Electrocardiographs record extracellularly and, therefore, detect the difference between parts of the external surface of the cell membranes with an action potential and parts which are at the resting membrane potential. The changes are best detected when the action potentials run parallel to the direct line between the two recording electrodes and are not seen at all when they go perpendicularly to the line between the electrodes. The right ankle is used to earth the patient. The P wave is produced by depolarisation of the atria. U waves are rarely seen outside textbooks.
Electrocardiography is used to detect changes in the electrical activity of the heart.
The abnormalities it can detect include death or damage of parts of the heart muscle so that the action potential follows an abnormal path over the heart and, therefore, changes the shape of the electrocardiogram. In complete heart blocks, due to destruction of the bundle of His, the QRST segments of the E.C.G. will occur less frequently than the P waves and there will not be a fixed relationship between the P waves and the QRST parts of the trace. The electrical axis of the heart and augmented limb leads The waves recorded in an ECG are vectors, having both magnitude and direction. A recorded wave will be largest when it runs parallel to the line joining the two recording electrodes and it will not be seen at all if it is running perpendicular to the line joining the electrodes. The magnitude of the recorded wave is the real magnitude multiplied by the cosine of the angle between the direction of the wave and the line joining the recording electrodes.