[Functioning of the anaesthetic conserving device: aspects to consider for use in inhalational sedation]

Halving the Volume of AnaConDa: Evaluation of a New Small-Volume Anesthetic Reflector in a Test Lung Model

BACKGROUND

Volatile anesthetics are increasingly used for sedation in intensive care units. The most common administration system is AnaConDa-100 mL (ACD-100; Sedana Medical, Uppsala, Sweden), which reflects volatile anesthetics in open ventilation circuits. AnaConDa-50 mL (ACD-50) is a new device with half the volumetric dead space. Carbon dioxide (CO2) can be retained with both devices. We therefore compared the CO2 elimination and isoflurane reflection efficiency of both devices.

METHODS

A test lung constantly insufflated with CO2 was ventilated with a tidal volume of 500 mL at 10 breaths/min. End-tidal CO2 (EtCO2) partial pressure was measured using 3 different devices: a heat-and-moisture exchanger (HME, 35 mL), ACD-100, and ACD-50 under 4 different experimental conditions: ambient temperature pressure (ATP), body temperature pressure saturated (BTPS) conditions, BTPS with 0.4 Vol% isoflurane (ISO-0.4), and BTPS with 1.2 Vol% isoflurane. Fifty breaths were recorded at 3 time points (n = 150) for each device and each condition. To determine device dead space, we adjusted the tidal volume to maintain normocapnia (n = 3), for each device. Thereafter, we determined reflection efficiency by measuring isoflurane concentrations at infusion rates varying from 0.5 to 20 mL/h (n = 3), for each device.

RESULTS

EtCO2 was consistently greater with ACD-100 than with ACD-50 and HME (ISO-0.4, mean ± standard deviations: ACD-100, 52.4 ± 0.8; ACD-50, 44.4 ± 0.8; HME, 40.1 ± 0.4 mm Hg; differences of means of EtCO2 [respective 95% confidence intervals]: ACD-100 - ACD-50, 8.0 [7.9-8.1] mm Hg, P < .001; ACD-100 - HME, 12.3 [12.2-12.4] mm Hg, P < .001; ACD-50 - HME, 4.3 [4.2-4.3] mm Hg, P < .001). It was greatest under ATP, less under BTPS, and least with ISO-0.4 and BTPS with 1.2 Vol% isoflurane. In addition to the 100 or 50 mL "volumetric dead space" of each AnaConDa, "reflective dead space" was 40 mL with ACD-100 and 25 mL with ACD-50 when using isoflurane. Isoflurane reflection was highest under ATP. Under BTPS with CO2 insufflation and isoflurane concentrations around 0.4 Vol%, reflection efficiency was 93% with ACD-100 and 80% with ACD-50.

CONCLUSIONS

Isoflurane reflection remained sufficient with the ACD-50 at clinical anesthetic concentrations, while CO2 elimination was improved. The ACD-50 should be practical for tidal volumes as low as 200 mL, allowing lung-protective ventilation even in small patients.

[New technical developments for inhaled sedation]

The circle system has been in use for more than 100 years, whereas the first clinical application of an anaesthetic reflector was reported just 15 years ago. In the circle system, all breathing gas is rebreathed after carbon dioxide absorption. A reflector, on the other hand, with the breathing gas flowing to and fro, specifically retains the anaesthetic during expiration and resupplies it during the next inspiration. A high reflection efficiency (number of molecules resupplied/number of molecules exhaled, RE 80-90%) decreases consumption. In analogy to the fresh gas flow of a circle system, pulmonary clearance ((1-RE) × minute ventilation) defines the opposition between consumption and control of the concentration.It was not until reflection systems became available that volatile anaesthetics were used routinely in some intensive care units. Their advantages, such as easy handling, and better ventilatory capabilities of intensive care versus anaesthesia ventilators, were basic preconditions for this. Apart from AnaConDa™ (Sedana Medical, Uppsala, Sweden), the new MIRUS™ system (Pall Medical, Dreieich, Germany) represents a second, more sophisticated commercially available system.Organ protective effects, excellent control of sedation, and dose-dependent deep sedation while preserving spontaneous breathing with hardly any accumulation or induction of tolerance, make volatile anaesthetics an interesting alternative, especially for patients needing deep sedation or when intravenous drugs are no longer efficacious.But obviously, the outcome is most important. We know that deep intravenous sedation increases mortality, whereas inhalational sedation could prove beneficial. We now need prospective clinical trials examining mortality, but also the psychological outcome of those most critically ill patients sedated by inhalation or intravenously.

Uncontrolled delivery of liquid volatile anaesthetic when using the anaesthetic conserving device

During patient sedation with liquid volatile anaesthetic, some problems may occur through a process called auto-pumping, defined as an expansion of bubbles inside the syringe, which can lead to uncontrolled anaesthetic delivery. The study examined how the temperature of liquid volatile anaesthetics (sevoflurane and isoflurane) and the presence of gas bubbles in the syringe affect the occurrence of auto-pumping when using the anaesthetic conserving device (ACD, AnaConDa™, Sedana Medical, Uppsala, Sweden). Four different circumstances for each volatile anaesthetic were tested with a bench study: volatile anaesthetic at room temperature or precooled with and without the presence of gas bubbles in the syringe. Liquid volatile anaesthetic was infused into the ACD via a syringe pump at a fixed rate and heated gradually until the temperature of the syringe surface reached 50 °C. A main-stream gas monitor was used to measure the expired fraction of volatile anaesthetic (F_E vol%). The occurrence of auto-pumping was observed only in the subgroups containing gas bubbles, with both anaesthetics. In these subgroups, the values of the expired anaesthetic gas fraction increased dramatically with the expansion of gas bubbles in the syringe (ΔF_E ranged from +1.6 to 2.4 vol% for sevoflurane and +2.3 to 3.4 vol% for isoflurane). Furthermore, when the heat source was removed, a substantial decline in anaesthetic agent values below the baseline was observed with both anaesthetics. The presence of gas bubbles in the syringe, especially when exposed to a heat source, may provoke auto-pumping with uncontrolled excessive anaesthetic delivery. If auto-pumping is suspected, the syringe pump must be stopped and the ACD removed from the breathing circuit at once.

