Reducing fresh gas flow (FGF) during general anesthesia is a simple strategy that leads to cost savings from reduced consumption of volatile anesthetics1 and to attenuation of atmospheric pollution from venting these greenhouse gases.2,3 We previously published results from a successful program to reduce the average time-weighted FGF <2 L/min during sevoflurane administration through a post hoc personalized e-mail feedback process.a4,5 Nair et al.1 achieved similar results using near real-time pop-up messages on the anesthesia information management system (AIMS) computer screen.
Some anesthesia providers may be reluctant to further reduce the FGF during sevoflurane administration because of Food and Drug Administration product labeling to not exceed 2 minimum alveolar concentration (MAC) hours at flow rates of 1 to <2 L/min and to maintain FGF at ≥1 L/min.b Health Canada also recommends against flow rates <2 L/min.c The theoretical basis for these recommendations is the inverse relation between FGF and peak concentrations of pentafluoroisopropenyl fluoromethyl ether (compound A) in the breathing circuit.6 Compound A is a sevoflurane degradation product that is generated from an interaction with some carbon dioxide (CO2) absorbents, particularly those containing strong alkali (e.g., potassium hydroxide [KOH] or sodium hydroxide [NaOH]); breakdown is enhanced when the absorbent is highly desiccated.7 Although compound A is nephrotoxic in rats,8,9 it has not been shown to have adverse renal effects in humans under clinical conditions.10–16 Litholyme® (Allied Healthcare Products, St. Louis, MO) and other absorbents lacking strong alkali do not produce compound A when normally hydrated and only subclinical concentrations when completely desiccated.17,18
Carbon monoxide (CO), another degradation product of both sevoflurane and desflurane resulting from the interaction of sevoflurane and strong alkalis,19 also is not produced by absorbents lacking NaOH or KOH.17
Because CO2 absorbents are available that virtually eliminate both compound A and CO production, making flow restrictions moot, we initiated a project to replace the reactive soda lime product we had been using (Sodasorb®; Grace, Columbia, MD) with a premium nonreactive formulation (Litholyme; Allied Healthcare Products). Litholyme contains calcium hydroxide with lithium chloride (LiCl) as a catalyst, does not contain either NaOH or KOH,d and has been reported not to produce either compound A or CO.e The objective of the switch to the nonreactive absorbent was to allow the reduction of FGF during sevoflurane administration to below the current package insert recommendation. The hospital approved the absorbent change based on anticipated (small) net savings (Table 1) and reduced emissions of greenhouse anesthetic gases.
We evaluated the following 4 hypotheses related to our intervention:
- Hypothesis 1: Average monthly FGF during sevoflurane administration over the intraoperative portions of cases (i.e., surgery begin to surgery end) and during the interval of agent administration would be reduced.
- Hypothesis 2: Measured sevoflurane consumption per minute of volatile agent administration would be reduced.
- Hypothesis 3: Cost savings, from a hospital perspective, due to the reduction in sevoflurane consumption would (modestly) exceed the incremental cost of the nonreactive absorbent.
- Hypothesis 4: Residual wastage in discarded 250-mL sevoflurane bottles would be <2.5 mL (i.e., <1% of a 250-mL bottle); i.e., reduced consumption during patient care would not be associated with greater wastage.
The Thomas Jefferson University IRB determined that this project did not meet the regulatory definition of human subjects research.
Conversion from Soda Lime to Litholyme and Provider Notifications
Anesthesiologists, nurse anesthetists, and anesthesiology residents (i.e., providers) were notified by e-mail on April 14, 2014 and May 6, 2014 about the impending switch from soda lime to the new absorbent (Supplemental Digital Content 1, http://links.lww.com/AA/B329). Over a several-week period, canisters containing exhausted soda lime were refilled with the nonreactive absorbent. After all canisters had been converted, we distributed e-mails to providers encouraging them to maintain 1 L/min FGF during the maintenance sevoflurane administration instead of increasing the FGF to 2 L/min after administering 2 MAC hours (Supplemental Digital Content 2, http://links.lww.com/AA/B330). An announcement was also made at Grand Rounds. The change in departmental policy was approved and endorsed by the chair and vice chair of the department as well as the vice chair for patient safety and quality improvement in the various communications to the department. As part of our routine feedback process (in place since April 2013 and ongoing), every fourth Monday at 5 AM, an e-mail was sent to every provider listing the average (nontime weighted) intraoperative FGF for his or her most recent 10 cases with each volatile agent (Supplemental Digital Content 3, http://links.lww.com/AA/B331). Percent deviations from target FGF values were provided, along with a reminder of the desired average FGF for each agent. No other efforts at goal setting or providing feedback to reduce FGF were made.
