Conduction system injury after aortic valve dilation in the dog single- versus double-balloon catheters. The range of morbidity induced by valvuloplasty is not fully known, but transient conduction disturbances are common. The authors performed aortic valve balloon dilatation on 10 closed-chest dogs with normal aortic valves, using a femoral cutdown approach and fluoroscopic guidance. Four were done with a single 15 mm balloon catheter, and in the other 6 two 12 mm balloon catheters were used. Balloons were inflated to 5 to 12 atms pressure with contrast solution. After several inflations the dogs were sacrificed, the hearts removed and examined. Gross examinations revealed subendocardial hemorrhage in the outflow tract in 5 of the 6 in which double balloons had been used. Microscopically, all aortic valve areas showed hemorrhage, mostly in loose connective tissues of the valve leaflets. The severity of injury appeared greater when two balloons had been used. Histologic examination showed definite injury to the myocytes of the left bundle branch in all 6 of the double-ballooned dogs, but in none of those subjected to the single-balloon procedure. During aortic valve dilation the only manifestation of conduction system injury was prolongation of the QRS complex in 3 of the 6 dogs in which a double-balloon catheter had been used. The results suggest that electrocardiographic conduction disturbances observed in patients undergoing aortic valvuloplasty may be the result of direct injury of conduction tissue and may be more likely to occur when larger balloons are used. 