Background: Diastolic dysfunction (DD) is a negative predictor in acute coronary syndrome (ACS). The 2016 American Society of Echocardiography (ASE) and the European Association of Cardiac Imaging (EACVI) DD guidelines (2016 ASE/EACVI guidelines) were validated in stable patients. We assessed the effect of implementing the 2016 ASE/EACVI guidelines in a population of patients with high-risk ACS by correlating invasively measured left ventricular diastolic pressures or pulmonary capillary wedge pressure within 8 h of the echocardiography estimated left ventricle (LV) filling pressures and determined how easily general cardiologists could apply the 2016 ASE/EACVI guideline parameters to see if this affects accuracy. Methods: Two groups of independent investigators, group FT, full-time echocardiography consultants, and group PT, part-time board-certified general cardiologists, blinded to each other and the invasive LV measurements, reviewed digitally stored echocardiography images. The researchers applied the 2016 DD guidelines definitions to 101 (24% females, mean age 57.2 ± 12.5, and a mean BSA 1.8 ± 0.2) high-risk ACS troponin I-positive patients admitted to the coronary care unit between December 2007 and February 2018. DD grade assessment using 2009 guidelines was obtained from the echocardiography reports. Results: The 2009 ASE/EACVI guideline results showed 0.58 sensitivity and 0.74 specificity. The 2016 ASE/EACVI guidelines in the FT group had 0.46 sensitivity and 0.74 specificity, and in the PT group, 0.45 sensitivity and 0.86 specificity. Conclusions: The agreement with 2016 ASE/EACVI guideline definitions in high-risk ACS patients was weak to intermediate. The 2016 ASE/EACVI guidelines classification and the non-invasive left ventricular filling pressure assessment were easy to implement compared to the 2009 ASE/EACVI guidelines. Our study validates the 2016 ASE/EACVI guideline algorithm in patients with high-risk ACS.