4The ECGAn upward deflection on the ECG represents depolarisation moving towards the viewing electrode, and a downward deflection represents depolarisation moving away from the viewing electrode.The P wave represents atrial depolarisation - there is little muscle in the atrium so the deflection is small.The Q wave represents depolarisation at the bundle of His; again, this is small as there is little muscle there.The R wave represents the main spread of depolarisation, from the inside out, through the base of the ventricles. This involves large amounts of muscle so the deflection is large.The S wave shows the subsequent depolarisation of the rest of the ventricles upwards from the base of the ventricles.The T wave represents repolarisation of the myocardium. This is a relatively slow process - hence the smooth curved deflection.
5ECG changes in myocardial infarction The changes in the ECG are seen in the leads adjacent to the infarct. In the first few hours the T waves become abnormally tall (hyperacute with loss of their normal concavity) and the ST segments begin to rise.In the first 24 hours the T wave will become inverted, as the ST elevation begins to resolve.Pathological Q waves may appear within hours or may take greater than 24 hr.Long term changes of ECG include persistent Q waves in 90%, persistent T waves. Persistent ST elevation is rare except in the presence of a ventricular aneursym.In non Q-wave infarcts, ST depression and T wave inversion occur without ST elevation.There may be ST depression in the leads opposite to the site of the infarct.In Type 1 DM a small infarct on ECG may hide large haemodynamic changes.