After the switch to the new absorbent, our anesthesia providers and technicians reported difficulty in assessing the (irreversible) color change in the Litholyme (from the included ethyl violet indicator) as a cue to replace the absorbent.f This resulted in an undesirably high prevalence of having to change the absorbent intraoperatively. We considered such changes undesirable because (1) this introduces the possibility of introducing a leak into the circuit when reseating the canister (even if a quick-change, self-sealing bracket is usedg); (2) end-tidal concentration of volatile agent can be substantially reduced after the change (Fig. 1); and (3) such activities represent a distraction from care of the patient. Therefore, we evaluated real-time inspired CO2 (PICO2) monitoring criteria to mitigate the need to replace the absorbent during delivery of anesthesia due to excessive rebreathing of CO2 (Fig. 1). We initially instructed providers to use PICO2 = 5 mm Hg as the criterion to ask an anesthesia technician to change the absorbent. However, this still resulted in too many intraoperative changes because the PICO2 rapidly increased during low FGF after reaching this threshold. We decided empirically that if the PICO2 reached 3 mm Hg during cases, the absorbent should be changed before the next case. To implement this, we added a notification to change the absorbent to the text message to turn over the operating room (OR), sent from our real-time decision support system to our anesthesia technicians when cases ended.h5 However, technician staffing was insufficient in the late afternoon and evening to refresh all exhausted canisters, resulting in too many intraoperative absorbent changes during first cases of the day. We added an alert to the anesthesia technicians the next morning, before the start of the OR schedule, to check the absorbent in the specified rooms where the last case in those rooms from the previous day had triggered a rebreathing alert. Finally, we sent an e-mail to providers asking them to place a note on the anesthesia machine requesting the absorbent to be changed if they noticed consistent rebreathing >3 mm Hg during the last case of the day (Supplemental Digital Content 4, http://links.lww.com/AA/B332). We subsequently evaluated formally the time to absorbent exhaustion after reaching the PICO2 = 3 mm Hg threshold (see below).
We extracted inspired CO2 (PICO2) and expired CO2 (PECO2) in mm Hg, oxygen, air, and nitrous oxide FGF in mL/min, inspired volatile agent concentrations (FiAGENT) as percentages, and liquid volatile agent consumption in mL from our AIMS (Innovian®; Dräger, Telford, PA) database (SQL Server 2008 R2; Microsoft, Redmond, WA). These parameters were transmitted by the monitors incorporated within our anesthesia machines (Apollo®; Dräger) to the local workstation running the AIMS client software, and median values over the prior minute were recorded in the central database at approximately 1-minute intervals. When a valid PICO2 or PECO2 measurement was unobtainable (e.g., due to an irregular capnograph), a null value was transmitted to the database. Only cases during which a volatile agent was administered were included in the analysis. Patient age, total case duration (interval from patient entry into and exit from the OR), and total duration of volatile anesthetic administration (interval from first FiAGENT >0.2 MAC to end of surgery) were also retrieved. The intraoperative period was defined as the interval between the recorded events Surgery, BEGIN and Surgery, END. For calculations of FGF, timestamps were normalized from the start of surgery, rounded to the nearest minute. Total FGF was calculated as the sum of the oxygen, air, and nitrous oxide FGF. For calculation of total sevoflurane consumed, measurements from the entire case were used. For determination of the start of a run of PICO2 ≥3 or ≥5 mm Hg, null values at 1-minute intervals were imputed as the most recent valid measurement. For example, if 3 consecutive values were 3, null, and 4, the missing value was set equal to 3.
