The electrocardiogram (ECG) is the surface recording of electrical activity of the heart as the cardiac muscle depolarizes and repolarizes (see Chapters 11 and 13). The recorded voltage (1–2 mV) is much smaller than that of the action potential, and reflects the vector sum of currents between depolarized and resting cells, thus providing both amplitude and directional information. Identification of intermittent events such as paroxysmal arrhythmias may require ambulatory ECG recording over 24 hours (Holter test), or an exercise tolerance test where workload is progressively increased to elicit events related to coronary artery disease for example.

Recording the ECG

The ECG is based around the concept of an equilateral triangle (Einthoven’s triangle; Figure 14a). The points of the triangle are approximated by placing electrodes on the right arm (RA), left arm (LA) and left leg (LL). The right leg is commonly used as an earth to minimize interference. The voltage between any two electrodes will depend on the amplitude of the current, which is related to muscle mass, and the mean direction of current; it is thus a vector quantity (Figure 14b). The greatest voltage and thus deflection is therefore seen when the wave of depolarization is directly towards or away from the respective electrodes. By convention, the ECG is connected such that a wave of depolarization towards the positive electrode causes an upward deflection, and the paper speed of the recorder is normally 25 or 50 mm/s.

The various combinations of electrodes are called leads (not to be confused with the cables connecting the electrodes). The three bipolar limb leads each approximate the potential difference (PD) between two corners of Einthoven’s triangle, and are essentially looking at electrical activity in the heart from three different directions, separated by 60°. Lead I measures the PD between RA (positive electrode) and LA (negative electrode); lead II, RA (negative) and LL (positive); and lead III, LA (negative) and LL (positive).

The unipolar leads use a single sensing electrode, and measure the PD between this and an indifferent electrode representing the average potential of the whole body (i.e. zero). Practically, this is obtained by connecting RA, LA and LL together, which approximates the centre of Einthoven’s triangle (i.e. the heart). The six precordial (chest) leads use a separate sensing

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