Determining the optimal technique for priming the breathing circuit for inhalation induction with Sevoflurane in Oxygen and Nitrous Oxide
Gautam Sehgal, MD and Yong G. Peng, MD
Department of Anesthesiology, University of Florida, Gainesville, Florida
Background: Inhalation induction has become almost the rule rather than
the exception in pediatric anesthesia practice. The goal of a satisfactory
inhalation induction is to minimize the duration of Guedel’s second stage of
anesthesia with its hemodynamic instability and airway irritability. High
concentrations of sevoflurane in a mixture of oxygen and nitrous oxide have
been used safely for this purpose. Priming the breathing circuit allows
maximal inspired concentrations of sevoflurane and nitrous oxide with the
first breath. We studied various fresh gas flow rates and breathing circuit
lengths during 10 minutes of circuit priming to identify the optimal
technique to maximize speed of induction and minimize waste of volatile
agent.
Methods: A Datex-Ohmeda S-5 Avance anesthesia machine with a Tech-7
sevoflurane vaporizer was connected to a Vital Signs Inc corrugated
expandable breathing circuit. A gas sampling line and a 1-liter latex bag
was connected to the Y piece of the breathing circuit. For each experiment,
the vaporizer was set to 8% sevoflurane. Concentrations of sevoflurane and
nitrous oxide were measured at 1 minute intervals from 1-10 minutes. At the
end of 10 minutes, the concentration of sevoflurane at the common gas outlet
of the machine was also measured to quantify vaporizer output. A fresh gas
mixture of 30% oxygen and 70% nitrous oxide was circulated through the
vaporizer at flow rates of 2.5, 5 and 10 L/min. The mean of three
experiments was used for data analysis. Experiments were performed with the
breathing circuit in the fully expanded and fully collapsed positions.
Results: Results indicate that vaporizer output decreases with high fresh
gas flows. The sevoflurane concentration at the common gas outlet was more
than 8%, 7.2% and 5.7% at gas flows of 2.5, 5 and 10 L/min, respectively. At
the T-piece, equilibration of nitrous oxide was fastest at the greatest
fresh gas flow. Equilibration times were 10 minutes for 2.5 L/min, 2 minutes
for 5 L/min and 1 minute for 10 L/min. Peak concentration and time course
for sevoflurane at the T-piece also depended on the fresh gas flow rate. At
2.5 L/min the concentration of sevoflurane approaches 8% only at 10 minutes.
Doubling the flow rate to 5 L/min increases the sevoflurane concentrations
at the T piece in a biphasic manner with an early peak of 6% at 1 minute
followed by a decline to 5% by 5 minutes and a subsequent increase to 7.1%
by 10 minutes. At 10 L/min fresh gas flow, high concentrations of 6.8% are
seen at the T piece at the end of 1 minute which gradually decline to 5.7 %
at the end of 10 minutes. The fully collapsed position of the circuit
allowed a more rapid rise in the concentration of nitrous oxide and
sevoflurane at the T- piece than the fully extended position. This
difference was most marked at 5 minutes and disappeared by 10 minutes.
Conclusions: With the Datex-Ohmeda S-5 Avance anesthesia machine, the use of
high fresh gas flows does not result in higher delivered concentrations of
sevoflurane. Vaporizer output decreases above 5 L/min. Priming a collapsed
circuit with intermediate fresh gas flow rates allows reliable delivery of
high concentrations of sevoflurane and nitrous oxide to the patient with
minimal delay and waste.
