Total Intravenous Anesthetic Versus Inhaled Anesthetic: Pick Your Poison

In this issue of Anesthesia& Analgesia, Sharma et al¹ revisit an old concern—occupational exposure to waste anesthetic gases (WAGs). They acknowledge that WAG is mutagenic and teratogenic, and suggest the use of a total intravenous anesthetic (TIVA) when there is a known pregnant provider in the operating arena. While health care worker safety seems settled, the authors give those of us in resource-rich nations a new perspective on specific challenges that face perioperative and maternity personnel in the developing world (India). It also highlights merely one of many reasons that TIVA should be the anesthetic choice where available. Beyond limiting occupational exposure, protecting public health is another.

OCCUPATIONAL HEALTH AND SAFETY

A recent survey of Indian anesthesiologists² found that a majority preferred to use nitrous oxide as a “carrier gas,” and WAG scavenging systems were only available for 33.5% of 166 respondents. These factors, combined with a lack of modern high-turnover heating, ventilation, and air conditioning (HVAC) systems, likely translate into a far higher concentration of WAG in the operating room than would be found in modern Western practice.

The fact that chromosomal change takes place in the presence of WAG is not in doubt. Multiple cytogenetic studies have demonstrated that health care workers exposed to WAG have chromosomal aberrations, and that cumulative exposure increases risks.^3,4 It is known that the developing embryo is most susceptible to teratogenic agents in the first trimester, when the primitive nervous system and neural crest cells are developing and migrating to their appropriate positions. Pregnant providers exposed to anesthetic gases, particularly nitrous oxide, are at higher risk for spontaneous abortions. An older meta-analysis⁵ of 19 studies found the relative risk to be 1.48 (1.9 when those studies with methodological flaws were excluded). A survey of Australian veterinary practices,⁶ where anesthetic gases are not scavenged, found a 2-fold risk of spontaneous abortions. For this reason, the National Institute for Occupational Safety and Health⁷ has set recommended exposure limits for anesthetic gases: 25 parts per million (time-weighted average) for nitrous oxide and 2 parts per million for the halogenated anesthetic agents (ceiling concentration not to exceed 1 hour). While engineering controls generally keep providers well below these limits in the developed world, individual practice may inadvertently increase exposure. As others have pointed out,⁸ if nitrous oxide and sevoflurane are used for inhalational induction and for maintenance (in cases involving supraglottic airway devices), the time-weighted average for nitrous can greatly exceed National Institute for Occupational Safety and Health limits even with modern scavenging and HVAC systems. While complete elimination of inhaled anesthetics may not be desirable (eg, for pediatric inductions, difficult intravenous access inductions, and difficult airway cases), otherwise avoiding nitrous oxide would reduce both environmental and occupational exposure.

We are familiar with the occupational hazards of inhaled anesthetics, but what about propofol? According to the Materials Safety Data Sheet,⁹ no special handling or engineering controls are required under conditions of product use. Propofol is not regulated by the Occupational Safety and Health Administration, further suggesting a high safety profile for health care workers that we have come to assume.

ENVIRONMENTAL HEALTH AND SAFETY

Beyond limiting occupational exposure, protecting public health is a new professional consideration for anesthesia practice. Climate change has been named the number 1 public health threat of the 21st century, leading to a global call to action within the health community.¹⁰ Modern health care itself produces significant fractions of national greenhouse gas (GHG) emissions—10% in the United States,¹¹ 7% in Australia,¹² 5% from the England National Health Service,¹³ and 4% in Canada¹⁴—with a combined release of 748 million metric tons of carbon dioxide equivalents annually. If a nation together, these health sectors would rank seventh in the world for GHG emissions. Just as caring for patient and occupational safety are essential to the anesthesiologist’s duty, so too is protection of public health from health care pollution.

Inhaled anesthetics are strong GHGs, hundreds to thousands of times more potent than carbon dioxide (the standard global warming unit). They are measurably accumulating in the atmosphere.¹⁵ They can account for 50% of surgical procedure life cycle GHG emissions,^16,17 and 5% of a health care facility’s carbon footprint. The National Health Service in England estimated that 2.5% of its GHG emissions stem from inhaled anesthetics alone.¹⁸ Nitrous oxide is additionally concerning due to its ozone depleting potential.¹⁹ Scavenging systems were designed to protect occupational health against indoor exposure, yet they simply vent WAG off of hospital rooftops to the outdoor atmosphere virtually unmetabolized and unchanged. When accounting for the entire life cycle (natural resource extraction, manufacturing, transportation, usage, and disposal), the global warming potentials of inhaled anesthetics are 4 orders of magnitude greater than a monitored anesthesia care-equivalent quantity of propofol (even accounting for plastic syringes and tubing and energy to run drug delivery pumps).²⁰ From the climate change perspective, TIVA is preferable to inhaled anesthetics²⁰ (especially desflurane and nitrous oxide).^16,18,20

