Volatile anesthetic agents have been used in the intensive care unit (ICU) for over 30 years.1,2 Their application as sedatives has been limited to patients suffering from severe reactive airway disease or status epilepticus.3-8 There has been a renewed interest in the use of volatile-based sedation within critical care environments given the potential benefits of shorter ventilation time, better pulmonary ventilation/perfusion matching, an improved sedation profile with precise control of dosing, organ protective properties (pre-and post-conditioning), and minimal systemic metabolism with lack of drug accumulation and sedative effects after administration is discontinued. Nevertheless, the use of volatile sedation has been practically limited by the requirement of a bulky expensive anesthetic machine, appropriate scavenging, and the need for the constant presence of an anesthesiologist.

Many of these issues have been overcome with the introduction of the Anesthetic Conserving Device (AnaConDa®, Sedana Medica, Sweden). This highly efficient lightweight and portable mini-vaporizer is placed between the breathing circuit and endotracheal tube and can be used with any type of ICU ventilator. The device is simple to use, and sedation can be easily monitored and titrated by the nursing staff.1 However, use of volatile agents requires scavenging to reduce ambient room contamination and staff exposure to trace amounts of volatile anesthetics. Scavenging systems within critical care environments are uncommon, which has been a major barrier to the use of these agents in this setting. Current Ontario Ministry of Labour guidelines based on the Occupational Health and Safety Act state that levels of halogenated volatile agents in the working environment should not exceed 2 parts per million (ppm).9

The aim of this study was to assess the efficacy of a simple scavenging system consisting of two Deltasorb® canisters attached in series to an ICU ventilator and used in conjunction with the AnaConDa device. Additionally, we assessed the concentration of postoperatively administered volatile anesthetic agents to which cardiac surgical patients and the local environment were exposed.

Methods

We obtained approval for our study from Health Canada and from the Research Ethics Board of the University Health Network after performing an initial safety study to meet their requirements. We then conducted a large randomized controlled trial comparing the use of propofol vs volatile-based anesthesia for postoperative sedation in cardiac surgical patients. This study was registered as a clinical trial (NCT01151254) and the results have been recently reported.10,11 In order to undertake this large randomized controlled trial, we were required to perform an initial safety study to meet the requirements for Health Canada and Research Ethics Board of University Health Network. All patients recruited within this study provided written informed consent. Patients scheduled for elective aortocoronary bypass grafting with grade 1 or 2 left ventricular systolic function were included in the study. Patients were excluded if they had a history of malignant hyperthermia, propofol infusion syndrome, severe kidney or liver dysfunction, or if they required sedation in excess of 14 hr or chest re-exploration. To assess the safety of volatile-based sedation, the first ten patients randomized to the volatile group were selected to participate within this sub-study. Reporting the data from our initial ten patients did not comprise the overall blinding of procedures. These patients received intraoperative volatile anesthesia and postoperative sedation within the cardiovascular ICU using the same agent with the AnaConDa device. Either sevoflurane or isoflurane was chosen at the discretion of the attending anesthesiologist (Table 1).

Table 1 Concentration of volatile agents measured in parts per million (ppm) at four points within the breathing circuit and patient room
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The AnaConDa device is a combined miniature vaporizer, microbial filter, and heat-moisture exchanger (Fig. 1a and 1b). The volatile agent is infused into the device by a syringe pump at a rate of 0.5-30 mL·hr−1 depending on the needs and requirements of the patient. The volatile agent evaporates within the device, which is placed between the endotracheal tube and the Y-piece of the breathing circuit (Fig. 2). The expired gas is resorbed and recycled on a carbon reflection layer. The overall efficiency of the AnaConDa device exceeds 90%, which accounts for the small infusion volumes required.2 The device has a dead space of 100 mL but a large internal surface area (Fig. 1b). This facilitates both rapid volatile vaporization and changes in concentration in response to the infusion rate which allow for quick changes in the depth of sedation. AnaConDa has an additional sampling port that allows breath-by-breath measurement of end-tidal concentration of volatile agents and expired gases. In our study, we used an A3 Anesthesia Gas Monitor® (Datex-Ohmeda, Madison, WI, USA). Postoperatively, patients were sedated to an end-tidal concentration of 0.1-0.3 minimum anesthetic concentration (MAC), aiming for a sedation score of 2-3 using the Sedation Agitation Scale.12 However, anesthetic doses of 0.6 MAC where used if necessary to manage inadequate sedation.

