Arrhythmias, or heart rhythm abnormalities, are also known as dysrhythmias. An abnormally fast heart rhythm (greater than 100 beats per minute) is called a tachycardia, and an abnormally slow one (slower than 60 beats per minute) is known as a bradycardia. Some arrhythmias have normal heart or pulse rates, but the heart rhythm is irregular, or the beat starts in parts of the heart other than the sinus (or sinoatrial) node.
A normal heart rhythm is referred to as a “sinus” rhythm because it originates in the sinoatrial node, the heart’s natural pacemaker. From there, it normally spreads to the right and left atria, followed by the atrioventricular node, and then to the right and left ventricles. Irregular heart rhythms, or fibrillations, can be arterial or ventricular. When a heart beat occurs earlier than normal, it is referred to as a premature contraction.
Two kinds of arrhythmia commonly associated with drugs are long QT syndrome and torsades de pointes (TdP). Long QT syndrome can be inherited, induced by abnormal levels of salts found in the blood such as magnesium and potassium, or induced by certain medications. TdP is a form of ventricular arrhythmia, which along with long QT syndrome, is associated with a prolonged QT interval. A QT interval is measured with an electrocardiogram and represents the duration of electrical activity that controls the contraction of heart muscle cells.
TdP may cause blackouts or even sudden death and occurs in people with genetic mutations in genes that control the expression of potassium or sodium channels. It can also occur as a complication of drugs that prolong the QT interval by blocking potassium channels. And while drug-induced TdP is relatively rare, with some drugs its incidence can be as high as 2-3 percent. Drugs with evidence of a risk of causing TdP include:
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In addition to the drugs listed above, the following drugs should be avoided by those with QT syndrome:
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In addition to the drugs on both lists above, the following drugs should also be avoided by those diagnosed or suspected of having congenital long QT syndrome:
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