO2. The O2/air (40% O2) sweep flow into the Memsorb™ was manually adjusted in an attempt to keep inspired CO2 concentration (FICO2) ≤ 0.8%. The following data were collected: FIO2, FETdes, FICO2, FETCO2, MV, fresh gas flow (FGF, O2 and air), sweep flow, and cumulative desflurane usage (Vdes). Vdes of the Zeus IE®-Memsorb™ combination was compared with historical Vdes observed in a previous study when soda lime (DrägerSorb 800 +) was used. Results are reported as median and inter-quartiles. A combination of manually adjusting sweep flow (26 [21,27] L/min) and MV sufficed to maintain FICO2 ≤ 0.8% and FETCO2 ≤ 6.0%, except in one patient in whom the target Zeus IE® FGF had to be increased to 0.7 L/min for 6 min. FIO2 and FETdes were maintained close to their targets. Zeus IE® FGF after 5 min was 0 [0,0] mL/min. Average Vdes after 50 min was higher with Memsorb™ (20.3 mL) compared to historical soda lime canister data (12.3 mL). During target-controlled closed-circuit anesthesia in patients undergoing robot-assisted radical prostatectomy, the Memsorb™ maintained FICO2 ≤ 0.8% and FETCO2 ≤ 6.0%, and FIO2 remained close to target. Modest amounts of desflurane were lost with the use of the Memsorb™. The need for adjustments of sweep flow, minute ventilation, and occasionally Zeus IE® FGF indicates that the Memsorb™ system should preferentially be integrated into an automated closed-loop system.

End of year summary 2019: anaesthesia and airway management.

This end of the year summary reviews anesthesia related manuscripts that have been published in the Journal of Clinical Monitoring and Computing in 2019. Anesthesia is currently defined as being composed of unconsciousness, immobility, and autonomic nervous system (ANS) control (Br J Anaesth;122:e127-e135135, Egan 2019). Pain is a postoperative issue, because by definition unconsciousness implies pain cannot be experienced. We first review work related to these aspect of the profession: unconsciousness (EEG, target control), immobility (muscle relaxants), and ANS control. Regaining consciousness has to be accompanied by pain control, and it is important to ensure that the patient regains baseline cognitive function. Anesthesia machine equipment, drug administration, and airway related topics make up the rest of published manuscripts.

The effect of isocapnic hyperventilation on early recovery after remifentanil/sevoflurane anesthesia in O2 /air: A randomized trial.

BACKGROUND: Isocapnic hyperventilation (ICHV) may hasten emergence from general anesthesia but remains inadequately studied. We prospectively determined emergence time after sevoflurane anesthesia of variable duration with and without ICHV. METHODS: In 25 ASA I-II patients, general anesthesia was maintained with one age-adjusted MAC sevoflurane in O2 /air and target-controlled remifentanil delivery. At the start of skin closure, the remifentanil effect-site concentration was reduced to 1.5 ng/mL, any residual neuromuscular block reversed, and once the remifentanil effect-site concentration had decreased to 1.5 ng/mL, remifentanil and sevoflurane administration was stopped, and the fresh gas flow increased above minute ventilation. Patients randomly received either normoventilation (n = 13) or ICHV (doubling minute ventilation while titrating CO2 into the inspiratory limb to maintain isocapnia [n = 12]). Three early recovery end points were determined: time to proper response to verbal command; time to extubation; and time to stating one's name. RESULTS: Demographics were the same in both groups. Recovery end points were reached faster in the ICHV group compared to the normoventilation group: time to proper response to verbal command was 7.6 ± 2.2 vs 9.9 ± 2.9 min (P = 0.03); time to extubation was 7.6 ± 2.6 vs 11.0 ± 2.4 min (P = 0.002); and time to stating one's name was 8.9 ± 2.8 vs 12.5 ± 2.6 min (P = 0.003). Within each group, duration of anesthesia only marginally affected the times to reach these recovery end points. CONCLUSION: Isocapnic hyperventilation only had a small effect on emergence times after anesthesia, suggesting that isocapnic hyperventilation may have limited clinical benefits with modern potent inhaled anesthetics.

