title: Non-invasive measurement of cardiac output using an iterative, respiration-based method.
abstract: BACKGROUND
  Current 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.
  
  
  METHODS
  Sequential 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].
authors.affiliation: Department of Physiology, University of Toronto, Toronto, Ontario, Canada Thornhill Research Inc., Toronto, Ontario, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Klein
authors.affiliation: Department of Anaesthesia, University Health Network, Toronto, Ontario, Canada.
authors.forename: L
authors.initials: L
authors.lastname: Minkovich
authors.affiliation: Department of Anaesthesia, University Health Network, Toronto, Ontario, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Machina
authors.affiliation: Department of Surgery, University Health Network, Toronto, Ontario, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Selzner
authors.affiliation: Department of Surgery, University Health Network, Toronto, Ontario, Canada.
authors.forename: V N
authors.initials: VN
authors.lastname: Spetzler
authors.affiliation: Department of Surgery, University Health Network, Toronto, Ontario, Canada.
authors.forename: J M
authors.initials: JM
authors.lastname: Knaak
authors.affiliation: Department of Anaesthesia, University Health Network, Toronto, Ontario, Canada.
authors.forename: D
authors.initials: D
authors.lastname: Roy
authors.affiliation: Department of Physiology, University of Toronto, Toronto, Ontario, Canada Thornhill Research Inc., Toronto, Ontario, Canada j.duffin@utoronto.ca.
authors.forename: J
authors.initials: J
authors.lastname: Duffin
authors.affiliation: Thornhill Research Inc., Toronto, Ontario, Canada Department of Anaesthesia, University Health Network, Toronto, Ontario, Canada.
authors.forename: J A
authors.initials: JA
authors.lastname: Fisher
mesh_terms: D000818:Animals; D001784:Blood Gas Analysis / Q000379:methods; D001944:Breath Tests / Q000379:methods; D002245:Carbon Dioxide / Q000378:metabolism; D002302:Cardiac Output / Q000502:physiology*; D023421:Models, Animal; D008991:Monitoring, Physiologic / Q000379:methods*; D010101:Oxygen Consumption / Q000502:physiology; D011659:Pulmonary Gas Exchange / Q000502:physiology; D015203:Reproducibility of Results; D012119:Respiration; D013552:Swine; D013815:Thermodilution / Q000379:methods; D013990:Tidal Volume / Q000502:physiology
chemical_list: D002245:Carbon Dioxide
keywords: carbon dioxide; cardiac output; monitoring; respiration