![]() ![]() Electrolytes: hypokalaemia, hypocalcaemia, hypomagnesaemia.Dysfunction of cardiac ion channels causing slow ventricular repolarisation. Different subtypes based on genetic mutation. Long QT syndrome: inherited condition.There are numerous causes of a prolonged QT interval which can be inherited or acquired. If the QT interval is prolonged there is an increased risk of developing polymorphic VT (Torsades de pointes) and sudden death. The normal QTc is different for Men and Women: One such formula is the Bazett: QTc = QT / √RR. Consequently, the corrected QT interval (QTc), which estimates the QT interval at a standard heart rate, may be calculated using a number of different formulas. The QT interval is affected by heart rate (lengthening with slow heart rates and shortening with fast heart rates). The QT interval represents the total time taken for ventricular depolarisation and repolarisation. The QT interval describes the time between the start of the Q wave and end of the T wave. In this type of myocardial infarction there is a dominant R wave in V1 similar to the findings in right ventricular hypertrophy. NOTE: a posterior infarction is not associated with Q waves on a normal 12-lead ECG. Lateral infarction: Q waves affecting leads V5-V6, I, aVL.Anterior infarction: Q waves affecting leads V2-V5.Inferior infarction: Q waves affecting leads II, III, aVF.Once Q waves have developed it is permanent. They can suggest previous infarction but do not give an indication of age. Q waves develop in ischaemia because of creation of an electrical window within the non-viable myocardium. Q waves indicate previous or current myocardial infarction. However, when Q waves appear greater than 40 ms (1 small square) in width, > 2 mm in depth and > 25% of the depth of QRS complex they are considered pathological. V5, V6) occur due to septal depolarisation. Q waves in the left ventricular leads (i.e. Q waves may be a normal finding due to septal depolarisation or indicate underlying ischaemia. Hypertrophy leads to an increase in electrical activity, which is seen as an increase in QRS complex height. We call the increase in mass either right or left ventricular hypertrophy. The height, or size, of the QRS complex is largely dependent on the ventricular muscle mass. For more information see our notes on Conduction and Rate & rhythm. In each of these cases, the width of the QRS complex increases (> 120 ms) due to depolarisation of the ventricles through a slower, abnormal pathway. The width of the QRS complex can change due to problems with conduction (e.g. V1) and the R wave predominates (R size > S size) in the left ventricular leads (i.e. The normal QRS complex is R) in the right ventricular leads (i.e. The QRS complex may be affected by ventricular muscle mass and conduction disturbances. ![]()
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