The Usefulness of Intraoperative Measurement of Beta Natriuretic Peptide in Evaluating Diastolic Dysfunction in Patients Undergoing on-Pump Coronary Artery Bypass Grafting, YEHIA H. KHATER, AHMED M. MUKHTAR, MOHAMMED A. BAKRY, RIHAM E. HANAFY and RAMI M. RAMZI