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.
The purpose of this paper is to analytically evaluate and compare two of the most common methods for measuring respiration-related heart rate fluctuations: linear detrended heart rate power spectral analysis and the Porges technique of filtered variance. Low-frequency power was removed from instantaneous 4-Hz R-R interval signals using either a first-order linear (linear/spectral technique) or a third-order polynomial (MPF-var technique). The signals were band-pass filtered and analyzed in both the time and frequency domains. Although in most cases the two techniques yielded substantially similar results, the MPF-var technique resulted in signal amplification at a few specific frequencies. The frequency range and effect to amplification of the MPF-var technique were dependent upon the polynomial size, sampling frequency, and frequency content of the signal.
The notion that vagal control of the heart closely parallels a variery of psychophysiological phenomena has become widely accepted; however, until relatively recently there has been no simple or noninvasive method of quantifying it. This study quantitatively evaluates and compares two of the most common methods for measuring respiratory-related heart rate fluctuations: Spectral analysis and the Porges technique of detrended, filtered variance. Low-jrequency power was removed from instantaneous, 4 H z RR interval signals using either a first-order linear (Spectral) or third-order parabolic (Porges) polynomial. The Porges technique resulted in signal amplification at a few specijic frequencies.The frequency range and effect of amplification were functions of the sampling frequency and the frequency content of the signal.
The validity and reliability of a new ambulatory impedance cardiograph (AZCG) was tested against the Minnesota Impedance Cardiograph (ZCG) during rest, orthostasis, and mental stress. Impedance cardiography allows noninvasive assessment of stroke volume, cardiac output, and systolic time intervals. A reliable ambulatory device would allow studies outside the lab. The devices were compared at two sites in healthy subjects. In both studies, the AZCG tracked changes across conditions closely with the ZCG (all Period x Device interactions were nonsignificant). Pearson rs, were .65 to .93, random intraclass correlation coefficients ranged from .80 to .98, indicating high degrees of shared measurement variance, and Cronbach's alpha indicated very good internal reliabilities (.91 to .99). Relative to the ZCG, the new AZCG appears to provide valid and reliable estimates of cardiac function at rest and during behavioral challenges in the lab.
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