An electrocardiogram (ECG) is a recording of the electrical activity of the heart. Electrodes are stuck to the skin of the chest and limbs, and they detect the small electrical changes produced as waves of excitation spread through the heart muscle. The ECG is plotted as a trace of voltage against time.A normal ECG has three main parts:P wave: depolarisation (excitation) of the atria, which leads to atrial systole.QRS complex: depolarisation of the ventricles, which leads to ventricular systole. It is large because the ventricles have much more muscle than the atria.T wave: repolarisation (recovery) of the ventricles, as they return to their resting state and relax.
heart rate = 60 ÷ R-R interval (s) For example, if the R-R interval is 0.75 s, the heart rate is 60 ÷ 0.75 = 80 beats per minute.Using ECGs in diagnosis. Doctors compare a patient's ECG with a normal trace. A resting heart rate above 100 beats per minute is called tachycardia, and below 60 beats per minute is bradycardia. In fibrillation the heart contracts irregularly and no clear P waves or regular QRS complexes are seen. An ectopic heartbeat is an extra beat that occurs early in the cycle. Damage to heart muscle, for example after a heart attack, changes the shape of parts of the trace.