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Purpose: We reported diastolic segmental left ventricular (LV) wall dysfunction starts from 50% coronary artery stenosis (CAS) last year. Since myocardial ischemia (MI) is said to start from 75% CAS, we tested a pig study to confirm if 50% CAS causes MI. The results showed that 50% CAS decreased both coronary artery flow (25.1±6.7%)(p<0.001) and segmental myocardial tissue O2 pressure (38.2±9.8%)(p<0.001). Systolic segmental LV wall function was evaluated using a strain rate (SR) function (see Table 1]). With this function, systolic segmental function decreases as Z increases. The Z values were -0.59±2.6 in normal pigs and 1.96±1.05 in 50% CAS pigs (p<0.0001). Based on these results, we studied the effect of 50% CAS on both systolic and diastolic segmental LV wall functions in clinical patients.Methods: Patients (n=86) who showed single CAS lesion on the same coronary artery by coronary angiography and agreed informed consents were enrolled in this study. Z values, peak E values and peak E/E time values (see Table 2]) were measured in each 4 CAS group [See Table].Results: [See Table] By SR analysis, systolic and diastolic segmental LV wall dysfunctions started from 50% CAS.Conclusion: In contrast to conventional wisdom, systolic and diastolic segmental LV wall dysfunctions are suggested to start from 50% CAS. The clinical significance of 50% CAS should be reconsidered.