The Effect of Beta-Sympathetic Blockade on Arterial Oxygen Saturation in Fallot's Tetralogy

In many patients with Fallot's tetralogy, the fall in arterial oxygen saturation that occurs during exercise can be reduced by prior administration of the beta-sympathetic blocking drug, pronethalol. In a few patients there is also a rise in arterial oxygen saturation at rest.During exercise, increased stimulation of beta-sympathetic receptors in the heart enhances myocardial contractility; this leads to more complete systolic emptying of the right ventricle, and increased systolic obstruction to ejection of blood into the pulmonary circulation by hypertrophied infundibular muscle. It is this dynamic component of the right ventricular outflow tract obstruction that is abolished or reduced by beta-sympathetic blockade. The phenomenon occurs most readily when valve or high infundibular stenosis is associated with infundibular muscle hypertrophy proximal to the organic obstruction, and can sometimes be demonstrated by angiocardiography.Beta-sympathetic blocking drugs may have a place in the treatment of cyanotic attacks in Fallot's tetralogy, and possibly in the long-term management of the patients in whom total correction has not yet been undertaken.