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Ambulatory Anesthesia: Technical Communication

The Effect of the Double Mask on Anesthetic Waste Gas Levels During Pediatric Mask Inductions in Dental Offices

Kurrek, Matt M., MD*; Dain, Steven L., MD; Kiss, Alexander, PhD

Author Information
doi: 10.1213/ANE.0b013e318290044e

Ambulatory anesthesia outside the hospital has experienced a rapid growth over the last decade, and it is estimated that currently >55% of ambulatory procedures in the United States are done in freestanding facilities.1 A significant portion of office-based general anesthesia (GA) for pediatric patients is performed in dental offices and involves mask inductions with inhaled drugs because anesthesia providers attempt to alleviate anxiety from needle sticks among both children and their parents.

Studies have linked high anesthetic waste gas levels to adverse health effects, especially reproductive health.2,3 To provide safe workplace conditions, the National Institute for Occupational Health and Safety has recommended relative exposure limits to be 25 parts per million (ppm) measured as time-weighted average for nitrous oxide (N2O), and also that no worker should be exposed at ceiling concentrations >2 ppm of any volatile anesthetic drug over a sampling period not to exceed 1 hour.a

Hospital operating rooms (ORs) are generally constructed to conform to so-called heating ventilation and air conditioning (HVAC) standards4 and are equipped with an augmented number of air exchanges that mitigate the accumulation of inhaled anesthetic drugs. Dental offices, however, are not constructed to hospital OR standards, and this could lead to an accumulation of anesthetic waste gases during administration of GA.

The administration of inhaled anesthetic drugs via facemasks (especially in high concentrations required for mask inductions of pediatric patients) can lead to significant pollution with waste gases in ORs.5 After a series of experiments with different local scavenging systems mounted at the head of the OR table, Reiz et al.6 from the University of Umeå in Sweden developed a special “double mask” that consisted of a second, slightly larger mask mounted outside on top of the actual anesthetic facemask (Fig. 1). This second mask was connected to a high-flow suction and was able to locally scavenge the gas leaking from the primary facemask underneath. This system proved to be very effective in an OR with approximately 15 air exchanges per hour as long as it was used with high-flow suction applied to the second mask (special fans producing 35 m3/h). The double-mask system, after receiving regulatory approval by Health Canada, became commercially available in 2011, but its efficiency has not been studied in dental office operatories where the air exchanges are not augmented and where each of the available suctions (high-volume evacuator) produces approximately 12 m3/h.b

Figure 1
Figure 1:
The Double Mask—Adapted with permission from Medicvent AB, Umeå, Sweden and Canadian Medical Ventilation.

Our aim was to determine anesthetic drug concentrations in dental offices during inhaled induction of anesthesia in children before and after the introduction of a scavenging double-mask system.


The study was approved by the Scarborough Hospital Research Ethics Board, and the need for a written informed consent was waived. Inhaled anesthetics were measured, as required by professional guidelines, during 2010 and 2011 in all 9 freestanding office dental facilities (standard office ventilation) in the Greater Toronto Area that required GA with mask induction. A double-mask system (Medicvent AB, Umeå, Sweden) was purchased in November 2011, and each of the 9 office-based dental facilities then had follow-up measurements done during similar cases but this time with the use of the double mask (using 2 high-volume dental suctions to provide a total scavenging flow of approximately 24 m3/h to the outer mask). All locations had functional anesthesia machine scavenging (vented outside of the building) and certified, leak-tested anesthesia machines. The costs of the sample measurement and the purchase of the double masks were funded by the operating budget of the anesthesia practice.

All cases were handled by the same anesthesiologist and consisted of mask induction with up to 8% sevoflurane in 50% N2O/O2, followed by nasotracheal intubation with an uncuffed tube, and maintenance with volatile drugs (50% N2O/O2). Patients were tracheally extubated under deep anesthesia.

Sample measurements were obtained and analyzed by a commercial laboratory (Pinchin Environmental, Mississauga, Ontario, Canada) through gas chromatography–electron capture detector, according to the modified Occupational Safety and Health Administration Method 166 (N2O), or through gas chromatography–flame ionization detector, according to the modified Occupational Safety and Health Administration Method 103 (volatile anesthetics). Badges were worn first thing in the morning of the study day by the anesthesiologist (for approximately 2 hours and 30 minutes) at chest height and then immediately transported to the laboratory. Each office was measured once on 2 separate days (one day using the conventional mask and then on another day using the double mask). After analysis, the laboratory calculated time-weighted averages and reported levels above 2 ppm (for volatile anesthetics) and above 25 ppm (for N2O) as elevated.

Descriptive statistics were calculated for all variables of interest. Continuous measures were summarized using median and interquartile ranges (IQRs) whereas categorical measures were summarized using counts. Comparisons between median N2O levels and median sevoflurane levels were performed using the Mann-Whitney test, and the upper 95% confidence limit on the percentage of offices that did not exceed the threshold value was calculated according to the Clopper-Pearson exact method for binomial proportions. A P-value of <0.05 was considered statistically significant. All statistical calculations were performed using SAS version 9.1 (SAS Institute, Cary, NC).


Anesthetic Gas Levels with Standard, Conventional Single Mask

The levels of N2O (Fig. 2) were above 25 ppm in 6 of the 9 office-based pediatric GAs (median N2O level: 40.0 ppm, IQR = 23.0–46.0 ppm, n = 9) and the levels of sevoflurane (Fig. 3) were above 2 ppm in all 9 of the office-based pediatric GAs (median sevoflurane level: 4.60 ppm, IQR = 3.10–7.00 ppm, n = 9).

