© B. Bo Sramek

HEMO SAPIENS'
Thoracic Electrical Bioimpedance (TEB) Technology
sometimes called the Impedance Cardiography (ICG)

B. Bo Sramek, Ph.D.

(For explanation of terms new to you, go to Glossary of Terms)

A new generation of TEB technology, discussed below, is an accepted, main stream technology for noninvasive, continuous measurement of global blood flow (Cardiac Index, CI - the global blood flow per minute, and Stroke Index, SI - the global blood flow per beat), respiration and a host of cardiodynamic parameters.

HEMO SAPIENS' proprietary TEB
is utilized in the TEBCO® OEM Module. TEBCO® (Thoracic Electrical Bioimpedance Cardiac Output) measures cardiac index (CI) and 9 other cardiodynamic parameters (stroke index, heart rate, respiratory rate, ventricular ejection time, pre-ejection period, ejection phase contractility index, inotropic state index, estimate of ejection fraction, end-diastolic index,...) noninvasively with the same clinical accuracy as the invasive thermodilution catheter, however, without its cost, risk, need for sterile environment and a skilled clinician. TEBCO's TEB measurement current is 300-to-400-times lower than the measurement current of any competitive TEB/ICG product. TEBCO® OEM Module packaged in a stand-alone enclosure with its own power source is also available as an external TEB module EXT-TEBCO® connected to a PC's via a serial or to COM1or to its USB port.

TEBCO® and/or EXT-TEBCO® also is a cornerstone of a series of HEMO SAPIENS' HOTMAN® Systems. HOTMAN® Systems represent the advanced trend in changing the cardiovascular medicine from monitoring patients (i.e., the reactive approach, responding to a cardiovascular disaster already taking place) to hemodynamic management of patients (i.e., the proactive approach, continually providing the physician with information what therapy to implement in order to maintain the normohemodynamic state). Even one of more expensive versions of HOTMAN® Systems (The Stand-alone HOTMAN® BAS System) still offers much better economics than a competitive ICG monitor, when used in a physician's office or in hospital.

Thoracic Electrical Bioimpedance (TEB), with a symbol Z [ohm], is an electrical resistance of the thorax to a high-frequency, very-low magnitude TEB measurement current. TEB utilizes a patient's thorax as an impedance transducer. The patient is connected to TEBCO®/EXT-TEBCO® via a patient cable attached to eight solid-gel, disposable electrodes (see Fig.1 below). The TEB measurement current is passed through the thorax in a direction parallel with the spine between a pair of electrodes placed on upper neck and a pair of electrodes placed on upper abdomen. On its way through the thorax, the TEB measurement current seeks the shortest and the most conductive pathway. As a result, the majority of the TEB measurement current flows through the thoracic aorta and vena cava superior and inferior - their black outlines are shown in Fig.1 below. The TEB measurement current produces a high-frequency voltage across the impedance of the thorax, directly proportional to TEB. This induced high-frequency voltage is sensed by two other pairs of electrodes placed inside the current path, i.e., at the beginning of the thorax (the line of the root of the neck) and the end of the thorax (the level of diaphragm - the xiphoid process level). These four sensing electrodes also detect four different vectors of the ECG signal. The Heart Rate, HR, is derived from the R-R intervals of the ECG signal. Due to anatomical shape of the thorax, a preferential placement for all eight electrodes is along the frontal plane - the thoracic widest dimension.

Fig.1: Location of the 8 electrodes along the TEB transducer - a patient's thorax. The top and bottom pair is a source and sink of the TEB measurement current, the inner pairs, located at the root of the neck (the beginning of the transducer) and the diaphragm level, i.e., the xiphoid process level (the end of the transducer), are used for sensing both the TEB signal and 4 different vectors of the ECG signal.

The TEB level (the Base Impedance, Zo)[ohm] is indirectly proportional to total content of thoracic fluids, however, it cannot identify individual conductance contributions of the intravascular, intra-alveolar and interstitial compartements. Instead of Zo, TEBCO™, therefore, measures and displays its inverted value, i.e., the Thoracic Fluid Conductivity, TFC [1/ohm], which is then directly proportional to the thoracic fluids content.

