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The Electrocardiogram (ECG) Ion fluxes across the cell membrane of cardiac muscle cells during waves of depolarisation and repolarisation result in voltage differences and currents on the surface of muscle cells. The electric currents spread through tissues surrounding the heart via body fluids. As a large number of cells are electrically active simultaneously in the cardiac muscle, the extracellular currents are strong enough to be able to record voltage differences between electrodes on the surface of the body. In this way, the electrical activity of the heart can be recorded using ECG. Although ECG does not directly measure the contractility of the heart or cardiac function, it can be inferred by the electrical activity recorded. It works by recording the difference in potential between a set of electrodes. There is no current flow between the electrodes if there is no difference in potential between them (i.e. the membrane is stable). As a wave of depolarisation starts to hit the membrane, there is a different between the positive and negative electrode. When the whole membrane is depolarised, there is no longer a difference. The membrane is then repolarised in the same direction, causing another different between the electrodes, then stability once more.
Dipoles and dipole vectors Dipoles exist within the myocardium. These are a result of ion movements, and current will flow between these poles. The currents between the two dipoles are elliptical and diminish as the distance from the dipole increases. A maximum voltage difference is recorded if electrodes are placed at opposite ends of the dipole. This is shown in the following diagram. There will also be a potential difference between points A and B. However, there will be no potential difference between points C and D as they are equally far from the ends of the dipole. The strength and direction of a dipole can be expressed as a vector, illustrated by an arrow, with the length being proportional to the voltage difference. The head of the arrow points towards the positive end of the dipole. During depolarisation or repolarisation, each cardiac muscle cell can be expressed as a small vector. The individual vectors can be summed into a larger vector. Two equal vectors of opposite directions will cancel each other out.
However, there is still a resultant vector during depolarisation and repolarisation of the myocardium. The size and direction of the resultant vector continually change, as the dominant pacemaker cell changes. The left side of the heart contributes more to the resultant vector than the right side. The last part of the heart to depolarise is the base of the left ventricle. The direction of the resultant vector is then only determined by electrical vectors in this part of the heart, and no other electrical currents are present in the rest of the heart. Bipolar recordings In bipolar readings, the voltage difference between two surface electrodes is recorded continuously as a function of time. There are three standard bipolar leads with the following electrode positions ...
Lead I - right foreleg (negative) and left foreleg (positive). RF- and LF+
Lead II - right foreleg (negative) and left hindleg (positive). RF- and LH+
Lead III - left foreleg (negative) and left hindleg (positive). LF- and LH+
The above leads are used in dogs and cats. In horses and cattle, typically only a base-apex lead is used. The recording is positive when the positive electrode is positive relative to the negative electrode.
The three leads can be thought of as an equilateral triangle surrounding the heart. Lead II usually sees the largest potential difference as it is placed almost at each end of the normal dipole. Three leads are used due to the changing dominant pacemaker cells, and the resulting change in dipole vector.
The standard ECG There are several features of a standard ECG ...
The P wave represents atrial depolarisation. It is a small negative deflection. It starts immediately before atrial contraction.
The QRS complex represents ventricular depolarisation. It is a large negative deflection. Ventricular contractin begins during the QRS complex. (N.B. atrial repolarisation occurs during ventricular repolarisation). The Q wave corresponds to septal depolarisation, whilst the ventricular depolarisation from endo- to epicardium is represented by the R wave. The S wave represents the depolarisation of the left ventricular base.
The T wave represent ventricular repolarisation. This may be positive, negative or biphasic but is consistent for individuals.
The PQ (or PR) interval represents the time taken for the electrical activity to pass through the AV node.
The isoelectric baseline of the ECG reading occurs when there is no current flow.
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