The dial setting on an anesthetic vaporizer
reflects the concentration of inhalation
agent delivered into the patient breathing
The dilution effect of rebreathing
describes some of the difference between
the vaporizer dial setting and the inspired
concentration presented to the patient.
Only an agent analyzer provides an
accurate inspired agent concentration
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The Dilution Effect
My vaporizer says 2%.
My analyzer says the inspired agent
concentration is 1.2%.
James H. Philip, MEE, MD
Brigham and Women’s Hospital, Boston, MA
Associate Professor of Anesthesia:
Harvard Medical School, Boston, MA
Madison, Wisconsin U.S.A.
From the Breathing System Series
What happened to the other .8%?
The Dilution Effect of Rebreathing
This Clinical Focus , produced by the Department
of Clinical Affairs, will describe the dilution effect
of rebreathing seen in circle anesthesia breathing
systems. Understanding the dilution effect of
rebreathing will assist the clinician to understand
the difference between the vaporizer dial setting
and the anesthetic agent concentration actually
presented to the patient.
Recent advances in inhalation anesthetic agents,
along with financial and environmental concerns,
have combined to heighten interest in reducing fresh
gas flows during inhalation anesthetic techniques.
With lower flow rates, the contribution that fresh
gas flow and rebreathed gases make toward each
breath is changed. This effect produces a difference
between the inspired inhalation agent concentration
compared to the vaporizer setting. This is the
Dilution Effect of Rebreathing, which is seen in
clinical anesthesia practice.
While the circle rebreathing system has been in use
for some time, the fresh gas flow (FGF) rates that
have commonly been used have allowed the dilution
effect of rebreathing to remain un-noticed. When
FGF is sufficiently high there may be little, if any,
rebreathing of exhaled gases. Under these
conditions, the dilution effect of rebreathing may
essentially be absent and the inspired concentration
more closely reflects the vaporizer dial setting.
Relationship of FGF, Minute Volume and
The Minute Volume (MV) is composed of the FGF and
the recirculated and rebreathed, exhaled patient
gases. At higher FGFs, the major constituent of the
MV is the FGF. As FGF is decreased, the exhaled
patient gases contribute a more significant portion
of the MV. The relationship between MV and FGF
determines how much of the rebreathed gases are
used for each tidal ventilation.
At higher FGFs less exhaled gases are breathed;
at lower FGF more exhaled gases are required to
provide the full tidal volume so more exhaled gas
is rebreathed. Similarly, if the MV is increased for
any reason there will be more rebreathing and,
consequently, a greater dilution effect of rebreathing.
Changes during inhalation anesthesia
At the beginning of inhalation anesthesia, the
clinician may elect to employ a high FGF. For some
patients this high FGF may far exceed MV thereby
reducing, or entirely eliminating exhaled gas
rebreathing. In essence, the fact that all gases are
composed of FGF makes the circle system function
like a non-rebreathing system. Since inhaled gases
are comprised mostly of the FGF and contain little
rebreathed gases, the inhaled concentration of agent
will closely reflect the vaporizer setting.
What is the actual inspired agent concentration?
The actual concentration of inhalation agent in
the inspired gases is a physical mixture of agent
concentrations in the fresh gas and the fraction of the
exhaled gas which is rebreathed. The exact inspired
concentration is difficult to compute
mathematically since it depends on many subtle
phasic aspects of circuit gas mixing with constant
fresh gas flow and breathing cycles. With
increasing dependence upon rebreathing (lower
FGF or increased MV), the value of the inspired
agent concentration will differ more from the
setting on the vaporizer. The need for accurate
agent analysis increases in these circumstances.
An accurate, calibrated agent analyzer is
invaluable and necessary.
How do I achieve the desired inspired agent
Assuming an accurate agent analyzer, the vaporizer
setting must be titrated to achieve the desired
inspired agent concentration. Depending on the
FGF and the MV this may require higher vaporizer
There are other factors, which may contribute to
differences between vaporizer dial setting and
reported agent concentration. Among these are:
• Tidal volume compensated ventilators
• Method of calibration used for the vaporizer
(oxygen versus air)
• Vaporizer type (conventional versus electronic)
• Type of agent analyzer used (peak-to-peak
versus mean inspired agent analysis)