Clint Eastwood’s longtime partner Christina Sandera died of a heart attack. The ‘Unforgiven’ actor and director, 94, was left shattered when his 61-year-old companion of 10 years died last week, and a death certificate has now revealed she was killed by cardiac arrhythmia – which causes the heart’s rhythm to become too slow, rapid or irregular. A representative from the Monterey County Health Department confirmed the cause of death. Atherosclerotic coronary artery disease was cited as an additional condition that led to Sandera’s death. It occurs when deposits of fat, cholesterol, calcium and …