By Alan Macy, BIOPAC Systems, Inc.
ICG is a measurement method that is primarily used to determine aspects of Cardiac Output (CO). CO is the total amount of blood, pumped from both the left and right ventricles of the heart for each of its successive systolic phases, over the course of one minute. The systolic phase, or systole, is when the heart muscle contracts and pumps blood from the chambers (left and right ventricles) into the arteries. Typical resting cardiac output is 4.9 liters/minute for an adult human female and 5.6 liters/min for an adult human male.
Left ventricular cardiac output is the CO from the left ventricle only and can be estimated via a non-invasive technique called Impedance Cardiography (ICG), which provides an approximation of Stroke Volume (SV). If SV and Heart Rate (HR) are known, then CO can be determined by:
SV * HR = Cardiac Output
ICG is measured by introducing a constant current (I) through the thoracic volume and then measuring the resulting voltage (V). The following equation is used to determine thoracic impedance (Z).
Z = V/I
Where V and I are the root mean square (rms) values of the measured voltage and supplied (known) current. ICG systems induce a constant magnitude, alternating current (I) through the thorax via electrodes. A separate set of electrodes (placed between the current electrodes) monitor the voltage (V) developed across the thorax. Because the magnitude of I is constant, V will vary in direct proportion to Z.
The applied current (I) flows through the volume of the thorax, which includes skin, skeletal muscle, the lungs, the heart and blood. Because blood is a conductor, the changes in blood volume during a cardiac cycle produce a measurable change in thoracic impedance. Researchers have developed equations for estimating left ventricular SV based on thoracic impedance, with the most widely used equation being that developed by Kubicek, et al. (1966):
SV = rho x (L/Zo)² x LVET x (dZ/dt Max)
SV = Stroke volume (ml)
rho = Resistivity of blood (Ohms·cm) which is assumed to be a constant 135 Ohms·cm (Quail, Traugott, Porges, & White, 1981).
L = Length or distance between inner band (voltage monitoring) electrodes (cm)
Zo = Basal Thoracic Impedance (Ohms)
LVET = Left Ventricular Ejection Time (seconds)
dZ/dt Max = Absolute value of the cyclic peak of the derivative of Zo (Ohms/sec)
LVET varies inversely with heart rate under a broad range of circumstances. LVET should not be treated as a constant in the Kubicek equation. Unlike Length (L), which is constant during a session, Thoracic Impedance (Zo) also varies some, although not as much as LVET. Blood resistivity (rho) also may vary with changes in hematocrit levels and red blood cell orientation, but that variation has largely been ignored because of the difficulties associated with having to draw blood to measure rho.
Impedance Cardiography (ICG) is an indirect measurement of Stroke Volume (SV). Studies have been performed to correlate ICG to SV, using thermodilution as reference and have found that accuracy improves when variables such as age, height, sex, waist circumference and weight are factored into the SV estimation. Accordingly, ICG measures are best interpreted as a relative, versus absolute, indication of SV. Even so, as a relative measure, ICG can provide a clear indication of important physiological processes associated with homeostasis and hemodynamic changes.
Research in psychophysiology, and other areas of physiology, focuses on the left ventricle because it pumps blood that supplies the metabolic requirements of striated (skeletal and cardiac) musculature and thus fundamentally supports behavior and exercise. For Impedance Cardiography (ICG), the left ventricle is principally important because the dZ/dt signal primarily indicates left ventricular blood ejection into the aorta. There is very little influence from right ventricular blood ejection in the ICG signal.
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