Madam Heng died of pneumonia. Her sons allege 'negligence and serious lapse of due diligence'. -- PHOTO: COURTESY OF PATRICK CHAN By Salma Khalik , HEALTH CORRESPONDENT The National University Hospital (NUH) is investigating how an elderly woman died while recovering from successful heart surgery. One doctor involved in her care has since resigned, and another has decided against pursuing specialist training to become a surgeon. Madam Heng Choon Noi, 74, was operated on by cardiothoracic surgeon Lee Chuen Neng on Oct 11, a Tuesday. Three days later, she was moved from intensive care to a general ward and then a surgical ward.