Model-based administration of inhalation anaesthesia. 2. Exploring the system model.

Item does not contain fulltextWe explored our model by displaying its new capabilities, testing its sensitivity to variations in input data and illustrating its use. Its multiple-gas character allows simulation of the mechanisms governing concentration and second gas effects. Simulating the volume of a standing bellows makes it possible to test algorithms for automated closed-circuit anaesthesia. Using desflurane, the model's sensitivity to changes in blood/gas partition coefficient (range 0.42-0.576), cardiac output and minute ventilation was analysed. The model was very sensitive to changes in blood solubility; other results agreed with those reported previously. An alveolar isoflurane tension of 1% atm was rapidly attained and maintained, even using 0.5 litres min(-1) of fresh gas, when isoflurane was 'co-administered' through a vaporizer set to 3.5 vol% and a single aliquot (1.25 ml liquid) injected into the expiratory limb. As a result of its credibility and capabilities, the model is to be tested in the clinical setting

A similar BIS value does not mean a similar depth of anaesthesia.

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Model-based administration of inhalation anaesthesia. 3. Validating the system model.

Item does not contain fulltextBACKGROUND: We quantified the predictive performance of our computer model of the administration of inhalation anaesthesia from a Datex-Ohmeda Modulus CD circle-absorber system. METHODS: In 50 patients, desflurane anaesthesia was maintained with a fresh-gas flow (FGF) of 0.5 litres min(-1) of both nitrous oxide and oxygen, preceded by fast (n=14) or slow (n=36) induction: FGF greater than total ventilation, Group F; FGF equal to 1.0 litres min(-1), Group S. The two versions of the model studied differed in the size of their inter-tissue diffusion, as 0 (version 1) and 3% (version 2) of the cardiac output was shifted from the viscera to adipose tissue. Model performance was judged by comparing measured and predicted gas concentrations in terms of three variables for each gas concentration in each patient: root mean squared error (rmse=total error), bias (mean predicted - measured) (systematic error), and scatter (error around bias). These variables were then averaged over all patients. These measures were calculated overall, and separately for each group and each stage (1 = induction or 2 = maintenance). RESULTS: Model predictions were in reasonable to very good agreement with clinically obtained data. Version 2 performed better than version 1. Differences between groups were not demonstrated. The model performed better for stage 2, but only for desflurane. In group S, results (mean (SD); as percentages of the measured values for nitrous oxide, oxygen and desflurane) in the order rmse, bias, and scatter were for end-tidal concentrations of nitrous oxide: 8 (4), 8 (5), 2 (1)%; oxygen: 11 (4), -10 (6), 2 (1.1)%; nitrogen: 0.9 (0.6), -0.8 (0.6), 0.2 (0.1) vol%; carbon dioxide: 1.8 (0.6), 1.8 (0.6), 0.2 (0.1) vol%; desflurane, stage 2: 8 (4), 4 (7), 4 (2)%, vs 15 (6), -10 (8), 9 (4)% for stage 1. CONCLUSION: Administration of inhalation anaesthesia can be based on version 2 of this model, but must be guided by active monitoring

Model-based administration of inhalation anaesthesia. 1. Developing a system model.

Item does not contain fulltextThis paper is the first of a series of reports on a system model for the administration of inhalation anaesthesia. We present the development and basic testing of the model. It is a multiple-gas model; it covers fresh-gas flow rates from basal to more than total ventilation and includes an actual, not an idealized, circle-absorber breathing system featuring a standing bellows ventilator. Kinetics of nitrogen, oxygen, carbon dioxide, nitrous oxide, inhaled anaesthetic agents and helium are described. Their partial pressures sum to the total pressure. Ventilation and cardiac output are treated as continuous, not cyclical. The model of the breathing system was empirically matched to the chosen one (a GMS absorber and 7850 ventilator (Datex-Ohmeda)). Predictions for the wash-in of isoflurane and the uptake of desflurane and isoflurane agree well with observed data. The results obtained by continuously checking total gas pressures, calculating mass balances and simulating the measurement of alveolar space by the closed-circuit helium dilution method support the mathematical credibility of the model. It thus merits further exploration

Repeated enflurane anaesthetics and model predictions: a study of the variability in the predictive performance measures

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A System Model for Halothane Closed-Circuit Anesthesia : Structure Considerations and Performance Evaluation

Contains fulltext : 21837___.PDF (publisher's version ) (Open Access

Model-based administration of inhalation anaesthesia. 4. Applying the system model.

