Embodiments of the subject application include a diagnostic apparatus including one or more antennas disposed on a thorax of a living body to direct radio frequency (RF) electromagnetic waves through tissue and output signals responsively to the waves that have passed through the tissue. The apparatus may also include processing circuitry configured to process the signals over time so as to measure one or more RF path characteristics of the RF electromagnetic waves. The RF path length may be defined by a length of time required for the RF waves to pass through the thorax to an antenna and/or pass to tissue and reflect therefrom to an antenna, based on the path characteristic to assess a fluid content of the tissue.