OBJECTIVE : Little is known about the dynamics of cardiac compression during cardiopulmonary resuscitation (CPR). The purpose of this study was to investigate the dynamics of chest compressions by analyzing movement of the right ventricular free wall excursion during CPR. METHODS : Patients older than 18 years with non-traumatic cardiac arrest who received CPR were enrolled. During CPR, transesophageal echocardiography (TEE) was performed and M-mode tracing was performed at the maximal compression area of the right ventricular free wall to measure the dynamics of cardiac compression. RESULTS : Twenty patients (mean age, 71 years; 14 males) were enrolled. The median compression depth of the right ventricular free wall was 34.4 (IQR 24.5-44.5) mm. The median duration of one compression-relaxation cycle was 0.59 (IQR 0.58-0.60) s, which comprised 0.23 (IQR 0.19-0.27) s of compression and 0.36 (IQR 0.32-0.39) s of relaxation. The median compression velocity was 162 (IQR 95-215) mm/s, and the median relaxation velocity was 93 (IQR 75-121) mm/s. Compression depth was linearly correlated with mean compression velocity (r = 0.882, p < 0.001). Compression velocity had a negative correlation with the ratio of compression-relaxation time (r=-0.711, p < 0.001). CONCLUSIONS : Maintaining high compression velocity is helpful in achieving adequate compression depth during CPR.