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.

Intraoperative stroke volume optimization using stroke volume, arterial pressure, and heart rate: closed-loop (learning intravenous resuscitator) versus anesthesiologists.

OBJECTIVE: The authors compared the performance of a group of anesthesia providers to closed-loop (Learning Intravenous Resuscitator [LIR]) management in a simulated hemorrhage scenario using cardiac output monitoring. DESIGN: A prospective cohort study. SETTING: In silico simulation. PARTICIPANTS: University hospital anesthesiologists and the LIR closed-loop fluid administration system. INTERVENTIONS: Using a patient simulator, a 90-minute simulated hemorrhage protocol was run, which included a 1,200-mL blood loss over 30 minutes. Twenty practicing anesthesiology providers were asked to manage this scenario by providing fluids and vasopressor medication at their discretion. The simulation program was also run 20 times with the LIR closed-loop algorithm managing fluids and an additional 20 times with no intervention. MEASUREMENTS AND MAIN RESULTS: Simulated patient weight, height, heart rate, mean arterial pressure, and cardiac output (CO) were similar at baseline. The mean stroke volume, the mean arterial pressure, CO, and the final CO were higher in the closed-loop group than in the practitioners group, and the coefficient of variance was lower. The closed-loop group received slightly more fluid (2.1 v 1.9 L, p < 0.05) than the anesthesiologist group. CONCLUSIONS: Despite the roughly similar volumes of fluid given, the closed-loop maintained more stable hemodynamics than the practitioners primarily because the fluid was given earlier in the protocol and CO optimized before the hemorrhage began, whereas practitioners tended to resuscitate well but only after significant hemodynamic change indicated the need. Overall, these data support the potential usefulness of this closed-loop algorithm in clinical settings in which dynamic predictors are not available or applicable.

Bispectral index-controlled postoperative sedation in cardiac surgery patients: a comparative trial between closed loop and manual administration of propofol.

BACKGROUND AND OBJECTIVE: Postoperative cardiac surgery patients are usually sedated according to clinical sedation scores. Electrophysiological data derived from electroencephalography, such as the bispectral index (BIS), have been reported to assess and quantify the level of sedation, although experience in these patients is limited. In the current study, we evaluated a closed-loop system - closed-loop anaesthesia delivery system (CLADS) - for postoperative sedation after open heart surgery using BIS. METHODS: Forty-one postoperative cardiac surgery patients in the age group 18-65 years were included. In the postanaesthesia care unit, they were randomly allocated to two groups: a CLADS group, which received a continuous infusion of propofol using CLADS, and a manual group, which received propofol at a rate manually adjusted by the clinician. Propofol was administered in both groups to maintain the BIS at a target of 70 for adequate sedation. Patients were weaned from mechanical ventilation and the trachea extubated after confirmation of haemodynamic stability, haemostasis, normothermia and mental orientation. RESULTS: The percentage of total sedation time during which BIS remained within +/-10 of the target value (BIS of 70 during sedation) was significantly higher in the CLADS group than in the manual group (P = 0.002). The assessment of performance parameters using median performance error and median absolute performance error indicated better performance in the CLADS group. Manual control required the propofol infusion rate to be changed frequently, taking up considerable time and attention of the clinician. CONCLUSION: Closed-loop delivery of propofol to control BIS for postoperative sedation is feasible and efficient after cardiac surgery.

Estimation of optimal modeling weights for a Bayesian-based closed-loop system for propofol administration using the bispectral index as a controlled variable: a simulation study.

