Chapter 10: Biopotential Signals

Biopotential Primer
Electrocardiography (ECG)
Electrodermal Activity (EDA)
Electroencephalography (EEG)
Electrogastrography (EGG)
Electromyography (EMG)
Electrooculography (EOG)
Impedance Cardiography (ICG)

Biopotential Primer

The principles associated with the biological generation of electricity were described, by physiologists Hodgkin and Huxley, in four papers that they published in 1952. The papers clearly noted the details of potassium and sodium ion movements as related to the generation of nerve impulses. Hodgkin and Huxley received the Nobel Prize for Physiology or Medicine in 1963 for their discovery.

Biopotentials are electrical signals (voltages) that are generated by physiological processes occurring within the body. Biopotentials are produced by the electrochemical activity of a type of cell, called an excitable cell. Excitable cells are found in the nervous, muscular and glandular systems in the body. When an excitable cell is stimulated, it generates an action potential, which is the essential source of biopotentials in the body. Excitable cell types, that contribute to biopotential generation, are:

1. Afferent (receptor or sensory) neurons that transmit signals from tissues and organs to the central nervous system

2. Efferent (motor) neurons that transmit signals from the central nervous system to effector cells

3. Effector cells, which include muscle cells and neuroendocrine cells, that instigate a physical effect on the basis of a received signal

4. Interneurons that exist entirely within the central nervous system, including the brain. Interneurons convey signals between afferent neurons and efferent neurons.

However, these classifications are quite broad. Detailed examination of neuron types and subtypes, in the entire nervous system, has resulted in the identification of many other classes.

An action potential is a transient change in electrical potential on the membrane surface of a neuron or effector cell. This momentary change occurs when the cell is stimulated, resulting in the transmission of an electrical (nerve) impulse through some part of the nervous system. The nervous system is a principal body communication system and consists of connected neurons that send electrical impulses between different parts of the body. The nervous system is comprised of the brain, spinal cord and peripheral nerves.

A electrical nerve impulse causes a movement of ions across the cell membrane of a neuron. The cell membrane of a neuron contains thousands of tiny proteins known as ion channels (gates). These gates allow either sodium or potassium ions to pass through. Usually, neuron gates are closed. A nerve impulse begins when a sensory input stimulates a neuron’s membrane, causing sodium gates to open, allowing positively charged sodium ions to flow to the inside of the neuron membrane. Accordingly, the inside of the membrane temporarily becomes more positive than the outside. This action is called depolarization. As the impulse passes, the potassium gates open, allowing positively charged potassium ions to flow to the outside of the neuron membrane. The action is called repolarization. At this point in time, the inside of the neuron’s membrane becomes more negative than the outside. The depolarization and repolarization of the neuron’s membrane is called an action potential (nerve impulse). Immediately after a nerve impulse passes is a period when the neuron is unable to conduct another impulse. This short time period is called the refractory period. During the refractory period, the sodium/potassium pump returns sodium ions to the outside of the membrane and potassium ions to the inside. After the refractory period, the neuron is determined to be in resting potential. Resting potential is established after repolarization. A traveling nerve impulse is a propagating wave of depolarization and repolarization. A traveling nerve impulse is the movement of an action potential, through connected neurons, as the series of ions channels open and close. A electrical nerve impulse travels much slower than electrons moving in a conductor. This speed of travel is known as nerve conduction velocity and can vary between 0.5 -120 meters/second, depending on type and location of the nerve.

From a system’s perspective, physical stimuli is sensed by afferent (receptor or sensory) neurons and transmitted to the central nervous system’s interneurons. Responsive neuronal activity is initiated and transmitted by the CNS via interneurons. The CNS may also self-generate neuronal activity to stimulate interneurons. Interneurons send activation signals through efferent neurons to effector (muscle or neuroendocrine) cells to initiate a physical response. Throughout this entire process, from sensory neuron to effector cell, action potentials are generated in copious quantities. Biopotential measurement is largely concerned with the measurement of these generated action potentials.