Figure 2
Figure 2:
All levels of nitrous oxide decreased and 0% of offices exceeded the limit of 25 parts per million (ppm) (95% upper confidence limit of 34%) using the double mask (median nitrous oxide level with double mask: 3.0 ppm, interquartile range = 2.3–4.7 ppm,n = 9).
Figure 3
Figure 3:
All levels of sevoflurane decreased and 0% of offices exceeded the limit of 2 parts per million (ppm) (95% upper confidence limit of 34%) using the double mask (median sevoflurane level: 0 ppm, interquartile range = 0–0.39 ppm,n = 9).

Anesthetic Gas Levels with Use of the Double Mask

All levels of N2O (Fig. 2) decreased, and 0% of offices exceeded the limit of 25 ppm (95% upper confidence limit of 34%) using the double mask (median N2O level with double mask: 3.0 ppm, IQR = 2.3–4.7 ppm, n = 9, P = 0.0055) and all levels of sevoflurane (Fig. 3) decreased and 0% of offices exceeded the limit of 2 ppm (95% upper confidence limit of 34%) using the double mask (median sevoflurane level: 0 ppm, IQR = 0–0.39 ppm, n = 9, P =0.0024).


Currently, many small surgical interventions continue to migrate from hospitals into office-based facilities, and this trend will likely increase in the years to come. There are a number of reasons for this trend, including greater patient and provider satisfaction, but it is ultimately perceived cost savings that has led to such an acceleration. It is simply more cost efficient to operate a small office-based facility compared with a large, full-scale hospital (with its attached emergency room, intensive care unit etc.), and several studies have now demonstrated that the risks of adverse events tend to be comparable between hospital-based ambulatory surgery and cases performed in freestanding ambulatory surgicenters and office-based facilities.7–10

The provision of dental care for very young children, especially with developmental disabilities or special needs often requires GA. Although it is possible to do an IV induction followed by total IV anesthesia after sedation with oral midazolam, this is not popular among many anesthesiologists because of associated delays in recovery and the increase in arousal distress.11 For this reason, a mask induction with inhaled anesthetics without oral premedication is frequently chosen for young pediatric patients.

When contemplating pediatric GA in dental office buildings, the practitioner has to be aware of the structural differences, because most of these facilities are not built as designated “anesthetizing locations.” One of the differences, for example, is a lack of hospital OR grade HVAC systems, designed to provide augmented air exchanges, and this may, especially during mask inductions for GA in pediatric patients, lead to significant exposure to inhaled anesthetic waste gas levels.

The retrofitting of dental offices with OR grade HVAC systems is likely not economically feasible (and in many cases probably structurally not possible) and may not be sufficient to reduce anesthetic waste gas exposure during mask use to acceptable levels without installation of additional scavenging.6 Dr. Reiz’s group in Sweden had previously experimented with augmented scavenging devices at the head of the operating bed, ultimately leading to the development of the double mask, however, applying significant suction flows to the outer mask (35 m3/h). Although a scavenging fan to supply this suction flow is commercially available together with the double mask, the unit is large and heavy, making it a less than ideal solution for small offices (and even less so for mobile office-based anesthesia).

An alternative suction, uniformly present in dental offices, is the high-volume evacuator used by the dentist themselves, of which there are usually 2 to 3 in each operatory (in addition to a low-volume suction). These high-volume suctions provide approximately 200 L/min (approximately 12 m3/h) and are vented outside the office. As they are only used by the dentist during the case (when the patient is already tracheally intubated), these suctions are readily available for the mask induction. Although the total flow provided by these suctions is lower than the flow of the commercial scavenging fan, it appeared to be sufficient to decrease anesthetic waste gas levels below threshold in our study.

The disadvantages of the double-mask system is its slightly more bulky setup, along with audible noise from the attached suction, both of which can be explained to the parents and the child and which did not present a problem in the authors’ experience. The masks can be reused with proper sterilization (as per instructions by the manufacturer) and are recommended to be changed after 5 years of use.c

In summary, we demonstrated in our study that the double-mask system, when used with dental high-volume suction in freestanding dental offices, was sufficient to decrease the exposure to anesthetic waste gas levels during pediatric mask induction in at least two thirds of offices when compared with the traditional mask, although the suction flow was lower than what was originally published.6 Larger studies are needed to demonstrate that this decrease in exposure to anesthetic waste gases would be true across all anesthetizing sites. This system may be useful for in-hospital ORs where, despite augmented air exchanges, the anesthetic waste gas levels may still exceed the recommended levels during pediatric mask inductions.


Name: Matt M. Kurrek, MD.

Contribution: This author helped design and conduct the study, collect the data, analyze the data, and prepare the manuscript. This author attests to the integrity of the original data and the analysis.

Attestation: Matt M. Kurrek approved the final manuscript and attests to the integrity of the original data and the analysis reported in this manuscript. Matt M. Kurrek is the archival author.

Name: Steven L. Dain, MD.

Contribution: This author helped design the study and prepare the manuscript.

Attestation: Steven L. Dain approved the final manuscript.

Name: Alexander Kiss, PhD.

Contribution: This author helped design the study, analyze the data, and prepare the manuscript. This author attests to the integrity of the original data and the analysis.

Attestation: Alexander Kiss approved the final manuscript.

This manuscript was handled by: Peter S. A. Glass, MB, ChB.


aOccupational Safety & Health Administration/United States Department of Labor. Anesthetic Gases: Guidelines for Workplace Exposure. Accessed August 13, 2012.
Cited Here

bPersonal communication, Mitch Beckwith, Air Techniques Inc, Melville, NY, 2012.
Cited Here

cMedicvent AB Pendelgatan 3 SE-904 22 Umea Sweden Double Mask Instruction Manual. Available at: Accessed August 13, 2012
Cited Here


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