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In the tradition of “everything old is new again”, the use of low (250–500 ml/min) and minimal (250 ml/min) flow is once again of interest to the anesthesia community. These techniques have been discussed and infrequently utilized, possibly for as long as there has been an organized science of anesthesiology. In previous generations, low and minimal flow anesthesia was primarily an academic demonstration tool of a very sophisticated few who sought to explore the pharmacokinetics and theories of volatile anesthesia and associated breathing systems. Given the nature of anesthesia equipment of the era, physical properties of available anesthetics and a lack of external administrative and cost pressures, it is not surprising that low and minimal flow anesthesia was relegated to a cult-like status, utilized by practitioners with a high tolerance for risk and exceptional interest in clinically irrelevant minutia. For the majority of the modern anesthesia era, higher flow techniques have been the mainstay of anesthesia practice, with most practitioners using fresh gas flows (FGF) of 2–4 liters per minute (LPM).When this author trained in the late 1980s, the use of flows less than 3 LPM was very uncommon and, as recently as the mid 2000s, I visited a prominent US institution still using flows of 8 LPM for the entirety of their cases! This institution, however, was an exception, but even today, most practitioners loathe decreasing FGF below 1 LPM.
At a gross level, the idea of low and minimal flow anesthesia is appealing. The use of minimal flows will necessarily decrease the loss of body temperature and drying of mucus membranes in the ventilatory tract, which occurs with the use of higher flows. Additionally, minimal FGF significantly decreases the amount of both fresh gas and volatile anesthetic waste to the atmosphere. Considered together, this rationale should result in a lower rate of airway inflammation and infection, decreased release of environmental pollutants, and savings in cost. Enhanced anesthesia system technology, better vaporizers, and inhaled anesthetics of lower solubility now permit anesthetists to deliver low and minimal flows with an acceptable margin of safety, so it logically follows that all anesthesia should be conducted at low and minimal flow. Or does it?
Clinical evidence arguing for low and minimal FGF is lacking. Although it has been demonstrated that utilizing FGF of 0.6 LPM results in a slow warming of the breathing mixture to 30°C within 1 h versus no warming seen at FGF >1.5 LPM, there is no evidence to suggest that this has an effect on maintenance of body temperature during surgery. Indeed, the vasodilation associated with the use of inhaled anesthetics, open body cavities, traditionally cool operating theaters, and cool intravenous fluids suggests that any maintenance of temperature by low and minimal FGF will be insignificant. Regardless of FGF, active warming and humidification of the breathing circuit, the use of heat/moisture exchangers (HMEs), warming of fluids, and convective and conductive warming of the patient's body are all commonly used and far outweigh any theoretic thermal advantage of minimal FGF. Similarly, there is very little evidence that the preservation of moisture in the airways associated with minimal FGF actually results in decreased perioperative pulmonary morbidity. Nevertheless, it is unlikely that humidification of FGF is a bad thing, so the use of HMEs, low flows, and other methods of gas humidification is reasonable, albeit not well supported.
Cost is certainly important in health care, particularly in this era of weak economies and limited resources. There is no doubt that use of low and minimal FGF results in lower cost. For example, an FGF of 2 LPM (0.5 LPM oxygen and 1.5 LPM air) during a 4-h anesthetic will result in usage of 120 l of O2 and 360 l of air. Annualized to 200 such sessions per year, this results in an annual release of 96,000 l of gas per operating theater (24,000 l O2 and 72,000 l air). Decreasing FGF to 0.5 LPM will decrease overall gas release to 24,000 l (12,000 l O2 – 250 ml/min required – and 12,000 l air). The perioperative cost of air and O2, however, while not free is difficult to quantify in overall hospital operations. A more germane example is to examine the cost of the anesthetic agent. In the United States (and many other parts of the world), desflurane is one of the most expensive inhalational agents. It is not yet available in a generic form and has a high minimal alveolar concentration MAC value. Recent studies are lacking, but a 1996 study demonstrated that desflurane's per hour cost was approximately 10 times that of isoflurane. Once again, using the example above, decreasing FGF from 2 LPM to 0.5 LPM would result in a savings of approximately $3000, based on current market prices. This may be more significant in the case of xenon, which has pharmacologic promise, but is prohibitively expensive. As xenon is currently still investigational, it is not clear that this element will ever become part of the anesthetist's armamentarium and at what price point.
