a QTc above 450 ms; and, in females, above 470 ms. If there is not a very high or low heart rate, the upper limits of QT can roughly be estimated by taking QT = QTc at a heart rate of 60 beats per minute (bpm), and subtracting 0.02 s from QT for every 10 bpm increase in heart rate. For example, taking normal QTc ≤ 0.42 s, QT would be expected to be 0.42 s or less at a heart rate of 60 bpm. For a heart rate of 70 bpm, QT would roughly be expected to be equal to or below 0.40 s. Likewise, for 80 bpm, QT would roughly be expected to be equal to or below 0.38 s. Abnormal intervals Prolonged QTc causes premature action potentials during the late phases of depolarization. This increases the risk of developing ventricular arrhythmias, including fatal ventricular fibrillation. Higher rates of prolonged QTc are seen in females, older patients, high systolic blood pressure or heart rate, and short stature. Prolonged QTc is also associated with ECG findings called Torsades de Pointes, which are known to degenerate into ventricular fibrillation, associated with higher mortality rates. There are many causes of prolonged QT intervals, acquired causes being more common than genetic. Genetic causes An abnormally prolonged QT interval could be due to long QT syndrome, whereas an abnormally shortened QT interval could be due to short QT syndrome. The QTc length is associated with variations in the NOS1AP gene. The autosomal recessive syndrome of Jervell and Lange-Nielsen is characterized by a prolonged QTc interval in conjunction with sensorineural hearing loss. Due to adverse drug reactions Prolongation of the QT interval may be due to an adverse drug reaction. Antipsychotics (especially first generation/"typical") haloperidol thioridazine mesoridazine chlorpromazine sertindole DMARDs and antimalarial drugs hydroxychloroquine chloroquine quinine Antibiotics macrolides fluoroquinolones Other drugs methadone vemurafenib pitolisant Some second-generation antihistamines, such as astemizole, have this effect. The mechanism of action of certain antiarrhythmic drugs, like amiodarone or sotalol, involve intentional pharmacological QT prolongation. In addition, high blood alcohol concentrations prolong the QT interval. A possible interaction between selective serotonin reuptake inhibitors and thiazide diuretics is associated with QT prolongation. Due to pathological conditions Hypothyroidism, a condition of low function of the thyroid gland, can cause QT prolongation at the electrocardiogram. Acute hypocalcemia causes prolongation of the QT interval, which may lead to ventricular dysrhythmias. A shortened QT