Almost all ECG changes detected in such processes occur as a result of changes in the concentration of extracellular calcium in the form of hypercalcemia or hypocalcemia. With hypoparathyroidism, the only obvious change is the lengthening of the QTc interval, which occurs due to the ST segment, since the duration of the T wave is normal. However, the configuration of the T wave can change: it can smooth out or become inverted. In such cases, the intravenous administration of calcium can cause temporary tooth T positivity. Such a change in repolarization is believed to be the result of changes in the sequence of ventricular repolarization due to the existence of two zones in the myocardium with different durations of action potentials. With hyperparathyroidism, the QTc interval is shortened in most patients and is directly proportional to the increase in plasma calcium level. Other changes are rare and completely nonspecific. Diabetes mellitus and pneumothorax on the ECG Even in the absence of hypertension, coronary heart disease or myocardial infarction, changes in the ECG in diabetes mellitus occur frequently and in separate studies they were observed in 50% of adult patients with diabetes. The most common changes are left AQRS deviations and non-specific repolarization disorders in patients without obvious heart disease. Such changes are more common in patients with type II diabetes and coronary disease than in coronary patients without diabetes. In one study, the VKG revealed changes in the QRS loop of the same type with focal intramyocardial fibrosis or with microinfarctions not recorded by a normal ECG. In diabetes with ketoacidosis, the ECG may reveal changes in the electrolyte balance (K +) that occur in this condition, although not all researchers agree with the role of the ECG in evaluating the treatment of ketoacidosis. Other repolarization changes, such as a negative T wave or ST segment elevation, which suggest pericarditis, sometimes occur. With panhypopituitarism, Addison’s disease and acromegaly, there are changes in repolarization, in particular, T wave inversion and lengthening of the Q — T interval. Spontaneous pneumothorax, especially on the left, is characterized by the following disorders: a) AQRS deviation to the right; b) QS configuration in lead V1-2 or even in V3 or V4; c) a decrease in the QRS complex in lead I, sometimes with an abnormal q wave in lead I and aVL; d) negative T wave of ischemic type in lead II, III and / or I, a, VL, V5 and V6. Changes in the ECG may be less pronounced, in particular if the ECG is recorded in a sitting position. After elimination of pneumothorax, changes on the ECG decrease or disappear. It has been suggested that the mechanism of electrocardiographic changes is a combination of specific cardiac rotation due to pneumothorax, and air entering the space between the heart and chest wall. It is useful to be aware of such electrocardiographic changes because they may resemble anterior wall infarction or other types of coronary heart disease.