Overview of Impedance Cardiography (ICG)



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 1960’s, 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 Works:

 
  •  An alternating current is transmitted through the chest.
  • The current seeks the path of least resistance: the blood
    filled aorta
    .
  • 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:
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Stroke Volume/Index (SV/SVI):

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).

Cardiac Output/Index (CO/CI):

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).

Systemic 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 equation:

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

Velocity 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 blood acceleration.

Thoracic 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.

Pre-Ejection Period (PEP):

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).

Left 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).

Systolic 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).

Left 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

Heart Rate (HR):

The number of heart beats per minute, measured from the ECG (R wave to R wave).

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