A new closed-loop control system for isoflurane using bispectral index outperforms manual control.
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BACKGROUND: Automatic control of depth of hypnosis using the Bispectral Index (BIS) can help to reduce phases of inadequate control. Automated BIS control with propofol or isoflurane administration via an infusion system has recently been described, a comparable study with isoflurane administration via a vaporizer had not been conducted yet. Our hypothesis was that our new model based closed-loop control system can safely be applied clinically and maintains the BIS within a defined target range better than manual control. METHODS: Twenty-three patients, American Society of Anesthesiologists risk class I-III, scheduled for decompressive spinal surgery were randomized into groups with either closed-loop or manual control of BIS using isoflurane. An alfentanil target-controlled infusion was adjusted according to standard clinical practice. The BIS target was set to 50 during the operation. The necessity of human intervention in the control system and events of inadequate sedation (BIS <40 or BIS >60) were counted. The number of phases of inadequate control, defined as BIS >/=65 for more than 3 min, were recorded. The performance of the controller was assessed by several indicators (mean absolute deviation and median absolute performance error) and measured during the skin incision phase, the subsequent low flow phase, and the wound closure phase. Recovery profiles of both groups were compared. RESULTS: No human intervention was necessary in the closed-loop control group. The occurrence of inadequate BIS was quantified with the mean and median values of the area under the curve and amounted to 0.360 and 0.088 for the manual control group and 0.049 and 0.017 for the closed-loop control group, respectively. In the manual control group nine phases of inadequate control were recorded, compared with one in the closed-loop control group, 10.3% to 0.5% of all observed anesthesia time. During all phases the averages of the performance parameters (mean absolute deviation and median absolute performance error) were more than 30% smaller in closed-loop control than in manual control (P < 0.05 between groups). CONCLUSIONS: Closed-loop control with BIS using isoflurane can safely be applied clinically and performs significantly better than manual control, even in phases with abrupt changes of stimulation that cannot be foreseen by the control system. (+info)
Time course of inhaled anaesthetic drug delivery using a new multifunctional closed-circuit anaesthesia ventilator. In vitro comparison with a classical anaesthesia machine.
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BACKGROUND: The aim of this study was to detail the time-course, defined as the changes in end-tidal drug concentration with time, and consumption of inhaled anaesthetics when using a multifunctional closed-circuit anaesthesia machine in various drug delivery modes, and to compare it with a classical anaesthesia machine using an out-of-circle vaporizer under high and low fresh gas flow conditions. METHODS: Using an artificial test lung, sevoflurane and desflurane time-course and consumption were compared when using the Zeus apparatus (Drager, Lubeck, Germany) with direct injection of inhaled anaesthetics or the Primus apparatus (Drager, Lubeck, Germany) using a classical out-of-circle vaporizer. Anaesthetics were targeted at 1 and 2 MAC end-tidal during 15 min. For both apparatus, out-of-circle high and low fresh gas control (FGC) and for Zeus, auto-control (AC) modes (fixed fresh gas flow at 6 and 1 litre min(-1) and uptake mode) were compared. Time to reach target, initial overshoot and stability at target, and wash-out times were compared. RESULTS: In FGC, an initial overshoot in end-tidal drug concentration is seen when using 6 litre min(-1) fresh gas flow and a slower time course is observed when using only 1 litre min(-1) in both apparatus. In auto-control mode, the time course of both sevoflurane and desflurane was very fast and not influenced by the changes in fresh gas flow. No overshoot at target was seen. At all settings, the wash-out times were faster when using Zeus than Primus. Inhaled anaesthetic consumption was lowest with the Zeus ventilator in uptake AC mode. CONCLUSION: A combination of the fastest time course and lowest consumption of sevoflurane and desflurane was found when using the Zeus apparatus in AC uptake mode. (+info)
Randomized comparison of three methods of induction of anaesthesia with sevoflurane.
