Bioimpedance Cardiac Output Monitoring
An interest in studying cardiovascular function during space flight
initially prompted investigations of impedance plethysmography as a noninvasive method
of determining cardiac output. The technique of bioimpedance
cardiac output monitoring was first described by Kubicek and colleagues
and is based on the changes in electrical impedance of the thoracic cavity that occur
with the ejection of
blood during cardiac systole.[758]
Their original
formula relates these bioimpedance measurements to stroke volume.
where SV = stroke volume
ρ = specific resistivity of blood
L = thoracic length
ZO2
= basal thoracic impedance
VET = ventricular ejection time
= maximum rate of
change in impedance during systolic upstroke
Cardiac output is computed from the product of the derived stroke
volume and heart rate. Over the years, this formula for deriving stroke volume has
been modified based on refined models of the behavior of thoracic resistivity.[626]
[759]
[760]
Various devices have been marketed commercially that measure cardiac
output by the bioimpedance method. Each requires the application of disposable electrodes
to the skin surface along the sides of the neck and lateral aspect of the lower part
of the thorax. Impedance measurements are made by applying a continuous small electrical
current across the chest. Patient height, weight, and gender are entered into the
monitor by the operator to allow calculation of the volume of the thoracic cavity.
Bioimpedance cardiac output is computed for each cardiac cycle and continuously
displayed as an average value over several heartbeats.
Validation studies comparing bioimpedance cardiac output measurement
with other methods have produced inconsistent results.[626]
[739]
[740]
[760]
[761]
[762]
[763]
[764]
[765]
[766]
Although many studies suggest that the bioimpedance method is accurate in healthy
volunteers, its reliability deteriorates in critically ill patients, including those
with sepsis, increased lung water, aortic regurgitation, and electronic cardiac pacing.
[760]
[761]
[766]
Because the bioimpedance method is an indirect measure of cardiac output with a
complex mathematical derivation, it is not surprising that this monitoring method
has not proved to be as reliable as some other techniques.
Recent reports of animal studies and small clinical trials have
described changes in signal-processing techniques that have improved the accuracy
of thoracic bioimpedance measurements.[767]
[768]
[769]
[770]
In
an
attempt to further improve signal quality, an alternative approach to bioimpedance
monitoring uses a specially designed endotracheal tube to measure electrical impedance
changes in the ascending aorta.[771]
Despite these
refinements, bioimpedance cardiac output monitoring has not found broad clinical
acceptance to date.
