Impedance Cardiography (ICG)
This examination method allows a real beat-to-beat measurement of the stroke volume and is, therefore, very suitable for haemodynamic monitoring, fluid management and cardio-vascular function tests. It is 100% non-invasive, easy to apply and operator independent.
In case of Impedance Cardiography (ICG) – often also named Thoracic Electrical Bio-impedance (TEB) – changes of electrical impedance of the thorax are measured. For this purpose usually 8 electrodes are placed on neck and thorax. The 4 outer electrodes (called current electrodes) are used to pass a very low constant and alternating current (1.5 mA, 86 kHz) through the thorax which is imperceptible to the patient and does not cause any physiological reaction. The 4 inner electrodes (called measuring electrodes) are placed between the current electrodes and measure the voltage which is caused when the current flows through the thorax. This voltage corresponds with impedance changes caused by pulse synchronous variations of the blood volume in the thoracic aorta and the alignment of the orientation of the erythrocytes when the blood is pumped out of the left ventricle of the heart into the aorta. In the result an impedance pulse wave (IMP) is recorded.
For the determination of the stroke volume the first mathematical derivative of the impedance pulse wave (IMP) is used. This curve is known as ICG wave form in which selected fiducial points are detected, such as the B and the X-points corresponding with the opening and the closing of the aortic valve, and the C-point which is the maximum of the ICG curve. Based on these curve points the stroke volume is calculated using an empirical equation.
The accuracy of an ICG device depends essentially on its ability to measure high quality ICG signals and the capability of its algorithms for the detection of the fiducial points to eliminate artefacts and to consider the high variability of ICG wave forms.
In case of healthy people having a low pulse wave velocity in the aorta the systolic part of the ICG wave form is not influenced by the reflected pulse wave so that the fiducial points represent correctly the physiological events which are the basis of the empirical equation for stroke volume calculation. But in the result of pathological changes the situation can be very different so that the standard ICG wave form can be superimposed by other processes such as
- activity of the right ventricle and the pulmonary system
- asynchronous work of aortic and pulmonary valves
- early reflection of arterial pulse wave, especially, in case of arterial stiffness
These influences often result in the appearance of curve points in the ICG wave form which could be, falsely, interpreted as the closing of the aortic valve so that the X-point is not detected correctly and the calculation of the stroke volume is inaccurate.
This is a general methodical problem of impedance technology (including velocimetry, bio-reactance and other derived methods) which can be solved only by using an additional signal. Therefore, the new and unique ACM (Arterial Compliance Modulation) - technology has been developed. For this purpose the arterial pulse waves taken from the ear lobe and/or the upper arm are used in addition to the standard ICG signal to detect the true X-point (closing of the aortic valve). In the result the accuracy of stroke volume calculation can be greatly increased.
To make the application of ICG even more simple the 4 TECT Sensor-Application has been developed whereby only 4 sensors instead of previously used 8 sensors are necessary.
|HR||Heart Rate||heart beats per minute|
|BP||Blood Pressure||pressure exerted by the blood on arterial walls|
|SV/SI||Stroke Volume / Stroke Index||amount of blood pumped by the left ventricle with each heart beat|
|CO/CI||Cardiac Output/Cardiac Index||amount of blood pumped by the heart in one minute|
|VI||Velocity Index||reflects the peak velocity of blood flow in the aorta during systole|
|ACI||Acceleration Index||eflects the maximum acceleration of blood flow in the aorta during systole|
|HI||Heather Index||contractility indicator|
|PEP||Pre-Ejektion Period||duration of electrical systole equal to isovolumetric contraction phase|
|STR||Systolic Time Ratio||ratio of electrical systole to mechanical systole|
|TFC||Thoracic Fluid Content||indicator of chest fluid status|
|TFCI||TFC Index||TFC, normalised to body size|
|PT||Propagation Time||propagation time of the pulse wave|
|PWVao||Pulse Wave Velocity||velocity of the aortic pulse wave|
|SVR||Systemic Vascular Resistance||the force the ventricle must overcome to eject blood into the aorta, estimate of "afterload"|
|SVRI||SVR Index||SVR, normalised to body size|
|TAC||Total Arterial Compliance||indicator of the degree of peripheral arterial stiffness / compliance|
|TACI||TAC Index||TAC, normalised to body size|