Fig. 1
figure 1

(a) AnaConDa® is a lightweight device placed between the ventilator circuit Y-piece and the endotracheal tube. The volatile agent is infused into the device via a syringe drive. A gas sampling port allows for continuous end-tidal agent and gas monitoring. (b) The volatile agent is evaporated within the device. The activated carbon layer recycles expired gas, which accounts for its high efficiency. AnaConDa possesses an in-built humidification/filter layer thus requiring no external device within the ventilator circuit. Illustrations are courtesy of J. Crossingham

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Fig. 2
figure 2

Shows equipment setup, volatile agent delivery, gas monitoring, and scavenging system. Volatile gas concentrations are measured at the end-expiratory port (1), post 1st Deltasorb (2), post 2nd Deltasorb (3), and near the patient’s head (4). Illustration is courtesy of J. Crossingham

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The scavenging system developed within our institution consists of two Deltasorb canisters (Blue-Zone, Concord, ON, Canada) assembled in series to an ICU ventilator (Nellcor Puritan Bennett 760, Vancouver, BC, Canada) (Fig. 3a). The proximal end is attached to the expiratory port of the ventilator, and the distal end is attached to the central scavenging port. The reservoir bag was installed after the second Deltasorb canister in order to avoid overpressurizing the system or applying excessive vacuum (Fig 3B). Each Deltasorb canister consists of a hydrophobic crystalline matrix of silica zeolite (Deltazite®).13,14 Deltazite adsorbent is an inert non-reactive inorganic high-silica-based filter with a unique uniform pore lattice structure that selectively captures volatile agents from the gas mixture passing through the Deltasorb canister. This compound acts as a molecular sieve and selectively adsorbs isoflurane, sevoflurane, and desflurane. The Deltasorb canister is a portable non-pressurized non-hazardous self-contained SS 316 cylinder with individually identified serial numbers for tracking and traceability. It is a passive device that contains proprietary Deltazite adsorbent. It is known to adsorb selectively by isolating diluted halogenated volatile inhalation anesthetics from anesthesia machine scavenging systems.13,14 Deltazite compound allows desorption of volatile agents that could potentially be recycled. After its use, the Deltasorb canister with its exhausted proprietary adsorbent matrix is exchanged by Blue-Zone Technologies and subsequently regenerated by their proprietary process where halogenated hydrocarbons are simultaneously extracted from the Deltazite.

Fig. 3
figure 3

(a) Scavenging system created with two Deltasorb® canisters connected in series. (b) Photograph shows a reservoir bag, which prevents the system from being overpressurized or applying an excessive vacuum

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A multigas infrared vapour analyzer (InfraRan Specific Vapor Analyzer, Wilkins Enterprise Inc., Worcester, MA, USA) was used to measure volatile concentrations within the scavenging system and in the patient’s room. The analyzer uses an infrared calibrated photometric beam to measure volatile concentrations in real time with values displayed in ppm. The operator determines which gas is to be analyzed and the machine is calibrated prior to performing each measurement. One hour after admission to the cardiovascular ICU, the concentration of anesthetic gas was measured using the analyzer at four locations; the expiratory limb of the ventilator, post 1st Deltasorb, post 2nd Deltasorb, and around the patient’s head (Fig. 2). Gas concentrations are recorded as ppm, and the average concentration at each location is reported as mean (standard deviation). In addition, we recorded any technical difficulties of the scavenging system or AnaConDa device until the patient’s trachea was extubated.

