Introduction: Recently, the effectiveness of the EA during CPR has been questioned. For some time, we have been analyzing the cerebral tissue oxygen saturation observed during CPR. During AHA 2014, we reported the possibility that an increase in the cerebral tissue oxygenation index (TOI) would predict the recovery of spontaneous circulation (ROSC). This time, we report the results of our clinical study about the relationship between the EA and changes in TOI. Methods: In 59 patients undergoing out-of-hospital cardiac arrest, the blood pulsation waveform and TOI in the brain were continuously monitored with the NIRO-200NX system (Hamamatsu Photonics K.K., Japan) using its pulse-observing mode (NIRO-Pulse mode). The total number of EAs was 86 and, as shown in the figure, the initial and terminal TOIs in the CPR before EA (2 minutes) and the initial and terminal TOIs in the CPR after EA (2 minutes) were measured. Results: The initial and terminal TOIs before EA were 37.6±7.2% and 40.0±7.2% respectively, and the increase in TOI during the CPR (αTOI) was 2.4±3.4%. On the other hand, the initial and terminal TOIs after EA were 37.6±6.6% and 44.0±8.4% respectively, and the αTOI was 6.5±4.8%. Discussion: Since the observation of blood pulsation is not possible with conventional cerebral oxygenation monitors, the relationship between the administration of CPR and changes in cerebral tissue oxygen saturation has not been made clear. However with the NIRO-Pulse mode, the changes in TOI caused by CPR are clearly observed. And, our observation that the terminal TOI and αTOI in the CPR after EA were significantly higher than those before EA suggests that cerebral tissue oxygen saturation is improved by EA. On the other hand, the increase in TOI caused by EA (αTOI) has a certain variation, which suggests the most effective time to administer epinephrine would exist and its effects would differ depending on each patient.