We compared Apollo sevoflurane consumption measurements with amounts obtained by weighing vaporizers before and after use20 through a Bland-Altman analysis and found these to be equivalent (Fig. 2). We determined that application of the Dion formula21 to estimate the sevoflurane consumption by integrating the product of FGF, and the inspired agent concentration was not equivalent to the values calculated by the proprietary Dräger algorithmi (Fig. 3). Therefore, our evaluation of average sevoflurane consumption before and after the change to the nonreactive absorbent was limited to cases performed using Apollo anesthesia machines, present in approximately 2 of 3 of our anesthetizing locations.
Records of sevoflurane and absorbent invoices were provided by the hospital and used to determine purchases by date of order before and after the change to the nonreactive absorbent. A full bag of either absorbent was used to refill a depleted canister, so the number of bags purchased was proportional to the amount of absorbent utilized.
Measuring Impending Absorbent Exhaustion
For each case, we determined whether a run of 3 consecutive PICO2 values ≥3 mm Hg occurred, and, if so, the interval between the start of the run and the start of the first run of 5 consecutive PICO2 values ≥5 mm Hg (which we defined as absorbent depletion, requiring replacement). If the end of surgery occurred with no run of 5 PICO2 values ≥5 mm Hg having occurred, the case was right censored at the end of surgery timestamp. We terminated measurement at this milestone because FGF is typically increased to approximately 10 L/min at this point, effectively ending any rebreathing, even if the absorbent was nearly completed depleted.
Measurement of Sevoflurane Waste from Incompletely Emptied Bottles
We wanted to rule out discarding partially full bottles of sevoflurane by providers as a potential source of wastage to put the anticipated reduction in sevoflurane consumption in perspective. To estimate wastage, convenience samples were collected in batches (i.e., every 2–4 days) from our surgical suites over a 3-week period. Providers were requested by e-mail and via occasional pop-up notices on their anesthesia workstations to cap and save any sevoflurane bottles that would otherwise be discarded in the trash after filling the vaporizer. The anesthesia technicians retrieved the bottles when turning over the ORs and placed them in a bucket in their workroom; providers also occasionally placed their empties here. The amount of liquid sevoflurane remaining was calculated by weighing each bottle with the residual agent 2 times, suctioning the liquid from the bottle, reweighing 2 times, and calculating the difference between the means for each bottle. The weights were converted to mL by dividing by the density of sevoflurane (1.51 g/mL at 20°C). An Acculab 333 digital scale (Sartorius AG, Göttingen, Germany), accurate to 0.1 g, was utilized. Average wastage ± SE per 250-mL bottle was computed.
We considered a difference of 100 mL/min in the mean sevoflurane FGF to be important. During the first N = 4 4-week batches, the mean sevoflurane FGF was 1892 mL/min and the SD among the batches was 32.5 mL. Using these values, N = 8 4-week batches before and after the absorbent conversion was estimated to provide 95% power to detect a 100 mL/min difference in FGF at α = 0.001 using the 2-sided Student t test.
The method of batch means,22–27 with 4-week batches, was used for all determinations, with 8 batches before the change compared with 8 batches after the change (reported as N = 8 of 8).j For financial calculations related to volatile agent and absorbent purchases (received several months after the FGF analysis was completed), 10 batches were used before and after (reported as N = 10 of 10).
Agent consumption and FGF data are presented as the mean ± SE. Statistical comparisons between groups were made using the 2-sided Student t test. For comparison with our target FGF of 1.25 L/min, the 1-group, 1-sided Student t test was used. For the analysis of the interval from reaching a PICO2 = 3 mm Hg to reaching a PICO2 = 5 mm Hg, the Kaplan-Meier survival statistic was calculated with absorbent exhaustion (i.e., 5 PICO2 values ≥5 mm Hg) considered equivalent to death. Statistical significance for all comparisons required P < 0.01. Systat® 12 (Systat Software, Inc., San Jose, CA) was used for all statistical calculations.