A typical neuron or effector cell has resting potential of around ‚Äì70 mV and a threshold potential of around ‚Äì55 mV. Action potentials are triggered when enough depolarization accumulates to bring the membrane potential up to the threshold potential. When an action potential initiates, the cell’s membrane potential quickly climbs to a peak of about +40mV, then drops back down, often dipping below and then returning to the resting level. The potential levels and timing relationships vary between neuron and effector cell types.

When recording biopotentials at the body surface, typically many action potentials are juxtaposed, weighted and recorded simultaneously. This is because the measurement electrodes are usually some distance from the activated volume and are large in comparison to the nerves and motor units under investigation. Furthermore, the activated tissue is surrounded by the volume conductor of the body. This surrounding volume conductor acts to variably superimpose and attenuate action potentials. Accordingly, surface biopotentials are usually 1mV or less, even though the source action potentials may be on the order of 100mV in p-p amplitude. With the use of fine wire or needle electrodes, however, measured biopotentials may be substantially higher. This result is primarily due to the proximity of electrodes to the activated volume and the increased specificity of smaller electrodes.

The body sources other biopotentials that are not directly related to action potentials. Examples include the corneal-retinal (corneoretinal) potential, skin potential and the electromechanical behavior of bone. In addition, many physiological processes in the body contribute to measurable impedance changes. Impedance changes are sensed by applying external excitation signals that convert impedance to voltage. These phenomena are usually considered to be indirectly generated biopotentials. Examples include electrodermal activity (EDA), cardiac impedance (stroke volume) and thoracic impedance (respiration). Another class of indirectly generated biopotentials include those in which the physiological process acts to modulate light energy, versus electrical energy. Examples include the optical blood volume density (PPG or BVP) and blood oxygen level (SpO2).

Electrocardiography (ECG)

Electrocardiography (ECG) results in a graph of electrical activity of the heart (cardiac) muscle as it changes over time. Cardiac muscle contracts in response to electrical depolarization of the heart muscle cells. The ECG is the measured sum, over time, of this electrical activity as it is recorded from electrode connections.

Willem Einthoven’s contribution to ECG recording technology was the development of the string galvanometer, that had improved sensitivity over the capillary electrometer. In 1908, Willem Einthoven published a description of a medically useful ECG recording system. This ECG system employed a specific lead connection method presently known as the Einthoven lead system. The Einthoven lead vectors are based on the assumption that the heart is located in a conductive volume. The Einthoven lead vectors are defined as:

Lead I: LA – RA
Lead II: LL – RA
Lead III: LL – LA

Where:

LA is Left Arm
LL is Left Leg
RA is Right Arm

The basic principle of the ECG is that periodic waves of propagated electrical stimulation cause cardiac muscle to alternately contract and relax. These electrical stimulation waves are transmitted from cardiac muscle cell to cell via gap junctions that connect between the cells. The electrical waves spread through the cardiac muscle cells because of cascaded changes in ions between intracellular and extracellular fluid. When the cardiac muscle cells are in a resting (polarized) state, the insides are negatively charged compared to the outsides. Cell membrane pumps maintain this electrically polarized state. Contraction of cardiac muscle is triggered by depolarization. Following depolarization, the cardiac muscle cells return to their resting charge. This process is called repolarization. These waves of depolarization and repolarization result in a changing electrical potential and can be detected by placing electrodes on the surface of the body.

The normal cardiac cycle begins with spontaneous depolarization of the sinoatrial (SA) node, a small volume of specialized tissue located in the right atrium. The SA node sets the rate of the heartbeat. The SA generated wave of electrical depolarization then spreads through the right atrium and across the inter-atrial septum into the left atrium causing the atria to contract. This contraction pushes blood into the ventricles. The atria are separated from the ventricles by a ring of non-conductive tissue. Accordingly, the only conductive path of electrical depolarization, from atria to ventricles, is through the atrioventricular (AV) node. The AV node is a cluster of cells in the center of the heart between the atria and ventricles. The AV node is a pathway that slows the electrical depolarization wave before it enters the ventricles. The wave of depolarization travels down the interventricular septum, through the His-Purkinje fiber network, to the left and right ventricles. The AV node delay gives the atria time to contract, before the ventricles do, to establish robust pumping action. In addition, with normal conduction, the two ventricles contract simultaneously to further maximize cardiac performance. After a volume of cardiac muscle cells depolarizes, the cells repolarize for the next cardiac cycle.

