title: Recent advances in and limitations of cardiac output monitoring by means of electrical impedance tomography.
abstract: BACKGROUND
  Currently, the monitoring of cardiac output (CO) and stroke volume (SV) is mainly performed using invasive techniques. Therefore, performing CO monitoring noninvasively by means of electrical impedance tomography (EIT) would be advantageous for intensive care. Our hypothesis was that, by means of EIT, it is possible to assess heart rate (HR) and to quantify changes in SV due to changes in ventilator settings.
  
  
  METHODS
  CO (HR and SV) of 14 pigs (32-40 kg body weight) was changed by incremental increases in positive end-expiratory pressure levels (0, 5, 10, 15, and 20 cm·H2O; ramp maneuver). This ramp maneuver was applied 4 times in each animal, yielding 43 evaluable single experiments. At each positive end-expiratory pressure level, SV was assessed by transpulmonary thermodilution using a PiCCO device. EIT data were acquired using a Dräger EIT Evaluation Kit 2.
  
  
  RESULTS
  The EIT-based SV-related signal, Z(SV) (in [AU]), showed only a weak correlation (after excluding 2 measurements) with SV(TTD) of r = 0.58 (95% confidence interval, 0.43-0.71). If Z(SV) is calibrated by the reference 1 time for each experiment (defined as SVEIT), the correlation is approximately 0.85 (95% confidence interval, 0.78-0.90). A possible reason for the moderate correlation is the unexpected scaling pattern, leading to amplification of the cardiac impedance signal, found in some animals. The scaling is probably due to the imperfect reconstruction (i.e., a change of sensitivity) of the EIT images or to a change in the position of the heart.
  
  
  CONCLUSIONS
  The hypothesis that EIT can be used to monitor CO and SV was confirmed, but further studies are required before this technique can be applied in clinical practice. HR was determined robustly and accurately. For SV monitoring, promising results were obtained in 80% of the experiments. However, unexpected scaling of the cardiac EIT signal causing inaccurate estimation of SV remains an issue. Before robust assessment of SV by EIT is suitable for clinical practice, the cause of and compensation for undesired scaling effects need to be investigated.
authors.affiliation: From the Institute for Medical Information Technology, RWTH Aachen University, Aachen, Germany; Department of Anesthesia and Intensive Care, Sahlgrenska University Hospital Gothenburg, Gothenburg, Sweden; and Department of Medical Statistics, RWTH Aachen University, Aachen, Germany.
authors.forename: Robert
authors.initials: R
authors.lastname: Pikkemaat
authors.forename: Stefan
authors.initials: S
authors.lastname: Lundin
authors.forename: Ola
authors.initials: O
authors.lastname: Stenqvist
authors.forename: Ralf-Dieter
authors.initials: RD
authors.lastname: Hilgers
authors.forename: Steffen
authors.initials: S
authors.lastname: Leonhardt
mesh_terms: D000818:Animals; D002302:Cardiac Output / Q000502:physiology*; D017097:Electric Impedance; D006339:Heart Rate; D008991:Monitoring, Physiologic; D011175:Positive-Pressure Respiration; D013552:Swine; D014054:Tomography / Q000379:methods*