Systems and methods for detecting a worsening of a patients heart failure condition based, at least in part, on a declining trend in a representative tidal volume value over multiple days. The tidal volume value may be a maximum tidal volume, and more particularly, a maximum tidal volume determined for an afternoon portion of each of the multiple days or a selected portion of each of the multiple days that excludes a night portion. The maximum tidal volume during these portions of the day may be more sensitive to changes in a patients respiration, particularly when a patient is expected to be more active, and thus, may more readily exhibit a declining trend when the patients heart failure status is in decline.