The net (average) path of electrical depolarization, through the heart, is known as the electrical axis. A fundamental ECG measurement principle is that when the wave of electrical depolarization travels toward a recording lead, this results in a positive voltage on that lead. When the wave travels away from a recording lead this results in a negative voltage on that lead. During repolarization, net wave activity towards or from a recording lead results in the opposite polarity as compared to depolarization.

The nature of the repolarization wave is different, from that of the depolarization wave, in that repolarization is not a propagating phenomenon. Cardiac muscle cell repolarization only occurs after the depolarization action potential duration, so cell repolarization does not depend on the repolarization of an adjoining cell. Assuming the action potential of all cells have equal duration, the repolarization wave would follow the same sequence as depolarization wave. However, in considering the entire extent of cardiac muscle, action potential duration is not identical between cells.

Meaning of ECG wavesM/h3>

P wave: Depolarization of the right and left atria
QRS complex: Depolarization of the ventricles
T wave: Repolarization of ventricles

Meaning of ECG intervals

PQ interval: Time period between start of atrial depolarization to start of ventricular depolarization
QRS duration: Time period of ventricular depolarization
QT interval: Time period between start of ventricular depolarization and end of ventricular repolarization
RR interval: Time period of complete cardiac cycle (inverse of heart rate)

Electrodermal Activity (EDA)

Electrodermal activity measurement is subject to electrically-related characteristics of the skin. There are two phenomena associated with this type of skin activity. One is an endosomatic (self-generated) skin potential, intrinsic to the skin. This potential is a consequence associated with dissimilar skin layers, placed in proximity, coupled with the presence and movement of interstitial fluids. An evoked skin potential response is called the sympathetic skin response. The other phenomenon associated with skin electrical activity is exosomatic and is observed when an excitation current is injected through, or an excitation voltage is applied across, attached skin electrodes. The excitation current or voltage stimulation is typically at constant (DC) level, or at low frequency, because the skin impedance incorporates a relatively high value of capacitance. The application of excitation current or voltage produces a corresponding voltage or current, respectively, that changes over time subject to physiological state. In these cases, the skin’s resistance, impedance, conductance or admittance is being measured, corresponding to the type of electrical stimulation used. Changes in skin potential or conductive qualities can be evoked by a wide variety of physical stimuli, including auditory, electrical, magnetic, olfactory, tactile or visual stimulation.

EDA Measurements – subject to electrical excitation type:

Skin Resistance – Direct Constant Current (not commonly used)
Skin Conductance – Direct Constant Voltage (most commonly used)
Skin Impedance – Alternating Constant Current (not commonly used)
Skin Admittance – Alternating Constant Voltage (sometimes used)

Typically, EDA is measured as skin conductance and a small direct constant voltage (0.5vdc) is used as excitation source across the skin electrodes. The skin admittance method, using alternating constant voltage, may provide some advantages in measurement because this method does not polarize certain types of skin electrodes during EDA recording. Skin conductance is measured in units of “Siemens”. The conductivity of the skin is small, so values are given in microsiemens (uS).

Changes in the electrically-related characteristics of the skin are correlated to eccrine sweat gland activity. Eccrine glands are in nearly all skin locations and are found, in highest concentrations, on the palms of hands, fingertips and soles of the feet. The preferred recording sites are the hypothenar / thenar eminences and the distal / medial phalanges of the index and middle fingers. EDA, measured as skin conductance, is a physiological signal that indicates increased sympathetic nervous system (SNS) activity. This aspect of EDA is representative of changes in the electrical conductance of the skin due to eccrine (sweat) gland activity. SNS activity increases sweat gland secretions. Eccrine glands only receive activation signals from the SNS, so increased EDA is an indicator of increased arousal.

There are two aspects to skin conductance measurements, namely phasic and tonic. Tonic value is the average skin conductance level (SCL), that is relatively consistent over time, and is related to arousal. The phasic component of skin conductance is the change of the skin conductance, over a short time period, in reaction to a stimulus. For a skin conductance response (SCR), the skin conductance rises in a short time period and then returns to tonic levels. SCRs are characterized by four different parameters: the response peak amplitude, latency of response, rise time to the response peak, and recovery time after peak.

