Background: Since the greater part of coronary blood flow takes place during the diastolic phase of each cardiac cycle, a time indicator of myocardial perfusion reserve, the ratio of diastolic to systolic duration (D/S ratio), was presented. The objective of this study was to evaluate the accuracy and precision, the biological implication, and the applications of D/S ratio. Methods: Multi-center pragmatic studies evaluating the time indicator of cardiac perfusion reserve were performed. Related experiments, clinical trials, and surveys were conducted at 5 centers. Results: The results showed that the measurement of D/S ratio is both accurate and precise; the mean values of D/S of all of the 3 species studied (human, rabbit, and rat) were greater than 1. These application studies on D/S ratio showed that a close negative correlation existed between D/S ratio and New York Heart Association Functional Classification (NYHA FC) (r = –0.659, p < 0.01); normal persons were mostly distributed at NYHA FC I and at high value of D/S ratio; the patients with cardiovascular disease were mostly at low value of D/S ratio; the difference of D/S between pregnant women with pre-eclampsia and either normal pregnant women or non-pregnant women were significant (p < 0.05); athletes had higher D/S ratio than non-athletes (2.04 ± 0.33 vs 1.82 ± 0.27, p < 0.01). Conclusions: D/S ratio has important biological implication, which is a safe, easy, reliable, and effective indicator, can be used to evaluate fitness levels, served as a pathophysiological marker for screening of cardiovascular disease (CVD), for predicting risk of cardiac events, and for evaluating the severity and prognosis of CVD.
 Abe, M., Tomiyama, H., Yoshida, H. and Doba, N. (2000) Diastolic fractional flow reserve to assess the functional severity of moderate coronary artery stenoses: Comparison with fractional flow reserve and coronary flow velocity reserve. Circulation, 102, 2365-2370. doi:10.1161/01.CIR.102.19.2365
 Xiao, S., Guo, X., Wang, F., Xiao, Z., Liu, G., Zhan, Z. and Sun, X. (2003) Evaluating two new indicators of cardiac reserve. IEEE Engineering in Medicine and Biology Magazine, 22, 147-152. doi:10.1109/MEMB.2003.1237516
 Cooke, G.A., Marshall, P., Al-Timman, J.K., Wright, D.J., Riley, R., Hainsworth, R. and Tan, L.B. (1998) Physiological cardiac reserve: Development of a non-invasive method and first estimates in man. Heart, 79, 289-294.
 Xie, M., Xiao, S., Liu, T., Yi, Q., You, F., Guo, X., Shao, Y., Huo, J., Du, D., Xu, D., Wu, W., Xiao, Z., Yang, Y. and Guo, W. (2012) Multi-center, multi-topic heart sound databases and their applications. Journal of Medical Systems, 36, 33-40. doi:10.1007/s10916-010-9443-x
 Huang, G., Xin, H. Shi, L., Huang, G., Xiao, Z., Xie, M., Yang, Y. and Xiao, S. (2010) A preliminary survey on two new indicators of infantile cardiac reserve. Labora- tory Medicine and Clinic, 7, 1304-1308.
 Shao, Y., Zhang, Y. and Liu, O. (2012) Using phonocardiography to investigate maternal cardiac reserve function in gestational hypertension and pre-eclampsia. Journal of Obstetrics and Gynaecology Research, 39, 53-60. doi:10.1111/j.1447-0756.2012.01897.x
 Xiao, S., Fang, X., Zhan, Z., Sun, X. and Xiao, Z. (2002) A clinical study in middle-aged and elderly persons on heart itself blood supply time during diastole with a non- invasive method. Chinese Journal of Medicine, 2, 961-962.
 Fraser, G.E., Luke, R., Thompson, S., Smith, H., Carter, S. and Sharpe, N. (1995) Comparison of echocardiogra- phic variables between type I diabetics and normal con- trols. American Journal of Cardiology, 15, 141-145. doi:10.1016/S0002-9149(00)80063-6
 Friedberg, M.K. and Silverman, N.H (2006) Cardiac ven- tricular diastolic and systolic duration in children with heart failure secondary to idiopathic dilated cardiomyo- pathy. American Journal of Car-diology, 97, 101-105. doi:10.1016/j.amjcard.2005.07.127
 Friedberg, M.K. and Silverman, N.H. (2006) The systolic to diastolic duration ratio in children with heart failure secondary to restrictive cardiomyopathy. Journal of the American Society of Echocardiography, 19, 1326-1331. doi:10.1016/j.echo.2006.05.024
 Friedberg, M.K. and Silverman, N.H. (2007) The systolic to diastolic duration ratio in children with hypoplastic left heart syndrome: A novel Doppler index of right ventricular function. Journal of the American Society of Echocardiography, 20, 749-755. doi:10.1016/j.echo.2006.11.014
 Alkon, J., Humpl, T., Manlhiot, C., McCrindle, B.W., Reyes, J.T. and Friedberg, M.K. (2010) Usefulness of the right ventricular systolic to diastolic duration ratio to predict functional capacity and survival in children with pulmonary arterial hypertension. American Journal of Cardiology, 106, 430-436. doi:10.1016/j.amjcard.2010.03.048
 Sehgal, A., Athikari-samy, S.E. and Adamopoulos, M. (2012) Global myocardial function is compromised in infants with pulmonary hypertension. Acta Paediatrica, 101, 410-413. doi:10.1111/j.1651-2227.2011.02572.x
 Koestenberger, M., Friedberg, M.K., Ravekes, W. and Nestaas, E. (2012) Non-Invasive Imaging for Congenital Heart Disease: Recent Innovations in Transthoracic Echocardiography. Journal of Clinical & Experimental Cardiology, S8, 2.