Characteristics of Cases Before and After the Absorbent Change
A total of N = 20,235 cases were analyzed (80.2% sevoflurane, 15.1% desflurane, and 4.7% isoflurane). There were no statistically significant differences before and after the absorbent change among any of the volatile agents for patient age, total case duration, duration of volatile agent administration, or inspired or expired agent concentration (Table 2). The average monthly number of cases in which desflurane was used for at least a portion of the case decreased significantly after the absorbent switch.
There was a nonsignificant trend for a reduction in the number of bags of CO2 absorbent purchased during the 10 consecutive 4-week intervals after, compared with before, the switch (N = 10 of 10; difference = −19.1; 95% confidence interval [CI], −40.8 to 2.63; P = 0.08).
Hypothesis 1: FGF During Sevoflurane Administration Was Reduced After the Absorbent Change
There was an immediate and sustained reduction in intraoperative FGF after the announcement of the initiative (Fig. 4). Intraoperative FGF was reduced for cases in which sevoflurane was administered by 435 mL/min (N = 8 of 8 periods; 95% CI, 391 to 479; P < 0.0001) (Table 3). Considering the portion of cases during which sevoflurane was administered, FGF was reduced by 372 mL/min (N = 8 of 8; 95% CI, 334 to 409 mL/min; P < 0.0001) (Table 3). Hypothesis 1 was accepted.
For cases where desflurane or isoflurane were administered, there was a nonsignificant trend for a decrease in the intraoperative FGF (Table 3).
Hypothesis 2: Sevoflurane Consumption Was Reduced After the Absorbent Change
Sevoflurane consumption per minute of administration decreased by 0.039 mL/min (N = 8 of 8; 95% CI, 0.029 to 0.049 mL/min; P < 0.0001) after the change to the nonreactive absorbent (Table 3). Based on the hospital’s wholesale cost for sevoflurane, this would be equivalent to a reduction of $0.89 per hour (95% CI, −$0.67 to −$1.11). Hypothesis 2 was accepted.
There was a negative trend but not a significant decrease in the average number of bottles of sevoflurane purchased during each of the 10 4-week intervals before, compared with after, the change to the nonreactive absorbent decreased from 180.8 ± 6.3 to 160.7 ± 8.5 (N = 10 of 10, difference = −17.1; 95% CI, −17.4 to 16.8; P = 0.07). This was in the context of no significant change in inspired or expired agent concentrations, duration of cases, or number of cases (Table 3).
Hypothesis 3: Net Hospital Savings Would Be Positive After the Absorbent Change
To compare the hospital expenses before and after the absorbent switch, sevoflurane costs were determined by multiplying the number of bottles purchased by the average wholesale cost ($95 per bottle). Similarly, absorbent costs reflected the number of bags purchased times the unit cost. The difference in mean cost for the sum of the sevoflurane and absorbent purchases for each of the 10 4-week intervals before and after the absorbent switch was −$293 (N = 10 of 10, 95% CI, −$2853 to $2266; P = 0.81). Hypothesis 3 was rejected.
We conducted a sensitivity analysis to explore the financial implications at other hospitals given a range of sevoflurane acquisition costs, the differential costs between the current and a proposed replacement absorbent, and varying percentage reductions in sevoflurane consumption over a realistic range of parameters (Fig. 5, Supplemental Digital Content 4, http://links.lww.com/AA/B332). Hospital cost savings, at best, are likely to be modest, given the relatively low hourly cost of sevoflurane administration.
Hypothesis 4: Sevoflurane Wastage in Discarded 250-mL Bottles Would Be <1%
Fifty-two bottles were collected on 8 different days from the storage bucket in the anesthesia workroom, with samples ranging from 4 to 11 bottles; 1 bottle was missing its cap and was excluded from the analysis. The average amount of residual sevoflurane per bottle was 0.67 ± 0.06 mL (N = 8 collection days; 95% CI, 0.54 to 0.81 mL per bottle; P < 0.0001 vs 2.5 mL).
Monitoring Absorbent Exhaustion
Once the PICO2 reached 3 mm Hg for at least 3 consecutive minutes, the absorbent became exhausted within 95 minutes in most (i.e., >50%) canisters (Fig. 6). Because the mean duration of volatile anesthetic administration was 146 minutes during cases where sevoflurane was administered (Table 2), this indicated that the absorbent should be changed between cases if there was persistent rebreathing of CO2 ≥3 mm Hg. Otherwise, there would be a substantive chance that absorbent exhaustion would occur during the subsequent case, requiring an intraoperative absorbent change.
