The binding of the class III and Ia drugs to the potassium channel is not use dependent. Thus, these drugs do not cause greater lengthening of the QT interval at more rapid rates than at slower rates. Indeed, the reverse is true, and the magnitude of the QT lengthening is often greater at slower rates than at the more rapid rates. This phenomenon has been called “reverse use dependency”. The graphs shown here illustrate the effects of rate on QRS duration and on QT interval in the absence and presence of class I and III antiarrhythmic drugs. Amiodarone causes less reverse use dependency than the other class III drugs,