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.

Prediction of readmissions and mortality in patients with heart failure: lessons from the IMPEDANCE‐HF extended trial

“…Pacemaker‐based devices were also found to be too insensitive to allow detection of small shifts in pulmonary fluid content in evolving HF . The physical basis of the limited effectiveness of such devices was described in the work of Charles et al …”

“…Pacemaker-based devices were also found to be too insensitive to allow detection of small shifts in pulmonary fluid content in evolving HF. 25 The physical basis of the limited effectiveness of such devices was described in the work of Charles et al 26 In the present study, we used the technique that eliminates noise impedance of the chest wall and calculates the net lung impedance. [11][12][13][14][15][16] The sensitivity of this approach is sufficiently high to allow detection of the pre-clinical stage of evolving pulmonary fluid accumulation and thus permits pre-emptive adjustment of treatment in LI-guided patients.…”

The Degree of the Predischarge Pulmonary Congestion in Patients Hospitalized for Worsening Heart Failure Predicts Readmission and Mortality

“…However, intrathoracic impedance measurements by the implantable devices did not improve outcome or decrease HF hospitalizations and outpatient visits in HF patients [21]. Charles et al [22] have described the reasons for the limited effectiveness of these devices. The device used in the IMPEDANCE-HF trial is capable of differentiating the LI signal from the noise signal and, therefore, allows more reliable assessment of lung fluid, leading to impedance-guided therapy.…”