Medical Physics Option Notes ECG - electrocardiogram

ECG The ECG is a quick and economical bedside test, which can yield invaluable information regarding cardiac function. It uses the fact that potential differences are produced by biological systems – these can be measured and used for diagnosis

Biopotentials A biopotential is a voltage generated inside the body and generally arises from salt concentration differences across cell membranes. These so-called 'membrane potentials' are exhibited by nerve, muscle and gland cells. Membranes are like dividing gauze between two parts of the body – they allow tiny particles through and stop bigger ones from transferring. Different concentrations are possible on either side.

Biopotentials All living cell membranes pass water, but the solute (solid bit – dissolved in the solvent of water!) transmitted depends on the state and type of membrane. For example, cell membranes of the large intestine pass even large molecules, whereas nerve fibre cell membranes are fine enough only pass the ions of NaCl and KCl (i.e. Na+, Cl- and K+).

An axon – nerve fibre – in its resting state Membrane potential approx 70 mV

Axon A nerve fibre, or axon, is a long, thin (a few micrometres in diameter) extension of a nerve cell (neuron), and consists of a central core of axoplasm, surrounded by a high-resistance membrane.

Axon In its resting state, there is a high concentration of negative ions and K + ions inside, and Na + ions outside the membrane. The balance is maintained by osmotic and mechanical forces. A resting membrane potential of about 70 mV is typical (you should know that!).

Action Potential When a nerve, muscle or gland cell responds to a stimulus, the membrane potential exhibits a series of reversible changes: depolarisation, reverse polarisation and finally repolarisation. This sequence constitutes an 'action potential'

Ion Movement through the Membrane When a nerve cell is stimulated, the cell membrane suddenly becomes permeable to Na+ ions which then move into the axoplasm from their higher concentration area outside The increase in positive charge inside the cell leads to a change in the membrane potential from about -70mV to +40mV (reverse polarisation).

Ion Movement through the Membrane Almost immediately, the membrane becomes impermeable to Na+ ions and permeable to K+ ions, which consequently leave their high concentration area inside the fibre and K+ ions, which consequently leave their high concentration area inside the fibre and move out, thereby restoring the original membrane potential of -70mV, (repolarisation). The Na+ and K+ ions are re-exchanged later during a slower recovery period.

The action potential of heart muscle Each complete cycle of: depolarisation (causing contraction), reverse polarisation and repolarisation (relaxation) corresponds to one heartbeat.

Propagation of the signal The depolarised (active) region of a fibre acts as a trigger, stimulating adjacent region to follow through the same action potential. How fast the action potential moves along the axon depends on several factors including type of cell, fibre diameter and temperature.

Detection of biopotentials Electrodes placed on the surface of the body measure the coordinated activity of a large group of cells and thus register the local action potential, which may be neural or muscular, depending on the electrode locations. Since body materials, particularly skin, are poor electrical conductors, the selection and preparation of electrode site are most important.

Detection of biopotentials For instance, a suitable site must be well-cleaned, hair-free and rubbed to remove some outer cells. A conductive, yet non-irritant, electrode paste is rubbed into the site to improve the electrical contact and the applied electrode is then held firmly in place using tape.

Preparing a patient for an ECG The patient must be striped to the waist to expose the chest (the doctor/nurse should keep the patient's lower half covered as much as possible, they may have seen it all before but should always treat the patient with the utmost respect). The patient's ankles also need to be exposed at this point.

Preparing a patient for an ECG If using a machine equipped with metallic stickers, it is important that the patient's skin is rubbed with an alcoholic swap before applying them, to ensure good electrical contact is made. If the machine is supplied with the older suction cups, then electrolyte spray must be applied to the areas of skin an which electrodes will be placed. Men with very hairy chests may require a gel based electrolyte for adhesion, or in extreme cases, shaving may be needed.

Applying the contacts Accurate chest lead placement is essential for ensuring quality ECG output. Misplacing leads may result in a change in ECG waveform, in turn this may cause the ECG trace to be misinterpreted. This is unacceptable, so chest electrodes must be placed methodically.

Applying the contacts The standard chest lead positions are as follows; V1 Fourth intercostal space, right sternal edge V2 Fourth intercostal space, left sternal edge V4 (Don't worry, not a mistake, place the fourth electrode before the third) Fifth intercostal space in the mid-clavicular line V3 I could describe this anatomically, but just between you and me, bang it half way between the second and fourth electrodes V5 Lies on the fifth rib in the anterior axillary line V6 On an imaginary horizontal line with V5 in the mid axillary line

The rest of the leads The four limb leads are easy. One on each arm (go for the wrist, have the metal plate of the clip facing the palm aspect, avoid bony prominences) and one on either ankle. Make sure the right clip is on the right wrist. The final electrode is the "neutral", it reduces AC interference and in all honesty could be applied to the patient's nose without any adverse effects to the ECG.

PQRST waveform The "P" wave corresponds to atrial contraction. The "QRS" complex relates to the contraction of the ventricles, it is much larger than the "P" wave due to the relative muscle masses of the atria and ventricles. The repolarization or relaxation of the ventricles can be seen in the form of the "T" wave, the repolarisation of the atria being masked by the "QRS" complex.

PQRST waveform The contraction of cardiac muscle is regulated in a centre located in the right atrium known as the sinus node. Cardiac muscle cells have a special property: they spontaneously depolarize at various rates (i.e. the charge of their membranes changes at a given rate without external stimuli). The resultant coordinated contraction leads to the ECG waveforms that need to be known for the examination.

Timing is important – it shows whether the heartbeat is normal of not ECG segment: P Normal range : wave less than 0.12s ECG segment: PR Normal range : 0.12s - 0.22s ECG segment: QRS Normal range : less than 0.10s