Electroencephalography (EEG)

EEG is the measurement of differences in electrical potential (voltage) between points on the scalp. These voltages differences are the result of ionic current flow within neurons, and glial cells, in the brain. The measured scalp voltages are primarily in the 200nV-200uV amplitude range, 0.01 Hz – 100 Hz frequency range, and are typically simultaneously recorded at multiple sites on the scalp. The EEG is usually collected using the 10-20 electrode montage. This arrangement of electrodes typically references left or right hemisphere electrodes to the left or right earlobe (or mastoid), respectively. However, a wide range of signal referencing methods are used depending on the situational requirements.

An EEG recording can be short or long-term. Shorter-term, signal-averaged, EEG measurements are typically used to evaluate precognitive and cognitive processes with respect to specific stimuli. These EEG signals have relatively low amplitude levels, typically 200nV – 2uV, so signal averaging methods are required to discern these signals from background concurrent EEG sources. Longer-term, non-signal-averaged, EEG measurements are used to discern the subject’s, real-time, conscious state. Different and observable wave rhythms are evident during such recordings. Commonly observed rhythms include: Delta, Theta, Alpha, Beta and Gamma waves. These waves can be continuously recorded and are in the range of 2uV – 200uV.

Delta waves (0.5-4 Hz) are considered slow rhythms and tend to be the highest in amplitude. Delta waves are normally the dominant rhythm in infants. Delta waves are found in sleeping adults and consistent Delta is considered abnormal in conscious adults. Delta waves are low-frequency EEG patterns that increase during sleep in the normal adult. Although Delta waves are generally prominent during sleep, there are cases when Delta rhythms are recorded from awake individuals. Delta waves may increase during difficult mental activities requiring concentration and other continuous-attention tasks. Delta rhythms depend on activity of motivational systems, in the brain, and participate in the identification of prominent phenomena. The presence and amplitude of Delta rhythms are highly variable within and between individuals.

Theta waves (4-8 Hz) are considered slow rhythms. Theta waves are normal in children up to 13 years old. Theta waves are found in sleeping adults and consistent Theta is considered abnormal in conscious adults. Theta rhythms are low-frequency EEG patterns that increase during sleep in the normal adult. Although Theta rhythms are generally prominent during sleep, there are cases when theta rhythms are recorded from awake individuals. Theta rhythms are involved in memory and emotional regulation. Theta waves will spike during emotional responses to frustrating events or situations. Theta waves are associated with the inhibition of elicited responses, in that Theta activity has been found to temporarily increase when a person is actively trying to repress a behavioral response. The presence and amplitude of theta rhythms are highly variable within and between individuals.

Alpha waves (8-13 Hz) are considered moderate rhythms and usually seen in the occipital regions of the head and are higher in amplitude on the dominant side. Alpha waves appear when relaxing and closing the eyes. Alpha waves are suppressed when opening the eyes or coming to an alert state. Alpha waves are the primary rhythm observed in relaxed adults and are present during most of life after 13 years old. Each region of the brain has a characteristic Alpha rhythm amplitude and the largest Alpha waves are from the occipital and parietal regions of the cerebral cortex. Alpha rhythms correlate with inhibitory processes in the brain. Reduced Alpha wave occurrence and amplitude, in relation to Delta-Theta activity, is associated with reduced inhibitory control over behavior.

Beta waves (13-30 Hz) are considered fast rhythms. Beta waves are typically observed on both sides of the head and are most evident frontally (executive function). Beta waves may be substantially reduced in areas of cortical damage. Beta waves are considered normal and are the dominant rhythm in patients whose eyes are open or are otherwise attentive to external stimuli or exerting specific mental effort. Beta rhythms also occur during rapid eye movement (REM) sleep. In this situation, the typical Alpha rhythm is suppressed and supplanted by Beta waves. This replacement of Alpha rhythm is called desynchronization, or “Alpha block”, because it represents a change in the synchronized activity of neural systems in the brain. It is thought that Beta waves represents arousal of the cortex to a higher state of alertness and may also be associated with memory retrieval.

