Ten fresh canine hearts were used to compare the peak left ventricular pressure required to disrupt prosthetic mitral valves sutured in place with horizontal mattress sutures with either subannular or supraannular pledgets. Each group consisted of 5 animals. A 29-mm Medtronic mitral valve was secured in the mitral position with ten pledgeted sutures. The aorta was cannulated and normal saline solution was infused into the left ventricle until valvar disruption occurred. The peak pressure and the location and mechanism of disruption were then noted. At the peak left ventricular pressure required for valvar disruption, no individual sutures were broken. Instead, in all specimens a subannular myocardial rupture occurred in the posterior portion of the mitral annulus along the extent of the atrioventricular groove. In addition, the posterior wall of the left atrium dissected upward subsequently. Significantly greater pressures were required in the group with subannular suture placement as compared with the supraannular group (354 +/- 37 versus 236 +/- 33 mm Hg; p less than 0.0007). These data suggest that placement of horizontal mattress sutures with pledgets in the subannular position is superior to the currently recommended method of supraannular suture placement in mechanical valves.