Finally, there is increasing concern in the literature over the environmental impact of release of halogenated inhalational anesthetics into the atmosphere and their theoretic global warming potential (GWP). This issue was first raised in the US journal Anesthesia and Analgesia in 2010 by Ryan and Nielsen. Using the infrared absorption of the inhaled agents, they performed mathematical projections and calculated potential GWP for the drugs. Nitrous oxide had the highest GWP, but of the inhaled agents, once again desflurane was the culprit. According to Ryan and Nielsen, a given anesthetic using desflurane equated to driving a car 200 miles, versus the same anesthetic using sevoflurane, which drove their theoretic car only 30 miles. This issue was picked up by the lay media, with some even calling for a ban on the use of desflurane for environmental reasons. In 2012, Anesthesia and Analgesia revisited the potential environmental impact of inhalational anesthetics with a series of articles and editorials devoted to the subject.[5–8] The drumbeat of climate change has led to increasing calls for anesthetists to consider the “environmental ethics” of their anesthetic choices as a part of perioperative planning.
The problem of the cost and environmental impact arguments, however, is a lack of perspective. In the cost argument, an entire year of low/minimal FGF results in a savings of only $3000 per theater. If a surgeon drops a disposable stapler on the floor, it costs $1000. To further illustrate the example from 1992, using isoflurane instead of desflurane in a hypothetical spine surgery lasting 10 h and costing approximately $50,000 would result in a savings of only $20. One of the most common fallacies in statistics is the annualization and multiplication of small results. Given enough operating rooms and enough time, it is certain that a calculation could demonstrate that using low/minimal FGF could save enough money to retire the US national debt and buy every citizen of India a new Mercedes. Ryan and Nielsen are also guilty of this sort of mathematic tomfoolery to beat the drum of anesthetic-induced climate change. As mentioned, their estimation of GWP of inhaled anesthetics is based on a mathematical extrapolation of the infrared absorption of the halogenated anesthetics. They then further extrapolate the number of anesthetics worldwide per year until the temperature starts rising and the polar icecaps melt. The only problem with their hypothesis is that no volatile anesthetic has ever been detected in the upper atmosphere in any quantity. That desflurane has a much shorter theoretic atmospheric half-life than carbon dioxide (CO2) and that, by virtue of its fluorination, it does not react with or degrade atmospheric ozone are the facts ignored by Ryan and company in their indictment of this drug. Even if the anesthetic-related GWP hypothesis is true, the amount of CO2 equivalent produced by all the desflurane used worldwide yearly is about one ten-millionth of global yearly CO2 production or 0.00001%. While this author does not claim to be an environmental engineer, I think it is fair to say that such a small amount is irrelevant and certainly not a reason to modify anesthetic technique.
In medieval times, theologic scholars debated the question of how many angels could dance on the point of a needle. In contemporary parlance, this phrase has come to signify an argument that consumes time and effort, but is meaningless. In a sense, the questions of the benefits of low and minimal FGF versus a 1–2 LPM FGF are similar. Without question, minimal FGF relies on well-maintained, modern anesthesia systems and good monitoring to achieve a reasonable margin of safety. Additionally, accumulation of metabolic and decay by-products of inhalational anesthetics, especially Compound A, in the case of sevoflurane, and the potential for exothermic reaction and fire in the circuit (again with sevoflurane) further compound the risks of minimal FGF. A circumspect review of all the issues involved, therefore, suggests that FGF below 1–2 LPM is unlikely to offer benefit that outweighs the risks of low/minimal FGF. The technique is useful as an academic demonstration of inhalational anesthetic pharmacokinetics and pharmacodynamics and it is important for practitioners to be well versed in its application. Nevertheless, for all practical purposes, low/minimal FGF will and should remain relegated to this academic use.