Recognition of a prolonged QT interval is complicated by the problems inherent in identification of T wave end and the inaccuracy of Bazlet's formula for correcting for heart rate. It may be preferable to judge the QT interval by changes in length or when it exceeds 50% of the R-R interval. QT prolongation can be due to rare hereditary abnormalities, to type Ia anti-arrhythmics such as quinidine, and to hypocalcemia or hypomagnesia. The importance of recognizing prolonged QT interval is that it is associated with PVC's, ventricular tachycardia and fibrillation (ie., vulnerability to lethal arrhythmia). A particularly woresome associated type of ventricular tachycardia is torsades de pointes (turning of the points).

The following is a list of conditions that can cause QT prolongation

• Hereditary syndromes (rare)
• Electrolyte/metabolic abnormalities including hypocalcemia, hypokalemia or hypomagnesia
• Intrinsic cardiac disease
• Medications including type Ia anti-arrhythmics such as quinidine and norpace; tri-cyclic anti-depressants, anti-histamines such as seldane, anti-cholinergic drugs such as cisapride (propulsid) and anti-fungals.
• CNS disorders
• Systemic illnesses
• Myocardial Infarction

Obtain blood chemistries including electrolytes, glucose, calcium, magnesium, BUN and creatinine. Take a careful medication history including non-cardiac drugs such as decongestants, anti-GERDs and antibiotics. Ask specifically regarding a family history of syncope or sudden death. If receiving Quinidine a level should be obtained and medications that can raise Quinidine levels should be considered.