We were able to demonstrate a significant and sustained reduction in both FGF and calculated average sevoflurane consumption per minute. Providers changed their FGF behavior promptly upon the conversion from soda lime to the nonreactive absorbent. However, they did not achieve the desired sevoflurane FGF intraoperative target of 1.25 L/min, only reaching 1.51 L/min (P < 0.0001 compared with 1.25). Given the small additional benefit that would accrue from achieving the FGF goal, we elected not to implement additional measures, such as sending intraoperative pop-up messages to the workstations when the FGF was >1.0 L/min, as described by Nair et al.1 The measured reduction in sevoflurane ordering (9.5%) was less than the reduction in sevoflurane consumption (13.2%). However, when judged from the perspective of the hospital’s variable costs, the project appears to have been cost neutral, as savings from sevoflurane reduction were offset by the additional cost of the premium CO2 absorbent.
The reduction in FGF has a societal benefit from reducing the venting of anesthetic greenhouse gases into the environment. However, in perspective,28 the global contribution of volatile anesthetics released into the atmosphere only represents approximately 0.01% of CO2 released from the fossil fuel consumption.29,30 Furthermore, we were unable to find a life-cycle assessmentk of Litholyme or Sodasorb, so we cannot estimate accurately the differences in the environmental impact between the 2 absorbents. It is not sufficient simply to assess the decreased venting of the anesthetics when choosing an absorbent, as this ignores potential cradle to grave differences due to manufacturing, processing, packaging, transportation, recycling, and disposal.
Managing the process of pre-emptive absorbent replacement to avoid intraoperative changes was unexpectedly challenging. There were behavioral issues related to changing exhausted absorbent at the end of the day. Our providers and anesthesia technicians found that a color change of the indicator was not a reliable cue for when to change the absorbent.l Thus, monitoring PICO2 (a parameter measured by capnometers) during volatile general anesthetics, setting and activating the alarm limit, and responding appropriately when excessive rebreathing is noted are necessary. We initially applied empirical criteria for levels and duration of rebreathing to trigger an alert to our anesthesia technicians to change the absorbent between cases or before the first case of the day and then modified these based on our analysis of impending exhaustion. Although changing the absorbent is neither difficult nor time consuming, intraoperative replenishment creates a distraction from care of the patient. In addition, a circuit leak may occur if the canister does not seal properly, caustic dust may be released from the process of emptying and refilling the canister, and the level of volatile anesthetic may be inadvertently reduced, with the potential for patient movement or recall. Although we have an automated system to notify our technicians to change the absorbent, providers also need to take responsibility to monitor the PICO2 and ensure that nearly exhausted absorbent is replaced before starting the next case.
Our study has several limitations. First, our results cannot be directly applied to other hospitals, because volatile agent and absorbent acquisition costs, patterns of FGF, percentages of volatile agent wasted from discarding partially full bottles, willingness to reduce FGF during sevoflurane anesthesia, and levels of volatile anesthesia provided will vary. In addition, the ability to distribute personalized e-mail FGF reports to providers (as described in our recent review article5) or to implement a near real-time feedback system1 likely is necessary to achieve the reduction in sevoflurane consumption. However, the principles we describe with respect to assessing the cost utility of a program incorporating a change to a nonreactive absorbent and reduction of FGF during volatile agent administration are relevant. Appendix A can be used to conduct a sensitivity analysis on the financial impact of a potential change to a premium CO2 absorbent over a range of input parameters and anticipated changes. Regardless, facilities should not expect large savings, because sevoflurane is relatively inexpensive, nonreactive CO2 absorbents cost substantially more than conventional formulations, and the opportunity to greatly reduce FGF is likely limited. Prefilled disposable or recyclable canisters containing nonreactive absorbents are much more expensive than bulk product, as used in our study, and would have further reduced the net savings.
