Acknowledgement
In writing this paper, I have benefited from the presence of my teachers and my colleagues. They generously helped me collect materials I needed and made many invaluable suggestions. I hereby extend my grateful thanks to them for their kind help. Particularly, I am deeply indebted to Professor Qian Zhai, my supervisor, who guided me throughout my writing of this thesis. She carefully read the whole draft and offered painstaking and precious criticism. Her standards of academic excellence have made my revision and exciting and gratifying experience. I also wish to sincerely thank my director, whose brilliant ideas and perceptive observations have proved immensely constructive. Furthermore, none of this would have been possible without the help of those individuals and organizations hereafter mentioned with gratitude: our department and its staff, the department of cardiovascular surgery of qilu hospital of shandong university and its staff.