Usefulness of monitoring congestive heart failure by multiple impedance vectors

Introduction: We investigated trends in intrathoracic impedance measured between multiple implanted electrodes for monitoring pulmonary edema secondary to congestive heart failure (CHF) in an experimental model. Methods: Biventricular ICDs were implanted in 16 dogs and 5 sheep. Continuous RV pacing (230–250 bpm) was applied over several weeks. Meanwhile, impedance was measured every hour along 4 intrathoracic and 2 intracardiac vectors. Four cardiogenic impedance vectors were also monitored. Cardiac function was assessed biweekly by catheterization and echocardiography. Left atrial (LA) pressure was measured daily by an implanted LA pressure sensor. Results: All animals developed CHF after 2–4 weeks of pacing as evidenced by changes in function (EF, 52 vs. 34%; LV end-diastolic volume, 65 vs. 97 ml; LV end-diastolic pressure, 7 vs. 16 mmHg; LA volume, 17 vs. 33 ml; LA pressure, 7 vs. 26 mmHg), clinical symptoms, or autopsy. Steady state impedance decreased during CHF: LV-Can, 17±9%; LV-RV, 15±8%; LV-RA, 13±6%; RV-Can, 13±8%; RVcoil-Can, 8±6%; RA-Can, 6±6%. Change in LV-Can impedance was greater than that of RA-Can, RV-Can, and RVcoil-Can (P<0.05). LV-Can impedance correlated well with LA pressure (r²=0.73), while RV-Can and RVcoil-Can were weakly correlated (r²=0.43 and r²=0.52, respectively). Changes in LV-RV and LV-RA impedances were also larger than those of RVcoil-Can and RA-Can (P<0.05). Meanwhile, all impedances were associated with circadian variability at baseline (5±2%) which diminished during CHF (2±1%); P=0.02. Furthermore, significant variations were observed in cardiogenic impedances during progression into CHF as evidenced by reduced peak-to-peak amplitude and increased fractionation of the signals. Conclusions: All impedance vectors decreased during CHF. Impedance measurement employing left heart sensors correlated well with LA pressure, an- d may improve detection of CHF onset compared to sensing by RA or RV leads alone. This approach has important clinical implications for managing heart failure patients in the ambulatory setting.