• M Klein, L Minkovich, M Machina, M Selzner, V N Spetzler, J M Knaak, D Roy, J Duffin, and J A Fisher.
• Department of Physiology, University of Toronto, Toronto, Ontario, Canada Thornhill Research Inc., Toronto, Ontario, Canada.
• Br J Anaesth. 2015 Mar 1; 114 (3): 406-13.

BackgroundCurrent non-invasive respiratory-based methods of measuring cardiac output [Formula: see text] make doubtful assumptions and encounter significant technical difficulties. We present a new method using an iterative approach [Formula: see text], which overcomes limitations of previous methods.MethodsSequential gas delivery (SGD) is used to control alveolar ventilation [Formula: see text] and CO2 elimination [Formula: see text] during a continuous series of iterative tests. Each test consists of four breaths where inspired CO2 [Formula: see text] is controlled; raising end-tidal Pco2 [Formula: see text] by about 1.33 kPa (10 mm Hg) for the first breath, and then maintaining [Formula: see text] constant for the next three breaths. The [Formula: see text] required to maintain [Formula: see text] constant is calculated using the differential Fick equation (DFE), where [Formula: see text] is the only unknown and is arbitrarily assumed for the first iteration. Each subsequent iteration generates measures used for calculating [Formula: see text] by the DFE, refining the assumption of [Formula: see text] for the next test and converging it to the true [Formula: see text] when [Formula: see text] remains constant during the four test breaths. We compared [Formula: see text] with [Formula: see text] measured by bolus pulmonary artery thermodilution [Formula: see text] in seven pigs undergoing liver transplantation.Results[Formula: see text] implementation and analysis was fully automated, and [Formula: see text] varied from 0.6 to 5.4 litre min(-1) through the experiments. The bias (between [Formula: see text] and [Formula: see text]) was 0.2 litre min(-1) with 95% limit of agreement from -1.1 to 0.7 litre min(-1) and percentage of error of 32%. During acute changes of [Formula: see text], convergence of [Formula: see text] to actual [Formula: see text] required only three subsequent iterations.Conclusions[Formula: see text] measurement is capable of providing an automated semi-continuous non-invasive measure of [Formula: see text].© The Author 2014. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

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