title: Thoracic impedance measures tissue characteristics in the vicinity of the electrodes, not intervening lung water: implications for heart failure monitoring.
abstract: The rationale for intrathoracic impedance (Z) detection of worsening heart failure (HF) presupposes that changes in Z reflect changes in pulmonary congestion, but is confounded by poor specificity in clinical trials. We therefore tested the hypothesis that Z is primarily affected by tissue/water content in proximity to electrodes rather than by lung water distribution between electrodes through the use of a new computational model for deriving the near-field impedance contributions from the various electrodes. Six sheep were implanted with a left atrial pressure (LAP) monitor and a cardiac resynchronization therapy device which measured Z from six vectors comprising of five electrodes. The vector-based Z was modelled as the summation of the near-field impedances of the two electrodes forming the vector. During volume expansion an acute increase in LAP resulted in simultaneous reductions in the near-field impedances of the intra-cardiac electrodes, while the subcutaneous electrode showed several hours of lag (all p<0.001). In contrast, during the simulated formation of device-pocket edema (induced by fluid injection) the near-field impedance of the subcutaneous electrode had an instantaneous response, while the intra-cardiac electrodes had a minimal inconsistent response. This study suggests that the primary contribution to the vector based Z is from the tissue/water in proximity to the individual electrodes. This novel finding may help explain the limited utility of Z for detecting worsening HF.
authors.affiliation: Christchurch Heart Institute, University of Otago, P.O. Box 4345, Christchurch, New Zealand, chris.charles@otago.ac.nz.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Charles
authors.forename: Miriam T
authors.initials: MT
authors.lastname: Rademaker
authors.forename: Iain C
authors.initials: IC
authors.lastname: Melton
authors.forename: Dan
authors.initials: D
authors.lastname: Gutfinger
authors.forename: Neal L
authors.initials: NL
authors.lastname: Eigler
authors.forename: Fujian
authors.initials: F
authors.lastname: Qu
authors.forename: Richard W
authors.initials: RW
authors.lastname: Troughton
mesh_terms: D000465:Algorithms; D000768:Anesthesia, General; D000818:Animals; D001795:Blood Pressure Determination / Q000379:methods; D002307:Cardiography, Impedance / Q000379:methods*; D003198:Computer Simulation; D004487:Edema / Q000473:pathology; D017097:Electric Impedance; D004566:Electrodes; D006325:Heart Atria / Q000473:pathology; D006333:Heart Failure / Q000175:diagnosis* / Q000473:pathology; D008168:Lung / Q000473:pathology*; D008991:Monitoring, Physiologic / Q000379:methods; D011654:Pulmonary Edema / Q000175:diagnosis; D012756:Sheep; D013997:Time Factors; D014867:Water / Q000737:chemistry
chemical_list: D014867:Water