Editor,—A 1 year old girl with pulmonary atresia developed an acute right hemiparesis. She had had an intact ventricular septum at birth. Ventricular septostomy and insertion of a Blalock-Taussig shunt were carried out at 1 week. She had otherwise been well and was developmentally age appropriate. At the time of the hemiparesis, she was only mildly polycythaemic (packed cell volume = 0.426). There were no new findings on praecordial …