Calibrating intrathoracic impedance for absolute lung fluid measurement

A machine-implemented method of estimating lung fluid content using an intrathoracic impedance measurement, the method comprising:

• receiving a first intrathoracic electrical response signal from a tissue of interest in a subject in response to a delivered first subcapture electrical test stimulus, the first test stimulus delivered and the first response signal obtained during a first subject state;
• receiving a first estimate of a fluid content value of the tissue of interest during the first subject state;
• receiving a second intrathoracic electrical response signal from the tissue of interest in the subject in response to a delivered second subcapture electrical test stimulus, the second test stimulus delivered and the second response signal obtained during a second subject state;
• receiving a second estimate of the fluid content value of the tissue of interest during the second subject state;
• pairing the first intrathoracic electrical response signal and the first estimate of the fluid content value to form a first pair of values and pairing the second intrathoracic electrical response signal and the second estimate of the fluid content value to form a second pair of values;
• and
• determining a relationship between the first and second pairs of values.