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BACKGROUND: Rebreathing will occur if a low gas flow and a Mapleson D circuit are used to induce anaesthesia with a volatile anaesthetic agent. This has the advantage that it allows ventilation to be sustained when consciousness is lost, and specific manoeuvres such as breath-holding or vital capacity breaths are not needed to facilitate induction of anaesthesia. However, if the fresh gas flow were too small, this would slow induction by limiting the rate of delivery of the anaesthetic agent. To assess the impact of fresh gas flow and rebreathing, we compared induction using sevoflurane 8% given by three different methods. METHODS: We randomly allocated 65 patients to receive induction of anaesthesia from either a Mapleson A breathing system with a fresh gas flow of 9 litre min(-1) (group A9), a Mapleson D system with a fresh flow of 6 litre min(-1) (group D6) or a Mapleson D system with a fresh flow of 3 litre min(-1) (group D3). We measured times for induction, end-tidal sevoflurane and end-tidal carbon dioxide. RESULTS: The median (quartiles) induction times were 58 (45, 72), 50 (42, 65) and 64 (52, 92) s in the groups A9, D6 and D3 respectively. Induction of anaesthesia took longer (P<0.01) and was more variable in group D3. In this group, end-tidal sevoflurane concentration at the time of induction of anaesthesia was lower (P<0.05). In group A9, end-tidal carbon dioxide was less (P<0.05). CONCLUSIONS: In adult patients allowed to breathe normally, prompt and consistent inhalation induction of anaesthesia with sevoflurane is obtained when fresh gas flow is limited to 6 litre min(-1) from a Mapleson D circuit, but smaller flows are impractical. (+info)
Effects of temperature gradient correction of carbon dioxide absorbent on carbon dioxide absorption.
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BACKGROUND: The effects of temperature gradients in CO(2) absorbents on water content and CO(2) absorption are not clear. We constructed a novel temperature gradient correction (TGC) canister, and investigated the effects of temperature gradient correction on the water content and longevity (time to exhaustion) of CO(2) absorbent using a simulated anaesthesia circuit. METHODS: Experiments were divided into two groups according to the type of canister used: the TGC canister (n=6) or the conventional canister (n=6). One kilogram of fresh CO(2) absorbent was placed into the canister. The anaesthetic ventilator was connected to a 3 litre bag and 300 ml min(-1) of CO(2) was introduced. Oxygen (500 ml min(-1)) was used as fresh gas. The anaesthetic ventilator was set at a ventilatory frequency of 12 bpm, and tidal volume was adjusted to 700 ml. RESULTS: Before the experiment, the water content of the fresh CO(2) absorbent in the conventional canister and TGC canister was 16.1 (0.9)% and 15.7 (1.1)%, respectively. After the experiment, the water content of CO(2) absorbent near the upper outer rim of the canister increased to 32.4 (0.7)% in the conventional canister, but increased to only 20.6 (1.3)% in the TGC canister (P<0.01). The longevity of CO(2) absorbent in the conventional canister and TGC canister was 434 (9) min and 563 (13) min (P<0.01). CONCLUSIONS: Temperature gradient correction prevented a local excessive increase in water content and improved the longevity of CO(2) absorbent. (+info)
Closed-loop control of mean arterial blood pressure during surgery with alfentanil: clinical evaluation of a novel model-based predictive controller.
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BACKGROUND: In contrast to hypnosis, there is no surrogate parameter for analgesia in anesthetized patients. Opioids are titrated to suppress blood pressure response to noxious stimulation. The authors evaluated a novel model predictive controller for closed-loop administration of alfentanil using mean arterial blood pressure and predicted plasma alfentanil concentration (Cp Alf) as input parameters. METHODS: The authors studied 13 healthy patients scheduled to undergo minor lumbar and cervical spine surgery. After induction with propofol, alfentanil, and mivacurium and tracheal intubation, isoflurane was titrated to maintain the Bispectral Index at 55 (+/- 5), and the alfentanil administration was switched from manual to closed-loop control. The controller adjusted the alfentanil infusion rate to maintain the mean arterial blood pressure near the set-point (70 mmHg) while minimizing the Cp Alf toward the set-point plasma alfentanil concentration (Cp Alfref) (100 ng/ml). RESULTS: Two patients were excluded because of loss of arterial pressure signal and protocol violation. The alfentanil infusion was closed-loop controlled for a mean (SD) of 98.9 (1.5)% of presurgery time and 95.5 (4.3)% of surgery time. The mean (SD) end-tidal isoflurane concentrations were 0.78 (0.1) and 0.86 (0.1) vol%, the Cp Alf values were 122 (35) and 181 (58) ng/ml, and the Bispectral Index values were 51 (9) and 52 (4) before surgery and during surgery, respectively. The mean (SD) absolute deviations of mean arterial blood pressure were 7.6 (2.6) and 10.0 (4.2) mmHg (P = 0.262), and the median performance error, median absolute performance error, and wobble were 4.2 (6.2) and 8.8 (9.4)% (P = 0.002), 7.9 (3.8) and 11.8 (6.3)% (P = 0.129), and 14.5 (8.4) and 5.7 (1.2)% (P = 0.002) before surgery and during surgery, respectively. A post hoc simulation showed that the Cp Alfref decreased the predicted Cp Alf compared with mean arterial blood pressure alone. CONCLUSION: The authors' controller has a similar set-point precision as previous hypnotic controllers and provides adequate alfentanil dosing during surgery. It may help to standardize opioid dosing in research and may be a further step toward a multiple input-multiple output controller. (+info)
Non-invasive metabolic monitoring of patients under anaesthesia by continuous indirect calorimetry--an in vivo trial of a new method.
