Desflurane and sevoflurane are expensive. These fluorocarbons also have an environmental impact. Low fresh gas flow rates help decrease the total amount of volatile anesthesia used for a case. When sevoflurane was first introduced the teaching was that low flows were not safe due to the formation of fluoromethyl-2,2-difluoro-1-(tri-fluoromethyl)vinyl ether (Compound A). Subsequent research demonstrated that this was a finding in animals that did not translate to humans. Even for cases as long as 17 h, no evidence for nephrotoxicity has been seen with low flow sevoflurane.
Low flows are not commonly utilized, in part because the fresh gas flow rates we choose are arbitrary. Dr. Samsun Lampotang, Department of Anesthesiology, University of Florida, Center for Safety, Simulation and Advanced Learning Technologies, Gainesville, FL and colleagues designed a simulation study to see if displaying recommended flow rates on the workstation monitor affected fresh gas flow rates without adversely affecting patients. Their findings were published in the November issue of Anesthesia & Analgesia, “Automated, Real-Time Fresh Gas Flow Recommendations Alter Isoflurane Consumption During the Maintenance Phase of Anesthesia in a Simulator-Based Study.”
The authors used a Dräger Apollo® anesthesia workstation, which has software that calculates the minimum amount of fresh gas flow needed to deliver as much drug vapor volume as is taken up by the patient from the circuit. Seventeen volunteer anesthesia providers were included in the study. The Dräger machine was not used at the authors’ institution at the time of the study. Study participants were asked to simulate care of a patient: ten minutes after they were ready, simulated incision would occur and the study ended five minutes later. Participants completed both a control scenario without the low flow display and with a Low Flow Wizard (LFW) display-enabled scenario. Participants were not told the nature of the study, though the LFW indications were explained and the LFW was presented as extra available information. Participants were told to use flow and vapor settings they felt were appropriate.
When the low flow indicator was enabled, participants reduced flow rates by more than 50%, from an average of 2.50 to 0.98 L/min. Inhalation anesthetic consumption decreased from 16.21 to 8.08 mL/h. Alveolar anesthetic concentration did not vary. The participants also completed a questionnaire after the study. All agreed with the importance of reducing anesthetic drug consumption. Despite evidence to the contrary, more than half of the participants stated that the minimum flow rates they would use for sevoflurane would be 2 L/min.
Information can guide practice. Most clinicians are aware of the basic risks of inhaled anesthetics (e.g., halothane causes hepatitis). However, there is less awareness of dogmas that have been replaced with facts (e.g., low-flow sevoflurane is safe). The study suggests that real-time guidance can help translate research results into clinical practice.
Additionally, educational resources such as this blog (aa2day.org) can draw practitioner’s attention to research findings that affect our clinical practice. A discussion about the nephrotoxicity of Compound A can be found in OpenAnesthesia.