Another limitation is that the very small amount of wastage of sevoflurane in discarded bottles may have been subject to a Hawthorne effect. However, providers were told that we were collecting bottles to assess sevoflurane usage, not to assess how much they were wasting. In addition, over the 3-week collection period, there was no trend for increased residual volumes in the bottles, as might have been expected due to desensitization had this effect been present. Thus, our application of the method of batch means to measure wastage was appropriate.
Another limitation of our approach is that measurement of agent consumption by the anesthesia machine is vendor and model dependent and may also depend on the ability of one’s AIMS to record transmitted values. For example, Dräger Tiro® anesthesia machines are present in our ambulatory and out-of-OR locations and lack the ability to calculate agent consumption. Weighing vaporizers to determine agent consumption is a cumbersome process. At least during low FGF anesthesia, the Dion method21 of estimating volatile agent consumption is not sufficiently accurate to gauge savings.
A final limitation is that the hospital purchasing records may not have reflected accurately the consumption of either sevoflurane or CO2 absorbent within each 4-week batch. There was limited space for storage of these materials in the hospital’s stock room, so deliveries were made every 1 to 3 days, based on inventory. However, there were considerable supplies stored in the anesthesia drug carts and in the anesthesia work rooms, thereby buffering the impact of any ordering shortfalls.
In summary, we showed that a department can successfully transition to a premium nonreactive CO2 absorbent in a manner that is at least cost neutral by reducing the sevoflurane FGF to below limits recommended by the package insert. This was achieved in part by monitoring electronically for when to change the absorbent and automatically notifying the anesthesia technicians. However, the potential for other departments to achieve the limited success we realized depends on their ability to collect and analyze the necessary data, local policies regarding FGF and absorbent replacement, their ability to alter provider behavior through monitoring and feedback, and local prices.
APPENDIX A. SENSITIVITY ANALYSIS WORKSHEET FOR CHANGING TO A NONREACTIVE CARBON DIOXIDE ABSORBENT
The net cost of changing to a premium carbon dioxide (CO2) absorbent is the difference between the savings realized through reduced consumption of volatile agent via reduction of fresh gas flow (FGF) and the additional cost of the nonreactive absorbent compared with current absorbent expenses.
There will be uncertainty as to the extent that these potential savings can be realized due to several factors.
First, the new absorbent may have a different CO2-absorbing capacity than the current absorbent, which needs to be accounted for in the projected cost estimate. For example, if the new agent costs 2× as much but lasts 2× as long, then, ideally, the switch would be cost neutral. However, the full benefit of additional absorbent capacity may not be realized, so that needs to be accounted for in the sensitivity analysis.
Second, the total reduction in volatile agent consumption may not be proportional to the reduction in FGF during the intraoperative portion of cases (begin to end surgery). This is because during the induction of general anesthesia, higher gas flows and vaporizer concentrations will be utilized, and when FGF is reduced early into the anesthetic, higher vaporizer concentrations will be needed to maintain the same expired agent concentration compared with previously, when higher FGF was used. These factors are related to the uptake and distribution of volatile agent and will be more pronounced for relatively soluble agents (e.g., sevoflurane and isoflurane) than with more insoluble agents (desflurane). For short cases, the impact of FGF reduction during maintenance on agent consumption will be reduced, compared with longer cases, as the percentage of agent consumed during the early uptake phase will be larger than for long cases.
The following worksheet and the associated graph can be used to estimate the range of potential savings (or loses) that may be associated with the change to a premium CO2 absorbent. Worked examples are provided based for 3 hypothetical hospitals with varying costs for sevoflurane and absorbents, using a range of projected savings.
To perform a graphical sensitivity analysis, construct a rectangle on the contour plot in Figure 6 with the vertices at the minimum and maximum values for the annual estimated incremental absorbent costs (x-axis) and the projected annual sevoflurane savings (y-axis). Each colored band represents the net difference between the estimated sevoflurane savings and the additional cost of the premium absorbent. The area within the rectangle represents the range of potential net savings or losses. The 3 scenarios (A, B, and C) presented above are overlaid on this graph.
Name: Richard H. Epstein, MD, CPHIMS.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript. He is the archival author.
Attestation: Richard H. Epstein attests to the integrity of the original data and the analysis reported in this manuscript and has approved the final manuscript.
