INTRODUCTION Acute chest pain is one of the most common causes for emergency medical care seeking. In some cases, it is considered as a manifestation of acute coronary syndrome (ACS). However, in patients with normal cardiac troponin (cTn) levels and a normal or nondiagnostic electrocardiogram (ECG) (non-ST elevation ACS (NSTE-ACS) low-risk patients), the incidence of stenotic coronary atherosclerosis is not high. Stress echocardiography in this category of patients is indicated for ischemia visualization. Analysis of heart rhythm, ECG changes, systemic and intracardiac hemodynamics, in addition to assessing local contractility, allows us to identify other pathological responses to stress. It is already known that their comprehensive assessment has predictive potential for long-term adverse events in patients with chronic coronary syndrome. THE AIM To identify non-ischemic phenotypes in low-risk patients with non-ST elevation acute coronary syndrome. MATERIAL AND METHODS A single-center, prospective, continuous study included 70 patients aged 54 (45; 63) years (60% men) hospitalized in the department of the regional vascular center in 2022—2023 with acute chest pain, normal or non-diagnostic ECG, normal cTn level, low (< 3 %) GRACE risk and no restrictions for exercise stress echocardiography. The patients had not previously verified coronary heart disease (which was an inclusion criterion) and had no wall motion abnormality at rest. Stress echocardiography on a horizontal bicycle ergometer was performed in the first 2 (0; 3) days of the hospital period. The tests were carried out in the absence of rhythmslowing therapy. The results of tests completed early for subjective reasons (muscle fatigue, failure) were not included in further analysis. To determine patient phenotypes based on stress echocardiography data, pathological responses to stress in the form of dichotomous variables were used to perform cluster analysis. The final group consisted of 50 of a nonischemic phenotype — those who did not belong to the cluster, which was characterized by the highest frequencies of wall motion abnormality, ST segment depression > 1 mm, and angina. To determine the potential significance of the identified phenotypes, the cardiac performance reserve (P/m) was additionally determined as the difference in P/m values, calculated using the formula 0,222 - CO - APBmean LVM where CO — cardiac output; APBmean — mean arterial blood pressure; LVM — left ventricular mass), at peak load and at rest. RESULTS Four patient phenotypes were identified that differed statistically significantly (p < 0.05) in terms of exercise tolerance, volumetric contractile reserve and heart rate reserve. Type 1 (n = 17) was characterized by a high frequency of combined decreases in all 3 indicators; type 2 (n = 11) — by the decreased exercise tolerance and heart rate reserve, normal contractile reserve; type 3 (n = 11) — by normal exercise tolerance and heart rate reserve, but decreased contractile reserve; type 4 (n = 11) — by normal exercise tolerance, contractile reserve and heart rate reserve. The decrease in contractile reserve (types 1 and 3) was consistent with a statistically significant lower increase in left ventricular ejection fraction at peak exercise (5 (2; 7) vs. 12 (8; 13), respectively; p < 0.001) and P/m reserve (1.0 (0.6; 1.4) versus 1.3 (1.1; 1.6), respectively; p = 0.010). Patients with decreased contractile reserve were statistically signif icantly older (60 (48; 68) vs. 47 (43; 54) years, respectively; p = 0.003). Type 4 included predominantly men (91 %). For type 2, no statistically significant differences in the clinical and demographic data of patients were identified. CONCLUSION In the structure of patients with low-risk patients with NSTE-ACS, the phenotypes are identified that are determined by a decrease in exercise tolerance, contractile deficiency and chronotropic reserve. Phenotypes are characterized by different levels of P/m reserve and may differ in the frequency of adverse events in the long-term period, and a decrease in contractile and heart rate reserve can be considered as potential targets for personalized therapy, however, in the diagnostic algorithm for ACS this remains a topic for further research.