The TEB variations and changes (DeltaZ) are produced by:

Fig.2: This picture documents the timing relationship between ECG, DeltaZ and dZ/dt signals: Myocardial contraction starts at the Q-time of the ECG QRS complex. The Pre-Ejection Period (PEP) [isovolumic contraction] is defined as the elapsed time between the Q-time of the QRS complex and the opening of aortic valve. The ejection phase, outlined by the Ventricular Ejection Time (VET), starts by opening of aortic valve and ends by its closure (S2-time). During the initial portion of ejection phase the aorta distends and the thorax, therefore, becomes more conductive; at the same time the velocity of blood increases, more erythrocytes are aligned so their planes are parallel with the main axis of aorta and, therefore, the blood becomes more conductive.

Fig.3: These are two simultaneous recordings of ECG and dZ/dt as displayed on all screens of HOTMAN™ System. To enable operator to confirm visually a detection of key points of the dZ/dt signal and acceptance of that specific heart beat for calculation of SI and other cardiodynamic parameters, the system adds to the dZ/dt signal an upward 2-pixle line where it detected the aortic peak flow [(dZ/dt)max], and adds a downward 2-pixle line where it detected the closure of the aortic valve (the S2 time in Fig.2).

The rate of cardiovascular TEB changes over time (dZ/dt) [i.e., the first derivative of DeltaZ] is an image of the aortic blood flow. Its maximum value, [(dZ/dt)max], is proportional to the aortic blood peak flow (please note the difference between TEB measured peak blood flow and peak velocity measured by Doppler technologies). The aortic blood peak flow is a mirror image of ejection phase myocardial contractility. This parameter's value is influenced both by the Frank-Starling mechanism (change in intravascular volume) and, pharmacologically, by inotropes. TEBCO® processes this parameter normalized by TFC and displays it as the Ejection Phase Contractility Index, EPCI:

The maximum rate of the second derivative of Z, [(dSuperscript2Z/dtSuperscript2)max], is, therefore, an image of the maximum acceleration of aortic blood flow - a true measure of inotropic state, essentially independent of preload and afterload (a detailed discussion and physiologic explanation of these phenomena can be found in the Chapter 7, Hemodynamics of the cardiovascular system, in the textbook Biomechanics of the Cardiovascular System and in the textbook Systemic Hemodynamics and Hemodynamic Management). TEBCO™ measures the level of inotropic state through another normalized parameter - the Inotropic State Index, ISI.

The timing landmarks on ECG (specifically the Q-time of the QRS complex) and on the dZ/dt signal enable measurement of the Systolic Time Intervals, namely the Ventricular Ejection Time, VET, and the Pre-Ejection Period, PEP.

The TEBCO™-measured parameters, i.e., the TFC, VET, EPCI, in conjunction with the Volume of Electrically Participating Tissues, VEPT (a function of patient's gender, height and weight), are used to calculate the Stroke Volume, SV, according to Sramek's Equation:

SV = VEPT * VET * EPCI

Note 1: This equation reflects the physiologic basis of SV determination: (a) SV is directly proportional to the physical size of a patient (i.e., to VEPT - body habitus scaling constant), (b) SV is directly proportional to duration of delivery of blood into the aorta (i.e., to VET), and (c) SV is directly proportional to the peak aortic blood flow (i.e., to EPCI).
Note 2: This equation corrected most of the deficiencies associated with the original TEB Kubicek's equation, used in the '70s.

The Systolic Time Intervals are then used to calculate an estimate of Ejection Fraction, EF, as

EF = 0.84 - 0.64(PEP / VET)

When SV is normalized by the Body Surface Area, BSA, the hemodynamically-significant blood flow parameter called the Stroke Index, SI, is calculated as

SI = SV/BSA

BSA [mSuperscript2] is a complex function of a patient's height and weight, calculated by TEBCO® from the DuBois & DuBois formula:

The perfusion significant blood flow - the Cardiac Index, CI, is then calculated as

CI = SI * HR

Please view the TEBCO® data presentation to see the four processed analog waveforms, the list of 10 measured cardiodynamic parameters and their physical dimensions.

Click here for the List of scientific papers related to new TEB utilizing the Sramek's equation.



TEB REPRINTS AVAILABLE FROM HSI:

Sramek BB. Hemodynamic and pump-performance monitoring by electrical bioimpedance: New concepts.
Problems in Resp Care 1989;2(2):274-290 (JB Lippincott)

Sramek BB. Thoracic electrical bioimpedance: Basic principles and physiologic relationship. Noninvas Cardiol 1994;3(2):83-88

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