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BACKGROUND: Oxygen uptake is an important form of metabolic monitoring for patients under anaesthesia. In critically ill patients oxygen uptake has been shown to provide valuable clinical information in directed therapy and acts as a useful monitor of cardiovascular dysfunction. A new method of continuous real time monitoring of metabolic gas exchange was tested in patients during anaesthesia. METHODS: Using a standard anaesthetic machine with attached semi-closed circle absorber system, oxygen uptake was measured continuously throughout surgery in 30 patients undergoing cardiopulmonary bypass surgery and compared with paired measurements made with the reverse Fick method. The method is an indirect calorimetry technique which uses fresh gas rotameters for control, regulation and measurement of the gas flows into the system, with continuous sampling of mixed exhaust gas. RESULTS: When compared with the reverse Fick method the oxygen uptake showed a mean difference (and sd) of 20.7 ml min(-1) or 12.1% (25.3 ml min(-1)) pre-bypass and 13.9 ml min(-1) or 8.1% (27.0 ml min(-1)) post-bypass. This bias is consistent with previous studies comparing oxygen uptake measured at the mouth against oxygen uptake by reverse Fick, which have shown a difference of approximately 10-15% accounted for by the consumption of oxygen by lung tissue. CONCLUSIONS: As the method allows continuous measurement of gas exchange and can be adapted to a modern anaesthetic workstation it is an attractive method for use in clinical setting. (+info)
Effects of carbon dioxide absorbent cooling and temperature gradient reduction on water condensation in the anaesthesia circuit.
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BACKGROUND: Large quantities of water condensation occur in the anaesthesia circuit during low-flow anaesthesia. We hypothesized that cooling of the CO2 absorbent would prevent water condensation. METHODS: To cool CO2 absorbent efficiently, we constructed a novel temperature gradient reduction (TGR) canister, which was cooled by a blower. Experiments were divided into three groups: the conventional canister group (control group, n=6), the TGR canister without cooling group (TGR group, n=6), and the TGR canister with cooling group (TGR cooling group, n=6). One kilogramme of CO2 absorbent was placed into the canister. The anaesthetic ventilator was connected to a 3 litre bag and 300 ml min(-1) of CO2 was introduced. About 500 ml min(-1) of oxygen was used as fresh gas. The anaesthetic ventilator was set at a ventilatory frequency of 12 bpm, and tidal volume was adjusted to 700 ml. RESULTS: The longevity of the CO2 absorbent was 437 (sd 7.8) min in the control group, 564 (13.8) min in the TGR group (P<0.001 vs control), and 501 (5.8) min in the TGR cooling group (P<0.001 vs control, TGR). Total water condensation in the anaesthesia circuit was 215 (9.4) mg min(-1) in the control group, 223 (9.7) mg min(-1) in the TGR group, and 47.7 (5.7) mg min(-1) in the TGR cooling group (P<0.001 vs control, TGR). CONCLUSIONS: TGR of CO2 absorbent with cooling is a useful and simple method to reduce water condensation in the anaesthesia circuit in low-flow anaesthesia, with a little increase in the longevity of the CO2 absorbent. (+info)
Fentanyl decreases end-expiratory lung volume in patients anaesthetized with sevoflurane.
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