It is often difficult to distinguish between coupled extrasystoles and parasystole, since there are many cases with variable but non-parasystolic extrasystoles. Extrasystoles with variable coupling and parasystolic ectopic beats are usually associated with electrocardiographic signs of cardiac disease, and are uncommon in normal subjects. The QT segment appears to be unaltered in patients with extrasystoles. In variable and fixed coupling, the ectopic beats tend to occur just after the T wave, possibly because of a supernormal phase of response to an ectopic focus. Digoxin is associated with both variable and fixed coupling of extrasystoles.