Familial sinoatrial node dysfunction. Increased vagal tone a possible aetiology.

Clinical and electrophysiological studies of a 13-year-old boy with sinus bradycardia revealed sinus node dysfunction. Long-term follow-up data of members of his family indicated familial sinus node dysfucntion. Increased vagal tone was present in all patients. It is suggested that excessive vagal discharge for a pronlonged time may be the basic mechanism of sinus node dysfunction in these patients.