The goal of this prospective observational study is to assess the diagnostic accuracy of the tissue Doppler imaging (TDI) S' wave in detecting acute coronary syndrome (ACS) in adult patients presenting to the emergency department (ED) with acute chest pain. This study focuses on patients aged 18 years or older, who require continuous cardiac monitoring but do not show ST-elevation myocardial infarction (STEMI) on their initial ECG. The main questions it aims to answer are: * Can TDI S' wave velocity serve as an early diagnostic marker for ACS in the emergency department? * How does the diagnostic performance of TDI S' compare with other echocardiographic markers (MAPSE, TAPSE, and diastolic parameters such as E, E', A, E/A, E/E')? * Do demographic factors (age, sex, BMI, echogenicity) influence the diagnostic accuracy of echocardiographic parameters for ACS? If there is a comparison group: Researchers will compare TDI S' wave velocity findings with the final adjudicated diagnosis of ACS (determined after 3 months) to evaluate its sensitivity and specificity.