Results

Six hundred eighty-eight patients were assessed for study eligibility and 176 patients gave written informed consent. Seventy-nine patients received volatile-based anesthesia and sedation. The first ten patients from this group were included within this sub-study. Recruitment was performed over two years commencing from September 2009 to August 2011. The concentration of volatile anesthetic measured at the four points is presented in Table 1. The mean concentrations at the expiratory limb, post 1st Deltasorb, post 2nd Deltasorb, and around the patient’s head were 8.2 ppm, 3.2 ppm, 2.1 ppm, and 0.4 ppm, respectively. No readings from within the patient’s room exceeded 1 ppm. We did not encounter any technical problems with the device or scavenging system during the study period.

Discussion

The results of this study show that volatile anesthetics can be administered for ICU sedation with minimal contamination within critical care environments when using the AnaConDa device in conjunction with a standard ICU ventilator and the Deltasorb scavenging system developed within our institution.

The risk of adverse events with chronic exposure to volatile agents has been debated within the literature with conflicting reports. Older studies have suggested that exposure to volatile anesthetics has potential harmful effects, including a greater risk of infertility and spontaneous abortion.15-17 More recently, a prospective trial has failed to show any association between adverse outcomes and exposure to anesthetic agents in operating rooms independent of the presence of scavenging18,19; however, many countries have recommended maximal allowable time exposure limits (Table 2). Our scavenging system used in conjunction with the AnaConDa device reduced the level of environmental contamination, which is acceptable to most Western countries and thus conforms to international safety standards (Table 2).9,20,21

Table 2 Accepted limits for occupational exposure to volatile anesthetic agents, as a time weighted average in ppm
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The AnaConDa device has been approved for use in the European Union since 2004. Many studies have shown the AnaConDa to be a highly efficient, safe, cost effective, and simple device to use.1,2,22 There are limited data assessing ambient room contamination, and the optimal scavenging system remains unknown. In accordance with guidance from the manufacturers of AnaConDa and the Canadian Centre for Occupational Health and Safety which governs workplace environmental exposure limits, we used a combined two Deltasorb active scavenging system in conjunction with nine room air exchanges per hour. However, in a recent case series by Marbini et al., they assessed the degree of ICU environmental pollution with the AnaConDa device using isoflurane with only ten room air exchanges per hour and no additional active scavenging.23 The authors showed peak isoflurane levels of 50 ppm at the expired port, which fell to less than 2 ppm around the patient’s head and the predicted nursing area. A second study by Sackey et al. in 15 patients during ICU sedation for 12-96 hr looked at ambient isoflurane pollution and consumption utilizing active scavenging and four room air exchanges per hour.2 Their scavenging system used a simpler ejector suction device that connected the ventilator to the hospital waste gas wall outlet system. Five patients within this series had no active scavenging and relied on room air exchanges alone. The mean levels of isoflurane pollution were less than 0.5 ppm in all patients. The full impact and requirement of each modality and the minimum number of air exchanges to reduce environmental contamination remain to be investigated.

There are several limitations to this study, including the small sample size and the delivered concentrations which rarely exceeded 0.1-0.3 MAC; however, the data are largely uniform with no significant outliers. Measurements were performed at one hour after ICU admission, as we anticipated this to be an adequate time period for volatile agents to reach equilibrium between the breathing circuit and the environment, and most cardiac surgical patients are also extubated within two to three hours of ICU admission. Thus, it is possible that ambient room contamination may have continued to rise beyond the one hour measurement time period. However, we tested the Deltasorb-based scavenging system within our operating rooms, which normally utilize much higher doses of volatile anesthetics (0.6-2 MAC), and the Deltasorb canister is changed only once per week. Our measurements showed that the regular ambient room concentrations never exceeded the current allowable safety limits (unpublished data). In addition to this scavenging system, there are nine room air exchanges performed each hour.

In summary, our study shows that the simple scavenging system developed within our institution together with the room air exchanges provides an efficient means to reduce ambient room contamination and staff exposure within the ICU when volatile agents are administered with the AnaConDa device. This has important safety implications with residual volatile anesthetic concentrations falling well below current guidelines and thus meeting Canadian and international safety standards. The results of this study will facilitate further research investigating potential benefits of volatile-based sedation within critical care environments.