Why would someone with aortic stenosis be at risk for sudden death?

Fixed cardiac output: With aorttic stenosis the amount of blood pumped by the heart is fixed below a specific threshold as it cannot increase the output even with increased demand as with exercise etc. Consequently, the heart , the brain or any other organ will not get the increased blood supply it needs and therefore sudden death from irregular heart beats, myocardial infarction or syncope and brain ischemia can kill.