Noninvasive assessment of cardiac performance by impedance cardiography: disagreement between two equations to estimate stroke volume.

Impedance cardiography, phonocardiography, and ECG were registered prior to, during, and after 10 min of gradual passive upright tilt in 35 normal male subjects. Stroke volume (SV) was estimated according to both the standard equation of Kubicek et al. and a newly introduced equation by Sramek et al. Sramek's equation estimated larger SV values throughout. It furthermore estimated a larger SV response to upright tilt: a bias of -17 ml (95% Cl: -20 to -13) was identified for Sramek's minus Kubicek's approach. Corresponding disagreement was identified for cardiac output and systemic vascular resistance derived from SV. A far better agreement was identified when the data were expressed as percent of the supine base line prior to tilt. Under such transformation, Sramek's approach estimated slightly smaller SV responses to tilt: a bias of 4.2% (95% Cl: 3.6 to 4.7) was identified for the percent response of SV to upright tilt for Sramek's minus Kubicek's equation. Data generated by these two equations are, nevertheless, not readily interchangeable nor comparable. As Sramek's equation estimated consistently larger SV values than Kubicek's equation, it is unlikely that Sramek's equation could solve the issue of the quantitative inaccuracy which remains intrinsic to the usage of Kubicek's equation in impedance cardiography.