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Journal of Clinical Monitoring and Computing (2004) 18: 325332 C Springer 2005CALCULATION OF O2 CONSUMPTION DURING
LOW-FLOW ANESTHESIA FROM TIDAL GAS
CONCENTRATIONS, FLOWMETER, AND MINUTE
VENTILATIONTakafumi Azami,1 David Preiss,2 Ron Somogyi,2
Alex Vesely,2 Eitan Prisman,2 Steve Iscoe,3
Andre M. De Wolf, 4 and Joseph A. Fisher,2Azami T, Preiss D, Somogyi R, Vesely A, Prisman E, Iscoe S, De Wolf
AM, Fisher JA. Calculation of O2 consumption during low-flow anesthesia from tidal gas concentrations, flowmeter, and minute ventilation.J Clin Monit 2004; 18: 325332ABSTRACT. We present the principles of a new method to calculate O2 consumption ( VO2) during low-flow anesthesia with
a circle circuit when the source gas flows, end-tidal O2 concentrations and patient inspired minute ventilation are known. This
method was tested in a model with simulated O2 uptake and CO2
production. The difference between calculated VO2 and simu-lated VO2 was 0.01 0.02 L/min. A similar approach can be
used to calculate uptake of inhaled anesthetics. At present, with
this method, the limiting factor in precision of measurement ofVO2 and uptake of anesthetic is the precision of measurement of
gas flow and gas concentration (especially O2 concentration in
end-tidal gas, FETO2) available in clinical anesthetic units.KEY WORDS. Oxygen consumption, modeling, low-flow
anesthesia.INTRODUCTIONMeasurement of O2 consumption ( VO2) during low-flow
anesthesia would provide a valuable monitoring function
as well as information about the minimum O2 flows required to close the circuit. Nevertheless, VO2 is seldom
measured in clinical practice because doing so is both laborious and imprecise. One can estimate VO2 by closing
the circle anesthetic circuit and adjusting the O2 flow to
prevent changes in volume of the ventilator bellows or in
the circuit reservoir bag; the O2 flow then equals VO2.This method is time-consuming and requires frequent adjustment of O2 flow. Interposing a metabolic cart in the
circuit automates the measurement but the method is still
subject to a major source of error, as metabolic carts do not
compensate well for changes in fractional concentration of
oxygen in inspired gas (FiO2) during inspiration.We present a simple, new approach for the accurate and
precise calculation of VO2 during steady-state low-flow
anesthesia with a circle anesthetic circuit. Our approach
uses already available information the gas flow and ventilator settings, and the output of a gas analyzer....