How does the heart respond to asphyxia ?

During asphyxia, your sympathetic system will be activated. When there is still some oxygen reserve in your lung and blood, your heart will be stimulated to pump faster and contract harder. This will likely last no longer than 5 minutes before the oxygen becomes depleted. After then, because no oxygen is supplied to the heart, the heart muscle will become ischemic. It means that the heart muscle cannot function properly (cannot contract, cannot relax). However, if oxygen is supplied at this time, the ischemia can be reversed. As this process goes on, the muscle will eventually infarcted (dead). Nothing can be done at this point to reverse the damage.