This part of the electrocardiogram is altered by many factors. Among these are drugs disturbances of electrolyte balance, certain endocrine diseases and most important of all myocardial ischaemia with or without subsequent infarction. The S-T segment and the T wave are also affected by primary myocardial disease-myocarditis or cardiomyopathy. Ischaemia causes a change in the electrical state of the affected area of the myocardium, and these results in elevation or depression of the S-T segment, depending on whether the sub-epicardial or sub-endocardial layer of muscle is mainly involved, and also on the position of the exploring electrode. Elevation is seen in acute pericarditis because of the widespread sub-pericardial damage. Depression is seen in ischaemia (e.g. during an attack of angina pectoris), in gross ventricular hypertrophy (probably due to a relative myocardial ischaemia or to fibrosis) and in bundle branch block.