EOY summary 2018.

Clinical monitoring and technology are at the heart of anesthesiology, and new technological developments will help to define how anesthesiology will evolve as a profession. Anesthesia related research published in the JCMC in 2018 mainly pertained to ICU sedation with inhaled agents, anesthesia workstation technology, and monitoring of different aspects of depth of anesthesia.

In vitro efficiency of 16 different Ca(OH)2 based CO2 absorbent brands.

Data directly comparing CO2 absorbents tested in identical and clinically relevant conditions are scarce or non-existent. We therefore tested and compared the efficiency of 16 different brands of Ca(OH)2 based CO2 absorbents used as loose fill or a cartridge in a refillable canister under identical low flow conditions. CO2 absorbents efficiency was tested by flowing 160 mL/min CO2 into the tip of a 2 L balloon that was ventilated with an ADU anesthesia machine (GE, Madison, WI, USA) with a tidal volume of 500 mL and a respiratory rate of 10/min while running an O2/air FGF of 300 mL/min. After the 1020 mL refillable container was filled with a known volume of CO2 absorbent (derived from weighing the initial canister content and the product's density), the time for the inspired CO2 concentration (FICO2) to rise to 0.5% was measured. This test was repeated 4 times for each product. Because the two SpiraLith Ca® products (one with and one without indicator) are delivered as a cartridge, they had to be tested using their proprietary canister. The time (min) for FICO2 to reach 0.5% was normalized to 100 mL of product, and defined as the efficiency, which was compared amongst the different brands using ANOVA. Efficiency ranged from 50 to 100 min per 100 mL of product, and increased with increasing NaOH content (a catalyst), the exception being SpiraLith Ca® cartridge with color indicator (performing as well as the most efficient granular products) and the SpiraLith Ca® cartridge without color indicator (outperforming all others). Results indicated a spherical or bullet shape is less efficient in absorbing CO2 than broken fragments or cylinders, which in turn is less efficient than a hemispherical (disc) shape, which is in turn less efficient than a solid cartridge with a molded channel geometry. The efficiency of Ca(OH)2 based CO2 absorbent differs up to 100% on a volume basis. Macroscopic arrangement (cylindrical wrap with preformed channels versus granules), chemical composition (NaOH content), and granular shape all affect efficiency per volume of product. The data can be used to compare costs of the different products.

The Anesthesia Workstation: Quo Vadis?

Ensuring adequate ventilation and oxygenation and delivering inhaled anesthetic agent to the patient remain core responsibilities of the anesthesia provider during general anesthesia. Because of the emphasis placed on physiology, pharmacology, clinical sciences, and administrative duties, the stellar anesthesia workstation technology may be underutilized by the anesthesia community. Target-controlled O2 and agent delivery and automated end-expired CO2 control have entered the clinical arena, with only cost, luddism, and administrative hurdles preventing their more widespread use. This narrative review will explain technological aspects of existing and recently introduced anesthesia workstations. Concepts rather than particular anesthesia machines will be addressed, but examples will mostly pertain to the more recently introduced workstations. The anesthesia workstation consists of a ventilator, a carrier gas and agent delivery system, a scavenging system, and monitors. Mainly, the circle breathing circuit configuration, ventilator, and carrier gas and agent delivery technology are discussed. Occasionally, technical details are provided to give the reader a taste of the modern technology.

Desflurane usage during anesthesia with and without N2O using FLOW-i Automatic Gas Control with three different wash-in speeds.