BACKGROUND: Implementing Bayesian methods in a model-based closed-loop system requires the integration of a standard response model with a patient-specific response model. This process makes use of specific modeling weights, called Bayesian variances, which determine how the specific model can deviate from the standard model. In this study we applied simulations to select the Bayesian variances yielding the optimal controller for a Bayesian-based closed-loop system for propofol administration using the Bispectral Index (BIS) as a controlled variable. METHODS: The relevant Bayesian variances determining the modeling process were identified. Each set of such Bayesian variances represents a potential controller. The set, which will result in optimal control, was estimated using calculations on a simulated population. We selected 625 candidate sets. Similar to our previous closed-loop performance study, we applied a simulation protocol to evaluate controller performance. Our population consisted of 416 virtual patients, generated using population characteristics from previous work. A BIS offset trajectory similar to a surgical case was used. RESULTS: We were able to develop, describe, and optimize the parameter setting for a patient-individualized model-based closed-loop controller using Bayesian optimization. Selection of the optimal set yields a controller performing with the following median absolute prediction errors at BIS targets 30, 50, and 70: 12.9 +/- 2.87, 7.59 +/- 0.74, and 5.76 +/- 1.03 respectively. CONCLUSIONS: We believe this system can be introduced safely into clinical testing for both induction and maintenance of anesthesia under direct observation of an anesthesiologist.

[Pros or cons of accessory anesthetic circuits. I. Arguments for their use]. / Pour ou contre les circuits anesthésiques accessoires. I. Arguments pour leur utilisation.

In addition to the circle breathing system, which represents the main circuit of the anaesthetic machine, the use of an accessory breathing system (ABS), either a partial rebreathing system according to Mapleson's classification, or a system including a non-rebreathing valve, is appropriate for the anaesthetic management of many patients, depending on their physical status, age, indication and duration of surgery. The same safety rules, namely full checking procedure before use of the system and monitoring of inhaled gases and end-tidal CO2 must be applied as for the main circle system. Potential complications resulting from non compliance with these rules cannot be considered valuable reasons for denying the use of breathing systems that have safely been used for decades in millions of patients.

[Pros and cons of adding an accessory breathing system to the main circle circuit. II. Arguments against their use]. / Pour ou contre les circuits anesthésiques accessoires. II. Arguments contre leur utilisation.

When compared to the circle system alternative breathing systems (ABS) are of no benefit. When the only indication of an ABS is emergency oxygen administration it should be connected to the O2 pipeline upstream from the flowmeter bank and the vaporiser. The use of an ABS for anaesthesia maintenance is no longer justified because of the difficulties in monitoring pressure, flow and concentrations of the gas mixture, the cost of gas and vapour administered at a high flow and the resulting pollution. The use of an ABS for very short anaesthetics is only acceptable if the administered gas mixture is monitored.

[Clinical evaluation of low flow anesthesia machine ACOMA ACM-10].

We studied low flow anesthesia using ACOMA ACM-10 anesthesia machine in 39 patients undergoing elective surgery. Randomly, 29 patients were allocated to low flow anesthesia group and 10 patients were allocated to high flow anesthesia group. After the induction, anesthesia was maintained with 2 l.min-1 of oxygen, 4 l.min-1 of nitrous oxide and sevoflurane (high flow anesthesia). After 15 minutes of high flow anesthesia, the total gas flow was reduced to 300 ml.min-1 of oxygen, 300 ml.min-1 of nitrous oxide or reduced to 400 ml.min-1 of oxygen, 200 ml.min-1 of nitrous oxide in the low flow anesthesia group. In the high flow anesthesia group, the high flow was maintained. The inspiratory oxygen concentration was maintained during anesthesia more than 35% in the low flow group and 33% in the high flow group. The absolute humidity in the inspiratory gas and the soda lime temperature were significantly higher during low flow anesthesia. The sevoflurane consumption in the low flow anesthesia group was reduced to less than 1-8th that of high flow anesthesia group. In conclusion, we could perform low flow anesthesia safely and economically with the ACM-10 anesthesia machine.

Priming of anesthesia circuit with xenon for closed circuit anesthesia.

