Mechanism of cyanotic spells in tetralogy of Fallot--the missing link?

The mechanism of cyanotic spells in patients with tetralogy of Fallot is not clear. Increases in infundibular contractility or hyperpnoea have been considered as the key factors, but this explanation appears inadequate. In this review, arguments are presented against these commonly held views. Preliminary evidence is synthesised in favour of another more plausible hypothesis that cyanotic spells may result from mechanoreceptor stimulation from the right ventricle. Increased contractility (due to catecholamines) and decreased right ventricular size (due to various factors) can trigger a reflex resulting in hyperventilation, some peripheral vasodilation without bradycardia, and this may initiate a spell. This mechanism explains most of the precipitating events and many other issues about cyanotic spells more satisfactorily.