title: Partial CO2 rebreathing indirect Fick technique for non-invasive measurement of cardiac output.
abstract: OBJECTIVE
  Evaluation in animals of a non-invasive and continuous cardiac output monitoring system based on partial carbon-dioxide (CO2) rebreathing indirect Fick technique.
  
  
  METHODS
  We have developed a non-invasive cardiac output (NICO) monitoring system, based on the partial rebreathing method. The partial rebreathing technique employs a differential form of the Fick equation for calculating cardiac output (QT) using non-invasive measurements. Changes in CO2 elimination (deltaVCO2) and partial pressure of end-tidal CO2 (deltaPETCO2) in response to a brief period of partial rebreathing are used to measure pulmonary capillary blood flow (Q(PCBF)). A non-invasive estimate of anatomic and intrapulmonary shunt fraction (Q(S)/Q(T)), based on oxygen saturation from pulse oximetry (SpO2) and inspired oxygen concentration (FIO2), is added to compute total cardiac output [Q(T) = Q(PCBF)/(1 - Q(S)/Q(T))]. The performance of the NICO was compared with iced 5% dextrose bolus thermodilution cardiac output (TDco) measurements in 6 dogs. Cardiac output was varied using dobutamine, and halothane, and by clamping of the inferior vena cava. Two hundred and forty-six (n = 246) paired measurements of TDco and NICO over a range of cardiac outputs (TDco range = 0.60-8.87 l/min) were compared using Bland-Altman analysis and weighted correlation coefficient.
  
  
  RESULTS
  The Bland-Altman technique yielded a NICO precision of +/- 0.70 l/min (13.8%) with a mean bias of -0.07 l/min (-1.4%) compared to TDco. The weighted correlation coefficient between TDco and NICO values was: r = 0.93 (n = 246).
  
  
  CONCLUSIONS
  The partial CO2 rebreathing technique for measurement of cardiac output is non-invasive, automated, and based on the well accepted Fick principle. The limits of agreement between NICO and TDco is within the recommended value for NICO to be a clinically acceptable method for cardiac output measurement. The results of this canine study show that NICO performed as well, and in some cases better, than other currently available non-invasive cardiac output techniques over a wide range of cardiac outputs.
authors.affiliation: Department of Anesthesiology and Bioengineering, University of Utah, Salt Lake City, Utah, USA.
authors.forename: D G
authors.initials: DG
authors.lastname: Haryadi
authors.forename: J A
authors.initials: JA
authors.lastname: Orr
authors.forename: K
authors.initials: K
authors.lastname: Kuck
authors.forename: S
authors.initials: S
authors.lastname: McJames
authors.forename: D R
authors.initials: DR
authors.lastname: Westenskow
mesh_terms: D000280:Administration, Inhalation; D000818:Animals; D001331:Automation; D002245:Carbon Dioxide; D002302:Cardiac Output; D004285:Dogs; D006801:Humans; D008962:Models, Theoretical; D008991:Monitoring, Physiologic; D013815:Thermodilution
chemical_list: D002245:Carbon Dioxide