Impedance pneumography: noise as signal in impedance cardiography

Thoracic impedance is modulated by events within the respiratory cycle, which represents a source of "noise" in impedance cardiography. Respiration itself, however, is a physiological rhythm of interest to psychophysiologists. We report here methods and validation for deriving impedance pneumographic measures of respiration from impedance cardiography signals, based on standard tetrapolar band electrodes. We recorded the change in impedance (delta Z), the first derivative of the change in impedance (dZ/dt), output from a strain-gauge respirometer, and criterion spirometry from eight healthy adults during rest, paced breathing, abdominal breathing, thoracic breathing, and a mental arithmetic task. Transfer function analyses revealed that a delta Zd signal (derived by integration of the dZ/dt signal) provided the best estimate of the criterion spirometric measure for all parameters (coherence, phase, and gain), accounting for almost 90% of the variance in respiratory waveform morphology. The results document the potential utility of impedance pneumography, as derived from standard impedance cardiography signals.