title: Biological variation of the cardiac index in patients with stable chronic heart failure: inert gas rebreathing compared with impedance cardiography.
abstract: OBJECTIVE
  In chronic heart failure (CHF), changes in cardiac function define the course of the disease. The cardiac index (CI) is the most adequate indicator of cardiac function. Interpretation of serial CI measurements, however, requires knowledge of the biological variation of CI. Because measurements of CI can be confounded by the clinical situation or the method applied, biological variation might be subject to the same confounders.
  
  
  RESULTS
  We prospectively included 50 CHF patients who met rigid criteria for clinical stability. CI was measured by both inert gas rebreathing (IGR) and impedance cardiography (ICG) in weekly intervals over 3 weeks-each measurement performed at rest (IGRrest/ICGrest) and during low-exercise 10 Watt pedalling (IGR10W/ICG10W). Intra-class correlation coefficients (ICCs), reference change values, and minimal important differences of CI were determined for IGRrest, ICGrest, IGR10W, and ICG10W. Impedance cardiography and IGR showed moderate agreement at rest (20% (6-36)) and good agreement at 10 Watt (-4% (-23-16)). Depending on time interval, measurement modality for CI, and mode, ICC ranged between 0.42 and 0.78, ICC values for IGR were lower than those for ICG. Reference change value ranged between 3 and 15%, and minimal important difference ranged between 0.2 and 0.5 L/min/m2. Values for IGR were lower at rest and higher at 10 Watt than those for ICG.
  
  
  CONCLUSIONS
  Non-invasive measurements of CI are stable over time. Measurement modalities for CI, however, are not interchangeable. Biological variation is less pronounced when obtained by ICG. The influence of low-level exercise on stability of CI depends on the measurement modality.
authors.affiliation: Department of Cardiology, Angiology, and Pulmology University of Heidelberg Heidelberg Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Täger
authors.affiliation: Department of Cardiology, Angiology, and Pulmology University of Heidelberg Heidelberg Germany.
authors.forename: Hanna
authors.initials: H
authors.lastname: Fröhlich
authors.affiliation: Department of Cardiology, Angiology, and Pulmology University of Heidelberg Heidelberg Germany.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Franke
authors.affiliation: Department of Cardiology, Angiology, and Pulmology University of Heidelberg Heidelberg Germany.
authors.forename: Karen
authors.initials: K
authors.lastname: Slottje
authors.affiliation: Roche Diagnostics International AG Risch-Rotkreuz Switzerland.
authors.forename: Andrea
authors.initials: A
authors.lastname: Horsch
authors.affiliation: Roche Diagnostics International AG Risch-Rotkreuz Switzerland.
authors.forename: Dietmar
authors.initials: D
authors.lastname: Zdunek
authors.affiliation: Roche Diagnostics International AG Risch-Rotkreuz Switzerland.
authors.forename: Georg
authors.initials: G
authors.lastname: Hess
authors.affiliation: Department of Cardiology, Angiology, and Pulmology University of Heidelberg Heidelberg Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Dösch
authors.affiliation: Department of Cardiology, Angiology, and Pulmology University of Heidelberg Heidelberg Germany.
authors.forename: Hugo A
authors.initials: HA
authors.lastname: Katus
authors.affiliation: Park University Ext LAFB San Antonio TX USA.
authors.forename: Frank H
authors.initials: FH
authors.lastname: Wians
authors.affiliation: Department of Cardiology, Angiology, and Pulmology University of Heidelberg Heidelberg Germany.
authors.forename: Lutz
authors.identifier: 0000-0001-7216-1004
authors.initials: L
authors.lastname: Frankenstein
keywords: Biological variation; Cardiac index; Chronic heart failure; Impedance cardiography; Inert gas rebreathing