Abstract: Impedance cardiography is a noninvasive technique for monitoring stroke volume, based on
sensing variation in the thoracic impedance, z(t), due to the blood flow. Time derivative of the
thoracic impedance is known as the impedance cardiogram (ICG) and is used for estimating
ventricular ejection time (Tlvet), the ICG peak ((-dz/dt)max), stroke volume, and some other
cardiovascular indices. Respiration and motion artifacts cause base line drift in the sensed
impedance waveform, particularly during or after exercise, and this drift results in errors in
estimation of the parameters. Objective of the research reported in this thesis is to investigate
techniques for removal of the artifacts from ICG for estimation of stroke volume and other
cardiovascular indices, without smearing the beat-to-beat variations.
A baseline restoration circuit and signal processing technique for suppression of
artifacts are developed and investigated. The baseline restoration circuit, based on amplitude
tracking, is developed for partly removing the artifacts for effective utilization of the input
dynamic range of the signal acquisition hardware. The signal processing techniques
developed and investigated are based on adaptive filtering and wavelet based denoising. A
signal related to respiration is sensed by a thermistor based airflow sensor and is used as the
reference input for the respiratory artifact cancellation. For a better approximation of the
respiratory artifact, cubic spline fitting is applied on the sensed impedance signal in
synchronism with the respiratory phases. Adaptive filtering is not suitable for suppression of
motion artifact because of practical difficulty in obtaining reference signal related to the
various motions causing variation in the thoracic impedance. A wavelet based denoising
technique, not requiring a reference signal, is investigated for removal of respiratory and
motion artifacts. These artifact suppression techniques are evaluated on signals with
simulated artifacts and signals acquired from several volunteers with normal health.
For validation of the techniques under a clinical setting, Doppler echocardiography is
used as the reference. The values of stroke volume estimated from impedance cardiography
were compared with those obtained from Doppler echocardiography, on beat-to-beat basis,
for subjects with normal health and ward referral patients. Artifact suppression resulted in
increased correlation, low scatter from linear regression, and a decrease in the mean bias and
the standard deviation of the differences, showing that the artifact suppression techniques can
be used with impedance cardiography instrument for continuous monitoring of stroke volume
and other cardiovascular parameters.