AGC® (Automatic Gas Control) is the FLOW-i's automated low flow tool (Maquet, Solna, Sweden) that target controls the inspired O2 (FIO2) and end-expired desflurane concentration (FAdes) while (by design) exponentially decreasing fresh gas flow (FGF) during wash-in to a maintenance default FGF of 300 mL min-1. It also offers a choice of wash-in speeds for the inhaled agents. We examined AGC performance and hypothesized that the use of lower wash-in speeds and N2O both reduce desflurane usage (Vdes). After obtaining IRB approval and patient consent, 78 ASA I-II patients undergoing abdominal surgery were randomly assigned to 1 of 6 groups (n = 13 each), depending on carrier gas (O2/air or O2/N2O) and wash-in speed (AGC speed 2, 4, or 6) of desflurane, resulting in groups air/2, air/4, air/6, N2O/2, N2O/4, and N2O/6. The target for FIO2 was set at 35%, while the FAdes target was selected so that the AGC displayed 1.3 MAC (corrected for the additive affect of N2O if used). AGC was activated upon starting mechanical ventilation. Varvel's criteria were used to describe performance of achieving the targets. Patient demographics, end-expired N2O concentration, MAC, FGF, and Vdes were compared using ANOVA. Data are presented as mean ± standard deviation, except for Varvel's criteria (median ± quartiles). Patient demographics did not differ among the groups. Median performance error was -2-0% for FIO2 and -3-1% for FAdes; median absolute performance error was 1-2% for FIO2 and 0-3% for FAdes. MAC increased faster in N2O groups, but total MAC decreased 0.1-0.25 MAC below that in the O2/air groups after 60 min. The effect of wash-in speed on Vdes faded over time. N2O decreased Vdes by 62%. AGC performance for O2 and desflurane targeting is excellent. After 1 h, the wash-in speeds tested are unlikely to affect desflurane usage. N2O usage decreases Vdes proportionally with its reduction in FAtdes.

In vitro performance of prefilled CO2 absorbers with the Zeus®.

Low fresh gas flows (FGFs) decrease the use of anesthetic gases, but increase CO2 absorbent usage. CO2 absorbent usage remains poorly quantified. The goal of this study is to determine canister life of 8 commercially available CO2 absorbent prepacks with the Zeus®. Pre-packed CO2 canisters of 8 different brands were tested in vitro: Amsorb Plus, Spherasorb, LoFloSorb, LithoLyme, SpiraLith, SpheraSorb, Drägersorb 800+, Drägersorb Free, and CO2ntrol. CO2 (160 mL min- 1) flowed into the tip of a 2 L breathing bag that was ventilated with a tidal volume of 500 mL, a respiratory rate of 10/min, and an I:E ratio of 1:1 using the controlled mechanical ventilation mode of the Zeus® (Dräger, Lubeck, Germany). In part I, canister life of 5 canisters each of 2 different lots of each brand was determined with a 350 mL min- 1 FGF. Canister life is the time it takes for the inspired CO2 concentration (FICO2) to rise to 0.5%. In part II, canister life was measured accross a FGF range of 0.25 to 4 L min- 1 for Drägersorb 800+ (2 lots) and SpiraLith (1 lot). In part III, the calculated canister life per 100 g fresh granule content of the different brands was compared between the Zeus and (previously published data for) the Aisys. In vitro canister life of prefilled CO2 absorber canisters differed between brands, and depended on the amount of CO2 absorbent and chemical composition. Canister life expressed as FCU0.5 (the fraction of the canister used per hour) was proportional to FGF over 0.2-2 L min-1 range only, but was non-linear with higher FGF: FCU0.5 was larger than expected with FGF > 2 L min-1, and even with FGF > minute ventilation FCU0.5 did not become zero, indicating some CO2 was being absorbed. Canister life on a per weight basis of the same brand is higher with the Zeus than the Aisys. Canister life of prefilled CO2 absorber canisters differs between brands. The FCU0.5-FGF relationship is not linear across the entire FGF range. Canister life of prepacks of the same brand for the Zeus and Aisys differs, the exact etiology of which is probably multifactorial, and may include differences in the absolute amount of absorbent and different rebreathing characteristics of the machines.

Journal of clinical monitoring and computing 2016 end of year summary: anesthesia.

Clinical monitoring and computing are essential during general anesthesia. As a result it would be impossible to review all the articles published in the Journal of Clinical Monitoring and Computing that are relevant to anesthesia. We therefore will limit this summary to those articles that are uniquely related to anesthesia. The topics include: anesthesia machines; ensuring the airway; anesthetic depth; neuromuscular transmission monitoring; locoregional anesthesia; ultrasound; and pain.

Automated gas control with the Maquet FLOW-i.

