Theory of Bioelectric Signals and Electrocardiogram

Cell Membrane Potential:

Tissues, fibre and muscles in the human body are all comprised of cells. Cells have fluid both internally, intracellular fluid, and on exterior surrounding them, extracellular fluid. Both extracellular and intracellular fluids have an abundant supply of ions, atoms containing an imbalance in the total number of electrons and proton and thus carrying a net negative or positive charge. The most general active ions found in human cells are Sodium Na+, Potassium K+ and Chloride Cl-. Such ions can pass via the membrane or wall of the cells via channels that have variable degrees of permeability to the various ions as shown in figure below. Furthermore, the channels have distinct degrees of permeability to ions travelling into and out of the cells. Because of different permeability in each and every direction, the concentration of ions is distinct inside the cells than outside.

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 Figure: Concentration of Ions inside and outside of Cells

In case of human cells below equilibrium conditions the:

  • Concentration of Sodium Na+, is lower within the cell than outside,
  • Concentration of Potassium K+, is higher within the cell than outside,
  • Concentration of Chloride Cl-, is lower within the cell than outside.

As the ions carry an associated electrical charge, the outcome is that the total charge within the cell varies from that outside the cell. This gives mount to a difference in potential across the wall of cell from inside to outside. This is termed as the cell Membrane Potential, Em, and can be predicted by the Goldman Equation as:

Em = (RT/F) ln {(PK[K+]ext + PNa[Na+]ext + PCl[Cl-]int)/(PK[K+]int + PNa[Na+]int + PCl[Cl-]ext)

Where:

R = Universal Gas constant, 8.314 JK-1mol-1
T = Temperature in degrees Kelvin
F = Faraday constant, 96.5 x 103 Cmol-1
Pi = Permeability of the cell membrane to the ith ion.
[I±]loc = Concentration of the ith ion either outside or inside the cell.

In animal and human cells this potential, that is measured across the cell membrane from inside to outside, is negative in polarity and consists of a value ranging from -40 to -100 mV, depending on the kind of cell. It is close to -90 mV in most of the human cells.

Action Potential:

Most of the cells in body, and in particular those related with nerve and muscle fibres, can be excited either chemically or electrically. An electrochemical stimulus can induce modifications in the permeability of cell membrane to various ions and cause the cell to become active. This signifies that the flow of ions across the cell membrane modifies abruptly and therefore also the volume of charge on each and every side of the membrane. This is accompanied by the corresponding abrupt change in trans-membrane potential and hence the cell becomes depolarised, sometimes encompassing a slight change in the potential in opposite direction to its equilibrium state. The cell will ultimately repolarise however generally at a slower rate than that at which it depolarises. This gives mount to the potential profile shown in figure below known as the Action Potential.

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Figure: A Typical Action Potential Profile

Once a cell becomes depolarised, the modifications in the conditions surrounding the cell can act as a stimulus to the adjacent cells and thus a corresponding activation of such cells occurs also. In muscle and nerve cells the impulse produced by depolarisation of the cells can be passed from one cell to the next through synapses and axons, and hence the stimulus passes all along a nerve or muscle fibre as a wave with the repolarisation wave following behind. This permits impulses to cause contraction of muscles to induce intrinsic bodily actions, like the movement of limbs and the beating of heart.

The Cardiovascular System:

The cardiovascular system of human body is essentially one of the heart acting as a pump to force the blood around body. Blood acts as a transport system to carry oxygen, nutrients and chemical agents to all organs, limbs and tissue in the body and also to transport waste products and toxins to organs for disposal. However, the heart really operates as a double pump and the circulatory system comprises of two separate circuits as shown in figure below. The heart consists of four chambers, the left and right atria on the top and left and right ventricles on the bottom.

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Figure: The Atria and Ventricles of the Human Heart

A simplified representation of circulatory system is as shown in figure below. Blood is gathered from all parts of the body into the right atrium, from where it is then transferred to right ventricle. The right ventricle contracts to force blood out of the lungs where carbon-dioxide is eliminated from it and fresh oxygen is absorbed. From lungs, the re-oxygenated blood travels back to the heart and into left atrium. This loop is termed as the pulmonary circulation.

Blood is then transferred to right ventricle, that contracts with strength to force the blood out beneath pressure to all organs and limbs in the body. Once nutrients and oxygen have been distributed through the blood to nourish all the cells around the body and waste products have been collected and delivered for the excretion, then blood returns to right atrium again. This second circuit is termed as the systemic circulation.

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Figure: The Heart and Circulatory System

The nonstop rhythmic pumping of heart is caused by the contractions of muscles within the walls of each chamber that pumps blood from chamber to chamber and all through the circulatory system. Such cardiac rhythms are controlled by specific mechanisms operating in the heart which transmit action potentials or electrical impulses all along nerve fibres to the cells in the muscles in order to activate them at suitable points in the cardiac cycle.

Electro-stimulation of the Heart:

Figure below shows the main elements of heart’s electro-conduction system. The sino-atrial (SA) node is a group of cells positioned in the upper right atrium. This node comprises special electrochemically stimulated cells that depolarise and repolarise rhythmically without the requirement for external influence.

