Linear elimination kinetics but non-linear pharmacodynamics in theophylline intoxication in a child

In an infant's acute theophylline intoxication following a medication error, plasma levels above 80 micrograms/ml were not associated with repetitive seizures, but also with heart rates above 200 bpm. The serum elimination kinetics were characterized by a first-order elimination process with a half-life of 15 h. Despite the exponential decay of plasma levels the heart rate remained at a high level during the first 20 h. This behavior is explained by a saturation phenomenon familiar in receptor and enzyme kinetics.