Xenon is an inert gas with a practical anesthetic potency (1 MAC = 71%). Because it is very expensive, the use of closed circuit anesthesia technique is ideal for the conduction of xenon anesthesia. Here we describe our methods of starting closed circuit anesthesia without excessive waste of xenon gas. We induce anesthesia with intravenous agents, and after endotracheal intubation, denitrogenate the patient for approximately 30 min with a high flow of oxygen. This is done to minimize accumulation of nitrogen in the anesthesia circuit during the subsequent closed-circuit anesthesia with xenon. Anesthesia is maintained with an inhalational anesthetic during this period. Then, we discontinue the inhalation agent and start xenon. For this transition, we feel it is unacceptable to simply administer xenon at a high flow until the desired end-tidal concentration is reached because it is too costly. Instead we set up another machine with its circuit filled in advance (i.e., primed) with at least 60% xenon in oxygen and switch the patient to this machine. To prime the circuit, we push xenon using a large syringe into a circuit, which was prefilled with oxygen. Oxygen inside the circuit is pushed out before it is mixed with xenon, and xenon waste will thus be minimized. In this way, we can achieve close to 1 MAC from the beginning of xenon anesthesia, and thereby minimize the risk of light anesthesia and awareness during transition from denitrogenation to closed-circuit xenon anesthesia.

[Evaluation of low flow closed anesthesia technique for oxygen, nitrous oxide and isoflurane using Engström Elsa Anesthesia System].

We studied low flow closed anesthesia technique for oxygen, nitrous oxide and isoflurane using Engström Elsa Anesthesia System in 47 patients undergoing elective surgery. In the low flow group, anesthesia was maintained with oxygen 300 ml.min-1, nitrous oxide 300 ml.min-1 and optimal concentration of isoflurane after endotracheal intubation. The inspired oxygen concentration was kept higher than 35% through the operation. In the minimal flow group, anesthesia was maintained with nitrous oxide 4 l.min-1, oxygen 2 l.min-1 and optimal concentration of isoflurane after intubation until the inspired oxygen concentration reached approximately 35%. Then the fresh gas flow was reduced to oxygen 250 ml.min-1 and nitrous oxide 250 ml.min-1. In order to maintain the inspired oxygen concentration at 35%, the flow of nitrous oxide should have been reduced as low as to 150 ml.min-1 after 240 min. The volumes of consumption of nitrous oxide and isoflurane in the low flow and minimal flow groups were reduced to one fifth and one third respectively, compared with those of the high flow group. In conclusion, we can perform low flow closed anesthesia safely and easily with this equipment.

Gas leakage in eight anaesthesia circle systems.

Eight currently used factory-new anaesthesia circle systems (Dräger Cicero, Dräger Sulla, Dräger AV1, Gambro Engström Elsa, Megamed 700A, Ohmeda Modulus II Plus, Siemens Ventilator 710 and Siemens Servo Ventilator 900 D with circle system) and a Megamed 077 which had been clinically used for 11 years were tested for gas leaks according to the Draft European Standard Anaesthetic Workstations and Their Modules. All measurements were performed using the Cicero ventilator developed by Dräger with its integrated test program for the detection of system leakage. All the factory-new systems showed leakage rates of less than 50 ml min-1 at a test pressure of 3 kPa (30 cmH2O). In the 'manual' position and with the soda-lime canister and the volumeter (or flow-sensor) included, the following leak rates were determined: Dräger Cicero, 5.0 ml min-1; Dräger Sulla, 22.8 ml min-1; Dräger AV1, 7.7 ml min-1; Gambro Engström Elsa, 33.4 ml min-1; Megamed 700A, 11.5 ml min-1; Ohmeda Modulus II Plus, less than 0.1 ml min-1; Siemens Ventilator 710, 0.3 ml min-1; Siemens Servo Ventilator 900D with circle system 985, 9.6 ml min-1; Megamed 077, 47.5 ml min-1. All anaesthesia breathing circle systems tested performed below the leakage limit of 100 ml min-1 proposed by the draft standard.