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Figure: The Electro-conduction System of Heart

Once the trans-membrane potential in cell reaches a certain threshold, the cell self-depolarises giving mount to an associated action potential, and then repolarises more slowly. It does this in a nonstop and rhythmical manner, therefore efficiently providing the electrical oscillator that repeatedly produces the trigger stimulus to operate the nerve fibres of the heart and the muscles of chambers to sustain a regular heartbeat. Whenever the SA node ‘fires’, the resultant electrochemical stimulus spreads across the muscles in the walls of right and left atria causing them to contract. Blood is thus forced out of the atria and into the lower ventricles on both sides of heart. The stimulus moves rapidly from the sino-atrial node towards the atrio-ventricular (AV) node in around 30 - 50 ms.

In order to permit the atria transfer, their contents to the ventricles prior to the latter contract due to the approaching action potential, the AV node operates as a delay unit slow down the transmission of the action potential by a further 110 ms prior to the stimulus is passed on by the AV node. The impulse is then transferred from AV node towards the ventricles through a branch of fibres termed as the Bundle of His that splits into right and left bundle branches as shown in figure above. Once the impulse reaches the right and left bundle branches it travels very fast through the Purkinje Fibres that excite the muscles in the walls of ventricles from the bottom upwards. The impulse can reach the furthest fibres just 60 ms subsequent to leaving the AV node. The action potential now causes ventricular contraction that forces the blood from the ventricles out into the pulmonary and systemic circulations. The excitation of such a big number of cells at similar time makes a significant electrical signal and a resultant electric field that is emitted outward from the heart to the surface of body. Such emanating electric signals can be detected by using electrodes positioned on the surface of body that is, on the subject's limbs or chest. The recorded electrical signal detected in this way is what is termed today as the Electrocardiogram or ECG signal.

The Electrocardiogram or ECG:

An idealised human ECG is shown in figure below. It can be seen that there are some distinct components that make up the whole signal profile which is measured over a single complete cardiac cycle.The major components are recognized as the P-wave, The QRS complex and the T-wave. Other segments and intervals that have a clinical significance from a diagnostic are defined. The amplitude of QRS complex of a signal measured on a subject’s chest is usually between 1 – 5 mV.

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Figure: An Idealised ECG Human ECG Signal

The various components of ECG correspond to various events taking place in the heart over a cardiac cycle.

The P wave is related with depolarisation of cells in the muscles of atria that cause the atria to contract and transfer blood to the ventricles.

QRS complex corresponds to sharp depolarisation of the cells in many and strong muscles of the ventricles. This period is termed as ventricular systole. The repolarisation of the cells in atrial muscles is masked by the QRS complex and can’t be observed independently.

The T-wave corresponds to the repolarisation of cells in ventricular muscles throughout their resting phase termed as ventricular diastole.

The duration, shape and rhythm of such components and of the segments among them can give invaluable insight into the state of the heart and cardiovascular system.

The initial record of a functional ECG recording was introduced by Willem Einthovencirca in 1904. Einthoven originally employed four jelled metal electrodes joined to the limbs of a subject for the recording procedure. This was later found that the electrodes could be moved to positions on the thorax devoid of loss of signal strength. Einthoven developed the Leads termed as the Einthoven triangle as shown in figure below, comprising Lead I, II and III as indicated that represent different pairings of electrodes, each providing  a distinct aspect of electrical activity in the heart.

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Figure: The ECG Einthoven Triangle Lead Configurations

Such lead configurations are measured as shown:

Lead I = LA – RA
Lead II = LL – RA
Lead III = LL - LA

On averaging the potential measured at the three major locations and employing the resultant as a new reference, three additional lead configurations termed as Augmented Leads can be obtained.

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Figure: The Unipolar ECG V-Lead Configurations

A further six leads, termed as unipolar V-leads, attached at a number of strategic positions around the chest give extra information on the electrical activity of the heart as shown in figure above.

A complete clinical ECG evaluation comprises of measuring all twelve lead configurations. All of the points employed to obtain such leads need electrodes of some sort to be joined to the body at accurate locations.

Electrodes:


Most of the biomedical measurements employ electrodes. Usually, an electrode measures the electrical potential at a location on the surface of body, generally relative to the potential at other position. They essentially execute the task of transforming the ionic current related with electrical activity in the body into electronic current that is fed into the input of a recording amplifier. Electrodes are most commonly used in the measurement of:

  • The Electrocardiogram or ECG, pointing cardiac activity,
  • The Electroencephalogram or EEG, point out brain activity,
  • The Electromyogram or EMG, pointing muscle activity and
  • The Electrooculogram or EOG, point out the activity of eye.

Initially the ECG was measured having the patient sitting with their limbs immersed in the cylinders filled with electrolyte solution, into which big metal rod electrodes were placed that were then joined to the recording machine.

  

Later, metallic strap-on types, that used a conductive paste to enhance contact with the subject’s body were developed. Such became smaller and more proficient with the metal ultimately becoming Silver-Silver-Chloride for best outcomes.

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Figure: Strap-on ECG Electrodes

Nowadays ECG electrodes are disposable, pre-jelled, self-adhesive, types generally employed in clinics and hospital wards and come in different sizes and shapes as shown in figure below. Illustrations of Electrodes in use for other measurements like EOG, EEG and EMG are shown in figure below and have essentially similar design requirements as those for ECG measurement and impose the same performance demands on recording amplifiers.

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Figure: Modern Disposable, Adhesive, Pre-jelled ECG Electrodes

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