Impedance (Z) is the resistance to the flow of electrical current and is measured in ohms. Blood and other fluids are excellent conductors of electricity and have low impedance, particularly compared to bone, tissue, and air. Blood and fluid in the lungs are the most conductive substances in the thorax. Thus, larger quantities of thoracic fluid/blood lower impedance and smaller quantities result in increased impedance. Impedance technology is based on Ohm's Law.
Impedance cardiography introduces a high frequency low magnitude alternating current through the transmitting or "injecting" thoracic electrodes (similar to an apnea monitor). "Sensing" or receiving thoracic electrodes measure the change in voltage associated with the pulsatile blood flow in the ascending aorta which occurs during the cardiac cycle.
Increased blood volume, flow velocity, and alignment of red blood cells (RBC) during systole reduces impedance. Conversely, the decrease in blood volume and flow, and more random configuration of RBCs during diastole causes an increase in impedance. Figure 1 represents transmission of electrical current through the aorta during systole and diastole. By measuring the impedance change generated by the pulsatile flow and the time intervals between the changes, stroke volume can be calculated.
The change in impedance is measured from the baseline impedance (Zo) that is the overall thoracic resistance to flow of electrical current. Zo predominantly reflects total thoracic fluid volume. The magnitude and rate of the impedance change is a direct reflection of left ventricular contractility. This change in impedance related to time (dZ/dt) generates a waveform that is similar to the aortic flow curve. Simultaneous recording of the ECG creates a timing-window for evaluation of each cardiac cycle (Figure 2).
Thoracic Electrical Bioimpedance (TEB)
A technique that provides noninvasive measurement of blood flow, by the analysis of the pulsatile changes in the electrical conductivity of the thoracic tissue. TEB is the measurement of blood flow in the thorax and has been identified to reflect the blood flow through the ascending aorta, i.e. left ventricular cardiac output.
Blood is the most electrically conductive substance present in the thorax. Thoracic impedance changes are the result of volumetric and velocity variations of blood in the ascending aorta. The volumetric changes of the blood and the periodic alignment of erythrocytes during systole provide the pulsatile TEB variation. This pulsatile change in the thoracic impedance represents the mechanical activity of the heart.
The patient is connected with 2 pair of BI-functional electrodes. The outer portion of the electrodes introduces a high frequency, low magnitude, alternating current across the thorax. The inner portion of the electrode measures changes in the resistance to that current.
Correct placement of the electrodes is essential. The dot electrodes define the thorax. The upper limits of the thorax can be described as the root of the neck. The lower limit is at the level of the xyphoid sternal junction, the point where the lower aspect of the sternum meets the xyphoid process. Each pair (upper and lower), need to be placed 180 degrees from each other (directly opposite one another). The black electrodes need to be placed at least 5 cm distal to the white electrodes.
The current across the thorax is modulated by the voltage change between the measured segment. This voltage change is proportional to the known segmental impedance as sensed by the dot electrodes. The signal is then electronically differentiated to produce a record of rate of impedance changes, demonstrated in the dZ/dt waveform.
The ECG and impedance electrodes are connected to the portable monitor and transmit the impedance data to the computer for signal processing and data measurement and calculations.