What about water and soil pollution? Propofol can find its way into the environment through spillage, improper disposal into municipal solid waste bin, and also unfortunately through direct dumping into the sewage system by those facilities that elect to classify it as a controlled substance. Propofol is detectable in hospital waste water.²¹ Glucuronidation is the major metabolic pathway of propofol, and it is unknown if metabolites are cleaved back into their active form after sewage disposal. Propofol is not classified as hazardous to the environment according to the Materials Safety Data Sheet,⁹ or under the dated US Resource Conservation and Recovery Act of 1976 that regulates pharmaceutical waste management on the US federal level. However, European regulations utilize the persistence, bioaccumulation, and toxicity (PBT) index to rank environmental risk of drugs. Propofol is not biodegradable in water, nor under anaerobic conditions. It has high potential for bioaccumulation. It is very toxic to aquatic organisms and may cause long-term adverse effects. Propofol was scored a PBT index of 6 out of possible 9 (the worst).²² Of note, the PBT index does not reflect concentration of drug in the environment. With the exception of controlled substances, most operating room drugs are incinerated, consistent with manufacturer recommendations for the treatment of propofol waste. Resource-poor settings tend to burn medical waste, however, in a largely uncontrolled fashion.

How does one begin to compare concern for anthropogenic GHG emissions to that of air, water, and soil pollution overall? Multi-attribute decision analysis²³ can aid decision-making tradeoffs of life cycle environmental and human health impact categories. When weighing both impact and time horizons, air pollution was ranked as the number 1 concern in the short term (0–10 years). However, for the medium term (10–100 years) and long term (100+ years), global warming dominated all the impact categories (Figure).

Figure.: Environmental impact importance by time horizon. Reprin ted with permission from Gloria TP, Lippiatt BC, Cooper J. Life cycle impact assessment weights to support environmentally preferable purchasing in the United States. Environ Sci Technol. 2007;41:7551–7557.²³ Copyright © 2007 American Chemical Society.

Promising technologies for WAG capture or destruction do exist and can lessen direct WAG emissions to the environment. However, captured drug storage is a concern as drug recycling has not yet been approved; importantly, capture technology does not handle nitrous oxide. WAG destruction technology can handle nitrous oxide and is starting to appear commercially in Sweden for labor and delivery facilities; however, it does not treat volatile anesthetics. Furthermore, WAG that is not suctioned through a scavenging system is not treated. Finally, such technologies require investment. Until such time as governmental approval for drug reuse, or carbon taxation with offset options—both of which could help return the investment of technology costs—regulatory mandate and/or professional society recommendation for WAG treatment are likely required to inspire innovation and broad adoption of such technologies. In the interim, reduction of inhaled anesthetic use and waste is most desirable. This is consistent with pollution prevention recommendations of the American Society of Anesthesiologists Environmental Task Force, which include use of lowest fresh gas flows, avoidance of nitrous oxide and desflurane, consideration for TIVA and regional anesthesia when feasible, and investment in WAG capture/destruction technology.²⁴

PRACTICALITY

Propofol is more expensive than inhaled drugs. While prices vary, in the United States, anesthetizing a 70-kg patient for 1 hour with propofol at 150 µg/kg/min would cost approximately $12.75 US dollars for the drug, while 2.2% sevoflurane at 1–2 L/min fresh gas flow would cost about $3.50–$7.00.²⁵ The start-up cost of installing a vacuum scavenging system and HVAC system in an operating room may be prohibitive in the developing world. TIVA may lessen the urgency of these costly renovations, despite it costing more than inhaled drugs. However, Sharma et al¹ reveal that propofol is not routinely used in India. In addition, recommending its selective use by/around pregnant personnel raises concerns for loss of confidentiality. Furthermore, often a woman of child-bearing age may not be aware that she is pregnant and may inadvertently be exposed to WAG when her fetus is most susceptible to harm. Routine use of TIVA would reduce these concerns.

CONCLUSIONS

Sharma et al¹ express a legitimate concern for pregnant providers in an environment that potentially exposes them to significant amounts of ambient WAG and suggest TIVA as an alternative. If TIVA (with or without regional anesthesia) is not a practical option in a setting that lacks modern WAG clearance systems or when patient conditions dictate otherwise, the elimination of routine nitrous oxide use and the use of the lowest possible fresh gas flows for anesthetic maintenance could also greatly reduce exposure and the potential deleterious effects to health care workers. There are multiple reasons to consider TIVA for the majority of nonurgent anesthetics beyond occupational exposure, related to improved patient care (reduced postoperative delirium, reduced postoperative nausea and vomiting, smoother emergence), and environmental concerns (GHG pollution and ozone depletion).²⁴ This is true even in the developed world. The effects of climate change are already being felt, and we are approaching a tipping point. While no anesthetic practice is without potential harm to the environment, we view the use of TIVA as one of many ways to better protect patients, health care workers, and public health. Protection of global health is part of our duty to first, do no harm.

DISCLOSURES

Name: Jodi D. Sherman, MD.

Contribution: This author helped with the writing of the article.

Name: Brian Barrick, MD, DDS.

Contribution: This author helped with the writing of the article.

This manuscript was handled by: Ken B. Johnson, MD.