This document describes, among other things, systems, devices, and methods
that use frequency domain analysis of a thoracic signal. One example uses
frequency domain analysis for discriminating between different pulmonary
physiological states. Examples of breathing states include normal
breathing, periodic breathing, Cheyne-Stokes breathing, obstructed
respiration, restrictive respiration, and pulmonary edema. The frequency
domain analysis may also be used for performing heart rate variability
(HRV) diagnostics. In one example, a frequency domain adaptive filter
implements a variable cutoff frequency for separating heart contraction
spectral content and other spectral content from lower frequency
respiration spectral content and other lower frequency spectral content.