Item does not contain fulltextBACKGROUND: We developed and tested a simple dosing strategy for rapid induction with isoflurane followed by maintenance under minimal-flow conditions, that is 0.5 litre min(-1) total fresh gas flow (FGF). An end-expired concentration was to be achieved within 5 min in a desired therapeutic window, that is 0.8-1.1 vol%, and to be maintained within it for at least 30 min. METHODS: With our new model we computed a three-stage regimen using one fixed vaporizer setting: 3 vol% isoflurane in a FGF of 3 and 1.5 litre min(-1), each for 3 min, and 0.5 litre min thereafter. The ratio of nitrous oxide:oxygen was, consecutively, 2:1, 2:1, and 2:3. We evaluated this scheme in 58 adult patients (body mass 74 (SD 13) kg), mostly during eye and ear, nose, and throat surgery. RESULTS: Measured oxygen (33-45 vol%) and nitrous oxide concentrations (66-50 vol%) evolved in accordance with those computed. In five patients with a median of body mass 92 kg (range 76-126 kg), inspired oxygen concentrations decreased to less than 30 vol%. End-expired isoflurane concentration entered the window after 2 min (range 1.0-5.67 min) and attained its maximum, that is 0.96 vol% (0.8-1.2 vol%), after 3.45 min (1.67-6.33 min). The mean end-expired concentration was in the desired window from 3-60 min and an average of 72% of individual measurements were within the window from 3-30 min. The scheme was adapted in six patients (excluded from analysis) because of hypotension. CONCLUSION: The regimen is easily remembered, reliable, and lends itself to alternative strategies, but must be guided by the monitoring of gas and vapour concentrations and haemodynamic variables

Moving beyond pain scores: Multidimensional pain assessment is essential for adequate pain management after surgery

Contains fulltext : 175628.pdf (publisher's version ) (Open Access)BACKGROUND: Clinical experience teaches us that patients are willing to accept postoperative pain, despite high pain intensity scores. Nevertheless, relationships between pain scores and other methods of pain assessment, e.g. acceptability of pain or its interference with physical functioning, are not fully established. Our aims were to examine these relationships. METHODS: A cross-sectional study was conducted on patients who underwent major surgery between January 2008 and August 2013. Using logistic regression, we quantified the relationships between movement-evoked pain scores on the numerical rating scale (NRS-MEP) and three dichotomous dependent variables: patient's opinion on acceptability of pain (PO: acceptable or unacceptable pain); nurses' observation of patient's performance of necessary activities to expedite recovery (NO: good or bad performance); a compound measure judging the presence of the clinically desirable situation of acceptable pain associated with good patients' performance (PONO: present or not). Using Receiver Operating Characteristics (ROC) analysis, NRS cut-off points were determined such that they best discriminate between patients having one versus the other outcome for PO, NO and PONO. RESULTS: 15,394 assessments were obtained in 9,082 patients in the first three postoperative days. Nine percent of the patients had unacceptable pain while having an NRS-MEP of 0-4. An estimated 47% (95%CI = 45%-49%) of patients with an NRS-MEP of 7 described their pain as acceptable on day one. Moreover, 33% (31%-35%) performed all required physical activities, and 22% (21%-24%) combined acceptable pain with appropriate movement. NRS cut-off points for PO, NO and PONO were five, four and four, respectively, but had insufficient discriminatory power. CONCLUSIONS: Our results suggest pain management should be guided by the many dimensions of the patient's pain experience, not solely by NRS cut-off points. Future research should evaluate the impact of such multidimensional pain assessment on patients' functional outcome

Ultrasound-guided needle handling using a guidance positioning system in a phantom

Item does not contain fulltextThe SonixGPS needle guidance positioning system provides navigation assistance to facilitate needle handling during ultrasound-guided procedures. Each of 20 inexperienced nurse anaesthetists performed 12 different ultrasound-guided tasks in a porcine phantom. Using both in-plane and out-of-plane approaches, they inserted a needle and made contact with metal rods at depths of 2, 4 and 6 cm. We compared their performances without and with navigation as paired observations. Using the out-of-plane approach, navigation yielded shorter execution times (26 s vs 14 s, respectively; p = 0.01) and fewer needle repositionings (8 vs 3, respectively; p = 0.001). Using the in-plane approach, the needle was more visible with navigation assistance: 24% vs 52% of execution time, respectively (95% CI: 44%-12%; p = 0.0025). Better needle visibility was associated with shorter execution times and fewer needle repositionings. Combining ultrasound-guided techniques with the needle guidance positioning system may reduce tissue manipulation, thus improving patient comfort and safety