Vardenafil-associated QTc changes: not merely a normal autonomic process.

To the Editor: The QT-RR interval relationship is too complex to be described by using simple correction factors. Because the regulatory and commercial impact of even minor errors in drug studies can be far reaching, these correction factors have come under scrutiny. The letter by Dr Barbey, “Vardenafil-Associated QTc Changes: Not Merely a Normal Autonomic Process,” emphasizes the important point that many facets of the autonomic state that are not readily discernible may come into play.1 The QT interval repolarization phase is activated largely by the sympathetic nervous system, whereas basal heart rate is controlled more through the faster-reacting parasympathetic system.2 When heart rate increases rapidly in response to a stimulus such as standing, parasympathetic control of the heart is rapidly withdrawn, resulting in a quick reduction in RR interval. On the other hand, the QT interval changes slowly