Survival after long-term isoflurane sedation as opposed to intravenous sedation in critically ill surgical patients: Retrospective analysis

BACKGROUND

Isoflurane has shown better control of intensive care sedation than propofol or midazolam and seems to be a useful alternative. However, its effect on survival remains unclear.

OBJECTIVE

The objective of this study is to compare mortality after sedation with either isoflurane or propofol/midazolam.

DESIGN

A retrospective analysis of data in a hospital database for a cohort of consecutive patients.

SETTING

Sixteen-bed interdisciplinary surgical ICU of a German university hospital.

PATIENTS

Consecutive cohort of 369 critically ill surgical patients defined within the database of the hospital information system. All patients were continuously ventilated and sedated for more than 96 h between 1 January 2005 and 31 December 2010. After excluding 169 patients (93 >79 years old, 10 <40 years old, 46 mixed sedation, 20 lost to follow-up), 200 patients were studied, 72 after isoflurane and 128 after propofol/midazolam.

INTERVENTIONS

Sedation with isoflurane using the AnaConDa system compared with intravenous sedation with propofol or midazolam.

MAIN OUTCOME MEASURES

Hospital mortality (primary) and 365-day mortality (secondary) were compared with the Kaplan-Meier analysis and a log-rank test. Adjusted odds ratios (ORs) [with 95% confidence interval (95% CI)] were calculated by logistic regression analyses to determine the risk of death after isoflurane sedation.

RESULTS

After sedation with isoflurane, the in-hospital mortality and 365-day mortality were significantly lower than after propofol/midazolam sedation: 40 versus 63% (P = 0.005) and 50 versus 70% (P = 0.013), respectively. After adjustment for potential confounders (coronary heart disease, chronic obstructive pulmonary disease, acute renal failure, creatinine, age and Simplified Acute Physiology Score II), patients after isoflurane were at a lower risk of death during their hospital stay (OR 0.35; 95% CI 0.18 to 0.68, P = 0.002) and within the first 365 days (OR 0.41; 95% CI 0.21 to 0.81, P = 0.010).

CONCLUSION

Compared with propofol/midazolam sedation, long-term sedation with isoflurane seems to be well tolerated in this group of critically ill patients after surgery.

A novel device for target controlled administration and reflection of desflurane--the Mirus™

The Anaconda™ system is used to deliver inhalational sedation in the intensive care unit in mainland Europe. The new Mirus™ system also uses a reflector like the Anaconda; however, it also identifies end-tidal concentrations from the gas flow, injects anaesthetics during early inspiration, controls anaesthetic concentrations automatically, and can be used with desflurane, which is not possible using the Anaconda. We tested the Mirus with desflurane in the laboratory. Compared with an external gas monitor, the bias (two standard deviations) of the end-tidal concentration was 0.11 (0.29)% volume. In addition, automatic control was reasonable and maximum concentration delivered was 10.2%, which was deemed to be sufficient for clinical use. Efficiency was > 80% and was also deemed to be acceptable, but only when delivering a low concentration of desflurane (≤ 1.8%). By modifying the reflector, we improved efficiency up to a concentration of 3.6%. The Mirus appears to be a promising new device for long-term sedation with desflurane on the intensive care unit, but efficiency must be improved before routine clinical use becomes affordable.

Efficiency and safety of inhalative sedation with sevoflurane in comparison to an intravenous sedation concept with propofol in intensive care patients: study protocol for a randomized controlled trial

Background

State of the art sedation concepts on intensive care units (ICU) favor propofol for a time period of up to 72 h and midazolam for long-term sedation. However, intravenous sedation is associated with complications such as development of tolerance, insufficient sedation quality, gastrointestinal paralysis, and withdrawal symptoms including cognitive deficits. Therefore, we aimed to investigate whether sevoflurane as a volatile anesthetic technically implemented by the anesthetic-conserving device (ACD) may provide advantages regarding ‘weaning time’, efficiency, and patient’s safety when compared to standard intravenous sedation employing propofol.

Method/Design

This currently ongoing trial is designed as a two-armed, monocentric, randomized prospective phase II study including intubated intensive care patients with an expected necessity for sedation exceeding 48 h. Patients are randomly assigned to either receive intravenous sedation with propofol or sevoflurane employing the ACD. Primary endpoint is the comparison of the ‘weaning time’ defined as the time required from discontinuation of the sedating agent until sufficient spontaneous breathing occurs. Moreover, sedation depth evaluated by Richmond Agitation Sedation Scale and parameters of patient’s safety (that is, vital signs, laboratory monitoring of organ function) as well as the duration of mechanical ventilation and overall stay on the ICU are analyzed and compared. An intention-to-treat analysis will be carried out with all patients for whom it will be possible to define a wake-up time. In addition, a per-protocol analysis is envisaged. Completion of patient recruitment is expected by the end of 2012.

Discussion

This clinical study is designed to evaluate the impact of sevoflurane during long-term sedation of critically ill patients on ‘weaning time’, efficiency, and patient’s safety compared to the standard intravenous sedation concept employing propofol.

Trial registration

EudraCT2007-006087-30; ISCRTN90609144