1.5.4 The cardiac electrical field
When a cardiac cell is depolarising or repolarising, different currents flow across the cell membrane at various points and a potential difference will occur between one part of the cell and another (Figure 1.12). This results in a ‘battery’ effect with the cell acting as a dipole. During flow of current along the cell surface, an external electrical field is set up around the dipole. However, when the cell is depolarised or repolarised and therefore at a resting potential, the membrane potential is not rapidly changing. Consequently, there is no difference in the membrane potential at different points along the cell surface and therefore no electrical field, despite the potential difference between the inside and outside of the cell.
The surface ECG
The changes in the electrical field around the heart can be detected by a gal(electrocardiograph) attached to the body surface. The electrois simply a voltmeter which records the potential difference between two electrodes. The link between a positive and negative electrode is called a bipolar lead. An electrocardiograph records the potential difference between electrodes placed at various points on the body surface. It does not record the depolarisation of each individual cell, but detects the sum of all the electrical fields which are present at any one time. When an electrocardiogram (ECG) is recorded, it is assumed that the heart can be represented by a single dipole, with the electrical activity producing a simple field around it. This is a major overbut does not invalidate the use of the technique. The points at which the ECG electrodes are placed are chosen to represent the electrical changes which are occurring in the heart, but a number of factors affect the potential difference between different areas of the body. The position of the heart within the body, the pattem of the spread of activation within the heart, the shape of the thorax, the conductivity of tissues between the heart and the electrodes, and the exact location of the body surface electrodes, all affect the body surface ECG.
Detection of the conduction process
The first electrocardiographers recognised the electrical activity of the heart at the body surface and gave the description P, QRS and T waves to the deflections on their voltmeters (Figure 4.3). These waves and complexes represent the depolarisation and repolarisation of the atria and the ventricles. Identification of these waves allows the clinician to determine that these processes have occurThe timing of the waves and their duration provides further information about the conduction process.
If conduction follows the normal pathway, the P, QRS and T waves will be recognised and have a normal relationship. If it follows a different pathway, the shape of the complexes, their timing, and the duration of the intervals between them, may change. The rate at which the QRS complexes occur indicates the heart rate. The regularity of the waves and complexes indicates the cardiac rhythm.
The concept of cardiac vector
The ECG voltmeter will have a positive deflection if the net direction of overall activity (vector) is towards the positive electrode of a bipolar lead, and a negative potential if it is away from the positive electrode. The voltage recorded will be largest when the vector is directly towards the positive electrode. If the direction of the maximum potential difference is at an angle to the lead axis, the deflection will be smaller (Figure 1.13). If the electrodes are positioned perpendicular to the vector of electromotive force, no potential difference will be detected. The sum of the electromotive forces changes in magnitude and direction from instant to instant during the course of the depolarisation and repolarisation processes. This is called the instantaneous vector. As the instantaneous vector is difficult to measure, the average direction and magnitude are more commonly recorded. The amplitude of the deflection indicates the magnitude of the vector and is also dependent on the mass of myocardial tissue which is depolarised. Multiple vectors from different groups of cells may cancel each other out when their electrical fields are summated before reaching the body surface.