Name: Franklin Dexter, MD, PhD.
Contribution: This author helped design the study, analyze the data, and write the manuscript.
Attestation: Franklin Dexter attests to the analysis reported in this manuscript and has approved the final manuscript.
Name: David P. Maguire, MD.
Contribution: This author helped prepare the manuscript.
Attestation: David P. Maguire attests to the integrity of the original data and approved the final manuscript.
Name: Niraj K. Agarwalla, DO.
Contribution: This author helped conduct the study and prepare the manuscript.
Attestation: Niraj K. Agarwalla attests to the integrity of the original data and approved the final manuscript.
Name: David M. Gratch, DO.
Contribution: This author helped prepare the manuscript.
Attestation: David M. Gratch attests to the integrity of the original data and approved the final manuscript.
Dr. Franklin Dexter is the Statistical Editor for Anesthesia & Analgesia. This manuscript was handled by Dr. Tong J. Gan, Section Editor for Ambulatory Anesthesiology and Perioperative Management, and Dr. Dexter was not involved in any way with the editorial process or decision.
a Patel N, Maguire D, Dexter F, Epstein RH. Reduction of fresh gas flow during administration of volatile anesthetic agents via monthly individualized e-mail feedback. 2014 Annual Meeting of the Society for Technology in Anesthesia. Available at: http://www.stahq.org/files/7013/9171/8017/13_Abstract_Epstein.pdf. Accessed January 3, 2015.
b Baxter. Sevoflurane, USP Package Insert. Available at: http://www.baxter.com/downloads/healthcare_professionals/products/SevofluranePI.pdf. Accessed January 3, 2015.
c Baxter Corporation. Sevoflurane Product Monograph. Available at: http://www.baxter.ca/en/downloads/product_information/SEVOFLURANE_PM_ENG_2013Nov26.pdf. Accessed January 2, 2015.
d Allied Healthcare Products. Material Safety Data Sheet: Litholyme. Available at: http://www.alliedhpi.com/images/Litholyme_MSDS.pdf. Accessed January 3, 2015.
e Allied Healthcare Products. Litholyme: a safer and more cost-effective carbon dioxide absorbent. Available at: http://www.litholyme.com/images/wp.pdf. Accessed January 3, 2015.
f Most of our absorbent canisters were cloudy from approximately 8 years of exposure to soda lime and were not able to be rendered transparent, despite vigorous cleaning. In addition, fresh nonreactive absorbent has a slightly purplish hue.
g Kuruma Y, Kita Y, Fujii S. Exchanging a CLIC absorber in the middle of surgery. APSF Newsletter, Winter 2013. Available at: http://www.apsf.org/newsletters/html/2013/winter/16ltr-CLICabsorber.htm. Accessed July 6, 2015.
h Gratch D, Maguire D, Epstein RH. Development of a system to pre-emptively identify impending carbon dioxide absorbent depletion during low fresh gas flow anesthesia to mitigate the need for intraoperative replacement during the case to follow. 2015 Annual Meeting of the International Anesthesia Research Society.
i Heesch R. Method for measuring the anesthetic agent consumption in a ventilation system. US Patent 20080029092 A1. Available at: http://www.google.com/patents/US20080029092. Accessed May 13, 2015.
j The method of batch means (bins) was used, as for analyzing most other operating room managerial data because mean fresh gas flows among successive cases represent a time series of correlated values.22–27 The data are correlated because (1) mean fresh gas flows vary among providers, (2) most providers perform multiple cases on days when delivering anesthesia, and (3) providers are often assigned over several-week intervals to work in the same areas with higher or lower prevalences of volatile anesthetic use. By aggregating cases over 4-week periods, the impact of such unmeasured relationships is mitigated.
k Wikpedia. Life-Cycle Assessment. Available at: http://en.wikipedia.org/wiki/Life-cycle_assessment. Accessed May 8, 2015.
l To put this in perspective, a competing nonreactive absorbent containing pure lithium hydroxide monohydrate in a solid-state matrix (SpiroLith®; Micorpore, Elkton, MD) lacks any color indicator.
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