Overview of Impedance Cardiography
ICG Technology Description
Impedance cardiography (ICG),
also known as thoracic electrical bioimpedance (TEB), is
a technology that converts changes in thoracic impedance to changes
in volume over time. In this manner, it is used to
track volumetric changes such as those occurring during
the cardiac cycle. These measurements, which are gathered
noninvasively and continuously, have become more sophisticated
and more accurate with the development of data signal processing
and improved mathematical algorithms.
This technology, orginally used
by NASA in the 1960s, has benefited from the advent of
the microprocessor and the better understanding of the cardiac
cycle, thanks to technology such as echocardiography and magnetic
resonance imaging. Today, noninvasive methods of measuring
of cardiac output are coming into clinical use on a larger
scale than ever before and are compared with other methods
such as thermodilution and the direct and indirect Fick methods.
|| How ICG
- An alternating current
is transmitted through the chest.
- The current seeks
the path of least resistance: the blood
- Baseline impedance
to current is measured.
- Blood volume and
velocity in aorta change with each heartbeat.
- Corresponding changes
in impedance are used with ECG to provide hemodynamic parameters.
ICG System Parameters
ICG offers noninvasive, continuous,
beat-by-beat measurements of:
information is available in PDF format. Click here to download.)
Stroke volume is the amount of
blood the left ventricle ejects in one beat,
measured in milliliters per beat (ml/beat). SV can be indexed
to a patient's body size by dividing by Body Surface Area (BSA)
to yield Stroke Index (SI).
CO is the amount of blood the
left ventricle ejects into the systemic circulation in one minute,
measured in liters per minute (l/min). To get CO, multiply Stroke
Volume (SV) by Heart Rate (HR). CO can be indexed to a patient's
body size by dividing by Body Surface Area (BSA) or afterload
to yield Cardiac Index (CI).
Vascular Resistance/Index (SVR/SVRI):
SVR is representative of the
force that the left heart must pump against in order to deliver
the stroke volume into the periphery. SVR is directly proportional
to blood pressure and indirectly proportional to blood flow (CO).
SVR is determined by the following
SVR = [(MAP - CVP) /
CO] x 80
SVR can be indexed to a patient's
body size by substituting Cardiac Index for Cardiac Output to
yield Systemic Vascular Resistance Index (SVRI).
SVR = [(MAP - CVP) /
CI] x 80
Index (VI) and Acceleration Index (ACI):
These two indices are both BioZ
specific parameters. VI is the maximum rate of impedance change,
and is representative of aortic blood velocity. ACI is the maximum
rate of change of blood velocity and representative of aortic
Fluid Content (TFC):
TFC is representative of total
fluid volume in the chest, comprised of both intra-vascular and
extra-vascular fluid. TFC is calculated as the inverse of the
baseline impedance measurement. Baseline impedance is directly
proportional to the amount of conductive material (i.e. blood,
lung water) in the chest.
PEP is the measured interval
from the onset of ventricular depolarization (Q-wave in an ECG)
to the beginning of mechanical contraction (first upslope of
the impedance waveform, B point).
Ventricular Ejection Time (LVET):
LVET is the time from aortic
valve opening (B point on the impedance waveform) to aortic valve
closing (X point on the impedance waveform).
Time Ratio (STR):
STR is is inversely proportional
to left ventricular function, and is calculated as the Pre-Ejection
Period (PEP) divided by the Left Ventricular Ejection Time (LVET).
Cardiac Work (LCW):
LCW parallels myocardial oxygen
consumption, and It is the product of blood pressure and blood
flow. LCW is determined with the following equation:
LCW = (MAP - PAOP) x
CO x 0.0144
PAOP is Pulmonary Artery Occluded
Pressure, or wedge pressure. Outside of direct measurement with
a Pulmonary Artery Catheter, a default value of 12 mm Hg can
be used because of PAOP's minimal effect on LCW determination.
LCW can be indexed to a patient's
body size by substiuting Cardiac Index for Cardiac Output to
yield Left Cardiac Work Index (LCWI).
LCW = (MAP - PAOP) x
CI x 0.0144
The number of heart beats per
minute, measured from the ECG (R wave to R wave).