Transient variations of transthoracic impedance as a predictor of heart failure and death in patients with implanted defibrillators

Background: Transient variations in physiological parameters may forewarn of life-threatening cardiac events, but are difficult to identify clinically. Implantable cardioverter defibrillators (ICD) designed to measure transthoracic impedance provide a surrogate marker for pulmonary congestion.

Objective: The aim of this study is to determine if the frequency of changes in transthoracic impedance (TTI) is associated with congestive heart failure (CHF) exacerbation and predicts mortality.

Methods: We followed 109 consecutive patients (pts) with ICDs (n=58) or CRT-ICDs (n=51) for a mean of 21.3 (+10.2) months. Using 80 ohm-days as a reference, we correlated the frequency of TTI changes above this index to CHF hospitalizations or death.

Results: There was at least one TTI threshold crossing in 79 (72%) pts over 23.3 months follow-up, with a mean of 1.8 ± 3.4 per year. There were 18 pts with CHF hospitalizations who had a mean of 4.3 TTI threshold crossings/year (S.D.=±7.3; median=2.8), compared to 1.3 (S.D.=±1.5; median=0.8) among pts without CHF hospitalizations (p=0.0006). Among 20 patients who died during follow-up, there were 4.2 (S.D.=±7.0; median=2.9) TTI threshold crossings/year, compared with 1.3 (S.D.=±1.3; median=0.9) threshold crossings/year among survivors (p=0.0004). Using Cox Proportional Hazard modeling, after adjusting for age, baseline EF, and number of shocks, TTI threshold crossing was an independent predictor of death (HR 1.72, 95% CI 1.26-2.36, p=0.001).

Conclusions: Increased frequency of TTI threshold crossings may be a useful predictor of transient risk for identifying a subgroup of ICD recipients at greater individual risk for death or CHF hospitalizations.