Noninvasive
Cardiac Output (NICO) measures can be determined by employing electrical
bioimpedance (EBI) measurement techniques. Electrical bioimpedance is the
characteristic impedance of a volume
of tissue and fluid. For Cardiac Output measures, the
relevant tissue includes the heart and the immediate surrounding volume of the
thorax, and the relevant fluid is blood.
The electrical
impedance of the thorax (Zt) can be thought of as composed of two impedance
types:
- The base impedance (Zo) corresponding to non-time varying tissues, such as muscle, bone and fat. Zo is measured when the pulsatile volume is minimal.
- The delta impedance (dZ) corresponding to time-varying fluid volume (blood).
Zt drops with
each pulsatile volume of blood ejected from the heart. dZ/dt is the magnitude
of the largest impedance change during systole.
Noninvasive
cardiac output amplifiers incorporate a precision high-frequency current
source, which injects a very small (400 microamp rms) current through the
measurement tissue volume defined by the placement of a set of current source
electrodes. A separate set of monitoring electrodes measures the voltage
developed across the tissue volume. Because the current is constant, the
voltage measured is proportional to the characteristics of the biological
impedance of the tissue volume.
BIOPAC data
acquisition hardware for recording real-time Noninvasive Cardiac Output
parameters include tethered amplifiers such as the NICO100C (or NICO100C-MRI
for fMRI environments), EBI100C, or SS31L, or the wireless BioNomadix BN-NICO.
Hardware
simultaneously measures impedance magnitude and phase, and may
incorporate an internal derivative function to output dZ/dt simultaneously with
Z; an AcqKnowledge calculation channel can also be used to determine
dZ/dt. The internal derivative function inverts the polarity of the dZ/dt
signal so that it displays a positive-going peak, coincident with negative
slopes indicated in Z, as per academic research convention.
Disposable
Ag/AgCl Paired Electrodes (EL500) are recommended for bioimpedance and cardiac
output studies. Paired electrodes provide fixed spacing between the dual
contacts and are pre-gelled with hypo-allergenic, wet liquid gel electrolyte
(10% chloride salt); the gel cavity is situated between electrode and skin
surfaces and helps reduce motion artifact.
The impact of
movement artifacts on recorded data is a significant methodological concern in
impedance cardiography; Ensemble Averaging tools in AcqKnowledge provide
an automated strategy to remove these artifacts. Watch a demo now!
