Explain the stages of an ECG trace.

The P wave is caused by the contraction of the atria, forcing the blood into the ventricles. The sinoatrial node, produces a wave of depolarisation which causes the atria to contact and therefore the p wave. The QRS complex represents the contraction of the venticles. The Q wave corresponds to the depolarisation of septum of the heart. Here, the atrioventricular node has conducted the wave of depolarisation down the septum after a short delay down the bundles of his (specialised conductive tissue). The R wave is the contraction of the main mass of ventricle, therefore has the largest peak.