Thoracic Impedance Measured Via Implantable Devices Provides Advanced Warning of Signs and Symptoms of Worsening Heart Failure in Addition to Hospitalization

Abstract

Background: Decreasing thoracic impedance (Z) measured via an implantable device (Insync Sentry, Medtronic) has been shown to be associated with heart failure (HF) hospitalization. However, most clinically relevant events (CRE) associated with worsening heart failure do not result in hospital admission. We examined the association of CRE with device-based Z recordings. Methods: Device-recorded Z and health record data from 69 CRT-D patients (73 ± 20 yr, 49 Male) were retrospectively reviewed. The diagnostic fluid index (FI) was automatically derived from consecutive negative deviations in the actual measured daily Z from the calculated basal or reference Z. Fluid Index values higher than the programmed threshold constituted a device classified event. HF hospitalizations, CRE's, (including documented signs and symptoms of worsening HF), acute changes in diuretic therapy (DT), and surgical device or lead revisions (LR) were identified for association with FI threshold crossings. Results: During the mean follow-up of 339 ± 46 days, 81 crossings of the nominal FI threshold (60 Ω·days) occurred in 48 patients. Four of 9 HF hospitalizations were preceded by FI threshold crossing (44%). CREs were correlated with 25 crossings in 19 patients. Ten CREs were not associated with a threshold crossing. Seven additional crossings were due to appropriate reductions in DT. Ten crossings in 6 patients were associated with LR. Thus, 35 crossings (43%) remained unexplained in 23 patients (33%). The average rate of unexplained FI crossings was 0.55/patient·yr (median = 0/patient·yr). Increasing the FI threshold from 60 to 80 Ω·days would have resulted in no loss in sensitivity to HF hospitalization but would have decreased the sensitivity to CREs from 82% to 61%. However, increasing the fluid index threshold to 80 Ω·days would have significantly reduced the number of unexplained crossings by 34% (p < 0.01). Conclusion: Intrathoracic impedance measurements performed via implantable devices provide important diagnostic information regarding clinically relevant events associated with worsening heart failure in addition to heart failure hospitalization. Individual optimization of programmable parameters may further increase the utility of the diagnostic feature.