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Background: A CRT-D device (Insync Sentry e Medtronic) monitors thoracic fluid accumulation via intrathoracic impedance (Z). The programmable fluid index (FI) is determined by the cumulative consecutive negative deviations in the daily Z from the calculated adaptive reference line. Increased FI is detected by a programmed threshold. Declining intrathoracic Z and FI threshold crossings have been associated with heart failure (HF) decompensation. However, the impact of surgical revisions of the device or lead on the clinical utility of the derived index has not been evaluated. Methods: Data from 5 patients (pts) with a CRT-D device who underwent a device pocket or lead revision were reviewed. Device-stored Z and FI data were evaluated for changes that correlated with revision date. Average Z was compared pre vs. 60 days post-revision. The number of days until the FI returned to zero was evaluated. Results: All pts demonstrated an abrupt decline in Z that correlated with revision date. This resulted in an increase in the FI and a subsequent threshold crossing. Daily Z gradually increased and the reference line declined in all pts post-revision. Daily Z remained significantly lower 60 days post revision (69 6 7 vs. 62 6 7 U; p!0.01; Table). The mean length of time for FI to return to baseline was 104 6 57 days. The difference between daily Z pre and post revision was correlated with the length of time until the FI returned to zero (r 5 0.97, p!0.01). Conclusion: Surgical revision of the device pocket or lead may result in a decline in Z. This decrease is likely due to transient pocket edema and may cause prolonged increase in the thoracic FI despite stable HF status. In these pts, monitoring of raw daily Z values, and not the FI, may be more useful for clinically managing fluid status. A programmable reset of the reference line may allow for more accurate fluid index data post device pocket/lead revisions.