Gamma waves (30-100 Hz) are considered fast rhythms. Gamma waves are present during mixed sensory processing, such as perceptual tasks that combine hearing and seeing. Gamma waves are also evident during short-term memory matching of recognized sights, sounds or sensations. A reduction in Gamma wave activity might be associated with cognitive decline, especially when compared to Theta wave activity levels.

Long-term EEG signal characteristics are related to consciousness. As conscious activity increases, the dominant EEG rhythms shift to higher frequencies. When asleep, with the exception of REM, the dominant EEG rhythms move to lower frequencies. In deep sleep, the EEG is characterized by low frequency Delta waves.

Short-term EEG measurements are typically focused on the nature of cognition. These types of recordings are typically performed in synchrony with well-controlled, environmental, stimuli. Signal-averaging measurements, of this type, are known as Event Related Potentials (ERPs). There are a range of ERP signals, stretching between 50ms to many seconds, with respect to the occurrence of the event. ERPs require signal averaging methods, to be observed, because their amplitude levels (0.2-2uV) are usually lower than the ambient EEG signal level.

ERPs can be generated by a wide range of stimuli. The stimulus event can be targeted to any specific human sense or combination of senses (auditory, visual, olfactory, touch or taste). ERPs indicate the superimposed potentials of neuronal activity that synchronously generate when a subject processes sensory information. There are two general types of human subject ERPs. Early ERP wave components occur before 100 milliseconds after the stimulus. Late ERP components occur after 100ms after the stimulus. Early ERPs are known as exogenous potentials and late ERPs are known as endogenous potentials. Exogenous potentials depend on neuronal sensory structure responses that occur prior to cognition. Endogenous potentials depend on cognitive responses to the sensory stimuli.

Slow Cortical Potentials (SCPs) are a class of ERPs that are characterized by slowly-moving, stimulus-related changes in the EEG signal. SCPs originate in cell concentrations in the top cortical layer of the brain. SCP waves (0.01-3Hz) are EEG signals that occur before and after stimulus presentation. They encompass such pre-stimulus measures as Contingent Negative Variation (CNV) and Readiness Potential (RP). The CNV is a slow-changing negative potential that happens between an initial warning and a “go” stimulus. CNV amplitude depends on the subject’s expectation of an imagined, forthcoming event. The RP is associated with the beginning of movement. The RP precedes movement and is identified as a slow-changing negative potential that maximizes with movement onset. The RP is observed when signal averaging the EEG, from the motor cortex, synchronously with movement.

EEG can be useful to:

Determine coma and brain death
Evaluate inhibitory control over behavior
Establish brain / computer interfaces
Monitor sleep physiology
Test afferent pathways via evoked potentials
Establish biofeedback situations
Evaluate anaesthesia depth
Locate seizure origin
Test drug effects for convulsion and seizure
Assist in cortical excision (surgery) of epileptic locus
Monitor human and animal brain development

Electrogastrography (EGG)

Gastric electrical activity consists electrical control activity (ECA) and electrical response activity (ERA). EGG is a measure of ECA (gastric slow waves or pacesetter potentials) that specify the frequency and propagation of stomach contractions. Electrical response activity consists of action potentials that are generated by ECA and induce the propagation of peristaltic contractions that originate in the lower portion of the stomach, including the antrum. These peristaltic waves of contraction increase in amplitude as they propagate toward the pylorus and establish the grinding capability of the stomach.

The surface measured EGG signal will be partially composed of a weighted average of gastric slow waves occurring in the stomach. The EGG may also incorporate ECG and respiration artifact. In addition the EGG may also show evidence of the slow wave of the small intestine. Because these signals can easily distort the EGG time-series data, spectral methods are typically used to isolate the gastric slow wave signals.

Spectral analysis of the EGG shows that gastric rhythms can be classified as normal, bradygastria, tachygastria and arrhythmia. The normal frequency of the gastric slow wave in humans is about 2-4 cpm, bradygastria is in the range of 0.5-2.0 cpm and tachygastria is in the range of 4-9 cpm. EGG arrhythmia may include a variety of abnormal rhythms, both consistent and irregular.

