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Clinical Overview

Environmental impact of pediatric anesthetic inhalational (mask) inductions

Sevoflurane Tec 7 Vaporizer Output as a Function of Fresh Gas Flow and Room Temperature

Tarone Thirawatananond, BS; Nikolaus Gravenstein, MD; Christopher Edwards, MD
Department of Anesthesiology, University of Florida College of Medicine, Gainesville, FL

Background

  • Unheated variable-bypass vaporizers are manufactured to perform optimally at a certain fresh gas flow (FGF) rate1.
  • As FGF increases beyond that rate it is more difficult for the gas exiting the vaporizing chamber to reach full saturation1,2.
  • This has implications in pediatric anesthesiology as it affects the speed and efficiency of inhalational induction.
  • Ambient temperature can affect the saturated vapor pressure within the vaporizer chambers and is posited to create an underperformance in rooms with cold temperatures1,2.
  • The aim of this study is to elucidate the variability of GE Datex Ohmeda Tec 7 sevoflurane vaporizer output in relationship to FGF rate and room temperature.

Methods

  • The experiment used an OR, thermostat, and anesthesia workstation.
  • Three GE Datex Ohmeda Tec 7 Vaporizers were used to dispense sevoflurane with the vaporizer dial set at 8%.
  • Inspired sevoflurane (Fi) was sampled at the elbow of a corrugated breathing circuit.
  • Fi was recorded at 15 second intervals across different FGF rates and two room temperatures (20.8°C & 25.3/25.6°C)
  • The external vaporizer temperature was monitored with a temperature probe adhered to the bottom of the vaporizer.
  • The external vaporizer temperature remained within ±0.1°C across all experiment runs.
Pediatric anesthetic inductions graph
Figure 1. Each line plot represents averaged Fi of sevoflurane collected across 90 seconds at different FGF rates and room temperatures. Error bars plotted at 95% confidence interval. Reference line at 8% for vaporizer dial setting.

Results

  • As FGF increases >5 L/min, the maximum Fi progressively decreases.
  • At FGF <5 L/min, more time is required to reach an Fi approaching 8%.
  • No significant differences in Fi were seen between the two temperatures tested.

Discussion

  • FGF of 5 L/min allows for both a rapid rise in Fi and achievement of an Fi of 8%.
  • FGF <5 L/min prolongs the initial rise in Fi.
  • FGF >5 L/min, prevents achievement of the 8% Fi set on the vaporizer dial.
  • FGF >5 L/min leads to additional waste with diminished anesthetic concentration delivered.
  • No significant difference in Fi within a temperature range routinely seen in ORs, likely due to the internal temperature compensation of the vaporizer.

Conclusion

  • Inhalational induction with a FGF of 5 L/min will be more rapid and efficient than using a higher or lower FGF.

Reference

  1. Datex Ohmeda Inc. Tec 7 Vaporizer-User’s Reference Manual. 1175 0013 000 06 02 A. 2002, Section 7.
  2. Boumphrey S, Marshall N. Understanding vaporizers. Continuing Education in Anaesthesia Critical Care & Pain 2011;11:199-203.