The FLOW-i anesthesia machine (Maquet, Solna, Sweden) can be equipped with automated gas control (AGC), an automated low flow tool with target control of the inspired oxygen concentration (FIO2) and end-expired concentration (FA) of a potent inhaled anesthetic. We examined the performance and quantitative aspects of the AGC. After IRB approval and individual informed consent, anesthesia in 24 ASA I-II patients undergoing abdominal or gynecological surgery was maintained with sevoflurane in O2/air with a target FIO2 of 40 % and a target sevoflurane FA (FAsevo) of 2.0 %. The AGC tool also allows the user to select 1 out of 9 different speeds with which the target FAsevo can be reached (with 9 being the fastest speed). Eight patients each were randomly assigned to speed 2, 4, and 6 (= group 2, group 4, and group 6, respectively); these three speeds were chosen arbitrarily. AGC was activated immediately after securing the airway, which defined the start of the study, and the study ended 60 min later. The following parameters were compared among the three groups: age, height, weight, FIO2, FAsevo, BIS values, heart rate, mean arterial blood pressure, fresh gas flow, and sevoflurane usage. Agent usage as reported by the FLOW-i was compared among the three groups. Patient demographics and maintenance FGF did not differ among groups. A very short-lived very high FGF (≈20 L min(-1) for 8-12 s) ensured that the target FIO2 was attained within 1-2 min in all patients. FAsevo was 1.8 % after 15, 10, and 6 min, and 1.9 % after 30, 20 and 15 min in groups 2, 4, and 6, respectively. Blood pressure, heart rate, and BIS values did not differ among the three groups. BIS values remained acceptable in all patients, even with the slowest speed. Cumulative agent usage differed among all three groups between 2 and 30 min (lower with the lower speed), and between group 2 and 6 between 35 and 60 min. AGC combines an exponentially decreasing FGF pattern with a choice of ramp functions for the end-expired target concentration of the inhaled anesthetic. Consequently, both FGF and the choice of speed become factors that influence agent usage. After 15 min, a 300 mL min(-1) maintenance FGF reduces agent usage to near closed-circuit conditions. This new addition to our automated low flow armamentarium helps to reduce anesthetic waste, cost, and pollution, while minimizing the ergonomic burden of low flow anesthesia.

Accuracy of inhaled agent usage displays of automated target control anesthesia machines.

Automated low flow anesthesia machines report how much inhaled anesthetic agent has been used for each anesthetic. We compared these reported values with the amount of agent that had disappeared by weighing the vaporizer/injectors before and after each anesthetic. The vaporizers/injectors of the Aisys, Zeus and FLOW-i were weighed with a high precision weighing scale before and after anesthesia with either desflurane in O2/air or sevoflurane in O2/N2O. These values were compared with the values reported by the cumulative agent use display tools of the respective anesthesia machines using a linear curve fit. Twenty-five measurements were performed in each group, except for the sevoflurane data with the Aisys that were available from another study (87 pairs). We also determined the amount lost by inserting and removing the vaporizers/injectors or by performing a machine checkout, corrected the measured amounts for these artifacts and repeated the linear fits. The average amount of sevoflurane and desflurane wasted by inserting and removing the cassette for the Aisys, Zeus, and FLOW-i were 0.21, 0.12, and 0.04 mL and 0.12, 0.61, and 1.13 mL liquid agent, respectively. The average amount of sevoflurane and desflurane wasted by the machine checkout with the Aisys, Zeus, and FLOW-i were 1.78, 0.21, and 1.67 mL and 2.39, 0.67, and 4.19 mL, respectively. Performance error of the displayed amount of agent use remained within 10 % of the weighed amount, expect for amounts less than 3 mL sevofurane with the FLOW-i and less than 20 mL desflurane with the Aisys and FLOW-i. Cumulative agent usage displayed by the Aisys, Zeus, and FLOW-i is within 10 % of the measured consumption, except for low consumption cases (<3 mL sevoflurane, <20 mL desflurane). The differences may be due to either measurement error or cumulative agent display error. The current results can help the researchers decide whether the displayed amounts are accurate enough for their study purposes. The extent to which these discrepancies differ between different units of the same machine remains unstudied.