Electromyography (EMG)

EMG is the measurement of the electrical potential generated by activated motor units. The simplest motor unit is a single motor neuron connected to a single muscle fiber, but usually many fibers are innervated by one motor neuron. A muscle fiber is also referred to as a myocyte or muscle cell. When a motor unit activates, the motor neuron impulse (action potential) is transmitted to the muscle fibers. The volume where the motor neuron connects to the muscle fiber is called the neuromuscular junction (motor end plate). After the action potential is transmitted across the neuromuscular junction, a corresponding action potential is evoked in all the connected innervated muscle fibers. The sum (average) of all these action potentials is a motor unit action potential (MUAP). An EMG can record a single MUAP, using invasive fine wire electrodes, placed close to the volume of interest. The surface EMG (sEMG) measures, multiple, superimposed MUAPs via non-invasive skin electrodes.

The primary function of muscle tissue is to generate forces and movements in the body. Muscles respond to electrical stimulus by contracting. Muscle tissue has three primary functions: general body movement and stabilization, transporting substances within the body and heat generation. A muscle consists of bundles of specialized cells (muscle cells) that are capable of contraction and relaxation. The three types of muscle are skeletal, cardiac and smooth. These three types of muscle have substantial differences in form and behavior. Even so, all three muscle types involve the movement of actin against myosin to create contraction.

There are roughly 650 skeletal muscles, found throughout the body, whose movements are controlled consciously by the prefrontal and motor cortex in the brain. In skeletal muscle, contraction is stimulated by electrical impulses transmitted by nerves. Skeletal muscle is a very uniform structure of elongated fibers. Skeletal muscle is relatively quick to relax and contract, and is controlled by the central nervous system. Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which periodically depolarize and contract. These contractions are propagated to other connected muscle cells. Cardiac and smooth muscles contract without conscious thought and are considered involuntary. Cardiac muscle is similar in form to skeletal muscle and is found only in the heart. A difference between cardiac muscle and skeletal muscle is that, in cardiac muscle, activation can propagate from one cell to another in any direction. Cardiac muscle has aspects similar to smooth muscle and skeletal muscle in that it has a striated structure (like skeletal muscle) but is also self-excitatory and under control of the autonomic nervous system. Smooth muscle is found within the walls of body structures such as the bladder, blood vessels, esophagus, intestines, lungs, skin, stomach, urethra and uterus. Smooth muscles are controlled by the autonomic nervous system and are the means by which the internal environment of the body is controlled. Smooth muscle has an indistinct arrangement of muscle fibers, are relatively slow to contract and relax, and are also self-excitatory.

Electrooculography (EOG)

EOG is an indirect measurement of eye movement and position. The EOG measures the potential that exists between the cornea (front of eye) and Bruch‚Äôs membrane (back of eye). Bruch’s membrane is a tissue layer that rests on the retina. This relatively stable electric potential is called the corneal-retinal (corneoretinal) potential, that is positive at the cornea and negative at the retina. The corneal-retinal potential is in the range of 2000uV to 5000uV. The corneal-retinal potential is the basis of the EOG and the Electronystagmogram (ENG). The ENG is a test to examine involuntary eye movements, based on EOG operating principles. The ENG is used to evaluate eye movements to determine the functionality of the occulomotor nerve and the vestibular nerve.

With the cornea constantly positive with respect to the retina, movement of the eye produces a shift of electrical potential as measured from surface electrodes on both sides of the eye. This measurement is the EOG. The EOG signal is typically between 400uV and 1000uV. The EOG can be observed by having the subject move their eyes horizontally, from side to side. Electrodes may also be placed above and below the eye to record vertical eye movements. There is an approximate linear relationship between gaze angle and the EOG output signal up to about a 30 degree shift. After this point, the relationship between measured EOG voltage and eye angle position becomes increasingly non-linear.

The EOG is primarily useful for the measurement of eye movement, especially saccades. Saccades result from central nervous system signals that are sent to the eye muscles. These signals result in the eyes making rapid, step movements after which the eyes become stationary at their new position. These step movements are known as saccadic eye movements (saccades). Saccades permit the eyes to make sequential, detailed, visual analysis at each new position. Typically, several saccades are made each second and each new eye position is determined by the autonomic nervous system, without conscious awareness.

Impedance Cardiography (ICG)

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)

Where:

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.