Each wave represents an electrical activity in a different region of the heart and in order to fully understand it, we must review the definitions of depolarisation, and repolarisation. Depolarisation is the reduction of the myocardial wall to a more positive value as a result of influx of sodium ions. Depolarisation leads to the generation of an action potential, which leads to contraction of the myocardium, resulting in systole. Repolarisation, on the other hand, is restoring the myocardial wall back to its negative resting potential, usually as a result of efflux of potassium ions. This usually puts an end to the contraction, resulting in diastole (Cason, 2014). Depolarisation and repolarisation of the myocardial wall can be represented by the following graph:
Now, it is understandable that P wave (represented by the colour green in figure 1) signifies depolarisation of the atrial myocardium. During this stage, the atrioventricular valves (mitral and tricuspid) are open, allowing blood to flow from the atria, into the ventricles. As the myocardial wall becomes more and more positive during depolarisation, an action potential is generated, resulting atrial systole, which is rep...
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... The P cells and the T cells. The P cells produce the electrical signal, while the T cells transmit the impulse to the right atrium, which is then passed on to the left atrium via the Brachmann’ bundle. This results in simultaneous systole of the two atria, squeezing the blood into the ventricles. Sinoatrial exit block is a result of failure of the T cells to transmit the electrical signal to the right atrium.
Depolarisation and contraction of the atria is represented by the P wave on the ECG. With sinoatrial exit block, the electrical signals are blocked, preventing propagation beyond the sinoatrial node. The node proceeds to depolarise as usual, but because some of the electrical signal is blocked before it leaves the sinoatrial node, atrial contraction is erratic. The P waves on the ECG appear to drop, as shown in figure 4 below (Bonow, Zipes & Libby, 2011).
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